Hammer Mill

A hammer mill carries out size reduction by using impact. Hammer mills consist of a series of four or more hammers hinged on a central shaft which is enclosed within a rigid metal case (fig.1). The material is fed at the top or center, thrown out centrifugally, and ground by the impact of the hammers or against the plates around the periphery of the casing.

Hammer mill
Fig.1: Hammer mill.

The material is retained until it is small enough to fall through the screen that forms the lower portion of the casing. Particles fine enough to pass through the screen are discharged almost as fast as they are formed.

A universal mill employs a variety of rotating milling elements such as a pin disk, wing or blade beater, turbine rotor, or hammer-type rotor. These elements are used in combination with either a matched pin disk (that may or may not rotate), a perforated screen, or a stator.

Applications of Hammer Mill

The hammer mill can be used for almost any type of size reduction. Its versatility, therefore, makes it popular in the pharmaceutical industry, where it is employed to mill dry materials, wet filter-press cakes, ointments, and slurries. Moreover, comminution is effected by impact at peripheral hammer speeds of up to 7,600 meters per minute. At this speed, most materials behave as if they were brittle. Consequently, brittle material is best fractured by impact from a blunt hammer.

A hammer mill can be used for granulation and close control of the particle size of powders. The size of the product is controlled by selecting the speed of the hammers, and the size and type of the screen. Speed is crucial. Below a critical impact speed, the rotor turns so slowly that a blending action rather than comminution is obtained. This results in overloading and a rise in temperature. Microscopic examination of the particles formed when the mill is operating below the critical speed shows them to be spheroidal, indicating not an impact action, but an attrition action, which produces irregularlyshaped particles.

At very high speeds, there is possibly insufficient time between hammers for the material to fall from the grinding zone. In wet milling of dispersed systems with higher speeds, the swing hammers may lay back with an increased clearance. For such systems, fixed hammers would be more effective.

Advantages and Disadvantages of Hammer Mill

Hammer mills are compact with a high capacity. Size reduction to 20 to 40 μm may be achieved with a hammer mill. However, to produce ultrafine particles, the mill must be operated with internal or external classification. This is because the inertial forces vary with mass as the inverse cube of the diameter. Consequently, small particles with a constant velocity impact with much less kinetic energy than larger ones. As a result, the probability that particles less than a certain size will fracture decreases rapidly. In addition, small particles pass through the screen almost as fast as they are formed. Thus, a hammer mill tends to yield a relatively narrow size distribution. Hammer mills are simple to install and operate. The speed and screen can be rapidly changed. They are easy to clean and may be operated as a closed system to reduce dust and explosion hazards.

Reference:

  • Aulton, M. (2018). Aulton’s pharmaceutics, the design and manufacture of medicines. Edinburgh. : Elsevier
  • Khar, R.,Vyas, S., Ahmad, F., & Jain, G. (2016). Lachman/Lieberman’s The Theory and Practice of Industrial Industrial Pharmacy. New Delhi, ND: CBS Publishers & Distributors Pvt Ltd