Operation of Tube Mills

GRINDING MANUAL

Operation of Tube Mills Hanspeter Fisch

MPT 01/14745/E

"HOLDERBANK“ Management and Consulting Ltd. Mechanical Process Technology

Table of Contents

1. INTRODUCTION........................................................................................................................1

2. TUBE MILL SYSTEMS (Fig.1)..................................................................................................1

3. MILL CONTROL........................................................................................................................3

3.1 Optimal mill load (Fig. 2)...................................................................................................3

3.2 Adjustment possibilities (Fig. 3)........................................................................................5

3.3 Process parameters (Fig. 4).............................................................................................7

4. MILL OPERATING PERFORMANCE.......................................................................................9

4.1 Mill efficiency (Fig. 5) ......................................................................................................10

4.2 Separator efficiency (Fig. 6) ...........................................................................................11

4.3 Mill ventilation, heating and cooling (Fig. 7) ...................................................................13

5. GRINDING AIDS (Fig. 8).........................................................................................................15

6. GRINDING MEDIA CHARGES...............................................................................................17

6.1 Grinding media charge compositions.............................................................................18

6.2 Ball charge calculation (Fig. 10) .....................................................................................19

6.3 Replenishment of ball charges ......................................................................................21

6.4 Ball charge management................................................................................................22

- 1 -

1. INTRODUCTION

The proper operation of a tube mill system is the key to get the highest benefit out of a given installation. A prerequisite is that the system and the process are appropriately set-up.

The operator has only a few adjustment possibilities, based on certain process values measured in the system and indicated in the control room, which allow to operate the grinding plant at its optimal efficiency.

This paper deals with the operation of tube mills for grinding cement. For grinding different materials the same principles can be applied, adapting the parameters correspondingly to these conditions.

2. TUBE MILL SYSTEMS (Fig.1)

Operating aspects

Ø Open circuit system:

• Fineness mainly adjusted by mill feed rate

• Mill ventilation rate limited by product fineness

Ø Closed circuit system:

• Fineness adjusted by separator settings

• Mill load controlled by fresh feed and rejects

• Mill ventilation, drying, heating separately adjustable

Ø Pregrinding with roller press:

• Coarse grinding shifted to roller press

• Pregrinding mainly of hard and dry components

• Pregrinding requires adjustment of first compartment length and ball charge composition (finer mill feed material)

• Mill operation influenced by feed fineness and feed rate from press

Ø Semi-finish grinding with roller press:

• Major part of coarse grinding shifted to roller press circuit

• Feed to tube mill quite fine (depending on press size and performance)

• Tube mill either open circuit- one compartment mill or closed circuit two compartment mill with small grinding media

• Tube mill feed fineness 1800 – 3500 [cm2/g] according to final cement fineness

• Intermediate bin with weighfeeder recommended to assure the best performance and a balancing of the two circuits.

- 2 -

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- 3 -

3. MILL CONTROL

3.1 Optimal mill load (Fig. 2)

Objectives of mill operation

Ø To establish and to maintain an optimal material load (= material level) within the 2 compartments

Ø To achieve an optimal particle size reduction along the mill to the discharge

Ø To achieve the shortest possible start time to normal operation and transition times from one cement type to another

Basis

Ø Experimental studies and experiences show the following important factors influencing the grinding:

• Material load in and over the ball charge:

− Best efficiency: 1st compartment: half of balls covered with material 2nd compartment: balls covered with ~ 50 [mm] of material These figures can not be checked during operation and can only be verified by measuring the material level over the ball charge after a crash stop of the mill.

• Material/grinding media tumbling behavior

− Best ball filling degree at a certain shell liner profile and mill speed

• Material residence time in mill influenced by: − Ball charge porosity − Material feed coarseness / fineness − Material (feed) temperatures − Material moisture and characteristics

Measures of control

Ø Establish the proper circulation load in the grinding system through changes in mill feed rate (based on existing ball charge composition and separator settings)

Ø Changes of the following parameters influence the flowability of the material within the mill (retention time, material level) and thus the material load:

− Water injection rate − Dosage of grinding aid

− Feed moisture and temperature − Cooling and heating within the mill (ventilation rate)

Ø Preconditions for a good performance are a proper selection of liner profiles, ball charge compositions and grinding media filling degrees, proper dimensioning of intermediate and discharge diaphragms.

- 4 -

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° C

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- 5 -

3.2 Adjustment possibilities (Fig. 3)

Each tube mill system has a certain number of control elements (adjustment possibilities) for operation of the grinding system.

The main control elements and the corresponding process values to check the adjustments as well as their influences are:

Control element Process parameter Influence on

Mill feed

− total mill feed − mill [kW] absorbed − mill material load

− bucket elevator [kW] − circulation load

− rejects flow meter [t/h]

− [%] of component − product composition − product quality

− grinding aid − dosing meter − flowability of material

− mill throughput

Temperature

− water injection − injection rate [l/h] − material temperatures

− material level

− hot gases − fuel rate [l/h] − material temperatures

− drying effect

− quality (gypsum)

− mill fan damper − mill exit temperature − quality aspect

− drying / cooling

− material transport across mill

Static separator

− guide vane position − filter dust fineness − product fineness

Dynamic separator

− separator speed − product fineness − product quality

− fan damper − separating air flow − product quality (PSD)

− separator efficiency

− fresh air damper − product temperature − product behavior

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°C (

Clin

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t/h

t/h

GA

l/min

l/h%

%

l/hl/h

%

%

l/h°CH

2O

%%

min

-1

- 7 -

3.3 Process parameters (Fig. 4)

Process parameters (indicated, measured values from the plant) are necessary to give important indications about how the system is performing and to allow for corresponding adjustments of the control elements.

It is of utmost importance that the indications are calibrated and correct. Wrong values lead to faulty and poor mill operation and performance! Usefulness of parameters

Ø Weighfeeder [%], [t/h]

• Very accurate

• Control quality and system load

Ø Electronic ear [%]/[dB]

• Valid for trend indication

• Varies with many material characteristics

• Mostly used for alarm purpose

Ø Mill motor power absorbed [kW]

• Indication of material load in mill (average 1st and 2nd compartment) Ø Bucket elevator power

• Good indication of total material through the mill

• Indication very delayed (retention time in mill) Ø Separator rejects flow meter [t/h]/[%]

• Best and fastest indication to adjust circulating factor and mill load

• Trends indication possible with corresponding instrumentation

Ø Temperatures [°C]

• Mill inlet – drying/cooling needs

• Intermediate diaphragm – drying efficiency, necessary hot gases or water injection 1st comp. for good material flowability - maintain temperature around 100 [°C]

• Mill discharge (product or air) − Indication of cooling/heating needs, ideal 100 – 120 [°C] − Air normally 5 [°C] lower than material

• Separating air – cooling effect in separator

• After cement cooler – cooling needs for final product

Ø Water injection [l/h]

• 1st compartment − Only with clinker temp > 100 [°C]

− up to 1/3 of total water injection

• 2nd compartment − control of mill discharge temp.

− up to 2/3 of total water injection Ø Static pressure mill discharge mbar – magnitude of draught Ø Fineness mill filter – if worse than the final product, adjust static separator or recirculate

to dynamic separator

- 8 -

Ø Fineness of product – adjust separator speed

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t/h

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l/hl/h

°CH

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%%

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°C°C

°C mb

ar

kW

°C °C

cm2/g

%R

°C

cm2/g

%R

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kW

Pro

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% dB

min

-1

l/h

- 10 -

4. MILL OPERATING PERFORMANCE

4.1 Mill efficiency (Fig. 5)

Fresh feed material

Ø The material characteristics of the mill feed influence considerably the mill performance. Corresponding actions must be taken to counteract negative influences.

Main problems:

• High/low clinker temperature

• Coarse/fine feed material

• Harder/softer components

• Changing feed moisture

Circulating load

Ø The circulation load, established by the separator rotor speed and the prevailing system parameters, must be maintained in order to keep an optimal material load in the mill. The control philosophy applied indicates what actions have to be taken to reestablish an equilibrium and an optimal grinding efficiency.

Start-up of the mill

Ø When starting-up a mill, the optimal mill load has to be achieved as quickly as possible to obtain a good grinding efficiency.

Ø An accelerated start procedure to reach the normal operation quickly can be done by a temporary increase of the mill feed rate and/or increase of the separator speed. The most important objective is to maintain a sufficient separator tailings flow rate and mill load from the beginning.

Stop of the mill

Ø For a routine stop, the mill should not be emptied. Stop the mill as quickly as possible after the stop of the feeders to keep the material in the mill.

Ø Emptying the mill is a loss of production and energy and leads to high wear of mill internals. Empty the mill only when necessary for maintenance purpose or dropping of the ball charge.

Transition of cement types

Ø The procedure for switching from one type of cement to another is similar to a start-up: do not fall short of the separator rejects flow and the material load in the mill. Determine an optimal/quick routine to change the feeder settings (feed rate and composition) as well as the separator speed (new fineness). Feed transition product to a special silo or to the silo with the lowest quality cement.

- 11 -

Operation of Tube Mills CC01-005.dsf Kma 06.06.00

Fig.5 Mill efficiency

[%R]acc.

[cm2/g]Blaine

mill length [m]

longitudinal sieving graph

GA[g/t]

°C[m3/h]

[%] [kW]

[t/h][%R,cm2/g][°C]

[°C][mbar][m3/h]

separator tailings[t/h][°C][%R]

Fresh feed

[t/h][%H2O][°C][% comp.]

[%R]acc.

[mm]

[m]

particle size1 10 25 50

- 12 -

4.2 Separator efficiency (Fig. 6)

Circulating load

Ø The circulating load shows the amount of material fed to the separator.

Ø The circulating load is generally determined by:

• Type and percentage of additives

• Cement fineness

• Existing ball charge composition The circulating load can be slightly changed during the operation by changing the mill feed rate or changing the separator settings. However, at a long run, the resulting circulation load, based on the factors above, will establish.

Ø The range of normal circulating loads is shown in the graph in fig. 6, based on OPC and high-efficiency separators. The adjustment is mainly done by the ball charge composition.

Ø High circulating loads tend to produce a narrow PSD (Particle size distribution) where as low loads lead to wide PSD.

Air flow/separator speed

Ø Modern separator systems offer the possibility to change the air flow rate. Basically, separators have to be operated with the maximum air flow for an optimal separating efficiency.

Ø The target product fineness is only adjusted by changing the separator rotor speed. The steps for an adjustment of the speed have to be minimal (e.g.: 1 – 2 [min –1]) to allow the grinding system to establish a new equilibrium.

Ø There might be cases, where the air flow rate should be reduced for e.g. product quality reasons. Lower airflow rates require lower separator rotor speeds for a given fineness, but tend to produce a wider PSD (= flatter slope acc. to RRSB) and vice versa.

Ø The separator efficiency is reduced at lower air flow rates (higher by-pass rates) resulting also in lower separating sharpness.

Cooling

Ø Separator systems with a filter (single-pass separators) have a possibility to open the fresh air damper to reduce the product and tailings temperatures.

- 13 -

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Fin

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] Bla

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3000

4000

5000

3000

4000

5000

Fin

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s [c

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] Bla

ine

[min

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A

[min

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[min

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[-]

Circ

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Load

- 14 -

4.3 Mill ventilation, heating and cooling (Fig. 7)

Ø The mill ventilation plays many roles, e.g.:

• Transport of fines through the mill

• Fluidization of material

• Heat carrier for heating and drying purpose

• Cooling the mill with fresh air

Mill ventilation rate

Ø The mill ventilation rate refers to the air speed over the ball charge. The normal range is between 1 – 1,5 [m/s] and is adjusted with the mill fan damper according to the needs.

Ø In case of moist feed material, the ventilation rate and air temperature have to be kept high enough to avoid condensation. Air temperature in the filter of ≥ 30 [°C] above dew point temperature is required.

Ø Higher ventilation rates are favorable in connection with the use of grinding aids.

Heating

Ø A good grinding efficiency is achieved when the material temperature at the intermediate diaphragm is around 100 [°C] as this guarantees a sufficient drying and a good material flowability. The correct temperature can be reached through heating (hot gases and/or hot clinker), adjustment of ventilation rate, water injection 1st compartment or the circulating load.

Ø Even if hot gases are used to maintain the temperature at the intermediate diaphragm, it might be necessary to inject water into the 2nd compartment to adjust the appropriate mill discharge temperature. This assures a proper gypsum dehydration and reduces the risk of coating in the 2nd compartment.

Ø Any drying process must be completed up to the intermediate diaphragm to avoid clogging of the slots and to guarantee a good flowability in the 2nd compartment.

Cooling

Ø Mill ventilation with fresh air helps to cool down the mill in case of feeding hot clinker. This enables to reduce the water injection rate.

Ø The air temperature at the mill outlet is generally 5 [°C] lower than the material temperature. If this difference is higher, this points to excessive false air at the mill discharge.

- 15 -

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- 16 -

5. GRINDING AIDS (GA) (Fig. 8)

Definitions and purpose Ø Grinding aids - Organic compounds (e.g. glycols, amines, amine-acetates)

⇒ Increase mill output and / or reduce spec. energy consumption Ø (Product -) Quality improver

- salts (e.g. NaCl, NaNO3), plasicizers (e.g. ligno sulfonates, amine-acetate))

⇒ acts as grinding aid and enhances / influences cement performance

? ?GA addition is only beneficial when savings of spec. energy costs are higher than GA costs and/or the additionally produced cement can be sold.

Effects of grinding aids Ø Increase of mill output

• Better material flow through mill • Reduce or avoid coating (remove interparticle attraction) • More efficient separation (less adhesion forces of particles)

Ø Reduced spec. energy consumption (higher output, better separation) Ø Reduces pack-set (better powder flowability of product)

Process Ø Storage with mixer (optional) – diluted with 40 – 60 [%] of water for higher dosing

quantity and to keep the admixture components soluble Ø Dosing/metering with control loops, ev. adaptation to production rate of mill Ø Solenoid valve in pipe ahead of feed point to prevent from dripping when the mill is

down Ø Feed generally onto mill feed material or injected into the 1st compartment (Optimal

distribution in mill feed). Avoid dosing GA into feed chute to avoid build-ups. Ø Changes in GA dosage affects the material level in the mill (change in material

flowability) → Changes in water injection show the same effect → Electronic ear changes signal

Ø GA needs an optimal dispersion in material in the mill (best in form of vapor in mill atmosphere) to be absorbed from particle surfaces and hence to dissolve agglomerations.

Ø A good mill ventilation improves activation of GA Ø GA shows also positive effects for grinding of slag

Tests for optimal dosage rate Ø Typical dosing range 100 – 300 [g/t] product (optimal quantity to be found by industrial

tests) Ø Typical test procedure

• Calibration on instrumentation • Start-up of mill system 3 – 4 [h] • Adjustment of parameters 2 – 3 [h] • Stable mill operation for test 8 – 12 [h] • Record all parameters • Tests with and without GA • Tests with various dosages and different types of GA

→ Adjust only one parameter – wait for the results

- 17 -

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300

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A

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cts:

bette

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- 18 -

6. GRINDING MEDIA CHARGES (FIG. 9)

6.1 Grinding media charge compositions

HMC established standard charges for various applications. The charges can be applied for wide ranges of cement types and finenesses. These charges are mainly based on OPC grinding to ~3’200 [cm2/g] Blaine. Analogue adaptations for different cases and based on performance test results (mainly longitudinal sieving tests) may be done. 1st compartment

Ø same composition for all types of circuits for medium hard and normal size (d80 ~12 [mm]) clinker

Ø Max. ball size to be adapted according to the formula in Fig.10:

∅ max [mm] Max. ball diameter

d80 ??m] Particle size at 80 [%] passing

K [-] Constant = 350 for steel balls

ρ [t/m3] Density material (cement comp.)

WI [kWh/t] Work index acc. To Bond

% n crit [%] Relative mill speed

DI [m] Internal mill diameter

Ø For preground feed materials, corresponding reduced particle sizes are applied 2nd compartment (based on classifying liners)

Ø Coarse ball charge, standard for closed circuit mill systems producing mainly “normal” OPC cements

Ø Fine ball charge, standard for closed circuit mill systems where finer cements are produced

Ø Ball charge for open circuit mill systems, where the ball charge in the second compartment must retain more material in the 2nd compartment

Grinding media sizes

Ø 1st compartment

• ∅ 90 – 60 [mm] without pre grinding

• ∅ 70 – 50 [mm] with pre grinding

Ø 2nd compartment

• ∅ 50 – 20 * [mm] for closed circuit mill systems

• ∅ 50 – 17 [mm] for very fine cements and open circuit mill systems

* practical experience in “Holderbank”

- 19 -

CC

01-0

09.d

sfK

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6.00

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9B

all c

har

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- G

uid

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Circ

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open

clos

edco

arse

fine

Bal

l ø%

9025

8035

7025

6015

506

611

4015

911

3022

1316

2526

1720

2030

5527

1715

0102030405060

9080

7060

5040

3025

2017

1st comp. 2nd comp.

icr

it

i80

Dn

%W

kd

18

max

.

Ø⋅⋅

ρ⋅

⋅=

Hol

cim

sta

ndar

d ch

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s:

Bal

l dia

met

er [m

m]

Percentage charge [%]

[mm

]

Max

. bal

l Ø1st

Com

p.

Ave

rag

eb

all w

eig

ht

[g/t]

spec

.b

all s

urf

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[m2 /t

]1st

Co

mp

:

wit

ho

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pre

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wit

h

p

reg

rin

din

g

2nd C

om

p:

CC

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7

800

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47,3

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or

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PC

~ 3

200

cm2 /g

)

fine

coar

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2nd c

omp.

1st c

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open

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3

coar

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e

10,2

13,0

- 20 -

6.2 Ball charge calculation (Fig. 10)

Ø Additionally to the standard charges, which are a result of experience, there are several methods to calculate a ball charge distribution. A good method is the calculation acc. to Bombled, which allocates the proper ball diameter to the particle size at each point along the mill axis, considering the particle size reduction progress.

Ø Methods according to Bombled, preconditions:

− Mill compartment with classifying lining

− Largest and smallest ball size to establish (∅ max, ∅ min)

− Use of grinding balls (not cylpebs)

− Formula for the Bombled curve:

? m [%] cumulated percentage of balls larger than diameter Dm [mm]

D0 [mm] max value = ∅ max * 1,1

D1 [mm] min value = ∅ min * 0.95

Dm [mm] mean value of ball size interval (e.g. ball ∅ 50, 40 ⇒ Dm = 45 mm)

m [-] constant ~1.7

[ ]%

1

1

100

1

0

0

−

−

⋅=Φ m

m

mm

DD

DD

- 21 -

CC01-010.dsf Kma 06.06.00Operation of Tube Mills

Fig.10 Ball charge composition

10

17

20

25

30

40

50

100

0 20 40 60 80 100

D0

45

35

27,522,5

D1

0

10

5

15

20

25

30

35

50 40 30 25 20

Ball size distribution acc. to Bombled

Bal

l dia

met

er [m

m]

Percentage of ball size [%]

Rel

ativ

e m

ass

[%]

Ball diameter [mm]

8

22

4470

8

14

22

26

30

mean of ball size interval

Bombled line Bombled limit size

- 22 -

6.3 Replenishment of ball charges

To maintain a good and constant mill performance, the ball charges have to be maintained in terms of composition, filling degree and clean charge (without scrap and deformed balls).

Methods to determine date for replenishment:

Ø Fixed intervals

Ø After reaching a minimum kW absorbed

Ø After reaching a minimum filling degree

The objective for replenishment is to maintain an optimal ball charge composition over a long time.

Proposed rules for replenishment of balls (compensation for wear):

Circuit Closed Open

Coarse Fine

Ball ∅ [mm] [%]

1st compartment 90 100

*) *)

50 15 10 15

2nd compartment 30 85 35 35

20 -- 55 50

*) with ball sizes ∅ 50-20 mm

6.4 Ball charge management

A sufficient and continuous mill efficiency can only be guaranteed if the changes of the ball charge (total quantity and quantity of ball sizes) are registered.

The statistics of the ball charges (ball charge management) allow to monitor the mill performance and to take corrective measures if needed. Also the balance of tons filled into the mill and the tons sorted out give the net and gross wear figures, which are important to assess the wear behavior of the ground material and the quality of the grinding media (judgement of suppliers quality and guarantees).

It is recommended to drop the entire ball charge, to classify and to regrade it:

− 1st compartment every year (sort out scrap, easy to classify due to large balls)

− 2nd compartment every 1 to 3 years according to the needs

The balls have to be classified according to sizes e.g. ∅ 40 [mm] → (all between ~31 – 40 [mm]), the deformed and too small balls as well as the scrap screened out. With the classified sizes and a respective quantity of new balls, a new complete ball charge is composed.

All quantities have to be weighed and registered.

- 23 -

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Corresponding papers

Operation of separators VA 93/4052/E

W. Zeller

Cooling and heating in the cement PT 99/14489/E grinding process

Hp. Fisch

B – Level Audit VA 91/5954/E

K.Breitschmid

Drying technology PT 96/14027/E

Hp. Fisch