Mod 13-Kiln Volatiles.pdf

Process EngineeringTraining Program

MODULE 13Kiln Volatiles

Section Content

1 CETIC “Volatiles” Group- Final Progress Report

2 CETIC Sub Commission “Behavior of Volatile Material in Kiln Systems

3 Investigation into Potential Low Temperature Volatilization

4 Factors Affecting Sulphate and Alkali Cycles in Rotary Kilns

5 Alkali Volatilization- A Review of Literature Available in 1977

6 A Study in the Volatile Cycles on HOPE # 2 kiln

7 Design and Experience with Bypasses for Chloride, Sulphate, and Alkalis

8 Kiln Gas Bleed Considerations

9 Ring Formations in Cement Kilns

10 Kiln Build-Up Meeting

11 Cement Seminar- Rings, Balls, and Build-Ups

12 Ring and Buildups in Cement Kilns

HBM Process Engineering ConferenceMinimization of Volatile Cycles

Blue Circle Cement

PROCESS ENGINEERING TRAININGPROGRAM

Module 13

Section 1

CETIC “Volatiles” Group- FinalProgress Report

Blue Circle Indusuies PLC

Internal memo

3 SEE BELOW:-

‘rot? C P KERTON

Your re:: Date 21 December 1994

Copies

Stijec CETIC “VOLAl-IUS” GROUP - FINAL PROGRESS REPORT

Herewith a copy of an English version of the final report from the CETIC GOUPwhich worked from 1990-1992, initially inspired by 3lue Circle’s initiatives.Copies of various French documents exchanged within the group are availablefrom me or from Cb.ris Hoit.

The text follows the order of the French original, intended for those who tookpart in the work. For those who have not been following progress so closely, thelogical order of reading would be to stan with Appendix 2, which is a publishedpaper based on the initial work of the group on cycles of chlorides, sulfates andalkalis driven from the burning zone. Next comes Appendix 3, which isTom Lowe-s’ account of some practical applications of these concepts whenburning petroleum coke. Returning then to the start of the main text, there is anaccount of further discussions and activity on this topic leading finally to

Appendix 1 which looks at capture of SO:, in colder parts of a kiln system. (Thoseconcerned soleIy with pollution abatemerit might well start here!)

Proposals for possible further work are listed and those relating to sulfurbehaviour will be pursued by joint exchanges of experiences from 1995.

Suggestions for further distribution within Blue Circle will be welcomed.

cf&L

I&,/ >c P KertonPatent & Information Service

Eric

To: ;P L Rover (2 copies)C P B ‘Turner

T M LowesC J HoltL P EvansR M MutterJ M LawtonP A Longman

ca: \?%+!\2112CETIC

Blue Circle Technical Centre

TC 94 049

CEl-lC SlJB-CO-ON

“BEHAVlOUR O F VOLATILE M A - IN KILNS”

FINAL. PROGFZES REPORT

MAY 1994

This report is strictly confidential within the Blue Circle Group.

Additional copies should not be released outside Blue Circle

without reference to the Technical Cenme.

Please apply to the hformation Servicesat tie adcks give0 below.

September 1994

S U M M A R Y

Together with an earlier report (ISTN 92/5) this text sets down the major findings of a

group which met from 1990 to 1993. For minor components volatilised in the burning

zone (alkalis and sulfates), there are several successes on production kilns in reducing

volatile cycles by attention to burning zone conditions, especially in relation to

chemical decomposition of C&I,. Correct diagnosis of conditions is assisted by

improvements to permanent on-line exit gas monitors and to suitable standardisation of

sampling procedures for dry process kiln enuy material. The full effects of dust cycles

in confusing results of sampling exercises remain to be established.

Sulfur volatilisation and absorption at lower temperatures have also been considered and

the conditions of temperature and atmosphere which aid reactions for SO2 capture by

various materials are outlined. Various permutations of actions are possible to abate

emissions and some will not succeed at all points in a production line or may require

moisture addition or increased residence time to improve their effectiveness.

Proposals for possible topics to continue this work are listed. (This is an English version

of the official French text).

=C O N T E X T S

Paee No

1 . Introduction

2 . Task of the Sub-Commission

3 . Cycles of minor elements generatedin the burning zone

4 . “Cold” cycles of minor elements

3. Trace elements

6. Future work

Figures

Appendix 1 SO? CAPTUREChknical IMechanisms far Sulfur Dioxide Absorption in CementKilns and other Industrial Abatement Plant.

Appendix 2

Appendix 3

1 1

14

15

TEXT FRO,M INTERNATIONAL SYMPOSIUM ON GASCL&WING AT HIGH TEMPERATURES.Behaviour of Volatile Materials in Cement Kiln Systems.

PAPER PRESENTED BY T M L0WE.S100% Pet Coke - Problems and Solutions.

1 . INTRODUCTION

The group has the following memberx-

HOLDERBANK

3LUE CIRCLE

CIMENTS LAFARGE

P BORKI

P KERTON (Animateur)/C HOLT

M DANDINE/K 30ULOT/M TOUSSSAINT/X DUPONT-WAVRLN

CIMENTS FRANCAIS B POLGE/G Bw BERGERY/G FLAMENT

ITALCEMENTI R TACCHINI’

C3R c MEYERS/J PARlSIS/P RENIER/M BRUYERE

CIMENTS D’OBOURG

ENCI

F LAMPROYEfR SPILLAERT

W VAN LOO/F ERE?JS

The foIlowing meetings too& place:-

1 . 1990 Autumn

2 . 1991 Spring

3 . 1991 Autumn

4 . 1992 Spring

5 . 1992 Autumn

6 . 1993 Spring

7 . 1993 Autumn

Maastricht

Greenhithe

St Antoing

Frangey

OrbY

Salerno

Obourg

ENCI

BLUE CIRCLE

CBWSCF

LAFARGE

HOLDERBANK

ITALCEMENTI

OBOURG

This year there was one meeting at Obourg on the 4 and 5 November 1993.

TC94049 2 Tt=chu.id Cease, .!%@cabm 1994

2 . TASK OF THE SUEKO~MMISSION

This is “to produce a state-of-the-art report concerning the behaviour of volatile

material in kilns.”

This work originates from

1 . The increased use of ail sorts of secondary raw materials and fuels.

2 . The trend to produce more low alkali cements.

3 . Emission regulations which are becoming more and more rigorous.

Furthermore, during the work, important implications for kiln output and cement quality

have been found in connection with control of cycles. Our principal recent activities

are covered under the three following headings, 3-5.

(A copy of an associated paper by T M Lowes is included at the end of this report,

having been presented at the same meeting in May 1994).

3. CYCLES OF MINOR ELEMENTS GENERATED IN THE BURNING ZONE

We have already produced a first text on “classical” knowledge for this group of cycles

(ISTN 92/5): this was distributed in 1992. Supplementary work is described here.

Given a sample of material and its chemical analysis, one might think that all would

become clear, but to calculate a chemical balance, it is also necessary to know the mass

flows which are involved in the calculation, and there are a number of methods of

deciding upon these. Each method has its own advantages and disadvantages, making

them more (or less) appropriate at different points in the burning line. It is important

to note that

1. In the past this sophistication has not generally been adopted

2. Various flow rates for hot raw meal entering the kiln can be calculated by

different means, each one giving a different “burning zone volatility”.

We have continued exchanges on these topics to improve our understanding. One

important parameter in the calculations is the flow rates of entrained solid particles

between preheater stages, in particular between the kiln and the preheater tower.

There are not many results available for this parameter (we note that there are not

always the same sets of results available to explain observations!).

As far as these “classical” cycles of minor elements are concerned, there is much

activity within member companies at present There are two fields of particular

interest:

Firstly, the characterisation and the behaviour of hot raw meal (including the use Of

modified geometry cyclones and special linings), and secondly the influence of

combustion and heat transfer on volatalisation - especially in the burning zone but also

in precalciners. The influence of the local atmosphere close to clinker granules in the

rc94049 4 TecbniGai Came. SenM I994

burning zone seems to play an even greater role than had been thought in the past:

there are several results where volatilisation has been significantly reduced by more or

less simple means (additional oxygen) and we trust that one day there will be a

somewhat deeper understanding than indicated in our previous report. This

understanding will also help us in applying results from the mathematical analysis of

cycles from CBR which has shown great variations in alkali and sulfate volatilities in

the burning zone in different kilns (even when taking account of the major volatilisation

of chloride which explains a proportion of these differences).

This year we have tried to bring together knowledge on:

The most recent studies on control of cycles and characterising hot raw meal

on the industrial scale;

Mathematical analysis of cycles and cyclone performance (from chemical

analysis of samples);

The effects of the flame and kiln atmosphere ‘on volatility, especially when

using petroleum coke.

Study of free energies indicates the compounds expected and which of them are the

most stable in the prevaiiing conditions of temperature, pressure and composition of the

solid liquid and gaseous phases. Mr Berard-Bergery distributed a summary of the talk

which he previously gave on this aspect of thermodynamics, He notes that without a

certain knowledge of this subject it is difficult to make progress, given the need to

explain the apparently contradictory results obtained from operating kilns. (We must

certainly bear in mind the fact that we need to consider dynamic equilibria, not only

static systems.) Thermodynamics allow us to determine the direction, intensity and

speed of transformations of physical systems as a function of conditions. The possibility

of a reaction is known from calculating the change in (Gibbs) free energy, if operating

at constant temperature and volume or of free enthalpy at constant temperature and

pressure. Examining the trend of this value as a function of temperature, the reactions

of formation or decomposition of a single chemical compound can be considered, and

the order of stability in a family of compounds (chlorides, fluorides, sulfates etc) can

be deduced. Hence, the reactions between one element and the compound of another

element can be foreseen. There are quite a number of diagrams of free enthalpy as a

function of temperature available (determined in the field of metallurgy) and also

information on partial vapour pressures which influence the equilibria.

Along a burning line it can be seen that for chlorides KCI is the most stable and easily

vaporises producing a chloride cycle. For fluorides CaF2 is the most stable at all

temperatures, and does not easily vaporise. For fluorides, phenomena are therefore

very different for those encountered with chlorides. For sulfates K2SO4 is by far the

most stable and CaSO4 the least. For C&30, stability also depends among other things

on the partial pressure of oxygen, so that the sulfate cycle brought about by the

decomposition and re-combination of &SO4 will be very different depending on the

oxidation/reduction conditions at different points along a burning line. Na20 is more

stable than f(20 and is found in clinker combined in the aluminate phase, whilst K20

tends to form &SO, if possible. It is interesting to see that sulfur, for example in the

form of SO,, can be captured by carbonates because sulfates are more stable. At low

temperatures sulfites can also be obtained.

CIMENTS FRANCAIS note that it is necessary to define the conditions in which a kiln

must operate to give the desired results, in the case of high fluxes of sulfur or alkali.

Results from CC3 confirm these ideas and they are being applied in other Works. Mr

Flament has told us that the text which he presented at the Berlin Congress gives a

good summary of his experiences at CCB.

At CCB, using 100% petroleum coke (ore-calcination with separate air at around 55-60%

kcal) a good kiln output is found when there is a higher and more constant oxygen level

at the kiln exit (1.5 - 2.0%), a greater fuel fineness (residue 3.6% compared with 5.0%),

a more stable flame shape, and a less severe burning regime - obtained by means Of an

examination and adjustment of geometry in the burner region. The geometrical aspect

helps to avoid too high a dust cycle (including cooler dust) and flushes, which can both

cause alkali capture and blockages (contrary to the ideas of certain plant suppliers).

&SO4 nodules are found in the interstitial clinker phase, and there is a need to consider

quality aspects further. There is a sulfate: alkali ratio of 3.5, which is acceptable in

the stable kiln regime: thus it is preferred to use only 50% petroleum coke for

production purposes, so as to reduce the possibility of entering into a potentially

difficult state. Coke brings 26% of the sulfur arriving in the kiln system.

The sulfate/alkali ratio is an important parameter to take into account, but it is not the

only one. It is also necessary to have a good and continuous analysis of kiln exit gases

to allow combustion conditions to be followed. It is preferable that gas analysis is

automatically corrected for oxygen level so as to indicate other changes more clearly.

It is equally necessary to continuously monitor precalciner combustion conditions by

means of supplementary analysis of relevant compounds in the gas phase.

The “intensity of combustion” in the burning zone is an important factor to understand

and use to control events. The decomposition of CaSO, in a reducing atmosphere is the

key mechanism. Each kiln has its appropriate oxygen level in the kiln exit gases, which

must be respected for a given sulfate input. The word “volatile” can lead some people

into error: whilst KCl and NaCl are present in the form of gaseous molecules,

thermodynamics indicates that K20 and Na,O decompose in the flame and re-combine

later. It can be useful to separately tabulate the calculated volatilities of KCl NaCl,

K$S02, NaSO,, CaSO, instead of only Cl, K,O, NaZO, SO3.

We have discussed the design and operation of by-passes, certain of which take a

significant dust burden at the kiln exit, which brings about difficulties in operation. In

Ciments Francais kilns the range of dust burdens is from some ZOO-1,000 kg of dust per

tonne of clinker. This dust burden reduces the performance of the lowest cyclone: if

the value of the dust burden is not known it can be difficult to interpret results. (It was

noted that Weber recorded low dust burdens in all the kilns which he analysed) Other

important parameters which must be known in order to diagnose a situation are the

chemical analyses of good samples of coal and of the hot material coming into the kiln

at the feed chute level.

Given the above-mentioned data, the measurement of CO? level at the kiln exit over

a certain period allows separate calculation of decarbonisation in the kiln and the

preheater/precalciner. The mixture of dust and new raw meal which comes into the kiln

can then be estimated from the loss on ignition. It is suggested that methods using

chemical tracers to estimate the material flow, for example K,O, can be falsified if the

level of dust cycle is not known.

It seems probable that the geometry of a plant has a marked effect on dust

entrainment. In the older generations of Dopol preheater, material fell a long distance

from the bottom cyclone into the kiln. The kiln entry material had a low loss of

ignition which could be falsely attributed to good decarbonisation. The more recent

Dopols have a side entry for material: a bypass can then expect to encounter a lower

dust burden. The older “lateral centrifuge” entry of F L Smidth also produces a poor

bypass efficiency, due to dust entrainment. High dust recycle levels also have the

inconvenience of increasing the probability of blockage

(Mr DuPont-Wavrin noted that the Berthold Company is supplying an X-ray detector t o

monitor material flow rates ex-cyclone).

It is useful to calculate the effect of dust cycles on thermal performance. A heat and

mass balance for each preheater stage allows the effect of dip tube geometry changes

to be observed. Opinions vary as to the appropriate choice for different stages, not to

mention the use or removal of cyclone exit flaps.

A variety of experiences have been reported regarding Hasle Vortex Finders. At ENCI,

excellent results have been recorded for over 3 years, whilst at CBR the tubes were lost

in 3 months- Ciments Francais have observed the same range of lifetimes. There is

consensus on the advantages of dense ceramic Hasle units in the kiln feed chute (with

a minimum of exposed refractory cement?,” when they are set up with a good

arrangement of air cannons. Some peopie have doubts as to their sensitivity to thermal

shock in other regions of kilns during heating and cooling.

ITALCEMENTI has described similar experience with a F L Smidth chloride bypass at

Picton, also used to assist the production of low alkali clinker. Here the high dust

burden (some 1,000 kg per tonne) causes the so-called “gas bypass” to have tbe

efficiency of a dust bypass. Tests are in hand at Nazareth with a purifier which

removes SO, by injection of raw meal and water (a Monsanto design). At Colaferro two

geometrically identical preheater kilns produce respectively some 1,900 and 1,150

tonnes a day. The higher output kiln is fired with a mixture of coke and coal and

produces build-up problems, whilst the other operates satisfactorily with 100%

petroleum coke. The only difference that has been noted in combustion conditions is

a higher secondary air temperature due to the use of IKN plates in the Fuller cooler.

Italcementi manages to use 100% petroleum coke on the Lepol process, even with 10%

over-grate firing, with emissions of SO, - except during build-up losses - having only

pyritic material as origin. The residue is 10% as for coal. On the dry process it is

necessary to drop the residue to 4% (or 5% for a coal/coke mixture). The precalciner

gas does not contain any SO2 when coke is used in the burning zone. The sulfur leaves

with the clinker, partly during occasional flushes.

In the past coke or anthracite was introduced into long granule-fed kilns. This helped

formation of a good burning zone, but nowadays it is found that there is also a high SO2

emission. This coke is now added to the main burner, a procedure which operates

satisfactorily if the burning zone is controlled via NO, monitoring to avoid the problems

which can be caused by the sudden arrival in the burning zone of build-ups detached

from internal cruciforms.

In the same way, LAFARGE has continued with a major programme of geometric

"centralisation" of burners, noting oxygen levels (typically some 3%) and SO, levels ex-

73.i~ re-sort Ls soicrlr confidemial w.rhio the Blue Ctie Group.

kiln as a function of flame momentum. Several Works keep the centralising mechanisms

on the kiln platform so that alignment can be corrected if there are changes after some

weeks of operation. In such circumstances 100% coke can be used (3-5% S) on the dry

process with a 5-10% residue.

Automatic kiln entry material sampling systems are generally installed with a view to

assuring safe operation (Pfaff) and Lafarge gave an account of an in-depth study at Port

la Nouvelle. The company was particularly interested in the impact of sampling

techniques on results. Here, there is a kiln fed at some 100 tonnes an hour (50% of the

heat energy coming from petroleum coke) with 3% oxygen ex-kiln. The levels of all

volatiles in the collected dust go down as a function of sample suction rate, reaching

a plateau. The dust Is really a mixture of fine material (high in volatiles and easy to

collect) and coarse material. Although the nominal isokinetic aspiration rate for the

probe was some 30 litres per second, it was not aligned with the gas fIow direction and

higher suction rates were therefore needed to obtain a representative sample.

Lafarge express the hope that a standard method can be written up, suitable for use

throughout the world. They have currently only two or three competent sampling teams

in their French group. It was noted that it is a bad practice to make use of large probes

and low capacity pumps to reduce blockages as far as possible.

CBR reported on SO2 levels in the kiln system at Antoing. The level of some thousands

ppm ex-preheater drops to 600 ppm at the stack. This loss is split 20% to the crusher,

20% to the mill and 60% to inleaking air. It was reported that at Rekingen the raw mill

is run at a reduced throughput, in order to allow SO2 capture t o continue throughout the

operating day.

A few supplementary results from the simplified mathematical model have been

distributed, with its application to the analyses of balance samples from various member

companies, so that volatilisation, entrainment and capture coefficients can be

calculated together with the performance of some cyclones.

ThLs remrr Ls snicdy codid~riaf w-if&in tie Blue CLv!e Grarw.

A significant range of values was noted, all calculated on the same basis. K20 is a good

tracer to determine raw meal entrainment by gases. For the calculated entrainment

values in the document distributed, one must consider the position and methods used to

collect the kiln exit dust samples - with a probe in the kiln, in the riser duct or ex-

bypass. Nevertheless we consider that it will be very useful to extend the tabIe of

results already obtained by sending further analyses to CBR to gather a common table

describing volatilities, An example of a Blue Circle kiln (poor flame with high

volatilisation) is the only one to have been added this year, and the model remains to

be more widely used.

At BLUE CIRCLE some thermodynamic data lead us to think that in a typical kiln gas

there is at llOO*C a sufficient reduction potential produced from 2,000 ppm of CO to

reduce CaSO, with a consequently much higher voiatility. A separate paper from Blue

Circle is appended, giving an account of UK experience with use of petroleum coke.

A paper from Blue Circle regarding the design of a bypass for a new Works with high

chloride raw materials was discussed. There was also an expected high content of

alkalis in the clinker, which could perhaps be reduced by the addition of even more

chloride. The performance of existing by-pass systems indicates that despite the fact

that (according to suppliers) there is a possible dust loss of some 200 to

250 g/Nm-’ in the gas extracted from the system it will be best to calculate with a

nominal level of 400 In such circumstances there will be a need for a raw meal

preparation system with a significantly higher throughput than normal.

Several remarks were made: there are examples of precalciners blocked by sulfur; the

handling of by-pass dust rich in CaCl, is much more difficult; is the fuel penalty per

percent of by-pass closer to 5% than the 1% used in this example? (This latter figure

was supplied by the company which intends constructing the proposed Works.)

The animateur was invited at short notice to give a paper during an International

Symposium of the Cleaning of Gases at High Temperatures in December 1993, which is

This remrr Ls sm’cr(;l cmfidemiai withiu tie Blue Cri-cle Group.

appended. Several specialist workers in this field have encountered problems of

blockages and build-ups which provoke their interest. (Note that data on the equilibrium

CO-CaSO~-CaS is given in "Sulphur Capture in Fluidised Bed Boilers: the Effect of

Reductive Decomposition of CS04”, by A Lyngfelt and B Leckner, Chemical Engineering

Journal, Volume 40. pages 59-69, 1989.)

4 . “COLD” CYCLES OF MINOR ELEMENTSLEMENTS

Information on the effect of internal cycles on emissions to the exterior has been

exchanged, avoiding (if possible) examination of equipment for capture of such emissions

which is left to the "Environment" sub-commission, liaising with its animateur. This

topic typically concerns cold cycles of SO2 formed at low temperatures in the

preheater, and capture of SO, emissions in long kilns and in the Lepol process involving

cycles which originated in the burning zone.

For this topic it seems that most information has already been treated by the

‘Environment” sub-commission: it remains to define more precisely the chemical

reactions which are involved and the domains in which these are the most (or the least)

effective. We are interested in establishing information about chemical efficiency of

absorption of SO2 as a function of conditions of atmosphere, humidity, temperature,

residence time, particle size, chemical composition etc.

The animateur noted the classification of absorption mechanisms given in two USA

papers: these concern tests carried out Davenport (Steuch) and at Lone-Star (Sheth)

The document sponsored by the British Pollution Inspectorate is also available. This

reviews published information on removal of trace gases. It covers a range of both

chemical species and reactive materials, The sections relating to SO2 capture appear

to provide a useful framework within which the cement industry’s experience can be

classified, see Appendix. Other industries are interested in the possible future use Of

sorbents with increased reactivity (cement kiln dust?) and in “regenerable” agents such

as calcium disilicate.

The reports of HOLDERBANK to the Environment Sub-Commission have indicated that

emissions of some 1,000 to 1500 mg/Nm3 of SO2 were reduced to close to 350 mg/Nm3

during the operation of the raw mill. The same effect could be obtained in direct

operation by the addition of Ca(OH)2 to the raw meal. This provides removal of 50%

SO2 at a stoichiometric level of 5; Polysius would suggest 80% removal at a ratio of

8. Not all users seem to have taken account of the need to use superstoichiometric

quantities of sorbents, which sometimes may react with only some 10% efficiency.

Work carried out in the UK by Lodge-Cottrell in the field of the electric power station

desulphurisation, has shown an efficiency of 25% for dry lime injection, rising to 50%

in the presence of moisture, and also that sodium based reagents had a genuine action

which was almost double that of calcium based sorbents (and that these could be

introduced as solutions by means of simple nozzles).

It was mentioned that Rekingen Works had modified its raw mill throughput so that it

operated 24 hours daily, thus capturing SO 3 to conform to emission regulations. The

Santa Cruz Works of Lonestar may do the same. In this class of activity, CIMENTS

FRANCAIS works on the basis of 50 to 75% absorption.

ENCI gave an account of experience with operation at different oxygen levels to reduce

SO2 emissions from its two-stage preheater kiln. The degree of sulfation of clinker at

Maastricht is 125%. This, along with other causes, produces an emission which must be

reduced to comply with new regulations During 3 weeks the oxygen level at the kiln

exit was altered from 1.5, 2.0, 2.5, 1.3, 2.0%. The results for SO2 level in the emitted

gases and SO3 in the raw meal and clinker are in agreement, showing that emissions can

be reduced. If the effects on quality and on kiln operation are acceptable, they intend

to buy new fans to guarantee sufficient oxygen level at maximum kiln output (with

fuzzy control). The re-installation of Magotteaux stirrers in the kiln has once again

given positive results after the last stop.

At OBOURG there is an emission problem similar to that at ENCI, which can be

resolved by a kiln exit oxygen level of 2 to 3 % but this could give a too high a chain

temperature and an unacceptable reduction in output. Oxygen has been added

(1,000 m3/h either by the primary air channel or beneath the flame) gaining 3 to 4

tonnes per hour of clinker at an acceptable chain temperature. The cost of oxygen is

some 3 Belgian Francs per cubic metre for a permanent irstailation, but this could be

This reqorr is snict(y confidential within the Blue Ckfe Group.

offset if cheaper (higher sulfur fuels can be used. It seems that at least a quarter, and

nore usually about a half of the SO2 disappears between the kiln and the stack, no

doubt by capture on dust. Given the large volume of gas produced by this wet process

kiln and its moist fuels, a new fan would be proportionately much more costly than for

ENCI.

At Obourg it seems likely that the longer kiln can satisfy emission limits through

control of excess air levels. For the other kiln, another method of reduction of peaks

of SO, is being studied; NaHCOS injection in the exit gas duct at the upper end of the

kiln. The trial installation from Solvay (about 400 kg/h of Na.I-IC03 powder) was leased

for longer trials. Chemical efficiency is 100%.

As already noted ITALCEMENTI is looking at a Monsanto system involving a water/meal

scrubber for SO,, with a cost expected to be only 20% for that of an “Untervaz” system-

LAFARGE has studied sulfur behaviour on a semi-wet Lepol grate. In the hot chamber

there is an excellent capture of sulfur coming from the kiln, but there is also

decomposition of pyritic sulfur on the grate. This starts at the transition from the cold

to the hot chamber (500-600'C) and is completed by the middle of the chamber.

TC9-4049 1 5 Tecfioical Centre. Senrember 1994

5. TRACE ELEMENTS

lTALCEMENTl has presented a summary of results from 30 kilns, seeking to determine

the amounts of 16 metals In stack dusts, and also of 5 inorganic micropollutants. At

Vibo works (precalciner), the raw meal is dosed with CaF2 to influence the

decomposition of strontium sulfate and so limit the undesired effects brought about by

SrO during alite formadon. On the Lepol process CaF2 also provides a less dusty

clinker and a reduced need for kiln system cleaning. No changes were noted in the

behaviour of other halogens or of alkalis.

OBOURG has provided, a list of balances over 2 years. BLUE CIRCLE has shown a

table for retention of various elements in a number of kilns as a percentage of the

quantity brought in by the kiln feed (including recycled dust). An examination of Blue

Circle’s conclusions regarding behaviour of trace elements considered elements as being

either non-volatile or partly volatile. 3-5 results were available for each element for

the wet, semi-wet and dry processes. The percentage of the input found in stack dust

was very low for the non-volatile elements (As, V and sometimes Cr - unless there was

enrichment from refractories), thus reflecting tie good efficiency of de-dusting and the

varied additional contributions from fuels. The highest proportions escaping with stack

dust were for cadmium, lead and thallium in semi-wet kilns. The levels noted were

influenced by the rate of removal of intemediate dusts produced in the kiln processes.

For the dry process there was less enrichment of cadmium and lead.

We have specifically looked at German work previously published in ZKG, where the

need to examine the chemical combination of elements is underlined. Mercury, for

example, must always be oxidised in a kiln system, so that the vapour pressure of the

uncombined element is not to be considered.

We have also received a supplement to the bibliographical list established in Autumn

1991, which concerns cold and trace element cycles. The published literature makes it

quite evident that cement kiIns are reputed to be potential origins of emissions of SO,,

Tl, Pb - and perhaps Cd and Hg, if these latter are found in the region. There is also

a certain interest in the upgrading of kiln dust,

TIC re,vrc is suictiv wnfide!zu*al’ witi tie BILE C’ric!e G~OLT.

TC94049 1 7 Techical Cam-e, ?&xBder i99d

FUTURE WORK

The Plenary session has asked for a new sub-commission to be set up in 1994/1995 to

follow the topics left from the existing group, with the addition of a study of corrosion

and the wider application of the mathematical model already developed. We must avoid

repetition of work already done, and coverage of subjects which the Maintenance Group

and the new Working Party on Flames are currently examining. (For corrosion,

depending on the activity of the Maintenance Sub-Commission, we might envisage a

search for methods of reducing its effects, as well as the identification of compounds

which have a strongly corrosive action).

Our group is convinced that there remains much to be gained for the companies of

CETIC in bringing together the specialists who are concerned with Chemical phenomena

within kiln systems and avoiding the study of topics which are more or less “legal”.

All are invited to consider a new division of chemical and engineering work with regard

to cycles, transport, clinkering, emissions, interaction with refractories, combustion,

sampling (gas and solid), etc etc.

A review by the Sub-Committee has produced a list of ideas, (see following page), these

points generally relate to factors which have an impact on quality, cycles, transport,

clinkering of emissions, refractory attack, combustion, sampiing of gas and solids etc.

Some of them are more relevant to groups in the Technical Commission. These ideas

remain for immediate discussion.

(NOTE : It was subsequently agreed to concentrate initially on topics related to the

behaviour of sulfur).

C P KertonTechnical CentreMay 1994.

LIST OF TOPICS

Environment:

Factors effecting organic emissions and their control.

Process:

Water injection in planetary coolers.

CO SO2, CaS04 balances.

Control of kiln build-ups and their chemical composition.

Effects of secondary firing on volatilisation/condensation in Lepol grates and

their control.

Methods of successful use of even higher levels of S in fuels.

IMethods allowing the retention of SO3 in clinker.

Control of chloride volatilisation.

Applications of CO, analysers.

Determination of dust cycles in kilns and heat and mass balances for each

preheater stage.

Optimum fineness of petroleum coke for kiln and precalciner burners.

Effects of volatile materials on the long term and short term stability of

kilns and their consequences for static and dynamic conuol strategies

(allowing early pro-active responses t o change).

Control strategies (anticipatory control) to restore operation of a disturbed

kiln (effect on throughput, environment, quality etc).

Bypass control.

Effects of geometry on cyclone operation.

Effects of volatile cycles and dust cycles on cyclone dip tubes and on various

refractories.

Control of sulfur cycles at low temperature.

Effects of heating and cooling on refractories and linings; success with rapid

regimes.

Effects of parameters other than CO on sulfur volatilisation.

Correlations of SO2 signals with other process parameters.

Effects of injecting and additional fuel at different points in the process (for

example solids at the kiln feed chute).

Distribution of trace elements throughout kiln systems, and methods of

control.

Interaction of volatiles and refractories.

Influence of SO3 on refractor+ life - both direct (chemical) and indirect

(perhaps a lower BZT).

Correlations between SOS and free lime in operating kilns.

Effects of V and Ni (coming from coke) on refractories.

Correct regulation for burners for different levels of coke fineness and coke

mixtures.

Maintenance:

Corrosion in colder zones of kilns (in relation to Cl and S)

Workplace hygiene and safety aspects from the point of view of volatiles

Product/Quality

Treatment and use of dust rich in volatiles

Methods for the internal use and/or upgrading of dusts which cannot be

dumped.

Effects of V and Ni (coming from coke).

Effects of SO3 and of sulfate/alkali ratio on clinker quality at different

levels of free-lime (is there an optimum level?)

Effects of marginally reducing conditions on quality.

Effects of halogens on cement behaviour (standards, etc).

Effect of clinker size grading on quality.

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I

X

ENCIRELATION MG/M'.SO~ - % 02

1. General

Information is presented here against the background of equipment/processes encountered outside thecement industry, where acid gas abatement may already be practised - and where ideas for transferto our industry may originate. Our most frequent needs are to improve SC+ capture by calciumcompounds in raw mills and/or compensate for the absence of this absorption when mills are notrunning. Whilst there is a 50 to 75% reduction in SC& levels in a number of cement Works whenhydrated lime is suitably added, some sites need to understand why they record drops of only 20 to40% and there also is an interest in better understanding the possibilities for using of alkalinesorbents. This note aims to provide suitable background information as an aid to better understandingwhen the complications and costs of the “Untervaz Solution” may have to be accepted.

2. Sorbent Injection

2.1 Basic Description of Technology and Principal Variations

Sorbent injection is used primarily in pollution abatement as a means of reducing emissions of sulphurdioxide (SO-,> and other acid gases, such as HCI and HF. Material is usually injected into the gasstream as a fine powder, where it reacts with the acid gases, generating a dry product for collectionin dust arresment equipment. Dry injection methods are particularly suitable for small boiler andincinerator plant or retrofit applications where the capital expenditure for other systems is pro-hibitively expensive. The efficiency of S&- removal is 40 - 80%, depending on the sorbenc used(most commonly calcium and sodium compounds).

The sorbent can be injected at various points in the plant, according to the temperarure and conditionsat which it is most reactive. The most common systems for boiler plant are:

Furnace injection of calcium based compoundsHeat exchanger injection of hydrated (slaked) lime (Ca(OH),,Post furnace injection of Ca(OHk at relatively high humidityPost furnace injection of sodium based compounds.

In a cement process, we may see:

“Classic” SG- capture in the lower stages of the preheaterHydrated lime injection at top of preheater or in conditioning tower (probably as slurry)Return of calcined meal to cooler parts of system (preferabiy with moisture)Wafer injection at suitable points to increase possibility of reactions with raw feedHydrated lime addition to raw millInjection of alkali compounds or solutions to exhaust gas ductSG- capture by limestone in raw mill in the presence of moisture.

(Some different possibilities for injection are envisaged for Lepol systems.)

2.2 Principle of Operation

The reaction between sulphur dioxide and dry sorbent is a heterogeneous one . (Reactions with moistsorbents are discussed later in Section 3.2). SC& molecules diffuse through the gas stream and areadsorbed on to the sorbent surface before diffusing into internal pores where chemical reaction occurs.

These mechanisms are exemplified in the classical kiln/preheater sulphate cycles.

Dry calcium sorbents react with sulphur dioxide as follows:

The first stage is calcination:

CaC03 + CaO + CG-

Ca(OH)z + CaO + Hz0

(These reactions occur at temperatures greater than 760 and 570°C respectively. Dolomitic limestonestarts decomposition at a lower temperature.)

The second stage is sulphation:

CaO + SG_ + Y.Q + CaSO,

With excess oxygen, complete oxidation to sulphate occurs at temperatures ahove 800°C. Below thistemperature a mixture of sulphate, sulphite and sulphide is formed. (The optimum temperature rangefor direct reaction with hydrated lime is listed as 130 - 180 “C.)

Sodium compounds react with SO, as follows:

2NaHCO,-Na&O, + CG- + Hz0

Na&O, + sa* + ‘ha- -, lN&-so, + co,

Sodium bicarbonate decomposes to sodium carbonate, which then reacts with S%- to form sodiumsulphate. The these reactions are significant at temperatures above 130 - 180°C; at the lowertemperatures in this range the favoured SO, reaction is directly with the carbonate, but at highertemperatures the thermal decomposition and sulphation reactions occur simultaneously. It is thoughtthat the good performance of this sorbent may be explained by the fia that a shrinking core ofbicarbonate is continuously decomposed, providing moisture at a fresh reaction interface for S%-arriving through a permeable outer shell of sulphate A similar scheme is seen for potassium.

23 Selection and design considerations

Dry sorbent injection is usually one of the cheapest abatement options for SG- removal, particularlyfor small or retrofit plant. as the capital cost is low. The choice of sorbent is a prime considerationand depends very much on availability. Calcium compounds used for dry injection are primarilynaturally occurring limestone or dolomite or hydrated compounds derived from these raw materials.Reactivity is dependent on pre-treatment as well as natural properties. The sodium sorbents ofinterest are sodium bicarbonate (NaHCO,) and sodium sesquicarbonate (NaHCO,.Npl_C0,.2H,0).These occur naturally as nacholite and trona respectively. In making a choice, process economics arehighly dependent on delivered price of reagent and rate of use, despite the initial lower cost andcomparative simplicity of operation of the dry post-furnace injection processes.

There are three temperature windows for calcium sorbent injection in boiler and incinerator systems,which also broadly apply to cement kilns. (Note that there is a temperature zone where neither groupof reactions is very effective, especially in dry conditions.)

Calcium sorbents can be injected directly into the furnace, where at temperatures of 1100 -1250°C the calcination and sulphation reactions can occur,

Calcium hydroxide Ca(OHX_ will react with SO, at about 550°C and hence can be injectedbefore the heat exchanger.

At high levels of humidity calcium hydroxide will react with SO, at even lower temperatures(5 - 15°C above the saturation temperamre of the gas, as discussed in Section 3.2). watercan be injected with the calcium sorbent into the duct between the heat exchanger andparticulate abatement equipment or elsewhere

Sodium sorbents are also injected in boiler plant to the duct between the heat exchanger andparticulate abatement device, where the temperature is in or above the range 130 - 180°C.

It is possible to inject sorbents at several points in the plant and to combine this technology with otherabatement options. (No accounts of multiple injection systems are known for the cement industry).Handling problems may be encountered and not all sources of lime are equally effective in reacting(at a given fineness) in the time available at the point in the process where introduction is feasible.Reactivity increases as the surface area of the sorbent increases (particle size decreases), up to 40m3/gm. The calcium:suiphur ratio is generally set at 2, but can be as high as 6, particularly for lowsulphur coals where the mass of sorbent is still comparatively low.

The reaction efficiency for limestone ranges from 40 - 50% at calcium:sulphur ratios of 2 - 4.Dolomites give greater conversion efficiencies, and this is attributed to the more open structure of thesorbent material which enables greater diffusion of gas into the pores of the sorbent. Conversionefficiencies of 70 - 80% are achieved with sorbents of hydrated lime Ca(OH), at 8OO”C, and sodiumcompounds also yield conversion efficiencies of 70 - 80%. (Use of a fabric Nter for particulateabatement is claimed to enhance SO, abatement efficiency by ca. 10% because the sorbent collectedon the filter bags continues to react-with SO, during particle filtering. Dry sorbent injection to thefilter bags after a cleaning cycle, is claims as an altemativelsupplementary method to enhancereaction.)

Superstoichiometric quantities of reagent are usually needed because efficiency of reaction is low, andrecycle may also be required. Efficiency may even be insufficient for elimination of either very highor very low SC& concenrrations at a realistic stoichiometric ratio. Use of “conditioned” (ca. 10%moist) hydrated-lime has been claimed to give improvements due to (a) breakage of particles whenbrought into contact with hot gases (so generating more surface area) and (b) cooling, which increasesreaction efficiency. There are no known reports from the cement industry on this point, or onreactivity of different sources of limestone or dolomite.

Limestone is the cheapest sorbent material currently in use. Lime (calcium hydroxide) is about 5 -6 times more expensive than limestone and trona/nacholite are generally ten times more expensivethan limestone. Some studies suggest N&HC03 becomes still more efficient at higher temperatures(e.g., up to 815 deg C); it costs 2 to 4 times as much as hydrated lime. although consumption maybe lower and there may be less residue to dispose of. (There is little or nothing known about the use.of alkali compounds at relatively high temperatures in the cement industry.)

Combustion systems using high sulphur coals yield the most promising results where SO, levels are2000 - 4000 ppm. At SC& concentrations < 1000 ppm the reaction is diffusion limited and it maybe more difficult to achieve desired levels of efficiency. (In general, this technology is consideredto be less efficient than the “wet” methods described later.)

In boiler systems, the added sorbent and its interaction with the fly ash, can cause fouling of surfaces.Also, higher particulate loadings, decreased particle size and increased electrical resistiviry of theparticles can impair the performance of collection devices. Handling and disposal of larger quantitiesof solid waste with properties different from fly ash or conventional scrubber sludge can be difficultand increase costs. For example, sodium salts are soluble in water and hence disposal requirementsare more stringent.

3.1 Future Developments

Research is continuing to enhance knowledge of-the appropriate mechanisms acting in these injectionprocesses, with a view to developing alternative, moreeffective sorbents. For example, alkali metaladditives in limestone enhance SO, abatement efficiency and early indications are that lime-containingwaste materials, such as carbide mud and sugar mill mud, react faster and have a greater sorptioncapacity. The use of regenerable sorbents such as calcium silicates is another possibility.

3. Spray Dryers

3.1 Basic Description of Technology and Principal Variations

Spray drying is a standard chemical engineering operation used to produce dry powders of controlledparticle size, density and moisture content Spray dryers are used in pollution abatement for thecontrol of acidic species in a flue gas stream. Droplets of reagent are contacted with the flue gas ina reaction chamber - probably a modified conditioning tower in a cement Works. Liquid iscontinuously evaporating from the droplets in the chamber during the neutralisation reaction and thedry reaction product can be collected at the base of the chamber or in the dust abatement plant.

A complete system consists of the spray dryer (atomiser and reaction chamber), associated slurry/liquid handling equipment, a particulate collection devise and soiids recycling equipment. There arethree types of atomiser in general use: rotary, two fluid or spray nozzles The reaction chamber canbe a tower or dust, and the flow of the droplets and flue gas stream are usually co-current. Limeslurries are most often used, but sodium carbonate/bicarbonate solutions are also acceptable.

32 Principle of operation

a) Lime spray driers

The atomiser generates dropiets of lime slurry which are injected into the flue gas stream in thereaction chamber. In the capture of sulphur dioxide, the chemical reactions which occur involvewater and are believed to be:

Liquid phase: CaC03 + Sa- + %H,O -. CaS03.%Hz0 + Ca-

Gas/liquid phase: Ca(OH)l + .Sa- + Hz0 - CaSO,.‘/iHIO + lXH,O

Sa- is absorbed in the aqueous phase of freshly atomised droplets forming suiphurous acid,where the reaction of SO-, with lime or limestone proceeds rapidly, forming calcium sulphitewhich may later be oxidised and form gypsum in the presence of oxygen and water.

As the droplets pass through the chamber, water evaporates to yield a porous particle whichhas a dry surface but a wet interior. Sa- diffuses into the wet sore of the particle and thereaction continues.

The reaction of SO, with lime in the absence of any moisture is slow. Consequently, in orderto extend the reactivity of the lime in the unit, the temperature near the exit is maintained justabove the saturation point of the gas.

As mentioned earlier (Section 2.2), these reactions can be involved in sorbent injection in cooler partsof a cement production line, for example when Ca(OH)z is injected to the preheater or Sa- reactswith limestone in the raw mill. In the absence of water, however, the reaction rate will be very slow- for example at the top of a preheater tower. Water injection to the preheater at Santa Cruz (without

adding any extra lime) was reported to allow 10-20% reduction in SO, levels. Failure to comparehumidity levels and/or use fresh lime may account for several differences in experience of SG-capture in kiln systems.

b) Spray driers using sodium salts

Dry S&- reacts with sodium carbonate/bicarbonate from low temperatures: and hence the requirementto enhance the reactivity by stringent control of temperature and humidity is not necessary.

3.3 Selection and Design Considerations

The principal design parameters for spray dryers are droplet size and distribution, and inlet and outlettemperature. Multiple atomisers are used in order to achieve an even distribution of droplets in thereaction chamber, and the droplet size has to be such that the rate of evaporation is fast enough toprevent formation of scale in the reaction chamber as droplets/particles strike and stick to the walls,but slow enough to enable the reaction to occur. High inlet temperatures enable more water or limeto be injected, and low outlet temperatures (slightly above the saturation point of the gas) optimisethe abatement efficiency of the spray dryer.

Fine sprays and concentrated reagents have shorter drying and reaction times. Water evaporates fromconcentrated reagents rapidly and hence the neutralisation reaction occurs mainly between the acidgas and the porous particle. Fine droplets ( < 100 pm) are used with size tailored to the residencetime of the flue gas and droplets in the chamber or duct. The residence time in a chamber is usuallyin the range 5 - 10 secs, with droplet size < 100 pm. For injection of slurry into a duct, reactionand drying times of 1 - 2 secs are typical. Residence times and evaporative heat available in anexisting cement plant conditioning tower or gas duct system may limit the amount of SO, which canbe scrubbed.

The choice of sorbent will depend on its cost and availability: sodium salts give better “once through”efficiencies, but lime has a cost advantage over trone/nacholite and the calcium based reaaion productis insoluble in water which renders disposal easier, should this be necessary.

Spray dryers have been successfully used in Europe for controlling emissions of acid gases, primarilyfor combustion plant and incinerators, using a lime sorbent which is recycled to improve its utilisationand achieving abatement efftciencies of > 99% and > 90% for HCl and SG- respectively.Efficiency can be enhanced by increasing the stoichiomeuic ratio for specific conditions oftemperature and humidity, but the gains are limited by sorbent utiiisation, sorbent solubility and wastedisposal costs. The Ca:S stoichiometric ratio is typicaiIy in the range 1 - 1.5 and liquid:gas ratiosin the range 0.027 - 0.04 l/m3. Spray dryers offer several advantages over wet scrubbing, especiallythe fact that a dry product is formed which is easier to handle and dispose of than a liquid effluent.The capital cost, maintenance cost and energy requirements for the spray dryer system are lower thanfor wet scrubbing plant although reagent costs are higher.

The particulate collection device can influence the operating conditions of the spray dryer. Acid gasremoval can continue in a fabric filter but care has to be taken to prevent blinding of the bags.Electrostatic precipitators, however, can operate at temperatures nearer to the saturation point of thegas, hence the spray dryer outlet temperature can be lower which improves its abatement efficiency.The dry product from the spray dryer (hydrated &SO,) can be used for landfill, processed to yieldanhydrite or pelletised to yield synthetic aggregate.

3.4 Scrubbers

“Absorption” is a process which involves mass transfer between a soluble gas and a solvent in acontacting device; chemical reaction may or may not occur In process design, both the chemistryof the system and the physical structure of the equipment must be considered. Unfortunately, wateralone is not effective at removing SO, from a gas stream because (unlike HCI) it is not very soiubie:an alkaline solution is needed The driving force for gas removal is the difference between the partialpressure of the soluble gas in the mixture and the vapour pressure of the solute gas in the liquid filmin contact with the gas. Mass transfer occurs by molecular diffusion across the interface and the ratedetermining step can be in either the gas or the absorbent phase. When the gas is very soluble orreacts chemically with a reagent in the sorbent, the process is “gas phase controlled”

Trace gas removal systems can be categorised by the solubility of the gas and by the reactivity of thesystem. SO-, is classed as “moderately soluble” in water (I-IF and HCI are very soluble), and sodiumsulphite and-alkaline compounds are used with some success as additional reagents. Efficiencies ofSC& removal of some 99% are attained in appropriate circumstances. There are no known accountsof the use of sodium sulphite solution for SO-, capture in the cement industry: usage in power stationsgenerally appears to be associated with systems which treat the resultant chemical products toregenerate the solution of sulphite sorbent, or systems in which the sulphite is used alongside otherreagents, providing an initial capture of sulphur as an alkaline compound for subsquent displacementreaction to form a more readily disposeable or saleable by-product.

Gas absorbers which attain gas/liquid contact by bubbling dirty gas through a liquid are suitable forabsorption processes which are “liquid phase controlled and those which involve spraying liquidthrough the gas stream are suitable for processes which are “gas phase controlled”. As absorptionis a rate process, the concentration gradient (driving force for the reaction) and the (high) surface areaof contact between the liquid and gaseous phase are crucial design parameters. The surface area isdetermined by the packing material or droplet size and this is usually achieved using packing materialswhich are coated with liquid or by droplet/bubble formation. The absorber design also has to providea means for renewing the liquid absorbent so that a high driving force for mass transfer is maintained-

Gas and liquid flow rates and pressure drop across the absorber influence the driving force, theefficiency and in some cases the surface area (droplet formation). A good gas absorber designremoves as much pollutant as possible in as small a space as possible. The choice of equipmentdepends on the abatement efficiency required, the energy and reagent requirements and the propertiesof the dirty gas stream.

c. P. KERTON,Blue Circle Te&nicai Centre, Greenhithe,&lay 1994 (Updated, July 1994).

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APPENDIX 2

TEXT FROM INTERNATIONAL SYMPOSIUM ON GAS CLEANING

AT HIGH TEMPERATURES

Behaviour of Volatile Materials in

Cement Kiln Systems

GAS CLEANING ATH I G H TEMPERATURES

Edited by .

?qv

(‘l’trc: usit ccnIc11I iidilslry convcnliun is loltuwrtt iii cxprcssinfi I’ snlls o f riinny clicrnicnlatialyscs ill Icrrris of cIxitlcs. c.g.. CaO, S O , , hl,O,. clc., u s u a l l y 018 a ‘loss [ICC” bas is . i.e.,allcr allowing, fnr ihc Ins5 ill wciglrl cvcnlually crpcricflcut rluc l o tlcslruclion o f clrbonalcs.clc.. during Iml lrcnlillcnl.)

“\‘01,A~1‘11.1~~S5” ANI) hllXTIIANIShlS 01; VOI.A’1‘II,ISA-I’ION

‘1.11~ pritlcitd volnlil~ ~ICIIIC~IS arc K, N;I, Ct. S. I n Ihc cast of’ r a w nnlcrints, ccrlain sutrurcor~~t~~ur~d~ (rulfirtcs or ort:anics) c:ul rratlily clcco~~rt~sclvolnlilisc Indow Goo”C, bul IllOSlvdalitc co~ntmunds in raw inalcriids o n l y c v a p o r a l c pnrlially and a l higtlcr Icnitxralurcs as111~ reed pa5scs low;rrtls llrc k i l n hrrnilig xonc. ‘I’hc rcsitluc rcmins i n lhc producl. cilllcri n s o l i d ~cd~~iou ill IIIC plincitd phscs ol IIIC clinker o r as cliscrcic cocrqm~ncls. Whilstalnml all fcctl cllloritlr. w i l l cvatwmlc, lcs.scr aniounls 0r ollicr c o m p o u n d s do s o , wliilsl incorrlr:i’tl. liitd vcrlalilrs arc :ilii~o’;l ;ilw;iys ciilircly cvnpor;ilcd during cmiiIiIislion.

lfv;rtKrrarrd volarilc~ rravcl hack up 11rc kiln wilt, 111c comlm~lion eases and cordcrrsc a sirmrt;;rnic comt~ountl’; (lilrcr;rlirrg I:~lwl Iical):

i)

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on llic Ucctl. li)rmitlg lhc his O r n rccirculaliiig inlcrnal votnlilc l o a d

n ;I lirrc rlrrsl r>r rcllrlc which is fin;llty Irappcd i n lhc g a s clrxning s y s l c m o rr a w m i l l arrtl hx~mcs p a r t or an cxlcrilal vnlalilc c y c t c . a s 111151 is parlly o rd$ly rl’lurncd ir1 tlrc sysic111

i i i ) 011 coltlcr strrl:rccs ill lhc syslcrii. linming llic Ixrsis O r hiltl~ups.

I’rcssurrs 10 cxploil cvcr more marginal rcscrvcs 0r r a w ni3lcrinls a n d fuels give rise l oirrcrcxirrg I;rmili;rrily willr llrc drm o f vohlilc spccics 011 process pcrlormarrcc. Wtrcricorrtlcrrsctt volalilcs rclurrr low;lrtls Ihc b u r n i n g zorrc, clctxriding o n Ihc overall clrcmicnlcr~nrliliorls and hrnint: ccrndiliorrs, llrry form a range or votalilc cnmpunds wlriclltlrcrr0clvcs cvapw;llc parli;rlly nri0 llrc cyctc only lids an utuilibriuni when ltx loiat quaillilyIcarirrt: llrc syslcm (itI clirrkcr ;md mm-rclirrrrcd tlusl) equals ltral cnlcrirrc lhc s y s l c m .

Alk;rlix ;md ~IIII~IICS ctrl~.rirrt! llrc trrrrrrirrp forrc irr trraclirc I:rrt:cly I’rlrrri :I \qr;lr;llc mr~llcr~5llll;llc t’ll;lr<, iiumiscilrlu wills Ilrc prLicip;rl fcrrilc Ilux. ‘Tlrc lrvct or volalilcs irrrccircul;rliorr i s sit;rrilic;mlly grcalzr ll1911 lhcir lol;d ralc or irrlroducliori am1 lhc s~rbsln~~ccsit1 llrc vapour ptrasc’ cnri 115 irr v;rrious slalcs or dissoci:llion a n d recomlrinalion. III gcncml,Itrc prc lcrrd clrloritlc corrrt~rmnd i s txrl:rsrium cllloritlc and only wlicrr lhcrc i s an cxccss IIIclrloriric lor cOcmic;rt crmrlrimrliori will) pol;lssium w i l l sodium ctrtoritlc lx hrmcd as : I

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~cmpcraturc in a WCI prnccsr k i ln , a considcrablc dab-bank c l I t.-6urcrncnls 1115 been buillup. The USC o f lnwrr a s h rucl lowcrcd c l i n k e r K,O l c v c l b y O.lS%. dcspilc 111~ inIroduclior1of n IiIIlc mnrc p&ssium IO lhc system (an cxlra 0 . I 16 o n clinker). T h e dusl - rclurncd IOlhc kiln - had bccomc more rich in alkalis, so lhal Ihe proportion or K,O broughl in by solidfuel fell lrom 24% I O IX% while ihai brough1 i n b y dust rcIIIrn rose from 18% I O 29%.This s~rp.gcs~s 1ha1 KzO incotporaIion in clinker no1 only dcpcnds on 1hc quan1ily inlroduccd11u1 also . a n d almvc all clsc _ o n IIIC type of malcrinl w h i c h b r i n g s i1 i n a n d perhaps o n 111~pnsiIimr whcrc i1 is injccIctl. On screening 111~ clinker al 20 mm. chcmic~l analysis showeda K,O coriccnlralinr~ snmc IO% Irighcr i n Ihc cn;~rsc f r a c t i o n . The clinker a lka l i COIIICIII hass~~cccssli~lly been rctlucccl i n lrials b y c a l c i u m chloride atldilinri nl l h c Ilamc.

Cnsc 6: A hil:h chlnridc coal ( - 0. I5 % Cl) can only bc ~isctl as a mix will1 anolhcr coal loavoid Imild-ullc wilh Iyliically 2 % Cl a1 Ihc ImI1nm Or cyclone 4 (lhc lowcsl in lhc prchcalcr~nwcr). hn;rlyTis of Inriltl~ul~s along IIIC kiln indica1ul chlnridc lc,vclr up IO 30% (al zero lossnn ignirinn) in 111~ coaIing Iron1 lhc base of cyclone 2 and 20% a1 SO 111 inlo Ihc kiln (tlcspilciit hs’i ih:m 5 % lrvcl i n Imllr lmi :md rold p;nI’i 0r Ilic k i l n ) .

C:1w 7: Atldinl; a sccontl ln~~hc;l~cr c y c l n n c s~agc IO a l o n g d r y process k i ln (a varinrIt of1hc I:igutc I prnccss, will1 a higher k i l n IcngIh/dian~cIcr ralio a n d a single cyclone slagcnhnvc i1) yicldcd vnrimrs build-up problems. To resnlvc ~hcsc, IIIC kiln gas cxi1 0, lcvcl wasiricrcA5rccl rrorll 0.5 In I .5 %, solid rd rcsiduc al 90 microns was rcduccd lo below 25 % andscvcral cmr~l~r~ss~d air “blaslcrs” wcrc inslallctl lo dislodge malcrial rrom lhe lower regionsoC 111~ prchcnlcr. T h c s c acrions improved IIIC siIuaIior1 and sul~scqI~cnIly addiIional mcasurcswcrc Iakcn: rcl~/l;iory slirrcrs wcrc addul l o I h c l i n i n g near the k i l n b a c k end, hurncr a i rvcltXiIy was incrcawd. a n d a “non-slick” lining was inslallcd in 1hc kiln cxi1 gas duel andcycl011c dip-lubes. ‘I’hcsc Crrm mntlc bcllcr ouIliu1 ralcs pnssihlc wilhoul b u i l d - u p s .

IIc~cnIly. a Iiil;hcr snll’ur rd hlcntl 113s been nscd, :~ccomp;mictl b y s l a g (S - I X) :Imonglhc r;~w niix coml~~ncnIs. I’rchcaIcr blockage problems rccurrcd, bu1 b u i l d - u p s c;m beavoidctl if the SO, l c v c l i n snmplcs taken Irnm the k i l n cnlry m a l c r i a l i s kcp1 b e l o w 2.5%b y lirnilirig lucl S conlcnl and sl;ig use i n lhc r a w m i x , provided lhal i n addilion l h c oxygenlcvcl at IIIC kiln back cm1 is kcp~ consis~cn~ly a1 or above 2%. Cnmpui~r c0nir01 or ihc ki lnIiclpr I n achicvc succc’i’i. rctlucinl: lhc vari:rhilily ol l h c 0, s i g n a l .

cxw Ii: I ;~li~n:ih~ry tl:11:1 on minor clcnic111s conlirm clvccls crlrxcrvr.tl ill ln:tclicc. I:rlrcr;~n~plc, suITur vnl;rIiliIy i n ;I s~;~rrtlarrI rcginrc (70% N,, 30% CO) is close I O 100% a1 0%0, OUI I~lls in ihc prcscncc Or 0,; r~~~~rihclcss ihc CrrCd Or 0, is much ICSS al 1400°C lhana l I2W”C. The volalilily 0r m i n o r c l c m c n l s i n l h c laboralory i s a l s o m u c h grcalcr r0rpowdcrrtl m a l c r i a l lhan rOr grarIulcs.

CXSC 1 2 : S O , 11~s l~ccn tmmit~rcd ;II lhc kilt1 Iwck cd IO dclcrwiw hrc:~l rules for avoitlinl:hltKk:igc Icndcncics. ‘I’hc S O , sil:n:rl i s ncpisy :~nd dillicull l o inlcrltrcl willroul a krtowlAl:cor IhC hiSlWy Or IllC SySlCltl. C.G.. a rcccn~ brcak;rway of sulfalc b u i l d - u p m a l c r i a l a r r i v i n gi n IIIC lmrning ~nnc c a n give a h i g h S O , s i g n a l a1 111~ k i l n back end dcspilc tlrc prc~cncc ola good flnnrc a n d ~cccl~~;~blc lcvcls of vnl;llilcs i n k i l n cnlry malcrial. I<ulcs l1avc lowcrccl111~ nun~l>cr (,I k i l n >I,,,,F ,w, yc:,r c;mcctl try I~ruln.:~tcr Irlcsk:rl;c 1’reun r~vc’r ‘)I) II) Icv\ 01:rnill. 1~151 lirlrc ln~~rrs Il:rvinI: :II\~I I:rllr~n loom :~r~nmtl 4,511 Jk‘r yc:tr 111 :rlr~url IIIII. ( ‘ I hi.11. lvcrt’:rl\o m:ijfn l::rin’; i n \I*rlt\ 1311wtl Ily rinl:\ :III~I Irr~3L;tw:1y\ ;II lhc kiln t’rllry rr.:ll.)

l3TECl~S OF C O N D E N S A T I O N

Cast Y: A prccalcincr kiln ran well will1 a Cl lcvcl in kiln cnlry maIcrial of 3 IO 4% (aboui The clrcc1 w h i c h rccirculaling vcd;IIilcs cxcrt o n b u i l d - u p rnrm:rIion a~ 111~ k i ln (gas) cxi10 . 5 % less 1har1 111~ K,O Icvcl) provided n o I r a c c of C O w a s indicald. (‘l’hcrc w a s aboul dcl~ntls ori coniposilion (governing Icmpccr;ilnrc or liquid lirrm;rIion :mcl Ihus lhc posilion andI . I % S O , i n I h c k i l n cnIry mc;ll i n Ihis siIuaIior1.) IC CO was tlc~cc~cd, Ihcrc was abnu1 2% h;trclncss or Iiiriltl~ul1s as well as Ilic surRcc Icnsirm and viscnsily Or Ihc liquid condc~is:ilc)S O , and SX K,O i n IIIC 1101 k i l n inlc1 l&cl. accompaniul hy b u i l d - u p s based o n SpurriIc a n d o n 1hc quanlily ( w h i c h gnvcrns ralc O r CurmaIion). ‘I’hc p l a n 1 p$omclry and IllC(2Caz(Si0,).CaC0,) and c u b i c K C I cryslals. ( I I i s gcncrally rccogniscd lhal regular k i l n ll~rougl~pul a l s o play a par1 i n m a k i n g clrccls mnrc or less prnnounccd. I n cxlrcmc cases.opcralion helps IO minimisc Ihc phcnonicnon 0r ccmcnlaliori by Ihc Crcczingl Ihawing Or par1 of 111~ k i l n P,aSCS ;ITC ‘bird” O F ‘by-passed’ Cnr qrrcnching a n d scparalc dc-tluslirr~ l ocliloridc-linsul tlclx,siIs.) rcmnvc volnlilcs . i n c u r r i n g linancial pcnallics i n plan1 cosl, complcxily a n d r0Ci use.

Cnsr In: T o c x n m i n c IIIC rcnsibiliry oC p r o d u c i n g a sulraic-rich clinker wiIhou1 insInlling a I n lhc pasl. s c v c r a l empirical limils have been l~roposcd rOr conccnlralions d volnlilcs

b y - p a s s . ICSIS WL 1m1 Inr some weeks will1 I h c objccIivc or reducing b u r n i n g roncvolaIili.~~tinn b y p l a y i n g o n process paramcIcrs a n d prnducing a IighIly mincraliscd clinkerwill1 higher volalilc rclcnlion. D u r i n g changcovcr ~hcrc was some I c n d c n c y l o rnrrn sol1build-ups in Ihc prchca~cr, bu1 wiIh ihc new regime csIablishcd ihcsc movctl lowards lhc kilnfcul chu~c wiIhou1 c a u s i n g m a j o r problems for k i l n opcniion. (EvidcnIly lhcrc a r cphcrIon1cna of boll1 shorI-Icrm a n d l o n g - I c r m sIabiliIy: o n c e a slablc b u r n i n g zonevolaIili.saIion is csIablishcd, in lakes Iimc f o r sIablc condiIions I O arrive higher u p the syslcmand in Ihc Iargc masses or malcrial in lhc build-ups and coalings already in cxislcncc.) Theapparcn1 b u r n i n g ~nnc IcmpcraIurc w a s rcducul b y almu~ 1 2 0 ° C . while K,O volalililydroplwl IrIlm 70% III 60% i n 111~ b u r n i n g ~onc a n d IhaI ol S O , Iron1 RO% Iowartls lhc ranl:cSO% I O GO%, provitlcd lhnl k i l n cxi1 oxygen l c v c l w a s kcp1 a b o v e 2 % . Thcrc wcrcin~provcn~cnIs in kiln outpul raIc and rucl consumpIion and lhc cxpcrimcnlal Works adoplcdccrlain Or Ihcsc changes during normal nl~ralinn rOr scvcral years.

C n s c I I : TCSIS wcrc c;Irricd 0111 invnlving variotrs N O , lcvcls ( t o indicalc llamcIcmpcralrIrc) a s well as chcmic~lly rcrhlcing rh0C contliliruIs.

so, : ‘I’hc r a l i o or S O , i n Slngc I V I n S O , i n r a w iri~l variedIypically Iron1 I.R IO 2.7 lnr 111~ higher lcvcls ol NO., and was3 . 0 [or a IOW 0, Icvcl. The clinker S O , c~nlcnl fell.

K,O a n d Na,O : In a parallel manner. for K,O 1hc ralin of l h c c o n l c n l i n S~ap,cI V IO Iha i n r a w meal varictl from 3.X IO 4 . 4 a n d rnr NazOrrom I .6 IO 2.0.

In gcncral, rultrcing cnndiIions incrcascd SO, lcvcl a1 Slagc IV by a hClOr 0r 2, also givinga lower clinker SO,.

~cmpcraturc in a WCI prnccsr k i ln , a considcrablc dab-bank c l I t.-6urcrncnls 1115 been buillup. The USC o f lnwrr a s h rucl lowcrcd c l i n k e r K,O l c v c l b y O.lS%. dcspilc 111~ inIroduclior1of n IiIIlc mnrc p&ssium IO lhc system (an cxlra 0 . I 16 o n clinker). T h e dusl - rclurncd IOlhc kiln - had bccomc more rich in alkalis, so lhal Ihe proportion or K,O broughl in by solidfuel fell lrom 24% I O IX% while ihai brough1 i n b y dust rcIIIrn rose from 18% I O 29%.This s~rp.gcs~s 1ha1 KzO incotporaIion in clinker no1 only dcpcnds on 1hc quan1ily inlroduccd11u1 also . a n d almvc all clsc _ o n IIIC type of malcrinl w h i c h b r i n g s i1 i n a n d perhaps o n 111~pnsiIimr whcrc i1 is injccIctl. On screening 111~ clinker al 20 mm. chcmic~l analysis showeda K,O coriccnlralinr~ snmc IO% Irighcr i n Ihc cn;~rsc f r a c t i o n . The clinker a lka l i COIIICIII hass~~cccssli~lly been rctlucccl i n lrials b y c a l c i u m chloride atldilinri nl l h c Ilamc.

Cnsc 6: A hil:h chlnridc coal ( - 0. I5 % Cl) can only bc ~isctl as a mix will1 anolhcr coal loavoid Imild-ullc wilh Iyliically 2 % Cl a1 Ihc ImI1nm Or cyclone 4 (lhc lowcsl in lhc prchcalcr~nwcr). hn;rlyTis of Inriltl~ul~s along IIIC kiln indica1ul chlnridc lc,vclr up IO 30% (al zero lossnn ignirinn) in 111~ coaIing Iron1 lhc base of cyclone 2 and 20% a1 SO 111 inlo Ihc kiln (tlcspilciit hs’i ih:m 5 % lrvcl i n Imllr lmi :md rold p;nI’i 0r Ilic k i l n ) .

C:1w 7: Atldinl; a sccontl ln~~hc;l~cr c y c l n n c s~agc IO a l o n g d r y process k i ln (a varinrIt of1hc I:igutc I prnccss, will1 a higher k i l n IcngIh/dian~cIcr ralio a n d a single cyclone slagcnhnvc i1) yicldcd vnrimrs build-up problems. To resnlvc ~hcsc, IIIC kiln gas cxi1 0, lcvcl wasiricrcA5rccl rrorll 0.5 In I .5 %, solid rd rcsiduc al 90 microns was rcduccd lo below 25 % andscvcral cmr~l~r~ss~d air “blaslcrs” wcrc inslallctl lo dislodge malcrial rrom lhe lower regionsoC 111~ prchcnlcr. T h c s c acrions improved IIIC siIuaIior1 and sul~scqI~cnIly addiIional mcasurcswcrc Iakcn: rcl~/l;iory slirrcrs wcrc addul l o I h c l i n i n g near the k i l n b a c k end, hurncr a i rvcltXiIy was incrcawd. a n d a “non-slick” lining was inslallcd in 1hc kiln cxi1 gas duel andcycl011c dip-lubes. ‘I’hcsc Crrm mntlc bcllcr ouIliu1 ralcs pnssihlc wilhoul b u i l d - u p s .

IIc~cnIly. a Iiil;hcr snll’ur rd hlcntl 113s been nscd, :~ccomp;mictl b y s l a g (S - I X) :Imonglhc r;~w niix coml~~ncnIs. I’rchcaIcr blockage problems rccurrcd, bu1 b u i l d - u p s c;m beavoidctl if the SO, l c v c l i n snmplcs taken Irnm the k i l n cnlry m a l c r i a l i s kcp1 b e l o w 2.5%b y lirnilirig lucl S conlcnl and sl;ig use i n lhc r a w m i x , provided lhal i n addilion l h c oxygenlcvcl at IIIC kiln back cm1 is kcp~ consis~cn~ly a1 or above 2%. Cnmpui~r c0nir01 or ihc ki lnIiclpr I n achicvc succc’i’i. rctlucinl: lhc vari:rhilily ol l h c 0, s i g n a l .

cxw Ii: I ;~li~n:ih~ry tl:11:1 on minor clcnic111s conlirm clvccls crlrxcrvr.tl ill ln:tclicc. I:rlrcr;~n~plc, suITur vnl;rIiliIy i n ;I s~;~rrtlarrI rcginrc (70% N,, 30% CO) is close I O 100% a1 0%0, OUI I~lls in ihc prcscncc Or 0,; r~~~~rihclcss ihc CrrCd Or 0, is much ICSS al 1400°C lhana l I2W”C. The volalilily 0r m i n o r c l c m c n l s i n l h c laboralory i s a l s o m u c h grcalcr r0rpowdcrrtl m a l c r i a l lhan rOr grarIulcs.

CXSC 1 2 : S O , 11~s l~ccn tmmit~rcd ;II lhc kilt1 Iwck cd IO dclcrwiw hrc:~l rules for avoitlinl:hltKk:igc Icndcncics. ‘I’hc S O , sil:n:rl i s ncpisy :~nd dillicull l o inlcrltrcl willroul a krtowlAl:cor IhC hiSlWy Or IllC SySlCltl. C.G.. a rcccn~ brcak;rway of sulfalc b u i l d - u p m a l c r i a l a r r i v i n gi n IIIC lmrning ~nnc c a n give a h i g h S O , s i g n a l a1 111~ k i l n back end dcspilc tlrc prc~cncc ola good flnnrc a n d ~cccl~~;~blc lcvcls of vnl;llilcs i n k i l n cnlry malcrial. I<ulcs l1avc lowcrccl111~ nun~l>cr (,I k i l n >I,,,,F ,w, yc:,r c;mcctl try I~ruln.:~tcr Irlcsk:rl;c 1’reun r~vc’r ‘)I) II) Icv\ 01:rnill. 1~151 lirlrc ln~~rrs Il:rvinI: :II\~I I:rllr~n loom :~r~nmtl 4,511 Jk‘r yc:tr 111 :rlr~url IIIII. ( ‘ I hi.11. lvcrt’:rl\o m:ijfn l::rin’; i n \I*rlt\ 1311wtl Ily rinl:\ :III~I Irr~3L;tw:1y\ ;II lhc kiln t’rllry rr.:ll.)

l3TECl~S OF C O N D E N S A T I O N

Cast Y: A prccalcincr kiln ran well will1 a Cl lcvcl in kiln cnlry maIcrial of 3 IO 4% (aboui The clrcc1 w h i c h rccirculaling vcd;IIilcs cxcrt o n b u i l d - u p rnrm:rIion a~ 111~ k i ln (gas) cxi10 . 5 % less 1har1 111~ K,O Icvcl) provided n o I r a c c of C O w a s indicald. (‘l’hcrc w a s aboul dcl~ntls ori coniposilion (governing Icmpccr;ilnrc or liquid lirrm;rIion :mcl Ihus lhc posilion andI . I % S O , i n I h c k i l n cnIry mc;ll i n Ihis siIuaIior1.) IC CO was tlc~cc~cd, Ihcrc was abnu1 2% h;trclncss or Iiiriltl~ul1s as well as Ilic surRcc Icnsirm and viscnsily Or Ihc liquid condc~is:ilc)S O , and SX K,O i n IIIC 1101 k i l n inlc1 l&cl. accompaniul hy b u i l d - u p s based o n SpurriIc a n d o n 1hc quanlily ( w h i c h gnvcrns ralc O r CurmaIion). ‘I’hc p l a n 1 p$omclry and IllC(2Caz(Si0,).CaC0,) and c u b i c K C I cryslals. ( I I i s gcncrally rccogniscd lhal regular k i l n ll~rougl~pul a l s o play a par1 i n m a k i n g clrccls mnrc or less prnnounccd. I n cxlrcmc cases.opcralion helps IO minimisc Ihc phcnonicnon 0r ccmcnlaliori by Ihc Crcczingl Ihawing Or par1 of 111~ k i l n P,aSCS ;ITC ‘bird” O F ‘by-passed’ Cnr qrrcnching a n d scparalc dc-tluslirr~ l ocliloridc-linsul tlclx,siIs.) rcmnvc volnlilcs . i n c u r r i n g linancial pcnallics i n plan1 cosl, complcxily a n d r0Ci use.

Cnsr In: T o c x n m i n c IIIC rcnsibiliry oC p r o d u c i n g a sulraic-rich clinker wiIhou1 insInlling a I n lhc pasl. s c v c r a l empirical limils have been l~roposcd rOr conccnlralions d volnlilcs

b y - p a s s . ICSIS WL 1m1 Inr some weeks will1 I h c objccIivc or reducing b u r n i n g roncvolaIili.~~tinn b y p l a y i n g o n process paramcIcrs a n d prnducing a IighIly mincraliscd clinkerwill1 higher volalilc rclcnlion. D u r i n g changcovcr ~hcrc was some I c n d c n c y l o rnrrn sol1build-ups in Ihc prchca~cr, bu1 wiIh ihc new regime csIablishcd ihcsc movctl lowards lhc kilnfcul chu~c wiIhou1 c a u s i n g m a j o r problems for k i l n opcniion. (EvidcnIly lhcrc a r cphcrIon1cna of boll1 shorI-Icrm a n d l o n g - I c r m sIabiliIy: o n c e a slablc b u r n i n g zonevolaIili.saIion is csIablishcd, in lakes Iimc f o r sIablc condiIions I O arrive higher u p the syslcmand in Ihc Iargc masses or malcrial in lhc build-ups and coalings already in cxislcncc.) Theapparcn1 b u r n i n g ~nnc IcmpcraIurc w a s rcducul b y almu~ 1 2 0 ° C . while K,O volalililydroplwl IrIlm 70% III 60% i n 111~ b u r n i n g ~onc a n d IhaI ol S O , Iron1 RO% Iowartls lhc ranl:cSO% I O GO%, provitlcd lhnl k i l n cxi1 oxygen l c v c l w a s kcp1 a b o v e 2 % . Thcrc wcrcin~provcn~cnIs in kiln outpul raIc and rucl consumpIion and lhc cxpcrimcnlal Works adoplcdccrlain Or Ihcsc changes during normal nl~ralinn rOr scvcral years.

C n s c I I : TCSIS wcrc c;Irricd 0111 invnlving variotrs N O , lcvcls ( t o indicalc llamcIcmpcralrIrc) a s well as chcmic~lly rcrhlcing rh0C contliliruIs.

so, : ‘I’hc r a l i o or S O , i n Slngc I V I n S O , i n r a w iri~l variedIypically Iron1 I.R IO 2.7 lnr 111~ higher lcvcls ol NO., and was3 . 0 [or a IOW 0, Icvcl. The clinker S O , c~nlcnl fell.

K,O a n d Na,O : In a parallel manner. for K,O 1hc ralin of l h c c o n l c n l i n S~ap,cI V IO Iha i n r a w meal varictl from 3.X IO 4 . 4 a n d rnr NazOrrom I .6 IO 2.0.

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FIGURE 3OXIDISED KILS SULFL.iR C Y C L E

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(fmm raw materials1

‘1‘115 cffccls o n cliukcr iuay be sutntnariscd a s f o l l o w s :

I’luxinl: aclioii:

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l cbangc al’ l i q u i d viscosily

6 allcraliun 0r sdxx Iciisioii OK l i q u i d

0 inihlilic;il~o~i ul’ crystal ulorlhology.

I’l~ass rclaliurls:

I lydraulic aclivily:

l lhe fL’;iclivtlics 01’ lhc cliukcr u~immls arc allcrcd b y sul~d mlulion ;uldlor by

the cffccrs o f crystal syinrilctry (IQli leu~pcralure stabilization o f poly~norphs)arid/or cl’fucls occurritrg during liydraliotl (e.g., coaling of cc~uenl particles by

insolubly salis).

( I I i s d i f f i c u l t lo isolale Ihcsc Ihrcc classes of cffccl i i i praclics.)

I I I gcuc~al. incorporalioo of a d d i t i o n a l clillkcr solC;~~e it1 a situatioll willi e~.ccss a l k a l i s y i e l d s

a inore dilficuli “iiIlI>ilfCl\l grithhbilily” with adviiolagcs i n the Iua~kcl or hlmvctl e a r l y

concrclc s1rc11g111 a n d workabilily.

I’IIc cf~cc~r or utimJr ~WII~~II~IIIS WI the vircosily a n d shrlace lensioll of l i q u i d phascr c a n

be cou~plrx. Luwcr viscosiliss e n c o u r a g e alilc ( c a l c i u m lrisilicale) forinalion. CalciuiuSUIT;IIL flux call. I~ow~!vsr. stabilise bclilc ( c a l c i u m disilicatc) antll~r CatIsc lhr prodtrcliori o f

clinker alilc will1 lime i n c l u s i o n s . I:urlher, iu clinkers with a l o w alk;lli CWICIII, bclilc

sI;tibilis;dicm due III cxccts S O , Icxls I~I tlilliuulr cl,lnl,iii;ll,ilily. S~rtm~ly c h e m i c a l l y fcducir$

comJilioiis I I I Ilic burriiug LOII(: cau Live a camcril will! par Ilow cliaiaclcrislics (tluc lo I’rsc

K;,O a n d N+O), p o o r workhility ( d u e I O IIIC i n c r e a s e d conlcnl o f Iricalcium aluminalc arid

ils rraclivily), poor slrcuglh (lower lricalcium silicalc conlenl) a n d Vilriill)lt colour.

A l k a l i s rclainrd i i i clinker a r e prcscm cilhrr a s slablr sulhles o r iibSlllbd i n llld silicate aridalulnitialr siruc~urcs: ihcsc inlltrencc IIIC bchaviour o f fresh cor~crele a n d m o r t a r d u e l o Ilieir

various solubililier. N;r20 hi\s a I~\WC matkcd ~ende~\cy than K,c) 10 ~OIIII wI~II~IIS illc a l c i u m alurnimilr. I:or clinkers with (ruular) ratios of sulfiilc:lol;~l alkalis below 0.3, iIIItloSla l l 111e sulh~e i s combimxl ill waltz soluble form, K$O,, beill& prcddomimmt. A pruporlion

ol’ the a l k a l i s ale i n s o l i d soluiiou itI IIIC c l i n k e r aluciiinalr phase and lhis has ati advorle

rlfccl ori the inirial cc~~~cnI rcaclivily illld Illus o n comxele a n d morlaf r h e o l o g y .

For ralios belwceii 0 . 5 a n d I .O, a c e r t a i n quanlily of langbrinilc i s a l s o formed (and no1 a l l

lltr a l k a l i s a r e wlul~lc). I:or r a t i o s alrove 1.0, si~nilicanl f r a c t i o n s o f lhr Sulfales a r e

c o m b i n e d williin Ihc silic; .md aluminalss o r as auliydrirc (CaSO,). which d~s~I~cs umrcslowly III~II a l k a l i sulf’ales, whilst llle fraclions of K,O aud NazO which are s o l u b l e ill w;ucrapproach I .O ;mJ 0.S. rcspxiivsly. al a raliu OI’:IIJOUI I.S. Al sitll’alc:;ilk:ili raliiJ9 ahvc I .SIrcntls a r c wmewl~al crralic. I’ur IWSI uoru~l clinkers IIW prilicip4 rulli~~c phase w i l l bcalhil~italilc (Iwlassitlrri/roditr[rl s u l f a t e ) with a m a x i m u m WN;I r a t i o o f 3 . 0 . ‘I‘l~is IIllasc i saccompanied b y m i n o r quaniiiirs o f K,SO, a n d c a l c i u m Iangbsiniic. Na$O, bciug hmd

o n l y Ibr u n u s u a l l y l o w K/Na raiios.

A s w e l l a s lhe s o l i d solulion rffecls a n d the f o r m a t i o n o f compou~ids d e s c r i b e d almvr,

v a r i o u s permulaliotis o f volalilcs ( e s p e c i a l l y in Itie p r e s e n c e o f f l u o r i n e ) c a n influcrux rhc

s~ructurs arid belnviour o f aliic aml bcliir crysials ( M o i r & Classtx, l!J92).

II is gc~icf;~lly ~.up~~‘xd IhI (t~llicr Ihcl~~rs ljcitlg cqii;~lJ ll~c cxlcul 1,1’ vt8I:tlili*:tlit~ll ~~L,I~:I\c’I

a s the Iliermal efficiency a l IIIC kiln iucrcascs. ‘I’liis i s prol~ably du1: Iu iltc lillliliug sllUcio f vapur wluralion b y a l k a l i c o m p o u n d s , a s c o n f i r m e d b y sludics of IIIC irca~msm o f k i l n

dusl i n a I00 IIII~ diarnslcr hidiscd bed (‘l’clinnr rf ul, 1978) lo e x a m i n e ihc fcasibili~y o fproduciril: cliriker ~~OIII WIII~III kilt1 flue dull will1 caplure of IIIC a l k a l i s distilled f r o m 111l:

b e d for p o s s i b l e use ill 111e ferliliscr i n d u s t r y .

I I i s su~gerlcd IIuI !II

V’ = -me..llilllM,1-.-.(1' - 1") WJ

v* i;i salurhtd vapour coiicciilralioii i i i Irai~sporl pscs ( k g / k g )p* = wluralrd vapour p r e s s u r e 1

OT au a l k a l i compouml ) ~:IIIIC imilsJ’ = g a s pcssur~ 1M, = niolrcul;ir wcigld III’ valmiir

M, = m o l e c u l a r weigh1 o f g a s .

Give0 malluzmalical cxprcssiims f o r S;~lt~rillI2~l vapour prcssltrcs ilS ~m~ciicms of lc~llpcralurua n d krmwlcdpz of kill) systru~ Icmpcralurc p r o f i l e s , the w~ura~cd val)uur conccmraiioll WI

be calculaled f o r exh a l k a l i compout~d a n d IIIIIS he maximurn qu;miiiits cval~ra~cJ f r o mllle feed lxx unit IllilSS of g a s e s . ‘I’lieri, considcrirlg the amoums ol’ gas passirlg Il~rou~:h lhckih al v a r i o u s temperatures, llx Iruc llllillllily ol volaiilcs Irar~sp~rlud per uuii m a s s o fc l i n k e r GUI be calc~tla~txl arld f r o m his krtowlcdp, ‘ideal* volaiilc cycles CarI lx tlsduccd.

For example, saturated vapour pressures at IZOO’C are (for IIIE prrc substances):

K C I 0. I8 am

K,SO, 0 . 8 x lo” illIll (0.6 x IO’j am with dccolnpition suI~Imssec1)Na$O, 0 . 1 3 x IO’] alIll (0.01 x IO” ~IIII with d e c o m p o s i t i o n s u p p r e s s e d )

‘1‘115 cffccls o n cliukcr iuay be sutntnariscd a s f o l l o w s :

I’luxinl: aclioii:

0 l o w e r lcmperalure o f firs1 l i q u i d phase f o r m a t i o n

l cbangc al’ l i q u i d viscosily

6 allcraliun 0r sdxx Iciisioii OK l i q u i d

0 inihlilic;il~o~i ul’ crystal ulorlhology.

I’l~ass rclaliurls:

I lydraulic aclivily:

l lhe fL’;iclivtlics 01’ lhc cliukcr u~immls arc allcrcd b y sul~d mlulion

;uldlor by

the cffccrs o f crystal syinrilctry (IQli leu~pcralure stabilization o f poly~norphs)arid/or cl’fucls occurritrg during liydraliotl (e.g., coaling of cc~uenl particles by

insolubly salis).

( I I i s d i f f i c u l t lo isolale Ihcsc Ihrcc classes of cffccl i i i praclics.)

I I I gcuc~al. incorporalioo of a d d i t i o n a l clillkcr solC;~~e it1 a situatioll willi e~.ccss a l k a l i s y i e l d s

a inore dilficuli “iiIlI>ilfCl\l grithhbilily” with adviiolagcs i n the Iua~kcl or hlmvctl e a r l y

concrclc s1rc11g111 a n d workabilily.

I’IIc cf~cc~r or utimJr ~WII~~II~IIIS WI the vircosily a n d shrlace lensioll of l i q u i d phascr c a n

be cou~plrx. Luwcr viscosiliss e n c o u r a g e alilc ( c a l c i u m lrisilicale) forinalion.

Calciuiu

SUIT;IIL flux call. I~ow~!vsr. stabilise bclilc ( c a l c i u m disilicatc) antll~r CatIsc lhr prodtrcliori o f

clinker alilc will1 lime i n c l u s i o n s . I:urlher, iu clinkers with a l o w alk;lli CWICIII, bclilc

sI;tibilis;dicm due III cxccts S O , Icxls I~I tlilliuulr cl,lnl,iii;ll,ilily. S~rtm~ly c h e m i c a l l y fcducir$

comJilioiis I I I Ilic burriiug LOII(: cau Live a camcril will! par Ilow cliaiaclcrislics (tluc lo I’rsc

K;,O a n d N+O), p o o r workhility ( d u e I O IIIC i n c r e a s e d conlcnl o f Iricalcium aluminalc arid

ils rraclivily), poor slrcuglh (lower lricalcium silicalc conlenl) a n d Vilriill)lt colour.

A l k a l i s rclainrd i i i clinker a r e prcscm cilhrr a s slablr sulhles o r iibSlllbd i n llld silicate aridalulnitialr siruc~urcs: ihcsc inlltrencc IIIC bchaviour o f fresh cor~crele a n d m o r t a r d u e l o Ilieir

various solubililier. N;r20 hi\s a I~\WC matkcd ~ende~\cy than K,c) 10 ~OIIII wI~II~IIS illc a l c i u m alurnimilr. I:or clinkers with (ruular) ratios of sulfiilc:lol;~l alkalis below 0.3, iIIItloSla l l 111e sulh~e i s combimxl ill waltz soluble form, K$O,, beill& prcddomimmt. A pruporlion

ol’ the a l k a l i s ale i n s o l i d soluiiou itI IIIC c l i n k e r aluciiinalr phase and lhis has ati advorle

rlfccl ori the inirial cc~~~cnI rcaclivily illld Illus o n comxele a n d morlaf r h e o l o g y .

For ralios belwceii 0 . 5 a n d I .O, a c e r t a i n quanlily of langbrinilc i s a l s o formed (and no1 a l l

lltr a l k a l i s a r e wlul~lc). I:or r a t i o s alrove 1.0, si~nilicanl f r a c t i o n s o f lhr

Sulfales a r e

c o m b i n e d williin Ihc silic; .md aluminalss o r as auliydrirc (CaSO,). which d~s~I~cs umrcslowly III~II a l k a l i sulf’ales, whilst llle fraclions of K,O aud NazO which are s o l u b l e ill w;ucrapproach I .O ;mJ 0.S. rcspxiivsly. al a raliu OI’:IIJOUI I.S. Al sitll’alc:;ilk:ili raliiJ9 ahvc I .SIrcntls a r c wmewl~al crralic. I’ur IWSI uoru~l clinkers IIW prilicip4 rulli~~c phase w i l l bcalhil~italilc (Iwlassitlrri/roditr[rl s u l f a t e ) with a m a x i m u m WN;I r a t i o o f 3 . 0 . ‘I‘l~is IIllasc i saccompanied b y m i n o r quaniiiirs o f K,SO, a n d c a l c i u m Iangbsiniic. Na$O, bciug hmd

o n l y Ibr u n u s u a l l y l o w K/Na raiios.

A s w e l l a s lhe s o l i d solulion rffecls a n d the f o r m a t i o n o f compou~ids d e s c r i b e d almvr,

v a r i o u s permulaliotis o f volalilcs ( e s p e c i a l l y in Itie p r e s e n c e o f f l u o r i n e ) c a n influcrux rhc

s~ructurs arid belnviour o f aliic aml bcliir crysials ( M o i r & Classtx, l!J92).

II is gc~icf;~lly ~.up~~‘xd IhI (t~llicr Ihcl~~rs ljcitlg cqii;~lJ ll~c cxlcul 1,1’ vt8I:tlili*:tlit~ll ~~L,I~:I\c’I

a s the Iliermal efficiency a l IIIC kiln iucrcascs. ‘I’liis i s prol~ably du1: Iu iltc lillliliug sllUcio f vapur wluralion b y a l k a l i c o m p o u n d s , a s c o n f i r m e d b y sludics of IIIC irca~msm o f k i l n

dusl i n a I00 IIII~ diarnslcr hidiscd bed (‘l’clinnr rf ul, 1978) lo e x a m i n e ihc fcasibili~y o fproduciril: cliriker ~~OIII WIII~III kilt1 flue dull will1 caplure of IIIC a l k a l i s distilled f r o m 111l:

b e d for p o s s i b l e use ill 111e ferliliscr i n d u s t r y .

I I i s su~gerlcd IIuI !II

V’ = -me..llilllM,1-.-.(1' - 1") WJ

v* i;i salurhtd vapour coiicciilralioii i i i Irai~sporl pscs ( k g / k g )p* = wluralrd vapour p r e s s u r e 1

OT au a l k a l i compouml ) ~:IIIIC imilsJ’ = g a s pcssur~ 1M, = niolrcul;ir wcigld III’ valmiir

M, = m o l e c u l a r weigh1 o f g a s .

Give0 malluzmalical cxprcssiims f o r S;~lt~rillI2~l vapour prcssltrcs ilS ~m~ciicms of lc~llpcralurua n d krmwlcdpz of kill) systru~ Icmpcralurc p r o f i l e s , the w~ura~cd val)uur conccmraiioll WI

be calculaled f o r exh a l k a l i compout~d a n d IIIIIS he maximurn qu;miiiits cval~ra~cJ f r o mllle feed lxx unit IllilSS of g a s e s . ‘I’lieri, considcrirlg the amoums ol’ gas passirlg Il~rou~:h lhckih al v a r i o u s temperatures, llx Iruc llllillllily ol volaiilcs Irar~sp~rlud per uuii m a s s o fc l i n k e r GUI be calc~tla~txl arld f r o m his krtowlcdp, ‘ideal* volaiilc cycles CarI lx tlsduccd.

For example, saturated vapour pressures at IZOO’C are (for IIIE prrc substances):

K C I 0. I8 am

K,SO, 0 . 8 x lo” illIll (0.6 x IO’j am with dccolnpition suI~Imssec1)Na$O, 0 . 1 3 x IO’] alIll (0.01 x IO” ~IIII with d e c o m p o s i t i o n s u p p r e s s e d )

‘I’lrc I~;III~IKJ~( capacity 14 air lo; vapours al lZ(XJ”C i s tbur: nil - 7(X1 g/1;: K,SO, - 1 g/1::Na ~SO,, < 0.S 1:/g. ‘I’hc cqj;lcily nl I?S(l”C i s ;IIJ~JIII I W O limes bil;llcr.

II cmi IljcrcInrc hc Iurcsccn Ilral (unless llic equilibrium vapour prcssurcs difCcr grally Iromurlur;jlrA values) lbcrc w i l l b c lilllc problem i n removing KCI from m a n y k i l n lluc dusls ina lluitliscd IJC~ witli a g a s llow ralc o f , say , 2 g per grammc o f dust, idll10ugb Ibe c a p a c i t yC0r sdhk removal m a y b c limilcd. The sa111c reasoning a p p l i e s 1 0 k i l n s , will1 wcl processkilns typically slmwinl: n ratio rjf a lilllc less 1ljan 2 g/g gas/solids in ~ljc burning z0rjc and‘pcrljapr 2 . 7 5 l;Ig ;II IIW b a c k end. will1 corrcsl~nding values C0r lljc d r y prtxzcss (wilboulI)rcc;jlcitl;lliolj) Or I .4 ~$1; and I .91 g/g.

I)c\pilc IIIC lacl IIWI qualilalivc diCCcrcr1ccs I~c~wcc~~ I W O k i l n s (OIJC d r y process a n d OIJC wet)arc rrllcclctl in s:jmplc calculations, Suclj “idcal” calcolalctl rccirculalinl; loads arc aboul IOIimcT lnrjicr lljn~i tllosc cricounlcrcd i n praclicc. TIJC probable reasons include:

3)

11)

cl

(1)

Cl

0

b)

In~~~w~~lclr ct~~~l:wl Iwlwccn g:iws :uul s o l i d s i n llir k i l n . wlicrc o n l y a sii1;1llI’raclitui 01 lltc s o l i d surracc i s cxlxjsrd a l ; I given lime. ( I I i s cxpcclcd llialIbcrc i% bcllcr coril;~cl i n llic cc~lrlcr dusly rqiojis Or lbc syslct~~.)

Vul:llilis:llion c:b;lr;lclcrislics of IIIC alk:lli-ccrlJl;lirliIlg IniItcritIs ;,I a givcu IJI;IIJI.i.c., conrljinnliorl i n m0rc co~~iplcx silicalcs a n d aluminalcs. 1~01 o n l y sijihks.I’raijspfjri 0r co~~jp~rintls wljicb arc condcnscd as/on solid dusl or rtl0lc.

I~~Ictluciion nl vap0ur prcssurc over scjlulions of a l k a l i c o m p o u n d s .

i~~lq~~c lrcnlmcnl d ibc Irnjjspjrl or ljcal and Or vnfxnjr willrin UJC bed Orclicikcr nodules i n Ihc kill).

I:orrnsliotj Or Olbcr compounds. e.g., CaSO,, dcpcndin~ on alknli:sullaic ralio.

I~rjs~al~lc opcralion or p r o d u c t i o n k i l n s : praclical c o n d i t i o n s a r c 1101 cxacllyIl~osc cxl~clcd k)r very l o n g lcrm slabilily Or lcmpcralurc a n d malcrial Ilow.

Tbc lurillcr tlcvclqjnrcn~ or a prctliclivc model w i l l Ijavc I O iakc accour~l o f such [actors, aswell as illc crrccls Or coni~xlsilion Or kiln almospbcrc. In rcccril ycnrs llicrc has been mucllinvcslig:llirjtj ol l;jclors /:nvcrninp, IIIC block;tgc of cyclones a n d lbcir pcc’k~rmancc i n biGIIcurpcralurc coal c~11iil~5lion proccsscs (itI Ilic Iiopc or prolccling Iurbinc blatlcs i n tlirrclcycle clcclric power gcucr;tlicln syslcms). Wljcn linic (or limesi0nc) i s injcclcd lo nbsorbS O , . 111~ COIJI~HJ~JI~S a n d Il~crrnodynamic crilcria invoked a r c cxaclly 1110s~ cncounlcrcd inIIIC CCIIKIII intluslry - particularly wl~cn rclalivcly l1igb cbloridc coals arc oscd. II is probableIllal Iljcrc i s now sullicicnl n c a d c m i c knowlcdgc IO bclicr Irc31 o u r silualion a n d a l l o wimprovctl mrxlcllinl; a n d undcrslanding. Anolbcr aspccl 10 consider is knowlcdgc acquiredrrnm sludy 0r ibc rcgciicraliori 0r CaO sorbcrils u.sul ror S O , s c r u b b i n g : a g a i n , dalap0lcnlially rclcvanf in k i l n sys~cms arc protluccd, l’or cxamplc, o n p r c s s u r c s o f S O , i n IIICsy~cm CaSO.,I C;rSI CnO i n IIIC prescncc ol v a r i o u s conccnlrations ol C O a n d C O , . II isImpcd Illal ibis paper promolcs cross-rcrliliulion bctwccn IIJCSC v a r i o u s fields or w o r k a n dlbc bcnly oC praclical knowlctlgc available i n ~ljc ccmcnl i n d u s t r y .

‘I’IIANI<S

‘l’ltis Ical. ori~;in;llly I~lsul o n v a r i o u s irjlcrn~tl amI crlcrr~l rqK)rIs, Ita\: l~ccrj atl;~l~lcd 01t lllcb a s i s of urchjl di.uzussion will1 Icclmical slall rrom a mrmbcr ol ccmcnl companies. 7‘banksarc given lo all involved, as well as IO lbc Directors of WIIC Circle Industries PLC ror lbcirpermission I O publish Ibis paper.

Clioi. G.-S. & Glaswr, 1:. I ’ . (19RR).‘I’bc Sulplrur Cycle i n Ccmcn~ K i l n s : Vap0urI’rcssurcs and Solid~l’liasc Slabilily O r llrc Sulpbalc I’ljascs. Ccr~~ct~l rind ConcrrlcIlcsc:~rcl~, IR. 367.371.

Kct~n. C . I ’ . & M u r r a y , It. 1. (1984). I’ortland CCIJIOJI I’rotluclicm. 111 Slntcljtrr nntlI’crlonnnnrr oC CWICII~S. c c l . I ’ . Ilarncs. Appliccl Scicncc I’ublisltcrs I.id. Ihrkirj~. ~IJ 705.7 . x

M o i r . (i. K . & Gksscr, I:. I ’ . (lYY2). hlincrnliscrs, h4otlificrs a n d Activators i n ~ljcClinkcring I’rr~css. In 9111 It~lrrwtliowjl Corjgrcsi ~IJ lljc Cljcjrjislrg or CCIJJCII~. NCWDribi, I n d i a , 1 9 9 2 : Congress Itcporls, Vnlumc I , National C o u n c i l r0r Ccmcnl a n dIluildirj~ Malcri:tls, New Dcllji, p p . I2S- 154.

Rilzmann, I l . (1971). C y c l i c I’bcrtomcna i n Rotary K i l n Systcrr~s. ~cjrlclll-l(nlk-Gip(, 24.338-343.

I’cllmar, n.. Kljor, I . I I . . k Gregory, S . ( 1 9 7 9 ) .Glps, 3 I , 288-290.

I’roccrsing or K i l n Dusi.

APPExmX 3

PAPERPRESENTED BYT MLOWES

100% Pet Coke - Problems and Solutions

100% PET COKE - PROBLEMS AND SOLU-I-IONS

“Good morning Lady and Gentlemen,

The citie of my paper is 100% Petcoke - Problems and Solutions.

It will give information on how Blue Circie Cement has moved from zero to 40%petcoke over a 3 year period, indicating the technological problems that need to beovercome for any works that seeks to fire 100% petcoke.

Before beginning my presentation I would like to thank Gerard Flament of CCB andJean Pierre Piliard of Ciments Lafarge for their help and information during thisdevelopment phase within Blue Circle Cement.

The paper mainiy deals with the dry process. However, the conclusions apply to wet,LepoI and long dry processes.

Slide 1

This overhead indicates BCC’s approach to petcoke. Firstly, prior to 1991 there waszero use of petcoke because ic was coo much trouble and low ash coal was much“NICER”. However, in 1992 due to a significant recession, we moved up to 30%replacement and in 1993 40% overall on 10 Works. Some Wor:ks using none, 100% usageat times on a Lepol process, 80% on the large semi-wets at Northfleet and 65% on awet process at our Masons Works.

In 1993 the prices increased, consequently the financial benefit accrued to the projectwas only the same in 1993 as 1992.

In 1994 there has been a slight market improvement, some Works can se11 everythingthey can make and consequently we are already failing to meet the 1994 plans for useco petcoke.

Slide 2

This slide gives some indications of why one should use petcoke. Firstly, its price can.be up to, and sometimes even more than, 50% less than the coal price per GJ. It canimprove cement quality, if there is an excess of alkalies over sulphates. In addition, ifyou are coal mill limited on cement olant output it can, with the appropriate Hardgroveindex, increase coal mill capacity.

In addition, it can be claimed to help the environment as petcoke needs to be burnt andif it is burnt in conventional power stations the SO, emission {vi11 increase, whereas inmost Of our processes it is significantly retained. However, the real reason for usingit is that in a recession, the full kiln OUtpUt is not reouired and consequently operationalcosts are at a premium and petcoke can make a significant reduction in OperatiOnalcosts.

Slide 3

This siide indicates reasons for not using petcoke. Firstly, it can increase SO?emissions, this even applies to a certain extent within a dry process as, for example,even in a precalciner betueen 6000 and 10000 ppm SO, is the maximum that can beabsorbed before bypass into the preheater sys=m occurs.

Due to the extra S02/S03 going into the system I don’t need to celi anybody there issignificant increase in potential for ring formation and blockages. In addition theblending operations are increased unless one is reaily firing at 100% petroleum coke..AIso there can be coal mill fineness problems having to meet the lower 90 micronresidue and there is more variability in sulphur and Hardgrove index. in addition, someinvestment may be required.

Slide 4

This slide identifies the main problem of using petcoke which is controlling the S02/S03cycles which leads to blockages and increased SO? emission.

This depends on the total SO3 input, the Na30 equivalent, the molar ratio of SO3 toalkaiies in the clinker, the combinability temperature of the clinker and in addition tothis, how well the kiln is controlled and the overall flame conditions that exist withinthe kiln.

Slide 5

This slide shows some facts concerning peccoke usage. Firstly, for price reasons themain interest lies in the higher sulphur petcoke which typically varies between 44% andS+% suiphur. For a dry process kiln with 800kcal/kg using 100% of a 5% sulphurpeccoke, this is equivalent to around 1.33% SO3 on clinker.

As most Works are limited to around 0.6 Nay0 equivalent, this corresponds to amolecular ratio of 2 to a clinker SO3 of 1.5%. -

.Above a molecular ratio of I, calcium iangbeinite, which is a salt of alkali and calciumsulphate with 2 molecules of calcium sulphate for 1 of potassium sulphate, forms inaddition to calcium sulphate.

Calcium lambonite decomposes at around 152O’C leaving alkali and calcium sulphate.Calcium sulphate decomposes at 145O’C. However, these decomposition temperaturesare both in the presence of excess oxygen.

If there is in excess of 2000 PP?J of CO in contact with caicium sulphate above a1000°C it would break down to SO3 and calcium oxide and hence exacerbating anyparticular problems one has with SO2 emission and preheater/kiln blockages.

For a molecular ratio greater than 2 in stage 4 raw meal, this generally is recognisedas producing hard deposits.

Slide 6

This slide shows what conditions you need to avoid and what conditions you need to haveto be able to use 10096 petcoke.

Firstly, the conditions that need to be avoid& are chemical reduction in the burningzone, Over-burning and low Na,O equivalent. For 1009/o petcoke one needs to have agood flame, a readily combinable raw mix and a good kiln control of free lime and backend oxygen.

Slide 7

This slide shows what are the requirements of a good flame and uses the CEMFLAIME1 information fairiy extensively.

One needs to ensure that the burner has adequate momentum, e.g. around 7N/MW. Thisis to ensure that there is adequate air available to combust the voiatiles and carbon ofthe peuoleum coke on a micro mixed basis before the petcoke comes in contact wirhthe burning zone at between 2 and 3 kiln diameters, depending on wheKher one has ahigh swirl or a zero swirl burner.

It is importam Khar the ignition takes places very near the burner, a 60% bluff body willhelp to promote Khis by removing the jet establishmenr. region and creating an internalreverse flow zone. In addition, a secondary air temperature of greater than 800°C isrequired to achieve this early ignition.

Petroleum coke is a by-product of a process which essentially means that all of thelower temperature volaciles have been extracted and therefore peuoleum coke needsto reach around 900°C before its first volatiles can be released.

TO cope with this lower volatile release rate and notentially a less porous stiumre ofthe carbon as a result of its formation process, a’90 micron residue of around 6-7% isrequired, following the general rule of +90 micron residue being 50% of the volatilematter.

In addition, it is absolutely essential that a uniform enKrainmenK of secondary air takesplace into the flame, particularly ensuring that the secondary air coming under theburner is not SKarved. To ensure the maximum possibilities a burner cenually lined u?the kiln is most important.

Even if all of these criteria are met, one will still operate in reducing conditionpromoted by unburned carbon and CO if there is not adequate conuol of the back endoxygen. A recommended level is around a minimum of 3.5%.

Slide 8

This slide shows results from the CE:VFLAME 1 trials which indicates impact of backend oxygen on CO for both a medium volatile coal and flexicoke, with 90 micronresidues of 1246 and 2%.

T’he measurements are made at the back end of the kiln simulator which is equivalentCO an L/D of 12. As you can see, for the flexicoke, co ensure r.har: rhere is a CO levelless than 1000 PPM, one is essentiaiiy looking for 349’0 oxygen. A similar condition isi?Ot necessary for a medium voiatiie coal where ic does look char: in Che a-?;% regionthe conditions for less ihan 1000 ?Phl can be achieved.

Slide 9

This slide shows a typical situation on our dry process Works at Hope which indicatesthe impact of hard burning and CO/oxygen on stage 4 SO3. Hope Works has a clinkerMolar Ratio of 2 with a British Coal.

This was for a high volatile coal with a 90 micron residue of around 15% with the burnerlined centrally up the axis and a momentum of at least 7NI1MW.

It can be seen that hard burning increases the amount of SO3 in stage 4. However, italso increases the amount of NaZO equivalent and consequently not until one gets abovearound 1300 ppm of NOx is there a significant increase of excess SO3 over alkalies dueCO the thermal breakdown of calcium sumhate in the burning zone.

However, the most interesting point is the impact of back end oxygen OR the level ofCO and its consequent impact on stage 4 SO3.

It can be seen that once the back end oxygen drops down beiow 1.8%, i.e. to 1.4%, theCO increases from 500 porn to 2500 ppm and a corresponding increase of stage 4 SO3from 3.2% to 4.5%.

Under the conditions of the experiment, around 24 times the Na30 equivalent in theclinker was appearing the in stage 4 raw meal. Consequently, fo; a molar ratio Of 2,this corresponded to around 3.75% S03.

It can be seen that under reducing conditions promoted solely by the lack of back endoxygen, the SO3 in stage 4 increased to 4.5% which is a totally unacceptable level andblockage problems occur on the Works at this ievei.

The Works normally seek to run at around 3% stage 4 SO;.

Slide 10

While a good flame is important, alone it is not enough.

Good kiln controi is required, i.e. using Linkman, to avoid continual SO3 recycling. Forexampie, if one has a blaster operadon that breaks down deposits, if once it reaches theburning zone the conditions are SO hot that 90% of the SO3 is sent back into thepreheater system, obviously the blaster operations are not being suitably supplemented.

In addition, it is very important to have a combinability temperature essentially lessthan 1500°C. Also, one operates at round about I-I?XJ free lime, or else a similarsituation exists as to that I referred to previously under the need for good kiln conuol.Otherwise a continual recycling of the SOTin the syste.m wili occur leading topermanent problems associated with blockages and breakaway.

In addition, it is important that one continually checks what is achieved for the stage4 material. Preferably, this is supplemented by an SO2 probe that can be used tomonitor what is happenin g in the back end of the kiln and even be linked into the highlevel control. For example, 3000 ppm of SO7 equates to around an extra 1.05% SO3 onstage 4 raw meal.

Slide 11

And finally, in conclusion, now we know how to do it, how can 100% petcoke be usedwith lower NOx emission, bearing in mind this normally mea&u running into on the vergeof reducing conditions?

The answer is, I don’t know at the moment. However, CEMFLAIME 2 which is due totake place at the end of this year, will provide the answer.

Thank you”

--__-._---_- ---

100% PET COKE - PROBLEMS AND SOLbTIONs]_... -_-_.. _--_-_.- --- ------.---- .-.. -__-

o BCC's approach to Pet Coke

- Prior to 1991 zero use too much trouble, low ashcoal much “NICER”

- 1992 - 30%

- 1993 - 40% 100% Lepol, 80% semi wet, 65% wet.

- 1993 prices increased

- 1994 market improvement

- 1994 budget 35%

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I-_--..---___ ____ - __...-- _-__- . .._ _.. -I__-__-_--. -

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1 100% PET COKX - PROBLEMS AND SOLUTIONS /I-- -.-_-. ..-._.- . ..__._ _-. -. -_.-.._ ._.. - .__... _ _-. _- ,_.___ ,.__ -___ _.------_-_--.---__-- -___--. -- 1

l Why use Pet Coke?

A Price - up to 50% < Coal/Gj

A Improve cement quality

A Increase coal mill capacity

A Help the enviroment

A Recession, fxlll kiln output not required

r --- -_- _ -____. --__ ___.______ _.__ __,_____-_I__., _ -._____ ____ _ __._ _ __-- I/ 100% PET COKE - PROBLEMS AND SOLUTIONS.-.-_ - -..-- -_.-.- . .- . ..-.--- . ..--.. I .._ _._- ~----.-__.--.- -.----.-. _------ ,I--.----

o Why not use Pet Coke?

A Increase in SO2 emission

A Increase in problems with rings and blockages

A Blending operations increased

A Coal mill fineness problems

A S ,and Hardgrove variability

A Investment may be required

.---__-__-.__-_- -.-__ --__ ..-- . . .._.- -_,.-..-. __.._______ -__- _---

11000/o PET COKE - PROBLEMS AND SOLUTIONS1 _.--.-__-- ____.-_ --_.-. __--.. ___ .._....-... -.. .__- - -__.__ ._. ._-_____. --____-__---

o Main “PROBLEM”

A Controlling SO2/SO3 cycles which leads to increasedblockages and SO2 emission

0 Depends on:-

A Total SO3 input

A Na20 equivalent

A Clinker Molar Ratio

A Combinability Temperature

A Kiln Burning Control

A Hame Conditions

.-- -------- -_---- -- --

100% PET-COKE - PROBLEMS AND SOLUTIONS]--- -_.--- -___--.-. ------- .__

A 5% S Pet Coke - 100% usage - 800 kcals/kg - 1.33% SO3 onclinker

A For 0.6 Na20 eq. MR = 2 for Clinker SO3 of 1.55

A Above a MR of 1, K2S04.2CaS04 - Calcium Lambiniteand CaS04 form

A Calcium Lambinite decomposes at 152OC, CaS04 -145OCin Excess 02

A 2OOOppm CO at above 1OOOC breaks down CaS04

A MR >2 in Stage w ray meal produces hard deposits.I ,

-._-.- ---__-_--------.-... ----

FT COKE - PROBLEMS AND Sx-j---- _- --_--- - - _--~____.--- __.____ -_.. -_.__._. _ ,_ __-.- -.--. ..-- - - --.--

l FOR 100% Pet Coke need to AVOID:

A Chemical Reduction in Burning Zone

A Over-burning

A Low Na20

o For 10096 Pet Coke need to IXAVE:

A Good FLAME

A Combinability < 145OC

A Good Kiln Control of Free Lime and Back Endoxygen

--.----------..-.. _ --...-_ ---..... --. ..-. ------

1 100% PET COKE - PROBLEMS AND SOLW=].-.. -. _---_. - -..-_--..- -.__.._ - _. . .._ -._.._-_ ..__ -_----_.. ---- . . ..-- - - - - -

l Good FLAME as per CEMFLAME 1

A Burner Momentum - 7 N/MW

A Bluff Body 60%

A +9Ou residue 50% VM

A Central Burner

A Secondary Air > 800C

A Back End Oxygen - 3.5%

I !

I i

<

100% PET COKE - PROBLEMS AND SOLUTIONS- - - - -. . - c . -. .--~-.-- -.--- _-- -_--.-.-- ----

------.- ._-.-- - .-.-... .-. - ___._ _.__. ____ . . ._-------P-w-- .--.-...-...---------- ------I

0 Good FLAME alone not enough need:

Ilr. Good Kiln Control - Linkman to avoid continualSO3 recycling

A Raw meal residue low enough for goodcombinability

A 14.5 % Free Lime.

A Continual checking of performance

A SO2 probe to monitor and control, eg 3000 ppm= 1.05% SO3 on Stage IV raw meal

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- . . - .__- _-__.,. - _ - _ -.-- _..--___ ..__ .-- .__- --_----___-

-COKE - PROBLEMS AND SOLUTIONS]I.-.- _-~ .____. __.--,. . ..: ._I -.-... ---. . - -. .I __.__ _... _,_. ..__.____ ---... I_-_-

o Now we know how to do it!

0 How can 100% pet coke be used with lowNbx emission

CEMFLAME 2 has the ANSWER

Blue Circle Cement


Module 13

Section 2

CETIC Sub Commission “Behavior ofVolatile Material in Kiln Systems”

ISTN 9215

CETIC ~~-C~~~~IssIOh’ “BEHAVIOtiR OF VOLATILE lMATERL4~ J.N =NSYSTEMS”

REVIEW OF “CLASSICAL” ECNOWLEDGE ON &mOR ELE&fEiiS - JUNE 1992

Summary

This report is based upon various documents originating within 3lue Circle and from externalsources, supplemented by accounts of experience in Works operated by CETIC membercompanies. A review is given of factors governing the behaviour of minor feed constituents(Cl, K, Na, S) in cycles originating i!t the kiln burning zone and of the consequent effectson process plant performance. Recirculating loads of volatile species are formed and theseare often implicated in the formation of build-ups, coatings and bloc.kages in cooler parts ofa kiln system. Some relevant practical experience is listed and implications for productquality are summarised.

In a given plant, burning zone temperature and atmosphere are the dominant driving forcesfor these cycles. A key area for action to gain control lies in the selection and preparationof fuels, burner settings and raw materials. Process control (including dust management) canalso be important, especially as regards selection and maintenance of sensors which givedirect or indirect information on burning zone atmosphere (i.e., chemically reducing oroxidising with respect to the volatiles). Several dry process Works have found useful resultsfrom study of the results of sampling and analysis of kiln entry meal on a regular basis.

Differences between pIants may arise from design features or from characteristics of rawmaterials and fuels. Various modifications :o details of process design and operation/ controlmay be used to alleviate plant difficulties or modify clinker quality, depending on the localpermutation of inputs and temperatures which is involved. For exampie, changes may bemade to cyclone geometry or “non-stick” linings added, or benefits may be found fromalterations to raw meal or fuel chemistry. These factors may also be considered whenselecting new equipment.

The prospects for further improvements in understanding and modelling of processphenomena associated with volatile cycles have improved in recent years with the completionof relevant thermodynamic studies in other areas of technology. The addition of secondaryfiring or precalcinarion can significantly alter the behaviour of some kiln systems, andappiication of the improved academic knowledge to some relevant situations may be worthyof encouragement.

IS-m 9215

CETIC SUB-COiMiullSSION ‘BEHAVIOUR OF VOLATDLE amTERI4L.S IN KILNSYSTEMS”

REVIEW OF “CLASSICAL” JKNOWLEDGE ON IMINOR ELEMENTS - JrJiuE 1992

1.

2 .

3 .

4 .

5 .

6 .

7 .

CONTENTSPage No.

INTRODUCTION 1

VOLATILES: WHAT ARE THEY AND WHAT ARE THEMECHANISMS OF VOLATILISATION? 1

CYCLIC NATURE OF VOLATILJZS N KILN SYSTEMS 3

3.1 EFFECTS OF VOLATILE CYCLES3.2 WET AND (LONG) SEMI-WET PROCESS3.3 LEPOL PROCESS3.4 DRY PROCESS

CHANGES N PROCESS CONDITIONS WHICH CAN INFLUENCEBERAVIOUR OF VOLATILES 7

4.1 PARAMETERS NFLUENCNG VOLATTLE RECIRCULATION4.2 ITALCEIMENTI4.3 CBR4.4 LAFARGE4.5 OBOURG4.6 CMENTS FRANCAIS4.7 ENCI4.8 HOLDERBAlNK4.9 BLUE CIRCLE

EFFECTS ON CLINKER 1 3

Sulfate retentionReducing conditionsFundamental aspects

TOWARDS A MODEL OF VOLATILE CYCLES 14

EFFECTS OF CONDENSATION 17

CETlC SUB-COIMMISSION “BEHAVTOUR OF VOLATILE IMATERIALS IN KILNSYSTEMS”

REVIEW OF “CLASSICAL” KNOWLEDGE ON mOR ELEMENTS - JUNE 1992

1 . INTRODUCTION

This text, originally based on various reviews available within BCI and on external literature,has been adapted on the basis of discussion within the CETIC “Volatiles” Sub-Commission.

During an examination of the available literature, some 190 apparentIy relevant articles wereidentified in “Chemicai Abstracts” for the period 1967-1991. It is interesting to note that amore detailed examination showed that 60 of these articles covered not only topics relevantto kiln operation, the environment and associated laboratory tests, but also:

l the production of mineral&d clinkers (despite efforts to exclude this topicfrom the list)

a low temperature clinkering by chloride addition

l the cement/sulfuric acid process

0 the use of waste materials in kiln feed.

65% of these articles came from. Eastern Europe, suggesting that there may be a knowledgebase in the countries formerly behind the Iron Curtain. These subjects remind us that volatilecomponents are nor merely bad news for cement producers, but may have useful effects onproduct performance and on economics in certain circumstances.

2 . VOLATI-LES: WHAT ARE THEY AND WHAT ARE THE MECHANISIMS OFVOLATILISATION?

The concern is with elements or compounds which partially or entirely evaporate in the kilnand which are transported towards cooler zones in kiln gases. Some can escape via the stackor are trapped in precipitator dust. Others condense as they leave the kiln and/or react withor condense on the feed in the preheater system.

The principal volatile elements are : K, Na, Cl, S, Pb, TI, Cd, V, Hg, Zn and As. Theelements F, V and As can be classed as “moderately volatile” and the others as“significantly”.

They originate from inorganic and organic compounds in raw materials and fuel. In the caseof raw materials, certain compounds can readily volatilise at a temperature below 600 deg.C, especially mercury, thallium and sulfur (if present as sulfide or in organic combination),but in generai volatile compounds in raw materials only evaporate pat-My as the feed passesalong the kiln system and through the burning zone.. -The residue will be retained in theproduct, either in solid solution in the principal ‘phases of the clinker or as discrete

compounds - normally alkali chiorides and sulfates. T’ne typical proponions of volatiles inraw materials which evaporate while the feed passes through the !tiIn system are as foIIows(“primary volatiliries”, according to certain writers):

c/oso3 6 0 - 9 0K2O 30 - 70

NazO 20 - 40Cl, Hg 96 - 9 9

F 1 0 - 40Pb, n 6 0 - 9 9

C d , V, Zn IO - 20

In contrast, fuei voiatiles are almost always entirely evaporated during combustion.

Evaporated volatiles travel back up the kiln in the gas phase where:

i> they condense on the feed and form the basis of a recirculating volatile loadin the kiln system

ii) or they condense as a fine dust or fume which is finally trapped in theprecipitator and which wiI1 then become part of an external volatile cycle ifdust is partly or wholly returned to the kiln system

iii) or they condense on a colder surface in the kiln system and form the basis ofa build-up

iv) or they escape through the stack either in the gas or vapour phase or as a finefume.

The remainder of this text concentrates on the cycles of minor elements (Cl, K, Na, S)originating in the burning zone. The cycles of trace eiements and/or cyIess hot parts of the system are not examined in detail.

When volatiles condense on raw meal and are returned towards the burning zone, dependingon the overall chemical conditions and burning conditions, they form a range of volatilecompounds which themselves evaporate partially and are partly retained in the clinker - eitheras discrete compounds or in solid solution in the clinker phases.

In general, chloride preferentially forms potassium chloride (KCl) and only when there is anexcess of chloride over the needs for chemical combination with the available potassium willsodium chloride (NaCl) be formed as a recirculating volatile species.

A large part of the alkalis and sulfate entering the burning zone is in practice present in theform of molten sulfates, forming a separate Iiquid phase immiscibIe with the principal ferriteflux.

2

The typical proportions of voiatiie compounds which evaporate in the burning zone (in theabsence of reducing conditions) are as follows:

%K,SOq 40 - 60

NaiSO? 40 - 602CaSO,.K$O, 40 - 100

K,O in solid solution 60 - 90NaZO in solid solution 20 - 40

K C 1 97 - 99NaCl 96 - 99

CaSO, 80-100

In consequence, a cycle of recirculating volatiles develops, which only finds an equilibriumwhen the total quantity leaving the system (in clinker, dust and stack Iosses) equals thatentering the system (in raw materiais and fuel). Various tables and figures are appended,showing melting and boiling points and vapour pressures.

At this point the level of volatiles in recirculation can be significantly greater than the totalmte of introduction of volatiles. The volatiles in the vapour phase can be in various statesof dissociation and recombination. Alkali sulfates evaporate congruently, that is to say thatthey disappear entirely when heated for a long period, whilst &SO, decomposes and leavesresiduaI CaO (in oxidising conditions). CaO:CaS03 melts can form. Typical recirculatingvolatile loads expressed as % of the total quantity introduced are as follows:

ChlorideK2O

Na,Oso3

%

5ooiJ200 - 650150 - 200200-800

The effect which these recirculating volatiles exert on the formation of build-ups at the kiln(gas) exit depends on the composition (which governs the temperature of liquid formation andthus the position and hardness of the build-up) and on the quantity (which governs the rateof formation). The dependence on vapour pressure on temperature differs for various speciesand the “relative volatility” (or rank order of volatility) can vary in the cold and hot zonesof the kiln system.

In the past, several empirical limits have been proposed for the concentrations of volatilesadmissible in a kiln, e.g., 0.03% Cl on clinker for preheater kilns. Today there is atendency to prefer to specify concentrations tolerable at the kiln inlet (in hot kiln feed at zeroloss on ignition). By way of indication, the concentrations of volatiles which can be toleratedin the lower stages of a preheater are typically given in the following ranges (exceptionsbeing of special interest for study):

5%Cl 1.2 - 1.8

so3 2.5 - 4.5Alkalis (eq. Na,O) 2.5.- 3.5

3

The FL.5 encrustation index

R = Total molar S inoutTotal K,O input + 0.5 (Total NazO input)

can be used as an indicator of the potential nature of any build-up, as follows:

R31 Hard build-ups based on SO,0.7 < R < 0.9 Relatively soft build-ups (easily removed)

R < 0.5 Carbonate-based build-ups in due course.

Most suIfates condense in the range 9000 - 1100 deg. C. The presence of fluorine canaggravate build-up problems due to the formation of fluoride compounds and. their aid to theformation of various silicates.

Potassium chloride alone condenses between 800 and 900 deg. C (and sodium chloride at aslightly lower temperature). Build-ups can develop in the kiln feed chute or in the riser ducttowards the bottom stage of the preheater.

There is an optimum temperature for the capture of SO, by a freshly cakined raw meal(e.g., 880 deg. C in one study). Primary condensation is expected to be in the form of liquidalkali sulfates. It is often interesting to calculate the composition of the sulfate phase in kilninlet material, with its addition of KCl. Fusion in the system Na,SO,/ K$O,/ CaSOJ KC1begins at below 700 deg. C. Liquid films on dust particles are The origin of build-ups andas the thickness increases, internal temperature drops and new equilibria may be establishedand compounds formed.

There is less literature on the phenomena governing condensation than on evaporation. Theeffects of atmosphere and the implications of precalciner operation are perhaps worthy ofstudy, especially for sulfate compounds which can be present in various states of oxidationand which can react with water vapour to form bisultites and bisulfates. The characteristicsare as follows:

ChemicalState

Fuel and raw material Clinker Vapour

Oxidiseds4+, 9’

Sulfares, e.g.,gypsum, anhydrite,alum, etc.

Alkali sulfatesand aBali/alkalimetal sulfites

SO2 and (atlow temp.)so3

Neutral, S Elemental S

Reduced,S2-

Pyrites, marcasiteorganic sulfides

Oldhamite, CaS,complex suiftdes ofcalcium, aluminium, etc

Non-volatiie

The oxidised sulfur cycle is illustrated among the appended figures.

3 . CYCLIC NATURE OF VOLATILES N JSXN SYSTE-MS

3.1 EFFECTS OF VOLATILE CYCLES

The volatilisation and condensation of volatiles in a kiln produce two undesirable effects onthe kiln process:

l the formation of build-ups - possibly blockages and the possible emission ofSO, from the system

a transfer of heat away from the burning zone towards cooler parts of thesystem.

There are also effects on the clinker, which may be summarised as fofollows:

Fluxing action:a lower temperature of first liquid phase formationl change of liquid viscosityl alteration of surface tension of liquid0 modification of crystal morphology.

5

Phase re!arions:0 the relative thermodynamic stabilities of the clinker minerals can be altered by

solid solution effects.

Hydraulic activity:0 the reactivities of the clinker minerals are altered by solid solution and/or by

the effects of crystal symmetry (high temperature stabilisation of polymorphs)and/or effects occurring during hydration (e.g., coating of cement particles byinsolubie salts).

(It is difficult to isolate these three classes of effect in practice.)

3.2 WET AND (LONG) SEMI-WET PROCESS

There is an obvious external cycie (dust return) as well as an internal cycle which has,perhaps, received less attention in the literature.

In an example involving a coal with a sulfur content of 4.5% (not very differ&t from thelevel for a blend of coal and sulfur-rich petroleum coke), there was a high internal cycle andsignificant stack losses. In one BCI kiln these represented 42% of the quantity introducedto the system and were 5 times higher than the proposed BATNEEC concentration.

Typically, FLS expect 30% or more of the sulfur entering a wet process kiln to escape viathe stack. The external cycle can be influenced by the proportions of dust returned to thesystem by various routes.

3.3 LEPOL PROCESS

The volatile cycle is more complicated on account of the additional external cycles due todust from riddlings and cyclones, even though the basic situation seems similar to that forwet process kilns. However, there is a major opportunity for a volatile bleed (especiallySO,) by disposing of riddlings and cyclone dust as well as that from precipitators.

3.4 DRY PROCESS

As indicated above there are both internal and external volatiIe cycles.

In general, sulfur escaping the preheater is found in the form of dust rather than SQ andforms part of the external cycle involving the raw mill, conditioning tower and precipitator.Further, it is expected that SO2 in the gas phase will be absorbed in the lower stages of thepreheater, at least if there are no adverse conditions (e.g., chemically reducing conditions).

When sulfide is present in the feed, a significant fraction will be lost from the preheater asSO, (e.g., 30 - 50%) and partly absorbed in the raw mill and the precipitator system.

6

4. CHAtYGES N P R O C E S S CO?iDITIONS rvHICH C&Y INFLUEYCEBEfIAVIOLJR OF VOL;ITILES

4.1 PAJiUMETERS NFLUENCNG VOLATILE RECIRCULATION

The principal parameters which influence volatile behaviour are as follows:

temperaturetimetype/compositionconcentrationdiffusion towards the solid surface (as controlled by clinker nodule size andflux content and composition)gas/solid contactother reactionskiln atmosphere (laboratory tests suggest that water vapour plays a role, aswell as the “traditional” kiln gas components).

(For summary, see table among appended figures.)

Once a kiln is in operation, the main parameters available for the kiln operator toalter/control are th.e temperature and the atmosphere in the kiln. The rate of gas flow seemsto be of secondary importance. Process design and local chemical conditions also play apart, determining the total quantities introduced to the system, chloride content, relativeconcentrations, combinability, fuel and to the firing system parameters and mixing withinthe clinker bed (with exposure of nodules to gas flow at the surface).

It is often difficult to distinguish chicken from egg in the industrial operation of a kiln. Forexample, when CBR changed from gas (or indeed, to 92s) as a fuel in Canada in the past,there were effects on volatile cycles. This is to be expected due to the presence of a higherlevel of water vapour in the combustion products (e.g., coke < 5 % , lignite > 10%) and thehigher vapour pressures of alkali hydroxides (see attached figures), but such changes arealways accompanied by other alterations to the quantity and composition of volatilesintroduced and/or the raw meal chemistry. The effect of nodule size on volatile behaviourseems equally. impossible to separate from that of the effects of liquid from alkali melts onclinker size grading (as discussed in past CETIC work).

It is important to note that, in general, no-one measures “typical” volatile balances (saveglobally): available detailed balances are usually gained when there is a need to investigatea non-typical kiln system due to some problem or another. A number of illustrations follow,taken from the experiences of participants in the CETIC Group.

4 . 2 ITALCEIHENTI

When CaC12 was introduced via the flame (0.3% on clinker) to volatilise K,O in a Gepolpreheater kiln of 2000 t/d capacity (Stockertown, USA), it was noted that operation couldcontinue for many hours with kiln entry levels of Cl of,around 3% with somewhat reduced_-

7

loss on ignition (heat pump effect). This indicates the capacity of this type of preheater totolerate a little more Cl than cyclone preheaters (although long term results from Stockertownare still awaited).

4 .3 CBR

With chloride injection on a precalciner system (with bypass), there was a marked effect onCl and alkali levels in kiIn inlet material but not on sulfate. (The same was true atStockertown.) Full system equilibrium was not reached for three days. At another Works(about 60% heat energy input at the precalciner and no bypass), an upper sulfate limit on kilninlet material of 3% was established in order to avoid any build-up problems (althoughperhaps not corrosion problems!). In this latter case the sulfate level was lower in thepresence of chlorine and of potassium (see later data); at the same time there was also acertain reduction in SO, loss from pyritic material in the preheater. It is known that thisprecalciner rapidly forms build-ups above about 91.5 deg. C. In a wet process kiln, theformation of rings can be followed with the she&scanner and the flame setting altered toeliminate them (provided that action is taken within 2 to 3 hours).

At Lixhe, various phenomena were noted when returning to coal firing after the use of gas,including a reduction in sulfate cycles and in the level of decarbonation at the preheater towerexit. This latter phenomenon results from a lower heat release within the preheater, notablyfrom CaS04 recombination. The thermal effect is of the order of 75 kcal/kg with the inverseeffect in the burning zone. Blue Circle have calculated the magnitude of this effect as 109kg/kcal at Hope Works in the past. At Lixhe (dry process) the sulfate level in the kiln entrymaterial can be reduced (and that of chloride raised) by increased burner momentum: wearof the burner tip allows the level to rise again in due course. A “non-stick” kiln feed chutelining from Hasle also gives good results.

Also at Lixhe, a number of interesting relationships have been established from the resultsof three years’ operation (with analysis for volatiles in kiln inlet material twice per shift).For example:

No. of kiln srops per monsh = -5.3 f 2.7 x %S03 (in hot meal)for cyclone blockages)

I?* = 0.88

%S03 (in hor meal). = 4.8 - 0.36 x % K,O (in hor meal) R’ = 0.68

It was noted that the input of used tyres at the kiln back end gave good results in mechanicalremoval of build-ups.

4.4 LAFARGE

A compilation of volatilisation levels was made in 1985 for all company kilns. Thecompany’s calculation of “volatility“ is somewhat different from that often encounteredelsewhere. The following results (which have changed little since that time) wereobtained:

8

%so3 K2O

Na,O

I-w 56 34 14

Dry 80 6 9 2 6

Precalciner 55 49 5 5

(Retention levels are close to 100 % for preheater kilns and at lower levels for the otherprocesses, being related to the level of dust disposal.) If is noted that Lepol (and wet)process kilns give Iower volatilisation levels than preheater kilns and that precalcinafionseems to influence these phenomena. Nevertheless, due to the small quantities of dustinvolved, the enrichment of volatiles in precipitator dust is highest for the Lepol process.(Italcementi note that for balances determined on 23 kilns the enrichment factor for dustcompared with raw material is close to one for SO,, K,O and NazO in the dry process, butbetween 9 and 11 for the Lepol; in the case of chloride the comparison is between a factorof 8 for the dry process and 123 for the Lepol; the values for long, granule-fed semi-drykilns are intermediate.)

In examining the performance of a new low-NO, burner with a higher momentum than theprevious one, it was noted that the CO signal could be made to disappear (for the same C+level) with less decarbonation at the preheater exit and a higher level of kiln drive power(Amps). These effects were accompanied by an apparently higher BZT (with less clinkerfree-lime), less gaseous SO2 at the kiln exit, and kiln inlet material with less SO, and ahigher loss on ignition at 1000 deg. C.

4.5 OBOURG

Here the clinker KzO level is used as a control parameter for the wet process tiln and aconsiderable data-bank of measurements has beeri built up. At the end of 1990 the use ofa lower ash fuel began and the clinker K,O level dropped from 0.69% (1990 mean) to 0.55%(mean for January/February 1991), despite the introduction of a little more potassium to thesystem (equivalent to. 1.43 % on clinker instead of 1.34%). Evidently the dust - returned tothe kiln - had become more rich in alkalis. It can be shown that the proportion of $0brought in by the solid fuel fell from 24% to 17.5% while at the same time that brought inby the dust return rose from 18.5% to 29 %. This observation leads to the conclusion that$0 incorporation in clinker does not only depend on the quantity introduced but also - andabove ail else - on the type of material which brings it in and perhaps on the position whereit is injected. On screening the clinker at 20 mm the same chemical analysis was found foreach fraction with the exception of K,O, where the concentration was about 10% higher inthe coarse fraction.

Here the clinker alkali content has successfully been reduced by chloride addition at theflame. Ciments Franc$s have also demonstrated this effect with CaC12 addition to slurry(3.5% K,O in the dust in comparison with 2.7%). Blue Circle also once succeeded indemonstrating the efficacity of this method (accompanied by non-return of part of the dust)

9

in lowering clinker K,O ieveis by a half in a long kiln, despite the fact that some questionsremained about the most appropriate place to introduce the chloride; it must be noted thatthere can be problems with the flow behaviour of precipitator dust above a certain chioridelevel.

4 . 6 CIivIENTS FRANCAIS

On adding precalcination to a Lepol grate with (stoichiometrically) more sulfur than alkalisin the system, an increase was noted in sulfate and alkali levels at the grate and especiallyin the dust and nodules beneath the grate. More potassium sulfate was found in the clinkerwith precalcination; CaSO, was found in the cyclone dust and somewhat less in theprecipitator dust, arising from kiln dust which had travelled across the layer of nodules onthe grate. There was a little more SO2 emission with precalcination. On another Lepol kilnno such effects had been found on adding precalcination; at Frangey, Lafarge had noted asomewhat higher recycle of potassium and sulfates with Lepol precalcination. (At this latterWorks, with use of chloride-rich substitute fuels, it is found necessary to carry out samplingover periods of at least a week to determine consistent volatile balances.)

To use a high chloride coal (-0.15% Cl) on the dry process it has been found necessary toprepare a mix with another coal to avoid build-ups with typically 2% Cl at the bottom ofcyclone 4. Analysis of build-ups along the kiln indicated that chloride levels reached 30%(at zero loss on ignition) in the coating from the base of cycione 2 and 20% at 50m into thekiln (despite its less than 5% level in both hot and cold parts of the kiln); run-out of materialfrom a stopped kiln gave chloride levels rising up to 0.7%.

During a past experimental campaign of burning chlorinated wastes, it was noted that thesimultaneous presence of chloride and sulfate at high levels could give rise to emissions ofHCI and SOz. Apart from this, the volatiles were all assimilated in the c!inker (burned atlow temperature with free lime levels of up to some 13% and formation of CaCI,.C,S) orin the dust (and probably also trapped in refractories).

4.7 ENCI

After the addition of a second stage to the preheater, various build-up problems wereencountered. In 1985/6 the kiln exit 0, level was increased from 0.5 to 1.5%, the solid fuelresidue at 90 microns was reduced to below 25 % and several “Cardox” units were installed.These actions improved the situation and in 1987 20m of “Magotteaux” stirrers wereinstalled, the burner air velocity was increased to 100 m/s and a Hasle “non-stick” lining wasinstalled in the duct and cyclone dip-tubes. These efforts made an output rate of some 110t/h possible without build-up problems.

More recently petroleum coke (3.5% S) has been fired (first at 4.5 t/h and afterwards 6.5t/h), accompanied by oxygen addition at the flame and, finally, by the use of slag (S - 1%)as a raw mix component (5%, and then 10%) with yet more coke (8.0 to 8.5 t/h, i.e.,- 60% of fossil fuel energy). At the start of 1991 the production rate was 120 t/haccompanied by preheater blockage problems. During ,199l it was deduced that problemsr

10

with build-ups could be avoided if the SO, level in the kiln entry material was kept beiow2.5%. This was possible with a coke input rate of about 8 t/h and a slag level of 10%provided the oxygen level at the kiln back end was kept consistently at 2%. If theseconditions are not met, then kiln operation rapidly runs into problems. The “Fuzzy Logic”control system helps to achieve success, as the 0, signal standard deviation has fallen from0.45% to 0.25% and that for the SO, in hot kiln-feed from 0.6% to 0.2%.

4 . 8 HOLDERBAhX

In the past, Iaboratory data have been gathered on minor elements and these confirm theeffects that are now more widely known. For example, suIfur volatility in a standard regime(70% Nz, 30% CO) is close to 100% at 0% 0, but falls in the presence of 0,; neverthelessthe effect of O2 is much less at 1400 deg. C than at 1200 deg. C. The volatility of minorelements in the laboratory is also much greater for powdered material than for granules.(See appended illustrations, taken from external literature.) Currently there is interest in rawmeal morphology and in the distribution of volatiles in the meal at the start of clinkering.

A precalciner kiln system in Spain ran for many years with a Cl level in the kiln entrymaterial between 3 and 4 % (that is to say about 0.5 % less than K20) and with no trace ofCO. In this case there was about 1.1% SO, in the kiln entry meai and 4% K,O and noproblems, but if CO was present there was about 2% SO, and 5% I(,0 in the hot kiln feedaccompanied by Spur&e-based build-ups and cubic KC1 crystals. (It is also recognised inCiments Franc@ that regular kiln operation helps to minimise the phenomenon ofcementation by the freezing/thawing of chloride-based deposits.)

While burning wastes at Clarkesville (wet process), it is found necessary to ensure that theclinker Cl content is always kept below 0.3 %, otherwise the kiln becomes unstable. AtOrigny there have been an enormous number of kiln stops caused by preheater blockageswhich, when sampled, do not contain many volatiles. This effect seems to have its originin a liquid phase formed by calcite arising from a chalk with an extreme level of finenesswhich can decarbonate and recarbonate very rapidly. The problems have been much reducedby altered cyclone geometry and helped a little by the use of a mechanical cleaning device.

4.9 BLUE CIRCLE

On one precalciner kiln it is difficult to find “typical” volatilities. For every determinationthere are almost always different values (40% for SO, and 50% for K,O changed to 25%and 4O%, for example). One can imagine that this is caused by variations in the nature ofthe raw materials and the content in the kiln feed of sulfate (and sulfite) captured in the mealafter initial low temperature voiatilisation in the preheater.

In order to examine the possibility of producing a sulfate-rich clinker (2% SC&) using certainavailable resemes of material and without installation of a by-pass on a new kiln, rests werecarried out for about three weeks on a dry process kiln at another site (-35 t/h). Theobjective was to reduce the volatilisation in the burning zone by playing on processparameters and producing a lightly mineral&d clinker (-- 1% K,O, -0.15% NazO, and

1 1

- 0.15 % F, as usual at that Works, but with double the usual level of sulfate) and with asilica ratio a litT.ie lower than usual (-2.7 instead of -3.2), accompanied by a change inalumina ratio from 2.8 to 2.2. During the changeover there was some tendency to form softbuild-ups in the preheater, but with the new regime established these moved towards the kilnfeed chute without causing any major problems for kiln operation. The apparent burningzone temperature was reduced from about 1500 deg. C to 1380 deg. C, while K20 volatilitydropped from 70% to 60% in the burning zone and that of Sq from 80% towards the range50% to 60% provided that kiln exit oxygen level was kept above 2%. There wereimprovements in the output and fuel consumption of the kiln and, in fact, the experimentalWorks adopted certain of these changes during its normal operation for several years, untilthe asrival of demand for low alkali clinker.

At Hope Works (dry process), tests were carried out involving various NO, levels as wellas reducing conditions.

so, : The ratio of SO3 in Stage IV to Scl, in raw meal variedtypically from 1.8 to 2.7 for the higher levels of NO, and was3.0 for a low 0, level. The clinker SO, content fell.

KzO and NazO : In a parallel manner, for K,O the ratio of the content in StageIV to that in raw meal varied from 3.8 to 4.4 and for NazOfrom 1.6 to 2.0.

In general, reducing conditions increase SO, level. at Stage IV by a factor of 2, giving alower clinker SO,.

As already described in a paper to the CETIC Technical Commission, at Cauldon Works(and later in other dry process Works) SO, has been monitored at the kiln back end todetermine the local rules for avoiding blockage tendencies. The SO, signal is noisy anddifficult to interpret without a knowledge of the history of the system, e.g., a reCent fall ofsulfate build-up material arriving in the burning zone can give rise to a high SO, signal atthe kiln back end despite the presence of a good flame and acceptable levels of volatiles inthe kiln entry material. At Dunbar there has been success in reducing the number of kilnstops per year caused by preheater blockage from over 90 (1987) to less than 10 (1989), losttime hours having also fallen from around 450 per year to about a hundred. (There werealso major gains in stops caused by rings and breakaways at the kiln entry seal.) There isno need to keep such monitoring equipment in permanent operation once the rules areestablished, but it expected that renewed investigations will be needed each time theconditions of operation change. In practice it is now found that with this know-how sulfur-rich petroleum coke can be used (to a certain level) even on dry process kilns which in thepast have given problems with just coal firing - but in several cases the build-ups seem tohave moved from the preheater into the kiln (where they are destroyed). In a general mannerit can be supposed that there are problems of both short-term and long-term and stability:once a stable burning zone volatilisation is established, one must wait for stable conditionsto arrive higher up the system and in the large masses of material which form the build-upsand coatings already in existence.

1 2

During tests of SO, monitors at the exit of a long kiln with filter cake feed, it was noted thatthe signal usually remained stable (below 100 vpm). But, when the oxygen level fell therewas an inverse correlation between the 0, and SO, signals. In this case levels of around1000 vpm SO, were reached, with considerable variations; it is supposed that cycles movedfurther up the kiln (nearer to the analyser) during low oxygen periods.

At Plymstock Works (dry process), when changing the BZT from 1390 to 1500 deg. C theratio of SO, in the fourth stage to that in the raw feed rose from 1.2 to over 4.0. Similarresults were obtained ‘at Lichtenburg Works (South Africa).

It is noted that a better understanding of volatile recirculation is useful for kiln operationbecause back end oxygen indicators can sometimes be misleading.

At Mason’s Works (wet process), raising the 0, level gave higher levels of SO, and K20retention in clinker (concentrations rising from 0.18 to 0.54% SO, and from 0.33 to 0.80%K20). Also for the wet process (Westbury Works):

Kiln 1, “low momentum” flame,2 % 0, - 2500 mg/Nm3 at kiln exit.

Kiln 2, “good momentum” flame,2 % 0, - 2 5 0 mg/Nm3 SO, at kiln exit1 % 0 , - 1350 mg/Nm3 SO, at kiln exit.

In practice alkalis are controlled on the wet process (by means of the LINKman system):

Masons: % alkali target

< 0.550.55 - 0.7

> 0.7

NO, set-point@pm at precip.)

5 5 04OO-500

300

Ravena (USA):- alkalis controlled by NO, set-point- sulfate/alkali ratio controlled by 0, set-point.

5. EFFECTS ON CXANKER

Sulfate retention: In general, changin,0 from a situation with excess alkalis offersadvantages.

In general, a higher SO, content:improves early strengthimproves workabilityproduces a more difficult “apparent grindability”.

Increased ciinker alkali levels can also be associated with sulfate retention for Lepoi and wetprocess kicilns, especially if there is already an excess alkali content.

13

The effects of minor components on the viscosity and surface tension of liquid phases canbe complex. Lower viscosities encourage alire formation. Calcium sulfate flux can,however, stabilise belite and/or cause the production of clinker alite with lime inclusions.(Sulfate liquid systems are capable of influencing ionic transport and chemical combinationdespite the limited solubility of the principal clinker compounds.)

In clinkers with a low al*ka.li content, there is the possibihty of belite stabilisation (difficulicombinability) due to excess SO,.

Reducing conditions: Reducing conditions in the burning zone can give a cement with poorflow characteristics (due to free K,O and NazO), poor workability (due to the increasedcontent of C3A and its reactivity), poor strength (lower C,S content) and variable colour.

Fundamental aspects: At some future date, the production of lower LSF and/or mineral&dclinkers may be of interest.

Alkalis retained in clinker are present either as stable sulfates or absorbed in the silicate andaluminate structures. NazO has a more marked tendency than K,O to form solutions in CIA.For ciinkers with (molar) ratios of sulfate:total alkalis below 0.5, almost all the sulfate iscombined in water soluble form, K$O, being predominant. A proportion of the alkalis arein solid solution in the clinker C,A and this has an adverse effect on the initial cementreactivity and thus on concrete and mortar rheology.

For ratios between 0.5 and 1.0, a certain quantity of langbeinite (2CaSO,.K$OJ is alsoformed (and not all the alkalis are soluble). For ratios above 1.0, significant fractions of thesulfates are combined within the sihcates and aluminates or as anhydrite (CaSOJ, whichdissolves more slowly than alkali sulfates, whilst the fractions of KzO and Na,O which aresoluble in water approach 1.0 and OS, respectively, at a ratio of about 1.5. At sulfate:alkaiiratios above 1.5 trends are somewhat erratic. For most normal clinkers the principal sulfatephase will be aphitalite with a maximum WNa ratio of 3.0. This phase is accompanied byminor quantities of K-$0, and calcium langbeinite, Na,S04 being found only for unusuallylow WNa ratios.

As well as the solid soiution effects and the formation of compounds described above,various permutationsof volatiies (especially in the presence of fluorine) ‘can influence thestmcture of alite and belite crystals. (A comprehensive review of recent work forms part ofthe text of G K Moir and F P Glasser at the 1992 International Congress on CementChemistry in New Delhi.)

6 . TOWARDS A MODEL OF VOLATILE CYCLES

Various empirical volatility factors have been proposed and used with a certain measure ofsuccess. This section considers the possible approaches to a more fundamentally basedmodel.

It is generally supposed that (other factors being equal) the extent of voiatilisation decreasesas the thermal efficiency of the kiln increases. An explanation may lie in the fact that this

14

is due to the limiting effect of vapour saturation by al,kaIi compounds. Studies by 3lue CircIeof the treatment of kiln dust in a 100 mm diameter fluidised bed tend to confirm thishypothesis.

This study examined the feasibility of producing a low quality clinker from flue dust withcapture of the alkalis distilled from the bed (for possible use in the fertiliser industry).Saturated vapour pressures at 1200 deg. C are (for the pure substances):

KC1 0.18 atmK2SO4 0.8 x 1O-3 am

(0.6 x 10m3 atm with decomposition suppressed)Na,SO, 0.13 x lo3 atm

(0.01 x 10s3 atm with decomposition suppressed)

The transport capacity of air for vapcur at 1200 deg. C is thus

KC1 700 g/gK2S04 4 g/g

Na2S04 x0.5 g/g

(The capacity at 1250 deg. C is about two times higher).

It can therefore be foreseen that (unless the equilibrium vapour pressures differ greatly fromsaturated values) there will be little problem in removing KC1 from many kiln flue dusts ina fluidised bed with a gas flow rate of, say, 2 g per gramme of dust, although the capacityfor sulfate removal may be limited.

(A wet process kiln typically operates with a ratio of a little less than 2 g/g gas/soiids in theburning zone and perhaps 2.75 g/g at the back end; the corresponding values for the dryprocess are 1.4 g/g and 1.94 g/g).

It is suggested that

V” = (I’*) (M,L_(P - p*) c-q

v* = saturated vapour concentration in transport gases (kg/kg)p* = saturated vapour pressure )

of an alkali compound ) same unitsP = gas pressure >M, = moIecuIar weight of vapourM, = molecular weight of gas.

Given mathematical expressions for saturated vapour pressure as a function of temperatureand knowledge of the temperature profile in the kiln system, the saturated vapourconcentration can be calculated for each alkali compound and thus the maximum quantitiesevaporated from the feed per unit mass of gases. In considering the amounts of gas passing

15

through the kiln at vaiou~ temperatures, the true quantity of volatiles transported per unitmass of clinker can thus be calculated and from this knowledge, “ideal” volatile cycles canbe deduced. (It is to be noted that, paradoxically, when alkali addition allows BZT to bereduced then blockage probIems can be lessened due to the dominance of temperature in theevaporation mechanism).

Despite the fact that qualitative differences between two kilns (one dry process and one wet)are reflected in sample calculations, such “ideal” calculated recirculating loads are about 10times larger than those encountered in practice. The probable reaSons are:

Incomplete contact between gases and solids in the kiln, where only a smallfraction of the solid surface is exposed at a given time. (It is expected thatthere is better contact in the colder dusty regions of the system.)

b) Volatilisation characteristics of the alkali-containing minerals at a given plant.(Alkalis seem to be lost more e&y from silicates and aluminates than whenpresent as sulfates).

cl TEuISpOIt Of compounds which are condensed ~/on solid dust or fume.

d) Reduction of vapour pressure over solutions of alkali compounds.

e> Inadequate treatment of the transport of heat and of vapour within the bed ofclinker nodules in the kiln.

0 Formation of other compounds, e.g., &SO,, depending on the alkaksulfateratio.

Unstable operation of production kilns, so that practical conditions are notexactly those expected for very long term stability of temperature and materialflow.

The further development of a predictive model will have to take account of such factors, aswell as the effects of.composition of kiln atmosphere. In recent years there has been muchinvestigation of factors governing the blockage of cyclones and their performance in hightemperature coal combustion processes (in the hope of protecting turbine blades in directcycle electric power generation systems).

When lime (or limestone) is injected to absorb SO,, the compounds and thermodynamiccriteria of interest in combustion systems are exactly those encountered in the cementindustry - particularly when relatively high chloride coals are used. It is probable that thereis now sufficient academic knowledge to better treat our situation and allow an improvedmodelIing and understanding. Another aspect to consider is knowledge acquired from studyof the regeneration of CaO sorbents used for SO, scrubbing: again, data potentially relevantto kiln systems are produced, for example, on pressures of SO, in the system CaSO,/ CaS/CaO in the presence of various concentrations of CO and CO, (see appended figures).

1 6

(It is interesting to note that volatile condensation has an effect on the kiln power signal usedfor process control. Any perturbation of the chemical composition of the kiln feed whichraises volatile content will increase kiln Amps; a control strategy seeking a constant Amp&resignal would have the effect of reducing burning zone temperature, yielding under-burnedclinker with a relatively high volatile content. A strategy using a constant fuel feed ratewould be equally inadequate, due to the depression of burning zone temperature producedby the increased volatile load. The best control of product quality should result from asystem based on observation of the peak clinker temperature, that is to say, indirectiy by kilnexit NO, control.)

7 . EFFECTS OF CONTXXSATION

The most probable primary condensation is in the form of liquid alkali sulfates, Melting inthe ternary system Na$SO,/ K,SO,/ CaSO, starts at below 800 deg. C. Addition of KC1increases the range of suIfate compositions which is liquid of this temperature and allowsformation of liquid melts even below 700 deg. C. Deposits on the feed can provokechemical reactions; they can equally cause adhesion and - as with deposits on surfaces -initiate build-ups. (Direct condensation as calcium langbeinite is not expected ontherrrtodynamic grounds.)

While the literature tends to agree (although not totally) on vapour pressures of pure a&licompounds, information for the more complex species of interest in cement kilns is morerare. Studies at Aberdeen University have produced self-consistent results for sulfosilicate,sulfoaluminate and langbeinite. The order of volatility alters with temperature. As indicatedearlier, liquid alkali sulfate systems have a poor dissolving power for most of the principaloxides of cement clinker. However, they have low viscosities and a low surface tensionagainst silicates and thus cover and englobe these particles very effectively. It seems likelythat the small quantity of silicate which is dissolved has a high mobility, so that the liquidsare effective at producing a reaction (for preference towards C2S at 700 to 800 deg. C.).Stabiiisation of carbonates has been suggested (CaCO, can dissolve in the liquid phase in thepresence of alkali sulfates in the range 880 - 900 deg. C., forming a liquid rich in CQ andthe presence of fluoride can cause further complications - but equally (in combination withcertain concentrations of other compounds) certain advantages as far as clinker quality isconcerned.

C P KERTON

June 1992 (English version, December 1992).

)jats~c;ld w D, Trans. hwhy Svc.. 66 (81, I966 - 1973. (197W.

rcrry R 11 & Chil!on C II, Chemical Engineer’s tlandlmk, McGraw-Ililt,

New York (S\h cJi\ion 1973).

Temperature (deg. C

/

~r3pt-1 to confirm that proportion volatilised for 5oiven hating regime is dm-xteristic of raw mix

(i.e., (fiat volatilisation reacfion is first order with rap+% to alkali content).

ha from Palmer K & Bayik 0, pc~ R. epon M-117(1952] ud from W&J H. Rock ProdUCu 45 (2j.M - 68 (19413

/II

iIIf

i

I350 -

Temperature (deg. Cl

MeI tinq point (“C)

ea -c 8 0 0

ca 440-#SO

edso-904

m9ou-954

CID 7 954

Meltinn &ae of (a) the CaSO4-wic SO -NazS04 System

and (b) the Effect of 6 KC1

(From fue .air---c------ ,wpp- ------4~p--

.

Oxidized P o r t i o n o f t h e S C y c l e R e l a t i v e t o t h e Ki ln .Dot ted pa ths r epresent c i r cu la t i on in the vapour phase ,solid l i n e s i n t h e s o l i d p h a s e ( s ) .

SLJAlAl,~l~Y 01: 1~AC’I’O1ci \\‘IIICII INi~LUi:NC~ ‘IIll!IIl~l~VIOU1~O1~~lLNOI~VOLA'I'IL~COMI~UNDS1N1WaN SYSTEMS

1.

2 .

3 .

4 .

5 .

6 .

7 .

8.

9.

LO.

11.

12,

Burning zone temperalure(alkali vapour pressures) - level

- variations

Temperature profile of burning zone

Comp&ition of alkali liquid systems in theburning zone

Atmosphere in burning zoneglobally reducinglocally reducingwater vapour

Clinker size grading ’

Clinker llux content (density)

Thermal efficiency (gas flow rates)

Prehea’ter system designverticalcold areas/air inleaksanti-build-up lininggeometrysolid/gas loading

Precalciner: designoperation

Composition of alkali phase in prehealer

Dust return

By-pass system

MAJOR SECONDAItY I’OSSll3LE CONTICOLINFLUENCE INFLUENCE ACTIONS

00

:

0

0

0

0

0

0

:0

0

0

Soft burning (including fluxes and mineralisers).Controlled burning.

Flame/burner settings.Fuel Characteristics.

Selection of raw feed chemistry(sulfate/alkali ratio).

000

0

0

00

0

Burner/flame settings.Fuel characteristics.Coal/coke fineness.Choice of fuel (solid, liquid, gas).

Selection of raw feed chemistry(sulfate/alkali ratio) - including flux.

As above.

Process design/seleclion,

Process design/selection.Elimination of air inleaks / Insulation.Design.Type of precalciner.Throughput.

Selection of precalciner.Control of precalciner.

Selection of raw mix components.Selective quarryinglbcneficiation,

ConHol.

Add.

0 = rrrrcertoin

X Drnper 1, PCA Rcjwt MRS.68 (19.54).

0 Gtrr C B Kcil I:, ‘I’IZ (4). 7 _ 9 (I9Mj).

HOURS LOST!xo

4 0 0

300

2 0 0

100

0

DUNBAR-HOURSLOST

NO. OF STOPS

. _

PREHEATER BLOCKAGES RINGS/KIN INLET BUILD-UP

1 9 8 7 1988 1989

100

80

6 0

4 0

2 0

0

DUNBAR-KILNSTOPS

PREHEATER .BLOCKAGES RINGSMILN INLET BUILD-UP

C2K53

l C

Log plot of the total pressure, in atmospheres,of rhe decomposition products of various

sulphates occurring in the cement kiln.Abbreviations:

C A s= 3Cail 0 .CaSOC5S,S = 2Ca Si0&aS04

&$33 = 2CaS&k2S044

Choi G-S & Ckser F P. Cement & Ccmxete Research 18. 367 - 374 (1988).

Phase diagram at atmospheric pressure for the systemcontaining the solids CaSO,, CaO and CaS, as well usthe gases SO,, CO and CO,. The reducing potenti equalsp(CO)lp(COJ. Region A is where Cu# and CuSO,are the only solids; region B Fras &SO, repkced by CaS(see text). Contours of equd p(SOJ are shown.c

ayhurst A N & Tder R F, I Inst. Energy 64. 212 - 229 (fgg1).

Blue Circle Cement


Module 13

Section 3

Investigation into Potential LowTemperature Volatilization

INVSSTICATION IN'@3 PV!ZNTIAL LOW TFXPEXATUEE VOUTILISATIOH

WITEIBT?BP~O&B3D OxFo3DH!.3wOKgSKIL9, PBSCALCINEB

AHDPREHUTERS-fSPERS

Objectives

1) To identify potential lov temperature volatile compounds in the rawmaterials of the'new Oxford Works,

2) To assess the quantities of volatile8 present ia the gas and Baterialstreams at partiizular positions vitbin the system, in particular vithinthe preheater and precalciner sreas.

3) To establish whether these calculated values will affect the conolusionson the size of bypass required for the nev Oxford Works.

The rav materials for the proposesnd sulphates (up to 1.546 So3 and

,xford Works are fairly rich in alkalis,% 820). A bypass has been proposed l ti

bleed off a proportion of the kiln'& to prevent these alkalis and sulphatescausing problems in clinker quality and build up vithin the preheater andprecalciner, The design of this bypass has been discussed extensively in thetechnical note STH 41/73. This note covered almost all possible condition8which could affect the design of the bypass. flowever one area of uncertaintyhighlighted by SlY gl/lj vns the possibility of significant raw materialvolatility (;,3C%) at low temperatures (<loooOC). Engineering B & D verecontacted as the problem was of a similar nature to the work carried out inthe investigation idto the processing of cement flue dusts.

This note applies'the theoretical prediqtions of the previous flue dnstwork together with other infonrration available fmm pubUshed literature tothe potential problem of low temperature volatilisation vithin the Oxfordpreheater/precalcin#r system.

Conclusions

1) Of the potential problem causing cornKC1 and sulphur'contaiuin& compound8p"

unds investigated (K2SO4, Ba$O4,l?a$O4 could be effectively dis

cussed a8 a low temperature volatile (see Table 1).

2) If the precalciher ia operated at normal temperstures (-SW'C) thenthe amount of Kj$SO4 in the vapour phase is insignificant. flovever at

3)

4)

S)

6)

7)

8)

9)

10)

higher teaPeratures (*- 95OOC) a significant amount (.-low4 Xgs per

Kg clinker of K2SO4 can be in the vapour phase. Tnns a high temp-erature in thi precalciner should be avoided as it vould have adetrimental effect on kiln operation.

The vast Gjoritp of K2SO4 and rJa$Oq will be in the condensed phaseon the suspended solids at the bypass position. This would mean thatbleeding gas alone at the bypass position vi11 not reduce the quantityof alkalis in the kiln system. The most effective uay of reducingalkalis would be to bleed off the dust on which the alkalis condense.

The quantity of KC1 in the precalciner for a precalciner operatingtemperature of 850 - 9ooOC is high (1-a on clinker) but this-isconsistent with the values predicted in SE7 81/13 and; F&RlIfSof normal vorks operating experience. Thik is taken into acconnt inthe bleed requiment.

The effect of sulphur is summarised Fn Table 2. This table demonetrateethat the source of sulphur, the kiln exhaust covsitlon and the presencesof other minerals determines whether the sulphur will be bled off by thebypass, recycled within the preheater, or exhausted from the stack.

If sulphur is present as calcium sulphate, or calcium sulphide tha t?ocontrolling parameters on the volatility are the gas temperature and gascomposition. Houever the effect of gas composition reduces with decreas-ing temperatare and the evolution of SO2 will not be large (7% IBX) attemperatures experienced &the kiln back end, and will notie sicpifi-cantly higher with reducing conditions than with oxidisiq conditions.

Low temperature volatilisation due to the presence of certain mineralorganic natter or iron pyrites will result in an increased SO2 emissionfrom the stack but should not have any detrimental effect on the wlphurcycle in the precalciner/preheater system.

Sulphur from the kiln fired fuel will tend to form a recirculating loadbetueen the kiln and preheater unless bled from the system via the bwss.

Sulphur from the precalciner fuel will form a recirculating load in thesame manner as sulphur from the kiln fuel. Houever this wxwt be bledfrom the system until the sulphnr has been recycled.

Approximately 9546 of the So2 in kiln and preheater gas streams will beabsorbed by the CaO in the kiln riser duct providing that the gas admaterials stream are in contact for sufficient time for an equilibriumto be reached. This time is very short due to large surface area of CaO.

Recommendationa

1) Further work should be carried out on the Oxford raw materials to estab-lish the form in which the sulphur is present in the kiln feed.

2) As the wunt of lov temperature volatilisation is dependant on theform of sulphur in the raw meal a standard method for determining rawmaterial volatility should be developed. Lf such a method were availableit could be carried out as a normal laboratory test on the raw mater!-'The information could then be used either for design information forfuture works, or as inforznation to the kiln controller to enable marefficient operation of the bypaas and kiln.

3) When the form of sulphw in the raw mterial has been established,a simulation of tjhe Oxford Works raw meal could be tried out at anexisting XI W works. (possibly Plymstock). %upling throughoutthe system would then show where, and at vhat temperature, volatilisationand condensation of alkali and sulphur containing compwnds occuza.

C O N T E X T S

Objective

Conclusions

Recomendations

1. RiTRODUCTION

2. THROREXCAL CONSIDEXATIONS

2.1 Volatiles Present in the Rack end Gas

2.2 Calculation of the Kaxidmii3 Amount of Katerial in

the Vapour Phase

3. SJLPZUR COFi’AlfJING COXF’OUXDS

3.1 Reaction of CaO and SO2

3.2 Effect of Reducing Conditions on the CaS/CaS04

Equilibrium

3.3 Effect of Iron Sulphides

3.4 Effect of Xinerals

3.5 Effect of the presence of organic sulphurC' icontaminous compounds

3.6 Effect of sulphur in the fuel

4. CONCLUSIONS

5. RECOXNENDATIONS

7) Estimated Alkali Vapour in the Kiln/Preheater

Gas per Kg of Clinker (no bypass)

2) Effect of Sulphur on Raw Material Volatility

7

7

8

8

9

10

11

1 2

13

74-15

16-23

a’. CRCULATION LIST 24

INVESTI~TION IF!$ POTENTIAL LOW TdI!XPERATURR VOIATILISATION

WITRIN T3Ei PROP$SXD OXFORD NEWWORXSKIE?, PFECALCIHER

ANDPREHEmmsYsTEt?s

Objectives

1) To identify potential low temperature volatile compounds in the ravmaterials of the hew Oxford Worka.

2) To assess the qua&ities of volatile8 present in the ~8 and material

streams at particular positions within the system, in particular VithiL

the preheater and precalciner areas.

3) To establish whether these calculated values vi11 affect the ccnclnsions

on the size of bypass required for the new Oxford Works.

1. INTRODUCTION

The Services Department of Research Division have produced a comprehensive

assessment of the siie bypass that would be necessary tc prevent process

problems due to volatile alkalis/sulphates within the pmposed Oxford new

Works kiln syste (STN 81/13). The report highlights problem sreaa m-icularly with reference to the effect of possible volatflisation vithin the

prscalciner or preheater system. The chemistry of the compounds concerned

is of a similar nature to tbat of materials studied by Eagineeriug Research

and Development of the processinc of cement flue dusts. This note apples

the theoretical predjctions of the previous flue dust work, together with

other infomation awilable from published literature, to the potential

problem of volatilisation in the Oxford preheater/precalciner system.

The assumptions nsed'in SIN St/I3 acknovledge that a certain amount of lov

temperature volatilisation may take place and allows for up to 3096 lov teap-

erature volatilisation with a bypass of up to 30% Rovever if low tenpersturc

volatilisation vere to exceed 3cq6, then the following problems could occur.

il The build up of a rec$rculating load within the preheater system

causing the kiln feed to have a concentration of volatile corn-

pnents which could exceed the design capacity of the bypass.

ii) The pssibility of material build up uithin the preheater system

causing blockage problems.

iii) The release of vapour particularly So2 at a temperature below the

temperature of the formation of free lime. This would mean that

the bulk of the SO2 would not be recaptured by lime but released

from the system in the stack exhaust gas, causing the bypass and

preheater design to be oversized.

2. THEORETICAL COEJDEFUTIONS

2.1 Volatiles Present in the Back end Gas.

The first stage of the investigation was to identify all the volatile

compounds present in the kiln exhaust gas;IL -.J. c

then from themdynamic

information on>heir ;zD ehavio.ar with temperature, compounds which could

cause problems could be identified. The standard volatile com_r>onentT

of X20, Na20, So3and CL were considered. The form in which these

components will recondense is well documented, and is listed below.

1) Chloride will recondense as KCl,

2) K20 remaining and Na20 ,will recondense as K2sD4 and Na2S04.

3) If there'is excess sulphate, as in the Oxford case, Cam4 will

be formed which preferentially forms the double salt (2 CaSO4g2s04) *

2.2 Calculation of the Maxfmum Amount of Material in the Tapour Phase

The solid gas phase equilibria of El, K2S04 and Ba2sD4 has been/

extensively studied at Barnstone by Khor?.

Prom curves of saturation vapour pressure tempera-e for KCl, %s04

and Na SO (see Fig. 1) the saturation mass of vapour in the exhaust2 4

gas can be calculated using the equation:

Where Ys equals the saturation mass of aUralA vamur in the exhaust

e-9

Pa is equal to the saturation vapour pressure (obtainable from Pig. 1)

P is the total pressure (atzn)

Mv i&-the molecular weight of the vapour component

Mg

is the molecular weight of the gas.

Taking the total pressure to be one atmosphere and the molecular weight

of the gas to be approximately 30 we obtain the curves shown in Pig. 2.

If we assume that 4@ of the fuel is burnt in the kiln and 6% in the

precalciner, if there is no 'bleed, the mass of gas to clinker can be

approximated to 0.55 Kg gas per Kg of clinker in the kiln and 1.7 X3

gas per Kg clinker in the precalciner. Using this information we can

obtain the CuIves shown in figure 3. which give the narimum vapouz

carrying capacity of the gases. The czurve at a higfier level in the

precalciner indicates the higher gas quantities in the precalciner.

By assuming the burning zone temperature to be 1450°C, the back errd

temperature to be 1050°C, and the precalciner to have a maximum tezp-

erature of 950°C Table 1, can be constructed. This shows that we -

would expect all the available KC1 to be vapouris-A hofnr- -a-h&

the kiln,? small amount of K.$04 to be'vapourised.&d a negligible

quantity of Na2SC4. If the precalciner is operated at a lower tv

erature (85O'C is more ty-pioal.) then the amount of K2SC4 vapourised

also becomes negligible.

This analysis recognizes KCI. as low temperature volatile, hoverer

this is also recognized in Sl!S 81/13, and the quantities of KC1 pre-

dieted in .the precalciner region by STB 81/13 (1.6% on clinker) com-

pares well the quantity predicted by this analysis over a temperature

range of 850 - 900°C (1-246 on clinker). The results of this

fundamental approach show that the assumptions made in Sn 81113

concerning chloride recirculation are reasonable. As the assump-tions made in STM 81/l-J ~~ncernina alkali recimlatian are stx-mn

to be reasonable bv this inventiaatlon. anv wtemlal aaerstinz

procuemz voula nave w ari8e rrom an cute-rnatlve source, prooablg

from sulphur contsining compounds. This is discussed in the

following section.

3. SUUHUR CONTldNISG COMEWXES .

Sulphur can be present in the kiln system from several different sources.

1) Calcium sulphate and calcium sulphide in the rair materials.

ii) Iron sulphides in the raw materials.

iii) Sulphur present in organic matter in the raw material.

iv) Sul?hur present in the fuel.

Ro similar partial pressure data to that used in the previous section wa

available for CaSO4'

CaS and iron sulphide. In this case a fundamental

approach considering the thermodynamics of the reaction of SO2 and CaO MS

considered.

This investigation was complicated by the fact that the equilibria and

rate of reaction are affected by the gas environment in which the reaction

takes place e.g. reducing or oxidising, and the presence of certain minerals

in the rav meal e.g. Si02, MgQ and FeO,

3.1 Reaction of CaO and SO2

Some of the many possible reactions of CaO and SO2 are listed belou3.

I) caso4(s) + yEI) + =2(s) + 3 O2(g)

2’ CaS04(S) + 'O(g) + ca'o(8) + =2(g) + co2(S)

3) c”s4(*) + @(*) + c”s(*) + 4 co(g)

4) C=s4(s) + 4co(g) + cas(8) + 4c02(g)

5) 3 -4(9) + C"s(s) + 4 ys> + 4 =2(g)

Beactions 2, 3 and 4 will only take place under reducing conditions

and these will be discussed later. Of the two remaining reactions

laboratory tests4 on the adsorption of SO2 on cement raw meal have

shorn that considerable adsorption of SO2 occurs in the temperatare

range 600 to goo’c according to the reverse of equstion (5).

6) 4 CaO(s) + 4 a2fg) 9 3 C=s4(s) + C=Scs)

From thermodynamic data we can obtain values of enthalpy, entmpy

and Gibbs free energy for the reaction at different temperatures.

Then using the Qan't Hoff isotherm the value of Kp (the equilbrium

constant) can be found.

DG = -2.303 %T Log K -P (2)

F'rom the stoichiometry and assuming the actitivities equal to unity,

it can be seen that Kp depends solely upon the partial pressure of

=2* From this knowledge we can obtain a curve (Figure 4.)&J ;,J/*L;& t h e

extent to which #e reactior6progreases to the right hand sic with_~.- _. --.----increasing temperature. Th2.s curve show that, at a kiln back end

temperature of 1050°C all the SO2 would be in the gas phase vhere as

over the precalciner temperature range (85O'C - 95O'C) there would be

between 2% to 1446 dissociation to fern S02. This is within the 3096

low temperature volatillsatlon allowed for in the report STH 81/13.

This equilibrium is for a static sitcation and does not consider the

removal of SO2 from the system or the rate at which the reaction

proceeds. In the kiln/preheater system the compounds are constantly

being removed and replenished and so the equilibrium is also depend-

ant on reaction rates and residence time of the compounds. This

analysis does shou however that the bulk of the SO2 will be generated

from the decomposition of CaSo4 at a temperature greater than the

typical temperature of the precalciner and so a bypaes at this Mgher

temperature would reduce much of the So2 available for reforming CaSO4

CaS in the precalciner.

3.2 Effect of Reducing Conditions on the CaS/CaSO, Equilibrium-t

If sulphur containing limestone is roasted in air all the salphur is

converted to C&O4 by the reverse of reaction (1)4.

7) CaO + So2 + 8 02 + Cas4

Reduction in the level of oxygen causes reaction (6) to be favoured

which has bee,n found to be the major reaction in cement kiln exhaust

gases. Further reduction of the level of oxygen causes reactions (2)

and (4) to be favoured. The extreme case of reducing conditions is

given by equation (3).

If CaS is present a further reaction can occur with carbon dioxide5

according to the equation.

8) CaS + 3 co2 = cao -+ so2 + 3co

!I'he temperature dependance of these reactions have been studied by

Turkdogen and Olsson5 who obtained expressions for the equilibrium

constants

10gpm2 (p~o/pco2)3 = - (20,000/T) + 9.27 (3)

lo~W2 (pC02/pcO) = - (‘9617/T) + 8.021 (4)

From these expressions the salphate/sulphide equilibrium diagrams can

be dravn vhich are shown in Figure 4, 5 and 6. These curves show that

adjustment of the ol;ygen potential of the kiln atmosphere will adjust

the level of sulphur retentio:n and that this effect is more sensitive

at higher temperatures. At the relatively lov temperatures, 95O'C and

3*5 Effect of the presence of orwic sulphur contaminoua..compou.nds

ii) the presence of potassium containing minerals very much enhances

the decomposition of CaYO4when compared to the effect of analogz

non potassium co&ainiry~ minerals. These obseroatione are valid

for temperatures below 12OO'C and at atmospheric pressure.

The presence of minerals which enhance evolution of s02, by the

same mechanism will obviously retard its recapture. This could

mean that SO2 is lost from the system via the stack in greater

amounts than that predicted by STN 61/13.

Another form in which sulphux can enter the kiln syetem is in the form

of vegetable matter. !&is vi11 evolve SO2 at very much lover temperatures

than mineral based%

. If this was the mjor form of sulphnr in the rav

materials, this uould mean that almost all the raw material sulphnr would

be lost via the stack causing the bypass tc be very much oversized. The

effects illustrated in sections 3.4 and j.5 can be investigated by eqeri-

merit and would ahow up as enhanced volatility at lair temperatures.

Experiments carried out on the Oxford raw material have not shown this fn

be the case.

3.6 Effect of sulphur in the fuel:

Sulphur in the fuel will be present in the kiln and precalciner gases a8

=2- From the discussions of their reaction of CaO and SO2 (section j.1)

it was stated that considera't)le amonnts of sulphur vould be absorbed at

600 to 900'~. In fact the peak rate of absorption for SO2 is at 850°C5,

Other factors affecting the amount of sulphur absorption are the concen-

tration of CaO and S02, the surface area of the CaO and the exposure time.. .In a kiln and preheater system the time for vhich CaO i8 exposed to So2

at a temperature where absorption can take place 18 very short. HoweV8r

the fineness of the CaO particles means that an enormous surface area of

CaO is presented to the gas stream.

This is generally the overiding factor and the reaction vi11 tend to

reach equilibrium very quickly. Hence in the stage IV preheater cyclone8

there is almost 100% absorption of SO2 by the feed.

With a precalciner the reaction is complicated by two streams of

gas and material, at different temperatures combining. The pre-

calciner gas stream will contain So2 from the fuel. The precalciner_..material stream carries‘bffsolid of increasing CaO content (see ?ig.

8). Eouever there will be little recombination of CaO with SO2 in

this stream, due to the gas temperature of approxinrately llOO°C.

(see Figure 4).

This will combine with gas and material streams from the kiln in the

riser duct from the kiln. (Por the RSP ONOQA system shorn in Figure

8, a mixing chamber is used),, This material stream will also contain

CaO and So2 from the kiln fuel and rav materials. The gas temperatare

is also around llOO°C and so there &ill also be very little combination

CaO with S02. As the gas and material progress up the riser duct the

endothermic production of CaO from CaCO3coolstthe gas and material

streams. This makes absorption of SC2 by the time more favourable.

The absorption should have reached a maximum at the point uhere the

gas and materials streams separate. The absorption at this point till

be close to the equilibrium <absorption for this temperature (-. 900°C)

at about 9% (see Fig. 4). This is borne out by operating experience

with suspension preheater kilns.

The presence of a bypass will mean that kiln produced SO2 will be bled

off immediately whilst SO2 from the precalciner will be recycled before

being bled from the system.

4. CORCLUSIOBS

1) Of the potential problem causing compounds investigated &So,, Sa2s04,

KC1 and sulphur containing compounds),Na2S04 could be effectively die+

cussed as a lov temperatie volatile (see Table 1).

2) If the precalciner is operated at normal temperatures &850°C) then

the amount of K2sD4 in the vaponr phase is insignificant. Sowever at

higher temperatures (- 950°C) a significant amount (u10W4 Kgs ger

Kg clinker) of K2S04 can be in the vapour phase. Thus a high temp-

erature in the precalciner should be avoided as it would have a

detrimental effect on kiln operation.

3) The vast majority of K SO and Na So will be in the condensed phase2 4 2 4

on the suspended solids at the bypass position. This would mean thatbleeding gas alone at the bypass position will not reduce the quantity

of alkalis in tha kil,~l system. The most effective way of reducing

alkalis would be to bleed off the dust on which the alkalis condense.

4) The quantity of KC1 in the precalciner for a precalciner operating

temperature of 850 - TCO'C is high (1-a on clinker) but this is-4%

consistent with the values predicted in ST3 01/13 and&rem results

of normal vorks operating experience. This is taken into account in

the bleed requirement.

5) The effect of sulphur is wbrised in Table 2, This table demonstrates

that the source of sulphur, the kiln exhaust composition and the presence

of other minerals determines whether the sulphur vi11 be bled off by the

bypass, recycled within the preheater, or exhausted from the stack.

6) If sulphur is present as calcium sulphate, or calcium sulphide the two

control&g parameters on the volatility are the gas temperature and gas

composition. 3ouever the effect of gas composition reduces with decreas-

ing temperature and the ev$ution of SO2 will not be large (1% mar) at

temperatures experienced annthe kiln back end, and will not be signifi-

cantly higher with reducing conditions than with oxidising conditions,

7) Low temperature volatilisation due to the presence of certain minerals,

organic matter or iron Fyrites will result in an increased SO 2 emission

from the stack but should not have any detrimental effect on the sulghur

cycle in the precalciner/preheater system.

8) Sulphur from the kiln fired fuel will tend to form a recirculating load

between the kiln and preheater unless bled from the system via the bmsa.

9) Sulphur from the precalciner fuel will form a recirculating load in thesame manner as wlphur from the kiln fuel. Houever this cannot be bled

from the system until the sulphur has been recycled.

10) Approximately 9546 of the SO2 in kiln and preheater gas streams will beabsorbed by tne CaO in the kiln riser duct providing that the gas and

materials stream are in contact for '&fficient time for an equilibrium

to be reached. This time is very short due to large 8urface area of CaO.

5. RECOI+2EDATIOBS

1) Further work should be carried out on the Oxford raw materials to estab-

lish the form in which the sulphur is present in the kiln feed.

2) As the amount of low temperature volatilisation is dependant on the

form of sulphur in the raw meal a standard method for determining rav

material volatility should be developed. If wch a method were available

it could be carried out as a normal laboratory test on the rau mterial.

The information could then be used either for design information for

future works, or as information to the kiln controller to enable zare

efficient operation of the bypass and kiln.

3) When the form of sulphur in the raw material hae been established,

a simulation of the Oxford Works raw meal could be tried out at an

existing BCI dry works, (poselibly Plymstock). Sampling throu&out

the system would then show where, and at what temperature, volatilisation

and condensation of alkali and sulphur containing compounds occura*

1)

2)

3)

4)

5)

oxford Works - Modern Dry process with sulp;lur Rypass - an

assessment of the technical risk. STN M/13. PA Loognarl,

D.S. Svift, March 1981.

Processing of Cement Flue dust.. PhD Thesis, Jaw Huei Wor,

December 1979.

Sulphur pollution from coal combustion. Effect of mineral

component on the thermal stabilities of sulphate ash dsd

calcium sulphate. Baker D.C. and Altar A. Ewironmental

Science and Technology. P!!ch 1981.

Recirculation problems in Rotary Kiln Systems.

H, Xitznann, Neubechun. Translation of Zement-Kalk-Gips (8)

338-343. 1971.

Desulphurisation of hot reducing gases vith calcined Dolomite

ET Turkdo6an & R.G, Olsson. Irorrcuz&ing & Steel %king 1978 Ho. 4.

Estiloated Alkali Vawur in the Kiln/Freheater

Gas per Kg of Clinker (no bypass

Position inSystem

WQ4 &3s/Kg Clinker

Na2w4 %s/Kg Clinker

IxL QdQTClinker

Burning zoneat 145OOC

Complete4.8 x 1O-2 1 . 4 x -210 Volatilisation

0.55 Kgs gas/ Expected

IKg Clinker

At 3ack Endat 1050°C

0.55 &P @S/ 8 x 1O-4 lo-5 5 . 6 -2x 10

per Kg Clinker

Precalcinerhigh temp. 95oOc -21.7 Kg3 es/ 5 I 10- 5 Megligible 4 . 6 I 10

Kg Clinker

Precalcinernormal temp.850°C

Negligible Negligible -21.7 &3s gas/

1 x 70

Kg Clinker

2.TAB=

Effect of Sulphur on Rw Fkterial Volatility

Source of Sulphur Conditions aff'ecting Likely effectreactions

Cam4 will be recycled being

CaS and CaSC4

Oxidising vapourised in the kiln and

condensing to form CaSO4’ in

the precalciner unless tha SO2level is reduced. CaS will

tend to form CaSC4and act in

CaS and CaSO4

Reducing Similar to oxidising cond-

itions except CaS will form

in the precalciner and the

So2 vi11 evolve at a sli&.tly

lower temperature.

FeS2 The bulk of the sulphur will

be converted to CaS but an

increased quantity of .sulphur

vi11 be lost from 'he system

via the stack.

Presence ofCaS and CaSO

4so* Fe203 Increased losses of So2 from

sodium and calcium the stack when corn-red with

montmorillonite pure CaS04

Presences ofCaS and CaS0

4Potassium containing Y&e SO2 in the stack exhaust

minerals when compared vith the effect

of SiO2 etc.

Cod/. l . . .

Source of Sulphur

Organic Sulphur

Sulphur the fuel

TABLE 2. Cont'd.

Conditions effectingreactions

LikeQ effect

A11 the sulphur vi11 be lost

via the stack exhaust.

0

Temperature ("C)

Figure It Saturation Yapour Pressure of El, K SO ank Na SO2 4 2 4

2 .FIGIJEE

Temperature @)

Figure 2: Saturation Mass of KCl, K SO2 4

and NapS04.Yapom in Air

-:18- :

__- .- -_--

_‘.- -.-

- --z

Blue Circle Cement


Module 13

Section 4

Factors Affecting Sulphate and AlkaliCycles in Rotary Kilns and the

Implications

ANDTHE IMPLIC?YXONS OFTZESE EZFFECTS KITH RESPECT ?o P-S CDNTROL

SYNOPSIS

i) To identify the fundmental and empirical relationships governing

sulphate and alkali cycles in rotarykilns.

ii) Using these relationships; to explain the kiln performance with

reference to clinker sulphate ark3 alkali retentions at bth

Northfleet and Hope M&s.

iii) In the long term, to learn kx4 best to control the rotary kiln process

with respect to sulphate and alkali levels, in order to qtimise the

prciluction of clinker that ineets current rmrket requirements.

Anexamii-la tion of alkali/sulphate cycles has 'keen carried out using

data frantrials carriedcutatNxthf1ee-t andF@e b&As. The fundamental

an3 e-rpiricxl relationships qzvernirig these cycles are discussed along with

deviations fran those relationships. The magnitude of the suggested fat-

tors ki-iich cause deviations fran the fundamental relationship is estimated

using data cbtained fran +mAs trials and ,&lished literature. This data

is then used to mre closely mcdel the real situation. The resultant r&e1

when refinedmaybe used to estimate the recirculating load of

alkali/sulphate (fran hereon described as wlatiles) within the rotary kiln

system for wxks where insufficient empirical data is available to

calculate the recirculating load using the traditional average crass balance

method. Using the fundamental relationships discussed in this report, the

requirements for a control stratq for rreintaining a'constant level of

volatiles in the kiln systxn is proposed. ll-Ls strategy is based a2 the

kiln X& signal.

i) A simple idealised wlatiles cycle within a rotary kiln can be dev-

loped frcmkncwledge

the kiln system.

of the temperature profile and gas flows within

ii) The rregnitude of the recirculating load of wlatiles in this simple

rwcdel is principally a functionof tb.e peak feed temperature. The

quantity of gas pr unit of clinker is another impxtant factor since

it determines the rraximum volatile recycling capacity of the kiln

gases: i.e. the wet process, through its higher energy demand, results

inrcrxe gas being available to

Conversely, a precalciner with

nwinff tm 90% dec~rlnnatinn &

carry a larger quantity of volatiles.

its smaller gas quantity within thekiln,

theburning of 60% of the a& cutside

the kiln in the preheater, is unable tc suppxt such ahighlevelof

volatiles as a suspension preheater !ciln or wet process kiln.

iii) The idealised cycle does rzot fully explain the cbserved situation.

However, through studying actual -rating data, the various effects

that influence recirculation canbe estimated and amxe realistic

model developed.

iv)

VI

vi)

Tne four principal factors that alter volatiles recirculation b&a-

viour fran the ideal cycle are the gas/solid mixing, kiln atmsphere

cqsition, dust insufflation and feed -ition.

The level of gas/solid mixing in the burning zone is a function of the

kilnvolume loading and the aznbined effect ofkiln speed and feed

residence time. The effect of pr mixingmuld appear tobe to

decrease the actual quantity of mlatiles recirculation to around

l/Sth to l/lOth that of the level predicted assuming an ideal cycle.

The gas/solid mixing tith reference to the recapture of volatiles

material is a function of the type of process e.g. suspension pre-

heaters are highly efficient gas/solid mixers at the kiln back end

whereas wet process kilns are rmch less efficient.

The effect of a reducing atrrosphere is to reduce thequantityof SO3

retained in the clinker and increase the So3 lost as SO2 via the

stack exhaust.

Dust insufflation will typically cause a large increase in the

volatile recirculating load.

vii) Volatile recirculation will tend to induce a cyclic pattern of beha-

viour for the quantity of mlatiles retained in the clinker *en

clinkeris~ttoaconstant~~limemntent.

The cyclic behaviour of the recirculating mlatiles causes changes in

the apparent burning characteristics of the clinker bynodifying the

quantity of flux in the pre-buming and burning mnes and also induces

a~geinfhequantityof~trequiredto~~aconstant

-iv-

recirculating load, i.e. the heat requirement for mlatilisation in

the burning zc)ne will increase *xith increasing mlatile content of the

feed.

This cyclic behaviour makes kiln axtroltc a&t-ant free lime

based on kiln amps exceedingly difficult. A amtrol strategy to min-

tain a am&ant level of volatile recirculation till rerrove this

cyclic behaviour. Ihis stability can be achieved by maintaining a

constant burning mne teqerature with a constant feed input. Mrk

carried out at I-b&pe Wrks has shown that the best relationship between

an observed kiln pa.rar&er and the level of recycle tas found to be

N&. This wouldman thataaxtrolstratqybased onK& shouldmin-

tain a ax&ant level of reci2miLation and allow the free lime wntent

to float inlinewith changes in the feed chemistry.

i) F'urtherwrk shouldbe carried cut to identify the mlue of E, the gas/

solid mixing factor, particularly wi"Lh reference to the kiln ,param+

ters of mL.xne loading, kiln speed and kiln angle. This factor, along

with thepeak feed temperature, wuld allowamre accurate estimate

of the level of bxning mne volatilisation to ba predicted.

ii) Ihe relationship between clinker m&hate and q should be further

studied to enanpss the effect of ,parameters such as B.E.02, feed ax-

position, coal consmrpticn and kiln cutput.

iii) The effect of oxidising or reducing atrmsphere cm mlatile cycles

should 'ZE studied further. Current theory and practical results indi-

cate an inverse relationship between the &antities of SO2 and 02 in

the gas stream andhence a direct relationshipbatween So3 in the

clinker and 02 in the gas stream.

iv) The effect of raw meal mineralogy shculd ke examined in order to

attempt to qmntify its effect m the ideal cycle.

VI The net effect of the thermal requirement for empczating and oHI-

densing circulated mlatiles r&is to be determined mre specifically.

This is of particular relevance where preheater cleaning causes sudden

surges of mterial with a high mlatile content to enter the 'kiln.

vi) Control strategies based on N& to mintain a azmstant mlatile

recirculation load should be develo,Ded and evaluated in preference to

those based mkiln amps and a amstant free Lime cmtent.

FIGURE t------we RECIRCULATING VOLATILE BALANCEHOPE WORKS 1977

S t a c k G a s S t r e a m c o a l

K2° 0 . 1 2 3 K2° 2.624 K2° 0.019Na20 0.033 Na20 0.180 Na20 0.005S O3 0.381 so 3 4.930

C l 0,018 C l 1 . 1 2 0

P 0.019F 0 . 0 0 0

I - - - -P r e c i p i t a t o r Duet

so3 0,480

C l 0 . 0 1 2

F 0 . 0 0 4

R e c i r c u l a t i n g L11 Volatilised i n 82 jXj

K2° 2.605

Na20 0.175so3 4.450C l 1 , 1 0 8

F 0.015

K2° 0 . 0 4 4

Na20 0.088

S O 3 0 . 0 4 0

C l 0 . 0 1 0

F 0 . 0 0 4

K2° 2.457Na20 O.l’t9

s o3 4.503

C l 1.092

F 0.015

K2° 3.125

Nu20 0.375

-IsR a w F e e d P r e h e a t e r F e e d

K2° 0.624 K2° 0.668Na20 0.218 Na20 0.226so.5 1.201 SO

31.2'11

Cl 0.009 C l 0.019

F 0.156 F 0.160

Na20 0 . 2 0 0

s o3 1.300

C l 0.003F O.lGO

83.446.7

77.11

99.78.6

Retenlion $- - -

q6.6

53.3%%,C

0.3

31.4

YCHNX%

ZCGC

ICC%

0

HOPE WORKS KILN : R E L A T I O N S H I P 6

21 I?102

Q

0

m/84/13

MD'IHE iMPLICATICNSOF'Z%SE EFFECTS WI'H RESPECT T0 PXCESS GXIWL

Page lb.

SXNOPSIS

OBJE~IVES

1.0 lzYmxum1m

Figure1

Figure2

wRecirculating Volatile Balance t30 ks 1977

raOpe Wrks No 2 Kiln vRelationship beween KilnExit W, and Clinker SO3 cbntSam Tine Base

2.0 FUNDAMENTAL RELATIONSHIPS ?G'FZTIG bQ LECYCLLES%

Figure 3 Idealised K2SO4 cycle for a Northfleet Kiln FeedLSF 98%

Figure 4 Idealised Cycle for a pe Wrks Kiln Feed

%3.0 DEXL..IONS FFXX¶ THE IDEAL CYCIE

3.1 Gas-solid Mixing

3.2 The effect ofcooler ,oart of

mixing and reaction rate in the

ical Trer& of So2 amxntratim in

of Kiln Feed Ccqosition

5

7

8

9

9

10

11

12

13

14

15

Page PD.

4.0 IKPLIC~TICNS OF WUTILE RECIRCUUVION To 'IWZ pKCl5S.S CCWjX)LOF KYI'ARY KIU?S

16

4.1 Burning Zone Temprature 1 7

4.2 Gas Qm-ktity 18

4.3 Xixing Efficiency 1 8

Figure 7

4.4 Gas solidrate

S02, NO2 and 02 axxentrations in the KilnExhaust (dry basis)

1 9

mixing in the cooler ,mrt of the kiln

4.5 Gas Clarqpsitions 2 0

4.6 -Kiln Feed CDnpsition 2 1

4.7 Ixlst Return 2 1

5 . 0 coNcus1ms 2 1

6.0 RECXMQXDATIONS 2 2

7.0 REFERENCES

APPENDIX 1

THEDRIES OF Q3NSTRUC!XoNOF IDEAL CYCE.S RM)CORREcrED IDEAL CYCLES

APPFxxx2

EEFECTOFREDJC CD!JDITIONS CN THE vOLU?ILIT'Y OF ALKALI SULE'HATES

%APPEHDIx3

HEN PIPE F KWTILE PECIRCUATIoN AT WPE KXKS

2 4

ANDTHE DIPLICATIONS OF'MESE EFEECTS KtTH RESPECf'T3 PKCESS CrXTWL

OE!JEmIvE

i) To identify the fundamental and empirical relationships governing

sulphate and alkali cycles in rotary kilns.

ii) Using these relationships; to explain the kiln ce with

reference to clinker sulphate and alkali re entions at b&h

No-fleet and I-Qpe Parks. b

iii) In the long term, the rotary kiln process

with respect to sulphate and plka in order to optinise the

prcduction of&clinker that nt mrket requiremnts.

1.0 1mmLUcr1oN

Kiln based su &ate and alkali cycles ("volatile cycles") have

\almys been of i.qx3rta.n to the cement raker, since the level of

recyclehasa 4 ficant effect co the kiln characteristics, 'the%

clinkerch ' try and the associated quality of the cement prcduced.

3Howaver, w1 e advent of dry process kilns these cycles have been

under r

Q

xtensive study due to build-up problems in the kiln ,pre-

heater an relatively high alkali levels in the clinker. i%re

rQ* enviromental pressures have added impetus to the study, due

to increasing interest in the level of q and s4, &ssiom fram the

kiln exhaust stack.

ANDTHE IMPLIC?YXONS OFTZESE EZFFECTS KITH RESPECT ?o P-S CDNTROL

SYNOPSIS

i) To identify the fundmental and empirical relationships governing

sulphate and alkali cycles in rotarykilns.

ii) Using these relationships; to explain the kiln performance with

reference to clinker sulphate ark3 alkali retentions at bth

Northfleet and Hope M&s.

iii) In the long term, to learn kx4 best to control the rotary kiln process

with respect to sulphate and alkali levels, in order to qtimise the

prciluction of clinker that ineets current rmrket requirements.

Anexamii-la tion of alkali/sulphate cycles has 'keen carried out using

data frantrials carriedcutatNxthf1ee-t a&F&e b&As. The fundamental

an3 e-rpiricxl relationships qzvernirig these cycles are discussed along with

deviations fran those relationships. The magnitude of the suggested fat-

tors ki-iich cause deviations fran the fundamental relationship is estimated

using data cbtained fran +mAs trials and ,&lished literature. This data

is then used to mre closely mcdel the real situation. The resultant r&e1

when refinedmaybe used to estimate the recirculating load of

This re,mrt piqwses an alternative .meticd of estirwiting t3e

recirculating volatiles based on the fundamental relationships

governing the process of recirculation. An ideal volatile cycle 'based

on these relationships is pro,oosed, along with ,mtulates for

deviations frcax this ideal cycle. Ran this rrcdel, a new estimate for

the internal cycle is developed for mrious kiln amditions.

These relationships are then used to support

controlstrategybased onmintaining clinker es levels, espe-

cially the sulphate level, at a constant va ue rather than to allcw

them to vary over a large range of values. b

& C Y C L E S2.0 FUNDAEEVTAL, fELATICNSHIPS AFFEcrrwG V3

The tm fmdamentalthe4

'c ,parameters which affect the

volatilization of any rmteria e the temperature and its imle frac-

tion in the gas stream (i.e. 3@al pressure). Inarotarykiln

system these are iranifeQ

the temperature of the feed and quan-

tity of gas atilabl to carry the volatiles. The relationships

between temperature,\m fraction and recirculation are discussed in

more detail ine

ndix 1, along with details of 'mw to construct an

ideal cycle. Ran these relationships we mn axstruct a diagram to

9ShGdhCWth erial is wlatilised and recondensed. Fbr the ,p.xpcse

of thi

9

rt the recirculation of mly one wlatile - K2SO4 - is

discuss , dlthough the same technique myba used for any mlatile.

Fi eQ

shows an ideal cycle diagram for Wrthfleet Mrks i%. 4 kiln.

In studying this figure, cry rmst mncieve the zaw feed being fed to

the kiln and rmving along the Xaxis until the feed temperature has

reached a pint at 5-6 'kiln diameters frm the nose ring where the

K2SO4 in the kiln feed is able to volatilise. The K2SO4 then begins

to vaporise into the gas stream where it attains the gas temperature

and is trans~rtedback along thekiln to apointtiere the gas tem-

perature falls to a level tiere the K2SO4 will reco e (and rem

-zfiiiLbine in the event of dissociation), at about 13 to 15 ' ters fran

the rose ring in the case of the first cycle.*

les will con-

tinue to build up the levels of volatile in the kiln bed until the

maximm quantity of K2SO4 tich can be is reached. This.

will occur at the &peak feed temperatur around three kiln diameters

frcm the nose ring.

example, consider

any excess K2SO4 in

with the clinker. Fbr

approximately 25 cycles

reached and K2SO4

culating load is 25% K2SO4 QI clinker.

etical kiln feed history is illustrated in figure

Theiqmrtantfhingtomtecnthiscycleisthe

ity of K2SO4 mlatilised. This is indicative of the

ility temperature of Hope tbrks kiln feed and the 1-r

1.92 kgs of gas per kg clinker in the burning zone and 2.75 kg of gas'

per kg clinker at the back end. E?qp Parks 'has 1.40 kg of gas par kg

clinker in tie burning zone and 1.94 kg of gas per kg clinker at the

back end.

100

90

80

70

80

50

b0

JO

20

10

0

t

t0

FiKurd - Iticaliscd KSO!, c y c l e f o r n NorthI’leet K i l n h’eed LSF 985\:

pdircction o f gas

4-edircctiorl ol’ feed

s o l i d t o

gas stream

K I L N DIflHETERS F R O M HOOD

LC

0

0

&MN113 NO X O3SIlI1UlOA tOSZW SSW

3.0 DJZVZ~TIONS FFCM TEE IDEAI; CicLE

The volatile rrms balance in figure 1 gives details of the maan

long tern recirculating ILoad at Wpe Wrks. If ma consider K3SO4,

the equivalent average quantity of K2SO4 recirculating is 1.94% which

is only l/lOth of that predicted by the thecretical It is

these deviations fran the ideal cycle which are in this

section, and, tiere pxsible, quantitative est f the effect of

these deviations the ideal cycle axe given. "

\The factors which are amsidered be of major iiqm2ance are

discussed in mre detail belcw. ?he ors are:-

i) The effect of gas/solid '+

the burning zone.

ii) The effect of gas/s&i g and reaction rate in the oc;oler

,"rts of the kiln.

iii) The effect of kiln atmsphere.

zone of a rotary kiln, vqmrisation till take

volatilised is in mntactwith the

unsaturated hut gas.. The rotary kiln is very inefficient at ~rcducing

this wntact, as the bulk of therraterial is contained in the feed bed

which presents cxiLy a szmllsurface to the gas strfxrnatany time.

For this reason a mixing factor (E) with dimensionless values between

0 andlneeds to be intrcxduced. The mlue of E will approach 1 tien

the gas/solid mixing is very efficient and allows all the feed +a acne

into cc&act with the gas. Thus the value of E represents the frac-

tion of the total solid surface ewsed to the gas stream. Using this

interpretation be can say that E will be a function of the volume

loading of the kiln: (a small~l.umeloading.presents wger surface

per unitmss of feed than ahigh volume loading) and the ccmbjned

Qu

function of kiln speed and kiln angle. A high . factor wuld be

expected with alowkiln slope and ahighki and vice versa.

Work is currently being carried cut has m aza-quter ncdelling as to

the precise value of E; the 'nest value ZLr hasedcnpresent

information is 0.1 to 0.2.

+The use of values of E .l to 0.2 implies a decrease in the

%maximum arw3unt of volatilisation the Iwrning zone by a factor of 5

to 10.00

3.2 TIE EFFECT OF GAS SOL ANDKEACI'ICNRATEIN'-!XEC0LERPARl'OF"

THEKIlxsi!Ls~

%This e ect is very mch a function of the type of process.

+For example, a dry process kiln &i&has intimate mixing of gas

solids at temperatures tiere reactions 6311 cccur very

ally all the volatiles till be captured. The mixing

of this part of the process can therefore be said to

apprwch unity. For a wt or semi-wet process, the tilk of feed

material is retained within the feed bed and therefore less intimate

mixing is likely resulting in a much 1-r level of Mlatiles being

captured (circa 50% efficiency).

This later case is illustrated in da'~a obtained fran an HES

bum carried out at Pamstone ibrksc21 see figure 3 here the

quantities of K20 and SO3 retained in porthole samples 1 taken from

the chain section outlet, and porthole 3 taken fra a point just prior

to the burning zone, show v-q little difference (i.e. change in K20

and only 0.5% change in SO3 content).. The stirring ction in the

9chain section of a long wet 'kiln does increase the mixin efficiency,

hence there is a sharp decrease in So3 content rtholelto the

stack of around 70% and in K20 mntent of aroundv906. It

%

is possible

that the actual mixing efficiency within a lon t kiln is likely to

be prirrarily a function of the chain

This section refers i 'tally to the oxidising potential of

the kiln atrrosphere and its-ifi?eff upon the volatilisation of sO3.

The fact that a reducingQ‘In atxrxphere as indicatedby alowbck

end oxygen content (generally

\

< 1.0%) gives rise to lu48er clinker SO3

andhigher exhaustSO2 n thehigher oxidistig conditionhas been

well documented

e

myle and Ferd41, B.-KG&~] and ~la~urne~~~.

Eetails of th possible rre&ani

&

'sins mncemedwith this process stay be

found in ' 2 tile the effect is illustrated in figure 6. The

values in

Q

figure 6 will be dependent upon the type of process

(e.g. wet ), feed and fuel -ition and burner design. The

feature is the shape of the curve i.e. a very rapid change

in SO2 content in the exhaust gas (andhence an equivalent change in

the SO3 in the clinker) for a sznall&ange in oxidisingpotential

FIQURE 5 BARNSTONE HES TRIAL NO. 6 AUGUST 1980 - VOLATILE RECIRCULATION

K2° 0.13

SO3 0.32

Feed

K2° I.77 Is o3 5.‘+3 I

I11A

Back Endof Kiln

Temp. 2-300 C ’ Temp. 800~ Temu

Ti?ICAL TRZXDS OF SO2 CO?;CE?XX4TION IY KIL!J EXYAUST

2000 1

I

1500

1000

2500 -

NZARLY ALL SULFUR?XTTED AS SO2

vY7

.?ROBABLE TREYD WL"r!LOU ALKALI FEED

----__

500

I.0 2.0 3.0 A.0 5.0

02 CO?lCEXTMTION (I)

Extracted from :

Doyle and r"er& A?$icatiocs of Flue Gas halysis to Cement i<llnCperation 9ock Products ::ov 1923

of the 'kiln gases from reducing to oxidisirg cmditions. A similar

pattern has teen observed at LXorthfleet Wr'ks during tests cm the So2

monitor where SO2 is practically undetectable at hack end oxygen

levels of qeater than 1.0% tit very quickly rises to levels of &me

500 ppn when back end oxygen falls below 1.0&[71. A change in SO2

concentration in the exlmust gas at Sbrthfleet of 0 500 ppn So2 is

the equimlent of approximtely 0.4% SO3 a2 clinker.b

vb

3.4 TE-E EFFFCI OF KILN FEED (XXKSITION

This can be categorised into tin0

i) &The effect of the rawrreal chemis

ii) ‘The effect of the rawrieal&

try after being rrcdified by

recirculation.

@YcementrawrEa1s

Tit

mly differ in the absolute quantities

of volatiles, but also tt*o meals -tith the same mlatiles amtent

may have very differe%Y' eralcgy. Mrk by Baker and r~tmarrC81' for

exarqle, has that certain minerals have a greater propensity to

encourage the ationof SO3 in particular. These mineralsinlpartanind ' ualcfiaracter to the particular rawmaal. and there iseno way as yet estimating their effect upon the recirculation pat-

tern ex4%

empirical techniques.

@irculation of the mlatiles themselves causes the content of

volatiles entering the burning zone to increase. This has tm affects

upn the burning characteristics of the raw meal:-

i) Both the armuntof flux within the burr&g zone and the

apparent length of the burning zone will tend 'io T. in

increase in kiln amps due to increased volatile load could, for

example, be mistaken for an indication of overburning and any

resultant reduction inburning zone terrrperature axld lead to

an increase in clinker free lime. An increase in fines within

the systemwillalso tend to encourage clinkerw

ctions

which in turn should decrease the free Ii, axtent.

77T7

ii) The evapration or disocciation of the wlatlles in the burning

XTzone will de-r-and a significant quantity high grade heat fran

the flame. ?%is high grade heatk

ver, is recovered further

dcxn the kiln as lw grade h t when the wlatiles recondense.

+Pa estimation of this 'heat ' effect" is shown in appendix

3. The overall effect * tiat the recirculating wlatiles make

%a significant demand (up 100 kcals/kg clinker) upon the heat

atilable in the zone andhence they tend to increase

the totalheatinputreqirementto the kiln.

\f&s could aggrevate each other and with the net

system, raw

mealch 'stry, and the oontrolstrategy smployed.

+

3.5 THE EFFECT OF RETURNED lxiZ?I

A rmjor differenc:e between the idealised cycle and the real

4.C

situation is, that although much of the recirculated mterial

condenses cm solids derived frcan the kiln feed, these solids are

generally held in suspension and therefore swept frcm the kiln in the

gas stream. If the dust is returned, especially by lation, then

the observed cycles should bqin to approach th

mixing factor (E) will begin to approach unity.

YThe greater the armmtof dust return , the greater the

recirculating load and hence the clos clinker mlatiles till

to the system (except for

at t-ratures within the

burning mne is considerably ter than cne at-sphere and hence, ir

%theory, should have an k-finite pacity for volatilisation).

e0

IMPLICATIONS OFKKx4T&E FECIRCUIATION~ PIE Pm C?N'I'floLOF

mARY IiIL~S

This s4

discusses the implications that mlatile

recirculatio has onkiln mntrol.

42

ijherever possible the individual

factors dis ed in the previous section are related to kiln cofkrol

parame sfo

uch as burning zone temperature and kiln feed rate. SOme

of the factors such as recapture of mlatiles will tend to be related

toc&-ss design rather than any &aarameter tNhich can be altered as

part of a control strategy. These should be rmted, as it is highly.

probable that any control strategy considering mlatiles recirculation

muldbe dependent to a certain extents the type of,orocess.

The quations given in table lcgl shm the iqxtance of

temperature as a factor amtrolling mlatile recirculation, because

the amunt of recirculation has a logarithmic relationship to

temperature. In a dxy process kiln, the usual strategy is to control

the kiln to produce clinker to a selected range of fr 'w

e cmntent

by mnitoring the kiln amps. This signal is a function of, amngst

vY7other things, the amount of flux present in the )u . However, in

section 3.4 it was pstulated that volatileT

irculation will mxlify

the armuntof flux in thekilnwith litt or m change in the fuel

&inputratetothekiln. Thekilnmps ' therefore be dependent to

sane degree cn the rragnitude of the tile cycle.

iii?

Akiln control

strategybasedmthe amceptof ' ining a steady level of kiln

amps is likely to disturb the%

'les cycle when recovering fran a

process disturbance khich '

-Q

in turn affect the ammtofkrning

zone flux and hence kiln For example, a perturbation in kiln

feedchenistrythat'%

eases mlatiles till tend to,increase 'kiln

Zip. react by reducing the BZT to maintain the

amps set pint the clinker produced would be both underburnt

in mlatiles.

Q tant coal feed strategy is similarly insufficient to

makltain le kiln control, as any increase in recirculating

Qvo1tiwill tend to depress the temperatures in the burning zone

andhence theheat inwtto the kiln will heed tobe wied in order

to control the solids temperature andhence the clinker free lime

content.

In order to mintain a specific temperature in the burning zone,

the best ,Darameter to consider is therefore the peak feed teqerature.

Unfortunately present day instrumentation is unable to supply a

reliable direct indication of ,oeak

method of establishing a peak feed

parameter qon which the peak feed

feed temperature. An alternative

temperature is to measure the

temperature depe fhe W

gas teqxxature. This maybe carried out indirectlyby&asuring a

parameter dependent upon the peak gas temperat

This will then be a function of the peak

that the feed rate is n-aintained at a constant level. The

relationship between NO, and clinker So4

as alreadybeen discussed

(see figure 2). Further evidence fo the relationship between wlati-

les content and N& is show in fi&

. Here the kiln exit So2

concentration is seen to foil exy step change in Q, highlighting

%the particularly strong correla ' between these tko ,oarameters.Clll

4.2 GAS CXJANTI'IY 00

%?This, as mention previously, is principally a process design

variable. Any on the mlatiles cycle due to gas quantity

changes assoc' ted with fuel changes are liable to be masked by the

greater s4

* ty to changes in the burning imne temperature.

The appr

Q

te strategy txxards gas quantity control should therefore

be-try mintdin constant gas flew through the kiln together with

a t feed rate.

4.3 MEQNG EFFICIENCY

This factor is essentially determined by the process design.

As discussed in section 3.1, the efficiency (E) is determined by a Ccnr

bined function of the kiln speed and 'kiln slo,pe which determines the

amount of effective surface area of feed within the burning zone. As

a consequence of the gecmetry of the circle, a significant change in

volume loading will only have a small change in the effective surface

area of the feed within the typical limits of kiln volume loading. As

thekiln angle is fixed, thekiln speed is the cnly cess'FIT

le para-

meter tich can have any influence ctl the wlatile cycle. It is

postulated that this influence should not be larF

rasmallrange

of kiln speeds as there are tw opposing effects. An increase in kiln

32speed would increase the rate at which effecti surface of feed is

exposed for volatilisation to take pla+L

but decrease the time

available for evapration. I t is p tulated that the likely effect

is for them to cancel each othera

'In speed is therefore

unlikely to be critical in a constant level of

volatile recirculation.

\This is similarly a function of process design. Changes due to

this effect mule

causetl in bet process kiln by altering the chain

design but ges during operation of the kiln are unlikely to affect

avolatiles r ure. (Gas wition tich affects this will be

discussa

the next section).

4-5 =fFF=As was stated in section 3.3 a &ange in oxidising~tentialof

the kiln gases can have a large effect cn the recirculation of SO3.

If a strategy of maintaining a constant wlatiles cycle is to be

achieved, a constant back end oqgen level will be necessary.

4.6 K!ZN FEED CMFOSITION

The effect a2 kiln mntrol of the kiln feed rrcdified by mlati-

les has already keen discussed with reference to temperature in sec-

tion 4.1. If%wever, even if a constant level of recycle is rmintained,

there stillmyke changes in the feed chemistry entering the'azming

zone due to changes in feed &en&try of the raw f cau edby

-9

inade-

qUate blending. Pius, as with the strategyofhrning a target

free line ken &anges in LSF of the feed muldv

instability, a

control strategy based cm mlatiles also r*es mw feed of

consistent chmistry.

4.7 DUST KEI'UEN

A dust return system tends rate 01 a discrete level (ie.

either on or off).

return systan till dis

tiles input will affect

culating load is

thatwuldbe affected

either turning cn or off a dust

cycles as the change in iciln wla-

Ps the size of the recir-

dependent, the mjor factor

- the mixing factor. 'Ihis muld

take cm anew%

and a mew volatiles recirculation equilibrium

bymaintaining a steady Wrning zone tmperature.

lised volatiles cycle within a rotary kiln can be deve-

kncwledge of the temperature profile and gas flows within

the kiln system.

ii) The magnitude of the recirculating load of mlatiles in this simple

rmdel is principally a function of the peak feed temperature. Another

factor, the quantity of gas per unit of clinker, is iqxxtant since

it determines tie mxirmm mlatile recycling capacity of the kiln

gases: i.e. the wet process through its higher

in more gas being available to otrry a larger

Conversely, a precalciner with its smaller gas y within the kiln,

owing to 90% decarbonation and the burning of -1 outside

%kthe kiln in the preheater, is unable to supp uch a high level of

volatiles as a suspension preheater ki wet L;rocess kiln.

iii) The idealised cycle does not the observed situation.

Ffowaver, through studying actual cpen

ing data the various effects

that influence recirculation estimated and a mre realistic

model developed.

iv) The four principal fa(;tors tit alter mlatiles recirculation baha-

\via= fra the ideal cyc .e are the gas/solid mixing, kiln atmqhere

fflation and feed amposition.

The level of solidmixing in the bnxning zone is a function of the

kiln mime4!ti1 '

residenQ

g and the ambined effect of kiln speed and feed

') e. l3e effect ofpr mixing wuld appear tobe to

the actual quantity of wlatiles recirculation, to around

l/lOth that of the level predicted assuming an ideal

cycle.

The gas/solid mixing with reference to the recapture of volatiles

material is a function of the type of process e.g. suspension pr*

heaters are highly efficient gas/solid mixers at the kiln 'back end

VI The effect of a reducing atmosphere is to reduce the quantity

contained in the clinker and increase the So3 lost as SO-2 via

stack exhaust. wvi) IXlst insufflation till typically cause a large in the

volatile recirculating load.

vii) Volatile recirculation will tend to indu a cyclic pattern of beha-

&viour for the quantity of volatiles retain in the clinker hhen

clinker is burnt to a amstantfree '

e?

wntent.

The cyclic behaviour of the re

@%z

ating volatiles muses changes in

the apparent burning characteris s of the clinker byrzdifying the

quartity of flux in the uming zones and also induces

a change in-the tomintain a constant

recirculating load.

whereas wet process kilns are mch less efficient.

of so3

the

QThis cyclic b&ml mkeskilnmntroltoconstantfreelimebasedcxlki e exceedingly difficult. Aamtrolstrategy to n-&n-

tain a 03 tan level of volatile recirculation will rezmve this

cyclic 43OUT. This stability can be achieved by titaining a

burning zonetmperaturewith aoonstantfeed input. Wrk

out at Hope Works has st-hown that the best relationship betm

an observed kiln parameter and the level of recycle was found to be

PI%. This muldman that a aontrolstrategybesed QIQ shouldmin-

tain a asnstant level of recirculation and allow the free lime content

6. FBXMGBDATIONS

i) E'urther mrk should be carried cut to identify the mlue of E, the gas/

solid mixing factor, particularly with reference tc the kiln parame-

ters of volume loading, kiln speed and kiln angie. This factor, along

with the peak feed temperature, kould allowarmre acwe estimate

of the level of burning zone tolatilisation to redicted.

T?Y7

ii) The relationship bst*en clinker sulphate an F.Dx should be further

\studied to envss the effect of ,mrameters s as 3.E.02, feed am-

position, ~1 consmption and kiln ou6

.

iii) The effect of oxidising or reducin sphere cc mlatile cycles

should be studied further. %zCurrent

4

ry and practical results indi-

cate an inverse relationship the quantities of SO2 and 02 in

the gas stream and hence ect relationship between SO3 in the

Qclinker and 02 in the gas s .

iv) 1The effort nf raw ek , ergiccv &c&d be examined~~im-order to

atterqt to quanf;

%its effect m the ideal cycle.

VI The net eff

e

the thermal requirement for evaprating and cm-

densing c' cul ed vclatiles reeds to ke detemiined mre specifically.

This is&icular relemnce where preheater cleaning causes sudden

SIX f mterial with a high volatile cmntent to enter the kiln.

c3vi) Control strategies based on Mx to maintain a constant mlatile

recirculation load should be developed and emluated in preference to

those based on kiln amps and a constant free lime ontent.

7. F33ERExcEs

1)

2)

3)

4)

5)

6)

7)

9)

10)

Weber P. Heat Transfer in Fbtary Kilns with due Regard to Cyclic

Processes and Phase Formation. ZKG tiglish Special Edition 1963.

Longman P.A. Swift LG. Oxford Mrks -~twem Dry Process with sulphur

By-Pass - An asseswnent of the technical Risk.‘ SI'N 81/13.

Rogers A.R. Assessment of the potential to q-tin&sewperformance by

application of improved prczess sensors : tinit *

Kiln September 20-24th 1982. TN 82/28. vY7

of I-bps wxks

Coyle B.W. and Fenk F.W. Applications of%

gas analysis to wnent

works operation. FwkPrcducts, Nov19 .

Brmm A.W. 4.2Retention of Alkali and SuLph in Clinker. FkSXCh

Division Repx~tis 1 and 2 SP-65/33

Chadbume J.F. 9

, SR-65/7/M&3.

Continuous i% 'tar' g of Gaseous Emissions an Cement

Kilns. Paper presented to4

d Meeting of the fir EMlution

Control Association, Jun 9.

QLorimer A.D.J. cklline 802 nitoring : Assessment of LXXJV and

Electrcchemical cell%terns at Northfleet : May to October1984.

m/84/21.

EBkr D.C. andkA. 8ul.phu.r mllution &an coal ambustion.

Effect of ' Cxnponents cn the Them&l. Stabilities of sulphated

Ashand

@@

cium S&hate. EZlvironmental Sciences and Technology,

March .

m

. The processing of Cement Flue Ibst, PhD Thesis, ?he

University of Aston in Birmingham, December 1979.

Haspel D.W. Le@ Grate Mt Transfer bechanisns. TM. IxJH.005,

Weardale bbrks, 1973.

11)

12)

13)

Gq2pelle M. Analyser 2.3nk at the ?A &3vre lbrks. CETIC mviromt

Sub-m-mission September 1984.

Turkdqan E.T. and Olsson R.G. Desulphurisation of Wt Reducing Gases

with Calcined l%lmite. Iron Making and Steel ?&king No. 4, 1978.

Myers J.C. Sul&ur Balances in @m%t Kilns and ilers. !4Sc

Thesis, University of Texas, Austin, 1977.

D. stenson&De r 1984

@Y

(i)

APPENDLyl

THM)RIES OF FECIFCULATICN, 3-E CTINSI'X~ION OF IDEAL CYCLES

The volatile aanponents of aarnent raw mals and fuels are

vaporised ken the feed temperature is raised to a level tvhere the

vapour pressure of the volatiles bscunes significant. The wlatile

caqments will then impart a partial pressure to the gas stream. Cn

cooling, the vqmur pressure of the arnponents will decrease causing

condensation. E?y measuring the kapour pressure of certain cmpments

at different temperatures, enpirical relationships for the saturation

vapour pressure and temperature have been established. Equations

relating saturation vap0u.r pressure to temperature are given in

table 1, equations 2 to 7 and shown graphically in figures (i) to

(iii) C91.

The saturation repour pressure can bs used to calculate the

saturation vapcur mncentratiun by applyirlg Raoults Law:

vf = P* Mv (1)PT Gj--

where vf is the saturation concentration of bap3ur in the

carry% gas (kg/kg 9as) l

P* is the saturation vapour pressure (any pressure unit)

P is the pressure of the gas (same units)

Mv and m are rm1ecula.r lHeights of the vapour and gas reqpec-

tively. An example for the saturation axcentrations of I@1 and K2S04

in air is shown in figure iv.

Examples of bw to construct a volatile cycle using equations

ltc 7 is shcmlbs!low.

(ii)

The first stage in oonstnxting an ideal cycle is to obtain a

feed and gas temperature profile. For this, kxwledge of the ox-

binability of the particular feed for a particular targetted free lime

must be available. This can be cbtained fran empirical data. This

will give the pfaak feed temperature. In the exarrples shown below the

peak feed temperature for the wet process is lSOO°C and 1490°C for the

dry process. Azonedheatbalance rrcdel for the kiln can then be used

to derive a temperature profile. Fbr cur examples the temperature

profiles are sho+m in figures v and vi (the oscillations in figure (v)

are due to graph interpretation; thecurveshouldbemth). By

use of equations 2-7, the partial pressure of the volatile QsmFonents

maybe calculated. Equation lwill then give the saturation vapour

concentration. The mlecular weight of the gas is estimated frcm its

ccqosition. The calculations are sumnar ised in tables 2 and 3.

By mking assumptions for the gas quantities at various &mints

in the kiln, the actual amount of mlatile &per !kilc.grarn of clinker can

be found. 2-ii.s is s-ised in table 4.

Table 4 can be used to amstruct an idealised cycle as seen in

figures (vii) and (viii).

Ekplamtion of the idealised q&e

To explain the idealised cycle, an example of akiln to&ich

is fed the equivalent of 2.0% K2SO4 in the feed and fuel is 0311-

sidered. Cm the first cycle the feed will be tranqmrted by the

turning action of kiln until, in the case of the dry process in figure

(vii), a pk.nt4.5 diameters frm the rrxe ring is reached. Here the

feed reaches a teqerature where significant empration of K2SO4 Qn

(iii)

occur as K20 and sO3. The m:.atile afnpments are taken up 'by the gas

stream &ere they rapidly achieve gas tmperature and are tranqmrted

away fran the burning zone to the back end by the kiln draught. At

twelve 'kiln diameters the gas tenperature has fallen to a level tiere

significant condensation as K2SO4 on cccur. This is then inmx-

porated txck into the feed and transported fomds along with the

fresh K2SO4 fran the kiln feed.

On the second ideal cycle there is an quimlent of 4.0% ~2~04

in the kiln feed approaching the burning zone. The cycles will con-

tinue until a +.ntattw kiln diameters tiere the riexi~um allckable

evaporation determined by feed temperature is reached. Ebr the dry

process curve this is 16% m clinker after eight idealised cycles. Cn

the ninth cycle the K2SO4is raroved fran the kiln in the clinker as

the equivalent of 2.0% K2SO4.

The corrected cycle

The idealised cycles shown in figures (vii

followzd in practice.

) and (viii ) are rx3t

The factors of 'mixing efficiency in the burning zone and

backerd, dust loss and reducing conditions all account for deviations

fran the idealised cycle. Tables 5 and 6 give results of calculations

based m assumptions of suitable Mlues for these factors. Fran fhese

tables, figures (ix) and (x) are cbtained. 'Ihe solid lines cn figures

(ix) and (x) indicate the 1evels of mlatile corrected for the mixing

efficiency. The broken lines show further axmctions to decrease the

amount of material recaptured in the cooler parts of the kiln due to

dust loss and reducing conditions.

TABLE I- - ^ - - I - -Saturated Vapour Pressure of KCl, K SO and Na SO3--4- Z-4

Reference

LoglO (p,/atm) = - '47oo 5 9oo

Decomposition Suppreoscd

Loglo ( ps/atm) P ' - ' "O" 5 'O"

Loglo (po/ntm) 6.23

Hart and Laxton, 19671llalntead, 19701970(cf. (cf. Koaugi,Koaugi, 1967;Duboia et al, 19613)19613)

Cubicdfotti andDccomnPnit.lPn, ‘ Kuneahea, 1972

I Loglo (pa/Atm) = -y -t 7*oe1 I 1 4 0 0 1 4 0 0 -- 1 6 2 5 1 6 2 5

g 0 . 3 0t:uh

5“,g 0.25-

‘0Lla52 0.20 -

0.15 -

0.10 -

1200 1250

Temperature (“Cl

Saturated Vauour Pressure of SC1

6omce: JANAZ', 1960 or J.snz, 1969)

3-

.5-

.o-

S-

10 -

OS-

TJ,zzz ( ii)______---s-.-

Decomposition unsupprrsscd

A- DecomposXon suppfms*d

I I I 11150 1200 12so 13M) 1350

Temperature ["Cl

Saturated Vawur and Decomuosition ?ressures of K,sO.

ISource: Eart and Lax-ton, 1967; Salstead, 19701

1.c

0.'

0.f

z 0.1

ri)Tz

; 0.63::v&

g 0.5

Pv1

",1 0.4

;;:

0.3

0.2

0.1

0.0

?ICc?z (iii)------------

Temperature ("Cl

Saturated Vaoour and Deconuosition Pressures of Ha SO

(Source: Cubicciotti and Xeneshea, 1972)2-4

.Cu

C0.-

1V

Temper abre ( l C )

Saturation Capacity of XC1 ad IT SO Vau0u.r2 - - j - - - - - - - -in Air from llOo-l’jOo°C

FXCURL (v) F e e d uh ps tempeniturc p-oSi1.e :wL. t PI~oce:is_..-------- ------..__ _..--__ --^----_ -----..------------.--

6 - F E E D TENP6 - GM TEW

25002500

20002000

SD0SD0

E’IGUNL ( v i )..--------c-Feeh and- - - L - - d - -- - - L - - d - - gas temperature proTilL?proTilL? :I)ry:I)ry ‘rOCCS9- - - - - -- - - - - - ^^-CI________------L.-------^^-CI________------L.-------

0 -- FEELIFEELI TEHP

“.-.__-_

.-.~-‘.-7*‘- -2 o *-..-. .__ -.. _. _

-i’ --i+---- “--T- -.-+-- -+..--

+s-.-;‘s-- 1 21 2-,-,-- -I-. _-,

1010 1 11 1 14 15 IE

K I L N OIFlMETERS F R O M N O S E R I N G

0

1

2

3

4

5

6

7

8

9

1 0

11

12

13

14

15

1 6

Gast

WC

500 1250

600 1350

1150 1450

1500 1500

1870 1350

2om 1260

zcoo 1130

1950 10(X

1870 950

1710 910

1550 850

14.60 810

1370 axI

1330 730

1290 690

1250 650

1203 600

i

Sabrated Concentrationi.3 qas (!&kg p.5.)

Kcl K2So4 -2504

0.601

knit*

. .

45.27

5.62

2.46

1.16

1.99x10- 3

0.169

9.95

infinity

0

.25x10-3

0.232

I.

I,

9.95

1.45

0.281

0.111

0.040

0.0246

4.29

0:429

0.140

0.042

0.0239

0.0148 0.0131

3.67x10-3 .02~10-3

~27~10-3 -06x10-3

.2.$6

infinity

*,

2.93

0.474

0.091

0.046

0.023

9.65X10-3

4.71x10-3

3.9SXlo-3

9.59x1&4

K2SO4

3.07x10-3

0.031

0.099

0.169

0.031

3.94x10-3

1.45xlO'3

?ia2SO4

7.02~10-3

0.032

0.123

0.232

0.032

3.24x10-3

3.64xlo-3

T?%aLz 3. DE ?5cGss

soO f

Ciln

liameers

Feed.tcqY

WC

0 605 1370

1 aao 1417

2 1594 1460

3 2110 1416

4 2274 1273

5 2227 950

6 2106 925

7 1940 8%

a 1799 a74

9 1678 a56

10 157s a42

11 14a5 a30

12 1407 a21

13 1338 al4

14 1277 ma

15 1224 803

16 1177 798

-

Satc.zit+d Concemraticnia 9 (kq/kg 9s)

KC1 K2So4 -2So4 XL K2SO4 .*2SO4

0.434

infhity

,.

430.0

3.465

0.983

0.360

0.144

o.ffi14

0.0272

0.0125

;.03x10-3

3.02x10-3

0.737

infinity

6,

84

4,

a*

159.4

2.208

0.583

0.192

0.0700

0.026a

0.0108

t.59x10'3

2.05x10-3

L-lfinhy

I.

4.

*a

3.78

0.046

0.031

0.0204

0.0144

0.0107

a.4xo-3

6.a2x10-3

5.81x10-3

S.llxLO-3

4.57x10-3

4.17x10-3

3.79x10-3

0.0400 0.0423

0.0688 O.oaol

0.111 0.140

0.0681 0.079

0.0118 0.0101

TAaLz 4.

(xiii)

Sal 33LriJ Tl,ss

0

1

2

3

4

5

6

7

a

9

10

11

12

13

1 4

1s

1 6

11

12

13

1 4

15

16

2 0

KC1 K2g4

in5.nity

. .

514.0

6.6

6.1

4.65

2.07

2.12

1.67

1.37

1.16

1.02

0.96

0.03

0.75

infinity1.

817

324

170

5.72

9.04

LS.87

9.74

1.69

6.25

11.45

20.02

lo.a7

1.44

28.4 0.382

12.2 13.9

5.45 5.33

2.49 2.14

1.20 0.913

0.61 0.408

61s-.1n:sr+y

I.

732

14s

27.9

14.1

7.65

2.9s

1.G

1.21

0.29

Sas c3 solid

infizity

,a

13800

1719

752

354

0.44

2.17 1.7S

7.75 a.00

24.7 30.1

$2.23 33.0

7.75 a.0

2.4.a 2.06

0.44 0.264

33.9 42.3

12.2 12.3

7.32 7.31

4.53 4.01

2.65 2.1s

1.31 0.94

.%2SO4

Dry mess124 scc/x;$ :Ks ‘aup w

5 diamzter

1.43 kg/k; clizkz

Aker 3

Diemeters

1.99

wet prccess213 sx/lZi :s ?dUp to S

Oiaetezs

2.499 icg/kq di-k

After 5

&meters

3.059 kg/k;/cli*e

Rt:t

.c

+.h

so

50

40

so

20

10

0

FIGURE (viii)--1----1----- Idcnliscd cycle for NA2SOlt-----_----------^---__^___

6 - D R Y PROCESf.!l - Y E T PROCES

KILN DIRMETERS FROH NOSE RING

(xvi)

Table 5

Corrected Cycle for Mixing Efficiency

No. ofkilndiameter

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

11

12

13

14

15

16

! Concentration (% &per Kg TClinker

KC1

Infinit

8,

I,

0.8

.32

.22

.8

0.57

0.42

0.33

0.27

0.23

0.20

0.18

0.17

0.16

lilfinit

I,

856

340

Dry E2m !SS

K2SO4 Na2SO4

1.14 1.21

1.97 2.29

3.17 4.04

1.95 2.17

0.34 0.29

30.3

12.89

5.72

2.61

1.26

- -

40.1

14.6

5.60

2.25

0.96

1 179 1 0.64 [ 0,43

Concentration (S pr KgClinker

KC1

123

Infinit:

‘I

‘I

1,

146

29

5.50

2.82

1.53

0.59

0.29

0.24

0.06

Infinity

I,

15870

1975

865

IGet EYOI

K2SOq

0.434

1.55

4.94

1.55

0.496

0.09

39.0

14.0

9.02

5.21

3.65

ss

Na2SO4

0.35

1.6

6.02

11.60

1.6

0.412

0.053

49.2

14.7

8.41

4.61

2.47

TUSlimptions

%2 = 0.2

EBE m

= 0.95

= 0.85

hii)

11

12

13

14

15

16

11

12

13

14

15

16

11 146 192 la7 236

12 61.6 70 67.2 70.4

13 27.4 26.9 43.2 40.4

14 12.5 10.8 25 22.1

15 6.04 4.60 17.5 11.8

16 3.08 2.08 7.24 5.2

bxclusicn (1 per kg Clinker)cry prccess T

infinity4.

1027

408

215

36.4 48.1

15.4 17.5

6.86 6.72

3.13 2.70

1.51 1.15

0.77 0.52

121 160

51.2 58.4

22.9 22.4

10.4 9

S.04 3.a4

i.56 1.72

CmclL;sim (3 per kg Clhkr)Xet Praess

KU

infinityI .

19KXl

2370

l@iO

4aa*

QSO4 Xa2SO4

46.8 1 59.0

16.8 17.6

10.8 10.1

6.25 5.53

4.38 2.46

1.81 1.30

!x5; lass

= 20% cn

Clinker

156 197 Pfhdng

56 58.8 Cmditioixi

36.1 33.6 SQ3

20.a 18.4 RecoroirationS

14.6 9.88 decxe.asedby

6.04 4.32 75%

ca7biii

ZffeG

?ss.ui&ons

~‘IcilJlikb ( i x )..---------.wCorrc?cLcd Cplc i-'or KL'CO'I_____---.---- --^------.--.---

2 0

16

16

14

1 2

10

6

8

4

2

0 f0

< i.i;oLlt, 'ITJ GA@ - D R Y HIXINC FRCTOR

X

CJ- YET HfXfNG FACTORA

3. &if 'I'0 SOLIIJ- DRY DUST LOSS V

0 - YET DUST 1x - D R Y REDUCIx - Y E T REDUCI+ - D R Y CBH8IIy - Y E T CbHBIt

6----I

--w-e --I--- l\ I I

Ix \ ,,\ 1\ >(\\

\

I\ I

1\\ -I- ’ ’

\ , ’

‘Q x y\

\ J ’

-\ \ \1\ ’\ ’

\GL \

KILN DIRMETERS FROM NOSE RING

FIGUHC (x)------.e--- C o r r e c t e d Cplc l*‘or IJA2L;ch_-__..------- -^I-------c----

6 - D R Y tlfXINC FRCTCJR < SULID ‘I’0 cxs

2 - Y E T tiIX1Nf.i FRCTOR cA!i TO SOLID- D R Y D U S T LOSS

@ - YET DUST LOSSx - DRY REDUCINGx - YET REDUCING-t- - D R Y COMElINED‘f - YET COMBINED

K I L N DIRMETERS F R O M N O S E R I N G

Effect of Reducing Conditions cm the Volatility of Alkali Sulphates

The precise mechanisins by which reducing conditions within the kiln

remove SO2 frm alkali sulphates is not fully understood. Eiowever, it is a

real effect observed and documented by my other mxkers in this field, e.g.

Chatiume , 4 and by Brcx& fran within Blue Circle;

A similar chernicalprccess to the removal of SO2 is the desul@rising

of steel using lime. This has been extensively studied by Turkdogan and

01ss0n. 021

The reactions described by Tuxkdogan and Olsson involve only calcium

sulphate. However, for cur plrFoses we riay irake the reasonable assumption

that the alkali sulphates till undergo similar reactions, as is suggested by

their relative psitions in the periodic table of elfments.I

Calcium sulphate will deccqcse under the effect of heat alone by the

reaction :

CaSOq(s) = CSO(~) + SO2(g) 4 h$2(g) &Gs = 408 W ml-l

Wwever under reducing amditions the follwing reactions will be

preferred:

(1)

Ea.504 + cqg) = cao(s) + S02(g) + ~203~) AGs + 15 Id ml-l (2)

=04(s) + e(s) = =(s) + “co(g) 4@ + 297 kJ m31-1 (3)

C&04(S) -+ 4CO(g) = CELLS + 4C02(g) AGS - 183 kJ ml-l (4)

where&? is the c%ange in Gibbs F'ree E&qy at 25°C and L atmosphere

Reabsorption of SO2 till corer bythe reactions:

$CaOo) + 4SO2(g) = 3CaSO4(,) f c~s(~) ;lGs - 787 W x11-l

or in the presence of oxygen by the reaction:

-o(s) + SZ(g) + $02(g) = -Q4(s)

The wlue:GS is an indicator of the

occurring under standard conditions (289K

.GS - 409 kiJ id-1 (6)

pssibility of reactions

and 1 amsphere). If the mlue

ofSGs is negative, then therrm$mmically the reaction should proceed qmn-

taneous1y. Generally the mre Ixksitive,UY, the rmre heat is required in

order to m&e the reaction take place. Thus cne would expect reaction 4 to

be favoured rather than reaction 1.

This analysis takes m amount of the kinetics of the reaction.

Howaver, sane idea of the reaction kinetics under oxidising and reducing

conditions my be c&An& by mnsidering the relative reactivities of SO2

ami so3. ?hermlecule SCl3is ilDre reactive than sO2, both in terms ofkine-

tics and therrr&ynamics, so the preferential formation of So3 rather than

SO2 wuld lead tomre sulphurbeing in a reactive formtiere it muld

react with tie available GO, KS, t&20 etc. 'Ihe equilibriabetween So2

and SO3 have been studied by LMyers.C13] Fran the eqirical equations

obtained byMyers, the graph in figure 2 (i) can be axstru~ed. This

shms that the higher the percentage 02 present, the more likely is the for-

mationof SO3 andhence the formtionof sul+ates whichtill. be mptured

bythemcuningfeed. The lcwoqqen curves shas thatsulphurwill be

present as SO2 which can escape in the'kiln exhaust. Ektreme reducing

conditions lead to the formation of H2S reaqnisable by its “bad egg"

odour: inthis case therewuldbe virtuallymrecapture of sulphurbythe

on-feed.

(xx-ii)

00

%S

t0

00

00

0

0a

PDu

a-

n

::

4‘.

d

EQ

S’Q

I

20

s

NLlIS1J3hN

83

x

(xxiii)

APPENDIX3

Heat pipe effect of volatile circulation at F!op? ibrks

i) Latent Heats of Vapourisation/Disscciation N.B. V = VqxmrisationD = Dissociation

KC1 + 670 Kcals/Kg (v)

K2S04

Na2SO4

+ 563 Kcals/Kg+ 1805 Kcal.s/Kg

Obtained fran Khor+ 733 Kcals/Kg+ 2096 Kcd.s/Kg (A?

c91

caso4 t 675 Kc&/Kg (D)

Recirculating volatiles

K20 2.457 % on clinker

Na20 0.149 % on clinker

SO3 4.509 % on clinker

Cl 1.092 % on clinker

This is quivaledto

KC1 2.29 % on clinker

K2SO4 1.94 % on clinker

Na2SO4 0.34 % on clinker

CaS04 5.46 % on clinker

Therefore Heat pipe effect, assuming all mqxmznts disscciate fully, is:

KC1 16.46 Kcals/Kg clinker

K2SO4 10.92 Kcds/Kg clinker

35.02 Kcals/Kg clinker

Na2SO4 2.49 Kcds/Kg clinker

7.13 Kcals/Kg clinker

CaSO4 36.85 Kcals/Kg clinker

Total 108.87 Kcals/Kg clinker

Therefore rraxinnrm total heat pipe effect for I-bps is 109 Kc&s/Kg clinker.

Blue Circle Cement


Module 13

Section 5

Alkali Volatilization- A Review ofLiterature Available in 1977

ALKALI VOLATILISATION

A iU3VIEFw OP LITERATURE AVAILBBcg IN 1977

Information from 119 published references has been collated andcritically reviewed. The effect of alkalis on clinker properties and kilnoperation are briefly discussed, followed by an outline of the origins ofalkalis in raw materials and fuels and of the published physicalproperties of relevant pure compounds.

Known physical and chemical factors affecting volatilisation are assessed- a key objective of this review. Finally, information on thedistribution of alkalis in clinker is presented, together with an outlineof initial attempts at theoretically estimating recirculating loads and ofpractically reducing their magnitude.

From the review it is concluded that there is general agreement on thefactors affecting the volatilisation of alkalis. Volatilisation isincreased by smaller nodule size, lower flux content in the clinker,higher levels of chloride, lower levels of sulphur, higher temperatures,longer heating periods and by higher contents of water vapour in the kilngases. It Is not clear what the effects of other factors are (over theranges studied in the available literature), and it may be inferred thatthese are relatively small: gas flow rate and composition, alkali content,presence of fluoride, LSF and S/R and oxidising conditions in the kiln..Several of these factors appear worthy of further study.

There is, nevertheless, insufficient quantitative evidence to predict thedegree of volatilisation of alkalis from a given raw mix under particularconditions. The interplay of phase equilibrium and transport phenomena isprobably too complicated for it to be worth while attempting to simplycalculate volatilisations from known thermochemical data: more complexmodelling is needed.

In practical terms, the circulation of alkalis is affected as much bytheir condensation and deposition as by their volatilisation. It would beuseful to devote some attention to this hitherto somewhat neglected aspectof the phenomenon.

ALKALI VOLATILISATION

A REVIEW OF LITERATURE AVAILABLE IN 1977

C O N T E N T S

1. INTRODUCTION

ISTN S7/4

Page Nr.

1

2. EFFECTS OF ALKALIS 3

2.1 Effects of high alkali levels in clinker

2.1.1 Alkali-aggregate reaction

2.1.2 Air-setting of cement

2.1.3 Effect on strength

2.2 Effects of high alkali levels on the manufacturing process

2.2.1 Effect on kiln fuel consumption

2.2.2 Corrosion of refractories

2.2.3 Build-up and preheater blockages.

3. SOURCES OF ALKALIS

3.1 Alkalis in limestones

3.2 Alkalis in clays and shales

3.3 Alkalis in fuels

3.4 Methods of reducing alkali ,zontents of materials

4. PHYSICAL PROPERTIES OF ALKALI METAL COMPOUNDS

5. FACTORS AFFECTING VOLATILISATION

5.1 Physical factors

5.1.1 Nodule and particle size

5.1.2 Effect of the liquid content of the clinker

5.1.3 Gas flow rates

5.1.4 Duration and temperature of heating

a

11

12

Cont./...

-ii-

N r .Page

5.2 - Chemical factors

5.2.1 K20 and Na20 contents

5.2.2 Effect of chloride and fluoride

5.2.3 Effect of SO2 and SO3

5.2.4 Effect of water vapour

5.2.5 Alkali containing minerals

5.2.6 Effect of LSF and SR

5.2.7 Effect of kiln atmosphere

6. DISTRIBUTION OF ALKALIS IN CLINKER

7. ALKALI CIRCDLATION IN THE KILN SYSTEM

7.1 Estimation of alkali circulation

7.2 Reducing alkali circulation

7.2.1 Removal and treatment of flue dust

7.2.2 Bypasses on preheater kilns

7.2.3 Other processes for alkali reduction

8. CONCLUSIONS

9. REFERENCES

2 8

31

4 0

10. NAME INDEX TO REFERENCES

11. TABLE 1

ISTN 87/4

ALKALI VOLATILXSATION - A REVIEW OF LITERATURE AVAILABLE IN 1977

1. INTRODUCTION

The following report summarises published information available

in 1977 on the subject of the behaviour of volatile alkalis in

cement clinker manufacture.

.These materials are distinguished from other clinker-forming

materials by being solid at the lower temperatures encountered in the

kiln system but vaporising at higher temperature - for instance in the

kiln burning zone. The effect of this is that they circulate within

the kiln and preheater system and can, under.certain circumstances,

attain high concentrations. The quantitive effect on alkali levels in

the clinker on removing some of this recirculating material by a

by-pass may not be immediately predictable.

Together with the compounds of the alkali metals sodium and

potassium, it is also convenient to consider other compounds involving

sulphur, chlorine and (less commonly) fluorine, which have boiling

points within the same range and which can exhibit similar behaviour.

Interest in the behaviour of alkalis in cement manufacture first

arose in connection with the possibility of obtaining potash for use

as a fertiliser from clays, feldspars and other unconventional

materials (I). A considerable amount of work was done on this topic

in the United States during the First World War, as at that time most

of that country's supply of potash was imported from Germany. In this

application, it was necessary to ensure that as high a proportion of

the potash in the raw feed as possible was volatilised and recovered

in the flue dust, and that this potash was water-soluble: this latter

feature could be achieved by heating in an oxidising atmsphere (2).!

This m&hod of producing @ash was uneconmic when ample supplies of

potashwereavailable fromother sources andsomrkwas discontinued

after the early 1920's.

The problem of alkali levels in CempJls was once mre brought

into pmninencewheh it was suggested (3) thatcemants withhigh

alkali levels reactedwith certainaggregates, suchas scxw chertsr

shales and limestones, to produce destructive -ion- This led

various national specifying authorities to limit the alkali content of.

cementS, ark3 so it was necessaryto investigatein5ms of r&uciqg

levels present. In the 1950's, the use of the suspension preheater

for dry process kilns intr&ucedfurther problems as alkalis are

recirculated within the kiln/prehsater system quite efficiently, so

t&&clinker alkali levels cxnnotba reduced simplybydiscardingthe

flue dust (which rarely &ibits a separately-collectable fraction

rich in alkalis). FurLher, the high levels of recirculating alkali

salts tend to condense at critical pints in the system and create

build-ups. This high level of recirculation can be reduced (with soma

increase in fuel qmsuqtion) by bleeding off sune of the gas from the

kilnbackendanddiscar~ngtheassociatedltustburden. Thedegree

of bleedrequiredto produce given results cannot always be predicted

accurately.

Research has been undertak~~atvaxious establishuents, with the

aim of constructing a theoretical model for alkali behaviour in

kiln/preheater system and devising laboratory tests which can be

applied to raw materials to predict their behaviour in large scale

kiln systems under various circumstances. Possible areas for such

research are identified in the course of this review.

2. EE7?Ems OF ALKALIS

2.1 Effects of high alkali levels in clinker

2.1.1 AUcdli-aqqreqate reaction

It was found by Stanton in 1940 (3) that cracking in concrete

structures couldbeattributedto reaction betweenalkalisinthe

cemant and certain minerals in the aggregate, This phexxxmmn wds

investigated by a number of organisiations, notably the US Wrreau of

Reclamation (41, frcm which it became apparent that the problem had

occurred with aggregates containing amorphous or microcrystalline

silica. Examples of such aggregates are opaline cherts, siliceous

shales and limestones and scma types of andesite, rhyolite and

obsidian (5). As a result of this work it becama standard practice in

theUSAto specify a rfaximm value of 0.6% by weight for equivalent

Na20 content (Na20 + 0.658 K-20) for the cement to be used with such

aggregates. This limit was intrcxluced as a tentative revision to the

ASIM standard for cemant in June 1959 and in subsequent versions

(e.g. 6) it is included as a characteristic which can be specified by

the purchaser if he considers it desirable.

The mst serious alkali-aggregate reaction problem were found

in the USA- mainly on the West Coast and in Nebraska and Kansas.

Reactive aggregates were also found in North Germany

(Schleswig-Holstein) and Denmark (71, but flint gravels found in

Germany and the UK have not generally been considered to be reactive.

Alkali-aggregate reaction is also known to occur occasionally in other

countries, e.g. Portugal and New Zealand (8).

Alkali limits are included in the standard specifications of

some countries. For example Brazil (91, Peru (lO).and Venezuela (11)

followed the ASTM in setting an optional limit of 0.6% for (Na20 +

0.658 K20) where a reactive alkali is expected, and Mexico (12) has

set limits of 1.2, 0.9, 1.2, 0.8 and 0.9% for Types I, II, III, IV and

V cements reapectivsly, with an optional lower limit of 0.6% for use

in concrete with reactive aggregate.

It has, however, been suggested (13) that alkali contents below

0.6X, perhaps as low as 0.35X, can still lead to reaction in some

circumstances.

Potash and soda generally have an equivalent effect in causing

expansion (7) but under some circumstances (14) it appears that more

potash than soda can be tolerated without expansion.

The form of the alkali does not seem to make a great difference

(13) in the long term, although water-soluble alkali is liberated more

rapidly from crystalline than from glassy alkali-containing phases in

early stages of hydration. It is suggested that the significant

factor appears to be the concentration of hydroxide ions in the pore

solution (15).

2.1.2 Air setting of cement

Rapid air-setting of c-t in storage is due $ the' formation

of syngenite (X2SO4.CaS04.H20) (16, 17) and thus it is generally

desirable to keep the K20 level in the clinker as low as possible

where other factors indicate that air-setting-may be a problem.

2.1.3 Effect on strenqth

It appears to be generally accepted in the literature that, at

levels typically found in cm-e&s , increasing the aUcali.contentof

the clinker tends to increaseearlystrengths anddepress ultimate

strengths. Forexample,AS?Mtestson2in.~~cubesgavethe

following results for cemnts with different soluble K20 contents (and

similar levels of potential C3S, C3A and fineness) (18):

Cartpressivestrength at

Cexnt with Carwtwith0.03% soluble K20

(MPa)0.61% soluble K$

@Pa)

1 &Y 8.41 9.79

3days 17.58 18.34

7day-5 27.72 24.41

28 days 45.09 32.41

The sama investigation found, however, that clinker fran which

the alkali (0.57% Na20 equivalent, minly as potassium salts) had.been

remved by reburning with ammniumchloride had lowx strengths atall

tinkas thahthe samcemantreburned ina similarmannerwithoutNH4Cl_.

addition. DatxlreportedbyMussgnug (21) ix-ldicatedthestrengthof

high-alkali clinker as being 100% greater than that of low-alksli

clinker atlday, 2O%greater at3 days and similar at7days and

subsequently. These findings were basedon tests co alargentznber of

clinkers withalkalisulphatecontents beW Oand4.0%

2.2 Effects of high alkali levels on the manufacturing process

2.2.1 Effect on kiln fuel consm@ion

The evaporation of alkalis in the burning zone consums heat at

high mature, which is subsequently liberated at a lmer

Mature. Additionalfuelisrequiredtomaintainthesamburning

zone temperature under these conditions, and Weber (22) estimates thad

this can be up to 31 kcal/kg of clinker for a specific suspension

preheater kiln,12 kc&/kg for aIqolkiln and 5 kcal/kg for awt

process kiln, the difference being explained by the opportunity for

effective use of the low-grade heat liberated in each mse.

EIowever, this fuel penalty my not be realised in practice,

because the presence of alkalis will tend to pramte the fornation of

clinker minerals atalowar temperature, thus permitting the burning

zone temperature to be reduced, other things being equal (19, 20).

2.2.2 Corrosion of refractories

Thamstserious adverse effect of alkalis on kiln linings is

said not to be cWcal but machanical (23, 24, 25). AlJ6i.i salts -

notably K2SC4 and Kc1 - condense on the brick at teqeratures of

700-10bO"C and fill the pores, thus increasing the risk of cracking

and spalling with temperature changes. In a kiln used part of the

time for the nanufacture of white cemaht under reducing conditions, it

was found (261 that sulphides, e.g. KE'eS2, me deposited on the

brick. Although the sulphides did not damage the brick and possibly

even tended to strengthen it, under oxiding conditions theywere

converted to sulphates with highly deleterious results.

2.2.3 E?uild-up and preheater blockaqes

The volatile ca-qounds evaporated in the burning zone tend to

recondense on dust particles and in the cooler parts of the kiln

system. There my ba a considerable teqerature range in tiich these

canpounds are in the liquid state (see Section 2) and in parts of the

kiln system in this temperature range, severe build-up problems can

OCCUT. Places where this happens axe the inletendof the kiln and

the preheater cyclones (271, in which coatings can lead to much

impaired flow of materials and ultimately complete blockage (28).

"Rule of thumb" limits have been quoted (29) of 0.015% for chloride

and 1.0 for the mlecular ratio of sulphate to alkalis in the raw mix.

Although clogging problems are widespread with suspension preheater

kilns, it has boer~ claimed thatshaftpreheaters are notsubjectto

them (30).

3. souw=Es OF ALKALIS

3 . 1 Alkalis in limestones

Innormal cases,publisheddata suggests thatthelimastone in

the raw mix is only a minor source of volatile carpounds. Average

values for 345 American limastones are (31):

K20 0.33%

Na$ 0.05%

=3 0.05%

Cl 0.22%

Anumber of Lkrbyshirelimestones had higher SO3 contents~ in

the range 0.1 to 4.8% (32).

One~uld~alkaliproblems tobecanali.kelywhenusing

calcareousmai%rials of marine origin, such as coral andaragonite

mud. Suchmaterials are used in Hawaii and the southern UnitedStates

(33) for ewmple, but as far as is known no particular problems due to

high sodiumchloride contents are encountered.

Inecpdments carried outatawet-prccess xorks where sea_water was used to-h oyster shell rawmaterial and tom&e uq the

raw slurry (341, it was found that substitution of fresh water

decreased the Na$ levels in the clinker to scene &xx-k but also

increased the K20 content, so that only a sr&l reduction of total

alkalicontentcould be obtained in this way.

3.2 Alkalis in clays and shales

These mterials normally provide the bulk of the alkalis in the

raw mix. Minerals with high alkali contents include feldspars, for

example orthcclasewhich can contain I+ to17% X2Oand albitewhich

can contain up to12% Na2O,andclayminerals, such as micas and

illite (35). Analysis of 33 illites (36) showed a range of Na2C

contents.frcxn 0.05 to 1.05% with a maan of 0.27% and X2C contents fran

4.6 to 11.0% with a mean of 6.7%.

3.3 Alkalis in fuels

Coals contain alkalis both in themineralmtter,~~andX~

together forming 1 to 6% of a typical British coal ash, and a.s sodium

and potassium chlorides, either in the free stateor adsorbed on the

coal stitances. Chlorine levels can be up to 1% ;37). British coals

are typicallymlow in sulphur, as arecoals franthewastem United

States, and Ruhr coal contains about 2.8% So3 (35). Eastern LE coals

are higher in sulphur as the following analysis (%'by weight of dry

coal) confirms (38):

Illinois coal

w 0.15%

Nazo 0.12%

=3 10.48%

Cl 0.22%

(Ash 9.58%)

It has been suggested (39) that coal my contain iodine which is

found in flue dust, but this muld not ha expected to have a

significant effect.

Heavy fuel oils, for ewrrple No. 6 Fuel Oil Bunker C) include

about 1% of water containing dissolved salts,‘mainly sodium chloride,

so that atypical ashmycontain 32%Na20, ckesponding tocontents

of up to about 0.1% of the oil. Sulphur levels can be high, ranging

frcxn about 2% (SO31 for a low sulphur oil with 0.04% ash to 10% GO31

for a high sulphur oil with 0.02% ash (40).

Natural gas contains no alkalis. Sulphurlevels (mainly in tbe

form of H2.S) can ba quite high in the raw gas, but pipeline

specifications generally limit H2S contents to 4 ppxn by voluma to

avoid corrosion, so that rmst of this is removed (41). (4~ by

volume of H2S corresponds to about 18.6 ppn by weight of S03, or about

0.0015 g SC3 per 1000 kcal).

Wastemterials usedas fuelcouldcontain appreciable

volatiles, depending 'on the sour&. . Addition of chlorinated

hydrccarbon waste to fuel has keen proposed (42) as amans of

reduciiq clinker alkali levels.

3 . 4 Methods of reducinq alkali contents of materials

It does not appear to be possible to reduce the contents of

alkalis in clays and shales by any practicable nmns. A series of

experiments by the Portland Cement Association (43) investigated

leaching, flotation and heating methods and showed that the only

mtl-&.5 to reduce the alkalicontentof the mterialappreciably ware

(a1 heating at a temperature and LSF approaching those used in cemant

manufacture and (b) heating with concentrated mineral. acid.

4. Pl-lYSICALPROPElRTIES OFALSAJLI MILTALCOMPOUNDS

Melting and boil& teqeratures (at 1 amphere) for scma

potassium; sodium and calcium cq&nds a.re given in‘&ble 1. SOUrCeS

for these are (40) and (441, except where otherwise indicated. It is

not'clear how significant these repours are under practical kiln

co&iitioti, since various eutectics form; for example in the

K2SO4/aSOq/I@1systemthelmest fusion teaiperatures are in the range

650-700°C (26).

VaqoU.pressures have been investigated fran an early date (47).

Fig. 1 shows saturated repour pressures of KCH, &OH, KCl, &Cl, KF

andNaF (40) together with approximate values for K2S04 (48) and

Na2sOq (49); these latter should be treatedwith scma caution as they

are ex%rapolated beyond the range of validity claimad for the

equations but should indicate general trends. For further details of

sadim chloride see (51, for potassium fluoride SW [SlJ and for

potassium and sodium sulphates (52). A considerable amunt nf wxk on

the behaviour of potassium sulphate at high temperatures has baen

carried out in connection with its possible use as a seed in

magnetohydrodynamic power generation (53, 54). &an calculation of

the partial presssures of the VazTious species present at teqeratures

in the range 1500-2000'K and 1 amphere pressure when

sulphur-ccntaining fuel oil is burnt, it appears that the potassium

ctxnpcunds present in significant quantities muld be K-$04,, K$O3

andKOH.

Althouqn me vapour pressures \Flg.ll agree v1 general. terms

with the order of volatility observed in practice, one muld

anticipate considerable difficulty in using them to predict absolute

volatilisation levels at given temperatures. Plbst of the species

involved do not cccur as pure canpounds in the solid state and

pressures andteqeratures inside nodules of clinker, for instance,

are not wall known.

5. FxxxxsAFFE13TINGvoLATILIs~IoN

By "volatilisation" we m3n the departure fran heat-treated

arterial of a proportion of a stitance originally present in a raw

llliX. This heattreatmtzntmayba under labratoryccnditions, in which

case volatilised stitances are generally r-v&l fran the system, or

i n tendto condense and ba

recycled,'giving a lowar

obtain~under different

care.

overall %xJ%e of

circumstances must

5.1 Physical factors

5.1.1 Nodule and particle size

vclatilisation". Values

therefore be carpared with

It muld be expected that under certain conditions the rate'of

loss of volatile suketances from nodules could be limited by their

diffusion frcm the interior of the nodule to the surface. Experiments

by the-FCA with various sizes of nodules of raw mix and with rebmning

clinker nodules of various sizes tend to confirm this' idea (551,

althouqh the author suggests elsewhere (56) that in practice the

effect of nodule size my be small in cmparison with factors such as

temperature and residence~tima at-mature. This view is supported

bv a series of statistically-controlled experiments at NIIITsenrent

(57). Results of Goes Ad Keil (58) and Draper (55) have been

replotted on a lag/log scale (Fig.21 and agree (with the exception of

one value for lmn pellet diameter), indicating a relationship of the

form:

volatilisationd 1

.3'dJ-

Analysis of pieces of clinker of different s&as (59) showA K20

levels to be higher & the larger pieces.

Although it appears that the effect of nodule size on

volatilization is not very great in practice (for example, a variation

in pill size of f 25% would alter volatilisation by C 7.5% - lo%), it

could nevertheless be useful to investiuate it more thoroughly, both

to verify the relationship, which is based on very limited data, and

as a guide to the factors limiting alkali volatilisation under various

ccxnbinations of burning tirre and temperature.

Scma mrk has been &ne with beds of poxdered raw mterials of

various depths (55, 591, the results of which broadly parallel those

for nodulised raw mterials. No publications dealing'with the effect

of the size of individual particles on volatilisation have been found;

it seems reasonable to propose that below a certain size this muld

have little effect but it muld be useful for stamlardising

-imental results to put this ptulate to the test.

5.1.2 Effect of the liquid content of the clinker

Since the effect of increasing the liquid content is to reduce

the porosity of the clinker, this factor would be expected to lower

the rate of alkali volatilisation (58, 60, 61). Mussgzmg, howaver,

found no correlation baMaen total alkali (KS + Na$) and Fe203

contents of a number of clinkers (211 anda separate series of

experim31.ts showed that the A/F r‘t'a 10 was the least significant -of the

five factors studied (571, being overshadoWed by temperature, time,

sulphate content and pellet size. Itwmld'te of interest to

investigate the magnitude of this effect in store detail, as there

appears to be scrne disagreeman tas to its iqortance.

5.1.3 Gas flow rates

The effect of gas flow rates on the volatilisation of alkalis

has been studied to a limited extent only, for example by Jackson and

Morgan (621 who detected ho significant variation. I&oratory

vimants on volatilisation rates scmatimas incorporate means for

passing a gas canposition approximating to kiln gas over the raw mix

(551, for instance 77% N2, 20% CO2 and 3% 02 (56). So2 (see Section

4.2.3.) and water vapour (Section 4.2.4.) have been added to this.gas

in varying quantities, but no results of altering its flow rate have

been found. It is possible that this could be significant in saz. -cases, as significant variations in volatilisation with stirrins a bed

of powder have been reported (59).

5.1.4 Euration and temperature of heating

There is general agreemant that the degree of volatilisation

increa.%es with the tim andwith the Mrperature of heat treatment,

although it has been noted (60) that after heating fran 700°C to

14OO"C, little additional volatilisation takes place on subsequent

heating at 1400°C for a further 30 minutes. The effects of duration

and teqerature are thought to be highly significant and about equal

in rnagni.tGde (57).

The rate of volatilisation frcxn a particular raw mix under gives

temperature andother definedexternal &hditions has not been

investigated, buts- interesting conclusions can be drawn fran

additional analysis of reported data.

An assumption which is widely made, albeit usually implicitly,

is that the proportion of a species volatilised on heating for a given

time and at a particular temperature is characteristic of the raw mix,

i.e. that the volatilisation rate is proportional to the amunt

present (a). Thus:

-da=kadt:

- a0 r 1h = orkdfa-

a t J t I

In (at) = -kt(a,>

where at is the amount of alkali present at.time t and a0 the amount

present at the start of heating. If this assumption is correct, that

is, if the volatilisation reaction is first order with respect to K20

or Na20 content, plotting the log of the amount of alkali remaining

against time should give a straight line. Data from Palmer and

Baylees (34) and Woods (63) were analysed in this way and results for

selected mixes are shown in Fig. 3. It will be noted that although

the fit is very good in most cases, the line of best fit does not

always pass through the point corresponding to 100% original alkali

content at time t=O. To explain this, it is necessary to consider t

heating conditions which were applied. In (34) the samples were

heated for 20 minutes at 1800°F (980°C) to decarbonate kthem and

immediately transferred into a high-temperature furnace maintained at

2600°F (1427"C), in which they were keept for the appropriate length oi

time. The heating programme in (63) is not stated. In the case of a

predicted initial alkali level substantially below 100X, a possible

explanation is that part of the alkali is driven out while the sample

is being heated up to the experimental temperature. This appears to

be reasonable, although it is surprising that, if this is SO, no

correlation was found between the rate constant k and the initial

alkali level ao. This could be due to the presence in the mix of

chloride levels appreciably less than those needed to react with all

the K20 (the addition of chloride is known to increase volatilisatic

significantly - see Section 4.2.2.). The line shown for Mix 12 of

(34) with added CaC12 shows a very low level of K20’ remaining at the

start of heating at 1427"C, and a rate constant which, although the

highest of those observed, is not much higher than that of the same

mix without CaC12 (not shown in Fig. 3), - 0.0820 and - 0.0771

respectively. In the latter case, however, no volatilisation appeared

to have taken place at lover temperatures.

Another problem is presented by results such as those for Mix 5

of (34) and Mix 3564, 125O"C, of (63). In these cases, it appears

that volatilisation does not begin until some time after the start of

heating at the final temperature. In the case of Mix 3565 of (631, no

K20 volatilisation takes plz&e at llOO"C, at 12OO'C it begins only

after 60 minutes, at 125O'C it begins after 30 minutes and at 1300°C

and higher temperatures it starts immediately. This delay is perhaps

due to the alkali-containing materials initially requiring to react

with other components before the alkalis are released.

The correlation coefficients were recalculated in selected cases

including the point a = 100X;, t = 0. As would be expected, the

correlation was usually slightly improved where the original line of

best fit passed near this point, but where significant changes in k

and/or a0 resulted, the effect on the correlation was unpredictable.

It vould therefore be desirable to carry out experiments on the rate

of volatilisation, in which the actual alkali contents at t = 0 are

measured. If the relation suggested by the data already examined was,.

confirmed, this would mean that the proportion of alkali volatilised

under given conditions was independent of the absolute amunt present

(until; of course, all the alkali was lost).

An alternative way of indicating the propensity of a raw mix to

lose alkali on heating has been proposed by Palmar and Rayless (34).

This is the "resistance factor“ which is the sum of the percentages OI

alkali remaining after heating for 10, 30, 50 and 70 minutes at 2600'F

(1470°C). This empirical mathod appears to be suitable for cuqar'i3-g

raw mixes with each other, but does not permit the prediction of

volatilisations under other conditions. Its use has not been taken up

by other mrkers.

Maintaining clinker atan elevated temperature has been used as

a n&hod of reducing its alkali content (64, 65). A tE Bureau of

Reclamation specification for low heat, low alkali cement was ntet by

reheating the clinker frcm the kiln to 1427°C in a converted rotary

cooler- Na$ equivalents decreased frcm 0.889 to 1.19% in the

(loss-free) raw mix to 0.411 to 0.890% in the clinker and to 0.315 to

0.716% in the treated clinker. The heat input to the clinker treater

was abxt550 kcal/lq clinker,which renders themathodunattractive

in nomal circmnstances.

Data obtained by the Portland Cement Association (56) for a

number of kilns show that total alkali volatilisation tends to

decrease as the residence time of the mterials in the burning zone

decreases, but the effect of other factors is tcogreatto permit a

quantitative analysis.

The effect of mrature on volatilisation is canparable in

magnitude to that of tine (51). It has been stated that "the burning

of cement clinker takes place within a temperature range where a

canparatively small rise in the temperature will cause a noticeable

increase in the amount of alkalis volatilised" (66). It is therefore

said to be necessary to control the burning zone temperature

accuratfAy. Itcertainlyseems to be the case that there is a big

increase bet- ll.50 and 13OO'C (particularly bat- 1200 and

1250°C) in the percentage of the original K20 volatilised in one hour

(591, although in normal conditions one would expect rmst clinker

reach these teqeratures in a kiln. Kate constants have been

calculated fran the data of Wads (63) at a range of matures

to

f o r

K20 and Na20 volatilisation frcxn one raw mix and these are shown in

Fig. 4. The rate constant increases roughlylinearlywith

temperature, the effect of tgmperature being much more significant for

K2Othan for Na20. Itmightbeexpe&ed fromtheorythatlqeK~uld

be proportional to l/T ("K), but the limited data available do not

appear to confirm this and further exp~imznts muld be desirable.

The increase in K$ volatilisation in a given tima with temperature.-follows a logistic curve (591, since the absolute rate of

volatilisation after a given tima initially increases with temperature;%

(as the effect of the increase in kwith temperature wthe daninant

factor) and then declines again (as the amount of alkali remaining

becomes the dxninant factor).

'i>- ta ure at which volatilisation begins can be as low as..

700-800°C for El (21) but is rrore typically in the region of 1100°C.

5 . 2

The volatilities of the various species are discussed in mre detail

below.

Chemical factors

5.2.1 KS and Nag contents

It has already been suggested (in Section 4.1.4. above) that the

kinetics of alkali volatilisation are first order with respect to RIO

and Na$, and if this is the case the percentage volatilisation is

independent of the absolute levels. Hcmever,dataobtainedbytheFa

(67) suggest that reduction in alkalis in fullsize kilns increases

with alkalicontentof the rawmix. These findings are not

necessarilycontradictory,as clays andshales withhigher K$and

Na$ my, for ample ,contain thein inaless firmlyminbined form,

lost at loam temperatures, but this is certainly an area which xmld

repay investigation.

As far as the relative volatilisation of Na$) and K20 is

concerned, it appears (56) that volatilisation of Na20 is negligible

where Kg volatilisation is less than 30%, and is at about half the

level of additional K20 volatilisation above the 30% "threshold".

5.2.2 Effect of chloride and fluoride

The early studies on the recovery of potash fran cement flue

dust and other minerals (62) showad that the amount of KF volatilised

under given conditions could be increased by addition of calcium or

sodiumchloride. Itwas found to be~necessary to add just over the

quantity needed to combine with the K20 to give KCl. NaCl was rather

more effective in promoting volatilisation than CaCi2. With the

interest in reducing total alkali in the clinker, the effects of NaCl

and CaCl2 on Na20 volatilisation also were investigated, and Woods

-(63) found that although NaCl was indeed more errective in liberating

K20 than was CaC12, the Na20 content of the clinker was noticeably

increased by NaCl addition. The use of calcium chloride was studied~

in full size kilns by Holden (68), who found that the removal of

alkali by addition of calcium chloride could be estimated for CaC12

additions up to 1.5% by the empirical equation:

0.559 K, [CaClZIAr f 100 - L

where A, * total alkali removal expressed as Na20, K, = apparent

efficiency of utilisation of CaCl2, L = loss on ignition of raw

material and [CaClz] = % of CaC12 added (dry basis). The efficiencies

of CaCl2 utilisation observed were 27% to 34% (average 32%) for wet

process kilns and 45% to 56% (average 48%) for long dry process kilns.

Addition of 35% commercial grade hydrocholoric acid has been used as a

cheaper alternative to CaC12 where this was not readily available

locally (69). No particular problems were encountered in adding the

acid to the slurry tanks and control of the quantity added was better

than when flake calcium chloride was used.

Problems have arisen with low volatilisation and build-up

problems when CaC12 is used in long dry process kilns(70). Although

no published data are to hand, it would be expected that levels of

chloride addition would cause severe problems in preheater kilns and

muld probably have little effect on alkali reduction.

Contrary to the above results, laboratory tests on smrrles man

six works in Central Asia (71) showed that addition of NaCl did not

appreciably increase and inmstcases reduced the Na2C1contentof the

clinker, but that CaC12 was in rmst cases more effective than NaCl.

Although it is known that addition of chlorides decreases alkal

contents, further mrk needs to be Qne before the precise effect at

various temperatures can be predicted.

Eata on the effect of fluorides on alkali volatilisation are

spar=. woods (63) states that where calcium fluoride was added to a

raw mix to produce a high early strength cerrwt, the clinker mntaine

less potash than usual butahoutthe same amount of soda. He suggest

that addition of CaF2 &uld probably only be of value in pramting

volatilisation where the raw mix was high in KS but relatively ion i.

Na20. According to Sprung and van Seebach (721, theremdymmic

considerations favour the formation of calcium fluoride and alkali

matal sulphates rather thanalkali fluorides andcalciumsulphate.

The amunts of fluoride in rawmaterials are, however, small in mst

cases. Cost and possible toxicity muld probably make fluoride

addition unattractive as a means of increasing alkali volatilisatior

even if it were mre effective,

5.2.3 Effect of So2 and So3

The water-soluble alkali salts remaining in clinker are almst

entirely sulphates (16), chlorides having been lost through

volatilisation. The effect of sulphur compounds 0; the alkali content

of the clinker is therefore considerable, the sulphur deriving both

from the fuel and from sulphur compounds naturally occurring in (or

added to) the raw materials. The sulphur content of the clinker is

typically 50 to 65% of the total sulphur input from all these sources,

so that it is possible for the sulphur content of the clinker to be

higher than that of the raw feed (22). As the vapour pressure of

alkali sulphates is considerably lower than that of chlorides (Fig.1)

it would be expected that higher levels of sulphur in the clinker

would tend to prevent alkali volatilisation and this is in fact

observed (54, 56). The effect on K20 volatilisation is appreciably

greater than on Na20 volatilisation (62). K20, Na20 and chloride

contents of flue dust are reduced even in the presence of chloride in

the feed (73) - which can be advantageous in improving the efficiency

of electrostatic precipitation - so, it would seem, demonstrating that

alkali sulphates are formed or retained in preference to the more

volatile chlorides and oxides. Statistical examination shows that

SO3 content is second in importance only to time and temperature in

affecting alkali volatilisation (57). Operating data (56) from ITS

kilns show a relation between volatilisation of SO3 and K20, although

it is not clear what is the significance of this and there is some

evidence (56) that the effect of SO3 is greater at higher

temperatures. The equilibrium between calcium sulphate and CaO, SO2

and SO3 has been extensively investigated. At temperatures above

lOOO”C, combustion of sulphur compounds in the fuel results

predanhantly in SC2 (271, which can react with lime in the

temperature range 600-900°C.

34 CaO + 4 SO2. . 3 CaSO4 + CaS

The xraximum absorption of SC2 by raw meal was obtained at 880°C;

this is significant since a high proportion of the sulphur in the kiln

exit gas can be absorbed by raw meal in the preheater (74). Snaller

amounts of sulphur can also be absorbed byrawmterials, where these

are dried by kiln exit gases in the rawmill.

The decaqosition of pure C&O4 begins at about 900-lOOO'C, but

is initially very slow. Sax5CaSC4remainsunconvertedatabout

1385“C, which is the melting point of the CaO-CaSO4 entectic (75).

The decaqosition rate is much increased by the addition of other

materials, eg silica, alumina, kaolin and iron odde, and by the

presence of water vapour. -sition is also faster in a reducing

atmosphere, when CaS is formed. This fact has been used in processes

for mmfacturing cen?ent and sulphuric acid, by heating a mix of

anhydrite, coke and argillaceous materials in a rotary kiln (76).

- 4

3 - 4

+x +m2+cas

+c&-+4c.&+4=2

The lime reacts with the argillaceous ccqxxents to give clinker

minerals and the SC2 is separated fran the exit gas. It is necessary

to maintain oxidising conditions in the exit gas to avoid the

formation of elemental sulphur, for instance by reaction of the H2S

andSO2. It has been claimed that low-alkali ceinentcanbeproduced

in this prccess by adding calcium chloride in the usual way (77).

This 'is perhaps unexpected in view of the effect of even a small

percentage of added calcium sulphate on alkali volatilisation, and no

doubt results from dissociation of the CaC12, allowing alkalis to

evolve as chlorides.

Further mrk on this aspect should investigate in greater detail

the effect of varying levels of sulphate addition at various

temperatures and in the presence of other materials; also, the

relative effects of So2 in the kiln atxmsphere and calcium sulphate in

thefeed.

5.2.4 Effect of water vapour

Fran the relatively high vqxmr pressures of KOH and NaOH it

might be exqected that water vqour +muld pramte volatilisation and

this is confirmed by laboratory expariments by Goes and Keil (581,

which gave the following results

H2OiIlkilngas

% byvol.

0

5

1 0

K20 volatilisation with 30 min at

1330°c 14OOY% %

6 4 83

76 100

100 n.d.

Wcods (70) reports that satka works have claimad that spraying

small Zn-ounts of water (approx. 2ml/kg clinker) into the burning zone

directly above the flame my reduce the clinker alk$i content by

-0.1 to 0.2% (on clinke?z). This effect might have sme relevance to

the observed tendency for gas (andperhaps oil-fired) kilns to have

higher alJcali volatilisations than coal-fired kilns (18, 54, 65) since

more water vapour muld ba produced by ccanbustion of hydrocarbons.

(Spiers reports moisture levels in canbustion products as:

2 - 5% coke

10 - 15% Lignite

7- 9% coking coass

5 - 7% Anthracite

1 0 % - H.F.O. 1

5.2.5 Alkali-containinq minerals

Scdiumandpotassiumin rawmterials can occur ina number of

different minerals (see Section 2) and these can vary in their

tendency to lose alkalis (57). Elpertits with rawmaals containing

mica, illite and orthaclase feldspar (58) showad that alkali w lost

mre readily fran mica and illite than from feldspar at temperatures

of 900-1250°C. It is suggested that this my be due to difference in

bond energies for the various minerals. Another factor may explain

differences between minerals; for example in greensand (glauconite)

the silicate is hydrated, (62) leading to formation of K20 in the

presence of H$ when heated, allowing the formation of Koti, tiich has

a higher vapour pressure than KS. (see 4.2.4.)

It muld be useful to investigate the response to heat treatmant

of various potash and scda containing minerals in mre detail, and

this muld require sama a-cans of allowing for the effects of other

factors and also devising a quantitative msasure of the "availability"

of K2OandNa20 in theminerals.

5.2.6 Effect of LSF and SR

Although little reliable infomtion on this subject has been

found, such as there is indicates thatitwxld be useful tocarryout

further Czq&nmts. Ekperirrwts on remving alkalis frcmraw

materials by heating (43) indicated that there was no mlatilisation

on heating mixtures of clay or shale with 10% or less of limastone to

fusion and that alkalis wsre not released in significant quantity

until the amuntof lime approached that of a normal cenwt raw mix.

The data are shown in Fig. 5 where percentage loss of alkali is shown

in relation to the proportion of CaC03 in the mix, and in Fig. 6,

which shows alkali loss in relation to the TLSF - which has been

estirrated since the caqxxsition of the shale was not reported. It

will be seen fran these figures that the proportions investigaM do

not permit us to say whether ease of volatilisation varies with LSF

within the range normlly found, or whether aminimumlima content is

required, above which there is little variation. Earlier experiments

(62) had shown that KS could be volatilised frm greensand

(glauconite)/l~stone/cdlcium chloride mixes containing as little as

one third limestone, if these were heated to temperatures just below

that of fusion.

The effect of silica ratio on alkali volatilisation has not been

investigated. Tbare is scma evidence (68) of a higher, calculated c3S

content leading to lowar alkali levels in clinker, and it ms found

(61) that clinker alkali levels could ba reduced by replacing part of

the argillaceous mterial with sand. Howevex, in this case the effect

of increased S.R. was not distinguished fran the effects of lmer

alkali levels in the sand reducing &-se in the raw feed and the

necessary higher burning matures, which one wuld expect to ba

more significant.

5.2.7 Effect of kiln a-here

In normal centant cperations, as distinct fran the

cment/sulphuric acid process (see 5.2.3 above), the limited

frm laboratory tests (59) suggests that

greatly affected by the kiln amphere. The

a reducing amphere pramtes volatilisation at

perhaps slightly depresses it at 13OO"C, but the

information available

volatilisation is not

results indicate that

1200°C and 1250°C and

data do notreallyperm.ituseful conclusions to be drawn.

FundamantaJ.ly,a reducingatmxiphere is expectedtoprcm&e

volatilisation of alkali sulphates in the burning zone.

6. DISTRIEUTIOJJ7OF~SINCLIN?GZR

The distribution of alkalis is not revie++ed in detail, as the

literature on this subject is very extensive.

E'rcm the efforts to utilise flue dust during the First World

War, it had been noted (78) that potash in flue dust existed in

several different forms with different degrees of solubility in water

or acid, but no substantial progress k~s made until the 1930's and

1940's, when ah extensive programe of mrk GELS carried out at the

(US) Natioml Bureau of Standards, in cooperation with the Portland

Cement Association (79 - 84). Newkirk (85, 86) concluded that the

alkali-containing compounds M&A3 andKC2jSl2ere unstable in the

presence of SC3 andCaSO4 and reacted to give an alkalisulphate phase

which xas imiscible with other phases. For R$/SO3 mlecular ratios

<I, the SC3 reactedpreferehtiallywith the KS. The alkalisulphati

phase is imzdiately soluble in mter (87) although alkali fran other

phases is not.

Segnit (881 suggested that at least some, and possibly a

considerable part, of the soda in Portland cemnt occurs in the C$.

On the other hand, investigations at the Bulgarian Building Research

Institute appeared to suggest that both KS and Na-$ are predminantly

in the form of aluminates (89). Toropov and Dobrovolksky (60) support

Newkirk's opinion that most of the KS is in belit&; they excluded

clinkers with free alkali sulphate from consideration.

Pollitt and Brown (16) generally mnfirm Newkirk's findings on

alkali sulphates, noting that equilibrium conditions are not attained

in a kiln, so that works clinkers tend to contain mre calcium

sulphate thah do latxlratory clihkers. They also suggestthatscme

potash could cxcu~ in the C3A and that t&h potash and soda could

exist in alite in small quantities.

Azelitskaya and co-workers (90, 91) found that addition of

gypsum to raw mixes containing alkalis diminished t?ne effect of

alkalis, which (at a raw mix SO3 content of 0.6% or less) was to

decrease C3S content. With the addition of.5 to 10% gypsum to the raw

mix (which also raised the LSF and hence the C3S content), alkalis

were in the form of their sulphates. When investigating dust return,

Luginina and Shaposhnikova (92) found that the presence of high levels

of alkali sulphate tended to reduce alite formation and to give alite

crystals-with inclusions of belite and alkali glass and eroded edges.

Other work (93) confirmed that the addition of gypsum to

alkali-containing mixes tends to convert the alkalis to soluble

sulphates; in this work the optimum SO3 level in the clinker appears

to be about 4%.

Subsequent work by Abbassi (94) confirms the general trend of

previous investigations. Alkalis can exist in clinker separately, as

sulphates or carbonates, or in other phases: C3A dissolves soda

preferentially. However, all the phases can contain at least some

potash and soda, the amounts in the flux phase being below 0.2%. The

ratio of the K20 content of the C2S to that of the C3S is usually

about IO, the maximum K20 content found for C2S in this work being

1.9%. The Na20 ratio varies, but is about 10 for max. 0.6% for Na20

recorded in the C~S. More Na20 was found (relatively) in the C3A at a

max. of 1.7% (cp, 3.2% max. for K20).

7. ALK?GICIRCULATION INTKEKIINSYSTEM

7.1 Estimation of alkali circulation

Equations to give the concentration of a volatile cqnent i at

various points in the system have been derived by Ritmann for

Polysius AG (28, 95). According to these, in a system without a

bypass, the attmntof the ith caqmentrecycled frcxnthe kiln gases

to the incaning mterials (referred to as the adsorbed phase, since

this is believed to consist rtkainly of alkali carpounds condensed on

raw material particles) is given by

(Eli + Bri*) .ai.CiXi =

l- c2i.ai

ci represents the amount of the.cmponent in the raw maal

Bri* =Bri/Ci where Bri is the amount of the cmponent in the fuel

ffl i

5 2 i

ai

represents the volatilisation of i from the raw ma1

represents the volatilisation of i from the recycled naterial,

and

represents theextenttowhich the component is reabsorbed from

the gases in the preheater.

Note that Xi, Ci and Bri can be in any consistent units of ItaSS,

such as (but not necessarily) g/kg of clinker or per cent of clinker.

$li, f2i ad ai are fractions not percentages.

It is ~instructive to consider the assurrptions mde in deriving

this equation. If the various value& are shown in a diagram Ri is the

content of the ccmponent i in the clinker:

XiTar

Then if Xin and Xi(n-1) are the values of Xi after n and (n-l)

:ycles respectively:

Xin = Bri + Ci Eli + Xi(n-1) 2i

But in equilibrium conditions the amount of each component

circulating is constant,so that

xi = Bri + Ci Eli + Xi E2i-

ai

Xi s ai (Bri + cilli)

(1-a c2i >

or Xi 31 ai Ci (Eli + Bri*)

(1-a C2i)

.This assumes that the whole of the volatile component in the

fuel is volatilised and that no raw feed is lost to the stack (or

dumped) from the kiln/preheater system.

Using the same symbols and assumptions, it is clear that Ri (1

content of component i in the clinker) will be the amount not

volatilised, i.e.

Ri = (IL-&li) Ci + Cl-f2i) Xi. i

hence

Ri = (l-<li) Ci + (l-[*i)

= C i

(l-aiE2i )

E(l-Eli) + ai Bri" (1-{2;) f ai (<li-E2i II

If ws can neglect the volatile content of the fuel, for exaqle

for K20 and Nag if oil or gas is used, Bri* = 0 and

Ri = ci [ (l-Eli) + ai (Eli -czi) 1

(l-aic2i)

= Ci (l-Eli (l-ail 1( 7l-aiE2il )

If a fraction V of the kiln gases is by-passed, then the

volatile ccqonent i passing on to the preheater will be reduced to a

level Xi Cl-V)/ai, and the axrount recirculating will be Xi (1-V). We

can therefore represent the effect of a by-pass in the above equation

by replacing ai by a'i where

a'i = ai (1-V)

These assm@ions are cleiuly open to criticism. The concept of

differing volatilisations for raw mterial and recycled mterial is

swrted by soma qrtitalevidence (28) which suggests that

alkalis are rtore readily volatilised frcm recycled mterial than from

raw mterials, although other mrk (55, 59) indicates that alkalis in

flue dust are less readily volatilised than those in raw materials.

However, the values for any single ccqqent do not autamtically take

into account the amount of other components present. For example, the

volatilisation of KS will be. increased by the presence of chloride

and&creased by the presence of SO3. As these ccaqxments are lost

fran the system to different extents, the proportions in the

recirculating material willvaryas the system tends towards

equilibrium. Although the volatile content of coal ash is relatively

small, it is not necessarily all lost (96) and sma alkalis my reach

the clinker from this source. It is also possible that SO3 fran oil

fuel could be re-absorbed at relatively high ixqeratures. Ibere

seems no reason to assume that dust lost frcm the prekzker will

contain alkalifrcmnthekilngases only and thathoalkaliwill be

lost frcm the raw material (55). It appears that in practice the

factorsE1 a.udc2 are estimatedempirically in a particular case, so

that while these equations may be useful for predicting the effect of

varying the level of ah existing by-pass, for example, they muld

probably ba of little value in predicting the bahaviour of a raw

mterial in aproposed hew system.

Teoreanu and Puri (94) adopt different ratios which seem to be

less gene-rally applicable than those of Ritzmann. They &, hcmver,

intrcduce the possibility of non-equilibrium conditions in the kiln,

for instance where alkali-containing coatings build up over a period

and then break free.

It is clear that in addition to the factors affecting

volatilisation which have been discussed at scnte length abve, the

condensation and recirculation of alkalis are also important. One

mans which has been adopted for following alkali circulation is the

use of radioisotopes such as K42. Inanexperimantonawetprccess

kiln (981 it was found that the man number of cycles was about five,

the time for each being 110-120 minutes. This value is on the high

side, caqaredwith those derivedbycalculaticm frmalkalibalances

(99, 100). The efficiency of capture of dust by the preheatar system

varies according to the type. Ritmann quctes typical values for dust

collection efficiency for cyclone preheabars of 60-90% for stages

3 and 4 (1011. The total capture of alkali-ccntaining dust will

therefore be very high, and hence the alkali levels in'the flue dust

are very low (102) and the circulating load very high. & wet process

kilns a proportim of the alkali, say 40% of the KS (221, is

precipitated, and alkali levels in the flue dust are mderate (102).

The typical Le@ kiln (22) emits little dust and apparently traps

aln-ostno alkali in the grate: very high values have been reported

for the alkali content of the flue dust (22).

7.2 Reducinq alkali circulation

7.2.1 Rezmval and treatnwt of flue dust

Over a significant period of tima, the net gain of alkalis in

the kiln systmmustbe zero. Therefore, apart frm the small amunt

of alkali in the stack gas which is not precipitated in the dust

collectors, all the alkali put into the systemmustleave either in

the clinker or in the flue dust. Thus, it is only possible to reduce

the alkali level in the clinker to a level below that arriving in the

rawmeal if the flue dust is either dmped or treated to reduce its

alkali content before being returned to the process (55). In a wet

process kiln, discarding the dust reduced the equivalentNa20 in the

clinker fran 0.68% to 0.54% (68). As dust rermml will reduce the

amour&of circulating alkali, however, the rduction in clinker alkali

that can be achieved by discarding dust will be less than the alkali

content of the reamed dust.

Anumber of ways of treating the dust to reduce its alkali

contenthavebeenproposed. As the finer fraction contains a higher

proportion of alkalis (103,104) it should be possible to reduce alkali

circulation by returning only the coarser fraction of the dust,

although it is doubtful whether the differential muld often be large

enough to mke this procedure mrthwhile.

For reasons discussed above, there is a considerable variation

in water solubility of K20 between flue dusts, ranging in one study

(103) fran 2 to 96%. Because of this the efficiency of leaching

vaxies, but it has frequently baen used or proposed (105 - 108). I t

is obviously best suited for use with uet process kilns. Problerrs may

arise in disposal of the salt solution produced, as it is rarely

econanic to recover these salts as solids.

Another mans of reducing the alkali content of flue dust is to

heat it to, say, 1200°C. This can be done conveniently in a fluidised

bed reactor (109).

7.2.2 Bypasses on preheater kilns-

In preheater kilns - especially 4stage cyclone preheater kilns

-virtually all the alkalis in the gas franthe kiln are absorbed by

raw feed in the preheater. Thealkalicontent in the flue dust is

therefore very low and little reduction in the alkali content of the

clinker or in the circulatingloadcan be obtainedbyremving or

treating the dust. One way in which this could can be achieved is by

removing part of the gas and/or dust before it enters the preheater,

cooling it, collecting the dust and discharging the gas to the

a?mosphere (21). Problems tend to arise in practice with the txlild-w

of alkali. Although it muld be preferable to cool the gases by uater

spray, as the gas volumes needing cleaning are nmch less in this case,

there is so far little practical experience of this, ccoling by mixing

with cold air having been used in rmst cases (ll.0). It appea.rs that

the effect of a bypass is greatest up to about 20% bleed (991, above

which the increased fuel consumption, which is about 40 kcal/kg of

clinker at 10% bleed (211, becomes excessive in relation to the alkali

reduction obtained. Suggested bypass levels (ill) are 2-4% to reduce

operational problem, eg blockages, and S-15% to reduce alkali levels

intheclinke.r. A possible modification proposed to the bypass (112)

is to pass the bled-off gas through an initial cyclone and return the

coarse dust (with a low alkali content) direct to the kiln. Although

this means thatmre gas has to be bled off tomintain the same level

of alkaliremval, the quantity of dust to be collected is much less.

Incorporation of a bypass enables the maximum permissible

chloride content of the raw fe& (see Section 1.2.3.) to be increased

frcxn 0.015% to about 0.075% at 10% bypass (29). As wxld be -ted

the effect of

Na$. It has

itnaybecaae

volIxrrs of gas

a bypass in reducing K2Ois greater thau the effect on

baen sugges& that if high levels of bypass are needed

desirable to use a precalciner system (ll3, ll4) as the

passing through the kiln is thenmuch reducedandthe

increase in fuel consuqtion at high bypass levels (eg 50 to 100%) is

less important.

Although alkali problems are n-c&often eucounteredwith cyclone

preheaterkilns,bypassescanbeusedwithgratepreheaterkilnsdlso

(IS, IX). In this case, the cleaned bypass gas can be used for

7.2.3 Other processes for alkali reduction

Same alternative n&hods for reducing alkali circulation have

beenproposed. One of these (ll7, 118) involves using excess hot air

fran the cooler for drying pellets on a grate preheater. The kiln

gases~~dbepassedthro~htfiecdlciningpartof thegrateandthen

passed toa dust-collecting systemtogether with the cooler air which

hadbeen passed through the other part of the grate.

An alternative mathod of lowaring alkali circulation in

suspension preheater kilns which has been proposed (although not yet

used in practice) is to pass some or all of the kiln exit gases over a

cooled surface (ll9) to produce a deposit of condensed alkalis and

high-i dust. Possible modifications to this process include the

use of~water-cooledtubes or of bdies exchanged betwzn the kiln

system and a cooling/alkali remval plant.

8. CONCLUSIONS

From the review it is concluded that there is general agreement on thefactors affecting the volatilisation of alkalis. Volatilisation isincreased by-smaller nodule size, lower flux content in the clinker,higher levels of chloride, lower levels of sulphur, higher temperatures,longer heating periods and by higher contents of water vapqur in'the kilngases. It is not clear what the effects of other factors are (over theranges studied in the available literature), and it may be inferred thatthese are relatively small: gas flow rate and composition, alkalicontent, presence of fluoride, LSF and S/R and oxidising conditions inthe kiln. Several of these factors appear worthy of further study.

There is, nevertheless, insufficient quantitative evidence to predict thedegree of volatilisation of alkalis from a given raw mix under particularconditions. The interplay of phase equilibrium and transport phenomena isprobably too complicated for it to be worth while attempting to simplycalculate volatilisations from knoun thermochemical data: more complexmodelling is needed.

In practical terms, the circulation of alkalis is affected as much bytheir condensation and deposition as by their volatilisation. It would beuseful to devote some attention to this hitherto somewhat neglectedaspect of the phenomenon.

RRC/TGB/JLMC/D31:CPK/LEX3211.03.87:12.02.R8.

T. G. BURNHAM.

1.

2.

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118. POLYSIUS GMEi: Process andapparatus for producing -toflowalkalicontent.British patent 874,818, published 10th August 1961.

119. VEIGEZ, J.F.: Problems of alkali reduction in the Htildt system.Sympsosium on alkali remval and problem, Milwaukee, Wis., 1959.Portland Cetrent Asscciation report M-158, 1960,

RRC/TGB/JMLC/b3111.03.87.

TABLE 1

Melting andimiling pints of selected calcium, potassiumand sodiumc-unds.

Camp&d Mslting point("Cl

Eoiling Point("Cl

CaCO3 C!ecoqases 899 (calcite)

--cam2 772 - 782 > 1600 - 2000

c*2

Ca(OHI2

Cd0

1386 - 1423 (45)

deccmpses 580

2570 - 2600

2500

2850 - 3000

- 4 1450

KP3 891 ckcqoses

KC1 776 - 790 1407 - 1500

ET 846 - 858 1505

IUJH 360 - 380 1320 - 1322

KiQ

K2Si03

K2Si$5

K2s04

*a2C03

N&l

NaF

NaOH

*aZo

Na2=2o5

Na2SiO3

Na4SiO4

*a2=4

ckarposes 350

976

1015 * 10

1069

851

801

988 - 997 (46)

318

874

1088

1018

890

1689

dL3XiVpSeS

1413

1695

1390

sublines 1275

RRC/+IGB/JMLC/b3111.03.87.

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Blue Circle Cement


Module 13

Section 6

A Study in the Volatile Cycles on HOPE#2 Kiln

A series of seven tests have been conducted at Hope Works during which the

No. 2 kiln was operated at selected, controlled levels of kiln back end

NOx (equivalent to burning zone temperature) and oxygen in order to study

the effects on volatile cycles of these different regimes. Each detailed

study period lasted at least twelve hours and samples were collected from

various points around the kiln and preheater system for analysis.

CONCLUSIONS

The test results indicate that the chemical proportions of the minor

constituent volatile components at Hope lead to the following cycle

levels in stage IV meal.

The typical fluoride cycle level is 1.2 to 1.4 times the input to

the system and does not change substantially as the kiln temperature

or atmosphere is modified, over the range studied.

The chloride cycle is 20 to 30 times the total chloride input and

increases slightly with increasing temperature, although this may be

due to improved kiln stability at higher temperatures. The chloride

cycle is not modified by vide changes to kiln atmosphere.

3. The chloride cycle is a low temperature cycle and cannot be

significantly modified by variation of kiln conditions.

4 . Chloride in the cycle combines with potassium where available.

However, at low temperatures - equivalent to NOx levels of below

1,000 ppm - some sodium is also involved.

5 . The total potassium cycle is approximately 3 times the input level,

however about two thirds of this is present in Stage IV in conjunction

with chloride and so cannot be controlled except by incorporation of a

bleed sys tern.

6. About one third of the potassium in Stage IV is either feed based on

its first passage through the preheater or derived from a potassium

sulphate based cycle. This cycle ratio varies from 1.05 (minimal

recycle) at 900 ppm kiln NOx level to 1 .25 at 1,400 ppm NOx (25%

r e c y c l e 1. This portion can be controlled by operation at low

temperatures.

7 . The sodium cycle level varies between 1 .2 and 2.0 times that in the

rarnneal over the temperature range examined.

8 . At low temperatures some sodium becomes involved in the low

temperature chloride cycle and this boosts the sodium cycle by

about 10%.

9 . The majority of the sodium in the system is involved in a sodium

sulphate based cycle. This is strongly temperature dependent, and

the rate of increase also appears to be increasing with temperature.

Over the temperature range investigated the cycle rose from 1.35 times

t h e l e v e l i n t h e feed at low temperatures to 2.0 times at high X0x

l e v e l s .

10. The total sulphur cycle is temperature dependent and rises from 1 .5

to 2.0 times that of the input over the temperature range investigated.

11. The alkali sulphate cycle has already been summarised in Points 6 and

9 and makes up about 35% of the total sulphate in Stage IV material.

These cyclic levels can be seen to be lower than the total sulphur

c y c l e l e v e l s .

12. The calcium sulphate recycle is strongly temperature dependent and

rises from about 2.5 times the level in the feed at lov NOx levels to

4 times at high (1,450 ppn) NOx levels.

13. The calcium sulphate cycle is also increased by a move into a

reducing kiln atmosphere. At about 1,200 ppm NOx the cycle

increased from 3.1 under oxidising conditions to 5.5 times the feed

level under reducing conditions.

14. For potassium and sodium there are indications of a slov but steady

increase in losses of these components from the preheater system

as temperatures increase. This may become part of an external

cycle or may be lost to atmosphere. This could be established by

a longer term study of the levels of these components in the

precipitator and stack dusts.

15. Sulphur is in overall balance within the system belov NOX levels

of 1,200 to 1,300 ppm, but above this level the loss increases

sharply with rising temperature. Again this may become part of

an external cycle or may be lost from the systen, hovever in this

exercise no precipitator or stock dust samples were collected so this

cannot be confirmed.

16. When the kiln a:mosphere moves into reducing conditions the losses

of alkalies and sulphur from the kiln system increases sharply.

In viev of these conclusions it is recommended that, at Hope:

1. Kiln back-end oxygen level is maintained at 2.0 to 2.52

(as measured).

2 . Kiln back-end NOx levels are maintained below 1200 ppm, although

the most appropriate NOX level will depend on detailed raw mix

chemistry.

3. Communication between kiln burner and personnel cleaning the

preheater be improved.

4 . During cleaning operations short term reductions in raw meal feed

vhich avoid the inception of reducing conditions will be

b e n e f i c i a l i n t h e l o n g e r t e r m .

N O T E : C y c l e = quan:ity i n S:age I V - o n a l o s s f r e e b a s i s

quantity in feed on a loss free basis

F u e l i n p u t l e v e l s f o r a l k a l i e s , fluorides and sulphates have not been

taken into account as the levels are relatively lou and constant, as

indicated from the small number of fuel samples that were analysed.

The chloride input from coal has been taken into account.

1. INTRODIJCTIOfi

Since the development of the suspension preheater based dry process for

cement manufacture the study of the inherent cycling effects of the

potentially volatile components, which are present as minor constituents

of the raw materials and fuels, has become increasingly important. In

the less thermally efficient plants a natural loss of a portion of these

volatile components occurred in the waste gases so automatically

controlling the recirculation levels within the burning system and the

l e v e l i n t h e c l i n k e r p r o d u c t . P r e v i o u s l y t h i s l o s s o f v o l a t i l e

components has also been higher than would be found today due to the

capabilities of the available pollution control equipment, as the

volatilised components condense on fine dust particles in the cooler

z o n e s o f t h e k i l n . I n the suspension preheater, with its greatly

increased surface contact between gas and particle and repeated

separation of gas and particle, the recovery and retention of

volatilised components will be almost complete. This leads to

increased proportions of these components in clinker and to the

development of operational problems in the pyro-processing stage due to

the quantities of the potentially sticky components that build-up

through cyclic processes. These effects limited the immediate impact

of the suspension preheater technology in the industry as a whole for a

number of years. Nevertheless increasing fuel prices and the

requirement for even larger process units provided the incentive for

identifying the causes of and solving the problems associated with

volatile component cycles 1~3. In general this work has been

targetted at controlling alkali levels in clinker and/or maintaining

the recirculation levels wi:hin such boundaries that allow reasonable

kiln operation, whilst the solutions have been limited to indirect

methods of control; modification of the raw mix, change of fuel,

installation of a bleed system, manual or automatic cleaning of areas

where excessive bui Id-up occurs .

During the developaent phase of the high level control project at Hope

Works it became evident that considerable changes in the volatile cycle

were produced as the combustion chamber conditions were modified. The

more consistent control and greater understanding of process conditions

vithin the kiln that vas developed through the reliable measurement and

interpretation of kiln back end NOx level in association vith the more

conventional instrumentation offers the potential for deliberate modi-

f i c a t i o n a n d c o n t r o l o f t h e v o l a t i l e c y c l e s b y v a r i a t i o n o f p r o c e s s

conditions on a plant scale. This would then permit the minimisation

of troublesome volatile cycles and hence lead to improved kiln stability

and operating times.

Consequently, it was decided to investigate the exient of variations in

the volatile cycles vhich could develop by operating a kiln under a

number of temperature and gas compositional (oxidising/reducing) regimes.

Over the course of the last tvo years, as operational restraints and

manpower availability have permitted, a series of seven studies have

taken place on No. 2 kiln system at Hope Works. In these studies three

different kiln back end NOx levels (800 to 1,400 ppm) and three kiln

atmosphere conditions (reducing to high excess oxygen) were targetted

as shovn in Table 1 . Previous vork 4j5y6 has shovn the general

relationship betveen kiln h’Ox level and burning zone temperature.

Although it is not possible to equate a particular NOx level vith an

actual definitive material or flame temperature at the fron: end of the

k i l n , t h e t y p i c a l r e l a t i o n s h i p f o r N o . 2 k i l n a t H o p e i s shovn i n F i g . 1 .

For control purposes the advantages of NOx measurement are that above a

base level NOx generation is proportional to the temperature of the

burning zone - v i t h i n s p e c i f i c l i m i t a t i o n s , - t h e r e i s a v e r y f a s t

reaciion t i m e , t h e s i g n a l s u f f e r s r e l a t i v e l y l i t t l e p r o c e s s n o i s e , a n d

the true reading is unaffected by front end dust cycles.

2 . VOLATILZ CYCLES

2.1 General Considerations 7,8,9,10

The minor components that are generally considered to be involved

in major volatile cycles are the chloride, alkali, and sulphur

s p e c i e s , with some attention also being paid to fluorides. Prime

control of the volatile component cycles is performed in the

design stage of a Works project through the selection of rav’

materials and fuels in order to optimise the relative and absolute

levels of the potentially volatile components and, if necessary,

the inclusion of a bleed system. Once these factors have been

defined the volatiles will develop internal and external cycles

during the pyro-processing stages; an internal cycle being

totally within the kiln and preheater system, vhilst an external

cycle will leave the system but be returned after a time lag (for

instance with the precipitator dust). Where processing condition2

are kept steady, the cycles will continue to develop uniil

equilibria are reached, at which time the total amounts of

volatile entering the system will be balanced by the quantities

leaving the system.

The degree of volatilisation and the rates at which the

equilibrium are established will depend on:

(i)(ii)

( i i i )

(iv)

(VI

(vi)( v i i )

( v i i i )

( i x )

(xl

The species, their chemical forms, and concentrations.

The volume of gases.

The intimacy of contact between gas and solid.

The vapour pressures of the salts.

P o s s i b i l i t y o f d i s s o c i a t i o n o r f u r t h e r r e a c t i o n .

Rate of diffusion to and from solid/gas interfaces.

Degree of saturation of gas.

K i l n a:mosphere.

Kiln temperatures.

Time/temperature profile of material within the kiln.

lost of these factors are to some degree inter-related, and so in

norma 1 operation the only methods available to control the degree

of volatilisation will be the kiln internal atmosphere and

temperature.

The major cycles will be internal cycles between the burning zone

and preheater where an individual cycle time of twenty to thirty

minutes would be expected. Smaller quantities of volatile

components may be involved in the longer term external cycle which

develops via the precipitator dusts and volatile collection in the

raw mill . These would be expected to have much longer cycle

times of up to twenty four hou:s.

2 . 2 F l u o r i d e s l1

Fluorides can be found naturally in raw materials or be

deliberately added in small quantities to the raw mix. In low

proportions its mineralising action has a beneficial effect on the

burning process. Generally it has a low volatility and causes

few operational problems, although a level of above 0.25Z in

clinker may lead to setting problems particularly in winter.

However, cases have been cited vhere a hard dense build-up has

developed in preheaters, where the build-up contains high (over II)

p r o p o r t i o n s o f f l u o r i d e .

2 . 3 C h l o r i d e s l2

Chlorides are derived from the raw ma:erials and the kiln fuel.

Either source will only have very small quantities of inherent

chloride, but the high volatilities of these compounds together

with the high collection efficiency of the cyclone preheater

systems will lead to the development of a greatly enhanced cycle.

The chlorides have a high affinity for the alkalies in general and

potassium in particular. This property - together with the high

volatility - has been used in wet process kilns to control clinker

K20 levels by addition of CaC12 to the rav mix o: fuel, which

leads to loss of KC1 with the exhaust gas from the kiln. In the

suspension preheater, however, the volatilised material is

recaptured within the system unless a bleed is utilised between

the kiln and riser duct. It is generally considered that no more

than 3X of the chloride passing from the preheater to the kiln

vi11 l e a v e the system with the clinker. Although considerably

higher levels have been noted in individual samples of clinker

this is probably due to a ‘push’ of kiln feed, or a semi flush

situation as thermodynamic consideration indicate that no chloride

should pass through the burning zone. On many SP kiln systems

some degree of preheater cleaning is necessary on a regular basis

and this may help to control the chloride cycle by forcing the

kiln conditions into a situation which permits a brief increase in

clinker chloride level (i .e s reduced material temperature,

increased material loading and flux level).

Small amounts of chlorides will also leave the preheater system

with the waste gas stream. Results of detailed balances at Rope

and Plymstock suggest that the preheater loss can be of a similar

o r d e r t o t h a t i n c l i n k e r . Taking the total loss of chloride from

the system as between 2 and 5% of the feed to the burning zone -

it would then be expected that a circulating load of 20 to 50

times the rota1 chloride input vould develop.

2 . 4 A l k a l i e s 13p l4

The major source of alkalies will be the raw mix; notably the

clay component although minor quantities can arise from :he fuels.

The initial free alkalies will behave in one of three ways:

1. Remain in the material being processed and become

incorporated in the clinker constituents that are being

formed . This happens to Na20 to a greater degree than K20.

2 . 3e converted into different compounds - chlorides, sulphates,

carbonates, hydroxides - by reaction with other constituents

of the raw mix.

3 . Diffuse to the surface of the process material and volatilise.

I n i t s ini:ial s t a t e K20 b e g i n s t o v o l a t i l i s e o v e r a w i d e r a n g e o f

temperature, depending on the form of clay in which it was

incorporated, but irrespective of source it would be expected to

have volatilised almost completely at burning zone temperatures

although some may have been at least partially stabilised by

conversion to the less volatile sulphate form within the material

bed. Once volatilised it will react to form chlorides and

sulphates at the rear of the kiln. These will then deposit on

d u s t p a r t i c l e s . I n i t i a l l y , Na20 i s l e s s v o l a t i l e t h a n K20 d u e t o

its higher bond energy and so a greater proportion of the kiln

feed Na20 would be expected to pass through the burning zone in

c l i n k e r w i t h o u t p a r t i c i p a t i n g i n t h e v o l a t i l e c y c l e s . Volati l ised

Na20 will react with SO2 and SO3 to form sulphates towards the

r e a r o f t h e k i l n , and with chloride vhere this species is present

i n e x c e s s o f K20. Where alkali is present in excess of chloride

and sulphate, alkali carbonates will be formed. Each of these

alkali compounds will deposit in a liquid state on the surface of

dust particles in the cooler zones of the kiln and lower preheater

stages and will enter the volatile cycles as the dust is separated

o u t i n t h e c y c l o n e s . Direct contact onto kiln or preheater

surfaces may lead to the development of build-up. The compounds

will then re-enter the kiln where the degree of volatilisation

will depend on the species and the kiln conditions. V o l a t i l i t y

decreases from chloride to carbonate to sulphate, and hence

sulphates are more likely. to pass through the burning zone.

Nevertheless the likely range o f burning zone temperatures cover

the thermal area in which alkali volatilities are likely to

increase significantly with rising temperature.

2 . 5 S u l p h u r 15s16~17

Sulphur can enter the system in a number of forms from either fuels

or raw materials. A limited amount may evaporate in the upper

preheater stages and escape from the system in the exhaust gases.

In general SO2 and SO3 can form in the high temperature areas and

b e t r a n s f e r r e d t o t h e g a s p h a s e . I n t h e c o o l e r a r e a s o f t h e k i l n

back end and preheater system, sulphates will form and re-enter

the material stream. Preferentially alkali sulphates will be

produced with excess sulphate combining with free lime or calcium

carbonate that is available in these areas. k’here r e d u c i n g

conditions exist the equilibrium will favour the existence of SO2

a n d SO3, so restricting the formation of sulphates. In this

situation loss of sulphur oxides by way of the stack may increase,

but where the gas stream passes through the raw mill the majority

of the sulphur oxides would be expected to react with the high

active surface calcium compounds which are produced in the milling

process. This will :hen return to the kiln in the raw mix as part

o f a n e x t e r n a l c y c l e .

The high boiling points of the alkali sulphates would indicate

that relatively low levels of volatilisation would be expected.

Bowever, dissociation may occur, particularly under the reducing

conditions which can exist to some degree within the burning zone.

Calcium sulphate also has a high boiling point, but is even more

susceptible to dissociation, so a higher recirculation of sulphate

from this compound would be expected. As CaSC& cannot recycle as

a compound the lime from this compound remains in clinker as free

lime - making burning more difficult - whilst the SO3 is carried

in the gas stream CO the kiln back end where it reacts to form

a l k a l i o r c a l c i u m sulphate.

3 . PROCEDURZS

The intention was to operate the kiln under controlled back end oxygen

and kiln temperature regimes, w i t h k i l n backend NOx l e v e l b e i n g u s e d a s

an indication of change in temperature. The selected target &OX levels

were 800, 1,200 and 1,400 ppm. For each of these temperature conditions

two back end oxygen levels were targetted; 1.8% (moderate) and 2.5%

(norma 1 oxygen 1 . At Hope Works there is a positive offset of 0.6 to

0.8% for the measured value of kiln back end oxygen as compared to the

real value due to the type of sampling system. In addition to these

s i x t e s t s , a further run was conducted at 1,200 ppn~ NOx with CO evident

at the kiln back end - reducing conditions in the burning zone (Table 1).

Each test run lasted at least twelve hours, over which period samples of

preheater feed, kiln feed (ex Stage IV cyclone) and clinker were taken

a t h o u r l y i n t e r v a l s . Coal samples were also taken periodically. Each

sample vas analysed for major oxides, fluoride, chloride, tr’a20, K20 and

SO3 using an XRF analyser. Ultimate and calorific value analyses were

also performed on :he coal samples.

Throughout each test period and, in most cases, for a period before the

test began, kiln operating conditions were monitored. Aver age kiln

conditions, and the range of conditions for each test are detailed in

T a b l e 2 . Tne results of chemical analyses of Stage IV material are

presented graphically for each test to show the variation with time

together with selec:ed process data in Appendix I.

During each test a number of plant measurements - air flovs, shell, gas

and material temperatures - vere completed, and a kiln operation log

maintained in order to build up sufficient information for heat and mass

balances. This raw data is summarised in Appendix 2, but in the context

of the alkali cycle studies this information is relatively meaningless

as the dominant factor in the overall heat balance vas the raw meal feed

rate vhich varied significantly from test to test, notably as a function

of whether the raw mill was in operation. Additionaliy, o v e r t h e t i m e

period covered, modifications were made to the top stage cyclones which

changed the external dust cycle for the system.

4 . EX?ERI.%ESTAL F!XSULTS

4 . 1 F l u o r i d e s

Average fluoride levels on a loss free basis for each test are

given in Table 3 together with the ratios of Stage 4 material and

c l i n k e r f l u o r i d e l e v e l s t o t h e f l u o r i d e l e v e l o f t h e p r e h e a t e r

feed material. The level of fluoride in Stage IV is generally

1.15 to 1.45 times that in the preheater feed indicating low

l e v e l s o f r e c i r c u l a t i o n . At such low levels of recirculation and

i n i t i a l i n p u t i t i s d i f f i c u l t t o b e d e f i n i t e a b o u t t r e n d s , b u t

plots of this data - Figure 2 - i n d i c a t e i n i t i a l l y a s l i g h t

decrease in average recirculation with increasing temperature,

folloved by a climb back to the original level. This pattern has

been seen before in experimental burning of mineralised clinkers

on the dry process. Experience here suggested that fluoride

volatilisation is more dependent on the potential flux level in

the mix than on material temperatures (Table 4). It is known that

an increase in flux level vi11 reduce component volatilities due

to changes in diffusion rate and reduced surface area, and so it

is possible that in the longer term over the lower temperature

range the effect of increasing flux level vith temperature has a

greater negative effect than the straight forvard volatility/

temperature considerations for this component.

At higher temperatures the recirculation rises back up to its

previous level, as temperature effects take precedence. These

trends are confirmed by the way in vhich fluoride levels in

clinker and preheater feed are similar at low temperatures, but

clinker levels lower than those of the preheater feed at higher

temperatures. Plots of Stage IV material fluoride level against

X0x level for each individual test (Figure 3) also suggest a

slight increase in recirculation with temperature over this short

time s c a l e .

There is no indication of any change in fluoride recirculation

level being brought about by variation of the kiln back end oxygen

l e v e l .

These observations cannot cover the situation where there is an

excess of alkalies over sulphate. In this instance alkali

fluorides can form from CaF2 and so a different trend would be

anticipated.

4 . 2 C h l o r i d e s

Initial comparison of the proportions of chloride in the Stage IV

material with X0x level for individual tests suggests a stable or

s l i g h t l y f a l l i n g l e v e l o f c h l o r i d e i n S t a g e I V w i t h i n c r e a s i n g

temperature (Figure 4) . However, higher X0x levels were generally

associated with higher raw meal input levels and a lower coal to

c l i n k e r r a t i o o v e r t h e s e t e s t p e r i o d s . This gives a lower total

chloride input to the system and so the lower Stage IV levels would

seem to reflect the lower input quantity. A S c h l o r i d e i s v o l a t i l e

at relatively low temperatures it can evaporate before the burning

tone, and so has a short cycle time. Figur? 5 compares the test

average da:a for the ratio of Stage IV to total input chloride

quantities and shows a slowly increasing chloride cycle vith

temperature, but this cycle is basically a large cycle with this

ratio falling between 20 and 30. There are also indications that

at the higher NOx levels the chloride cycle had not reached

equilibrium as Stage IV chloride levels were still rising with

t i m e a t r e l a t i v e l y s t e a d y NOx l e v e l s .

During the time period i n which these tests took place the Works

were still maintaining a regular preheater cleaning campaign to

control preheater build up, and the regular seni-flush situations

vhich this incorporates may have helped to control the S:age IV

chloride levels in the lower half of the predicted range.

Certainly it has been noted at Elope t h a t o c c a s i o n a l l y c l i n k e r

s a m p l e s c o n t a i n h i g h (+ 0.1X) l e v e l s o f c h l o r i d e . Furthermore,

t h e h i g h e s t t e s t a v e r a g e c l i n k e r c h l o r i d e l e v e l (0.162) vas n o t e d

in Test 1, during vhich period the kiln was unstable.

This is no evidence that modification of the kiln atmosphere by

moving into reducing conditions causes any change in chloride cycle

as the Stage IV to total input chloride ratio for Test 5 is very

similar to those from Tests 1 and 2 which had similar NOX levels.

4 -3 Potassium

The potassium (K20) cycle measured in the Stage IV cyclone during

these tests vas betueen 3.5 and 6.5 times the input level from the

rav meal. AS the proportion in coal is relatively small and vi11

not vary very much it has been ignored in these studies in order to

s i m p l i f y t h e e x e r c i s e . Figure 6a presents average data for each

test for the ratio of K20 in Stage IV material to that in the

preheater feed, plotted against average NOx level. For the normal

oxygen tests, this K20 ratio can be seen to be increasing (from

3.85 to 4.4) as NOx rises from 900 to 1,400 ppm. For the moderate

o x y g e n t e s t s t h e p i c t u r e i s l e s s c l e a r v i r h t h e r a t i o s f o r t h e lov

(Test 3) and high NOx (Test 6) :ests being similar vith a sharp dip

in Test 1, at the medium NQx level . It has previously been

suggested that K20 vi11 pre ferentially react vith chloride to form

a lov temperature cycle. It is also noticeable that the test

which does not fit in with the general picture - Test 3 - also has

a very high chloride level in Stage IV. Assuming that the

preferential reaction of K20 in Stage IV is vith all the available

chloride it can be seen that 65 to 752 of the potassium in Stage IV

will be bound to chloride. This cycle is a low temperature cycle

and can only be controlled by use of a bleed system.

K20 vhich has not reacted with chloride will nov react to form

sulphates. Table 6 also includes the ratios of K20 as sulphate in

Stage IV to K20 in the preheater feed. In this case the level of

K20 as sulphate in Stage IV rises from 1.05 times the K20 level in

the preheater feed at low #Ox levels to 1.24 times the preheater

feed level at high NOx levels. At both moderate and normal oxygen

levels this ratio increases as kiln NOX level rises. At the lower

NOx levels (850 to 900 ppm) the indications are that potassium

sulphate passes through the kiln without becoming involved in an

internal cycle. In fact at these temperatures, it is likely that

some Na20 becomes involved in the chloride cycle, as insufficient

potassium was measured in Stage IV to totally account for the

measured chloride level. These ratios are plotted in Figure 6b,

and the trend shovs up clearly. The K2S04 ratio for the reducing

conditions test (tes: 5) is slightly higher than those of the other

tests at 1,200 X0x level, but this increase is too slight to be

convincing proof of an increased cycle under reducing conditions.

Figure 6c plots the average ratios for each test between the K20

levels in clinker and preheater feed. This clearly shows that at

low NOx levels the system is in equilibrium in terms of K20,

however as the NOx level rises the output level of K20 falls belo=

that of the input. By an EiOx level of 1,400 ppm this inbalance

is approaching 202. Bowever, with the onset of reducing

conditions this inbalance approaches 50f. There art two possible

explanations for such an offset betveen output and input levels.

Firstly, it is possible that a build up of concentrated K2SO4 and

KCL is developing; this material sets on the surfaces vi:hin the

preheater and so prevents the material re-entering the cycle.

Secondly, there is a loss of K20 from the system. This may pass

to atmosphere through the stack, or be collec:ed within the raw

mill and precipitators and be returned to system as part of a long

term external cycle.

4.4 Sodium

Test average data for Xa20 levels - on a loss free basis - around

the kiln system are presented in Table 7, showing that the Na20

level in Stage IV is between 1.6 and 2 times that in the preheater

f e e d . Figure 7a plots the ratio of Stage IV Na20 to preheater

feed Na20 against NOx level. This suggests that initially the

r a t i o f a l l s s l i g h t l y a s NOx r i s e s b e f o r e c l i m b i n g a g a i n . In the

previous section it was suggested that at low NOx levels some Na20

reacts with the chlorides so becoming involved in the low

temperature cycle. Whilst only low proportions of the available

Ka20 - possible 10% - become involved in this way, it can still

s i g n i f i c a n t l y i n c r e a s e t h e t o t a l Na20 c y c l e . I n F i g u r e 7b t h e

r a t i o o f Na2S04 i n S t a g ? IV - as calculated from a chloride,

potassium and sulphate balance - to preheater feed Na20 is

p l o t t e d . This shows an increase in ihe cycle derived from Na2SO4

as NOx increases. The data suggests that the Stage IV Na20 level

- as SO4 - rises from about 1.35 times the level in the preheater

f e e d a t l o w NOx t o a b o u t 2 . 0 t i m e s a t t h e h i g h NOx l e v e l . T h e

results of moderate and normal oxygen level tests suggest that the

recycle can be reduced slightly by operation at higher oxygen

leve 1s , although this is not born out by the low oxygen test.

AS with K20, clinker output and preheater input Na20 levels are

Salanced a t l o w NOx l e v e l s , b u t a s t h e NOx l e v e l r i s e s a b o v e 1 , 3 0 0

t h e l e v e l i n c l i n k e r f a l l s q u i c k l y ; being betveen 20 and 30% lower

than the input level at 1,400 ppm NOx. This suggests that above a

certain temperature l e v e l Na2SO4 r e c i r c u l a t i o n l e v e l s a r e s t r o n g l y

temperature dependant. As Na20 is almost completely present as

Ira2SO4, this makes it possible to control and in some cases signifi-

c a n t l y r e d u c e t h e fia20 r e c y c l e b y conirol o f k i l n c o n d i t i o n s .

4 . 5 S u l p h u r

The level of sulphur - calculated as SO3 - measured in Stage IV

material and in clinker was very variable during the course of

each test as the levels react strongly to changes in both kiln

temperature and atmosphere. However, average values for the

samples taken around the kiln system for each test - given in

Table 8 - show a more consistent picture. The ratio between

sulphur as SO3 in Stage IV and in raw meal is plotted against kiln

NOx level in Figure 8a and shows that over the temperature range

investigated the total sulphur cycle rises from 1.6 to 2.6 tonnes

the sulphur input from the raw meal, rising as the NOx level

r i s e s . There is also a substantial increase in cycle once

reducing conditions are encountered - from 2.0 to 3.0 at

1,200 ppm NOx.

In the previous two sections the alkali sulphate cycles have been

discussed. AS these cycles increase only slowly with temperature,

these effects have been removed from the sulphate cycle by

calculating the amount of sulphur likely to be present as alkali

in raw meal and Stage IV and subtracting these from the total

quantities. The remaining figure can be considered to be the

sulphur which takes part in a calcium sulphate derived cycle; at

ilope this makes up approximately two thirds of the sulphur in

Stage IV. In this case the ratio rises from about 2.5 at low NOx

levels to around 4 at an NOx level of 1,450 as decomposition of

CaSO4 i n c r e a s e s . This cycle also increases quickly as the kiln

moves into reducing conditions; from about 3.1 to 5.5 under the

t e s t c o n d i t i o n s .

As in the previous two sections at low NOx levels the sulphur

output in clinker is approximately in equilibrium with that in the

raw meal, whiist as NOx rises above 1 ,300 ppm clinker sulphur

levels are lower than the inputs. Study of the data from

individual tests shows that after a sharp rise in temperature the

sulphur cycle initially responds very quickly but takes several

h o u r s t o s t a b i l i s e . This suggests that if the kiln is held stable

at higher temperatures the recycle ratio is likely to rise even

above the levels measured to date, and an external cycle will

develop. In reducing conditions the clinker to raw meal sulphur

ratio also drops substantially, from equilibrium to 0.55 when

operating at a 1,200 ppm NOx level at the kiln back end.

5. GZERAL DISCUSSION

The results indicate that with improved control of the kiln and

operation at lower temperatures and under steady oxidising conditions

it would be possible to reduce the alkali levels in Stage IV by a

moderate amount, and the sulphate level significantly. Over the

temperature range investigated the K20 level in Stage IV material was

reduced by about 10% at the lowest temperatures, whilst Na20 and SO3

levels in Stage IV material both fell by over 302. It must also be

recalled that the highest kiln back end NOx level that was targetted

was 1,400 ppm. Shortly before this test work began the kiln was

commonly run at an NOx level of about 1,800 ppm, and when an NOx

monitor was first installed NOx levels of about 2,500 ppm were common.

At that latter NOx value SO3 levels of up to 7.0% were recorded as

compared to the figures of 2.4 to 2.7 obtained during the low

temperature test work. A similar effect on Stage IV SO3 level was

observed during the new Oxford Works Simulation Trials at Plymstock in

1980 lg, as shown in Figure 9, although in this case extra iron was

added to the raw mix to permit the large reduction in burning zone

temperature. Overall the general trends and size of cycle for each

component fit in with theoretical studies for the vapour pressure v.

temperature relationships (Figure 10).

During the early days of high level control test work it was noted that

raising the measured kiln back end oxygen level from its previous normal

level of about 1.5% to around 2.3 to 2.5% resulted in considerably more

stable kiln operation. This was presumably a two tier effect, as a

normal level of 1.5% gave the potential for the kiln to dip into

reducing conditions when any disturbance occurred. The first effect

of this would be to modify the flame and hence temperatures around the

system. Secondly the reducing conditions vould strip volatiles -

notably sulphate - from the system, so upsetting the equilibrium and

modifying the quantities of low temperature flux and increasing the

likelihood of high levels of build up around the kiln system. This

e f f e c t o f o x y g e n o n s u l p h u r l e v e l s in clinker has also been noted at

Northfleet Works 2o d u r i n g t e s t i n g o f an SO2 continuous measurement

probe, and at Westbury. In the first case oxygen levels were

deliberately raised in order to stabilise clinker SO3 levels to improve

the consistency of cement quality, whilst in the latter the oxygen

level was reduced in order to increase the sulphate and alkali cycles,

so allowing the control of alkali levels on these wet process kilns.

Stabilization and reduction of the volatile cycles will lead to more

c o n s i s t e n t c l i n k e r q u a l i t y , and there will be a lower and more

consistent heat sink on the front end of the kiln resulting from the

e v a p o r a t i v e l o a d 21 3 22. However, this may be partly offset by the

reduction in low temperature flux level at the rear of the kiln.

Further study would be necessary to investigate this.

In the past most methods for the prediction of volatile recirculation

have been based on the oxide input levels, without taking real account

of their inter-dependancy and the forms in which the recirculation

develops. The results of this exercise clearly show the need for a

more detailed breakdown into the components of the volatile cycles.

kiiilst early studies by Research Division did take this approach, 23.

The installation of ‘Linkman’ high level control systems will give

increased kiln stability and permit improved studies of volatile

cycles . A further exercise on a dry process site with excess alkali

would be of benefit in expa riding t h i s s t u d y t o c o v e r t h e compl?te r a n g e

of possible volatile components.

6. PR..C?IC.AL I.QLICATIOWS FOR ROPE WORKS

Tne da:a shows that the major significant cycle at 'dope is calcium

sulphate based. This cycle increases markedly with increasing

temperature and also increases greatly with the on-set of reducing

conditions. Consequently it can be minimised by operation at lov kiln

temperatures and in an oxidising environment. It is not, however,

simply enough to avoid CO at the kiln back-end, as significant reducing

conditions can occur within the burning zone before an obvious increase

in CO is noted at the kiln back-end. Previous work has shovn that a

water wash sampling system tends to give an offset in oxygen analysis

and that this offset will depend on the water flow rate through the

sampling system. In general during the test periods this offset fell

betveen 0.6 and 0.8; a kiln back-end oxygen reading of 2.5% vas

equivalent to a true reading of 1.9% (offset 0.6). In order to

minimise recycle it is advisable to maintain the kiln back-end oxygen

reading - from the existing kiln instrumentation - between 2.0 and

2.5%. Kiln front end temperatures should be at the lowest level

compatible with steady kiln operation and will depend on the rav mix

chemistry and condition of the firing system, but is likely to

generally fall in the back-end NOx range of 800 to 1000ppm.

It is significant that the periods when kiln operation is most likely

to fall outside these ranges are during the routine cleaning

operations, vhich is also the time when the concentrations of volatile

components within the kiln are likely to be the highest. Extra

attention to kiln operation during these periods should result in

minimisation of volatile retention, vhilst lack of attention is likely

to cause the majority of the potentially volatile components cleaned

from the preheater to be driven back and redeposited within the system.

In order to fully control kiln operation at these times it is necessary

that the kiln burner be notified before cleaning commences.

7 . COSCLCSIONS 6 KECOM?fENDATIONS

The test results indicate that the chemical proportions of the minor

constituent volatile components at Hope lead to the following cycle

levels in stage IV meal.

1 . The typical fluoride cycle level is 1 .2 to 1 .4 times the input to

the system and does not change substantially as the kiln temperature

or atmosphere is modified, over the range studied.

2 . The chloride cycle is 20 to 30 times the to:al chloride input and

increases slightly with increasing temperature, although this may be

due to improved kiln stability at higher temperatures. The ch lor ide

cycle is not modified by wide changes to kiln atmosphere.

3 . Chloride in the cycle combines with potassium where available.

Eowever , at low temperatures - equivalent to NOx levels of below

1,000 ppm - some sodium is also involved.

4 . The chloride cycle is a low temperature cycle and cannot be

significantly modified by variation of kiln conditions.

5 . Tne total potassium cycle is approximat ely 3 times the input level,

however about two thirds of this is present in Stage IV in

conjunction with chloride and so cannot be controlled except by

incorporation of a bleed system.

r0 . About one third of the potassium in Stage IV is either feed based on

its first passage through the preheater or derived from a potassium

sulphate based cycle. This cycle ratio varies from 1.05 (minimal

recycle) at 900 ppm kiln NOx level to 1.25 at 1,400 ppm NOx (25%

recycle 1. This portion can be controlled by operation at low

temperatures,

7 .

8 .

9 .

10 .

11.

1 2

1 3

The sodium cycle level varies between 1 .2 and 2.0 times that in the

rawmeal over the temperature range examined.

At lou temperatures some sodium becomes involved in the low

temperature chloride cycle and this boosts the sodium cycle by

about 10%.

The majority of the sodium in the system is involved in a sodium

sulphate based cycle. This is strongly temperature dependent, and

the rate of increase also appears to be increasing with temperature.

Over the temperature range investigated the recycle rose from 1.35

times the level in the feed at low temperatures to 2.0 times at

h i g h NOx l e v e l s .

T'ne total sulphur cycle is temperature dependent and rises from 1.6

to 2.0 times that of the input over the temperature range

investigated .

The alkali sulphate cycle has already been summarised in Points 6

and 9 and makes up about 35% of the total sulphate in Stage IV

material . These cyclic levels can be seen to be lover than the

t o t a l s u l p h u r c y c l e l e v e l s .

Tne calcium sulphate recycle is strongly temperature dependent and

rises from about 2.5 times the level in the feed at low NOx levels

t o 4 t i m e s a t h i g h ( 1 , 4 5 0 ppm) NOx l e v e l s .

The calcium sulphate cycle is also increased by a move into a

reducing kiln atmosphere. At about 1,200 ppm NOx the cycle

increased from 3.1 under oxidising conditions to 5.5 times the

feed level under reducing conditions.

14 . For potassium and sodium there are indications of a slow but steady

increase in losses of these components from the preheater system

as temperatures increase. This may become part of an external

cycle or may be lost to atmosphere. This could be established by

a longer term study of the levels of these components in the

precipitator and stack dusts.

15. Sulphur is in overall balance within the system below NOx levels

of 1,200 to 1,300 ppm, but above this level the loss increases

sharply with rising temperature. Again this may become part of

an external cycle or may be lost from the system, however in this

exercise no precipitator or stack dust samples were collected so

this cannot be confirmed.

16. When the kiln atmosphere moves into reducing conditions the losses

of alkalies and sulphur from the kiln system increases sharply.

In vieu of these conclusions it is recommended that, at hope:

1. Kiln back-end oxygen level is maintained at 2.0 to 2.52

(as measured 1.

2 . Kiln back-end NOx levels are mainta

the most appropriate NOx level will

chemistry.

ined b e low 1200 ppm, although

depend on detailed raw mix

3 . Communication between kiln burner and personnel cleaning the

preheater be improved.

4 . During cleaning operations short term reductions in raw meal feed

which avoid the inception of reducing conditions will be

beneficial in the longer term.

REFERENCES

1.

2 .

3 .

4 .

5 .

6 .

7 .

a .

9 .

G. Mussnung 'Contribution to the Alkali Problem in Suspension

Preheater Kilns (Bumboldt Kiln)‘. Annual Meeting of the Heat and

Energy Committee of the German Cement Makers Association,

8 December 1961, Dusseldorf.

'Coal and the US Cement Industry, Parts 1 and 2.' World Cement

Technology, Jan/Feb. 1982 and March 1982.

Weber 'Heat Transfer in Rotary Cement Kilns' Section 4.5.

3aspe1, Lorimer, Southern, Taylor 'Blue Circle High Level Kiln

Control' IEEE 1987.

A. Lorimer 'Fuel Savings Arising from the Operation of High Level

Control at Elope Works', Research Division, Process Development Dept.,

F N 86/68.

Haspel, Taylor, Kerton 'High Level Kiln Control based on NOx Monitoring'.

Paper to Cetic Chemical Commission Mee:ing, Liege, June 1985 (ETN 85/21).

Locker, Spring and Opitz 'Reactions associated uith kiln gases.

Cyclic Processes of Volatile Substances, Coatings, Removal of Rings'.

VDZ Congression on Process Technology of Cement Manufac:ure, 1971.

Davis and Longman 'Design and Experiences with Bypasses for Chloride,

Sulphate and Alkalies'.

Bra-, A.W. 'Chemical Consequences of High Efficiency Systems' Group

Technical Conference 1976.

10. Danoe and Steuck 'Behaviour of Volatile Matter in Cement Kiln Systems'

FLS Publication.

11. Hawkins and Wilson 'Fluorine Containing Phase causing Blockages in

Suspension Preheaters'. American Ceramic Society, Pacific Coast

Regional Meeting, Los Angeles, Oct. 31, 1977.

12. Polysius Review NO. 58 'Chlorine and its Behaviour in Preheater Kilns'.

13. Goes and Keil 'The Behaviour of Alkalies in Cement Burning'.

Tonindustrie-Zeitung 84(8) 125-133, 1960.

14. H. Carlson 'The Behaviour of Alkalies in Cement Raw Materials during

the Burning Process'. Rock Products Cement Industry Operations

Seminar 1965.

15. Goldman, Kreft and Schutte 'Cyclic Phenomena of Sulphur in Cement

Kilns' World Cement Technology, November 1981.

16. Hatano 'The Behaviour of Sulphur in Suspension Preheater Kiln Systems'

Zement Kalk Gips, l/1972, p. 18-19.

17. Etoc P 'The Cement Kiln - An efficient Trap for Sulphur'

Ciment, Beton, Platzer, Chaux (701) 210-213, 1976.

18. Ritzemann 'Recirculation Problems in Rotary Kiln Systems' ZKG(8),

338-343, 1971.

19. ‘-Lxperimental Prodquction of a Minor HESC at Plymstock Works, in order to

Investigate The Potential froblems Associated with The Production of a

Similar Material at a New Oxford Works' Eng.RD TN 81/13.

20. A Lorimer 'On-line SO2 Monitoring: Assessmen: of RDUV and Electrochemical

Cell Systems at Northflee:: May to October 1984'. Research Dept. Em 84/21.

21. D W Haspel 'Cement Kiln Alkali/Sulphate Cycles under Equilibrium

Conditions', Research Division, Process Development Dept., Fh'/'84/93.

22. D G Stenson 'Factors affecting Sulphate and Alkali Cycles in Rotary Kilns

and the implications of these Effects with Respect to Process Control'

Research Dept. ETN/84/13.

TABLE 1 - EST TARGET CONDITIONS

Test No.

1 1,200 1.8

2 1,200 2.5

3 800 1.8

4 800 2.5

5 1,200 AS required

6 1,400 1.8

7 1,400 2.5

NOx level BE02 level co- - -

Nil

Nil

Nil

Nil

Positive

Nil

Nil

TASL.E 2 - TEST AVERGE CONDITIONS

Test No.

1

2

3

4

5

6

7

Average

NOx l e v e l

1,244

1,214

9 9 5

a68

1,223

1,462

1,365

Range of

NOx values

573 t0 1,938

840 to 1,587

5 2 7 to 1,472

543 to 1,330

9 0 9 to 1,418

1,008 to 1,780

1,100 to 1,608

Average BE02

level

1.67

2.25

2.11

2.47

1.57

1.37

2.72

Range of Ave CO

BE02 value level- -

1.20 to 2.05

1.94 to 2.67

I .50 to 2.80

I .37 to 3.08

0.9 to 1.76

0 . 9 to 2 . 2 8

1.88 to 3.14

TABLE 3 - AtERAGE FLUORIDE LEVELS FOR EACH TFiST (LOSS FREE BASIS)

Test No.

I, Fluoride in

raw meal

Z fluoride in

clinker

1 2 3 4- 5 6 7

0.109 0.109 0.109 0.109 0.155 0.186 0.185

0.105 0.105 0.13 0.11 0.16 0.16 0.16

2 fluoride in 0.126 0.128 0.154 0.142 0.179 0.235 0.218

Stage IV

Ratio o f fluoride in 1.16 1.17 1.41 1.30 1.15 1.26 1.18

Stage IV to raw meal

Ratio o f fiuoride in 0.963 0 -963 1.193 1.009 1.032 0.870 0.845

clinker to raw meal

TABLE 5 - TEST AVE.RXE CHLORIDE LEVELS (LOSS FREE BASIS)

Test No. 1 2 2 4 5 6 7

1. % Chloride input 0.060 0.060 0.063 0.064 0 -059 0.059 o.oc+

2. X Chloride clinker 0.16 0.14 0.009 0.017 0.008 0.008 0.010

3. % Chloride in Stage I V 1.295 1.405 1.702 1.419 1.346 1.519 1.627

4. Ratio 3:l 21.6 23.4 27.0 22.2 22.8 25.7 25.4

TABLE 6 - EST AVER;IGE KpO LEVELS (LOSS FREE BASIS)

Test

1. % K20 in raw meal

2. % K20 in c linker

3. % K20 in Stage IV

4. % K20 in SCage IV

as chloride

c2. % K20 in Srage IV

as sulphate

Ratio 3:1

Ratio 5:l 1.158 1.223

Ratio 2:l 0.86 0 -89

1 2

0.65 0.622

0.565 0.551

3 4 5 6 7

0.623 0.636 0.651 0.646 0.678

0.64 0 -615 0 -465 0.53 0.56

2.505 2.618 2 -749 2.452 2.387 2.811 2 -976

1.752 1.851 2.089 1.781 1.585 2,028 2 -133

0.753 0.761 0.66 0.671 0 -802 0 -783 0.843

3.85 4.21 4.41

1.059

1.03

3 -85 '5

1.054

0.97

3 -667

1.232

0.68

4.351 4.389

1.212 1.243

0 -82 0.83

Test

TAELE 7 - TEST AVERAGE NA20 LEVELS (LOSS FREE BASIS)

1 2 3 4 5 6 7

1 . % Na20 in raw meal 0.218 0.20 0.202 0.202 0.233 0.231 0 -277

2 . 2; Na20 in clinker 0.211 0.207 0.221 0.227 0.172 0.195 0.196

3 . % fia20 S t a g e I V 0.329 0.294 0.327 0.323 0 -330 0.471 0.499

4 . % Na20 in Stage I V 0.005 0.006 0.033 0.052 0.00 0.011 0.00

as chloride

5 . % Na20 in Stage I V 0 -324 0.288 0 -294 0.271 0.330 0.460 OTFFJ

a s sulphate

Ratio o f 3:1 1.509 1.470 1.619 1.599 1.416 2.039 1.801

Ratio o f 5:1 1.488 1 .44 1.455 1.342 1.416 1.99 1.801

Ratio o f 2:1 0.97 1.03 1.09 1.12 0.74 0 -84 0.71

TABE 8 - TEST AVEUGE SO3 LEVELS (LOSS FREE BASIS)

Test

1 . 2, SO3 in raw meal

2 . % SO3 in rm (CaO)

3 . % SO3 in clinker

4 . % SO3 in Stage IV

5. % SO3 as CaS04 as

in Stage IV

Ratio 4:l

R a t i o 5:2 3.043 3.199 2.416

R a t i o 3:l 0 -985 1.035 1.023

% a s CaSO4

1- 2 3 4 5 6 7

1.53 1.369 1.417 1.419 1.473 1.668 1.696

0.696 0 -582 0.627

1.507 1.417 1.449 1.485 0.838 1.400 1.425

3.176 3.193 2.356 2.710 4.433 4.426 4.005

2.118 1.862 1.515 1.812 3 -326 3.166 2.644

2.076 2.142 1.663 1.910

2.932

1.047

66.9

3 .OlO 2 -653 2 -361

5.373 3 -856 3 -470

0.569 0.84 0.84

66.7 58 .3 64.3 75 .o 71.5 66 .o

X

HOPE WORKS No 2 KILN: RELATIONSHIP -BE;TwEEN NOx AND LX&T

x

x

x hourly ATerAg datafrom 3/5/85

0 daily AVel%gt dAtA

August 7985

I r I

730 1350 1400Burning Zone Temperature ( "cl (av.Worka pyru rending)

I

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634 PLY?WJ!OCK WORKS.. VARIATION OF SDLPHATE CYCLE WITH BZT

2c.b I I I300 4 0 0 500

so3 ex ntnge IV a8 of total SO, in feede (meal + coal)

X?PEE;DIS !

GUPHICAL REPRESESTATIOTS OF VARiATI04‘S IK

STAGE IV MATERIAL CklEYISiKV 0% A?i IYDIVIDOAL TEST EASIS

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APPENDIX 2

PLANT .?EASUREiHEATS

.9/?/37

3

3-27

l220

23-o

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t

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q I-?

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33t BLC,I 822 I

I

2

3

4

s

6

‘31

APPENDIX1

Material %Feed % B.E.

Na20

K20

S O 3

cao (caSO4

Cl

If

O-2

0.6

1 . 3

0.37

0.4 (x2)

6.0 (x10)

6.5 (x5)

0.74 W)

F

MJOI -

0.19

EOPE h7XK.S ALKALI CYW

BuRNI% ZONE c.kimIcAL LOAD

TADER 4UILIBRIUd COhDITIONS

% Prcduct 1982 clinker AbsoluteAverage % Recycled

0 . 2 0.25

0.6 0.55

1 . 3 1 . 3

0.37 0.37%

0.2% (xl)

5.4% (x9)

5.2% (x4)

0.37% (xl

2.0%

* Eetemined

13.17%

fran sulphate balance.

Assuming the average heat requirement to vacourise - deccxqmse alkalis

averages 800 kcals/kg (600 to 1000 kcals/Ig[l] 1 then average aBali

vapourisation load on th,n burning zone is 100 kcah/kg.

[ll Vapourisation only - No decmpos ition of alkali salts.

m 84/93

Over a period of years, various extimtes of the heat required to

drive off the alkali/sulphates in the burning zone have been made. The

majority are in the order of 100 kcals/kg. However ETIU 83/13 has sho+.n that

this can easily tax-y on a short term basis by i 50 kcals/lcg due to a minor

disturbance in-the Burning Zone Tarcjerature. This note references scxne of

the various source& on this subject.

1.

2.

3.

"Heat Transfer in Rotary Kilns" Weber. P.

ZIGS Spcial Publication X0 9.

Alkali cycle equivalent to 100 Kcals/kg clinker.

Lepol Grate Heat Transfer Mechanism

D-W. Haspel June, 1973

ikxaxA 005

At least 70 kcals/kg clinker of heat are recovered as a resu+lUor the

condensation of alkalis/recrmbination of SO3 on the Lepl grate.

Hope Works Alkali Cycles - See Appendix I.

The mean heat required to vapourise the alkali/sul?hates at Eope is in

the order of 100 kcals/kg. A further 10 kcals/kg is involved in the

melting/fusion of the salts.

The exothermic clinkering rcuctions cccuring in the burning zone are

equivalent to -97 kcals/kg (typical 1982).

APPENDIX II

HOPE 5XRKS CLINERING REACTIONS

~thermic Reactions:-

Heat of Form&ion

C3S

C2S

C3A

C4M

-126.2 kcal/kg

-174 kcal/kg

- 3.7 kcal/kg

-20.1 kcal/kg

Total

Hope % (1982 Av)Clinker Canposition

62.7

9.2

10.4

7.8

k&b/kg clinker

-79.1

-16.0

- 0.38

- 1.57

90.1% -97.05 km&/kg clinker

Note heat required to prcduce C3S fran C2S is +2.8 kcals/kg reactant

Endothermic Reactions:-

Alkali Recycling tlO0 kcals/kg

Latent heat of Z&ion (circa 10% of vapourising load) + 10 kcals/kg

ll0 kcals/kg

Net (Ekothermic/~doth~c~ + 13 kcals/kg

Useful Heat in Gases and Clinker Dust

Useful Heat in Gases (Sp Heat xA T) circa 0.17 x T = 17 kcals/kg/lOO°C

Useful Heat in Clinker (SP Heat x A T) " 0.30x T = 30 kcals/kg/lOO"C

APPENDIX Iv

Z-IBM! DISmmCE ?!SSCC3ATED WItTYI KILB CLEYNFG (HOPE)

Area to b= cledned. 8m (circumference) x 5m (length)

= 4Om*

AssumS thickness circa l/4 - l/3 m

Total rr&erial to be cleaned - circa 10m3

Assuming S.g of 0.8 - 1.0, Material hocked down 8 to 10 totes, of

which circa 20% - 30% are alkalis.

With additional heat requirenent to m&e clinker 300 kcaI.s/kg

This material is equivalent to 3 'Lonnes of clinker

(distrilxlt& over two hours?)

or5 tOMeS of feed

D. W. Has@

November 1984

APPENDIX III

PHYSICALC0NsmNE ASSCCIATEDWITE! ALKALI CYCLES

Latent Heat of Vapxrisation/Eemqmsition

Kcl +670 kcaldkg W)

K2s04 +563 kcals/kg W)

+1805 kcals/kg (D)

Na2SO4 t-733 kcals/kg W)

+2096 kcals/kg (D)

CasOi +675 kcaldkg (D)

v = VapxrD = Dissociate

Later&He&of Fusion

KC1 +96.6 kcals/kg (M-P. 722°C)

K2SO4 + 57.6 kcals/'Kg (M-P. 1069°C)

Na2SO4 + 42-S kcals/kg (M.P. 884°C)

Blue Circle Cement


Module 13

Section 7

Design and Experience with Bypassesfor Chloride, Sulphate, and Alkalis

DESIGB AND EXPERIENCE WITB BYPASSES FOR CBLORIDE, SULFATE .tvl, ALKALIS

By Xessrs P.. Davis and P-A. Longman

Blue Circle Industries, United Kingdom

1. INTRODUCTION

During the production of Portland cement clinker, some of the

minor components $n the raw materials and fuel, notably the alkalis,

sulphur and chloride compounds are volatilised in the kiln. These com-

ponents are then swept with the kiln gases to the cooler parts of the

kiln system uhere they condense. In the less thermally efficient pro-

cesses, such as the wet and long dry process, a significant proportion

of these volatiles can *escape' from the system either in the prccipi-

tator/5ag house dust or up the stack. In the Lepol or XL process,

which we have successfully operated for more than 20 years, fairly

high levels of volatiles can also be tolerated. This is partly

because the nodule grate is not a particularly effective scrubber and

partly.because precipitator and intermediate cyclone dust can be

discarded thereby lowering the volatile recirculating load.

Similarly, in two stage preheater processes moderate volatile inputa

can be tolerated.

However, in the more thermally efficient processes, such as the

four stage suspension preheater process, greater amounts are "trapped"

in the feed as It passes down the preheater and are carried back into

the kiln, where a proportion again evaporates. This repeated evapora-

tion and condensation leads to the gradual formation of an internal

volatile recirculating load until a steady state is reached(l).

-2-

When the first suspens'ioti.preheater processes were:-built In-tt.

1950's the importance of recirculating‘volatlles~was m.t fully appr

ciated and- a- number bf plants simply could- not run.&ecausc.of build

and blotikages at" thi Wla inlet and in. the--louer, stages: of the prc-

heater.

I,n some instances-the clinker produced uaa of unacceotable

qualitv because af its hinher alkali content in comoarlson ulth that

produced in the simpler, but leas thermally efficient processes.

Over the yeears, as a result ef studies made by both the plant

svooliers and the cement manufacturers (see references) an

understand$ng has emerged of the factors affecting the formation of

the_ volatile recirculatinn load in a four stage ausuension oreheater

process, and of the effect such a recircnlatlon has on plant ~4

mance and cement quality.

As a reaul.t, It is possible, when employing the four stage

suspension preheater process for cement manufacture to predict the

maxImum levels of chloride. alkalis and sdohate that can be tolerate<

in the raw materials and fuel. Where these levels are just exceeded

the situation can often be recovered, ad kiln operation and cement

qiiaLitp,ptob1en!s. avoided, by extr$cti-ng a proportion of the kiln gases

'befbre. tA#! preheatar such-.tI& a .sufficient proportion of the unwanted

volatile! components- are "bled" qut the system flowever, operation of

such a."bJeed" or Wla bypass,also depletes. the preheater of some of

Its he,- uith,the.reault. that -on-a susoension preheater process

the kiln bypass has to be llmited$to_about 25% otherwise there is

insufficient heat for effective preheating(2).

-3-

Bith the introduction of precalciners in the mid 1970's it vas

generally recognised that this limitation could now be overcome and

that by burning a substantial proportion of the total fuel in the pre-

calciner it would-be possible to bleed all of the kiln gases, if

necessary, and still achieve sufficient decarbonation and calcinatioa

in the preheater(3r4,S)* Moreover, whereas in a suspension preheater

process a fuel penalty of about 4 kcal kg'1 of clinker is incurred for

each 1% bypass, in the precalciner process this is roughly half at

around 2 kcal kg-l.

At that time, therefore, the precalciner pt'ocesi looked par-

ticularly attractive in this respect and partly for this reason'gained

videspread adoption. Since then operational'experience and results

have been obtained vhich have cast doubt on the effectiveness of the

kiln bypass on this type of process.

In Blue Circle Industries, in addition to operating a number of

conventioaal suspension preheater processes with no bypasses, UC have

experience in running one suspension preheater process fitted ulth a

small bypass to remove cnlorlde and two precalciner processes fitted

vith bypasses to remove sulphur and alkalis respectively.

In this paper, we outline the chemical factors which determine.

the volatile recirculating load in a four stage preheater process and

the effects this can have on kiln operation and cement quality. Our

experience in operating both four stage preheater and precalclner

kilns with and vithout by$asses and how the bypass performance com-

pares vith design predictions is-also discussed.

-4-

CFEMICZLL, PACTORS

2.1 Volatile Components

The two main factors which vi11 dictate uhether a kiln bppasz

necessary, and if so hov large, are:-

i) The total quantities of alkalis (sodiumGiAd potassium), color

and eulphur present in the rav materials aad fuel which vi11

determine the overall composition of the clinker.

ii)..,The volatility of these components in the kiln system which WI

determine-the nature of the volatile recirculating load.

The main source of alkalis is the clay and shale phases in the

secondary rau materials and in general the presence of al'alis in t1

primary calcarcous raw material or the fuel arises from coataf"~ -'ic

wtth argillaceous materials.

Chloride is normally present as alkali chloride in the raw

materials and fuel but the presence of organic chloride compounds in

coal or other fuels is possible.

Sulphur can occur as a variety of organfc and inorganic compounti

in both fuels and raw materials and in a variety of oxidation states.

The volatility of these components in the kiln system Is complex

and is dependent on the follouing interrelating factors:-

i> Whether the volatile components occur in the fuel or the raw

materials.

- 5 -

ii) The relative amounts of the total alkalis, sulphur and chloride

entering the system.

iii) The burnability of the mix, uhich can be influenced by the

overall chemistry of the clinker., the fineness of the raw feea

'and whether mineralisers such as fluorine are present.

iv) The burning environment in the kiln system, which vi11 be

influenced by (iii) above, the level of free lime required, the

nature of the fuel and firing system employed and whether carbon

and/or sulphlde is present in the raw materials.

2.2 Volatile Cycle

In practice, the volatile recirculating load in a kiln system

vi.11 gradually build up to equilibrium and it is customary for

researchers and workers in this field to estimate the final balance of

volatlles recirculating by regarding the build up in cycles. A number

of papers have been published on this subject (6-9) and this is an

approach which we have been using for some 20 years, uith the

assistance of computer programmes for the past 10 years.

In our current computer model the first cycle is regarded as

where fuel and feed first enter the kiln system. In general the vola-

tiles In the fuel are assumed to evaporate completely as the fuel com-

Susts, while of the volatiles in the raw materials, only a proportion

are assumed to evaporate - the remainder being retained in the

clinker.

-6 -

Of the volatiles that have been evaporated a certain propor:

(generally most) are assumed to condense on the feed in the prehe

such that In the secood cycle the material entering the burning zt

contains both recirculating volatile compounds and volatile compel

present in the raw materials.

For the second cycle a proportion of the volatlles in the raw

materials will again evaporate in the burning zone and the rentaind

stay in the clinker. Similarly, a proportion of the recirculating

compounds will also be evaporated uhile the remainder are retained

ihe clinker. With each ensuing cycle, therefore, the quantity of

volatile compounds recirculating and the quantity retained in the

clinker gradually builda up until the total quantity entering the

system via the feed and fuel equals the total quantity leaving c

system 'via the clinker and stack.

A simple illustration of this is shoun in Figure 1. Rouever,

tsoastruct such a balance it is necessary to have a knowledge of the

recirculating compounds being formed and the volatllities of both

these compounds and those present in the raw materials. Below, ue

.summarisf the information we have acquired from both laboratory

investigations and studies of full scale plant of the behaviour of

chlorine, sulphur and alkalis in suspension preheater processes. It

was on these data that we based the design of our bypasses for our

precalciner plants, and uhich in the light of our more recent

experiences now require modifying.

-7 -

2.3 Chloride

Virtually all the chloride input to a kiln system vi11 be volati-

lised at burning zone temperatures and in general less than 3% of that

entering the burning zone will leave via the clinkerc2).

During the process of evaporation and being swept down the kiln

the chloride will preferentially react with potassfrrm to form

potassium chloride and in general sodium chloride vi11 only form if

there is an excess of chlorine over potassium.

A very small percentage of this alkali chloride may escape up the

stack as fume or fine particles of dust but the bulk will condense out

on the feed.

By itself, potassium chloride till largely condense out in the

temperature range of 800 - 900°C and as a result it can often cause

blockages and form hard deposits in the back end chute or in the riser

pipe to the lower preheater stage. If sodium chloride is also present

the temperature range over which the chlorides vi11 condense till be

extended generally dounwards i.e. 700 - 900°C and in terms of kiln

operation this may present fever problems. A similar effect occurs in

the presence of sulphates.

2.4 Sulphur

The behaoiour of sulphur in the kiln is more complex than that of

chloride and is strongly influenced by the form of the sulphur and the

burning conditions.

-a-

In general, sulphides present in the raw materials wfll‘read~

oxidise and evqorate such.that most if not all are driven off by

time the feed reaches the burning zone. Calcium sulphate, whether

present in the raw materials or. fotmed in the kiln cycle, although

not particularly'oolatil,e, readily dissociates at burning zone tur

peratures particularly under:prevailing reducing conditions, and y

only a small proportion of this sulphate- is retained in the clinke:

Alkali sulphatea similarly dissociqte id's reducing cvirooment.

However, in an oxidising enviromaent they are'nore stable with the

result that up to 40%. of the total alLd.1 sdphates entering the

burning zone map be retained In the clinker,

It follows that the bulk of the sulphate retained in the clinks

is alkali based - either as alkali sulphate - @i?la)2S04 or as, It

alkali calcium sulphates such as calcium langbeinite 2CaSOf,.K2suq-

This is illustrated 19 Figure 2. . In contrast the sulphate evaporate

may be alkali sulphates, dissociated sulphates or oxidised sulphide.

This mixture vi11 condense and/or react over a tide range of tex-

peratures as it passes to the cooler parts of the kiln. SO2 gas vfl

readily react uith calcium oxide in the decarbonated feed at the bat

end of the kiln but any uhich is not trapped in this ny say well pa:

through all of the preheater and escape up the stack(lO*ll).

Most sulphate compounds will condense at temperatures of

900-1100°C. In our expeience if the alkalis and sulphates are more

or less in balance this condensation forms a light loosely bound depa

sit in the fourth stage and kiln inlet which can be scoured away by

-9-

passing feed or if necessary easily removed. Aowever, if there is an

excess of sulphate over alkalis the deposit can be more sticky and

harder to remove. Densified layers of calcium sulphate and sulphosi-

licates 2(2CaO.S102)CaS04 (Figure 3) may form which vi11 adversely

affect kiln performance and. require special cleaning

facilities(12r13). If fluorfne is present In the rav materials the

situation may be exacerbated by it promoting the formation of inter-

mediary compounds such as the fluorinated calcium dumim:e

(CllA7.CaF2), spurrite 2(2. CaO .SiO2 )CaC03 and various calcium sulpho-

silicates.

2.5 Alkalis

In general a substantial proportion of the recirculating alkalis

will be associated uith either the chloride or sulphate as discussed

above.

Houever, there ulll always be a tendency for some alkalis, par

ticularlp Na20, to be retained in solid solution in the main clinker

phases(14). The proportion of alkalis retained in this uap in the

clinker will increase if there is an excess of alkalis over sulphate

entering the system or if reducing conditions prevail in the burning

zone. Reductioa will lead to dissociation and preferential loss of

SO2 although with severe reductioa, alkali iron sulphide compounds

such as ?ZeS2 may form in the clinker which results in a greater

retention of alkali and sulphur in the clinker than is otherwise

expected.

In general up to 50% of allalis entering the burning zone as

alkalis oxides may be retained in the clinker.

- 10 -

If there is an excess of al.kali over sulphate uhere condensation

occurs at the back end of the kiln hard alkali carbonate based depo-

sits may form.

PRACTICAL IWLICATIONS

3.1 Kiln Operatioa

Published data, coupled vith our OM experience, suggests that

the formation of blockages and hard'deposits in the fourth stage and

kiln chute are likely to start presenting problems when the level of

chloride or sulphate in the feed material leaving stage 4 exceeds 1.5%

or 3.5% respectively. In addition such levels of sulphate may also

enhance ring formation in the kiln itself thereby restricting output.

The quantity and nature of the volatile recirculating load will

obviously vary from Works to Works, but in general practical terms in

order to avoid exceeding the above limits it will be necessary to

restrict the total volatile input to the system, when &pressed as a

percentage of the clinker output, to 0.025 - 0.030% chloride and

around 1.5% sulphate.

Normally, the alkalis vi11 be bound up ulth the chloride and

sulphate and the levels that can be tolerated in the material leaving

the fourth stage till be dictated by these components. If alkali is

predominantly present as alkali carbonate, however, we vould expect to

be able to tolerate up to at least 3% alkalis in material leaving

stage 4. Thus in terms of kiln operation it may be possible to

- 11 -

tolerate a total alkali iIIQUt to the SysteZI expressed as a percentage

of the clinker output of 1.2% equivalent xa20 without running into any

serious problems.

'3.2 Cement Quality

In terms of cement quality the obvious factor which may dictate

limiting the quantity of alkalis retained in the,clinkar vi11 be the

necessity to manufacture a low alkali cement in order to avoid expan-

sive alkali aggregate reactions occurring in the concrete. In this

respect the ASTM limit of'O.6% equivalent Xa20 is generally recognised

worldtide. Aowever, even if the productioa of a low alkali cement is

not required, it may still be necessary to limit the level of al!calis

retained in the clinker. For example alkali sulphates vi11 shorten

setting times, enhance the early strength of the cement at the expense

of its late strength, and will render the cement more prone to air

setting during etorage. These effects vi11 normally become 'noticeable

-Jhen around 1% alkali sulphate is present in the clinker and will be

significant at the 2% level.

If there is an excess of alkalis over sulphate then the alkalis

will enter into solid solution in the main clinker phases and in

general late strength will be depressed. In this situation If the

clinker is also subjected to reducing conditions then the resultant

cement will be prone to adsorbing moisture from the atmosphere and its

flowability properties uill noticeably deteriorate. This effect is

illustrated in Figure 4.

- 12 -

If there is an excess of sulphate over alkalis then the formation

of calcium langbeinite (2CaSQ.K2S04) will enhance strengths at all

ages.

If high levels of sulphate are retained in the clinker this till

limit the level of gypsum addition that can be made at the c&en&

mill. This in turn will lead to an increase in the milling energy

required to grind to a given surface area or cement strength.

Although chloride is not usually retained in the clinker in

appreciable quantities, there is always a risk where the chloride

input is high that unstable kiln operating conditions will lead to

periods of production where high levels are retained in the clinker.

This could present problems if reinforced and/or prestressed concrete

is prepared from this cement since the chloride may migrate through

the concrete and lead to corrosion of the reinforcement.

Iu the U.K. it is recommended that prestressed concrete contains

less than 0.06% chloride when expressed as a percentage of the cement

component. In practice, after allowing for the contribution from the

aggregates and water, this means limiting the amount of chloride in

the cement to less than 0.03%.

3.3. hyuass Design

Should a kiln bypass be required to control the quantities of

volatiles recirculating in the kiln or retained in the clinker, then

this Is nornally located at the kiln inlet. The objective is to bleed

off the kiln gases containing volatiles without removing excessive

quantities of dust.

- 13 -

Some plant suppliers have found the best location for such a take

off is from the riser immediately above the kiln and along the line of

its axis. Houever, promising results have also been obtaihed with

bypasses located on the side of the riser and fitted with a deflector

which keeps the hot gases from the kiln apart from the incoming feed.

The hot gases bled from the system ulll, of course, need to be

quenched to freeze the volatilised material, and then cooled and

filtered. As close to the take-off point as possible, the..gas should

either be quenched tith cold air to a temperature of 250°C and

dedusted in a glass bag filter, or air quenched to around 4OO"C, water

cooled to 150°C in a conditioning tower and dedusted in an electrosta-

tic precipitator. Variable speed fans are preferred for the quench

air and filter fans to give a reduced electrical power consumptioa

should it be found possible to operate at less than the design bypass

percentage. Tfius the capital kost of the bypass and its ancillary

equipment, whilst only a small proportion of the total cost of the

plant, is nevertheless significant.

In sizing the bypass, the procedure we have adopted is to

construct a similar volatile recirculating load to that illustrated in

Figure 1 but with a proportion of the volatiles now bled from the

system. This is illustrated for the suspension preheater and pre-

calciner process in Figures 5 and 6 .respectlvely. For these calcula-

tions it is assumed that the bypass is 100% efficient i.e. a 50%

bypass vill bleed off 50% of all of the volatiles approaching it in

the gas stream. By running our computer programme for different

- 14 -

bypass levels it is then possible to plot the stage 4 andclinker

volatile contents against X bypass and from these plots read off the

level of. bleed required.

3.4 Bypass Operation

In addition to the capital cost, a bypass facility vi11 also

incur running and maintenance coats. As mentioned previously there

vi11 be a fuel penalty and this is illustrated for both the suspension

preheater akd the precalciner processes in Figure 7. Bypass dust will

need to be handled and disposed of and inevitably there vi11 be a

depletion of rav material reserves.

At this stage, therefore, it is uorth considering some of the

alternative eolutions. In terms of kiln operation a slight excess of

volatiles above the limits discussed in section 3.1 can often be dealt

with by installing poking facilities to remove hard deposits in the

riser pipe and in the lower stages of the preheater. -An imbalance

between sulphate and alkalis can be restored by the deliberate addi-

tion of an appropriate source of volatile compound to the feed. Uh

chloride inputs can often be avoided, for example, by purchasing a

special lov chloride coal which although generally more expensive than

the normal fuel employed we have found that in some cases its use is

cheaper overall than operating a bypass.

In tens of cement quality modifications to the setting and

strength characteristics of the cement caused by the presence of

alkali compounds can be countered by adjusting the main chemical para-

meters. ?or example it say be possible to offset the effects of

alkali sulphate by raising the silica ratio.

- 15 -

4.

If a Works is only required to manufacture low alkali cement for

some of the time then It is conceivable that alternative raw materials

sufficiently low in alkalis could be acquired for this purpose or it

may be more economic to use selective quarrying. For example, Blue

Circle Industries are presently converting an old Lepol process plant

to a precalciner. The level of sulphur in the kiln feed uas such as

to require a sulphur bypass in the updated process. Aowever, detailed

geological investigation showed that a large proportion of the sulphur

was contained vithin a well defined, approximately lm thick, band in

the shale quarry. In terms of both capital and operating cost it was

shown that it would be-more economic to discard this high sulphur

material by selective quarrying: therefore, no bypass has been

installed.

In some instances rather than operating a bypass to remove vola-

tiles it may be more convenient to make further additions to the feed

in order to produce a fully mineralised clinker with enhanced cement

propertiee. This is a technology which ue have developed and suc-

cessfully applied in Blue Circle Industries (15-17).

PRACTICAL EWERIENCE

4.1 Suspension Preheater Plants without Bypass Facilities

Blue Circle Industries currently operate a number of suspension

preheater plants without bypass facilities. Samples of kiln feed,

fuel and clinker are regularly analysed on the Works and on several

occasions samples have been taken from the stages of the preheaters ia

order to construct a volatile balance. For the purposes of this paper

-de have selected four plants to illustrate the range of our

experience.

- 16 -

The total volatile input at these plants derived from both the

raw materials and fuels and expressed on a clinker basis is as

follows:-

Plant lA 1B 2 3 4A 4B

Fuel Oil coal coal coal Gas Oil

Total S as SO3 ,0.90 0.95 1.70 0.45 0.1s 1.05

K20 1.30 1.30 0.65 0.35 0.60 0.60

NaZO 0.20 0.15 0.20 0.10 0.50 0.50

Cl 0.02 0,ozs 0.02 0.04 0.00s 0.00s

Further details of these plants, the typical levels of volatiles

found in the stage 4 feed and the typical equivalent ??a20 content of

the clinker are shown in Figure 8.

Plant 1 was originally oil fired but in 1981 it was converted to

coal firing. By necessity a low chloride coal is used 1:; order to

avoid having to install and operate a chloride bypass. With both

fuels the alkali and sulphate input is high but more or less in

balance uith the result that although build ups do form in the riser

and fourth stage they are relatively easy to remove.

The resultant clinker, however, has a high equivalent Na20 cou-

tent, the bulk of which is present as alkali sulphate. Although the

productioo of a low alkali cement is aot necessary to counter the

adverse effect the alkali sulphates have on the late strength of the

cement the Works have traditionally incorporated some sand in their

mix to lover the total alkali input and raise the silica ratio. Hore

recently, mineralisers have been successfully used at this Works to

- 17 -

achieve both a further Improvement in late strengths and a reduction

in the level of alkali retained in the clinker.

Plant 2 is also fired uith a specially purchased low chloride

coal in order to avoid a high chloride recirculating load.

The raw materials "are relatively low in alkalis with the result

that a low alkali clinket,uith an equivalent Na20 of less than 0.60%

can be easily produced. Unfortunately, these rau materials also con-

tain fluorine and appreciable quantities of sulphur and uhile both

these components can be limited by selective quarrylng their presence

leads to the formatioa of hard back end deposits uhich require regular

removal. !4'hile such removals do create a certain amount of kiln

instability this is not excessive. Overall this plant demonstrates

that vith comprehensive cleaning facilities high volatile recir

culating loads can be tolerated, and the formation of hard back end

deposits can be successfully controlled, vlthout having to resort to

the operation of a kiln bypass.

A further consequence of the presence of fluorine, hovever, is

that the early activity of the oement is depressed with the result

that, in viatet in particular, setting times are extended and any

placed concrete has a tendency to "bleed". Various solutions to this

problem have been tried but have either been impractical or unecono-

mic. More recently, however, this problem has been overcome by the

use of mineralisers.

Plant 3 is fired ulth a locally available coal. As a consequence

it3 chloride Input is high and would normally require the operation of

- 18 -

a small chloride bypass. Houever, the sulphate and alkali inputs are

both relatively low, and more or less in balance, vith the result that

by operating facilities to regularly clear away deposit? which form in

the riser and stage 4 the need to bleed any of the kilns' gases has

been avoided. The resultant cement has a very low alkali coatent well

belov 0.60% equivalent Xa20.

Plant 4 contains bath an oil fired kiln and a gas fired kiln.

30th kilns have a lov chloride input but a high alkali input which

results in the equivalent Xa20 in the clinker exceeding 0.60%.

However, as extended cements are mainly produced at this Works this

does not present any problem. . In term of kiln operation, the gas

fired kiln has an excess of alkalis over sulphate uhile the oil fired

kiln has an excess of sulphate over alkalis. Both kilns form deposits

in the riser and fourth stage which are easily removed.

Construction of the volatile recirculating load for each of the

above 'kilns has indicated that the proportion of volatiles evaporated

in the burning zone of each is similar. In all cases (Figure 9) vlr

tually all the chloride, approximately 85% of any excess sulphate over

alkalis, around 65% of alkali sdphate and about 60% of excess alkalis

over sulphate was evaporated.

4.2 Chloride Bypass - Plant 5

In addltioo to the above suspension preheater processes, a

further one is operated in the Blue Circle group *with a small bypass

to remove chloride. Up to 1975 this plant uas oil fired, thereafter

it has been fired with natural gas.

- 19 -

The raw materials employed on this 53orks comprise a high grade

limestone, a marl and a sand. Both the limestone and marl contain

chloride with the result that the total chloride input to the kiln

system, as a percentage of the clinker output, is around 0.10 - 0.15%.

In designing the bypass for this plant it uas assumed that both reten-

tion of chloride in the clinker and loss up the stack uould be negli-

gible. As a consequence it was predicted (Figure 10) that a kiln

bypass of some 8 - 10% would be required to lower the concentration of

chloride in the stage 4 feed to an acceptable level. In practice,

however, up to 0.01% chloride can be retained in the clinker vhile a

somewhat smaller amount leaks up the stack uith the result that the

actual bypass operated is in the range 6-8X.

Volatile and mass balances have been constructed for this plant

with both fuels. These have indicated that:-

i> the proportions of volatlles evaporated in the buhling zoue are

similar to the other suspension preheater plants operated in the

Blue Circle Group

ii) the bypass is also effective at bleeding sulphur and al!calis from

the system

iii) changing from oil to natural gas did not affect the performance

of the bypass although some change in clinker chemistry and

cement quality and grindability as a result of the lover sulphur

input was noted.

4.3 Alkali Sypass - Plant 6

This is a 2500 tonnes per day oil fired precalciner process uhich

- 20 -

came on stream in December 1981. The tav materials consist of a high

grade limestone, a siliceous limestone, a clay and iron oxide. The

total volatile input, as expressed as a percentage of the clinker out-

put, i s : -

%

Total S as SO3

K20

Nat0

Cl

0.75

0.75

.0.3s

0.01

Use of these raw materials produces a mix with a relatively high

silica ratio of around 3.5 and a difficult combinability.

Consequently it was assumed when designing this plant in the late

1970's that the degree of evaporatioo which would be achieved in the

burning zone uould be at least the same as that found in our suspen-

siou preheater processes.

Against this background and assuming losses up the stack would be

negligible it was predicted that the bypass required to produce a lov

alkali cement would be 30% (Figure 11). At the time all the main

plant suppliers agreed with thi's prediction.

However, since the plant has been in operation difficulties have

been encountered in producing a low al'kali cement. Close monitoring

of the process, during a series of trials carried out In conjunction

with the plant supplier, has established that while the bypass is

currently limited to only about 25% the main reason for this failure

- 21 -

to produce a low al'kali ceaent is that the degree of evaporation

a'chleved in the burning tone is lower than *was expected. Further

investigation has indicated that the main reason for this is the very

much higher solid to gas ratio achieved ia. a precalciner'kiln which

restricts the sueeping action of the gase&*).

On this basis (Figure 11) it is estimated that the bypass would

have to be increased to around 60% in order to produce a suitably low

al!=11 clinker.

.AI.temative solutfoas have been considered.. Attempts to increase

the volatility in the burning zone by raising the lime saturation and

silica ratio of the feed, or by burning more fuel in the rotary kiln

rather than in the precalciner were only partially successful. Adding

chloride to the system to enhance the volatility of the alkalis or

selective quarrying of the raw materials to lower the alkali input

have been considered but are expensive. At present the Works

have found the least expensive solution is to

low alkali clay and produce low al'kali cement

pending modification to the bypass.

4.4 Sulphur Bypass - Plant 7

This is a 1200 tonnes per day coal fired precalciner process

buy In an alternative

on a campaign basis

which was first lit up in September 1982. The raw materials consist

of a high grade and a low grade limestone. Both these and the fuel

employed contain appreciable quantities of sulphur *with the result

that there Is a gross excess of total sulphate over al:kalis entering

- 22 -

the system. The total volatile input, as expressed as a percentage of

clinker output is:-

Total S as SO3

K2O

Na20

Cl

I

2.30

1.05

0.65

0.01

These raw materials produce a mix uith a relatively lou silica

ratio and an easy combinability and it was anticipated that volatility

in the burning zone could be low. In addition, from the form of the

sulphur in the rau materials it vas recognised during the design stage

that this might be volatillsed in the upper stages of the preheater

and escape up the stack, Comercial decfsioas dictated that the plant

had to be ordered before the rav materials could be comprehensively

tested and consequently when sizing the bypass we decided to take the

possible conservative view that only a limited quantity of SO2 may

escape up the stack and that the volatility of the excess sulphate

over alkalis entering the burning zone as calcium sulphate could well

be dissociated to the same degree in a precalciner process as in the

suspension preheater process. Accordingly, ue estimated (Figure 12)

that the bypass would have to be at least 30X and that cleaning faci-

lities similar to those employed at our plant 2 may also be required.

At the same time our calculations showed that while the clinker

sulphate content would be sufficiently lowered (Figure 13) to pemit

an acceptable gypsum addition, the allcall content of the clinker

- 23 -

(Figure 14) uould remain high. Fortunately, the production of a low

al'kali cement is not required at this Works'and, although the alkalis

are mainly present as alkali sulphatcs, their effect on the cement

strength is also not critical (indeed it is beneficial) as the

majority of the cement produced at this Works is extended with an

addition of natural pozzolan. However, the presence of alkali sulpha-

tes uas expected to lead to air setting problems necessitating

appropriate counter measures.

In general the plant supplier agreed ulth these conclusions.

Their calculations shoved that while a lover bypass would probably

suffice, a 25% bypass should be installed to cater for all even-

tualities and therefore they raised no objection to our proposal to

install a 30% bypass.

Ho-vet, uhen the plant was fully commissioned in November 1982

it became clear that the Win could be successfully operated wlthout

having to bleed any of the kiln's gases. Since at that time the fuel

cousumption guaranteed by the plant supplier still needed to be

checked opportunity was taken while performing this exe&se to take a

range of balance samples at different bypass settings. The results of

this investigation are shovn by the dotted lines on Figures 12-140

The main points to emerge from this evaluation were:-

i> around 50% of the total sulphide present in the rav materials MS

evaporated in the upper stages of the preheater and escaped up

the stack

- 24 -

ii) the proportions of volatiles evaporated in the burning zone were

substantially lower than in any of our other plants.

As a consequence, acceptable lev.$ls of total sulphate in both the

stage 4 material and the clinker are obtained without having to resort

to bleeding any of the kiln's gases* Hovever, the level of alkali

sulphates in the clinker Is high and as expected the Works have been

forced to counter airsetting problems.

Follouing these observations we have developed a laboratory test

method for assessing the potential for sulphur to burn off over the

temperature range likely to be encountered in the upper stages of a

preheater. 'By subjecting raw materials to this test irom a number of

sites ue have shown that this cannot be readily predicted from the

chemistry alone but is dependent on a number of factors including the

form the sulphur occurs in the raw naterials, the temperature at which

the calcareous component in the feed starts decarbonating and the

atmosphere in the preheater, which may itself be influenced by the

composition of the feed.

DISCUSSION XND CONCLUSIONS

1. In modern, high thermal efficiency dry process kiln systems, con-

tinuity of kiln operatioa and clinker cement quality are more

susceptible to adverse effects of volatile components in the kiln

feed and fuel. Wet, long dry and Lepol or ACL process plants are

not so sensitive.

2. In some circumstances it has been found that such effects can be

more economically countered by selective quarrying, or by the use

- 25 -

of alternative raw materials or fuel, or by adjusting or minera-

Using the chemistry of the clinker, or by adopting intensive and

automated techniques for clearing build ups from the riser pipe,

rather than by resorting to a bypass.

3. The adoption of precalciner processes has made it practicable to

employ bypasses of up to 100% of the rotary kiln exhaust gas,

compared tith a practicable limit of about 25% vith a simple

suspension preheater system.

4. However, with a precalciner process only about 40% of the fuel

is burned in the rotary kiln itself and recent experience shovs

that the volatilisation of alkalis and sulphates in the kiln are

lower than in normal preheater kilns. This reduces the effec-

tiveness of the bypass where the objective is to limit the al'kali

or the sulphate content of the, clinkers.

5. While siting a bypass to remove chloride is relatively easy, that

required to remove alkalis and sulphur Is more complicated and is

dependent on the interaction of these compounds and their volati-

lity in both the preheater and the rotary kiln and is therefore

more difficult to size.

6. -Although plant data provides a number of useful reference points

from which future predictions can be based, there is also a need

for the properties of the raw materials and fuel to be e!UplOyed

to be thoroughly evaluated in the laboratory.

- 26 -

For our part in Blue Circle we have both a comprehensive modern

laboratory which can carry out such investigations and coaslderable

background experience and data from the operatioa of plants.

6. ACKXOWLEDGEHENTS

The authors wish to thank Blue Circle Industries PLC for their

permission to publish this paper and to colleagues and Associate corn-

panics vithin the Blue Circle Group for their contributions.

7. REFERENCES

1. Reactions associated vith the kiln gases: Volatile cycles, build-

ups, ring removal.

F.W. Lecher, S. Spmng C D. Opitz.

Zement-Kalk-Gips, 1972, 25, (l), l-12.

2. Bypass systems for preheater and flash calciner kilns-

N.U. Biege h L.J, Parsons . '

Pit and Quarry, 1978, 2 (L), 91-97.

3. Second generation precalclnlng with bypass alternatives for

alkali control.

F.I. Kohanowski h J.L. Shy.

Proceedings of the 13th International Cement Seminar,

Chicago, 1977, 98-108.

4 . Eeductioo of alkali and sulfur content of clinker by kiln bypass

in flash calciner systei.

J. Tjarshawsky & E.S. Porter

In Process technology of cement sanufacturing,-

VDZ Kongress L977, 652-659.

- 27 -

5. New cement plant in Abu-Dhabi uith precalciner 'kiln process with

100% kiln gas bypass system.

N. Xakamura & 2. Tojo.

Clments, BiGtons, Platres, Cbaux, 1982, (7381, 268-274.

6. Redustioo of alkali and chlor%e cycles in the suspensioa pre-

heater kiln.

3. Schliiter

Zement-Kalk-Gips, 1972, 25, (l), 20-22

7. Raw meal preheater and alkali problans.

B. RItzmann.

Proceedings of the Eighth International Cement Industry Seminar,

Chicago, 1972, 39-47.

a. Cyclic behaviour of volatile components in dry process plants for

burning cement clinker.

W. Danowski h U. Strobel.

Silikattechnik, 1977, 28, (2), 40-43.

9. Hethod for predicting cyclic behaviour of deterious substances in

cement 'kilns.

W. Kreft.

Zement-Kalk-Gips, 1982, 35, (9), 456-459.

10. The behaviour of sulphur in cement clinker burning.

S. Sprung.

Tonindustrle-Zeitung, 1965, 89, (5/6), 124-130.

- 23 -

11. The behaviour of sulphur in the suspension preheater kiln.

H. Eatano.

Zement-Kallc-Cips, 1972, 2, (l), 18-19.

12. Investigatioos of the formation of rings in rotary cement kilns.

3.93. Sylla,

Zement-Kalk-Gips, 1974, 27, (LO), 499-508.

13. Xing Formation in Rotary Cement Kilns.

D. Opitz.

Schriftenreche der Zementundustrie No. 41, 1974.

14. The distribution of al'kalts in Portland cenent clinker.

H.W.U. ?ollitt h A.W. Broun.

Proceedings of the Fifth Internatioaal Symposium on the Chemistry

of Cement, Tokyo, 1968, Vol.1, Part I, 322-333.

15. U.K. Patent No. 1498057.

16. Use of mineralisers to produce high strength cement.

G.R. Long.

Proceedings of the Fifth International Conference on cement

rnicroscapy. Nashville, 1983, 86-98.

17. Improvements in the early properties of Portland cement.

G.K. Yoir.

Proceedings of the Royal Society discussion meeting,

London, 1983, 127-138.

- 29 -

3.8. Processing of 'kiln dust

B. Tettmar, S.R. Khor and S. Gregory

Process Technology of Cement Manufacture

WZ Koogress 1977, 658-663.

FIG 2. Calcium Langbeinite (arrowed blue) with associated alkalisulphate (arroued red) in production clinker.

FT.G 3. Laths of calcium sulphosilicate (arrowed yellow) within densecal clum sulphate rich deposit from sintering zone in kiln.

FIG 4. Dark chain like droplets (arroued white) of atmosphericmoisture occurring on the surface of a polished section of a clinkermanufactured in a reducing environment.

.Ti-

cW

,*iu

-b .

-

!-,y

4T.i =i

‘t!I G!-a

.3

‘1, cr,

+L ‘2

z:..:

iyL

LiL

I-

!Z-

1

:i27.;

&I.

ax.-

L‘I-‘z-

.-

:ri&

.--....... . . . . . . . . .

,_

i--.-l.-.:, ..!.

II-- f.,,.... . . .._I

~ _ .. ..LIl-l-

ail.._

..... . . . . . . . . . .._.__..

- c

Figure 7-f&l Consumption vs % BypassI_-

11001100

7373ss--33* 1000iii2g.I

P51 g00

8-43A.23.g 8 0 0

%cdi-L

/

/

/ ;//Suspens ion

000 Precalciner KilnAir separate)

High Dust Loss - - - -Low Dust Loss

10 20 30 4 0 50 60 70 8 0 90 100% Bypass

00

00

00

LnLnO

u3LnLo

a3

a3

cn

m-c

n4

- 4

Figure $-Typical Kiln Volatilities of E3CI Suspension Preheater Process (No Bypass)- - -.. - -

100

90

80

7 0n

20

10

0

Chloride

AlkaliSMphate A

6s 3,

Llr

‘lkali S(other than Alkali Sulphate

Sulphate(other than Alkali Sulphate)

QS-

(u

kI

co .

Fiqure IO-Chloride Level 4 Stage 4 vs % Bypass in PLd 5

11

1 0

9

8

7

6

5

4

3

2

1

Predicted Level

- A - - - Actual Level

4 6 0 1 0 12‘10 E&pass

ommendeBypass

Figurell, -E%quivaleht NazO in Clinker (Ye) vs ‘lo Bypass in Plant 6.

Predicted levels for suspensionprahealer kiln volati(ities---I- Revised prediction using Bypass trial data

Actual levels from Bypass trial

mendat ion

40 50 .60z!!LBBy~

80

Predicted lev& for different kiln volatibtiesw - - L --- Actual iwels from Bypass trial

&commended

1 I I 1 I , , 1 1 1 J

0 5 1 0 15 20 25 30 35 40 45 50 55‘lo Bypass- -

FJgure14--E4uivaient NgzO in Clinker(%) vs % Bypass in Plant 7

2.0 -

1.8 -

I.6 -

f-#+zdicted IQVQ~ for different kiln volatilitibs_c-m-- “Actual IQVQIS. from Bypass trial.

0.6 -

o-4 -

ecommended Bypass

o-2 -

0’ I I I I I 1 I 1 I 1 10 5 10 15 20 25 3 0 35 40 45 5 0 55

B y p a s s‘lo

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Blue Circle Cement


Module 13

Section 8

Kiln Gas Bleed Considerations

KILN GAS SLEED CONSIDERATION!fj

2 .

The process under consideration wiil need a chloride bleed in order thatregular kiln operation can be maintained. With the present raw materials,the required bleed level will be 30 to 50% of kiln gases. At this bleedlevel, clinker equivalent soda levels will be very high, at 0.9 to 0.95%, butcould be reduced by 0.1% by use of a bleed of up to 75%.

Other options for clinker alkali reduction are the use of alternative rawmaterials, which is also likely to reduce the bleed requirement for kilnoperation, or addition of CaCl, which will have a major effect on alkalilevel but will require a larger gas bleed.

Dust loss with the bleed is likely to be 200 to 250 gnns/Nm’ of gasextracted from the system but design should assume a level of 400grms/Nma.

INTRODUJCTION

Since the development of the suspension preheater based dry process forcement manufacture, the study of the inherent cycling effects of thepotentially volatile components - which are present as minor constituentsof the raw materials and fuels - have become increasingly Important. Inthe older, less thermally efficient processeS, a natural loss of a portionof these volatile components occurred in the waste gases, soautomatically controlling the recirculation levels within the burningsystem and the level in the clinker product.

In the suspension preheater, with its greatly increased surface contactbetween gas and particle and repeated separation of gas and particle, therecovery and retention of volatilised components will be almost complete.This leads to Increased proportions of these components in clinker andwithin the kiln system; which can lead to the development of operationalproblems in the pyre-processing stage due to the quantities of potentiallysticky components that build-up through cyclic processes. Wherenecessary, a portion of the kiln gas is bled off in order to remove aportion of the volatile components and so control the levels of thesecomponents either in clinker because of cement quality requirements orin the kiln system because of the potential to cause blockages within thepreheater.

The minor components that are generally considered to be involved inmajor volatile cycles are the fluoride, chloride, alkali and sulphur species- although other elements do also become involved in cycles to a muchlesser degree (V, As, Pb, TL, Cd, Hg, Zn), these are not important for thisstudy.

1

Prime control of the volatile component cycles is performed in the designstage of a works project through the selection of raw materials and fuelsin order to optlmfse the relative and absolute levels of the potentiallyvolatile components and, where necessary, the inclusion of a bleedsystem. Once these factors have been defined, the voiatiles will developinternal and external cycles during the pyre-processing stages; an internalcycle being totally within the kiln and preheater system, whilst anexternal cycle will leave the system but be returned after a time lag (forinstance with the precipitator dust). Where processing conditions are keptsteady, the cycles will continue to develop until equilibria are reached, atwhich time the total amounts of volatile entering the system will bebalanced by the quantities leaving the system.

The degree of volatilisation and the ratez at which the equilibrium areestablished will depend on:

(a)

b)

(cl

(4

(e)

(0

kc)

(h)

(i)

(j)

The species, their chemical forms and concentrations.

The volume of gases.

The intimacy of contact between gas and solid.


Possibility of dissociation or further reaction.



Kiln atmosphere.

Kiln temperatures.


Most of these Factors are to some degree inter-related and so in normaloperation the only methods available to control the degree ofvolatilisation and eventual concentration in clinker and kiln system willbe the kiln internal atmosphere and temperature, and the proportion ofgas bleed from the kiln exit.

The major cycles will be internal cycles between the burning zone andpreheater where an individual cycle time of twenty to thirty minuteswould be expected. Smaller quantities of volatile components may beinvolved in the longer term external cycle which develops via theprecipitator dusts and volatile colIection in the raw mill.. These would beexpected to have much longer cycle times of up to 24 hours.

Any kiln gas bleed from a suspension preheater system obviously has a

2

3 .

3.1

3.2 Fluorides

fuel penalty associated with it. For a standard preheater, it is generallyconsidered that a 30% bleed approximately represents the maximumeconomically justifiable level, whilst in a precalciner the equivalent kilnexit gas volume can be between 30 and 45% of that of a simple suspensionpreheater which makes bleeds of up to 100% justifiable under certaincircumstances. Because of this factor, control of the cycles for improvedprocess operation is easier on a precalciner than on a suspensionpreheater. Conversely control of the proportions of the volatiles inclinker is more difficult due to the reduced volatilisation that occurs inprecalciner kilns - a consequence of factors (b) (c) (f) (i) and (j).

GENERAL FUZVIEW OF PROPERTIES OF VOLATILE COMKNENTS

General Comments

In practice, the proportion of each potentially volatile compound whichevaporates within the kiln can vary significantly depending on the speciesand the type of process. Typical ranges that have been reported in theliterature are set out in Table 1, whilst Table 2 details melting and boilingpoint data for the major components. In the past, empirical limits havebeen proposed for the total concentrations of volatile input to a kilnsystem but modern practice is to specify the concentrations that can be

- tolerated in the lower stages of a preheater. The maximum reportedranges that are generally accepted at this point as being unlikely to causeany operating problems are:

chloride 1.0 to 1.5%so, - 2.5 to 4.5%alkalies 2.5 to 3.5%

It is possible to operate successfully with significantly higher levels ofindividual components, however, the overall effect on kiln operation woulddepend on the relative proportions of individual compounds and the effortput to cleaning the preheater interior. The individual components arediscussed in the following sub-sections.

Fluorides can be found naturally in raw materials or be deliberately addedin small quantities to the raw mix. In low proportions its mineralisingaction has a beneficial effect on the burning process. Generally, it haslow volatility and causeS few operational problems, although a level ofabove 0.25% in clinker may lead to setting problems, particularly inwinter. Cases have been cited, however, where a hard dense build-up hasdeveloped in preheaters where the build-up contains a high (over 1%)proportion of fluoride.

3.3 Chlorldq

Chlorides are derived from the raw materials and the kiln fuel. The highvolatilities of these compounds, together with the high collectionefficiency of the cyclone preheater systems, will lead to the developmentof a greatly enhanced cycle. The chlorides have a high affinity for thealkalies in general and potassium in particular. This property, togetherwith the high volatility, has been used in kilns (commonly on the wetprocess, occasionally on the SP process) to control clinker K,O levels byaddition of CaCl, to the raw mix or fuel, which leads to loss of KC1 withthe kiln bleed or the exhaust gas from the kiln system. In the suspensfonpreheater, the volatilised material is recaptured within the system unlessa bleed is utilised between the kiln and riser duct. It is generallyconsidered that no more than 3% of the chloride passing from thepreheater to the kiln will leave the system with the clinker. Al*thoughconsiderably higher levels have been noted in individual samples ofclinker, this is probably due to a ‘push’ of kiln feed, or a semi-flushsituation as thermodynamic considerations indicate that no chlorideshouldpass through the burning zone. On many SP kiln systems some degree ofpreheater cleaning is necessary on a regular basis and this may help tocontrol the chloride cycle by forcing the kiln conditions into a situationwhich permits a brief increase in clinker chloride level (i.e. reducedmaterial temperature, increased material loading and flux level).

Small amounts of chlorides will also leave the preheater system with thewaste gas stream. Taking the total loss of chloride from the system asbetween 2 and 5% of the feed to the burning zone, it would then beexpected that a circulating load of 20 to 50 times the total chloride inputcould develop in a system without a kiln gas bleed.

No reports of low temperature chloride volatilisation within the preheaterhave been identified.

3.4 Alkalies

The major source of alkalies will be the raw mix; notably the claycomponent, although minor quantities can arise from the fuels. Theinitial free alkalies will behave in one of three ways:

(1) Remain in the material being processed and become incorporatedin the clinker constituents that are being formed. This happens toNa,O to a greater degree than K,O.

(2) Be converted into different compounds - chlorides, sulphates,carbonates, hydroxides - by reaction with other constituents of theraw mix.

(3) Diffuse to the surface of the process material and volatilise.

4

3.5

In its initial state, K,O begins to volatilise over a wide range oftemperature, depending on the form of clay In which It was incorporatedbut irrespective of source, it would be expected to have volatilised almostcompletely at burning zone temperatures, although some may have beenat least partially stabilised by conversion to the less volatile sulphateform within the material bed. Once volatilised it will react to formchlorides and suiphates - chlorides preferentially - at the rear of the kiln.These will then deposit on dust particles. Initially Na,O is less volatilethan K,O due to its higher bond energy and so a greater proportion of thekiln feed Na,O would be expected to pass through the burning zone inclinker without participating in the volatile cycles. Volatflised Na,O willreact with SO, and SO, to form sulphates towards the rear of the kiln andwith chloride where this species is present in excess of K,O. Where alkaliis present in excess of chloride and sulphate, alkali carbonates will beformed. Each of these alkali compounds will deposit in a liquid state onthe surface of dust particles in the cooler zone of the kiln and lowerpreheater stages and will enter the volatile cycles as the dust is separatedout in the cyclones. Direct contact onto kiln or preheater surfaces maylead to the development of build-up. The compounds will then re-enterthe kiln where the degree of volatllfsation will depend on the species andthe kiln conditions. Volatility decreases from chloride to carbonate tosulphate and, hence, sulphates are more likely to pass through the burningzone. Nevertheless, the likely range of burning zone temperatures coverthe thermal area in which alkali voiatilities are likely to increasesignificantly with rising temperature. In general, precalciner kilns havesignificantly lower burning zone temperature than are common in otherprocesses and, hence, alkali sulphate volatilisation in particular is lowerin precalciners than in other processes.

SulDhur

Sulphur can enter the system in a number of forms from either fuels orraw materials. A limited amount may evaporate in the upper preheaterstages and escape from the system in the exhaust gases. In general, SO1and SO, can form in the high temperature areas and be transferred to thegas phase. In the cooler areas of the kiln back-end and preheater system,sulphates will form and re-enter the material stream. Preferentiallyalkali sulphates will be produced with excess sulphate combining with freelime or calcium carbonate and hydroxide that Is available in these areas.Where reducing conditions exist, the equilibrium will favour the existenceof SO, and SO,, so restricting the formation of sulphates. In thissituation loss of suiphur oxides by way of the stack may increase butwhere the gas stream passes through the raw mill, the mafority of thesulphur oxides would be expected to react with the high active surfacecalcium compounds which are produced in the milling process. This willthen return to the kiln in the raw mix as part of an external cycle.

The high boIIing points of the alkali sulphates would Indicate thatrelatively low levels of volatilisation would be expected. However,dissocfation may occur, particularly under the reducing conditions which

5

can exist to some degree within the burning zone. Calcium sulphate alsohas a high boiling point but is even more susceptible to dissociation, so ahigher recirculation of sulphate from this compound would be expected.As CaSO, cannot recycle as a compound, the lime from this compoundremains in clinker as free lime - making burning more difficult - whilstSOS is carried in the gas stream to the kiln back-end where it reacts toform alkali or calcium sulphate.

4. BLEED REOUIREMENT

The proposed new line is based on a precalciner process. The raw mixcontains such high levels of chloride and alkali that the system would beinoperable without a kiln gas bleed. In addition, the alkali level in theclinker could potentially restrict the markets open to this material.

A series of raw mix designs have been considered and associated with thiswas a series of calculated material analyses based on the i3CIvolatilisation model, showing the effect of gas bleed levels between zeroto 100%. The results of these calculations are presented agafn in Table3. ,Mixes 6 and 8 represent the proposed range of clinker chemistry andfrom the material analyses these would have a chloride content in the rawmeal of approximately 0.22%. Figure 1 plots the effect of kiln gas bypasson the chloride level in the material passing from preheater to kiln andindicates that a bypass requirement of 33% is anticipated in order torestrict the chloride level at this point to 1%. Figure 2 indicates theeffect of increased chloride content in the raw meal, with chloride levelsof 0.25% and 0.30% requiring 38% and 48% bypasses respectively. Thelevels of chloride bleed would also control the levels of alkali and sulphatein the kiln inlet material to acceptable levels for kiln operation. With thelevel of bleed necessary for the control of chloride in order to maintainsatisfactory process operation and with the proposed raw mix, alkalilevels in clinker are likely to be about 0.95% Na, equivalent, as indicatedin Figure 3.

Such an alkali level is likely to be acceptable in many areas but may notbe acceptable in parts of the world where ASR is a significantconsideration. Lower clinker alkali levels can be achieved in one of threeways:

(1) Increased bypass level. Increasing the bypass level to 75% wouldbe expected to reduced the clinker equivalent soda level tobetween 0.8 and 0.85% depending on the mfx (Figure 3). In thiscase the majority of the reduction would be in the K,Oconcentration due to its higher volatility.

(2) Use of alternative raw materials. The use of an alternative rawmaterial to clay could significantly reduce the input of alkalies andchloride to the process. As an example, mix 11 in Table 3considers the effect of using 2% of a European bauxite in the mix.

6

This would reduce the cIay usage by about WY% and increase thesandstone requirement but the overall effect on the volatilecomponents is to reduce the concentrations by between 25 and40%. The immediate result of this is to reduce the kiln gas bleedrequirement for chloride control from 33% to 22%. At this lowerbleed level a clinker equivalent soda level of about 0.7% would beanticipated, whilst high bleed levels would permit further reductionin a similar manner to case 1 (e.g. 50% bleed 0.6% equivalent soda- the exact effects would obviously depend on the full chemicalanalysis of the material used).

(3) Addition of CaCl when lower alkali clinker is required. This couldbe used to make a separate quality clinker as needed. The chloridewould preferentially react with alkali and thus increase thevolati$ty of this species. However, this route would require thatthe bleed system has the potential for operation at higher levels -possibly up to 100% - during such periods of manufacture.

OTHER EFFECTS OF KILN GAS BLEEDS

As the kiln gas bleed removes gas and material at high temperature, thisobviously has a significant heat penalty on the process. A first orderestimate of the fuel penalty can be obtained from Fig. 4. The dust thatis extracted from the process has a high volatile content and so cannot bere-used within the kiln system. Consequently any dust drawn out with thegas bleed will effectively increase the raw meal to clinker factor and sorequire increased capacity in all the stone processing and mealpreparation stages through to the kiln feed point. Recently the majormanufacturers have all been making significant efforts to reduce theamount of dust extracted for a given gas bleed percenta Q e. Currently,most will design for a dust loss of 150 to 250 grms/Nm of gas bleed,whilst users are commonly reporting figures of 150 to 400 gnns/Nm’ withextremes of up to 600 grms/Nm’. The effect of this range on dust lossfrom the system and consequent extra raw meal requirement for the twoproposed plant sizes is shown in Table 4. It is assumed that in normaloperation the dust loss will be approximately 200 to 250 grrns/Nma butthe design parameters for raw meal preparation should allow for up to 400grms/Nm’.

AD JL/JAS

7

TABLE 1: VOLATWY FLANGES

(a) Primary Volatilitks

%

SO, 60-90

K,O 30-70

Na,O 20-40

Cl, l-b 96-99

F IO-40

01) Specific Volatflkies

‘56

KS4 40-60

NaS4 40-60

2CaSO,.Ks, 40-100

K,O In solid 60-90solution

Nai in solid 20-40solution

i

! I

Pb, Tl SO-99 KC1 I 97-99

Cd, V, Zn

I

I O-20 NaCI 96-99

CASO, 80-100

Primary volatilities indicate reported range of volatilities irrespective offull chemical form.

Specific volatilities indicate the reported range of volatllities for themore common compounds.

TABtE 2 : MELTING 6 l3OLlNG POWI- DATAT

Melt&g Point Boiling Point D&XX&lOC Oc OC

SoDiUM

Chloride 800 1440 - 1465 over 1200

Sulphate 884

Carbonate 850 - 854 dissociates

Hydroxide 319 - 328 1390

IfOTA9XJ-M

Chloride 700 - 768 1407 - 1411 over 1200

Sulphate 1075 1689

Zarbonate 894 - 897 disszlates

bIy&oxide 320 1320

Calcium Sulphate 1297 1350

bJ.9r1113

-1.93

000.430.510.600.229

14.05

:::43.2135.05

0.090.33

cz9

96.02.41.5

14.493.342.2342.w3.73

0.130. $1

22:

93.02.6

13.943.492.X4.923.35

0.140.5.30.560.227

w.0

LO1

1

2020.510.800.330.138

0.42

0

950.550.390.910.367

0.41

m100.60

zo:a7

000.490.7a0.810.331

0.41

Y,50.470.90.630.241

0.54

co1 0

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0.57

5450.570.820.860.W

0.43

0.81 0.66 0.4715.16 2.4a 1.331.27 1.16 1.W11.036 1.91 0.67

m100.590.870.900.368

0.45

0.430.87

z

0.260.250.590.U0.75

000.9O.Bc0.360.34

0.39

2020.4s0.Y0.410.234

0.540.40 0.42

0.440.9O.%0.387

0.Y

3.713.43. w7.64

f-43L3lj-61L 076Lrn

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0.33

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0.491.431.100.71

22.25.123.4265.60

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TABLE 4 : WI%CT OF DLJZED LJZVEL AND DUST LOADING

I

-7

-

c

Plant output 5500 tpd I 7500 tpd

Bleed % Dust Load Actual Dust Los Raw Meal Equivalent Actual Dust Loss Raw Men1 Iquivolentgrm/Nm3 .

Av tpd Max tpd Av tpd Max tpd Av tpd Max tpd Av W Max tpd

30 100 78.4 82.5 117.4 123.6 106.9 112.5 160.1 168.5200 156.8 165 234.8 247.2 213.8 225 320.2 337.1300 235.1 247.5 352.2 370.8 320.6 337.5 480.3 505.G400 313.5 330 469.7 494.4 427.5 450 640.4 674.2500 391.9 412.5 587.1 618 534.4 562.5 800.6 842.7600 470.3 495 704.5 741.6 641.3 675 960.7 1011.2

50 100 130.6 137.5 195.7 206 178.1 187.5 266.9 280.9200 261.3 275 391.4 412 356.3 375 533.7 561.8300 391.9 412.5 587.1 618 534.4 562.5 800.6 842.7400 522.5 550 782.8 824 712.5 750 1067.4 1123.6500 653.1 687.5 978.5 1030 890.6 937.5 1334.3 1404.5600 783.8 825 1174.2 1236 1068.8 1125 1601.1 1685.4

70 100 182.9 192.5 274 288.4 249.4 262.5 373.6 393.3200 365.8 385 547.9 576.8 498.8 525 747.2 786.5300 548.6 577.5 821.9 865.2 748.1 787.5 1120.8 1179.8400 731.5 770 1095.9 1153.6 997.5 1050 1494.4 1573500 914.4 962.5 587. I 618 1246.9 1312.5 1868 1966.3600 1097.3 1155 1643.8 1730.3 1496.3 1575 2241.6 2359.6- J

-__-_=

Three potential raw mixes were selected for the bypm calculation (on the basis ofkiln operation and quality).

(1) For the situation where a bypass was in operation, the fuel consumptionwa5 assumed to rise by PKcallKg per 1% of kiln gzs bleed from thesystem and 1% dust was assumed to be lost by 10% bypass in operation(precalciner process). The fuel consurnptions and dust losses on clinkerbasis were estimated as follows:

(2)

(3)

The estimated clinker analysis has been calculated for the situation wherea bleed of 0, 10, 20 50% and 100% of kfln gasa fs in operation.

When a bleed is In operation, the levels of volatiles leaving the system inthe clinker was calculated on the basis of the following assurnptiom

0) All the recirculating voIatiIes are potentially available to leave thesystem via a bleed.

(ii) The following volatillsation rata were assumed:

NaCl 99 ExcessK,O 35KC1 99 E x c e s sNa,O 35Kfl.4 40 Ca!Xl, 40KG04 40 tCasOq.K&04 40

The total volatile input is the sum of the volatlles from the raw feed andfrom the fuel expraed on a clinker basis.

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HEAT CONSLJt.4P1ON()I cd/kg NETT)

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SUSPENSION PREHEATF~~ K I L N(100% F U E L IN)ECTION INTO THE Kl

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IUST LOSS EX K ILN *I. ON CLINHtmH

I 1 I I

30 L O 50 G O 70 n o

‘1. G A S BLEED O F F A T ItIE K I L N DACK EN0

90

Blue Circle Cement


Module 13

Section 9

Ring Formations in Cement Kilns

Ring formations in cement kilnsGreg PalmerPlant Superintendant, Queensland Cement Ltd, Australia

IntroductionRing formations in cement rotary kilns have beenknown to induce operational instability and restrictgas flow to the point where production must bestopped. Rings that can cause severe operationalproblems in long wet kilns have been found in thefollowing areas:

Transition zone

Chain area

Kiln discbarge area

To prevent rings forming it is necessary to under-stand the mechanisms that assist in formation.There are a number of mechanisms, detailed byRumpfc2), that can lead to ring formations, however,there are many factors. that contribute or aid amechanism. Due to this complexity it is notpossible to study each factor in isolation. Toovercome this problem investigations rely upon anempirical approach, past history and industrialobservations.

Using a combination of both XRF chemicalanalysis and petrographic analysis, it is possibleto determine the mechanism most likely to cause abuild-up. This paper discusses transition zone andkiln discharge rings that have caused operationalproblems over the last few years. The investigationfound that the rings were formed by a combinationof combustion thermodynamics and freeze/thawmechanisms. There was no evidence of any alkalisulphate or sulphate spurrite induced rings.

Mechanism of ring formationThe main mechanisms for rotary cement kiln ringswere categorised by Rumpfm into the followinggroups:

0 melting or softening of the surface due tofriction

0 melting or freezing from the loss or input of heat

0 interlocking of particles that have been built-upby particles held together by surface forces

0 interlockirg of fine needle like particles

0 electrostatic attraction

The source of material can also be categorised asthere is a number of streams available, as shown inFigure 1, that can effect the composition.

Thus, it can be seen that ring formation is adynamic process with a number of streamsaffecting the composition at any point and anumber of different mechanisms which can bindt h e m a t e r i a l t o g e t h e r . T h i s m a k e s a n yinvest igat ion extremely di f f icul t , as i t is notpossible to hold all but one parameter constant sothat its effect can be evaluated. To overcome thisproblem investigations rely upon an empiricalapproach, past history and industrial observations.In rotary kilns most ring formations that cause

process instability are caused by mechanisms 2.3and 4.

Freeze-thaw mechanismsThe freeze/thaw mechanism is used to advantagein the burning zone of cement rotary kilns, in thiszone coating protects the refractory lining fromaggressive chemical attack and reduced thethermal energy loss from the kiln. In fact, in theburning zone, coating will build-up to a steadystate level of some 20 to 30cm in thickness. This isachieved as the rotary action of a cement kiln is anideal environment where temperature fluctationsand tumbling feed will expose a cooler surface tofreeze any liquid.

The f reeze/ thawing mechanism is tota l lydependent on the composition present and thetemperature in the vicinity of the solid. If thetemperature increases or the composi t ionchanges, for example by chemical reaction, then itis possible that the mineral system may reach theeutectic point or congruent melting point at adifferent position along the kiln axis.

The composition of the mineral system providesvaluable information as historical interpretation ofthe concentration levels can be linked to varioustypes of rings. At a fixed temperature, the mineralconcentration determines whether or not theeutectic or peritectic points will be reached. In anindustr ia l environment, i t is not possib le toquantify the exact mineral system that may exist atany one moment in a dynamic- kiln environ-ment. Mainly because the concentrat ions off lux ing and minera l is ing e lements such asalkalies, sulphates and chlorides are changing dueto volatilisation or condensation. Thus, as theconcentrat ions vary, the posit ion of l iquidformation will also vary along the kiln axis. Animportant source of alkali, sulphate and chloride isfrom the electrofilter dust being returned to thekiln. This dust can contain concentration levelsmany times that of kiln feed which, and if notmonitored, can cause rapid concentration build-up.

The role of alkali salts and alkali aluminatesMany investigations into the role of alkalies,sulphate and chlorides in ring formations by freezethawing have been undertaken and are welldocumented(4). (5). (6). (71. Choi and Glassern discussthe role of sulphur in c l inker product ion bysynthesising, calcium langbeinite (CaL), calciumaluminosu lphate (&A&) and ca lc ium silico-sulphat’e (C&,S) from chemical reagents. Thepresence of sulphur has both beneficial andadverse affects: it improves burnability by actingas a fluxing agent.

Choi and Glasse@) found that at low tempera-tures, below l,lOO’C, the evaporation process is

WORLD CEMENT DECEMBER 1590

dominated by alkali sulphates. However, this is indirect contradiction to Holderbank investigationswhere the low temperature evaporation process isdominated by alkali chlorides. However, the latterprocess is more in line with what is observed in anindustrial process.

Alkali sulphates and chlorides have possibly thelowest eutectic point in the clinkering system:K2S04-Na2S04 binary system has a eutectic pointof less than 900%. The source of alkali andsulphates is either from volatilisation from the feedentering the burning zone or a sulphur rich fuel. Aspreviously mentioned, from 1 ,lOO% upwards, thevolatilisation process is dominated by alkalisu lphates and chlor ides. Since the onlymechanism of ‘bleeding’ the system in a long wetkiln of alkali sulphate or chlorides is by dust loss orsubstition in the clinker phases, it is easy to seehow concentration can quickly reach criticalleve ls .

As the alkali salts are transported, via the gasstream towards the feed end of the kiln the dewpoint temperature wi l l be reached al lowingcondensation of alkali sulphates. In the zonewhere alkali sulphates can remain liquid its lowsurface tension will ensure that it wets the clinkercrystals present. The ident i f icat ion of alkal isulphates can be done both chemically and micro-scopically. If sulphate is a major mechanism, thenit would be expected that the percentage SOS,Na,O or K20 found in a rin,g would be several timesthe amount found in clinker. Also, if a sample isprepared for microscopical observation, then alkalisulphates can be high-lighted by potassiumhydroxide etching, which makes the identification

Table I The Chemical analysis of the different rings

reasonably straightforward.The ferrite and aluminate phases, C4AF and &A,

and alkali aluminates must be considered in ringformation as they are a significant component inthe clinker matrix, accounting for around 17-20 percent of the total composition. With an aluminamodulus of 1.38 in a pure &A-C4AF mix, theeutectic point will be reached at 1338OC. However,substitution of Na+l and K+l ions can occur in theferrite and aluminate phases, but this does notnecessarily mean that the eutectic point willdecrease. In fact Bogue(16j reports for the ternarysystem K20-CaO-A1203 the eutectic point is above1300%. The KA-CdAF-C2F system was similarshowing a eutectic point, again above 1300°C. Astudy of the different systems in which Na+l couldbe susbtituted into &A or C4AF similarly showedthat the eutectic point was about the same level asif K-7 was subst i tuted. So even though thealumina and the ferrite components will determinethe potential liquid phase, it is the presence ofsulphate and chlorides which will determine theeutectic point or congruent melting point at thelower temperature range in a cement kiln.

Spurrite formationSpurrite formation should also be considered atthis stage as it has long been associated with ringformations. Spurrite or sulphate spurrite has beenrecognized as a contributor in ring formation by theintergrowth or matting of the distinct prism shapedcrystals, The formation of spurrite Ca; (SiO&C03, has been extensively studied;(l). (lo). (l’) andrecently Bolio-Arced and Glasser@b investigatedthe role of mineralisers H20, F and Cl in the

Kiln No

Location (m)

S a m p l eDate

Na,O

K20

so3

p205

LOI

6

Clinker2116l89

0 . 7

0.11

0 . 5 3

0 . 0 6

0 . 3 2

6 6 6 5

3Om-36m 21m O m 3 0 m

Ring Ring Nose Ring Ring2611189 717189 1 o/5/89 27111185

0 . 7 0 1 .5 0 . 7 0 . 5 3

0.14 0 . 5 0.11 0 . 1 0

0.55 2.60 0.57 0 . 7 7

0.11 0.10 0.12 0 . 1 2

0.71 3.22 0.56 0 . 3 2

S R

AR

L S F

% Liquid (Lea)1338°C AR> 1.38

% Liquid (Lea)1338=X AR < 1.38

14cvc

2 . 6 7 2.63 2.29 3.52 2 . 5 6

1.65 0.86 1.21 1.46 1 . 4 1

9 4 . 4 8 3 . 3 98.4 9 5 . 7 8 1 . 2

21.3% r. 18.1% 26.4%

2 4 . 3 I+; 1:: 19.1 27.1

Flaw Meal Fly Ash

lnsutflated oust

e

Coal AshAlng

Clinker Oust

VolatIlizeed Elements Combustion Gas

Figure 7. The source of material can also be catagorised asthere is a number of streams available that can effect thecompos i t ion .

formation of spurr i te. A number of react ionmechanisms have been proposed for the formationof spurrite with the reaction frequently beginningwith either C&S or C$S.

It is generally accepted that spurrite is not- stable above 900%. Thus, if the alite mechanism is

responsible for spurrite formation, then the alitemust be transported to the cooler region in the gasstream. For alite crystals transported this way, it isonly a matter of time before the C3S will react withCO2 or SO3 to form spurr i te. Bolio-Arceo andGlassero) also studied the mechanism of spurriteformat ion f rom C2S with CaF2 and CaClz asmineralisers in the reaction species. It was foundthat at least 0.2% CaC12 or CaFl was required. Therole of spurrite in ring formation can easily bemisunderstood, particularly if CaFz is available, forwith these compounds present, a liquid phasewould be produced, raising the question, ‘Which

or the adhesion by freeze/thawing? Investigationsconducted at Holderbank suggest that r ingformat ion is a combinat ion of sol id i f icat ionbinding the spurrite crystals which further assistswith crystals growth.

Regardless if spurrite is suspected of being thebinding mechanism, then microscopic examin-ation will easily identify these crystals by their longprismatic, needle-like shape. However, it must bepointed out that investigations(‘l) have shown that.the concentration of spurrite can vary radiallythrough a ring and, at or near the hot surface theconcentration of spurrite can be very small. Itwould therefore be recommended that a radicalcross-sect ion of a r ing be sampled whenexamining for spurrite.

comes first - the interlocking of spurrite crystals rings.

Ash ringsAsh rings are worth mentioning briefly as they havebeen historically associated with ring formations.In coal fired kilns ‘ash’ rings have been reported toform in the transition zone and cooler inlet areas.These rings are formed as liquified ash, from theflame, settles on the surface of clinker crystalscreat ing areas wi th h igh SiO;, and low CaOconcentrations. In these areas only belite can beformed. Holderbank has reported that the chemicalcomposition of these rings is similar to ordinaryclinker. Microscopically, these rings are veryporous wi th both ash and c l inker part ic lesaggerating to form particles in the range of 100pmto 350pm. Cooling by rotation of the kiln or fromcolder secondary air will ensure solidification ofthe liquid phase. The presence of belite streakscaused by the welding together of the crystals byash drops is commonly associated with these

SF7 Clinker.

c.5 CIinker

Figure 2. SiO,-Al,O,-Fe,O, Ternary diagram.

industrial observations and interpretationsRings forming in Kiln 6 have, on occasions, causedsevere operating problems to the point where ithas been necessary to stop production. The ringswere forming in the upper transition zone and atthe kiln discharge (nose ring). It was initially feltthat rings forming in the transition zone weresulphate rings caused by excessive SOs beingreturned in the electrofilter dust. While the ‘nosering’ formations were caused by dusting, as theclinker cooled by secondary air at approximately500°C, it would transform belite from the beta togamma type. The dilatation of the belite crystal asit undergoes this transformation will causedisintegration of the crystal. However, the gammabelite transformation is uncommon as a high LSFwill cause a slight excess of lime to remain inbelite which acts as stabiliser.

Extensive investigations “‘sing both XRFanalysis and petrographic examination, werecarried out on the rings and the followingdiscussion shows that a completely differentmechanism was responsible.

The chemical analysis of the different rings isshown in Table 1, while Figure 3 and Figure 10 formpart of the petrographic examination.

Transition zone ringsThe chemical analysis of the K6 ring at the 30-36mmark on 26 January, 1989, shows no abnormallevels of alakali, SO3 or Cl, in fact the concen-trations are similar to normal clinker. The silicaand alumina ratio are 2.63 and 0.86, respectively.While the silica modulus is normal for ordinaryclinker the alumina modulus is very low. Plottingthe alumina and silica modulii on the ternarydiagram, Figure 2, shows that the composition hasa tendency to form rings; however, this tendency isonly slightly more than normal SR clinker. WhileSR clinker is known to form a thicker coating in theburning zone, it could not be said that unwantedring formation was associated with this type ofclinker production. The percentage liquid phasepresent in the sample, as determined by Lea(ln* islow and calculated at 12.4 per cent. Even thoughthe sample has a low percentage liquid the highiron content means that the matrix viscosity will below and effectively wet any surrounding particles.Petrographic examination is shown in Figures 3and 4; two different sections of the ring at differentmagnifications. In this section the belite has beenetched blue, alite is etched brown and the closedpores can be seen as black areas. The very densenature of the ring is evident by the close packingstructure of the different phases and a very lowopen porosity. Figure 4 shows that the matrix isferrite rich with the ferrite phase being identifiedby its unetched light grey appearance. There is nosign of any alkali sulphate and as the ferrite phasehas a low viscosity it is the most likely bindingagent in this case. The freeze/thawing of the kilnfeed by the tumbling motion is further enhanced inthis area by the increases or decreases in the heat

Figure 3. K6 Ring 3Q36m.

Figure 4. K6 Ring 3036m.


flux profile due to combustion thermodynamics.Mathematical modelling of the heat flux profilealong the axial length of the kiln shows an areafrom 20m to 35m where, given the right chemicalcomposition, the eutectic point could easily bereached as a result of a changing temperatureprofile. The heat flux profile is that of a long lazyflame with the flame temperature dropping from

1460°C, at 20m to 129OOC at 26m then increasingagain-to 13OOOC at 29m. Thus, the most likelymechanism for ring formation is an increasingflame temperature combined with a higherpercentage of liquid ferrite phase. The liquid phasewould be solidified by the tumbling kiln charge’.




The chemical composition of the K6 ring at the21m mark on 7 July, 1989, has a silica and aluminaratio of 2.29 and 1.21 respectively. Plotting thesemodulii on the ternary diagram, figure 2, showsthat its composition has the greatest tendency toform a ring. The percentage liquid phase, approxi-mately 23.4 per cent, is higher than the ring formedat the 30-36m mark, but only marginally higher thannormal clinker at 1,338OC. The alkali content,KzO(0.5%), NazO(1.5%) and S03(2.60%) are higherthan typically found in clinker. liowever, even atthese concentrations it would be unlikely thatalkali sulphates could be the binding mechanism.Petrographic examination shows that the ring is

very dense with almost no porosity. Also, as wouldbe expected for material in this section there wasalmost no alite with belite being the major phasepresent. The belite crystals varied in size from 10 to50 pm; the large crystals would suggest that atemperature around 1500°C was reached. Thebelite structure was internally disorganised, withwhat appears to be alkali sulphate on or near thesurface. Figure 5 shows a section of the ring with aband of idiomorphic periclase, round greyishcrystals, which is indicative of the ring being incontact with a dolomite lining. Associated with thepericlase is a large amount of free lime. The matrixis predominately ferrite with a small amount ofcoarse aluminate. Figure 6 is a higher magnifi-cation view of the sample further away from thepericlase band. The shape and internaldisorganisation of the belite make it impossible to

interpret; however, it is again most likely that thebinding phase is the ferrite rich matrix. The heatflux profile reduces significantly from 20m to 25m,thus any small changes in the axial position of thefeed would expose the feed to large changes in thesurface temperature. As mentioned earlier theflame temperature changes from 1460°C at 20m to1290°C at 26m, but it must be remembered thatthese predictations are based on a mathematicalmodel and in the real environment the combustionthermodynamics could shift the heat flux profile inany direction along the kiln axis. Again, it wouldappear that a varying heat flux profile in the regionof 20m to 27m is inducing ring formation by afreeze/thaw mechanism of a ferrite rich matrix.

The chemical analysis of the K5 ring gives analumina and silica modulii of 1.41 and 2.56,respectively. Plotting these values on the ternarydiagram, Figure 2, shows that the ring formationtendency is not all that dissimilar to ordinaryclinker. Petrographic examination of the ring, asshown in Figures 7 and 8, reveal a very densestructure with some closed pores (black areas) andeven less open pores (mat grey in colour). The mostevident feature in Figure 7 is the nesting of belite,blue etched crystals, which could be attributed toan inhomogeneous feed. Figure 8 shows a highermagnified view of the ring. The alite crystals,etched brown, have a maximum size ofapproximately 50pm but many small alite crystalsare also present. The matrix is predominatelyferrite rich, particulariy in areas of belite clusters.Some aluminate can be seen as a slightly darkermat grey colour. Similarly, the mathematicalmodelling of the heat flux profile in this kiln is verysimilar to Kiln 6 with an increase in the flametemperature around the 30m mark. Thus, it wouldappear that the same mechanism is responsiblefor the rings. That is varying flame temperatureassisting a high liquid content which is beingfreezed by a tumbling kiln ‘charge’.

Nose ringsampleThe sample labelled ‘nose ring’, 10 May 89, has achemical composition which is very similar toordinary clinker. The percentage sulphate is theonly minor constitutent that could be consideredslightly high. The silica and alumina modulii are3.52 and 1.46, respectively and plotting these

Figure 9. K6 ‘Nose’ Ring.

figure 10. K6 ‘Nose’ Ring.

values on the ternary diagram, Figure 2, shows thatthe ring composition has the least tendency of allthe samples to form a ring. Petrographicexamination of this sample, is perhaps the mostinteresting, and typical views are shown in Figures9 and 10. The sample has a very open porousframework of crystals cemented together by aninterstitial matrix of fine aluminate and ferritephases, as shown in Figure 9. The major phasepresent is an idiomorphic alite with a size rangingfrom 240pm to 100pm. The larger aiite shows signsof decomposition with cracking and secondarybelite formation, see Figure 10. The belite crystalson the other hand appear mainly as clusters withcrystal size ranging from 10 to 50pm. The etchingof belite shows areas of different reactivity whichis believed to be caused by alkali sulphate on ornear the crystal surface(15,.

Both the size of the alite and belite crystalswould suggest that a sintering temperature of atleast 1500% was reached. In addition, the alitedecomposition would suggest that the coolingrate, between the burning zone and the kilndischarge, was too slow. Thus, if the kiln coolingzone was relatively long and hot then hot clinkerwith a mobile matrix will crystallize as it meets thecooler secondary air as it leaves the kiln. The fact

proposed mechanism in this case would be a ver)hot burning zone combined with a hot cooling zoneso that as the clinker leaves the kiln, any clinkerdust re-entering the kiln will adhere to a mobilematrix phase and solidify on contact or as it comesinto contact with coler secondary air.

ConclusionsIt is concluded that ring formation, in Kiln 6, iscaused by freeze/thawing brought on by acombination of combustion thermodynamics anda low alumina ratio slurry. Mathematical modellingof the kiln heat flux has shown that the flameappears to be long and lazy with an increase in theflame temperature around the 30m mark. Thechemical composition has also shown that thealumina modulus is consistently low allowing for agreater percentage of liquid phase to be present atlow temperatures and that it is very near theeutectic point of the &A-C4AF system. It is alsopossible that an inhomogeneous feed may furtherexacerbate the problem.

There was no evidence to suggest that alkalisulphates or chlorides formed part of the bindingmechanism.

References

(1) OPITZ, D - Ring and Coating Formation in Cement KilnsSchr i f fenre ihe der Zement indust r ie , H:41/1974. V e r e i n dDeutscher Zementwerks EV Dusseldorf.(2) RUMPF. H - Prooerties. Bindino. Mechanisms and Strenothof Agglomerates Aufberei?ungs-Tec’knik. No 3. 1970. ”(3) LONG, G R - Deleterious Raw Materials and Their Effects onClinker Burnability. Proceedings of the fifth fnternationafConference on Cement Microscopy March 14.17,1983. NashvilleTennessee USA.(4) SPRUNG, S - influence of Process Technology on CementProperties. Translation ZKG No 10 ~577 1986.(5) CHOI GANG-SOON and GLASSER. F P -The Sulphur Cyclein Cement Kilns: Vapour Pressures and Solid-Phase Stabil ity ofthe Suiphur Phases. Cement and Concrete Research V78 ~3671988.(6) SYLLA, H M - Untersuchungen zur Bildung von Ansatzr ingerin Zementdrehofen. Zemenf-Kalk-Gips No 10 ~499 1974.(7) STRUNG, J. KNOFEL. 0. DREIZLER. S, DREIZLER. I, andBERGISCHGLADBACK. Influence of Alkalies and Sulphur onthe Properties of Cement Parts I. II and Ill. Zemenl-Kafk-Gips No5 ~130 1985.(8) MACGREGOR MILLER. F - Dustv Clinker and GrindabilitvProblems. Rock Products ~152 April i98O.(9) BOLIO-ARCEO. H and GLASSER. F P - Formation ofSpurrite Cae(SiO&COJ. Cemenf and Concrete Research V20(2)p301 1990.(10) SYLtA, H M - Investigations on the Formation of Rings inRotary Cement Kilns. Zement-Kalk-Gips (10) p499 1974.(11) BECKER. F and SCHRAMLI. W - Build-up of Rings Causedby Spurrite Formation. Cement and lime Manufacrure V42(9)p91 1969.(12) HOFMANNER, F - Microstructure of Portland CementC l i n k e r . Holderbank Managemenf and Consul t ing L td .Holderbank Switzerland 1973.(13) IMLACH. J A - Determination of the Cause of RingFormation in Kiln No 5. Darra. Holderbank Management andConsultina Ltd. Reoort No MA8613355iE.(14) IMLACH. J A 1 Analysis of Cause of Ring formation in Kiln6 at 98-121 Feet from Kiln Outlet. Holderbank Management andConsultino Ltd. Reoort No MA8913613lE.(15) IMLACH: J A and MISTELI. 8 - Texture Evaluation of Ringand Slab Samples Taken from Kilns 5 and 6 at Darra.

#Holderbank Management and Consul t ing Lfd. Report Nothat solidification take:s ulace as the materialdischarges from the kiln is evident by the rapid MA89l758lE.

cooling of the matrix. Petrographic examination of (16) BOGUE. R H - The Chemistry of Portland Cement.

the matrix shows it is composed of evenlyReihhold Publishing Corp. 1965.

distributed fine ferrite and aluminate crystals. The(17) LEA, F M - The Chemistry of Cement and Concrete 3rdedition. Edward Arnold (Publishiers) Ltd. 1983.

WORLD CEMENT DECEMBER 1990

Blue Circle Cement


Module 13

Section 10

Kiln Build-Up Meeting

KILN BUILD-UP MEETING

5 May 1998 - Fairfield House, Stanhope

Present: P.GreenoJ. CollinsonD.BelemyA.BainbridgeK.Atkinson

M.MutterL.EvansA.Edwards

J.WilsonS.Dodd

A meeting was organised to review the causes of kiln build-ups at Weardale, Cookstown and Dunbarworks. Each works has specific build-ups and the aim of the meeting was to review any commonalityin the build-ups and methods for reducing the frequency of stops. A review of each works past andpresent problems was the basis of the meeting.

Weardale Works

Three areas of build-up appear at Weardale

(iii)

Lepol roof and kiln inlet chute, with material often falling off the roof and into the kiln.Kiln back-end, often a long build-up of around 20 feet long and up to 2 foot thick. Feed canoften be held up in this area causing problems with overloading of the riddlings system.Back of the burning zone - this. material can often be reached and removed by water jetting.

The works are currently using open cast coal with a sulphur level of 0.9 - 1 %, and around 30 %petcoke with a sulphur level of around 5 %. Recently water has been injected into the flame at around 5litres per minute, which has had the benefit of tightening the flame; The water injection has also hadthe effect of softening and reducing some of the build-up in the burning zone. It is proposed to increasethe water addition rate progressively to attempt to remove build-up further back in the burning zone.

The back-end build-up is thought to be alkali-sulphate based. Sulphate to alkali molar ratio is runningaround 1.5 - 1.7 on OPC and 1.7 to 2 on SRC. Slag is also being used in the raw mix, which is having anumber of effects:

(i) Kiln outputs have increased as the level of slag used has increased.(ii) The slag tends not to create as much build-up.

The slag addition is reducing the overall sulphate input as it is replacing higher sulphate shale. Theworks is also attempting to select the best shale to minimise sulphate input.

Additional problems are experienced with the production of SRC, where nodule breakdown on thegrate is an issue leading to a dusty kiln and increased bed blinding. On occasions the kiln can build upin 48 hours on an SRC run.

Cookstown Works

Cookstown reported that whilst in the past many similar areas of build-up occurred, steps have beentaken to reduce their formation. Since 1992 the cyclone dust has been removed from the system andmixed with the CKD. This material is then transferred to the cement mills and incorporated into thefinal product. It was commented that if the cyclone dust is returned to the process then the build-ups re-occur very rapidly. The benefit of this is two-fold, the first effect is to reduce bed blinding and thesecond is to reduce the sticky alkali-sulphate material entering the kiln and forming a build-up.

Grate/chute build-ups still occur at Cookstown, although their effect has been minimised by theinstallation of silicon carbide refractory on the roof (around half of the length of the above calciner)

and sides of the Lepol grate, and also in the chute. There are no reports of any difficulties in keepingthe silicon carbide in these areas. The grate and chute have around 14 air cannons installed althoughtheir effectiveness has not been quantified. The chute and hearth were also modified around the sameperiod to allow an increased area for gas flow.

1997 saw a significant number of stops due to long, tapered build-ups through the burning zone. Thesewere often hard build-ups, and due to hardness and the shape of the taper, were difficult to remove byany other method apart from stopping the kiln and digging out the build-up. These build-ups wereassociated with the burner itself (a new FCT pipe) and the position of the burner. Since the re-installation of the burner and its alignment, only two burning zone build-ups have occurred, both beingrings as opposed to tapers. The result of this is that the rings can be shot away and a kiln stop avoided.

Dunbar Works

Dunbar works manages to achieve a good burning zone coating (although sometimes a little deficient atthe nose ring). The build-up problem would appear to be around the 35 and 45-metre length from theburner, with distinct rings being formed at these points. There is no real build-up in the riser, and thereis no riser-cleaning or air cannons. The effect of the rings is to create a dam in the kiln causing rawmeal to be held back and spill out of the back end seal.

Amongst recent solutions are the speeding of the kiln to 4 rpm to move the material away from theseal, modification of the seal to reduce spillage and a rubbing ring to be installed later in the year. Theworks have been trying to identify the cause of the rings. The’ stage 2 inlet oxygen is controlled at 4 %in an attempt to reduce sulphate volatilisation. Observations are that there is a link between NOx andspillage, and that less build-up is present if the fuel split between kiln and precalciner is kept around the40/60 % (worse spillage at 45/55 % split). The build-up is sometimes claimed to be clinkered.

Discussion

Having identified the types and areas of build-ups in each of the kilns, it is important to classify thetypes of build-up being formed. It would appear that the experiences of Dunbar could be a differentproblem to those at the back of the kiln at Weardale (and those previously experienced at Cookstown).

Dunbar need to evaluate the composition of the build-up to identify whether it is due to volatilerecirculation or whether the rings are due to clinker being carried to the back of the kiln and depositingon the sticky melt in the kiln. Such build-ups can be characterised as a dense yellow/brown material,made up of material with fine particles. The principle of analysing the “building blocks” - the size ofparticles in the build-ups - will be pursued by Technical Centre on samples of build-up received in thefuture.

On a kiln where cooler control is poor, and fine, dusty clinker is periodically produced due to over-burning, such build-ups can be expected.

In terms of sulphate control for Dunbar, much of the raw material sulphate is lost in the preheater toweras it is present in the raw meal as sulphite. This acts as a bleed for sulphur in the system, although somesulphate is returned to the system via the GCT dust. If the build-ups turn out to be sulphate based thenfurther examination of the volatile cycles is required.

Cookstown build-up is currently in the burning zone, and as previously stated, tend to be rings asopposed to tapers. It is likely that the rings are originating from the return of the dust from the coolerwhen the air cannons are fired on the IKN. The solution here would be to attempt a redesign of the inletaround the IKN to reduce the use of the cannons and therefore the amount of dust returned to theburning zone.

This leaves a comparison of the three Lepol kilns to review how to reduce the back-end build-ups atWeardale. The obvious difference is that the cyclone dust at Cookstown is completely removed fromthe system whereas this is not carried out at Weardale. When this method was tialled at Weardale, the

cyclone dust was removed from one kiln and returned to the other kiln. The kiln with cyclone dust, notsuprisingly, performed poorly. The kiln without cyclone dust resulted in a reduction in output with ahotter kiln. Whilst the trial was possibly not conclusive, the removal of dust from the cyclones andaddition to the cement mill at Weardale will potentially increase the cement alkali level above the 0.6soda equivalent level.

All of the riddlings are returned to all three kilns. Riddlings are often screened on other Lepol plants sothat the smaller fractions, which have the higher volatile contents, can be removed from the kiln systemand further reduce volatile cycles. It is proposed to trial this at Cookstown with the results being fed onto Weardale.

Cookstown comment that removal of the CKD and cyclone dust will reduce the soda equivalent in thecement by around 0.02. It is also commented that the amount of cyclone dust and the level of alkalis inthe clinker could reduce by bleeding off this material. In the trial carried out at Wear-dale in removingthe cyclone dust, around 2 tph material was collected, with around two thirds of the material having tobe dumped in the quarry.

The quantities and qualities of the materials around the system need to be reviewed and compared withCookstown, to examine whether the dust could be returned to the cement mills. This will also allow asulphate balance to be carried out to examine the extent of circulation around the system. Therecirculation of the volatile materials - alkalis and sulphates - is related to effects in the burning zonesuch as flame impingement and reducing conditions in the flame, as well as the high sulphate to alkaliratio. Calcium sulphate will decompose in the burning zone due to contact of the flame with thematerial bed in the presence of CO, and also excessive temperatures in the burning zone. Therefore it isimportant to ensure that a tight flame can be produced to avoid the flame touching the bed.

This, in principle, can be achieved by the use of a bluff body in the burner pipe. It was agreed to trialsuch a device when an appropriate design is provided by the Technical Centre. The position and angleof the burner will also be review. Both burners are in different positions at present but this has notshown any significant differences in operation. If the burner is pushed into the kiln the secondary airwill be better entrained and produce a narrower flame.

Coal mill and oxygen control at Weardale will also be reviewed so a more stable operation in theburning zone can be achieved. NOx control is currently used as a control parameter and variations inthis parameter, along with the SO2 at the back of the kiln can also give indicators as to the extent of therecirculation in the system.

An action plan of areas to review has been compiled as a result of these discussions.

2.

3.

4 .

5 .

6.

7.

8.

9 .

Process Parameters

02, NO,, S02, CO, (stack and back end).

Kiln feed, temperature profile and suction profile.

Secondary air temperature, coal feedrate.

Cement quality

- sulphate - max 3.5%- alkalis - max 0.6%

Cement quality - slump- early strength/reactivity- 28 day properties

Effect on product properties of any consideredchange.

Alkali bleed via auxiliary stack.

Kiln/cooler exhaust/dust to burning zone.

Lepol best practice for dust return/build-up.

10. Secondary firing.

1 . Cookstown max speed 1.8Weardale max speed ?Does this affect build-up?

2 . Silicon carbide on grate areas/kiln areas - SCbricks.

3. Grate chute design at CKN - compare withWeardale and hearth design.

4 . Grate Operation

- bed depth and variation (fixed speed)- doors open/closed- pan angles

5. Recirculation fan - use/benefits/effects.

6 . Microphone on grate - information on bed blinding.

Raw Meal(either for SR or OPC)

1. Fluorspar addition- benefits?- where in process?

2. Secondary r.m.‘s- Shale ‘Z’ usage- S l a g- PFA (up to 3% shale replacement -

nodule problems) .+ nodule strengthener to increaseof pfa

- increased low stone usage

3. Raw meal residue- now- with secondary r.m.‘s

4. Chemistry of particle range in raw meal.

5. Raw meal variability

use

- effect- improve

1.

2.

3.

4.

5.

6.

7.

8.

9.

Coal/petcoke mix - impact of fuel sulphur - waterInjection

Flame- bluff body- firing pipe position in relation to nose ring- firing pipe angle- secondary air temperature

Coal Residue

Coal ash composition and % in coal.

Fuel blending in coal store.

Fuel composition - frequency of sampling.

PF moisture < 1%.

Coal mill control.

NOx/kiln burning strategy (N.B. control of kiln withlow NOx burner?)

10. Kiln camera - Quadtech?

11. Shell temperature for build-up.

12. Sulphate balance/SO2 recirc/grate material SO4sample.

13. Inleaking air through system.

14. Temperature profile through grate - comparisonbetween works.

15. Removal of coal mill cyclone off cooler.

1 . Quantity of riddlings.

2. Quality and psd of riddlings.

3. Screening - possible without breaking up nodules?

4. Position of riddlings return - back onto grate vs kilnchute.

5. 1 Kiln no riddlings return, other riddlings from bothkilns - high alkali clinker.

6. Nodule strength improvement.

Cyclone Dust

1.

2.

3.

Quantity of dust - OPC and SR.

Quality of dust- chemistry- particle size- hot/cold cyclones- efficiency of cyclones between kilns -

compare with CKN. Vortex finders incyclones.

Return of dust to cement mills- how much

4. Cost of tipping dust? Can it be sold?

5. Return to other kiln.

6. Return all/proportion to noddy pans/elsewhere.

7. Predict effects of all/partial removal.

8. Pelletizing of dust.

9. Intermediate fan efficiency.

Blue Circle Cement


Module 13

Section 11

Cement Seminar- Rings, Balls, andBuild-Ups

Table of Contents

Page

1. INTRODUCTION

2. LOCATION OF RINGS

2.1 Classification

1

2

2

3. THEORETICAL ASPECTS OF RING AND DEPOSITFORMATION 4

4. CHARACTERISTICS OF VARIOUS RING ANDDEPOSIT TYPES 5

4.1 Exhaust Fan Deposits 54.2 Slurry Rings 64.3 Cyclone and Grate Preheater Deposits 74.4 * Meal Ring (Calcininq Ring) in Long Kilns 84.5 Middle Rings in Large Preheater Kilns 94.6 Sinter Rings (excl. coal-ash rings) 104.7 Coal Ash Sinter Rings 104.0 Clinker Rings / Cooler Inlet Deposit 124.9 Kiln Charge Balls 12

5. METHODS OF REMOVAL/ELIMINATION 13

TABLES 1 - 7

APPENDICES I - III

RINGS, BALLS AND BUILD-UPS

1. INTRODUCTION

Rings and deposits are accumulations of solidmaterials (from the powdery kiln charge) in therotary or static sections of clinker productionlines. They have been encountered since the earliestdays of rotary kiln production, with each develop-ment in process technology, e.g. grate and cyclonepreheaters, grate cooler, bringing with them theirown specific type of deposit.

Rather than being of academic interest, ring anddeposit formation has an appreciable influence onplant operations, frustrating-operations personnelby their impairing or even impeding production, andannoying the company management by lowering produc-tion (and sales) and increasing production costs.

As a direct consequence of rings and deposits, thegas and material flow through the kiln is restricted,resulting in a reduced kiln output. Especially inthe sinter zone, the presence of rings can interferewith combustion of the fuel and can result inimproper combustion. From time to time unstablerings and deposits can break away leading toblockage or mechanical damage in the cooler, or incyclone blockages. The partial shedding of coatingfrom the exhaust fan blades results in severe vibra-tion which mostly requires a short shutdown forcomplete removal. The breaking of a ring almostalways causes a flush of material into the burningzone and a temporary loss of stable kiln operations.

The formation of deposits in cyclones results inextra costs for the labour needed to remove thedeposits by poking. The introduction of air canons(big blasters) provides a method for their regularautomatic removal and has been installed in Groupplants with persistant preheater blockages.High pressure water jets may also be employed.

In the worst cases, a complete shutdown is necessaryto allow entrance to the affected area and mechani-cal removal of the blockage with compressed airdrills. This shutdown invariably weakens the sinterzone refractories, and accelerates the next shutdownfor rebricking.

2. LOCATION OF RINGS

2.1 Classification

Unwanted build-ups may be classified with regardto the type of material from which they areformed, either sintered or unsintered. Withinthese two groups the various types can beclassified as follows:

unsintered:----------. exhaust fan deposits. cyclone and grate preheater deposits. slurry or mud rings. meal rings

sintered:--------. middle ring. sinter ring. clinker ring. "snowman" in grate cooler. kiln charge ball

Process technological characteristics of suchbuild-ups e.g. kiln type, location, temperature ofgas and kiln charge can be seen in Table 1.Material technological characteristics e.g. stateof kiln charge, enrichment in various elements,and type of texture are summarized in Table 2.

The location of the various types of the aboverings and deposits can be seen in Fig. 1.

CYCLONE

PREHEATER KILN

GRATEI -

PREHEATER

KILN- -

WET KILN c

.

\

/

/

MUD BALLSSLURRY RING

/

RINGS AND BUILD- UPS IN

DIFFERENT KILN SYSTEMS

ASH RINGSL/D - 5-7

/

COATING ON

INLET TO

COOLER

MIDDLE RINGS LARGE DRY KILNSMEAL RINGS (LONG KILNS)L/D - 7-13

SINTER RINGSL/D - 2-7

CLINKER RINGSL/D- O-2

3. THEORETICAL ASPECTS OF RING AND DEPOSIT FORMATION

Although of much practical significance, littlequantitatively based, fundamental knowledge isavailable on the formation of deposits from solidssuspended in gas streams. In a qualitative way,however, the more important features of such pro-cesses are known.

The formation of a deposit is always a dynamic pro-cess in which the factors responsible for formationoutweigh the forces of degradation. In general, thestronger the forces of destruction, the more unli-kely the chance of deposit formation, but when thisdoes occur, a strong, hard to remove agglomerationis the result.

After the transport of material to the area of depo-sition, a definite force is required to make itadhere to the wall. This can range in magnitude fromthat caused by turbulence within the gas stream,increasing to centrifugal forces when the streamchanges direction, to that due to mechanicalpressure. Whereas preheater deposits involve thefirst two, mechanical pressure certainly plays a partin ring formation within the rotating kiln.

The forces according to Rumpf considered to causedeposit formation can be grouped as follows:

A - melting or softening of surface due to frictionor collision

B - melting or freezing due to addition or removalof heat

c - interlocking of aggregates built up of finerparticles held together by surface forces

D - interlocking of long fibrous particles

E - electrostatic attraction

The mechanisms B, C and D are the ones encounteredin kiln operations. In general, the finer thepowder, the greater the tendency towards agglomera-tion, and in many cases the absence of particlesunder a critical size (e.g. 5 urn) ensures freedomfrom deposition.

4. CHARACTERISTICS OF VARIOUS RING AND DEPOSIT TYPES

Tables 3 and 4 contain a list of typical proper-ties of rings and build-ups encountered within the"Holderbank" Group plants (with full chemical analy-sis being provided in Appendices I, II and III).Included are such factors as volatile element con-centrations and moduli of the deposited materials.As an indication of the texture, the size of thepores and the particles or aggregates of particles,from which the materials were built up, is given. Inmany cases the mineralogical composition is alsogiven.

4.1 Exhaust Fan Deposits

In the case of kilns with pressure filter systems,in which unfiltered dust-laden gas passes throughthe exhaust gas fan, deposit formation causesproblems. These arise when the deposit falls offone blade, and brings the rotating fan out ofbalance. Deposits of up to 3.5 kg/blade are knownto occur.

Characteristic properties:

Exhaust fan deposits, composed of the finest rawmeal particles are usually red-brown, hard andquite brittle. They exhibit a compact layeredstructure and have a very low porosity of 8%.

Their chemical and mineralogical composition isbasically that of the raw meal but often theplate-shaped clay particles are preferentiallydeposited parallel to the blade surfaces. Due totheir long stay in the system, fan deposits areenriched in the volatile components K20, Na20,so3. Typical values include the following:

LSF 20 - 100SR 1.0 - 1.5K20 + Na20 2.1 - 3.0 %SO3 4.7 - 6.0 %

The SO3 is usually present as anhydrite (CaS04).

Bindinq mechanism:

In this case, the temperature is such that liquidphase involvement - aqueous or molten salts - canbe ruled out.

The dust particles, because of the fan rotation,strike the blade surfaces with a high velocity andare so compacted. As the texture of the surface,after even a short time in operation, possessesundulations in the order 0.5 - 20 urn, the smallerdust particles can be mechanically "locked-on".Subsequent development of the deposit follows byan identical mechanism.

4.2 Slurry Rings (including mud balls)

Characteristic properties:

These occur in long wet kilns and are composed ofthe partially dried kiln charge somewhat enrichedin alkalis and S03. They are soft and can usuallybe broken - and hence prevented - by heavierchains. The H20 content lies between 20 and 30%, arange in which clay materials exhibit a sticky,plastic consistency. The content of the alkaliswhich greatly increases the tendency to adhesion(influence on rheological properties) can be up to10% K20 + Na20, and about the same level of S03.In many cases, balls form (in addition) on thechain links by the same mechanism. A typicalexample of a mud ball is plant I in Table 4.

Bindinq mechanism:

The binding mechanism is the well-known ability ofclays to form a sticky, plastic mass when con-taining the correct quantity of H20, and to hardenon the further water loss. To this mechanism mustalso be added the crystallization of K2SO4 solu-tion and the further strengthening of the struc-ture by formation of CaS04. Photo la gives anexample of such a ring.

4.3 Cyclone and Grate Preheater Deposits

Characteristics:

These deposits form on the roofs, walls, outletand riser pipes of cyclone preheaters, in the hotchamber of grate preheaters, and vary considerablyin appearance and homogeneity. In general, theyhave a light colour varying from cream to brown topink, indicating that the component particles hadnot been heated higher than 1200°C. In some cases,darker zones of harder burnt material can beobserved. Depending on their place of deposition,they range from a dense, compact, definitelylayered structure, hard to break to a porous (30%)material with only moderate strength with lessobvious layering. The former type is typical ofcyclone cones and discharge pipes while the latteris to be found in the transition and swirl cham-bers. Soft deposits can, however, also be found inthe cyclones.

From a chemical viewpoint, this deposit typeusually is characterized by a concentration of thevolatile elements in the following range:

K20 = 1 - 30% so3 = 1 - 35%Na20 = 0 - 2% Cl = 1 - 25%

In some cases, therefore, deposits can occur withno appreciable increase in concentration. Typicalanalyses found for deposits are given in Table 3and Appendix I.

The mineralogical composition of preheater depo-sits differs as would be expected from that of theraw meal in that the clays are essentially decom-posed, and a reaction to form intermediateminerals has taken place. Minerals containing onlythe volatile elements can also be found.

Amongst the minerals found in preheater depositsare the following:

raw meal: calcitequartz

CaC03SiO2

normal intermediate phases:

free limepericlasemayenite

CaOfMgOC12A7

typical deposit phases with low melting point:

sylvite KC1halite NaCllangbeinitearcanite

2CaS04 . K2S04K2S04

typical deposit phases without melt involvement:

carbonate spurrite 2C2S . CaC03sulfate spurrite 2C2S . CaS04anhydrite CaS04

Formation mechanism:

The binding substance in this deposit type is thelow melting point Na20, K20, SO3, Cl based com-pounds. These are molten in the kiln gas and aredeposited on the cyclone walls and pipes, or firston dust particles which then themselves are depo-sited out of the gas stream in these areas.Cooling on contact or with increasing thicknessresults in an appreciable strengthening of theoriginally sticky deposit. Because of the exten-sive duration of stay in the kiln system, a reac-tion takes place with gaseous CO2 and SO3,resulting in the formation of lath shaped spurriteand sulfate spurrite which additionally strengthenthe texture. Typical textures for unsintered, pre-heater and kiln inlet deposits can be seen inphotos lb - 1d.

4.4 Meal Ring (Calcining Ring) in Long Kilns

Characteristics and formation:

The meal rings, often called "calcining rings" inlong kilns, are in their properties and mechanismof formation very similar to those of preheaterdeposits in heat exchanger kilns. This is perhapsnot surprising in that both build-ups occur in thesame temperature zone. Meal rings are mostly lesstroublesome than preheater deposits because often,due to their relatively poor strength, thermalfluctuations, kiln deformation and the action Ofthe material stream, they fall off periodicallyunder their own weiqht. A typical example of a

4.5 Middle Rings in Large Preheater Kilns

Characteristics:

Unlike meal rings, middle "rings" are dense (finegrained) of low porosity, very hard and seldomfall off during operations. Although termed as aring they are rather more elongated, like a band,being often some 15 - 20 m long extending from 7to 11 diameters from the outlet, e.g. 35 - 55 mfor a 5 m Qj kiln. Unlike previous types, thisdeposit is clinker-like in colour indicating itbeing composed of well burnt kiln charge. Perpen-dicular to the direction of deposition, the finelayered structure can be seen showing the cur-vature of the kiln shell.

The chemical composition of middle rings is verysimilar to that of clinker. This is surprisingbecause considering the long duration of the stayin the kiln, no increase in concentration of thealkalis or SO3 takes place, and often the ringshows lower volatile element values than forclinker. Typical analyses of a middlering are given in Table 5.

The minerals found in middle rings are the clinkerminerals alite, belite, aluminate, ferrite andfree CaO, the alite having often decomposed intomicroscopically mixed belite and free CaO,resulting from the temperature at the site of thering being under the lower stability temperatureof alite (i.e. 1260°C).

Formation Mechanism:

The mechanism of bonding is the freezing of theclinker alumino-ferrite melt. Due to a long coolflame, the clinker has a tendency to be fine, andthe smallest clinker particles of 150 - 450 urn arecarried back by the flame and deposited onto thekiln wall in a zone where temperatures of below1250°C exist. The particles immediately freeze inplace, and because the kiln charge is still fine,it does not possess sufficient abrasive action toremove the growing ring. The typical compactstructure of a middle ring can be seen in photo le.

4.6 Sinter Rings (excluding coal-ash rings)

Characteristics:

These occur at the beginning of the sinter zonesome 4 - 5 D from the kiln outlet. They aregreyish-black in colour, strong and are (usually)agglomerations of small clinker pellets andclinker dust. No layer structure is obviousbecause of the presence of large pores and voids.

In general, the chemical composition is that ofthe clinker with no appreciable concentration ofvolatile elements.

From a mineralogical viewpoint, the normal clinkerminerals alite, belite, aluminate, ferrite andfree CaO are observed, with reactions to formbelite and CaOfree, spurrite and belite beingfound with increasing depth in the ring, i.e.decreasing temperature, similar to the case ofmiddle rings.

Bonding mechanism:

The bonding is created by the freezing of the alu-mino ferrite clinker liquid in the case of puresinter rings. This phenomenon occurs especially atthe beginning of the sinter zone, where the liquidphase is just starting to form (approx. 1280°C).Due to the rotation of the kiln, the charge inthis zone freezes with each kiln revolution: a newwet layer sticks on, and with time a thick depositbuilds up consisting of particles of less than 1 mm.

4.7 Coal Ash Sinter Rings

Characteristics:

In kilns fired with a high ash content coal,sinter/coal ash rings can form at 7 - 8.5 D fromthe kiln outlet. They are dense, often layered andsometimes glassy in appearance and built up fromparticles some 150 - 250 urn in size. They arerather less dense and have larger pores and voidsthan middle rings. Photo 1f gives an example ofthe microstructure of such material, showing thecoal ash layers.

From the viewpoint of their chemical and minera-logical composition they are essentially similarto clinker, exhibiting the minerals alite, belite,aluminate, ferrite and free CaO. With decreasingtemperature (increasing ring depth) reactions toform spurrite and calcite take place, and also thetransformation of alite -> belite + CaOf and /3-beliteerbelite. Details of the chemistry andmineralogy are given in Table 6 . No enrichmentof the volatile phases can be observed. Because ofthe enrichment in coal ash, the belite content ishigher than that of the clinker, and tends to befound in layers.

Formation mechanism:

The bonding medium here is the sticky molten coalash particles and perhaps to a slight extent, thealumino ferrite clinker liquid phase occurring bya mechanism such as in Fig. 2 showing thetypical build up during kiln rotation.

Fig. 2: Mechanism for ring formation

ash layer(sticky)

kiln charge

a)

ash layer+ sticking kiln

charge

kiln charge

b)

ash layer /kiln charge /ash layer

kiln charge

c)

The molten coal ash droplets adhere to the exposedkiln lining at a point and temperature at whichthey are still partially fluid and sticky. Whenthis sticky layer passes under the kiln charge oneach rotation, it is assumed that a single layerof the still very fine kiln charge adheres to it.Because of the presence of fine crystalline aliteand the overall occurrence of liquid phase, itmust be assumed that the material temperature atthe position of the ring lies above 126O'C.

The alite crystals are very small and certainlymuch smaller than those of the clinker. Because ofthis, it can definitely be said that the ring isnot formed from clinker dust blown back down thekiln.

4.8 Clinker Rings / Cooler Inlet Deposit (snowman)

Such rings and deposits are formed from normalsize clinker granules and have a high porositycontaining many voids. They are usually nottroublesome to kiln operations as they can easilybe removed. Their composition and mineralogy isidentical to clinker, but in some cases, rings ofup to 3.5% K20 and 3.0% SO.3 have been observed.

The mechanism of bonding is the freezing of theclinker liquid phase as the clinker passesthrough the cooling zone (ring) or on falling downthe chute into a grate cooler, grate kilns beingusually operated so as to have no cooling zonewithin the kiln itself.

4.9 Kiln Charge Balls

Kiln balls occur in cases where a tendency to mealor sinter ring already exists and can be up to 1 min diameter. The chemical composition is, thus, animportant factor. They are usually found upstreamof meal or sinter rings.They are usually made up of already calcinedmaterial and can have a porosity of up to 558,consisting of many fine pores. Often they consistof a hard dense porous core, surrounded by themajority of porous material. The core usually is apiece of coating from say the lower heat exchangersor the transition chamber, and often has a com-position different from the kiln charge in thearea of formation. Differences in composition canbe seen in Table 7.

The mechanism of meal ball formation can be due toeither, or a combination of the following factors:

- stripping and subsequent "balling" of old,excess coating

- agglomeration enhanced by available clinkerand/or salt melt

- ring section acts as a dam, retaining "pieces"of material for long periods. Radial growth ofthe pieces occurs by compaction and adherenceof fresh surface due to continual rollingaction of the pieces/balls over the charge.

In most cases, no liquid phase participation insufficient quantities is possible so that theballs behave like a snowball and by their ownpressure material sticks to the surface. Thismechanism is similar to that of deposition on theexhaust fan blades.

5. METHODS OF REMOVAL/ELIMINATION

An important prerequisite for minimizing the ten-dency to form objectionable coatings and rings, isstable kiln operation. This applies to the com-position, fineness and feed rate of the raw materialand fuel, and burning zone heat control.

The tendency to form coatings in the kiln is reducedby lowering the dust load of the kiln gas.

Objectionable coatings and rings which are formed asa consequence of high concentrations of various cir-culating elements can be obviated by appropriatereduction of the cycles in question.

This can be achieved by:

- Employing different raw materials and/or fuelwith lower concentration of the offendingelement. This is generally not practicable.

- Control of the raw meal milling so as to reducethe concentration of the very fine particles ofsizes under 20 urn.

- Intervention into cyclic process by eitherdiscarding dust in which the circulating elementshave become concentrated, or by means of abypass installation which extracts a portionof the kiln gas.

The penetration of false air into the preheater andkiln inlet chambers should be avoided, as such coldareas will act as sites for preferential build-up.

In order to reduce the tendency to form sinterrings, it is in the first place necessary to reducethe proportion of fusible matter in the clinker,i.e. the lime standard and silica modulus should beincreased. "Coating-inactive" bricks have alsoproved successful in certain cases, in reducing thetendency toward sinter ring formation.

In coal-fired kilns a coal with a normal ash contentshould be employed as coals having ash contents of40% are characterized by a very high tendency toring formation. No general approach can be given tothe effectiveness of other measures, e.g. alterationto firing conditions, as these represent variableswhich are peculiar to the particular installation.

Clinker rings can be avoided by shifting the flamefurther back, thus increasing the clinker tem-perature at the kiln outlet. As a result of this,however, the "stickiness" range of the clinker isshifted towards the cooler inlet. Coatings can thenbe formed on the cooler inlet chute. This is par-ticularly problematic with satellite coolers. Ininstances where this occurs with grate coolers,these coatings can be eliminated by the use of watercooled plates on the inlet chute.

Table 1: 7 r' AND LOCATION OF RINGS AND COATING(according to Opftr, 1974)

type kiln type(s) location temperature ('C)gas charge

coatfng a l l exhaust fan 128/180 -blades

slurry rlng long wet drying zone 150/300 < 100

coatfng dry stage 3 & 4 700/1100 700/800(preheater)

coating Lepol grate walls/roof of 1000/l 100 --hot chamber

coating long dry Inlet chamber lOOO/llOO --

meal ring preheater calcining zone 1100/1400"("%8""middle ring long dry II 41400

wet 1) #I 41400 <1200

kiln charge a l l calclnfng <1400 <1200ball zone

slnter ring a l l beglnnlng of 1400/1600 1250/1350. sinter zone

slnter zone a l l slnter zone 1600/1800 1350/l 450coatfng

clinker rfng a l l end of slnter 800/1600 1200/1400zone

coating grate cooler 600/800 1200entrance

Table 2: GKNPRAL MATERIAL CHARACTERISTICS OF VARIOUS RING TYPES

K

:n

all

NL

DS

w/P

DL

WLDSDL

A L L

A L L

A L L

A L L

Enriched in TextureChbractarBase Material

LocationType

-----Clin-ker

Ne,KInt.Med.

Packing

den8e

d e n e e

pOr0ll*

poKOU8

porou8

deneeI.

porou8

poroun

pcwOU8

prwOU8

PorOU8

'P8rtlcle'8ire

fine

R a wHeal

+Coating EXh8U8t fanBlade8

Slurry ring Drying Lone t(du8t

t t fine

-

t

-

t

t

t

t

--

t-

t

Coating Stage Ji4 fin8

fineCoating Halls/roof ofhot chamber

t

Coating t-

t fineInlet chamber

Calclnfng zone. l

� *

t

t*

flnel

.

Heal ringHiddle ring t

+ 4’

Kiln chargeball

+Calcining zone fine

largeSinter ring Beginning of

8inter xone

I

t

t

tSinter tone largeSinter xonecoating

-

End of sinterLone

Cl lnkerring

Coeting

large

large

-

Grat8 COOl cr

l coating center

lrblt 3 : PROPERTIES ff BUILD-UPS EXAMINED

AlktllSulphulRJtio

Gtnerrl tppttrrnctModlIlIVolrtllt Eltatnts Ttrturt

Poresire (jIltI

S R An LSF Strtngtl Colour

.ocrtlaalrnl

A

A

A

6

c

C

0

E

B

F

HtntrtlogyColour

,f strtrksK20 Bld.lJnlts

fpd

.I - 10

LtytrSwpltI

0.16 1.76 2.71 Yellw rtdlbrwn1st cyc 7.6 I - 1062

66

124

149

339

180

196

103

104

69

1.6

0.36 2.05 2.77

stron9

strong11.4 0.16 24.0 ytllw rtd/brwnInd cyc dtnseporous

dtnstporous

denseportus

VI- 5I - 10

-

Cl- 32- 6

10 - 20

1: : 2:

portsfllltd

portsfllltd

portsfilled

10 - 20

2: : 5:

KCI-200chrvgt 1-5

KC1 cont.chrrgt 2-3

KC1 cont.chrrgt O-5

5 - 10

20 - 30

Iu.KCl cant20 - 30

1.8

1.09

0.85

0.02

2.25 3.44

2.8 2.5

WJk

strong

yellow

allw/irwn-yellow

none

dtrL brown

4.6

1.3

0.09

0.65

2.56

36.0

0.61

lrd cyc" .

Ith c c'Pjpt r

' (Cone.

strong

strong *td/brom

mtdlum brown

wrk brown

0 . 8

0.19

3.6

28.0

23.6

0.67 2.4 1.69

0.38 3.33

0.07 1.46

1164.91 3.01

2.31 3.2

1.70

1.45

2.26

2.6

none

none

blttlgrttn

none

none4.66 2.62 2.36. 31.5

3 . 6

4.0

0.16 strongitllwlrwn

1.59

1.3

157

196 strong none

0.69 3.2 2.13 114 WtJk none

5.69 62 weak none

0.68 75 strong ,rd/broua blrck

User'lpt

" 10.2

1 . 3

4.6

17.6

0.08 1.61

2.5

-i-Kma or: crlcltt,su p atrite, KC1

spur-

minor: s ur-Tltc,Cr tIi

porars IO - 1s

2: : 3:

>l100

:flnnltt.

.

6.4

1.6

6.9

0.19

1.1

0.34

0.27 2.09

1.48

1.65

I4E0

6tnsrporous

dtnstporous

Table 4 : PROPERTIES ff RIMiS EXAMINED

TextureVolrtlla Elements nodull Generrl appearance

AH LSF Strength Colour

hlkrllSulphurRatio

0.44

0.14

Hlnerrlogy.ocrtlonPlant RingTYPL

MUd

Ball

Heal

SR PoreShe (pm)

Bld.Unltsfpd

K20 ma20 SO3

strong

strong d--He or - Sulphrtepurrlte

#(nor - C2S, C4AF,mite. vrrloussulphrto

2- 3

100

100

110 a

100 a

4.1

2.18

0.40

0.16 14.2 2.36

1.40

1.46

(*let,(bitt)

2- 5

50 - 100

50 - 100

150 - 450

50 - 100

yellow/brown

yellow/brown I

110

10

9 2

00

g;; c25, C3A. C4AF 150 - 250

50

0.16

0.12

0.06

0.07

1.56

0.84

1.6

2.32

K

F

Middle

Middle

45 l

37 a 2.13

2 .4

strong

rtrong

strong

strong

0.13

0.71

1.01

0.21

0.01

0.05

Et;; CZS, C3A* C4AF

C3S. C S,spurrl et

C3A, C&Of

200

100 - 150

Ash/Slnter

Ash/Slnter

100 - 200

200

150 - 300

1.01

0.34

0.24

0.03

1.14

1.21

01

7932.5 8

I EP0.20 1.56

Ash/Slnter

Et& $5, C3A, C4Af 200 - 500brow/9wN 2.88 a018.5 a 0.95 0.20 0.56

TABLE 5 : Typical analyses of a middle ringat various depths

sample ring ring ring clinkerinternal middle external

Loss on iqn. 0.47 1.82 0.94 0.37

Si02 23.0 17.8 21.9 21.9

Al203 4.9 5.1 5.5 5.6

Fe203 3.0 2.9 *-. 3.1 3.2

CaO 66.4 70.7 66.8 66.1

M90 # 1.1 1.0 0.95 0.95

so3 0.13 0.07 0.27 0.48

K20 0.16 0.09 0.30 0.35

Na20 0.06 0.06 0.12 0.14

TiO2 0.25 0.23 0.25 0.25

m203 0.02 0.02 0.02 0.02

p205 0.24 0.21 0.20 0.20

Cl' 0.01 0.01 0.01 0.01

Total 99.74 100.36 100.01 99.57

LS 92 122 96 93

SR 2.91 2.23 2.55 2.49

AR 1.63 1.76 1.77 1.75

TABLE 6: Chemistry and.Mineraloqy of a (coal ash) ring

ciln point

depth in ring

chemicalanalysis:

si02u203-203CaOM90K20Na20so3Loss on ign.-

Total

LSS RA Rfree lime

ninerals det.{wt. %)alite

p< and pbelit

liquid phase(aluminate +ferrite)

carbonatespurrite+CaOf

E\belite

texture:

average poresize (esti-mated) p

29.5 32.5

under hotsurf ace

24.65 24.61 21.775.68 5.70 5.484.23 4.29 3.53

61.27 61.92 65.310.49 --0.79 0.640.58 0.34 0.170.36 0 . 0 3 0.200.15 0.27 0.342.61 1.63 2.48

100.0

78.1 79.0 93.72.49 2.46 2.421.34 1.33 1.551.0 4.0 2.7

nJ 30

ry35 - 40

ni 15

Id15

< 5

400 - 800 400 - 800

middle

99.68

d 15

N 55 - 60

N 15

h/ 10

45

near shell fI

100.0

d 4 0

rJ 30

d 15

& 15

II

100 - 300 1

TABLE 7: Chemical composition of core and rim of mealball and the kiln charge composition at thecorrespondrnq zone

Ik i l n b a l l kiln charge

rim core calcininqzone

Loss on iqn.I

3.9 4.3 1 1.9I

Si02 I 20.9 I '-17.2 I 21.2

A1203 I 5.6 I 8 . 0 I 5.7

Fe203 ' 2 . 8 4 . 4 2.7

Ca0 63.0 62.5 62.1

s03 1.9 1.7 2.2

K20 I 2.1 I 1.5 1 2.3'I

Cl' I 0.1 I 0.1 I 0.1I

LSF 94 1 0 4 I 91

SR 2 . 4 1 . 4 1 2.5I

APPEND

IX I

APPENDIX II: CHEMICAL ANALVSIS OF RISER PIPE AND KILN INLET DEPOSITS

MA-Mat. Nr.I

50'393 I 53'550I

PlantI

DI

G

LocatlonI

Rlser Plpe

Loss on Ign. 11.8sio2 9.2A1203Fe 03Ca 6

:*2"49:3

MgO 1 . 2SO3 15.9K2D 3.6Na20 0.98TiOMn2 6 3

0.090.07

; :- 05 0.11 2.7

Total 100.65

:"R 1.61 1.59LSF 1 5 7

10.17.4f*$

46:l

2:::

z50:150.010.095.0

1;*:3:4

5:::

E8.40.190.150.030.052 . 1

252:;'10.6

4:*;0:834.6

: 7.6

it*:10:os0.380.27

0.5712.4

2:39:82.3

27.6

:*:r0:190.160.080.09

99.7 102.82 101.23 98.83

2.47 3.12 2.09 1.481.31 2.13 5.89 0.68196 114 52 95

54’155 1 54’118 53’956

A I I3

Kiln Inlet (OS)

H

KilnInlet (DG)

APPENDIX III: CHEMICAL ANALYSIS OF RINGS FROM,ROTARY SECTION OF KILN

MA-Mat. Nr. 53’952 52'040 52'037 52’178 49’643 45’061 50'524

Plant I K F M 1 L N J

Location 100-l 10m 45m 37m 32.5m 35m 18.3m 1OOm

Loss on ign.SfO2Al203;;a03

I? t.3so3K2DNa20ii02Mn2D3p

10.714.7

ii:;52.30.79

t::0.400.180.060.230.23

0.4723.04.93.0

66.41.10.130.160.060.250.020.240.01

2:::

FL!63:l2.10.770.720.070.310.040.080.05

1.6324.6

45::61.90.790.270.340.03

0.1222.84

35:;63.90.921.071.07

' 0.240.360.150.24

8.1121.34.5

5;::

:.:60:950.280.280.020.080.28

2:::

:*:50:81.13

14.22.180.160.220.040.240.01

Total

CaOfSR .

EF

98.89

2.191.48110

99.74 100.44 99.68 100.25 100.56 99.38

27-h1:63

92

2.73 2.462.32 1.3388 79

El1:8488

2.881.5588

0.622.361.467 8

PHOTOTABLE 1: S.E.M. MICROGRAPHS OF DEPOSITS

1 a)l0J.u

I I REM 84/108

Mud ball in chains

1 b) eY REM 84/506

Compact cyclone 2 deposit

IOU/lc) u REM 841545

KC1 crystals

ld) ,xm , REM 84/69

Spurrite crystals

1 el REM 84/79 1 f) REM 84/72

Blue Circle Cement


Module 13

Section 12

Rings and Buildups in Cement Kilns

B U R N I N G I S S U E S

The problems caused by rings andbuild-ups in a kiln system always cre-ate turmoil and frequently a loss of

production. Most of these problems can becontrolled if not eliminated.

Figure 1 indicates the major causes ofrings and build-ups. There must be a goodevaluation programme, which includes areview of the literature. When this is accom-plished, there are definitely solutions to theproblems of rings and build-ups in the kiln.Frequently the solution requires forgettingsome preconceived ideas.

This section will not cover all of the ringsand build-ups that can occur, but will

address those most frequently encountered.There are problems associated with theburning of waste fuels which can be attrib-uted to flame position, alkalies, chloridesand sulphur.

Figure 2 shows a basic understanding ofair volume changes attributable to changesin temperature. Our experience indicatesthat many people tend to forget this rela-tion. They comments “I didn’t increase theair flow,” “the flame looks like it is on theload. It wasn’t yesterday; someone musthave moved the burner.”

Figure 2 shows the increase in volumecaused by temperature changes. One cubic

MAJOR CAUSES fig.11 . Overheating2. Slow clinker quench3. Fuel impingement on the burning zone

load4 . Long flame5 . Chlorides6 . Suiphur7 . Potassium8 . Mechanical restrictions

EVALUATION PROGRAMME.1. Sampling2. Care of sample (temperature, air and

moisture considerations)3. Samples which are consistent and

representative4. Documentation of conditions before and

during time of the problem

SOLUTIONS1. Alter raw materials and fuel2. Control internal alkali, sulphur and chloride

cyclea. Install a kiln gas by-pass for preheaterand calciner kilnsb. Do not return as much total dust,especially where precipitator fields dischargeto individual conveyorsc. Determine time cycle for build-upAdjust kiln burner (permit clinker quench,shorten burning zone length, eliminate fuelimpingement on the load, locate burner onkiln center line and slope)

4. Adjust kiln material and gas temperaturep r o f i l e

5. increase kiln rotational speed6. Install kiln internal restrictions such as

dams or orifice rings7. Maintain the secondary air temperature

consistently

foot of air at 100°F weighs about0.071 pounds. The weight of onecubic foot of air seems insignifi-cant, but at each of these plottingthe weight of air is the same(0.071 pounds), only the volumehas changed. 4.4 cubic feet of airat 2000°F still weigh 0.071pounds.

We often hear the question,“Where is the best place to sam-ple the kiln discharge hood pres-sure?” But the real question is“where on the hood does thesample point (or points) give apressure reading that permits rel-ative control?” That is, where arethe conditions today similar towhat they were yesterday? Thenext question is “what is the cor-rect kiln discharge hood pres-sure?” The kiln discharge hoodshould be at a slightly negativepressure to permit observationby instruments or persons with-out undue overheating and dustyconditions. From the standpointsof good housekeeping and main-tenance, the hood pressureshould be slightly negative. Thisvalue should be

inches w.g., -0.01, -0.1, -0.15, etc. Quite pos-sibly the most serious effect on hood pres-

sure sampling over the years has been ourattempt to “bum on the nose.“ All changesin fuel ignition are immediately detected byhood pressure changes-there is no damp-ening effect as there is when the flame isaway from the nose.

Figure 3 shows the different pressureconditions found in the kiln discharge hood.This is why more than one sample point isneeded, with all manifolded together toserve as one sample source. The result is themeasurement of an average pressure.

It is interesting to note the effect highersecondary air temperatures have on the kilndischarge hood pressure. The increase ofsecondary air temperature increases the vol-ume of air as well as the velocity. Thisincrease of velocity tends to drive the sec-ondary air and dust toward the top of thehood. This condition always creates a dust-ing and puffing at the top of the hood overthe kiln, whereas the bottom side of the kilnmay be at slightly negative pressure.

A change in secondary air temperaturecan move the flame position up or down.Certainly, a change of secondary air temper-ature wilt alter the fuel ignition rate, but theconcern in this example is the positioning of

determined by trial rand error for each fig.3 POSITIVE

system. It is alwaysadvisable periodi-

POSITIVE PRESSUR

tally to review theselected set point todetermine if condi-tions have changed.

NEGATIVE

Once the desiredkiln discharge hoodpressure is selected,that is the target,whether it is 0.05


secondary air temperaturei s increased. Velocitythrough the cooler throatincreases to 1275 feet perminute. This increase ofvelocity tends to raise theflame path, which usuallycauses the burning zone tocool off and the calcinedmaterial to flush into theburning zone.

These examples showwhy it is more importantto maintain a constant sec-

the flame path. ondary air temperature than to attempt toFigure 4 shows the burner positioned on reach the highest possible temperature.

the kiln centre line and slope. Thisposition has been adjusted duringoperation to compensate for the sec- fig.6

r - - - - - x -ondary air’s tendency to lift theflame path. In this example the intentis to direct the flame tip on the kilncentre line and slope. Figure 4 indi-cates that the average secondary airtemperature is 1000°F. The volume ofsecondary air passing though thefixed throat area has a velocity of 900feet per minute.

The system in figure 5 is identicalto that in figure 4 except that the sec-ondary air temperature has been

clinker and past a thermocouple sensor.Methods for aspirating air from the clinkercooler have proved to be impractical pri-marily because of wear created by theclinker dust. Quite possibly the calciner kilnsystem permits the most accurate measure-ment of combustion air temperature. Thecalciner kiln system aspirates combustionair from the clinker cooler as tertiary air forthe calciner.

In spite of its inaccuracy, the thermocou-ple placed in the clinker cooler throat hasbeen accepted as indicating a usable relativesecondary air temperature for day-to-daykiln operation. This method of detecting sec-ondary air temperature is fine if we remem-

ber thatm It is a relative temperature and

may read much higher because ofradiated heat from the clinker.

n It may increase or decrease,depending upon changes in theclinker cooler bed, without muchreal change in air temperature.Fluctuation of secondary air tem-

perature is one of the major causes ofrings and build-ups. The kiln flameand location must be controlled tomaintain a stable operation. A stablekiln operation should create the pat-tern of coating and a ring formation

reduced to 700°F. The velocity of the sec- Normally, attempts to achieve the maxi- shown in figure 7. This drawing shows onlyondary air through the clinker cooler throat mum secondary air temperature produce a small amount or no coating from the burn-has now been reduced to 715 feet per cyclical operation of the kiln. This promotes ing zone to the kiln discharge end. The ringminute. The flame path has been lowered the production of clinkerand the tip is no longer on the kiln centre burned in a reducing atmos-line and slope, resulting in fuel impinging phere, slow quench of theon the load. The problem of fuel impinge- clinker minerals, dusting in the C ,I ’ment on the load is definitely more pro- kiln discharge hood and kiln ., ‘, .,nounced when the burner is adjusted to turn ring formation.the flame toward the load. Microscopic It is also important to recog- :analyses often indicate that the clinker was nise that the secondary air tem-produced in a reducing atmosphere on this perature recorded by mostdate, whereas the day before this was not plants is a relative temperature.the case. The secondary air temperature

Figure 6 shows what happens when the is usually detected by placing athermocouplesomewhere near the that forms 80-115 feet from the kiln dis-clinker cooler throat area. charge end is in the area where calcination isThe value indicated by this complete and the liquids begin to form. Themethod of sensing not only location of this ring depends upon the burn-measures the air tempera- ing zone length. It is formed because of theture, but it also detects coexistence of calcined material, a smallradiated heat from the amount of liquid, and material still in theclinker and the flame. A solid phase. This creates prime conditionstrue secondary air temper- for build-up. The ring does not adhere to theature is measured by aspi- refractory, is not dense and is, very fragile. Itrating a portion of the set- breaks up and falls out when the kiln tem-ondary air away from the perature is changed by alterations to the cal-


cining zone and material preparation. It such as a decrease in sec-may fal l out when f lame length and locat ion ondary air temperature, maychange. crea te the condi t ion where the

This r ing i s regarded as an asse t because f lame t ip i s projected throughi t serves as an or i f ice that increases the gas the load ( f ig 9) . This lengthen-

ing of the flamecauses fuelI I

LONG FLAME impingement onthe load , bu t a l socauses the conical -long flame ringbuild-up shown in figure 8. presence of a nose r ing indicate the presenceWhen this ring is detected, it of slow quench. The nose ring permits acan be broken up and dropped very s low quench of the c l inker because theout by shortening the flame. mater ia l i s pooled when i t passes out of the

_ _ ----.-I- This type of ring can also be burning zone. Quick quench of the clinkerprevented with a short flame minerals must be completed wi thin the ki ln

ve loc i ty a t i t s l oca t ion . Th i s t ends to ho ld with its tip directed on the kiln centre line or it will not be achieved.back and mix aerated material. While the and s lope . Slowly quenched c l inker causes the C?Sring is present kiln operation tends to be sta-ble, with less material flushing into theburning zone. If all conditions remain sta-ble , the r ing remains and ass i s t s opera t ion .I t does no t g row subs t an t i a l l y a s t he s t ab l eoperat ing t ime increases . For several daysof ten the r ing fa l l s out , k i ln opera t ion maybe cyclic and it is difficult to keep the rawload out of the burning zone. We have of tenreviewed kiln operators’ logs and found thisto be a common scenario.

Figure 8 shows the ring caused by a longflame. This ring may also be formed when

Another example of a long flame is to rever t back to C,S and free lime. Furthers low coo l ing causes the C,S finthe beta s ta te or high- tempera-ture form) to change to thegamma state of C,S (a low-temperature form). T h egamma form of C2S is a dustand no longer forms a nodule.

\ This dust is picked up by the

NOSE RING flow of air and carried backKILNDISCHARGE SLOW QUENCH into the kiln where it enters the

--I_--- burning cycle again . The s lowquench cyc le con t inues as long

as the nose ring persists to act as a dam. Thesuspended par t ic les re turn ing wi th combus-t ion a i r a re eas i ly preheated because the sur-face area is maximised. The l iquid availableat the kiln nose permits adherence of thedus t pa r t i c l es , and the bu i ld ing o f the noser ing con t inues .

I- t i e . 1 2SLOWQUENCH

the flame path is directed into the load. In shown in fig 10. In this case the flame tip isthe latter case, the kiln may have experi- at least directed on the kiln slope and paral-enced stable operation with the flame lel to the kiln centre line. There are appar-

ently sufficient liquids avail-ab le to p roduce a s t i cky env i -ronment which promotes thedevelopment of a material bal l .Bal l s which are 6 to 12 inchesin diameter have been foundin the middle of the calciningzone. A few of these ballsgrow to diameters of 6-8 feet .The larger bal ls look alarming

di rec ted toward the load , and th i s loca t ion when they are f i r s t seen pass ing through themay be sat isfactory as long as the f lame t ip burning zone. Burning with a shorter f lameis not on the load. However , when the f lame length prevents additional balls formingtip is directed into the load, any change, unless they a re caused by a h igh concentra-

tion of alkalies, sulphur, andchlorine.

fig.10

i P----KILNKILNDISCHARGE

The nose ring (fig 11) hasbeen described as an “ashring.” Some ki lns operate wi tha nose ring most of the time.This tends to restrict clinkerdischarge from the kiln.Microscopic evaluations ofclinker produced during the

Figure 12 shows an example of a snow-man on the clinker cooler back wall. Somesnowmen grow tall enough to reach theburner pipe. Generally, the larger the kiln,the larger the snowman. Depending uponthe ins ta l la t ion procedure of refractory overdead grates, some kiln systems form snow-men on the c l inker cooler s ide wal l near thethroat . The snowman bui ld-up i s caused bythe same problem that promotes the noser ing bu i ld -up - tha t i s , s low quench of thecl inker .

Microscopic evalua t ion of c l inker showswhether the mater ia l was s lowly or quicklyquenched and whether C,S changed fromthe beta to the gamma s ta te . Both the nosering problem and the snowman build-upcan be eliminated by adjusting the kilnburning operation so that the clinker isquickly quenched within the kiln.


We have learned to live with a dusty kiln temperature increased from 2600°F to level which removes a similar amount fromdischarge hood, especially in larger kilns. 2750°F and NOx fell from 750 ppm to 350 the cycle. If the long wet and dry kilns useThe old small wet-process kilns were sel- ppm. The clinker went from slowly an electrostatic precipitator, the dust col-dom dusty because the fuel consumption quenched to quickly quenched. The clinker lected in the final fields can be wasted as

HIGH SULPHUR RING T H I C K N E S S C A N B E 1

KICNDISCHARGE

cooler snowmenwere eliminated,the kiln dischargehood cleared andwe could see theflame and burn-ing zone. In addi-tion the 28-dayc o m p r e s s i v es t r e n g t h sincreased by 600psi over a 90 dayperiod withoutany increase offineness.

high alkali, sulphur, and chloride material.The electrostatic precipitator works well

as a kiln gas bypass system for the long wetand dry kiln systems. Since the solidifiedalkali, sulphur, and chloride particles arevery small, they are concentrated in the finalfield of the precipitator, and are easily sepa-rated and removed from the system.

Figure 14 shows some areas in the sus-pension preheater where problem build-upsoften occur. As we proceed up the preheaterin the direction of the kiln gas flow, the firstproblem area is at the kiln feed shelf. Thisproblem on a preheater kiln is either causedby leakage of ambient air into the system or

Figure 13 displays a ring formation by operating with a high level of carbonwas high and we could not gain quick which occurs in the calcining zone or the monoxide in the exit gas. Ambient air leak-enough ignition to burn on the nose. Thispromoted the quick quench of clinkerwithin the kiln. We also found that the oldwet kiln produced the most reactive clinker,which permitted a lower fineness for similarcompressive strength levels.

Our most recent experience of puttingthis flame technology into practice was witha large wet kiln. It was necessary to removelarge snowmen from the clinker back wall.These snowmen were giants, lo-12 feet highand 6-8 feet in diameter at the bottom. Thekiln discharge hood was so dusty that wecould not see the nose of the kiln. The noserefractory had to br replaced every sixmonths and the nose castings every 12months. The kiln burner was adjusted to

area where thegas temperature issufficiently low topermit condensa-tion of sulphurand chloride com-pounds. This ringis a part of thealkali, sulphur,a n d chloridecycle. All kilnshave a variety ofrings in this area:some consist of asmall amount ofPU”kY coating.With larger rings

shorten the flame: this reduced the burning (fig 13) the kiln has to be shutdown to phys-zone by about 45 per cent. The burning zone ically remove the build-up. The elimination

of the cause nor-mally requires a

fig.14 fuel change, suchas a lower sulphurfuel, a n d thereturn of less kiln

VELOCITY dust. If a micro-scopic evaluationof the clinker indi-cates productionin a

HATES ANi

reducing

T E M P E R A T U R Eatmosphere, theburner should be

BYPASS QUENCH Al RIN RISER adjusted to elimi-

nated fuelimpingement onthe load. This willpermit a higherclinker sulphur

K I L N G A SBYPASS

fig.15

L 19OO’F T O 21OO’F

age causes a localised condensation of alkali,sulphur and chloride compounds. Thesechemicals are vaporised in the burning zoneand exit as a kiln gas until temperature con-ditions are sufficiently low (about 1800°F) tocause condensation to the liquid state.Normally, the preheater kiln exit gas tem-perature is above the condensation point.When ambient air leaks into the kiln feedend housing there is a localised cooling ofthe kiln gas at the leakage source that resultsin build-up at that point. A different type ofcalciner kiln system build-up at the feedshelf and feed end housing walls can also becaused by leakage air. This build-up iscaused when the kiln feed is nearly calcinedand there are C,AF liquids present.However, if the gas dust concentration issufficiently high, the liquid will adhere tothe dust particle rather than to the surface ofthe wall, thereby preventing a build-up.


AP = 2.0' KI LNthe kiln bypass

75O*F T O ilOO°F I i - - + - -induced draught fan.

GAS Alkali, sulphure

!

BYPASS and chloride com--0.5. TO -1.5” pounds create no

-2.5" TO -3.5” o

QUENCH CHAMBER m -Ibuild-up problems ifthey exist in either

I ‘.

7

7-J

em -1900’F T O 2100’Fthe gaseous state orthe solid state.

QUENCH AIR ’

i

/However, if theyexist in the liquidstate, they behave

also ensure that the

quench air exits to

fig.16 like water on dust.The secret to efficientkiln gas bypass sys-

This situation can be artificially dupli- tern operation is taking a portion of the kilncated by the introduction of dust from the exit gas at plus 1900°F and instantaneouslyStage III cyclone material discharge and/or quenching it to about 750°F. This permitscreating a rough feed shelf surface which the alkali, sulphur and chloride compoundscauses the a splashing of the feed out intothe gas stream. Dust re-entrained in the kilnexit gas by a rough feed shelf surface willincrease the dust lost through a kiln gasbypass system, so the feed shelf must have asmooth surface when running a kiln gas by-pass system.

Figure 15 shows a build-up above thekiln gas bypass take-off and within thequench chamber. The build-up in the kilnriser above the kiln gas bypass take-off iscaused by the leakage of quench air fromthe quench chamber. Proper sizing of thebypass quench chamber inlet can ensurethat quench air does not enter the riser duct.

The example in fig 16 shows the parame-ters used for design and adjustment of thequench chamber inlet. A two-inch pressureloss through the quench chamber inlet will

to pass from the gaseous state directly to thesolid state without passing through the liq-uid state. Some designers and operatorsquench to higher temperature levels, i.e.900°F to 1100°F. Our experience has foundmore potential for build-ups in the quenchchamber at these higher temperatures.

The kiln gas bypass system appears to workbest when the quench chamber and kiln riserduct take-off are placed above the kiln. As thegas and dust exit the kiln, the dust is thrownagainst the feed shelf while the gas is turnedupward. This separates dust particles from thekiln exit gas stream. The cleaned gas tends topass on the kiln side of the riser duct for a shorttime. Figure 17 shows the desired quenchchamber position and fig 18 indicates thedesired operating parameters for a kiln gasbypass quench chamber. In our experience aquench chamber operated with these parameters will not product any build-ups, and will

NCH CHAMBER

GAS AND DUST

DJST -GAS ---c

operate with no dust in the bottom of thechamber.

Kiln gas bypass dust collector material con-tains 0.520 per cent of clinker, 20-25 per centSo,, and 4.5-5.0 per cent K20. If the percentageof sulphur as SO3 is less, for example 16 percent, the bypass system is taking too muchdust from the kiln riser, etc.

There are always answers to problemswith rings and build-ups. The solution isusually found when the attitude of theoperator is that “we cannot continue to livewith this problem.” JI

This paper was first presented by the author,Floyd C Hamilton, of Hamilton Technical Serviceslnc, Roanoke, Virginia, for the National LimeAssociation meeting, St Louis, Missouri, UnitedStates, October 1997.

Blue Circle Cement


HBM PROCESS ENGINEERSCONFERENCE

• Minimization of Volatile Cycles

MINIMISATION OF VOLATILE CYCLES

1 . SUMMARY

Concentration of minor components within kiln systems due to volatile cycles can lead tosignificant operational problems on all types of process, with consequent loss of output. Aportion of some of these volatile materials is also lost to the atmosphere, and minimisation of this“leakage” is also increasingly becoming an environmental concern. understanding of the factorsthat effect the magnitude of these cycles is an essential part of improved kiln control leading toeffective control of the cycles. Successful implementation requires co-operation between chemist,process engineer, mechanical engineer and operators.

2. INTRODUCTION

A limited number of minor components in the raw mix and/or fuel can become highlyconcentrated within the cement kiln system and then create operational problems. The minorcomponents that are generally considered to be involved in major volatile cycles are the chloride.alkali (Sodium and Potassium) and sulphur species - although other elements do also becomeinvolved in cycles to a much lesser degree (such as F l V, As, Pb, Tl Cd, Hg and Zn) they havenot been identified as causing operational problems and so will not be considered at this time.These substances are present in the raw materials in low proportions in a variety of forms, but arelikely to evaporate or decompose under the temperature regimes found in the burning zone. Oncethis happens they become associated with the gas stream and cool as this losses its heat to thematerial bed, until they either condense or react to form compounds that will condense. At thispoint they are present in the gas stream as potentially sticky liquids and will adhere to any surfacewith which contact is made; this can be a particle surface or a vessel side wall. Once the liquidhas condensed on to a particle this can still stick to any wail with which contact is made until thetemperature drops to a sufficiently low level for the liquid to solidify. In the suspension preheaterand Lepoi processes the temperatures at which the potential compounds are liquid coincide withthose found close to the kiln hearth and lower preheater stages (SP kiln) or above calciner (Lepolkiln), and this leads to material building up on the walls in these areas, or to ring formation at thevery back of the kiln. In the older long chained kilns, excessive volatile cycles can contribute toring formation. In either situation, at best this reduces the duct or kiln dimensions and so causesincreased pressure drop and - probably - extra dust generation, whilst at worst it reduces output,increases kiln instability, and causes blockages to develop in the preheater system: the end resultbeing significant kiln down time.

In general, in the more thermally efficient processes (precalciner, suspension preheater and Lepol)most of the volatiles will condense in the preheater and only a small fraction will condense on theprecipitator dust or escape up the stack. In contrast in the less thermally efficient processes (wetand long dry) a higher proportion of the volatiles will pass through the kiln system and condenseon the precipitator - or bag house - dust or escape up the stack. Commonly where volatilescondense onto a dust the finer dust fraction will develop a higher concentration of the volatilecomponent, due to the higher surface area of the finer dust.

Where a component is partially volatilised in the burning zone and then partially recombines intothe material stream within the kiln/preheater system it is possibie for a large amount of thecomponent to continuously recycle around the kiln system. This is called an internal cycle.Where some of the component is collected with a dust stream externally to the kiln system andthen returned to the kiln, this is referred to as an external cycle.

3. GENERAL REVIEW OF THE PROPERTIES OF THE MAJOR VOLATILECOMPONENTS

3.1 Chlor ides

Chlorides are derived from the raw materials and the kiln fuel. The high voiatilities of thesecompounds, together with the high collection efficiency of the cyclone preheater systems, will leadto the development of a greatly enhanced cycle. The chlorides have a high affinity for the alkaliesin general and potassium in particular. This property together with the high volatility has been

used in kilns (commonly on rhe wet process, occasionally on the SP process) to control clinkerK2O levels by addition of CaCl2 to the raw mix or fuel, which leads to loss of KCl with the kilnbleed or the exhaust gas from the kiln system. In the suspension preheater, the volatilised materialis recaptured within the system unless a bleed is utilised between the kiln and riser duct. It isgenerally considered that no more than 3% of the chloride passing- from the preheater to the kilnwill leave the system with the clinker. Although considerably higher levels have been noted inindividual samples of clinker, this is probably due to a “push” of kiln feed, or a semi-flush situationas thermodynamic considerations indicate that no chloride should pass through the burning zone.On many SP kiln systems some degree of preheater cleaning is necessary on a regular basis andthis may help to control the chloride cycle by forcing the kiln conditions into a situation whichpermits a brief increase in clinker chloride level (i.e. reduced material temperature, increasedmaterial loading and flux level).

Small amounts of chlorides will also leave the preheater system with the waste gas stream.Taking the total loss of chloride from the system as between 2 and 5 % of the feed to the burningzone, it would then be expected that a circulating load of 20 to 50 times the total chloride inputcould develop in a system without a kiln gas bleed.

No reports of low temperature chloride volatilisation within the preheater have been identified.

3.2 Alkalies

The major source of alkalies will be the raw mix; notably the clay component, although minorquantities can arise from the fuels The initial free alkalies will behave in one of three ways:

1) Remain in the material being processed and become incorporated in the clinkerconstituents that are being formed. This happens to Na2O to a greater degree than K2O

2) Be converted into different compounds - chlorides, sulphates, carbonates, hydroxidesby reaction with ofner constituents of the raw mix.

3) Diffuse to the surface of the process material and volatilise.

In its initial state, K2O begins to volatilise over a wide range of temperature, depending on theform of clay in which it was incorporated but irrespective of source, it would be expected to havevolatilised almost completely at burning zone temperatures, although some may have been at leastpartially stabilised by conversion to the less volatile sulphate form within the material bed. Oncevolatilised it will react to form chlorides and sulphate - chlorides preferentially - at the rear of thekiln. These will then deposit on dust particles. Initiallv Na2O is less volatile than K2O due to itshigher bond energy and so a greater proportion of the kiln feed Na2O would be expected to passthrough the burning zone in clinker without participating in the volatile cycles. Volatilised Na2Owill react with SO2 and SO, to form sulphates towards the rear of the kiln and with chloride wherethis species is present in excess of K2O Where alkali is present in excess of chloride and sulphatealkali carbonates will be formed Each of these al “-=smmthe surface of dust particles in the cooler zone of the kiln ad lower preheater stages and will enterthe volatile cycles as the dust is separated out in the cyclones. Direct contact onto kiln orpreheater surfaces may lead to the development of build-up, The compounds will then re-enterthe kiln where the degree of volatilisation will depend on the species and the kiln conditions.Volatility decreases from chloride to carbonate to sulphate and, hence, sulphates are more likelyto pass through the burning zone. Nevertheless, the likely range of burning zone temperaturescover the thermal area in which alkali volatilities are likely to increase significantly with risingtemperature. In general, precalciner kilns have significantly lower burning zone temperature thanare common in other processes and, hence, alkali sulphate volatilisation in particular is lower inprecalciners than in other processes.

3 . 3 Sulphur

Sulphur can enter the system in a number of forms from either fuels or raw materials. A limitedamount may evaporate in the upper preheater stages and escape from the system in the exhaustgases. In general, SO, and SO, can form in the high temperature areas and be transferred to thegas phase. In the cooler areas of the kiln back-end and preheater system sulphates will form andre-enter the material stream. preferentially alkali sulphates will be produced with excess sulphatecombining with free lime or calcium carbonate an$li@ru&&5li~-i5Xailablein~e~~.--~~~r-‘l-----‘t’:----~~a~~~~~~~~~~~~~, so

restricting the formation of sulphates. In this situation loss of sulphur oxides by way of the stackmay increase but where the gas stream passes through the raw mill the majority of the sulphuroxides would be expected to react with the high active surface calcium compounds which areproduced in the milling process. This will then return to the kiln in the raw mix as part of anexternal cycle.

The high boiling points of the alkali sulphates would indicate that relatively low levels ofvolatilisation would be expected. However, dissociation may occur, particularly under thereducing conditions which can exist to some degree within the burning zone. Calcium sulphatealso has a high boiling point but is even more susceptible to dissociation, so a higher recirculationof sulphate from this compound would be expected. As CaSO4 cannot recycle as a compound,the lime from this compound remains in clinker as free lime - making burning more difficult -whilst SO, is carried in the gas stream towards the kiln back-end where it reacts to form alkali orcalcium sulphate.

4. CONTROL OF VOLATILE CYCLES

As indicated earlier the major species involved in volatile cycles are chlorides, alkali and sulphurbased components. As the development of a volatile cycle depends on the evaporation ordissociation and condensation of a range of compounds the meiting boiling and dissociationtemperatures of these compounds will be of major significance, whilst volatility characteristics willalso be of importance. Boiling and melting point data is presented in figure 1, and volatility datain figure 2. As chemical analysis gives details of the volatile components in the feed materials ofin material from within the system in terms of the primary oxides or elemental forms - IWO, K20,SO3 and Cl - the volatilities are normally expressed in the same terms. and the ranges of volatilityfor each that are commonly found within the cement kiln are given in figure 3. These can beconsidered to be the primary volatilities for these components, and in each case the tabulatedfigures show a wide range of volatilities. This doesn’t help to extend our potential understandingof what is happening within the kiln.

The volatiles however will be present in the material bed in the kiln as compounds for each ofwhich the volatility will depend on physical characteristics such as those set out in figures 1 and2. As indicated in section 3 there will be an order of priority (ease) with which these compoundswill form. Therefore it is possible to study the development of a volatile cycle in terms of thephysical properties of these compounds. The specific volatilities that have been found for eachof the compounds that are likely to form are set out in figure 4. This shows for instance, thatalkali in combination with chloride is more likely to develop a cycle than that combined withsulphate. In the same way, sulphate combined with calcium is more likely to volatilise than thatpresent in an alkali form.

Where volatiles are present in the raw materials some degree of cycle is bound to develop.However, the overall cycles can be governed to a degree by optimising the relative proportionsof each volatile component to maximise the potential for the formation of those compounds withthe lower specific volatilities. The prime control of the volatile component cycle is performed inthe design stage of a works project through the selection of raw materials and fuels in order tooptimise the relative and absolute levels of the potentially volatiie components and, wherenecessary, the inclusion of a bleed system. However, whenever changes are made to a sourcingof raw materials (including fuels) the potential affection in the volatile balance needs to beconsidered.

Where processing conditions are kept steady, the cycles will continue to develop until equilibriaare reached, at which time the total amounts of volatile entering the system will be balances bythe quantities leaving the system.

The degree of volatiisation and the rates at which the equilibrium are established will depend on:

(a) The species, their chemical forms and concentrations.

(b) The volume of gases.

(c) The intimacy of contact between gas and solid.

(d)

(e)

(f)

(g)

(h)

(i)

(j)


Possibility of dissociation or further reaction.



Kiln atmosphere.

Kiln temperatures.


Most of these factors are to some degree inter-related and so in normal operation the onlymethods available to control the degree of volatilisation and eventual concentrations in clinker andwithin the kiln system will be the kiln internal atmosphere and temperature, and the proportionof gas bleed from the kiln exit or material bleed from the system.

As indicated above, it should be possible to modify the volatile cycle by variation of the kilninternal atmosphere (oxygen level) and burning zone temperature. In the late 1980’s a series ofkiln trials were conducted at Hope Works to investigate the effect of these kiln conditions (BZTand BEO,) on the volatile cycles. This study covers the situations of significant chloride, alkali,and sulphate inputs. The major conclusions were:-

1 . The chloride cycle is 20 to 30 times the total chloride input and increases slightly withincreasing temperature, although this may be due to improved kiln stability at highertemperatures. The chloride cycle is not modified by wide changes to kiln atmosphere(Figure 5).

2 . Chloride in the cycle combines with aetassiu~availahle, However, at lowtemperatures - equivalent to NOx Eve1.s‘ om p$ii - some sodium is alsoinvolved.

3. The chloride cycle is a low temperature cycle and cannot be significantly modified byvat-ration or rum conanions.

4. The total potassium cycle is approximately 3 times the input level (Figure 6), howeverabout two thirds of this is present in Stage IV in conjunction with chloride and so cannotbe controlled except by incorporation of a bleed system.

5 . About one third of the potassium in Stage IV is either f& based on its first passagethrough the preheater or derived from a potassium sulphate based cycle. This cycle ratiovaries from 1.05 (minimal recycle) at 800 ppm kiln NOx level to 1.25 at 1,400 ppm NOx(25% recycle). This portion can be controlled by operation at low temperatures (Figure7).

6.

7.

8.

9 .

10.

11.

12.

13.

14.

15.

The sodium cycle level varies between 1.2 and 2.0 times that in the raw meal over thetemperature range examined.

At low temperatures some sodium becomes involved in the low temperature chloride cycleand this boosts the sodium cycle by about 10%.

The majority of the sodium in the system is involved in a sodium sulphate based cycle.This is strongiy temperature dependent, and the rate of increase also appears to beincreasmg with temperature. Over the temperature range investigated the recycle rosefrom 1.35 times the level in the feed at low temperatures to 2.0 times at high NOx levels(Figure 8).

The total sulphur cycle is temperature dependent and rises from 1.6 to 2.6 times that ofthe input over the temperature range investigated (Figure 9). Low oxygen levels increasethe suiphur cycle.

The alkali sulphate cycle has already been summarised in points 6 and 9 and makes upabout 35% of the total sulphate in Stage IV material. These cyclic levels can be seen tobe lower than the total suiphur cycle levels (Figures 7, 8 and 9).

The calcium sulphate recycle is strongiv temoerature dependent. The quantity of SO, as%&(ii$L~V rises from^-“aoout 2.5 times tne level in the feed at low NOx levels to 4times at high (1,450 ppm) NOx levels (Figure 10).

The calcium suiphate cycle is also increased by a move into a reducing kiln atmosphere.At about 1,200 ppm NOx the level of SO, as CaSO4 increased from 3.1 under oxidisingconditions to 5.5 times the feed levei under reducing conditions (Figure 10).

For potassium and sodium there are indications of a slow but steady increase in losses ofthese components from the preheater system as temperatures increase. This may becomepart of an external cycle or may be lost to atmosphere. This could be established by alonger term study of the levels of these components in the precipitator and stack dusts.

Sulphur is in overall balance within the system below NOx levels of 1,200 to 1,300 ppm,but above this level the loss increases sharply with firing temperature. Again this maybecome part of an external cycle or may be lost from the system to the atmosphere,however in this exercise no precipitator or stack dust samples were collected or SO2emission measurements made so this cannot be confirmed.

When the kiln atmosphere moves into reducing conditions the losses of alkalies andsulphur from the kiln system to the atmosphere increase sharply.

Overall the results show that the chloride cycle and its associated alkali cycle cannot becontrolled by process conditions. Cycles of alkali present in the form of alkali sulphate can becontrolled and minimised by careful control of the burning zone to the lowest practicaltemperature. Calcium sulphate based cycles can also be minimised by burning to lowtemperatures, but in this case careful control of the kiln atmosphere {back end 02 level) is alsonecessary to ensure that the on-set off reducing conditions is not possible.

While these tests were conducted on a suspension preheater kiln, the critical area is the burningzone and so the general results will be equally applicable to other processes.

The susceptibility of calcium sulphate to increased volatilisation under even borderline reducingconditions also emphasises the potential for the sulphate cycle to be affected by the condition ofthe firing system, by the type of fuel, and by cooler operation. Low secondary air temperature,a low volatile fuel, poor coal drying, or low momentum in the firing pipe are likely to promoteslow initial combustion, in which case significant combustion is likely to continue to occur oncethe jet has fully expanded. This may give reducing conditions immediately above the material bedand encourage sulphate volatilisation. Where this is possible it is recommended that the firingpipe is aiigned along the axis of the kiln in order to ensure the maximum kiln length is availablefor jet expansion. A “cool” flame is also likely to produce a long burning zone which will holdthe potentially volatile materials for a longer time period within the temperature range at whichvolatilisation could occur; this will also promote higher degrees of volatilisation.

5 . CONCLUSIONS

From a Process Engineer’s point of view, the easiest way to minimise kiln volatile cycles is topush the problem to the Works Chemist and expect the raw materials to be selected to give theminimum practical inputs of potentially volatile materials. This also requires the levels that doexist in the raw materials to be balanced to give the maximum opportunity for the formation ofcompounds with relatively low specific volatilities.

The quantities of material recirculating can also be controlled by the selective removal of a dustfraction from the process. Examples of this are the selective dumping of the finest precipitatordust fraction on the wet process at Ravena, and the dumping of Lepoi cyclone dust at Cookstown.

The magnitude of the volatile cycles can also be reduced by the careful control of kiln conditions.In general volatile cycles will increase slowly with increasing burning zone temperature or length.There will also be a major increase in sulphate cycle if reducing conditions develop close to thematerial bed. This increase will start to occur well before a significant increase in kiln backendCO level becomes apparent. This emphasises the need to possess optimised fuel preparation andfiring systems and an efficient clinker cooler.

Mod 13-Kiln Volatiles.pdf

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