03 Ball Mills

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Ball mills

Michael Müller-PfeifferResearch 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 investigation7. Case studies

8. Summary



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



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 coarsercements suitable

High fineness cements cannot be produced

Fineness of cement is only adjustable by feedmass 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 havemore technical problems

Larger space requirement

Higher investment cost (+ 25 to 30 % incl. housing)



Structure

1. Introduction




bearing)



8. Summary



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












• The relative mill speed is the relation of actual mill speed tocritical mill speed


%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



Ball filling level

%100164.1068.1

=

igballfillin D

h

Recommended ball filling level:

1. Chamber: 26 – 32 %

2. Chamber: 24 – 28 %



Structure

1. Introduction




bearing)



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)

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



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



Ball charge

• New arrangement of ball charge

• Removal of worn and undersized grindingballs

• Recommended once a year



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



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



Structure

1. Introduction




bearing)



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 cheaperinstallation

• 1 % energy saving



Fixing of liner plates

boltless

bolted

semi-bolted




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




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




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




• 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 %



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 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 %



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



Mill inlet

• Material shall enter the ball chargedirectly behind the mill inlet

• Separation of air and material feeding



Mill drive technology

Girth gear and pinion drive

Central driveRing 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 farhigher capacities transmittable)



Central drive

Advantage: Low maintenance

Disadvantage: 50 % more expensive than singlegear and pinion drive

With co-rotating planetary stagetransmittable power is limited to5000 kW



Ring motor

Advantage: Unlimited drive capacity

No gear

Low required space

Rotational speed continuously

adjustableLowest maintenance requirementand highest availability

Disadvantage: High investment costs



Mill bearings

Slide shoe bearings

Trunnion supports



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



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 mill3. Ball charge and ball filling level


bearing)


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 thefree cross section:

Open circuit mills: < 1.2 m/s

Closed circuit mills: < 1.4 m/s



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



Water injection chamber 1

air

water

w a t e r

a i 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 % andthroughput increases up to 40 % are possible



Structure

1. Introduction



bearing)



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

S



Structure

1. Introduction



bearing)



7. Case studies

8. Summary

1 Ch b B ll h t



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



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



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



0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10


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



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



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




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




0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 11


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




Cause: blocked slots in the outletdiaphragm

Effect: Ball hits partly are absorbed

high energy consumption

low throughput





0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10


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



Too low material filling level

Cause: Too high ball filling level

Effect: high energy consumption

high wear

Bad material supply in the mill inlet



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



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


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



1. Introduction



bearing)



7. Case studies

8. Summary

Ball mills - summary



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



Thank you for your attention!

Any questions?

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03 Ball Mills