COMMINUTION

BY: ABIGAEL L. BALBIN

 Liberation of the valuable mineral from its associated gangue is
accomplished by comminution
 Comminution: Size reduction of particles until the valuable mineral
and gangue are produced as separate particles.

 Comminution involves ;
1. Crushing ( compression or impact)
2. Grinding (attrition-abrasion and impact)

 Crushing reduces the r.o.m ore to a level that grinding can be

carried out (Crushing produces the feed of grinding)

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Crushing : compression of the ore
against rigid surfaces, or by impact
against surfaces in a rigidly constrained
motion path. Usually a dry process.
(Reduction ratio = 3 to 6 each stage)
Grinding : Abrasion, attrition and impact
of ore by the free motion of the media
(rods, balls, pebbles).Usually a wet
process. 3
 PRINCIPLES OF COMMINUTION
 Most minerals are crystalline in nature in which the atoms
are regularly arranged in 3-dimensional arrays.The physical
and chemical bonds are holding them together.Such inter
atomic bonds can be broken by tensile or compressive
loading.

 Distributionof load (stresses) are not

uniform in ore as it consists of variety
of minerals.

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Distribution of stresses depend upon;
1. Mechanical properties of minerals.
2. Presence of cracks or flaws in the matrix.

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Minerals are assumed as brittle , but
crystals store energy without
breaking , and they release energy
when the stress is removed.( Elastic
Behaviour )
When fracture occurs , some of the
stored energy is transformed into
free surface energy.( Formation of
new active surfaces) 6
IMPORTANT
 Brittle
materials relieve the strain energy by crack
propagation.
Tough materials can relax strain energy by the
mechanism of plastic flow where the atoms or molecules
slide over each other by distorting the shape of the
material.
 Crack propagation can be inhibited by meeting crystal
boundaries.That is why fine-grained rocks are usually
tougher than coarse-grained rocks.

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 Theenergy required for comminution is reduced in the
presence of water and further reduced by chemical
additives. This is due to the lowering of the surface
energy and reduce the bond strength
When an irregular particle is broken
by compression or crushed, the
products are
➢ coarse particles resulting from
tensile failure;
➢ fines from compressive failure
near the points of loading.

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Impact breaking : Due to the rapid
loading, the particle absorbs more
energy and tends to break rapidly, by
tensile failure. The products are often very
similar in size of shape.

Attrition(shear) : Produces more fines

due to the particle-particle interaction.
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COMMINUTION THEORY
 Comminution theory is concerned with :
- Relationship between energy input, and
- Particle size of product against feed
 IMPORTANT : Most of the energy input is absorbed by
the machine itself (it is lost by heat and sound). Only a
very small fraction of energy is used for breakage. E.g.
<1% of total energy.

 Allthe comminution theories assume that the material 10

is brittle and no energy is absorbed.
 Rittinger’s Theory (Oldest Theory)
Energy consumed in the size reduction is proportional to
the area of new surface produced.
𝟏 𝟏
𝑬= 𝑲( − )
𝑷 𝑭

K: Constant
E: Energy input
P: Product particle size
F: Feed particle size
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 Kick’s Theory

The work required is proportional to the reduction in volume

of the particles concerned.
𝑭
𝑬 = 𝑲 𝒍𝒏
𝑷

𝑭, 𝒅𝒊𝒂𝒎𝒆𝒕𝒆𝒓 𝒐𝒇 𝒕𝒉𝒆 𝒇𝒆𝒆𝒅

𝑹𝒆𝒅𝒖𝒄𝒕𝒊𝒐𝒏 𝑹𝒂𝒕𝒊𝒐, 𝑹 = 𝑷,𝒅𝒊𝒂𝒎𝒆𝒕𝒆𝒓 𝒐𝒇 𝒕𝒉𝒆 𝒑𝒓𝒐𝒅𝒖𝒄𝒕

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Bond’s theory
-Energy input is proportional to the New
Crack Tip length produced.
-The crack length is unit volume is inversely
proportional to the square root of the
diameter of the particle.
-For particle size (in practical calculations),
the size in microns which 80% passes is
selected as criteria.
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 𝒌𝒘𝒉 𝟏 𝟏
𝑾𝒐𝒓𝒌 𝑰𝒏𝒑𝒖𝒕 , 𝑾 = 𝟏𝟎𝑾𝒊 −
𝒕 𝑷𝟖𝟎 𝑭𝟖𝟎

Where:
P → Diameter in microns which 80% of the product passes.
F → Size in microns which 80% of the feed passes
𝑾𝒊 → Work index:

- Comminution parameter = Resistance of material to

crushing and grinding ,
= kwh/sh.t required to reduce the material from infinite size
to 80% passing 100m.
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 SUMMARY

Kick’s
Law is reasonably accurate above 1
cm diameter.

Bond’s Theory is reasonable in the rod and

ball mill size.

Rittinger’s
Law is applicable in the fine
grinding (10-1000m).
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THANK YOU! KEEP SAFE
GRINDABILITY

Refers to How easy to crush and grind the

materials.

Grindability data are used to evaluate

crushing and grinding efficiency.
The most widely parameter to measure ore
grind is the Bond Work Index (Wi).

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For most naturally occuring raw materials,
grindability change with particle size (e.g.
Minerals break easily at the boundaries but
individual grains are tough, then grindability
(resistance) increases with the fineness of
grind).

Consequently, work index values are generally

obtaing for some specified grind size.

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Standard Bond Test is time consuming. It
is based on constant screening out of
undersize material in order to simulate
closed-circuit operation.
Comparative Method: To determine the
grindability of an ore by using a
reference ore of known grindability.

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 Procedure: Reference ore is ground for a
certain time and the power consumption
recorded. An identical weight of the unknown
(tested) ore is ground for a length of time such
that the power consumed is identical with that
of the reference ore .

 10 10   10 10 
Wr = Wt = Wir  −  = Wit  − 
 Pr Fr   Pt Ft 
 10 10 
 − 
Wit = Wir Pr Fr 
 10 10 
 − 
 Pt Ft 
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 IMPORTANT : In comparative method , the reference
and test ores must ground to about the same size
distribution.( in order to get reasonable Wi values)
 Asthe efficiency of grinding is varying with the types
of equipment, work indices obtained from
grindability test on different sizes of several types of
equipment, using identical feed materials are used to
compare the efficiencies of the machines.

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 IMPORTANT : The equipment with highest work indices
and the largest power consumption has low
efficiency.
- Jaw and Gyratory Crusher and Tumbling Mills
have highest Wi and power consumption.
- Intermediate power consumers are Impact Crushers
and Vibration Mill
- Smallest energy consumer-Roll crusher (apply
steady , continuous compressive stress on the
materials)

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HARDGROVE GRINDABILITY INDEX
DETERMINATION

HI = 6.93  Wt + 13
HI : Hardgrove grindabili ty Wi = 435
0.91

index ( HI )
Wt : Weight of the ground product passing
200 mesh size
 Procedure:

- 50 gram sample (-14+25 mesh )

- 60 revolution (total)=20 rev/min
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- Grinding ring weight=64 lb.
HARDGROVE GRİNDABİLİTY OF VARİOUS
MATERİALS

MATERIAL HI MATERIAL HI
Antracite 21-50 Graphite(amorph) 73
Barite 116 Graphite(crystalline) 47
Bauxite 39-76 Hematite 96-35
Chromite(African) 35 Limestone 54-78
Chromite (Turkish) 62
Talc 67-130
Clay 97
Sulphur (raw) 104
Coal (bituminous) 37-105
Turkish çay seam Rutine 26
87
Mica 7
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Turkish pirinç seam 102