2. Importance of Size Reduction
In industries that process raw material in the solid state or use
solid material in the processing of fluids, reduction in the size of
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solid particles is frequently required. In the production of
gypsum plaster, the raw gypsum rock is removed from the
quarry in large blocks, sometimes five feet in diameter. It must
be reduced to particles fine enough to pass through 100 mesh
screen in order to provide sufficient specific surface for
hydration to take place rapidly. This means a reduction in size
from 60 inch to 0.005 inch. Pigments in paints must be very
fine in order to give good coverage when applied to surface.
Sodium chloride used in cooking is also an example of size
reduction.
3. How reduction is done
Reduction in size involves the production of small mass units
from larger mass units of the same material. It is an operation
which causes fracture to the larger units. This fracturing or
shattering of larger units is accomplished by the application of
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pressure. All true solid materials are crystalline in nature i.e. the
atoms in the individual crystals are arranged in definite repeating
geometrical pattern, and there are certain planes in the crystal
along which shear takes place more rapidly. The pressure applied
must be sufficient to cause fracture by shear along these
cleavage planes. If the shear along these planes results in
deformation but not rupture, the deformation is called as “Plastic
Deformation”
It appears that the best method of causing rupture to take place
in solid material would be the application of shearing loads.
However, the orientation of crystals in solid matter is usually so
irregular that the direct application of compressive loads is just
as effective as shearing loads. All equipment for size reduction of
solids uses compression or shearing or both as disrupting forces.
4. Objectives of size reduction
The purpose of size reduction is not only to make “little ones out of
big ones” when the effectiveness can be measured by the degree of
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fineness of the product, but also to produce a product to the desired
size or size range. The size required for various products may vary
widely, and hence different machines and procedures are involved. A
size range entirely satisfactory for one purpose may be highly
undesirable for another, even when the same substance is involved.
Example:
Powdered coal is widely used for firing industrial furnaces, and lump
coal is also fed into furnaces by mechanical strokes. But powdered
coal could not be used in the stoker and lump coal could not be used
in the equipment designed for firing pulverized or powdered coal.
This unit operation has a number of advantages and purposes. Some
of the objectives are as follows:
5. 1. Increasing rate of chemical reaction
We know that the rate of the chemical reaction is directly
proportional to the area of contact of the reacting species i.e.
greater the surface area of the particles, greater would be the
products. Size reduction is thus very helpful in reducing the
larger lumps to very fine size in order to bring the greater area,
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thus increasing the rate of chemical reaction.
2. In mixing
More intimate mixing of solids can be achieved if the particle
size is small, also in solid- liquid mixing more precise work of
mixing is possible.
3. In coating pigments
Color and covering of a pigment is considerably affected by the
size of the particles.
4. To make material handling easy
It is very difficult to transport the very heavy particles, thus for
easy conveying from point to point, size reduction is necessary.
6. 5. To dissolve solid in solvents
It is a common practice that is to dissolve a lump of sodium
chloride in water is not so rapid in this case, we break the lump
with spoon and make it smaller and smaller to dissolve it in
water, thus lesser the size of solid particles, greater will be the
dissolution.
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6. To dry the material
To get more rate of drying, a solid material is reduced in the size
because we know greater the surface area, greater will be the
evaporation, because evaporation is a surface process, thus by
reducing, that is by making more surface area we get more rate
of drying.
7. To separate valuable minerals from the ore
It is almost impossible to get a valuable material say gold from
its ore without the use of reduction of the material. How it is
possible to get gold from the interior or under the surface
without breakages of the valuable metal and get the solid metal,
however if it is possible, but in most cases after reducing the
size, by the chemical etc, we achieve the valuable. In an iron
sand mixture, we may use iron separator.
7. Operating variables in Size Reduction
The operating variables in size reduction are as follows:
Moisture contents:
The moisture content of solids to be reduced in size is
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important. When it is below 3% or 4% by weight no particular
difficulties are encountered. When moisture content exceeds
about 4%, most material become sticky or pasty with the
tendency to clog the machine. This is particularly true in the
coarse and intermediate stages of grinding. A large excess of
water (50% or more) facilitates the operation by washing the
feed into and product out of the zone of action by furnishing a
mean for transporting the solids about the plant as a
suspension or slurry. Wet grinding is mostly confined to the
fine stage of reduction.
Reduction ratio:
It is the ratio of the average diameter of the feed to the
average diameter of the product. Most machines in the
coarser ranges of crushing have a reduction ratio from 3 to 7.
Fine grinders may have a reduction ratio as high as 100.
8. Free crushing:
In free crushing, the crushed product with whatever fines
have been formed is quickly removed after a relatively
shorter time interval in the crushing zone. The product may
flow out by gravity, be thrown out by centrifugal force. This
method of operation prevents the formation of an excessive
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amount of fines by limiting the number of contacts.
Choke feeding:
In choke feeding (the opposite of free crushing), the crusher
is equipped with a feeder hopper and kept filled (or choked)
so that it does not freely discharge the crushed product. This
increases greatly the proportion of fines produced and
decreases the capacity. In some instances, choke feeding
may result in economy of operation eliminating one or more
reducing stages because of the large quantity of fines
produced. This method is used only when a comparatively
small amount of material is to be crushed and when it is
desired to complete the whole of the size reduction in one
operation.
9. Closed Circuit Operation:
If the oversize material is returned to the crusher, the operation
is termed as closed circuit. Closed circuit operation is
economical with respect to crushing power.
Open circuit operation:
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If no material is returned for re-crushing, the operation is called
as open circuit.
Oversize
Material
Feed Primary Screening Secondary
Crusher Crusher
Product
Fig 01: Open circuit operation
