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8/8/2019 03 Ball Mills
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Ball mills
Michael Müller-PfeifferResearch Institute of the Cement Industry (Düsseldorf)Seminar Grasim, 28 February 2007
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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 investigation7. Case studies
8. Summary
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Ball mill for dry grinding (e.g. cement)
Coarse grinding chamber33 % of total grinding path length
Lifter plate lining
100 mm – 60 mm balls
Fine grinding chamber67 % of total grinding path length
Classifying plate lining
50 mm - 15 mm balls
Intermediate diaphragm
Discharge diaphragm
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Open circuit ball mill grinding plant
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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 coarsercements suitable
High fineness cements cannot be produced
Fineness of cement is only adjustable by feedmass flow
At big mills very high mill temperatures
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Closed-circuit ball mill grinding plant
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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 havemore technical problems
Larger space requirement
Higher investment cost (+ 25 to 30 % incl. housing)
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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 investigation7. Case studies
8. Summary
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Movement of grinding media in a tube mill
Relative mill speed in %
20 40 60 70 80 90
B a l l f i l l i n g
l e v e l i n %10
20
30
40
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Movement of grinding media in a tube mill
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Movement of grinding media in a tube mill
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Movement of grinding media in a tube mill
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• The relative mill speed is the relation of actual mill speed tocritical mill speed
Movement of grinding media in a tube mill
%100=
critical
actualrelative
n
nn
i
critical
i
critical
Dn
Dgn
298.42
230
=
=
• The critical mill speed is that speed of rotation at which the ballsstick to the mill shell and do not fall, that means the centrifugalpower neutralizes the force of gravity
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Ball filling level
%100164.1068.1
=
igballfillin D
h
Recommended ball filling level:
1. Chamber: 26 – 32 %
2. Chamber: 24 – 28 %
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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 investigation7. Case studies
8. Summary
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Necessary ball size
Big particle
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Necessary ball size
Big particle
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Necessary ball size
Small
particle
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Necessary ball size
Small
particle
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Ball charge – Size distribution chamber 1
Example for ball size distribution for ball mills in closed circuit with a high
efficiency separator (without pregrinding)
1590-20253025-%
Average ball-weight
(g/pc)
5060708090100DBall
(mm)
1. Chamber
100 mm-balls are only necessary in case of
• Very coarse clinker or
• Small diameter mills
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Ball charge – Size distribution chamber 2
Example for ball size distribution for ball mills in closed circuit with a high
efficiency separator (without pregrinding)
4915232420104%
Average ball-weight
(g/pc)
172025304050DBall
(mm)
2. Chamber
Some 50 and 40 mm-balls are necessary to ensure the comminution of
oversized particles passing the intermediate diaphragm
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Ball charge
• New arrangement of ball charge
• Removal of worn and undersized grindingballs
• Recommended once a year
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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 [-]
s p e c . p o w e r c o n s u m p t i o n [ k W h / t ]
0,0
0,4
0,8
1,2
1,6
2,0
2,4
t h r o u g h
p u t [ t / h ]
15 %
20 %
25 %
throughput
spec. power consumption
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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 [-]
s p e c . p o w e r c o n s u m p t i o n [ k W h / t ]
0,0
0,4
0,8
1,2
1,6
2,0
2,4
t h r o u g
h p u t [ t / h ]
15%
20%
25%
throughput
spec. power consumption
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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 investigation7. Case studies
8. Summary
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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
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Lifter lining
Tasks:
• Protection of the mill shell
• Good ball lifting
• Throughput increase
• Wear resistant
(high chromium cast)
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Classifying lining
Tasks:
• Protection of the mill shell
• Good ball classification
• Throughput increase
• Wear resistant
(high chromium cast)
8 Kg
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Classifying lining – new Magotteaux Xclass
Advantage:
• Reduction of lining weight
• Faster and cheaperinstallation
• 1 % energy saving
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Fixing of liner plates
boltless
bolted
semi-bolted
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Fixing of liner plates
bolted
Advantage: mechanically stable
insensitive in case ofbreakage of a plate
simple installation
Disadvantage: plates must bemanufactured with theaccurate dimension
boreholes in the mill
shell must be accuratelypositioned
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Fixing of liner plates
semi-bolted and boltless
Advantage: easier manufacture
boltless lined mills don’tspread
Disadvantage: plates must be
manufactured with theaccurate dimension
sophisticated installation
sensitive in case of
broken plates
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Intermediate diaphragm
Tasks:
• Prevent passing of grinding balls intothe other grinding chamber
• Prevent passing of oversized particlesinto the fine grinding chamber
• Material transport:
• Separation of air and material
• Material shall enter the ballcharge in 2. chamber directlybehind the diaphragm
• Control of material filling level in thecoarse grinding chamber
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Intermediate diaphragm
• Consists of several cast segments foreasy installation
• Double wall partition with 2 front wallsand lifter scoops
• Central hole for mill ventilation
• Entrance side: Slot plates with slotwidth of 6 to 10 mm
• Slot 2 times wider at the exit side
• Active surface 5 – 10 %
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Intermediate diaphragm – Separation of air and material
transport
Old design New design
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Intermediate diaphragm – Separation of air and material
transport
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FLS-Combidan diaphragm
Screen plate
Coarse grate
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Discharge diaphragm
• Design comparable tointermediate diaphragm
• Central hole for millventilation
• Entrance side: Slot plates
with slot width of 6 to 10 mm• Slot 2 times wider at the exit
side
• Active surface 5 – 10 %
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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
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Mill inlet
• Material shall enter the ball chargedirectly behind the mill inlet
• Separation of air and material feeding
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Mill drive technology
Girth gear and pinion drive
Central driveRing motor
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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 farhigher capacities transmittable)
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Central drive
Advantage: Low maintenance
Disadvantage: 50 % more expensive than singlegear and pinion drive
With co-rotating planetary stagetransmittable power is limited to5000 kW
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Ring motor
Advantage: Unlimited drive capacity
No gear
Low required space
Rotational speed continuously
adjustableLowest maintenance requirementand highest availability
Disadvantage: High investment costs
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Mill bearings
Slide shoe bearings
Trunnion supports
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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 useof hot gas (e.g. raw mills)
• High gas velocities in mill trunnion
• Difficult material feeding
• Higher investment costs
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Slide shoe bearing
Advantage:
• Easy alignment
• Simple foundation
• Reduced Length
• Less weight of mill shell
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Structure
1. Introduction
2. Movement of grinding media in a tube mill3. 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
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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 thefree cross section:
Open circuit mills: < 1.2 m/s
Closed circuit mills: < 1.4 m/s
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Water injection
• Usually water injection from the discharge sideinto chamber 2 (warmest part of the mill)
• In case of high clinker temperature also waterinjection into chamber 1 possible
• It is better to cool the balls than to cool the air
• Right positioning of the water injection is important
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Water injection chamber 1
air
water
w a t e r
a i r
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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 % andthroughput increases up to 40 % are possible
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Structure
1. Introduction
2. Movement of grinding media in a tube mill3. 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
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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
S
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Structure
1. Introduction
2. Movement of grinding media in a tube mill3. 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
1 Ch b B ll h t
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1. Chamber - Ball charge too coarse
Cause: Too big ball charge
Effect: Inefficient grinding
Number of ball impacts onmaterial too low
1 Ch b B ll h t
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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
R e s
i d u e i n %
2000 µm
1000 µm
400 µm
200 µm
90 mm
40 µm
Chamber 1 Chamber 2
1. Chamber - Ball charge too coarse
1 Chamber Ball charge too fine
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1. Chamber – Ball charge too fine
Cause: Very big clinker pieces,
Too small ball charge
Effect: Coarse material particles arenot sufficiently comminuted and
accumulated in front of thediaphragm
1 Chamber ball charge too fine
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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
R e s i d u e i n %
4000 µm
2000 µm1000 µm
400 µm
200 µm
90 µm
40 µm
Chamber 1 Chamber 2
1. Chamber – ball charge too fine
2 Chamber Ball charge to coarse
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2. Chamber – Ball charge to coarse
Cause: Too big grinding balls,
Effect: Inefficient grinding
Low throughput
High energy consumption
2 Chamber Ball charge to coarse
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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
R e s i d u e i n %
4000 µm
2000 µm
1000 µm
400 µm
200 µm
90 µm
40 µm
Chamber 1 Chamber 2
2 Chamber – Ball charge too fine
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2. Chamber – Ball charge too fine
Cause: Too small grinding balls,worn grinding balls (ball scrap)
Oversized particles passinginto the 2. chamber
Effect: Oversized particles are notcomminuted and accumulated
in the 2. chamber
2 Chamber – Ball charge too fine
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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
R e s i d u e i n %
4000 µm
2000 µm
1000 µm
400 µm
200 µm
90 µm
40 µm
Chamber 1 Chamber 2
Too high material filling level
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Too high material filling level
Cause: blocked slots in the outletdiaphragm
Effect: Ball hits partly are absorbed
high energy consumption
low throughput
Too high material filling level
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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
R e s i d
u e i n %
4000 µm
2000 µm
1000 µm400 µm
200 µm
90 µm
40 µm
Chamber 1 Chamber 2
Too low material filling level
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Too low material filling level
Cause: Too high ball filling level
Effect: high energy consumption
high wear
Bad material supply in the mill inlet
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pp y
Cause: High air velocity in mill inlet
Bad material supply
Effect: Material is sucked into the mill
The first meter of the coarsegrinding chamber get lost
Bad material supply in the mill inlet
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pp y
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
R e s i d
u e i n %
10000 µm4000 µm
2000 µm
1000 µm
400 µm
200 µm
90 µm
Chamber 1 Chamber 2
Structure
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1. Introduction
2. Movement of grinding media in a tube mill3. 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
Ball mills - summary
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y
• High grinding energy consumption - less than 10 % oftotal grinding energy is used for comminution
• Optimal adjustment of the ball mill (ball charge, ballfilling 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
Summary
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Thank you for your attention!
Any questions?