Ball mills are tube mills used for grinding materials like cement and involve several components. The document discusses the movement of grinding media in ball mills, optimal ball charge and filling levels, key components like linings, diaphragms, drives and bearings, and factors that influence mill operation such as ventilation, water injection and grinding aids. Case studies are presented to illustrate problems that can occur with improper ball charge sizes and material filling levels.

1. Ball mills
Michael Müller-Pfeiffer
Research Institute of the Cement Industry (Düsseldorf)
Seminar Grasim, 28 February 2007

2. Structure
1. Introduction
2. Movement of grinding media in a tube mill
3. Ball charge and ball filling level
4. Components of ball mills (linings, diaphragms, mill inlet, drive,
bearing)
5. Mill ventilation, water injection and grinding aids
6. Mill investigation
7. Case studies
8. Summary

3. Ball mill for dry grinding (e.g. cement)
Coarse grinding chamber
33 % of total grinding path length
Lifter plate lining
100 mm – 60 mm balls
Fine grinding chamber
67 % of total grinding path length
Classifying plate lining
50 mm - 15 mm balls
Intermediate diaphragm
Discharge diaphragm

4. Open circuit ball mill grinding plant

5. Open circuit ball mill grinding plant
Advantage: Simple plant layout with minimal cost
Little space requirement
Simple process control
Disadvantage: High energy consumption – only for coarser
cements suitable
High fineness cements cannot be produced
Fineness of cement is only adjustable by feed
mass flow
At big mills very high mill temperatures

6. Closed-circuit ball mill grinding plant

7. Closed-circuit ball mill grinding plant
Advantage: Lower power consumption than an open circuit ball mill
High cement fineness achievable
Cement fineness can be controlled with separator adjustment
Disadvantage: Closed-circuit mills are more sophisticated and can have
more technical problems
Larger space requirement
Higher investment cost (+ 25 to 30 % incl. housing)

8. Structure
1. Introduction
2. Movement of grinding media in a tube mill
3. Ball charge and ball filling level
4. Components of ball mills (linings, diaphragms, mill inlet, drive,
bearing)
5. Mill ventilation, water injection and grinding aids
6. Mill investigation
7. Case studies
8. Summary

9. Movement of grinding media in a tube mill
Relative mill speed in %
20 40 60 70 80 90
Ball
filling
level
in
%
10
20
30
40

10. Movement of grinding media in a tube mill

11. Movement of grinding media in a tube mill

12. Movement of grinding media in a tube mill

13. • The relative mill speed is the relation of actual mill speed to
critical mill speed
Movement of grinding media in a tube mill
%
100
=
critical
actual
relative
n
n
n
i
critical
i
critical
D
n
D
g
n
298
.
42
2
30
=
=
• The critical mill speed is that speed of rotation at which the balls
stick to the mill shell and do not fall, that means the centrifugal
power neutralizes the force of gravity

14. Ball filling level
%
100
164
.
1
068
.
1
=
i
g
ballfillin
D
h
Recommended ball filling level:
1. Chamber: 26 – 32 %
2. Chamber: 24 – 28 %

15. Structure
1. Introduction
2. Movement of grinding media in a tube mill
3. Ball charge and ball filling level
4. Components of ball mills (linings, diaphragms, mill inlet, drive,
bearing)
5. Mill ventilation, water injection and grinding aids
6. Mill investigation
7. Case studies
8. Summary

16. Necessary ball size
Big particle

17. Necessary ball size
Big particle

18. Necessary ball size
Small
particle

19. Necessary ball size
Small
particle

20. Ball charge – Size distribution chamber 1
Example for ball size distribution for ball mills in closed circuit with a high
efficiency separator (without pregrinding)
1590
-
20
25
30
25
-
%
Average ball-weight
(g/pc)
50
60
70
80
90
100
DBall
(mm)
1. Chamber
100 mm-balls are only necessary in case of
• Very coarse clinker or
• Small diameter mills

21. Ball charge – Size distribution chamber 2
Example for ball size distribution for ball mills in closed circuit with a high
efficiency separator (without pregrinding)
49
15
23
24
20
10
4
%
Average ball-weight
(g/pc)
17
20
25
30
40
50
DBall
(mm)
2. Chamber
Some 50 and 40 mm-balls are necessary to ensure the comminution of
oversized particles passing the intermediate diaphragm

22. Ball charge
• New arrangement of ball charge
• Removal of worn and undersized grinding
balls
• Recommended once a year

23. Ball filling level – coarse feed material
0
6
12
18
1,5 2,0 2,5 3,0 3,5 4,0
L/D-Ratio [-]
spec.
power
consumption
[kWh/t]
0,0
0,4
0,8
1,2
1,6
2,0
2,4
throughput
[t/h]
15 %
20 %
25 %
throughput
spec. power consumption

24. Ball filling level – fine feed material
0
6
12
18
1,5 2,0 2,5 3,0 3,5 4,0
L/D-Ratio [-]
spec.
power
consumption
[kWh/t]
0,0
0,4
0,8
1,2
1,6
2,0
2,4
throughput
[t/h]
15%
20%
25%
throughput
spec. power consumption

25. Structure
1. Introduction
2. Movement of grinding media in a tube mill
3. Ball charge and ball filling level
4. Components of ball mills (linings, diaphragms, mill inlet, drive,
bearing)
5. Mill ventilation, water injection and grinding aids
6. Mill investigation
7. Case studies
8. Summary

26. Movement of grinding media in ball mills
Coarse grinding chamber: Movement with cataract portion – lifter plate lining
Fine grinding chamber: cascade movement – smoother lining profile

27. Lifter lining
Tasks:
• Protection of the mill shell
• Good ball lifting
• Throughput increase
• Wear resistant
(high chromium cast)

28. Classifying lining
Tasks:
• Protection of the mill shell
• Good ball classification
• Throughput increase
• Wear resistant
(high chromium cast)
8 Kg

29. Classifying lining – new Magotteaux Xclass
Advantage:
• Reduction of lining weight
• Faster and cheaper
installation
• 1 % energy saving

30. Fixing of liner plates
boltless
bolted
semi-bolted

31. Fixing of liner plates
bolted
Advantage: mechanically stable
insensitive in case of
breakage of a plate
simple installation
Disadvantage: plates must be
manufactured with the
accurate dimension
boreholes in the mill
shell must be accurately
positioned

32. Fixing of liner plates
semi-bolted and boltless
Advantage: easier manufacture
boltless lined mills don’t
spread
Disadvantage: plates must be
manufactured with the
accurate dimension
sophisticated installation
sensitive in case of
broken plates

33. Intermediate diaphragm
Tasks:
• Prevent passing of grinding balls into
the other grinding chamber
• Prevent passing of oversized particles
into the fine grinding chamber
• Material transport:
• Separation of air and material
• Material shall enter the ball
charge in 2. chamber directly
behind the diaphragm
• Control of material filling level in the
coarse grinding chamber

34. Intermediate diaphragm
• Consists of several cast segments for
easy installation
• Double wall partition with 2 front walls
and lifter scoops
• Central hole for mill ventilation
• Entrance side: Slot plates with slot
width of 6 to 10 mm
• Slot 2 times wider at the exit side
• Active surface 5 – 10 %

35. Intermediate diaphragm – Separation of air and material
transport
Old design New design

36. Intermediate diaphragm – Separation of air and material
transport

37. FLS-Combidan diaphragm
Screen plate
Coarse grate

38. Discharge diaphragm
• Design comparable to
intermediate diaphragm
• Central hole for mill
ventilation
• Entrance side: Slot plates
with slot width of 6 to 10 mm
• Slot 2 times wider at the exit
side
• Active surface 5 – 10 %

39. Lifetime of wear resistant products of ball mills (12 % Cr)
• Mill inlet (liner plates): 12,000 – 17,000 h
• Lifter plates: 35,000 h
• Intermediate wall: slot plates: 13,000 h
back board: 19,000 h
• Classifying liner plates: 60,000 h
• Outlet wall: slot plates: 20,000 h
• Grinding balls: 40 g/t

40. Mill inlet
• Material shall enter the ball charge
directly behind the mill inlet
• Separation of air and material feeding

41. Mill drive technology
Girth gear and pinion drive
Central drive
Ring motor

42. Girth gear and pinion drive
Advantage: Low investment costs
Little space requirement
Disadvantage: With single gear and pinion drive
capacity is limited to 5000 kW
(Double gear and pinion drive far
higher capacities transmittable)

43. Central drive
Advantage: Low maintenance
Disadvantage: 50 % more expensive than single
gear and pinion drive
With co-rotating planetary stage
transmittable power is limited to
5000 kW

44. Ring motor
Advantage: Unlimited drive capacity
No gear
Low required space
Rotational speed continuously
adjustable
Lowest maintenance requirement
and highest availability
Disadvantage: High investment costs

45. Mill bearings
Slide shoe bearings
Trunnion supports

46. Trunnion supports
Disadvantage:
• High loading of mill front walls (risk of damage by material fatigue)
• Temperature problems of the trunnion bearing in case of the use
of hot gas (e.g. raw mills)
• High gas velocities in mill trunnion
• Difficult material feeding
• Higher investment costs

47. Slide shoe bearing
Advantage:
• Easy alignment
• Simple foundation
• Reduced Length
• Less weight of mill shell

48. Structure
1. Introduction
2. Movement of grinding media in a tube mill
3. Ball charge and ball filling level
4. Components of ball mills (linings, diaphragms, mill inlet, drive,
bearing)
5. Mill ventilation, water injection and grinding aids
6. Mill investigation
7. Case studies
8. Summary

49. Mill ventilation
Tasks of mill ventilation:
• Dedusting
• Cooling
• Support of material transport
Air volume flow: 0.2 – 1.0 m³/kgthroughput
Limit values for the air velocity in the
free cross section:
Open circuit mills: < 1.2 m/s
Closed circuit mills: < 1.4 m/s

50. Water injection
• Usually water injection from the discharge side
into chamber 2 (warmest part of the mill)
• In case of high clinker temperature also water
injection into chamber 1 possible
• It is better to cool the balls than to cool the air
• Right positioning of the water injection is important

51. Water injection chamber 1
ai
r
water
water
air

52. Grinding aids
Specification:
• Reduce adhesive forces between fine particles
• Prevent coating of linings and grinding balls in the 2. Chamber
• Improve the flowability of cement
• Improve the separation efficiency in the classifier
• Water is the most simple grinding aid
• Surface active organic liquids are more effective
(e.g.Triethanolamin 0.02 - 0.06 %, Ethylenglycol 0.02 – 0.08 % )
• By addition of grinding aids energy saving up to 25 % and
throughput increases up to 40 % are possible

53. Structure
1. Introduction
2. Movement of grinding media in a tube mill
3. Ball charge and ball filling level
4. Components of ball mills (linings, diaphragms, mill inlet, drive,
bearing)
5. Mill ventilation, water injection and grinding aids
6. Mill investigation
7. Case studies
8. Summary

54. Mill investigation
• Control of condition of wearing parts
(lining, slot plates, etc.)
• Control of ball filling level
• Sampling at each meter of the grinding path
• Recommended once a year

55. Structure
1. Introduction
2. Movement of grinding media in a tube mill
3. Ball charge and ball filling level
4. Components of ball mills (linings, diaphragms, mill inlet, drive,
bearing)
5. Mill ventilation, water injection and grinding aids
6. Mill investigation
7. Case studies
8. Summary

56. 1. Chamber - Ball charge too coarse
Cause: Too big ball charge
Effect: Inefficient grinding
Number of ball impacts on
material too low

57. 0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8 9 10
Length of grinding path in m
Residue
in
%
2000 µm
1000 µm
400 µm
200 µm
90 mm
40 µm
Chamber 1 Chamber 2
1. Chamber - Ball charge too coarse

58. 1. Chamber – Ball charge too fine
Cause: Very big clinker pieces,
Too small ball charge
Effect: Coarse material particles are
not sufficiently comminuted and
accumulated in front of the
diaphragm

59. 0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8 9 10
Length of grinding path in m
Residue
in
%
4000 µm
2000 µm
1000 µm
400 µm
200 µm
90 µm
40 µm
Chamber 1 Chamber 2
1. Chamber – ball charge too fine

60. 2. Chamber – Ball charge to coarse
Cause: Too big grinding balls,
Effect: Inefficient grinding
Low throughput
High energy consumption

61. 2. Chamber – Ball charge to coarse
0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8 9 10 11 12
Lenth of grinding path m
Residue
in
%
4000 µm
2000 µm
1000 µm
400 µm
200 µm
90 µm
40 µm
Chamber 1 Chamber 2

62. 2. Chamber – Ball charge too fine
Cause: Too small grinding balls,
worn grinding balls (ball scrap)
Oversized particles passing
into the 2. chamber
Effect: Oversized particles are not
comminuted and accumulated
in the 2. chamber

63. 2. Chamber – Ball charge too fine
0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8 9 10 11
Length of grinding path in m
Residue
in
%
4000 µm
2000 µm
1000 µm
400 µm
200 µm
90 µm
40 µm
Chamber 1 Chamber 2

64. Too high material filling level
Cause: blocked slots in the outlet
diaphragm
Effect: Ball hits partly are absorbed
high energy consumption
low throughput

65. Too high material filling level
0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8 9 10
Length of grinding path in m
Residue
in
%
4000 µm
2000 µm
1000 µm
400 µm
200 µm
90 µm
40 µm
Chamber 1 Chamber 2

66. Too low material filling level
Cause: Too high ball filling level
Effect: high energy consumption
high wear

67. Bad material supply in the mill inlet
Cause: High air velocity in mill inlet
Bad material supply
Effect: Material is sucked into the mill
The first meter of the coarse
grinding chamber get lost

68. Bad material supply in the mill inlet
0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8 9 10 11 12
Length of grinding path in m
Residue
in
%
10000 µm
4000 µm
2000 µm
1000 µm
400 µm
200 µm
90 µm
Chamber 1 Chamber 2

69. Structure
1. Introduction
2. Movement of grinding media in a tube mill
3. Ball charge and ball filling level
4. Components of ball mills (linings, diaphragms, mill inlet, drive,
bearing)
5. Mill ventilation, water injection and grinding aids
6. Mill investigation
7. Case studies
8. Summary

70. Ball mills - summary
• High grinding energy consumption - less than 10 % of
total grinding energy is used for comminution
• Optimal adjustment of the ball mill (ball charge, ball
filling level, mill ventilation) is very important
• Regularly control of the condition of the wearing parts
(lining, diaphragm)
• Possible optimisation potentials:
• Ball charge
• Diaphragm
• Lining
• Water spray equipment

71. Summary
Thank you for your attention!
Any questions?