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Cement Milling I
Chris Holt
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A typical two chamber cement mill
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1 Feed
2 Division wall
4 Outlet grid
5 Lifters
6 Mill shell7 Lining plates
8 Internal water injection
9 Rotating seal for water system
10 Torsion shaft for central drive
A typical three chamber cement mill
1 24
8 8 1059
67
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Examples - chamber 1 linings
Mill rotation
- duolift type liner
- reverse step type liner
300
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Boltless lining
plate
Mill rotation
Mill shell
Card
Section through
a boltless plate
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Ripple type lining
2 1/8
rad
1 1/2
3 7/8
15/8 rad
3/161
/8
1 1/2
3 1/8 dia
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Classifying type lining plate
AA
B
B
A - A
139mm
60mm
B -B
308 mm
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FLS type spiral wave lining
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CARMAN LINING
SLEGTEN - MAGOTTEAUX TYPE
FLS TYPE
Examples of classifying type liners
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Lorain Bar lift lining
2 1/2
chamber 1
1 1/2
chamber 2
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Dragpeb type - Chamber 2 liner
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Division walls - diaphragms
1.- To control the level of material in each chamber to
prevent:
Under filling of material - wastes power.
Overfilling of material - wastes power.
2.- Separate the grinding media in each chamber, to optimize
the media grading to suit the material fineness.
3.- To prevent coarse material passing unprepared to the
following chamber.
4.- To permit sufficient cooling airflow/ventilation through
themill while preventing short-circuiting of material
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Pfieffer type double diaphragm
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PfeifferMaterial flow
control
Circumferential slotshave a self cleaning
characteristic
large central vent can
limit material charge
below potential motor
capacity, can reduce
maximum output
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Pfeiffer
3rd generation
diaphragm
(Tulsa)
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Breakage rate vs Particle size
Breakage rate for cement clinkerusing ball sizes 50 mm, 25 mm,
16 mm
Breaka
gerate
Particle size (microns)
20 100 1000
16mm
25mm
50mm
16mm
25mm
50mm
Typical axial sample data - open circuit milll
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Typical axial sample data open circuit milll
A v e r
a g e s i
z e o
f
t o p 5
% o
f p a r
t i c l e s
Bzllsizesmm
Size of normal ball grading
Average ball size presentOptimum ball size to break top 5%
10000
1000
100
mill length ===========>
2
5
10
20
50
100
200
500
1000
10 100
1 2 3 Mill chamber
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Comparison of ball gradings
Chamber 1 Chamber 2
Ba
llsizem
m
% of effective mill length - % of charge
Blue Circle(Current)
120
100
80
60
40
20
0
Base curve(FLS)
Blue Circle(Historical)
FLS
Blue circle
historical
current
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Use of fine mediaPreconditions and Considerations
1.- Satisfactory first chamber liningcondition.
2.- Design and condition of the mill outletdiaphragm.
3.- Material flow characteristics.4.- Milling temperature /
Coating of the
grinding balls.
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Temperature
Effect on gypsum dehydration Effect on milling efficiency
Excessive use of water sprays Customer complaints
Temperature conditions in a mill
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Temperature conditions in a mill
A High clinker temp
- no cooling
A With cooling
Temperature
(degC degF)
320
300
280
260
240
220
200
180
160
140
120
100
0 20 40 60 80 100
B Low clinker temp
- no cooling
B With cooling
160
150
140
130
120
110
10090
80
70
60
50
40
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Schematic - water injection
Central drive mill
O i i ill
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Open circuit cement mill -
Heat balance
Gypsum
4.5 t/h 25 C
Air 125C
Water Vapour 125 C
Cement 125 C
Convection600 Kcal/h/m2
Mill power
= 2208 Kw
Clinker 75 t/h 50 C
Air 25 C
Water
10 C
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Mill ventilationRecommended airflows for mill ventilation
Circuit Open Closed
Air changes/min 3 5Kg air/cement Kg 0.25 0.4
False Air inleak
1.- Estimate the air inleak by comparing temperatures at the
milloutlet and the filter outlet. Also measure airflows at the mill
inletto compare with the filter outlet flow.
2.- A typical inleak level over a filter system on a new mill is
about =30% of the flow passing through the mill.
