A.

THEORY OF COMMUNITION
Comminution is a sequence of crushing and grinding operations applied to progressively reduce
the size of a boulder or a lump. Crushing is applied to reduce the size of the run-of-mine lumps
or boulders to the size suitable for grinding. Blasting using explosives can be taken as the first
stage in comminution and involves the use of explosives such as mercury, fulminate, a salt of
fulminic acid and lead azide (Pb(N3)2) to excavate ore from their deposit beds.

The purposes of comminution are:

a. In a quarry, there is a need to obtain aggregate that consists of standard-size fractions
or specifications.
b. To make transport of the excavated materials to the plant by scrappers, conveyors and
chutes easier.
c. Ore is an aggregate of mineral value and gangue minerals and the mineral
value particles are intimately associated with the unwanted gangue minerals. Comminution
allows the “un-locking” or “liberation” of the desired valuable mineral(s) from the gangue
minerals so that the clean particles of the mineral value are separated from the gangue
minerals as much as possible.

1. The Theory and Principles of Comminution

Most minerals have their atoms arranged naturally in crystalline form, that is, the atoms are
ordered or arranged regularly in a three—dimensional array. The strength of an ore lump
depends on the nature of the chemical and physical bonds occurring between the atoms. The
bonds between the atoms must be extended until it breaks. A crusher can break a bond strength
above 150 MPa. It should be noted that concrete has a bond strength between 20 and 30 MPa.
For an ore to be crushed, a stress has to be applied to extend and break the interatomic bond
and the stress can be a compressive or a tensile stress. Different minerals have different hardness
values on the Mohs hardness scale. Therefore, if stress is applied to an ore, the different minerals
in the aggregate will break into fragments with different shapes and sizes. During crushing and
grinding, the applied forces may be tensile, compressive, shearing, attrition or impact.

The Mohs hardness scale classifies minerals based on their hardness in ascending order of
talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum and diamond. The
hardness levels are such that talc and gypsum can be scratched by a fingernail, calcite by iron,
corundum by diamond and diamond being the hardest cannot be scratched by any of the nine
other minerals.
In mining and quarrying, rocks and ore minerals are excavated from open and underground
mines. The typical operations are drilling and blasting, primary crushing and materials
handling. In some natural environments such as glacial, alluvial and marine, primary crushing
takes place naturally such as by ice covering, flow of water and natural attrition by erosion
leading to sand and gravel of different size distributions. Size reduction is carried out to liberate
mineral value from the rock hosting it and thus it is carried out until the liberation size that
ranges from 10 to 100 µm is attained. Wearing components endangers the processing machines
while noise and dust pose danger to the operators.

Since solid minerals are crystalline, they tend to break into innumerable fragments and shapes
upon being subjected to breaking stresses. Consequently, the challenge in ore comminution
is to ensure correct grinding that minimizes undergrinding and over-grinding. It has been
found that the separation processes become more efficient if the grinding curve for the feed
becomes steeper in the latter stages implying shorter or narrower fractions.
The comminution process is influenced by the ore grindability and wear profile, called the
work index and abrasion index, respectively. For example, the work and abrasion indices for
hematite are 13 8 ± and 0 5 0 3 . . ± ; respectively. The work index has effects on energy
requirement and size reduction, while the abrasion index influences the wear rate of the
grinding component.

High reduction ratios lead to in-efficient grinding and hence reduction ratios
recommended are 3–4, 3–5 and 3–5 for jaw, gyratory and cone crushers, respectively. For
the ball, rod and autogenous (AG)/semi-autogenous (SAG) milling, the reduction ratios
recommended are 100, 1000 and 5000, respectively.
It should be noted that the crushing reduction ratios depend on the type of material to be
crushed, either rock, gravel or ore such that ore responds with maximum reduction while
rock and gravel exhibit limited reduction. Primary, secondary and tertiary crushing typically
precede rod and ball milling. The High Pressure Grinding Roll (HPGR) is typically used as a
tertiary crusher or quaternary crusher followed by ball milling or vertimilling or as a pebble
crusher.
2. Comminution Theory

a. For the energy input in crushing and grinding, it has been established that only a small
percentage is used for particle breakage; the major part being lost as heat and noise in
the machine. Therefore, the small fraction of energy actually expended for particle
breakage will be more difficult to determine. For instance, for ball mills, it has been
found that less than 1% of the energy supplied for operation is used to actually reduce
particle size.
b. It is expected that a relationship should exist between the energy required to break the
material to be crushed and the area of the new surface to be produced. However, it is
not possible to separately determine the energy expended for creating the new surface
and thus the relationship cannot be expressly shown.
c. The comminution theories assume that all ore materials are brittle in
nature. However, some ores are plastic in nature and for such ores, energy is
expended for shape change which has nothing to do with the actual particle breakage.

3. Crack Propagation in Minerals

It has been shown by Griffith (1921)3 that materials fail by crack propagation when this is
energetically feasible, that is, when the energy of the new surface to be created is lower than
the energy released when the strain energy created by the comminution is relaxed. Surface
energy or interfacial energy or surface free energy quantifies the disruption of intermolecular
bonds that takes place when a new surface is created. It can be called the work per unit area
done by the force that produces the new surface.

B. PARTICLE SIZE ANALYSIS

Particle size analysis refers to the analysis of the shape, size and size fraction distribution range
of ore at every stage of mineral processing such as after crushing, grinding, separation and
product collection. After crushing and grinding, the aggregate of ore minerals with different
Mohs hardness values and locations within the ore lump will break into different sizes and
shapes. In particle size analysis, the range of sizes and shapes produced depending on the
severity of the comminution process will be determined.

1. Importance of Particle Size Analysis in Mineral Processing

Particle size analysis or screen distribution analysis or sieve analysis is an important procedure
in mineral processing for the following reasons :

a. It enables one to know the quality of grinding carried out on an ore using the
percentage undersize at a particular size as an indicator.
b. It enables one to know the extent of liberation of mineral value particles
from the gangue mineral particles at different size fractions. A microscopic
analysis or size by assay of the size fractions after crushing or grinding will
reveal the degree of liberation of the mineral value at each size fraction.
c. It provides the optimum ore feed size consist that will give us the maximum
efficiency in the mineral processing operation. Depending on the Moh hardness of
each mineral type in the ore, the crushed product will contain different particle sizes
and shapes after crushing depending on the initial size of the lumps.
d. It enables knowing the size ranges fractions at which any losses can occur in mineral
processing operation so that such losses can be reduced.

Since major plant decisions are made on a routine basis using the results of particle size
analysis, two important issues about particle size analysis have to be ensured:
a. The method applied should be reliable and accurate.
b. The sample used must be a representative of the bulk original (bulk) sample as
accurately as possible.

2. Particle Size and Shape

The primary purpose of precision particle size analysis of ore is to obtain quantitative data about
the size and size distribution of mineral particles in it. Ore particles can occur in regular shapes
such as spheres or cubes or irregular shapes. For regular shapes, the exact size can be uniquely
assigned while for irregular shapes it is not possible to assign them an exact size because
“breadth”, “length”, “width”, diameter or “thickness” dimensions cannot be uniquely
determined unlike for regular particles. For instance, for spherical and cubical particles, the
exact sizes are the diameter and the length of a side, respectively.

3. Methods of Particle Size Analysis

Some of the methods of particle analysis are:

i. Test sieving—this is suitable for both dry and wet analysis

ii. Laser diffraction—this is also suitable for both dry and wet analysis
iii. Optical microscopy—this is suitable for only dry analysis
iv. Electron microscopy—this is suitable for only dry analysis
v. Elutriation—this is suitable for only wet analysis
vi. Sedimentation (gravity)—this is suitable for only wet analysis
vii. Sedimentation (centrifuge)—this is suitable for only wet analysis