Mechanisms of Size Separation: Size separation is useful since particles are sorted into categories solely based on size, independently of other properties such as density, surface, etc. It can be used to classify dry or wet powders and generates narrowly classified fractions. While doing this mechanical sieving equipments used works on the principle of agitation, brushing, and centrifugal force.
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| Mechanisms of Size Separation |
Agitation Method
A single sieve or sieves in a set may be agitated in
different ways.
(i) Oscillation:
In an oscillation case, the sieve is mounted in an inclined frame or rack that
oscillates back and forth. This is the most common and simple mechanism in
which material rolls on the surface of a sieve and sometimes forms a ball. This
movement is achieved by the ordinary eccentric movement of the rotating motor
shaft.
(ii) Vibration:
Sieves in this mechanism are mechanically or electrically vibrated at high
speed allowing powder material to pass through. The vibrations are either
sinusoidal or gyratory. Sinusoidal vibration occurs at an angled plane relative
to the horizontal. The vibration in a wave pattern is determined by frequency
and amplitude. This motion can cause sieves to move up and down or left to
right.
(iii) Gyration:
The sieves are kept on rubber mounting which is attached to a rotary eccentric
flywheel. This moves desired amplitude with the required intensity to induce
motion in the material. Rotary motion allows particles to spin and pass through
the meshes. Gyratory Vibration occurs at near level plane at low angles in a
reciprocating side-to-side motion.
Brushing Method
Material to be processed for size separation using
sieving is passed through sieves by use of brushes in different directions. For
motion in circular sieves, the brush is rotated at the center of the sieve
while in horizontal cylindrical sieves it rotates around its longitudinal axis.
The brush helps to keep the meshes clean. This method is suitable for wet,
greasy, and sticky materials.
Centrifugal Method
In the centrifugal method, a vertical cylindrical type
mechanical sieve is used. It consists of a high-speed rotor inside the cylinder
that throws particles on the sieve by centrifugal force. A current of air
generated due to the movement of the rotor causes particle separation. To aid
particle separation further, an air jet can be fixed in the cylinder.
Other mechanisms of particle separation are
gravitational force, density, and electrostatic force. Gravity is a physical
interaction in the sense that when the material is thrown from the sieve causes
it to fall to a lower level. Gravity also pulls the particles through the sieve
media. The density of the material relates to material stratification. The
classification, in this case, is based upon the weight of the material.
Electrostatic force can be applied to a sieve when particles are extremely dry
or wet that helps to sieve.
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