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Ball mills

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

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

Ball mill for dry grinding (e.g. cement)

Intermediate diaphragm Fine grinding chamber 67 % of total grinding path length Classifying plate lining 50 mm - 15 mm balls

Coarse grinding chamber 33 % of total grinding path length Lifter plate lining 100 mm – 60 mm balls

Discharge diaphragm

Open circuit ball mill grinding plant

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

Closed-circuit ball mill grinding plant

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)

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

Ball filling level in %

Movement of grinding media in a tube mill

10 20 30 40 20

40 60 70 80 Relative mill speed in %

90

Movement of grinding media in a tube mill

Movement of grinding media in a tube mill

Movement of grinding media in a tube mill

Movement of grinding media in a tube mill

•

The relative mill speed is the relation of actual mill speed to critical mill speed

nactual 100% n relative = ncritical • 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

ncritical =

30

2 g Di

ncritical =

42.298 Di

Ball filling level

ballfilling = 1.068 1.164

h Di

100%

Recommended ball filling level: 1. Chamber: 26 – 32 % 2. Chamber: 24 – 28 %

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

Necessary ball size

Big particle

Necessary ball size

Big particle

Necessary ball size

Small particle

Necessary ball size

Small particle

Ball charge – Size distribution chamber 1 Example for ball size distribution for ball mills in closed circuit with a high efficiency separator (without pregrinding) 1. Chamber DBall (mm)

100

90

80

70

60

50

Average ball-weight (g/pc)

%

-

25

30

25

20

-

1590

100 mm-balls are only necessary in case of • Very coarse clinker or • Small diameter mills

Ball charge – Size distribution chamber 2 Example for ball size distribution for ball mills in closed circuit with a high efficiency separator (without pregrinding) 2. Chamber DBall (mm)

50

40

30

25

20

17

Average ball-weight (g/pc)

%

4

10

20

24

23

15

49

Some 50 and 40 mm-balls are necessary to ensure the comminution of oversized particles passing the intermediate diaphragm

Ball charge

• New arrangement of ball charge

• Removal of worn and undersized grinding balls • Recommended once a year

Ball filling level – coarse feed material 18

2,4

20 %

2,0

25 % 12

1,6

spec. power consumption

1,2

6

0,8 throughput 0,4

0

0,0 1,5

2,0

2,5

3,0

L/D-Ratio [-]

3,5

4,0

throughput [t/h]

spec. power consumption [kWh/t]

15 %

Ball filling level – fine feed material 18

2,4

15% 2,0

25% spec. power consumption

12

1,6

1,2

6

0,8

0,4 throughput 0

0,0 1,5

2,0

2,5 3,0 L/D-Ratio [-]

3,5

4,0

throughput [t/h]

spec. power consumption [kWh/t]

20%

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

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

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

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

8 Kg

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

Fixing of liner plates bolted

semi-bolted

boltless

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

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

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

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 %

Intermediate diaphragm – Separation of air and material transport

Old design

New design

Intermediate diaphragm – Separation of air and material transport

FLS-Combidan diaphragm

Screen plate

Coarse grate

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 %

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: • Outlet wall: • Grinding balls:

60,000 h

slot plates: 20,000 h 40 g/t

Mill inlet

• Material shall enter the ball charge

directly behind the mill inlet • Separation of air and material feeding

Mill drive technology Girth gear and pinion drive Central drive

Ring motor

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)

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

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

Mill bearings

Trunnion supports

Slide shoe bearings

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

Slide shoe bearing

Advantage: •

Easy alignment

•

Simple foundation

•

Reduced Length

•

Less weight of mill shell

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

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

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

Water injection chamber 1

water

air

water

ai r

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

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

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

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

1. Chamber - Ball charge too coarse Cause: Too big ball charge

Effect: Inefficient grinding Number of ball impacts on material too low

1. Chamber - Ball charge too coarse 100 Chamber 1

Chamber 2

2000 µm 1000 µm 400 µm 200 µm 90 mm 40 µm

90 80

Residue in %

70 60 50 40 30 20 10 0 0

1

2

3

4

5

6

7

Length of grinding path in m

8

9

10

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

1. Chamber – ball charge too fine 100 Chamber 1

90

4000 µm 2000 µm 1000 µm 400 µm 200 µm 90 µm 40 µm

80 70

Residue in %

Chamber 2

60 50 40 30 20 10 0 0

1

2

3

4

5

6

7

Length of grinding path in m

8

9

10

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 100 Chamber 1

Chamber 2

4000 µm 2000 µm 1000 µm 400 µm 200 µm 90 µm 40 µm

90 80

Residue in %

70 60 50 40 30 20 10 0 0

1

2

3

4

5

6

7

Lenth of grinding path m

8

9

10

11

12

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

2. Chamber – Ball charge too fine 100 Chamber 1

Chamber 2

4000 µm 2000 µm 1000 µm 400 µm 200 µm 90 µm 40 µm

90 80

Residue in %

70 60 50 40 30 20 10 0 0

1

2

3

4

5

6

7

Length of grinding path in m

8

9

10

11

Too high material filling level

Cause: blocked slots in the outlet diaphragm Effect: Ball hits partly are absorbed high energy consumption low throughput

Too high material filling level

100 Chamber 1

Chamber 2

4000 µm 2000 µm 1000 µm 400 µm 200 µm 90 µm 40 µm

90 80

Residue in %

70 60 50 40 30 20 10 0 0

1

2

3

4

5

6

7

Length of grinding path in m

8

9

10

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

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

Bad material supply in the mill inlet

100 Chamber 1

90

Chamber 2 10000 µm

Residue in %

80

4000 µm

70

2000 µm

60

400 µm

1000 µm

200 µm

50

90 µm

40 30 20 10 0 0

1

2

3

4

5

6

7

8

Length of grinding path in m

9

10

11

12

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

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

Summary

Thank you for your attention! Any questions?