3 - Ball Mill Grinding

BALL MILL GRINDING

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Ball Mill Grinding

LEARNING OBJECTIVES

 At the end of the day the trainee shall be able to :  Describe the ball mill grinding process  Describe ball mill internal equipment and their impact on performance  Describe the process principles of mill separations and interpret the Tromp curve

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BALL MILL GRINDING PROCESS

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Ball Mill Grinding

ACTION OF THE BALLS Crushing is done by : Cascade effect : the most efficient grinding by rolling / crushing under pressure material entering into the charge Cataract effect : splitting up by impact balls falling onto particles 4

Ball Mill Grinding

CASCADING ACTION

Nipping Angle and Particle Size! 5

Ball Mill Grinding

CATARACTING ACTION

Crushing by Impact

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Ball Mill Grinding

RELEASE POINTS AND GRINDING Rotation Rotation

Re le ase Points Release points 1

2

3

1

2

3

Lift Lift Zo ne Zone Grinding Grinding Zone Zone 1 - Cascading (tumbling) 1 - Cascading (tumbling) 2 - Cataracting (free fall - max impact) 2 - Cataracting (free fall - max. impact 3 - Excessive Lift (impact on liner) 3 - Excessive Lift (impact on liner)

Liners into charge. Liners digsdigs into charge. Some Some slipslip occurs. occurs

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Ball Mill Grinding

GRINDING PROCESS

 Energy transmitted by the Balls • Mass

 Ball sizes, Specific Weight

• Speed  Movement, Mill Speed, Filling, Liners • Probability

 Number, Size, Material Filling, Contact Type

 Contact

 Cataract

- Impact, Low Number of contacts

 Cascading - Friction, High Number of contacts 8

Ball Mill Grinding

BALL CHARGE - POROSITY

Coarse Balls - Large Voids Low Retention

Fine Balls - Small Voids High Retention

 Average Ball Weight • Total charge weight / Total number of balls [kg/ball]  Specific Surface Area • Total surface area / Charge weight [m 2/t] 9

Ball Mill Grinding

GRINDING VERSUS FILLING

 Interparticle grinding occurs when the void space is properly filled

 The collision of balls causes a momentary high pressure compression Nip or Contact points where crushing takes place For filling ratios less than 0.6 there's steel to steel contact & no grinding.

Voids where interparticle grinding takes place

For filling ratios greater than 1.1, balls are pushed apart, cushioning impact.

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Ball Mill Grinding

SPEED AND LINERS TYPE

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Ball Mill Grinding

HOW A PARTITION WALL WORKS Feed End View of Partition without Grate Segments

Side View of a Partition

3. Material gravity drops through the center hub and into the next compartment. 2. Material is lifted by mill rotation.

.

1. Material fills the chamber, in between the slotted plates and blind plates

Material is lifted by mill rotation and is then gravity dropped into the next compartment.

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Gas Flow

BM Load Level

2 Material passes through the slots and fills the lifter chamber.

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BALL MILL

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Ball Mill Grinding

THE MILL

Depending on their function • Raw mill, additions (slag, limestone, ...) • Cement Mill (pure cement or cement with additions) • Coal Mill

The more adapted technology will be used • Air-Swept Mills (material is sorted by air) • Mill with end discharge - Compound Mill • Mill with center discharge - Birotator 14

Ball Mill Grinding

THE MILL

Air Swept Mill

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Ball Mill Grinding

THE MILL

End Discharge Ball mill with 2 Chamber

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Ball Mill Grinding

THE MILL

Bi-rotator

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Ball Mill Grinding

THE BALL MILL Intermediate diaphragm Outlet diaphragm Reject return Inlet head C2 liners C1 liners chute

Gas + dust outlet

Fresh feed

Discharge box

Outlet seal Material discharge

Shell Central or ventilation ring Fresh air inlet

Central Ventilation ring grate Supports

Trunnion

Feed Arrangements Mill Heads Mill Bearings Drive Types Mill Shell 18

Ball Mill Grinding

FEED ARRANGEMENTS

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Ball Mill Grinding

MILL HEADS

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Ball Mill Grinding

MILL BEARINGS - TRUNNION

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Ball Mill Grinding

MILL BEARINGS – SLIDE SHOE

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Ball Mill Grinding

MILL DRIVE TYPE – GIRTH GEAR

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Ball Mill Grinding

MILL DRIVE TYPE – CENTRAL DRIVES

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Ball Mill Grinding

MILL DRIVE TYPE – GEARLESS DRIVES

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Ball Mill Grinding

POWER RANGES OF MILL DRIVES

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BALL MILL INTERNALS

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Ball Mill Grinding

CONTINUOUS LIFT OR STEP LINERS

 For first compartments only  Better wear characteristics  Moderate lift ROTATION

Rotation

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Ball Mill Grinding The step size (lifting power) depends on  Mill rotation speed (% Critical Speed) Direction of rotation

 Volume load  Grindability of feed material BOLTLESS STEP CHAMBER 1 LINER  Less material spillage  Avoids bolt breakage

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Ball Mill Grinding

SINGLE WAVE & DUOLIFT LINERS Early release on backslope causes media to slip.

Rotation

 

For first compartments only



Racing is common, wear life is short

 

It has good lift



Duolift is designed for mills that had lorrain liners before

Negative back slope induces sliding

Duolift has a small hump in between lifts to prevent this. Wearlife is much better.

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Ball Mill Grinding

LORAIN BAR LIFT LINING

2 1/2”

1 1/2”

Chamber 1

Chamber 2

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Ball Mill Grinding

INSTALLED DUOLIFT LINERS

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Ball Mill Grinding

CLASSIFYING LINERS Diameter < 4.4m

Diameter > 4.4m

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Ball Mill Grinding

EXAMPLES OF CLASSIFYING TYPE LINERS

CARMAN LINING

SLEGTEN - MAGOTTEAUX TYPE

NATAL (THIN PROFILE TYPE) 34

Ball Mill Grinding

RIPPLE TYPE LINERS Grinding media to be used in 2nd chamber < 30mm

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Ball Mill Grinding

BEST PRACTICES ON LINERS

Refer to the Best Practice “Selection of Shell Liners”

Refer to the Best Practice “When to Change Shell Liners”

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Ball Mill Grinding

PARTITION / OUTLET WALL DISCHARGE & 2ND COMPARTMENT VIEW

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Ball Mill Grinding

PARTITION / OUTLET WALL

Double wall, full liftered design is the most common

Feed End View of Partition with Grate Segments Outer Grate Segment Inner Grate Segment Center Hub or Cone Center Screen (Mill Sweep)

Typical Chamber Filling Level

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Ball Mill Grinding

SLEGTEN ADJUSTABLE PARTITION

 Scoop Assembly

 Number of

open scoops regulates the material flow

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Ball Mill Grinding

CONTROLLING MATERIAL LEVEL IN C1 & C2

Conventional Material Transport

Effect of the Material Flow Control 40

Ball Mill Grinding

GRINDING CIRCUITS

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Ball Mill Grinding

OPEN CIRCUIT Gas Material

Dust Collector

Fan

Fresh Feed Mill

Finished Product 42

Ball Mill Grinding

OPEN CIRCUIT  Little or no control of fineness  Not adapted to high fineness  Possibility of over grinding  Higher temperature of products  Broad particles size distribution  Lower investment costs

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Ball Mill Grinding

CLOSED CIRCUIT Fan Dynamic Separator

Dust Collector

Fan Dust Collector

Static Separator

Rejects

Fresh Feed

Finished Product Mill

Material Gas

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Ball Mill Grinding

CLOSED CIRCUIT  Better regularity because of a real control of the fineness  High fineness is possible  Higher flexibility and possibility of optimisation by product  Possibility of higher mill ventilation  Better control of the temperature (cooling or drying is possible in the separator)  Higher output, better efficiency  Narrow particle size distribution  Higher investment costs 45

Ball Mill Grinding

DESIGN ASPECTS OF BALL MILL SYSTEM

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Ball Mill Grinding

THE BALL MILL Leff C2 Leff C1

 2.8 < L/D < 3.2  27% < Length of first compartment < 35% mill effective length  First Compartment Power – 8 to 10 kWh/t  Ball diameter in C1 from 100-90 mm to 70-60 mm  Ball diameter in C2 from 60-50 mm to 20-17 mm

Leff TOT D

Deff

L

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Ball Mill Grinding

ABSORBED POWER OF A MILL SLEGTEN FORMULA :

1.27

rpm 2.379 π P = L* J * Kj * * Fr * d * C * 4 Vcr Where : L rpm Vcr Fr J Kj d C

- Useful Length of mill (m) - Mill Speed (rpm) - The critical speed inside liners = (42.3/Fr^0.5) - Internal Diameter (inside liners) (m) - Ratio between the apparent ball volume and the internal volume - (1.36 – 1.2 * J) - Apparent density of load (t/m3) : C1=4.5 & C2=4.6 - Constant depending on the material and the liners = 11.262 – For clinker mill closed circuit with Slegten equipment = 10.70 – For Clinker + Slag = 12.16 – For raw mix 48

Ball Mill Grinding

NET AND GROSS MILL POWER Normal values for the ratio Net/Gross power 0.90

Normally for old and inefficient mills

0.92

Polysius Combiflex Drive

0.93

Girth gear driven mill

0.94-0.95

Modern central drive mill

Unexpected values 0.88

High power losses possibly due to mill chambers running empty.

0.97-1.00 Low power losses possibly due to charge expansion and/or build up of clinker nibs 49

Ball Mill Grinding

BALL CHARGE DESIGN – C1

 Emphasis on crushing and less on grinding  Typical top size: • 80 mm Ø if easy to crush, small feedsize • 90 mm Ø is the most common • 100 mm Ø in rare cases; very hard, coarse feed  Coarser ball charges, gives good crushing capability but: • Too porous - shorter retention • Less surface, less grinding • Can result in poor preparation for second chamber if you overfeed (usually forced to underfeed) • Extra wear 50

Ball Mill Grinding

TYPICAL BALL CHARGE FOR C1 Typical Recommendation 40 %

30 %

For Smaller Feed Size 40 %

25 %

20 % 20 %

15 %

10 % Size Ø 90 / 80 / 70 / 60 mm

Size Ø 90 / 80 / 70 / 60 mm

Bond : Equal Number of Balls 51

Ball Mill Grinding

BALL CHARGE DESIGN – C2

 Top size depends on how much preparation is done in the first chamber. Recommendation : 30 ... 40 mm

 Smallest size depends on the discharge grate slot size • Practical rule of thumb; smallest Ø = 2 X slot width • E.g. slot width = 8-10 mm; smallest Ø = 16-20 mm

 Non-classifying liners limits C2 to 3 sizes  Classifying liners allow a large variety of Ø’s  Best Practice “Ball Charge Optimization” 52

Ball Mill Grinding

RELATION BALL SIZE – PARTICLE SIZE

90 mm

30 mm

15 mm

40 mm

1 mm

0,1 mm

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Ball Mill Grinding

SLOT DESIGNS CRITERIA : C1 Slots –  From 6 to 8 mm C2 Outlet Slots –  Larger by at least 2mm than those of C1 outlet

With the slot flared out, material can pass freely without jamming

Beyond here, the slot begins to widen

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Ball Mill Grinding

MILL WATER INJECTION

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Ball Mill Grinding

Water Injection – Only if required INJECTION INLET/OUTLET

NO INJECTION

ºC

ºC

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Ball Mill Grinding

Water Injection – Only if required

ºC

ºC

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Ball Mill Grinding

Water Injection – Only if required

Inlet water injection

Diaphragm water injection 58

Ball Mill Grinding

Different arrangements to compensate for different trunnions

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Ball Mill Grinding

Different arrangements to compensate for different trunnions

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Ball Mill Grinding

Different arrangements to compensate for different trunnions

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Ball Mill Grinding

BALL MILL INTERNAL CONDITION

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Ball Mill Grinding

CONDITIONS OF THE LIFTING LINERS

 Purpose Insuring proper cascading of the balls and hence crushing of material  General Condition of Liners Plate conditions (breaks, chips, attachments)  Criterion Performance should be checked when step < 40% of initial step (wear=60%)  Corrective Measure Replacement

Step

Rotation

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Ball Mill Grinding

VOLUME LOADING – C1 & C2

 Purpose  Insure adequate size reduction  Insure proper ourput  Quick Estimate  Distance between charge and lower part of the trunnion  Distance between charge and lower part of the ventilation screen  Criterion  Between 28% and 34%  Corrective Measure  Regular recharging

Measure on 1/2 step or plate transverse center

Vertical Measurement

Middle Axis

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Ball Mill Grinding

LEVEL MANAGEMENT Specific Grinding Energy [kWh/t]

Volume Load vs. Specific Power 25 20 15 10 5 15

20

25

30

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Volume load [%]

Best Practice “Ball Charge Level Management“ 65

Ball Mill Grinding

BALL CHARGE CONDITIONS IN C1 & C2

 Purpose Evaluating ball charge wear  Detecting a mechanical problem (breakage of a diaphragm sector)  Criteria  Clean charge (no scrap)  Round balls (not deformed) and Clean of suitable size Charge  Corrective Measures  Ball charge sorting in case of contamination  C1 ball sorting (every 5000 - 7000 h)  C2 ball sorting (every 10000 - 14000 h) 

Polluted Charge

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Ball Mill Grinding

MATERIAL LEVEL IN C1 & C2

 Purpose  Limit

wear and insure a good particle size reduction  Detecting the presence of spitzers

 Quick Evaluation  Material

Overfilled compartment

/ ball level 67

Ball Mill Grinding

MATERIAL LEVEL IN C1 & C2

Criterion 

Material level = ball level over the entire compt.

Corrective Measures 

 

Change in the number of small balls 60 mm or 50 mm in C1 and 20 mm or 17 mm in C2 Scoop adjustments Control of the circulating load

Right level

Empty compartment 68

Ball Mill Grinding

MATERIAL LEVEL 

Maximum level (Ball charge is covered with material) fine material coarse material balls have contact balls loose contact ideal case in 2nd comp. very critical in 1st comp.



Normal level (Material is found in between balls) fine material coarse material balls have contact ideal case in 1st comp but still material between the balls



Too low level (few material in between balls) high specific energy wear and breakage on balls and liners 69

Ball Mill Grinding

BALL CHARGE CLASSIFICATION

Purpose

Adapts the charge granulometry to that of the material Criterion Progressive decrease of the ball size in C2 Corrective Measures Sorting and optimization of the ball charge Replacing liners Checking the size of slots • 6mm intermediate diaphragm • 8mm outlet diaphragm

Classified

Unclassified

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Ball Mill Grinding

Purpose  

VENTILATION SCREENS

Insure gas flow Preventing balls from passing between compartments

Good Permeability

General Condition 

Cleanliness

Criteria   

Maximum permeability Mechanical behavior with the 90 mm balls and screen# < 40 mm in C1 Screen# < 15 mm in C2 to prevent 20 mm balls from escaping

Total lack of Permeability

Corrective Measures 

Cleaning or replacement 71

Ball Mill Grinding

DIAPHRAGM SLOTS

 Purpose Providing outlet for the material in the compartments while retaining spitzers and balls Improving ventilation  General conditions Slot plugging Peening, Clearances

Scrap

C1 Side Clearance

Slot C1 Side Width Clearance

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Ball Mill Grinding

DIAPHRAGM SLOTS

Criteria C1 slots from 6 to 8 mm C2 outlet slots larger by at least 2 mm than those of C1 outlet Corrective Measures Filing and cleaning Diaphragm replacement

Peened slots

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Ball Mill Grinding

DIAPHRAGM SLOTS Rotation Rotation

Slots deformed from one end

Center

Deformation over slot’s entire length Center

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Ball Mill Grinding

SUMMARY

Conditions of Lifting Liners Performance has to be looked at if step < 40% of initial step (wear=60%)

Volume Loading : 28% to 34% Ball Charge Conditions : Clean, round shape and suitable size for the compartment C1 has to be sorted every 5000 to 7000 h C2 has to be sorted every 10000 to 14000 h 75

Ball Mill Grinding

 Material level

SUMMARY

 Material level = ball level over the entire compartment

 Ball charge classification

 Progressive decrease of the ball size in C2

 Diaphragm slots

 First compartment slots : 6mm to 8mm  Second compartment outlet slots larger by at least 2mm than those of first compartment

 Ventilation screens

 They have to have the maximum permeability to let the maximum gas flow through the mill while retaining the balls 76

Ball Mill Grinding

Target levels for material fineness before the intermediate diaphragm Sieve Size (mm)

Cumulative Residue (%)

2.3

5

1

8 - 14

Basis - sieve about 0.5 - 1 Kg of material if there are clinker nibs present. 77

Ball Mill Grinding

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