A hammer mill is an essential machine in the pharmaceutical and food processing industries. It can be used to crush, pulverize, shred, grind and reduce material to suitable sizes. In a hammer mill, swinging hammer-heads are attached to a rotor that rotates at high speed inside a hard casing.
Principle of Hammer Mill:
The working principle of the
hammer mill is simple to understand. The principle is illustrated in Fig. 1(a).
It only requires choosing an appropriate motor, crushing hammers/knives and
material to be crushed. It operates on the principle of impact between rapidly
moving hammers mounted on the rotor and the stationary powder bed. The material
is crushed and pulverized between the hammers and the casing and remains in the
mill until it is fine enough to pass through a sieve that forms the bottom of
the casing. Both brittle and fibrous materials can be handled in hammer mills,
though with a fibrous material, projecting sections on the casing may be used
to give a cutting action.
Construction of Hammer Mill:
Hammer mill has five main
parts. A fully assembled pharmaceutical hammer mill is shown in Fig. 1(b). Normally,
the number of parts may vary depending on the complexity of the machine design.
Every part of the hammer mill plays an integral role in the overall working of
hammer mills. However, the milling process mainly takes place in the crushing
chamber. It consists of a stout steel casing in which a central shaft is
enclosed to which four or more swinging hammers are attached. When the shaft is
rotated by the motor the hammers swing out to a radial position. On the lower
part of the casing, a sieve of the desired size is fitted which can be easily
replaced according to the particle size required. The material is crushed and
pulverized between the hammers and the casing and remains in the mill until it
is fine enough to pass through a sieve. Some mills consist of projecting
sections on the casing used to give a cutting action if fibrous materials are
to be processed. The hammer mills are available in various sizes, designs, and
shapes. In the pharmaceutical industry, they are used for grinding dry
materials, wet filter cakes, ointments, and slurries, etc.
Fig.1: Hammer Mill (a) Principle (b) Industrial Equipment
Working of Hammer Mill:
The feeding mechanism
refers to the process by which particles enter the crushing chamber. Depending
on the design of the hammer in the mill machine, it may use either gravity or a
metered feeding system. Metered feeding systems are used when product
uniformity is a major concern as they eliminate all possible variables that may
cause output product inconsistencies. A good example, in this case, is the
pneumatic rotary valve found between the feeding hopper and the crushing
chamber. In the gravity feeding system, the milling machines solely depend on
the gravitational force that helps to feed particles into the crushing chamber.
Users can switch ON/OFF
the machine from the control box. The operator may control the feeding system
or motor speed. Some pharmaceutical milling machines come with a display panel
where users can monitor all processes. This mill operates at a high speed that
may vary from 2,500 to 60,000 r.p.m. In most cases, hammers are mounted on
horizontal shafts where they may rotate either clockwise or anti-clockwise.
This may depend on the direction of the rotor rotation. A rotor is a rotating
shaft coupled to an electric motor. The hammers come in different styles and
shapes. Hammer mill’s crushing tools may be coupled directly to a motor or
driven by a belt. As opposed to direct connection, the belts can cushion the
motor from shock and allow for accurate speed adjustment. The output of a
pharmaceutical hammer mill varies broadly. Normally, the size of the particles
depends on the sieve variation. These hammer mills may have over 12 different
types of sieve meshes. Pharmaceutical materials that enter the systems are
reduced to very small particles due to the rotating hammers.
The basic working steps are as follows:
- Introducing material through the feed hopper: Materials with suitable physical properties that have been cut to the right size are selected. Depending on the design of the hammer mill it will move into the crushing chamber either by gravity or controlled/metered process.
- In the crushing chamber: The ganged hammers or chopping knives hit the material severally. These components rotate at high speed reducing materials to the desired size. Only particles whose diameter conforms to that of the sieve size pass through sieves. Otherwise, the hammers continue to hit these materials until they are reduced to the required size. Basically, within this chamber, the material is hit by a repeated combination of knives/hammer impact and collision with the wall of the milling chamber. Moreover, a collision between particles plays an instrumental role in this size reduction process. It is advised not to open the crushing chamber when the machine is operating.
- An outlet of the milling chamber: The outlet of the milling chamber has perforated metal sieves (bar gates). Depending on the size and design of the metal sieve, it allows the required size of particles to pass through while retaining coarse material. The material that passes through is the finished product called output.
Factors determining output and capacity of hammer mill:
Three aspects determine
the particle size of a hammer mill. These include hammer or cutting knife
configuration, shaft speed, and sieve size. In most cases reducing large
particles into small sizes may result in a fine or coarse finish. To obtain fine
particles key aspects such as size, a fast-rotating rotor speed, small sieve
size, and large or/and more crushing knives/hammers are preferred. For coarsely
finished output key aspects are few or/and small crushing hammers/knives, slow
rotor speed, and large sieve. The overall capacity of hammer mills depends on
many critical aspects that include characteristics of materials to be crushed,
nature or type of the crushing hammers or knives, number of rows of crushing
hammers or knives, and feed size.
Advantages of Hammer Mill:
- It is rapid in action and is capable of grinding many different types of materials.
- They are easy to install and operate, the operation is continuous.
- There is little contamination of the product with metal abraded from the mill as no surfaces move against each other.
- The particle size of the material can be easily controlled by changing the speed of the rotor, hammer type, shape, and size of the sieve.
Disadvantages of Hammer Mill:
- The high speed of operation causes the generation of heat that may affect thermolabile materials or drugs containing gum, fat, or resin. The mill may be water-cooled to reduce this heat damage.
- The rate of feed must be controlled carefully as the mill may be choked, resulting in decreased efficiency or even damage.
- Because of the high speed of operation, the hammer mill is susceptible to damage by foreign objects such as stones or metal in the feed. Magnets may be used to remove iron, but the feed must be checked visually for any other contamination.
Uses of Hammer Mill:
- Fibrous materials can be handled in hammer mills by cutting edges.
- Brittle material is best fractured by the impact of blunt hammers.
- It is capable of producing intermediate grades of powders of almost all substances.
- Powdering of barks, leaves, roots, crystals, and filter cakes.
- Useful for granulation where the damp mass is cut into granules by the hammers.
Make sure you also check our other amazing Article on : Factors Affecting Size Reduction