Important Stages of the Production Process - Introduction to Polymeric Composites with Rice Hulls - page 82

fed by chutes, which might constrict flow. In general, the feed flows adequately but
the crushed particles tend to compact and flow with difficulty. Intermediate storage
bins or containers may aggravate flow issues by causing compaction and bridging.
Theoretically, softer materials produce finer particles than harder materials. Nearly all
size-reduction methods result in some degree of fine particles. Hence, unless production
of very fine particles is the objective, it usually is more efficient to undertake size
reduction in stages, with removal of the desired product after each operation.
7.1.1 Equipment used for Reduction of Particle Size
Size-reducing equipment relies on compression or impaction. Compression is applied
via moving jaws, rolls or a gyratory cone. The maximum discharge size is set by the
clearance, which is adjustable. Impact-based equipment commonly uses hammers.
Continued size reduction increases the number of particles and the amount of surface
area per unit of volume. Size reduction also modifies the physical characteristics of
ingredients, resulting in improved mixing, pelletising and, in some instances, handling
and transport. In the case of production of polymeric composites, particle-size
reduction of rice hulls enhances blending of the mixture of rice hulls and polymer
matrix.
Rolls , in particular, can produce very fine particles. Rolls are used in flour milling,
in which crushing yields different-sized particles, allowing separation of purified
flours. Here, moisture content is important so that, for example, the bran is soft and
remains in large pieces, whereas the endosperm is brittle and fractures into small
granules. Corn can be separated from starch and other fibre by roller milling because
it selectively absorbs water and is made into flakes, whereas the starch fractures.
Roller mills accomplish size reduction through a combination of forces and design
features. If the rolls rotate at identical speed, compression is the primary force used.
If the rolls rotate at different speeds, shearing and compression are the primary forces
used. If the rolls are grooved, a tearing or grinding component is introduced. Coarse
grooves provide less size reduction than fine grooves do. There will be little or no
noise or duct pollution associated with approriately designed and maintained roller
mills. Their slower operating speeds do not generate heat and there is very little loss
of moisture. Particles produced tend to be uniform in size, meaning very little fine
material is generated. The shape of the particles tends to be irregular; more cubic or
rectangular than spherical. The irregular shape of the particles means they do not
pack as well. For similar-sized particles, the bulk density of material ground on a
roller mill will be ≈5-15% less than material ground by a hammer mill.
Impact mills use revolving hammers to strike incoming particles and to break or fling
them against the machine case. The hammers might be fixed, or more commonly,