Grinding-Fundamentals and Theory: Dr. Chandranath Banerjee Sr. Engineer and Lead Minerals and Separation Technology

Grinding- Fundamentals and
Theory
Dr. Chandranath Banerjee

Sr. Engineer and Lead
Minerals and Separation Technology
5/8/2019 1
Comminution process frame of minerals
Drilling (and blasting) is the technology of achieving primary

fragmentation of “in situ” minerals. This is the starting point for most
mineral processes with the exception of natural minerals in the form of
sand and gravel.
Crushing is the first controlled size reduction stage in the

process. This is the main process in aggregate production and a
preparation process for further size reduction.
Grinding is the stage of size reduction (wet or dry) where the

liberation size for individual minerals can be reached.
By further size reduction filler (mineral powder) is

produced.
Principles of Comminution
The size reduction process
Physics of Comminution
Value Curve
Jaw Crusher
Cone Crusher
Truck dumping ore into a
primary gyratory crusher Crusher head during installation
Cone Crusher- Principles
Closed Side Setting (CSS) + Eccentric Setting (Ecc.) = Open Side Setting ( OSS)
Reduction Ratio
The reduction ratio is the ratio of feed size to product size,
often with reference to the 80% passing size, that is,
reduction ratio is F80/P80
Crushing Unit Grinding Unit
Grinding Mill
Compression Impact Rod 100
Ball 1000
AG & SAG 5000
Jaw 3-4 HSI 4-8
Gyratory 3-5 Stirred Mill 100
VSI 2-5
Cone 3-5 HPGR 25
Problem 1
Design the Crushing stage – choose the appropriate crusher
Product size: P80 = 16 mm
Feed Size Distribution: F80 = 400 mm

Final crushing stage
• More than just crushing For many

rock and gravel crushing circuits the
final crushing stage is of special
interest.
• The final sizing and shaping will take

place in this stage influencing the
value of the final product.
• For hard, abrasive rock circuits Cone

crushers, Vertical Shaft Impactors
(VSI) or High Pressure Grinding
Rolls (HPGRs) can be used.
Problem 2
Given data:
Fresh feed to the circuit (N) = 200 t /h
Fraction finer than screen opening (n) =
20%
Screen opening (aperture)= 13 mm
Screen efficiency = 90%
Crusher closed side setting (c.s.s)= 10 mm
aData taken from Crushing and Screening Handbook
Selection of crushers
For soft feed and non-abarasive feed (low Bond work index) a horizontal
Impactor (HSI) is an option if the capacity is not too high.
For harder feed there is a choice between a gyratory or a jaw crusher, see
below.
Note: HSI can be used only if the abrasion index is lower and the plant does not
mind fines production. Otherwise, a jaw crusher is preferred for lower capacity
aggregate plants.)
Rule 1: Always use a jaw crusher if you can, jaws are the least capital
cost.
Rule 2: For low capacity use jaw crusher and hydraulic hammer for
oversize.
Rule 3: For high capacities (800-1500 tph) use jaw crusher with big
intake opening.
Rule 4: For very high capacities (1200+ tph use gyratory crusher.
Grinding Methods
Size reduction by crushing has a size limitation for the final
products. If we require further reduction, say below 5-20 mm, we
have to use the processes of grinding. Grinding is a powdering or
pulverizing process using the rock mechanical forces of impaction,
compression, shearing and attrition.
All crushers including impactors have limited reduction ratios.
Due to the design there is a restricting in retention time for the
material passing.
In grinding as it takes place in more “open” space, the retention
time is longer and can easily be adjusted during operation.
The two main purposes for a grinding process are:
To liberate individual minerals trapped in rock crystals (ores) and

thereby open up for a subsequent enrichment in the form of
separation.
Grinding Mills
To produce fines (or filler) from mineral fractions by increasing the
specific surface.
Ball Mill
Rod Mill
VERTIMILL Vibrating ball mill
SAG Mill
STIRRED MIL
AG Mill
Grinding Cost
Work Index
Specific energy for Grinding (Bond’s Law)
1 1
𝐸𝐺 = 10 𝑊𝑖 − kWh/t
𝑃 𝐹
𝑊𝑖 is known as the Bond Work Index and represents the work

required to reduce the ore from an infinite size to 100 μm.
Estimation of Work Index for crushers and grinding mills

Bond Pendulum test
Narayanan and Whitens rebound pendulum test
JKMRC drop test
Bond ball mill grinding test
Bond rod mill grinding test
How to Measure Bond Work Index
Effect of Ball Mill Charge
Ball Mill and Rod Mill- Operational Feature
Problem 3
Using the Bond equation, select the size of ball mill in

closed circuit with a cyclone for the following
conditions: F80 =600 μm, P80 =110 μm, Wi=10.5 kW /t,
T= 150 t /h.
Grinding vs. enrichment and upgrading
Comminution Circuit
Thank You
Question ??

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Grinding- Fundamentals and

Dr. Chandranath Banerjee

Drilling (and blasting) is the technology of achieving primary

Crushing is the first controlled size reduction stage in the

Grinding is the stage of size reduction (wet or dry) where the

By further size reduction filler (mineral powder) is

Crushing Unit Grinding Unit

Design the Crushing stage – choose the appropriate crusher

Product size: P80 = 16 mm

Feed Size Distribution: F80 = 400 mm

• More than just crushing For many

• The final sizing and shaping will take

• For hard, abrasive rock circuits Cone

To liberate individual minerals trapped in rock crystals (ores) and

𝑊𝑖 is known as the Bond Work Index and represents the work

Estimation of Work Index for crushers and grinding mills

Using the Bond equation, select the size of ball mill in

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