FULLER - CEMENT CHEMISTRY - HANDBOOK CEMENT CHEMISTRY Table of Contents 1. INTRODUCTION 1.1 INTRODUCTION 1.2 DEFINITIONS - cement, concrete, cement types, raw materials etc. 2. COMPOSITION 2.1 COMPOSITION - basic calculation/formulas - chemical shorthand etc. 2.2 MODULES 2.3 MINERAL COMPOSITION 3. TYPES OF CEMENT 3.1 TYPES OF CEMENT 3.2 CEMENT STANDARDS 3.3 CEMENT QUALITY - MAIN FACTORS 4. MANUFACTURE 4.1 MANUFACTURE OF CEMENT - grey, mixed and white cement - wet, dry and semi-dry process 4.2 RAW MIX 4.3 RAW MATERIALS 4.4 CHEMICAL COMPOSITION AND CONTROL OF RAW MIX 4.5 PHYSICAL CONTROL AND COMPOSITION OF RAW MIX 4.6 BURNABILITY OF RAW MIX 4.7 CLINKERISATION 4.8 INFLUENCE OF THE RAW MIX ON CLINKER FORMATION AND BURNABILITY 5.PROCESS AND KILNS 5.1 TYPES OF KILNS - wet, dry & semi-dry 5.2 WET KILN - main features - process 5.3 DRY KILN 5.3.1 LONG KILN 5.3.2 SP KILN 5.3.3 ILC-E KILN 5.3.4 ILC KILN 5.3.5 SLC KILN 5.3.6 SLC-S KILN 5.3.7 SLC-I KILN 5.4 ASH ABSORPTION 5.5 VOLATILE MATTER Chemistry Bible Rev.0; 7 Dec 00 1
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FULLER - CEMENT CHEMISTRY - HANDBOOK
CEMENT CHEMISTRY
Table of Contents
1. INTRODUCTION1.1 INTRODUCTION 1.2 DEFINITIONS - cement, concrete, cement types, raw materials etc.
2. COMPOSITION2.1 COMPOSITION - basic calculation/formulas - chemical shorthand
etc.2.2 MODULES2.3 MINERAL COMPOSITION
3. TYPES OF CEMENT3.1 TYPES OF CEMENT3.2 CEMENT STANDARDS3.3 CEMENT QUALITY - MAIN FACTORS
4. MANUFACTURE4.1 MANUFACTURE OF CEMENT - grey, mixed and white
cement - wet, dry and semi-dry process4.2 RAW MIX4.3 RAW MATERIALS4.4 CHEMICAL COMPOSITION AND CONTROL OF RAW MIX4.5 PHYSICAL CONTROL AND COMPOSITION OF RAW MIX4.6 BURNABILITY OF RAW MIX4.7 CLINKERISATION4.8 INFLUENCE OF THE RAW MIX ON CLINKER FORMATION
AND BURNABILITY
5.PROCESS AND KILNS5.1 TYPES OF KILNS - wet, dry & semi-dry5.2 WET KILN - main features - process5.3 DRY KILN
The evaporation of alkalies is larger when chloride is high. This is at times used
to increase the evaporation in the burning zone by adding CaCl2.
Sulfur is difficult to evaluate. Some sulfur in the raw mix is present free in various
organic compounds or in pyrites. Approximately, 50% of the sulphur burns off in
the top stages of the preheater tower. CaCO3 assisted by moisture catches some
of it in the rawmill. SO2 in the preheater also reacts with calcium carbonate with a
maximum around 800°C.
Sulfur in combination with alkalies behaves differently than SO2 from fuel. Excess
sulfur, sulfur not bound as alkali sulfates, can be calculated as:
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Excess S = 1000*%SO3- 850*%K2O-650*%Na2O < 250 g/100 kg clinker
where: All percentages are calculated on clinker basis
SO3 is total from rawmix + fuel
To ensure trouble free operation of a preheater kiln the following limits apply:
Table 12
Limits for Volatiles
Raw mix burnability Easy Hard
Na2O + K2O (% clinker basis) Max 1.5 % Max 1.0 %
Cl (% on clinker basis) Max 0.023 % Max 0.023 %SO3 (% on clinker basis) from raw mix + fuel ;
Or excess sulfur under 250 g/100 kg clinker
Max 1.6 % Max 1.0%
If the natural valves are insufficient, then a kiln bypass can be installed. The
bypass will take part of the kiln gas before the preheater and transport it to a
separate cooling and dedusting system. The bypass gas has to be cooled
immediately to 350oC to avoid clogging. The cooling takes place in a swirling
chamber with atmospheric air. Some dust will be removed with the bypass gas
(2-3% with 10% bypass.)
5.6 MAIN FEATURES DURING BURNING
Chemical control during operation of the kiln system is divided into the following:
-Feed composition
-Product quality of clinker
-Emission control
-Fuel
-Preheater
The raw meal must have the correct quality with little variation. The standard
deviation for LSF should be less than 1% and corresponding less than 0.1 for MS
and MA. Large variations will result in irregular kiln operation resulting in
problems with ring formation and coating in the preheater, as well as, requiring
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higher fuel consumption.
Performing a free lime analysis on an hourly basis monitors the product quality of
the clinker. The analysis can be made either on an average hourly sample or on
an hourly spot sample.
Environmental authorities stipulate emission control in many countries. The
plants have to control and continuously register plant emission of dust, SO2 and
other constituents in the exhaust gas. The results have to be reported back to
the authorities.
The type of fuel used in cement production is either pulverised coal, fuel oil,
natural gas, or waste products.
Pulverised coal is usually produced at the site in a coal mill that dries and grinds
the raw coal to a fineness of approximately 15% retained on the 0.09-mm sieve
and moisture content of 1 to 2 %. The residue and the moisture content vary
according to the type of coal. Some types of coal with high gas content have a
high tendency toward self-ignition, which has to be taken into account. Coal with
low content of volatiles like semi-anthracite has to be ground very fine to promote
ignition.
An important component in heavy fuel oil and coal is the sulfur content. The
sulfur has to be taken into account together with the alkalies. Sulfur content in
heavy fuel oil above 5% will usually cause build up problems. Fuel analysis
should be made regularly by either the supplier or the plant laboratory.
The preheater has to be kept free from coating that can clog the cyclone outlet or
increase the pressure drop in the riser duct. This can be followed by regular
sampling of the material going into the kiln and analyse for chloride, sulfur, and
alkalies.
6. HEAT OF REACTION & HEAT TRANSFER
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The chemical change from raw material to clinker requires heat for two reasons.
The first is due to the heat of reaction for the transformation to clinker.
Secondly, because the clinker process is not an ideal or 100 % effective process,
heat is lost from the kiln system as:
• Radiation loss from all outer surfaces
• Heat loss with the gasses from the kiln
• Excess hot air from the clinker cooler
• Heat loss with hot clinker
Heat effects the chemical reactions, the formation of solutions and changes in
the state of aggregation such as melting or vaporization. The heat effects are
called exothermic, when a reaction is accompanied by heat evolution. When heat
is absorbed, then the reaction is endothermic.
The dissociation of CaCO3, calcium carbonate, is a typical endothermic reaction:
CaCO3 CaO + CO2 – 422 kcal/g
The double arrow signifies that the reaction can be reversed. In the preheater,
this is called recarbonation. The order of magnitude of the heat of recarbonation
is normally evaluated from the temperature profile and the temperature difference
between the lowermost and second lowermost cyclone in the preheater tower.
When planning a new plant or when making a kiln conversion it is important to
know the heat of reaction for the process. The analysis is made in the laboratory
of the equipment supplier. Basically, there are the following heat changes:
Table 13
Reactions During Heating
Temp °C Reaction Heat change<100 Evaporation of free water Endothermic
100 - 400 Absorbed water evaporates Endothermic400 - 750 Decomposition of clay minerals,
Kaolinite metakaolinite
Endothermic
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600 - 900 Decomposition of metakaolinite to free reactive oxides Endothermic600 - 1000 Decomposition of carbonates to free reactive oxides Endothermic800 - 1300 Reactive oxides form intermediate or final clinker
minerals
Exothermic
1300 - 1380 Formation of clinker melt from aluminates and ferrites Endothermic1250 - 1500 formation of aliteC3S the principal clinker mineral Endothermic
The reactions within the kiln system take place at a slightly negative pressure
and in an oxidizing atmosphere. Reduction does not normally take place in the
kiln system apart from a reducing zone in the riser duct of the preheater to
reduce the emission of NOx. The first five reactions in the table take place rapidly
with a velocity determined by the transfer of heat from the gas to the solid
material. The last two reactions are determined first by the contact rate of the
reactive components present in the solid phases and later in the burning zone by
diffusion of the reactive components in the clinker liquid phase. The total of the
heat reactions during clinker formation is endothermic. An example of the heat of
reaction is:
Table 14
Heat of reaction Kcal/kgEvaporation of combined water 20
Decomposition of clay minerals 35Dissociation of carbonates 475
Formation of clinker minerals -130
Combustibles in the raw mix -15Total heat of reaction 385
The heat of reaction is the theoretical heat consumption for the kiln system.
Since the process is not ideal, heat losses exist in the system. The losses of
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heat come from the following:
♦ Hot exhaust gas
♦ Heat loss from surfaces of the kiln system, i.e. radiation loss
♦ Excess air from the clinker cooler
♦ Heat lost with clinker after the cooler
Some of the loss can be utilized for drying of the raw materials and of raw coal.
Table 15
Typical Heat Consumption for Different Systems
Specific heat consumption for kiln systems kcal/kg clinkerWet kiln 1400Long dry kiln 11001-stage preheater kiln 10002-stage preheater kiln 9004-stage preheater kiln 800Semi-dry kiln w/ preheater & calciner 9505-stage kiln w/ preheater & calciner 7255-stage preheater kiln w/calciner and latest cooler type 690
The dry process is always chosen unless the raw materials have moisture
contents above 20-30%.
The efficiency of the heat exchange in a cyclone is the same as the separation
efficiency due to the rapid heat transfer between material and gas. Usually, there
is only a temperature difference of 5°C between the exit gas and material leaving
the cyclone. There is, however, a variation in efficiency between the cyclones as
we go lower down in the preheater. This is due to the change in the design of the
cyclones. At the high temperature in the lower cyclones these cyclones do not
usually have an internal vortex. The vortex or central pipe is difficult to construct
in a material that will last at the high temperatures.
7.FUEL
7.1 TYPES OF FUEL
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The most common types of fuel are: coal, fuel oil and natural gas. The most
common fuel is coal with heavy fuel oil being second. Natural gas is used where
available and is an excellent fuel. Many waste products from a variety of
industrial sources are also used as a fuel source.
7.1.2 COAL
Coal is found all over the globe. Coal originates from plants, that over many
millenniums have been transformed into coal. The age of coal results in different
composition and quality. Anthracite and hard coal are old types, while lignite and
peat are younger types.
Raw coal is a combination of coal, ash and water. The carbon is the main
constituent in the coal, but there are also hydrocarbons, oxygen, nitrogen and
sulfur often as pyrites FeS2. Heating the coal in a non-oxidising atmosphere
drives out some of these constituents as gas also referred to as volatiles and tar.
The coal is then changed into coke. Younger coal has a higher gas content than
older coal. They are easier to ignite than the older coal. They are also prone to
self-ignition during storage.
Table 16
CLASSIFICATION OF COALSLignite Bituminous coal Anthracite
Total moisture % 40-50 5-10 0 – 3Volatiles % 40 – 50 10 – 40 5Hygroscopic water % 10-25 1-3 1Ash % 5-25 10-20 5-10Examples of commercial grades of coals
Chemical composition:
Carbon C % 56 70 78Hydrogen H % 4 3 2Sulphur S % 1 1 1Nitrogen + oxygen N+O % 19 3 2Heat value Gross Hs Kcal/kg 5120 6625 7100
Net Hi Kcal/kg 4820 6310 6900Combustion air and gasCombustion air required weight Kg/kg 7.1 9.2 9.9
volume Nm3/kg 5.5 7.1 7.6
Composition
of combustion
gases
Wet , 0%
oxygen
Total Nm3/kg 6.0 7.4 7.8
CO2 + SO2 vol% 17.8 17.6 18.9H2O vol% 10 6.5 4.5
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N2 vol% 72.2 75.9 76.6Dew point °C 46 38 31
Table 17
Typical Petroleum Cokes
Type %H2O %Volatiles %Fixed C %Ash %Sulfur Gross Heat
Value (kcal/kg)
Hardgrove
Green delayed 8 11 82 0.5 4 8000 60-100
Fluid 0 5 86 1.0 4 8000 25-30
Proximate analysis of coal:
The proximate analysis of a coal sample gives values for moisture, volatile
matter, ash and fixed carbon. It is performed under detailed laboratory
procedures, which can be found in other reference material. The volatile matter is
the portion of the coal when heated to 900
°
C without air is driven off as gases.
Fixed carbon is the residue remaining after the volatile matter is driven off. The
ash content is found by heating the sample to 800
°
C. The moisture in coal is
divided into free moisture and hygroscopic moisture, where the free moisture is
the moisture lost by air-drying. The volatile matter, fixed carbon, ash, and
moisture add up to 100%.
The amount and composition of ash varies from one coal to another. The amount
of ash and its composition has to be known, as it will be a part of the clinker.
Some coals have a very abrasive ash and a high wear index, which is of value
especially for vertical coal mills.
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Coal dust can be dangerous and cause explosions. It can also self-ignite at room
temperature. For that reason a safety index for a particular coal is assigned
according to the characteristics of the coal.
The common safety index is the ratio of % fixed carbon to % volatiles. A high
safety index means a lower chance for a coal dust explosion. The safety index
for coal varies from 1 for high volatile lignite up to 15-16 for petcoke and
anthracite.
Ultimate analysis of coal:
In the ultimate analysis of coal, carbon, hydrogen, sulphur, nitrogen and oxygen
are determined.
Chemical analysis:
In the chemical analysis of coal, the inorganic composition is determined on the
coal ash. The values are used for the calculation of the raw mix and clinker
composition.
Heat value:
The heat value or calorific value is important for the evaluation of the coal and for
the heat economy of the kiln. The difference between the gross and net heat
value is the heat of evaporation of the water from combustion and the
evaporation of the water. The approximate calculation is:
Hnet = Hgross – 5.85(9*%H + % Water in sample) [kcal/kg]
7.1.3 FUEL OIL
Fuel oil is used for cement kilns at many plants. The fuel oil most commonly used
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is a heavy fuel oil. The lighter fuel oil types are often used for lighting up kilns.
Typical analysis for oil is:
Table 18
TYPICAL FUEL OILSGas oil Light fuel oil Heavy fuel oil
The grindability is usually given as a hardgrove index number. The relationships
between the hardgrove index and the grindability in kWh/t for different types of
grinding applications are given below. The Hardgrove index for coals can vary
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from 40 to over 100 for very soft lignite or petcoke.
An important aspect for the choice between a ball mill and a vertical mill is the
wear index. Some coals have a high ash content with an abrasive constituent
resulting in a high rate of wear. In such cases, a ball mill might be chosen or a
vertical mill with stronger wear resistant parts.
Figure 3
9.2 DRYING OF COAL
Raw coal contains varying amounts of water. The raw coal has to be dried to
facilitate grinding and handling but the drying must not go beyond the limit of
safety. The coal is dried in the coal mill only to the recommended minimum
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H AR D GR O VE -G R IND A BI LIT IE S
0
5
10
15
20
25
30
35
40
4 0 50 60 7 0 8 0 90H ar dg rov e ind ex
spe
c.grin
dabi
lity- K
Wh/t
bal lmi ll, 5 %+ 0 .09 m m b allm ill , 10 % + 0 .09 m mbal lmi ll, 2 0% + 0.0 9 mm v ert ical m ill, 5 % +0. 09 mmver tica l m ill, 10 % +0. 09 mmV ert ica l m ill, 20% + 0. 09 mm
HARDGROVE-GRINDABILITIES
0
5
10
15
20
25
30
35
40
40 50 60 70 80 90
Hardgrove index
spec
.grin
dabi
lity-
KW
h/t
ballmill, 5%+ 0.09 mm
ballmill, 10% + 0.09 mm
ballmill, 20% + 0.09 mm
vertical mill, 5%+0.09mm
vertical mill, 10% +0.09 mm
Vertical mill, 20% + 0.09 mm
FULLER - CEMENT CHEMISTRY - HANDBOOK
residual moisture content to reduce the risk for fires and explosions. This
residual moisture content is found by using the graph in figure 4 below
according to the actual hygroscopic moisture of the coal. The plant laboratory
determines the hygroscopic moisture and residual moistures at various drying
temperatures. A detailed procedure to determine surface and hygroscopic
moisture of coal meal is found in another reference. Presented here is an
overview of the program.
Free moisture, or surface moisture, is the water lost by air drying a prepared coal
sample at an ambient temperature of 30°C. Further drying of the coal sample at
various temperatures between 30 and 105°C drives off the remaining water. The
ratio of the change in sample weight between 30 and 105°C divided by the
weight at 105°C, represents the hygroscopic moisture. A graph of the residual
moistures at various drying temperatures shows the correct operating
temperature for the outlet of the coal mill.
Usually, a coal mill outlet temperature of between 60 and 80°C is correct. The
inlet temperature to the coal mill can be as high as 300 to 350°C. The inlet
temperature depends on the quantity of air through the mill, and requires the dew
point of the outlet air after the coal mill to be 20°C higher than the outlet air
temperature to avoid condensation and clogging in the transport system following
the mill. For example, to dry one kg of coal with 10% moisture 1.2 kg drying air
at 300°C is required.
9.3 ASH CONTENT
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Figure 4Recommended moisture in coal dust
024681012
1 2 3 4 5 6
Hygroscopic w ater,%
Res
idua
l moi
stur
e,%
FULLER - CEMENT CHEMISTRY - HANDBOOK
The ash content in various coals differs in quantity as well as composition. It is
necessary to know the exact composition of the ash because it combines with
the raw mix in the kiln system. The ash analysis is performed by the plant
laboratory or by the coal supplier.
The ash content of coal will consume some heat during combustion due to
chemical changes of the minerals in the coal. This reduces the flame
temperature. In coals with a high ash content, it can be difficult to obtain a
sufficiently high flame temperature for the process.
9.4 GAS CONTENT
The gas content is important for the ignition of the coal. Anthracite and some
petcoke have low gas contents and consequently are very difficult to ignite. Only
very fine grinding can compensate for the lack of gas. Sometimes a very high
gas content can result in problems with the mixing air. This can impede ignition
and proper burning which can result in a low flame temperature.
9.5 MINOR COMPONENTS
Coal also contains alkalies, sulfur and chloride. These constituents have to be
taken into consideration for the calculation of the raw mix and of the clinker
composition. These volatile components will participate in the internal circulation
of volatile components in the kiln, calciner and preheater.
9.6 REQUIREMENT FOR AIR The combustion of fuel requires air to oxidise the carbon, hydrogen and sulfur in
the flame into the combustion products of carbon dioxide, sulfur dioxide and
water. In addition, a certain amount of excess air is used to obtain complete
combustion without formation of carbon monoxide. The reactions are:
C+ O2 CO2 + heat
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4H + O2 2H2O + heat
S + O2 SO2 + heat
The water content in the coal is evaporated.
The minimum amount of oxygen required for combustion, Omin, can be calculated