ball mills

© Confederation of Indian Industry

BALL MILLS


Ball Mills

� As old as Cement Industry

� Popularly used in many Cement

plants

� Tremendous Potential for energy

saving


Efficiency Of Ball Mills

�Efficiency of Ball Mill ?

� 90 %

� 50 %

� 25 %

Less Than 10 % !!


Ball Mills

� Energy saving Opportunities

� Grinding media Finetuning

� Installation of high efficiency

separators

� Installation of Pregrinding system

� High level Control Systems


Grinding Media Finetuning

� Mill Crash stop & samples at

different points

� Analyse at different sieves & plot

the curve

� Typical curve

� Steeper - Initially

� Gradually taper & flatten


Grindability Curve

-20

0

20

40

60

80

100

2 4 6 8 10 12

Axial distance from mill inlet

% Residue

1" 3/4" 1/2" 1mm 0.5mm 90 µµµµ 45 µµµµ -45 µµµµ


Grindability Curve

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Sieve size 25 mm 19 mm 16 mm

12 mm 6 mm 4.75 mm 2 mm

1 mm 72 # 170 #

% Residue


Optimise Ball Mill Charge

� Typical Grindability curve

� Steeper - Initially

� Flat towards end of Mill

�Whenever the curve is Flatter

� Grinding media to be supplemented


Other Aspects of Grinding Media

�With better grinding Media

(Hichrome) & Separator

� Possible to use lower size grinding

media in II Chamber

� Particularly applicable for Cement

Mills


Other Aspect on Grinding Media

� Increase in specific surface of grinding

media (m2/ton) – 10%

�Increased output

�Lower specific energy

� Example : Reduction of average ball size

20 – 50 mm (30 m2/ton) to 15 – 20 mm

ball size (40 m2/ton)

�Reduction in specific energy by 10%


Case StudyOptimise Grinding Media in Cement mill

�Installed Polycom as pregrinder and

product is fed to cement mill

�Excellent step

�Feed size of material 80% Less than 90

micron

� Max size of Grinding media ~20 mm

� Present size

Chamber 1 - 90 mm to 60 mm

Chamber 2 - 17 mm to 12 mm


�Utilizing the benefit of Poly com in ball mill�Reduction of feed input size to ball mill results in�Eliminates larger balls �Leads to increase in Output

�Atleast 10% – 15% increase in output



�Present size

First Chamber

2623%19%34%23%

MT60708090

Second Chamber

7135%45%20%

MT121517



�Recommended size

First Chamber

2612%27%35%26%

MT20253040

Second Chamber

7135%45%20%

MT121517



Annual Saving - Rs 30 Lakhs

Investment - Nil

Payback period - Immediate

Optimise Grinding Media in Cement mill


Installation of High Efficiency Separator

� Another Potential Area

� Various types available in Market

� High Efficiency Separator

� Better Particle size Distribution

� Higher capacity & lower SEC

� 20% increase in capacity

� 10-15% Reduction in Sp Energy Consumption

� Excellent opportunity for increasing capacity


Ball Mills

�Measurement of product fineness

� Residue on 212µµµµ, 90 µµµµ, 45 µµµµ

� Blaines

� Traditionally

� Raw meal & coal – Residue on 90 µµµµ

� Cement – Blaines

�Modern high efficiency separators (Ball mill &

VRM) cement mills

� Particle size distribution – close

� Blaines does not give a true measure of particle

size distribution


Experiment in a Ball Mill

4

8

12

14

16

20

2 40 5 6 7 8

Residue on 45µµµµ

Circulating Load

Same Blaines – 3000 cm2/g45µµµµ Residue – 20% to 0.8%Equivalent power – 24 units/ton to 36 units/ton


What should be the measure of Fineness ?

� Strength development of cement &

residue on 45 µµµµ

� If Blaines to be used as a measure

� At constant circulation load

� Cement

� Strength development – 3 µµµµ - 30 µµµµ

particles


What should be the measure of Fineness in Cement Mill ?

� Objective

� Maximise 3 µµµµ – 30 µµµµ particles

� Fine-tune the separator

� Get the PSD tested

� Establish a correlation between PSD

& 45 µµµµ residue / Blaines

� Fix up residue on say 45 µµµµ or Blaines

as a quick measure for reference


Installation of Pre Grinding System

� Ball Mill More Inefficient - Coarser size reduction (Crushing)

� Installation of a Pre Grinder

�Coarser Crushing gets shifted

�Ball Mill - Only fine grinding

� Pre Grinding Systems

�Horizontal Impact Crusher

�Roller Press

�VRM

� All 3 systems being used successfully


Installation of Pre-grinding system

�Impact crushers

�Most suited – Input size is high

� Crusher with screen

� Eliminates larger balls

� Leads to more efficiency & Output

�Successfully used in many cement mills

�10 – 15% increase in output


Pregrinder System – Roller Press

�Most popular pregrinder

�Used for both Raw grinding & Cement

grinding

�Roller press as a pregrinder in closed

circuit with a separator

� Upto 100% increase in capacity

� 30% reduction in specific energy

consumption


In a Raw Mill…..

� Installation of a Roller press

� Output increases

� 130 TPH to 190 TPH

� Energy saving – 7 units / ton

(34 units / ton to 27 units / ton)

� Excellent alternative for increasing

production & energy efficiency


VRM – As Pre-grinder

� Implemented in Cement Mills

� Increase in capacity (50 – 100%) &

reduction in specific energy to the

extent of 30%

� Simple system installed with substantial

benefits

� Attractive when combined with production

increase


Utilisation of VRM as a Pre-grinding System

� Good potential – VRM as pre grinder in

existing plants

�Augmenting capacity

�Reducing power consumption

�Less investment


Utilisation of VRM As a Pre-grinding System

�Present status – in a Cement plant

� 2 mills – 50 TPH – Ball mills

� 35 Units/Ton of OPC

� CAP requirement increasing


Utilisation of VRM As a Pre-grinding System

�Proposing to install VRM as a pregrinder

�Capacity - ↑↑↑↑ - upto 50%

�Specific energy consumption – 28 Units/Ton

�Energy Savings - Rs 1.50 Crores

�Investment - Rs 12 Crores

© Confederation of Indian Industry© Confederation of Indian Industry

Feed Polycom reject to Cement Mill 9

� Ball mills efficient for fine grinding

� Capacity increases with pre grinder

� Poly com installed as pre grinder for cement mills 5 & 8

� Excellent initiative


Feed Polycom reject to Cement Mill 9

� Polycom operated in closed circuit

� cement mill 9 feed – Fresh clinker


Feed Polycom reject to cement mill 9

Sep

Clinker silo

CM 9

CM5,8

Existing

200 TPH

200 TPH

400 TPH

200 TPH



Sep

Clinker silo

CM 9

CM5,8

Proposed

230 TPH

430 TPH

430 TPH

200 TPH


� Sieve analysis Material in Polycom circuit ( material passing under)

81

10

2

90

7196086Polycom reject

6296100100Polycom product

151846Fresh clinker

4520010006300Sieve



� Recommendations

� Utilize the Polycom to maximum extent by feeding the reject to cement mill 9

� Layout discussed / Diversion to existing system for emergencies

� Atleast 15 % increase in output possible

� Optimise Grinding media



Annual Saving - Rs 250 lakhs

Investment - Rs 200 lakhs

Payback period - 10 Months



Increase Grinding Chamber By 0.5 M By Shifting Diaphragm

�Rawmill Design

Drying - 2.5 M

Grinding - 4.0 M

�Rains Less & Low Moisture

�Possible to reduce drying chamber

�Diaphragm shifted by 0.5 M


Increase Grinding Chamber By 0.5 M By Shifting Diaphragm

�Grinding Chamber Increased

�Capacity Increased by 8 - 10 %

�Reduction in specific energy

consumption

Annual savings - Rs 7.20 Lakhs

Investment - Rs 4.80 Lakhs

Payback - 8 Months


Optimize grinding in Cement Mills

�In a 1 MTPA Cement Plant

�Particle Size Distribution (PSD)

Analyser

�Frequent checking of Samples

�Excellent initiative



�PSD Analysis

�0.3 µ to 400 µ

�<3 µ fraction distribution also

available

�Previous data studied

�< 3 µ fraction ~ 7.5 to 8.5%

�Distribution of < 3 µ analyzed



�Impact of higher < 3 µ fraction:

�Higher grinding power consumption

�Reduction in initial strength

�Flushing

�Possibility of crack formation for

same consistency

�Possibility of lowering < 3 µ fraction

explored



�Mill velocities reviewed

�CM 1 : 0.90 m/s

�CM 2 : 0.827 m/s

�CM ESP fan

�Adequate margin

�Operating at 50 – 60% rated speed

�Potential to increase mill velocity



� Similar exercises carried out in many plants

�Increase mill velocity

�Reduced Blaine for same strength

�Lower grinding power



�PSD Analysis

�2 – 3 µ fraction 2.5 to 3%

�Potential to attack first

�Recommendation

�Increase mill velocity in steps of

0.1 m/s

�Review bucket elevator load



Annual Saving - Rs 13.44 Lakhs


Control Systems for Ball Mills

� Conventional

� Open loop manual

� I Generation controls

� Based on Mill sound, BE load,

Recirculation

� Mill feed varied

� Fineness – Periodically tested

� Separator speed varied – Manually

� Limited success


Latest Control System

� Adaptive predictive control systems

� Intelligent, Smart & Adaptive operator

� System controls

� Mill sound kept constant by varying feed

� Mill sound set point

� Outlet elevator, separator rejects, mill outlet draft,

mill (kW)

� Blaines or Residue Predicted

� Separator speed varied to maintain fineness


Latest Control System

� Continuously measured Blaines is also fed

to the control mechanism

� Blaines prediction mechanism is refined

� Plants achieved a saving of 5 – 10% in

production & energy consumption

� Attractive payback period


Maximum ball size

The expression used for calculating the maximum ball size, given in millimetres, in a mill’s chamber is:

Where: Fk = sieve that retains 20% of the material fed to the chamber (mm)

S = density of the material fed to the mill (ton/m3)

Wi = Bond’s work index corresponding to the material fed to the mill (kWh/ton) Cs = percent of mill’s speed related to the critical speed (%)

Def = effective diameter of the chamber (m) K = factor that depends on material’s humidity and the kind of grinding bodies

K factor Ball Cylpeb Rod Ceramics

Dry 36 33 35 38

Humidity < 5% 34 31 35 38

Humidity > 5% 28 26 35 38


Maximum Ball size

� Max size in mm

25.4 * sqrt (f1 * Wi / 200 Cs) * s /(3.28 * D)

� where

� F1 is the sieve where 80% of feed passes in

micron

� Wi is the work index in kW /MT

� Cs is the critical speed in %

� s is the density of the material

� D is the effective dia of the chamber


Separator theory

SeparatorFeed , M , Rm

Reject , G , Rg

Product , P , Rp

M = P + G

M Rm = P Rp+ G Rg


Useful Formulae

� Circulation Load : Ratio of

separator feed to Separator

Product

� CL = M /P = (Rg – Rp) / (Rg – Rm)

� Circulation Factor : Ratio of

separator Reject to Separator

Product

�C = G /P = CL+1


Classifier efficiency

� Fraction of fines present in the

classifier feed that is recovered

with product

� % Recovery or Efficiency =

100 *(1/C) * (100-Rp) / (100 – Rm)


Tromp Curve

� Graphical representation of the

probability of a particle in the

classifier feed exiting with the

rejects.

� T % = (C-1)/C * 100 * (Rg / Rm)


Tromp Curve

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1.0 10.0 100.0 1000.0

Particle Size in microns

% R

eco

very

Vs R

eje

cts


Tromp Curve

�Apparent By pass – Delta

� Proportion of fines which are reject

� Ideal Separator – 0 %

� Normal – 5 -15 %

� Cut size D50 – particle size corresponding to 50 % rejection

� Kappa – D25 /D75 – Sharpness index –Between product and rejects

�Ideal – 1.0

�Normal – 0.5 – 0.6


S Type Circuit

CM3

SepSep


To Sum-Up

� Ball mills – Popular Grinding systems

� Potential for energy saving – High

� Possible scheme

� Grinding media optimisation

� High efficiency separator

� Pregrinding system

� High level control system

ball mills

Documents

grinding media increase

lower size grinding

flatter grinding media

cement industry

sieve size

reduction of average

present size chamber

curve typical curve