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Ball mills
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© Confederation of Indian Industry
BALL MILLS
© Confederation of Indian Industry
Ball Mills
� As old as Cement Industry
� Popularly used in many Cement
plants
� Tremendous Potential for energy
saving
© Confederation of Indian Industry
Efficiency Of Ball Mills
�Efficiency of Ball Mill ?
� 90 %
� 50 %
� 25 %
Less Than 10 % !!
© Confederation of Indian Industry
Ball Mills
� Energy saving Opportunities
� Grinding media Finetuning
� Installation of high efficiency
separators
� Installation of Pregrinding system
� High level Control Systems
© Confederation of Indian Industry
Grinding Media Finetuning
� Mill Crash stop & samples at
different points
� Analyse at different sieves & plot
the curve
� Typical curve
� Steeper - Initially
� Gradually taper & flatten
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
Optimise Ball Mill Charge
� Typical Grindability curve
� Steeper - Initially
� Flat towards end of Mill
�Whenever the curve is Flatter
� Grinding media to be supplemented
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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%
© Confederation of Indian Industry
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
© Confederation of Indian Industry
�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
Case StudyOptimise Grinding Media in Cement mill
© Confederation of Indian Industry
�Present size
First Chamber
2623%19%34%23%
MT60708090
Second Chamber
7135%45%20%
MT121517
Case StudyOptimise Grinding Media in Cement mill
© Confederation of Indian Industry
�Recommended size
First Chamber
2612%27%35%26%
MT20253040
Second Chamber
7135%45%20%
MT121517
Case StudyOptimise Grinding Media in Cement mill
© Confederation of Indian Industry
Annual Saving - Rs 30 Lakhs
Investment - Nil
Payback period - Immediate
Optimise Grinding Media in Cement mill
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
Utilisation of VRM as a Pre-grinding System
� Good potential – VRM as pre grinder in
existing plants
�Augmenting capacity
�Reducing power consumption
�Less investment
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry© Confederation of Indian Industry
Feed Polycom reject to Cement Mill 9
� Polycom operated in closed circuit
� cement mill 9 feed – Fresh clinker
© Confederation of Indian Industry© Confederation of Indian Industry
Feed Polycom reject to cement mill 9
Sep
Clinker silo
CM 9
CM5,8
Existing
200 TPH
200 TPH
400 TPH
200 TPH
© Confederation of Indian Industry© Confederation of Indian Industry
Feed Polycom reject to cement mill 9
Sep
Clinker silo
CM 9
CM5,8
Proposed
230 TPH
430 TPH
430 TPH
200 TPH
© Confederation of Indian Industry© Confederation of Indian Industry
� Sieve analysis Material in Polycom circuit ( material passing under)
81
10
2
90
7196086Polycom reject
6296100100Polycom product
151846Fresh clinker
4520010006300Sieve
Feed Polycom reject to cement mill 9
© Confederation of Indian Industry© Confederation of Indian Industry
� 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
Feed Polycom reject to cement mill 9
© Confederation of Indian Industry© Confederation of Indian Industry
Annual Saving - Rs 250 lakhs
Investment - Rs 200 lakhs
Payback period - 10 Months
Feed Polycom reject to cement mill 9
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
Optimize grinding in Cement Mills
�In a 1 MTPA Cement Plant
�Particle Size Distribution (PSD)
Analyser
�Frequent checking of Samples
�Excellent initiative
© Confederation of Indian Industry
Optimize grinding in Cement Mills
�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
© Confederation of Indian Industry
Optimize grinding in Cement Mills
�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
© Confederation of Indian Industry
Optimize grinding in Cement Mills
�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
© Confederation of Indian Industry
Optimize grinding in Cement Mills
� Similar exercises carried out in many plants
�Increase mill velocity
�Reduced Blaine for same strength
�Lower grinding power
© Confederation of Indian Industry
Optimize grinding in Cement Mills
�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
© Confederation of Indian Industry
Optimize grinding in Cement Mills
Annual Saving - Rs 13.44 Lakhs
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
Separator theory
SeparatorFeed , M , Rm
Reject , G , Rg
Product , P , Rp
M = P + G
M Rm = P Rp+ G Rg
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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)
© Confederation of Indian Industry
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)
© Confederation of Indian Industry
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
© Confederation of Indian Industry
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
© Confederation of Indian Industry
S Type Circuit
CM3
SepSep
© Confederation of Indian Industry
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