Hammer Mill

On the small scale, size reduction by Impact can be carried
out by the shattering of brittle substances with a hammer or with a pestle and
mortar.

Principle

A hammer mill is essentially a steel drum containing a
vertical or horizontal rotating shaft or drum on which hammers are mounted

The hammers are free to swing on the ends of the cross or
fixed to the central rotor

The rotor is spun at a high speed inside the drum while
material is fed into a feed hopper

The material is impacted by the hammer bars and is thereby
Shredded and expelled through screens in the drum of a selected size

Construction

The hammer mill consists of a central shaft to which four or
more hammers are attached

These are mounted with swivel joints, so that the hammers
swing out to a radial position when the shaft is rotated

The lower part of the casing consists of a screen through
which material can escape

The screen can be changed according to the particle size
required

Working

Material is fed into the mill grinding chamber through the
feed chute

It repeatedly is struck by ganged hammers which are attached
to a shaft rotators at high speed inside the mill chamber

The material is crushed or shattered by a combination of
repeated hammered impacts, collisions with the walls of the grinding chamber
and particle on particle impacts

Perforated metal screens or bar grates covering the
discharge opening of the mill retain coarse material for further grinding while
allowing properly sized materials to pass as finished product

Advantages

Easy to install, dismantle and clean

Scale up problems are minimum

Various types of feed stock can be handled using screen of
different sized

It occupies less space

It is versatile

Operated in a closed environment to avoid dust and explosion
hazard

The product can be controlled by variation of rotor speed,
hammer type, and size and shape of mesh

Operation is continuous. No surfaces move against each
other, so that there is little contamination of the product with metal abraded
from the mill

Disadvantages

The screens may get clogged

Product degradation due to heat building

Wearing of mill with abrasive materials

Unsuitable for sticky, fibrous and hard materials

Applications

Fine to moderate grinding of powders

Particle size may vary from 10 to 400 mm

Nonabrasive, brittle materials can be used as feed stock

Milling dry materials, wet slurries, ointments etc.