Important Stages of the Production Process - Introduction to Polymeric Composites with Rice Hulls - page 83

pivoted. Typically, the hammers can be reversed to provide added life before they
need to be replaced. In jet mills, particles strike each other as they are transported in
a stream of air or steam. For the initial reduction of the large materials, a rotating
drum propels the feed into the air, where the pieces strike each other and fracture.
Hammer mills reduce the particle size of materials by impacting a slow moving
target, such as a rice fibre, with a rapidly moving hammer. The target has little
or no momentum (low kinetic energy), whereas the hammer tip is travelling at
≥4,880 m/min and perhaps >7,000 m/min (high kinetic energy). The transfer of
energy that results from this collision fractures the fibres into many pieces. Sizing is
a function of hammer-tip speed, design and placement of the hammer, screen design,
hole size, and whether or not air assistance is used.
Because impact is the primary force used in a Hammer mill to reduce the size of
particles, anything that increases the chance of a collision between a hammer and
a target increases the magnitude of the collision or improves material output, and is
also advantageous for particle-size reduction. The magnitude of the collisions can
be escalated by increasing the speed of the hammers. If the drive speed and screen
size is kept constant, the increased hammer-tip speed obtained by increased rotor
diameter produces particles of smaller mean geometric size. Particles produced using
a hammer mill will, in general, be spherical, with a surface that appears polished.
The distribution of particle sizes will vary widely around the geometric mean, such
that there will be some large-sized and small-sized particles.
Ball, pebble and rod mills are rotating cylinders that are partially filled with metal or
ceramic balls, flint pebbles or rods. The units can produce very fine powders, such as
pigments for inks and paints. The crushing mechanism is a combination of impact
with the grinding media and shearing between the media and the cylinder walls.
A variation is a jar mill , in which relatively small ceramic containers holding some
grinding media are rotated on a machine. It is ideal for small batches of materials for
testing and for laboratory purposes.
For size reduction of polymer resin wastes, standard machines such as shredders,
granulators, pulverisers and other types can be used. If virgin polymer resins are
used, they can be used directly.
7.2 Drying of Rice Hulls
Paddy farming or rice farming is generally done on a large scale. Because of the sheer
volume of the harvests, a farmer after the process of milling (in which the grain is
separated from the hulls) is very likely to heap the hulls outdoors, out in the open