Energy and power requirements in comminution

During size reduction, the particles of feed material are first distorted and strained. The work necessary to strain them is stored temporarily in the solid as mechanical energy of stress, just as mechanical energy can be stored in a coiled spring. As additional force is applied to the stressed particles, they are distorted beyond their ultimate strength and suddenly rupture into fragments. New surface is generated. Since a unit area of solid has a definite amount of surface energy, the creation of new surface requires work, which is supplied by the release of energy of stress when the particle breaks. By conservation of energy, all energy of stress in excess of the new surface energy created must appear as heat.

Size Reduction of particles

The term size reduction is applied to all ways in which particles of solids are cut or broken into smaller pieces. Commercial products must often meet stringent specifications regarding the size and sometimes the shape of the particles they contain. Reducing the particle size also increases the reactivity of solids; it permits separation of unwanted ingredients by mechanical methods; it reduces the bulk of fibrous materials for easier handling and for waste disposal.

The common ways in which solids are broken are as follows:-

· Compression: It is used for coarse reduction of hard solids, to give relatively few fines. Ex- nutcracker.

· Impact: It gives coarse, medium, or fine products. Ex- hammers.

· Attrition or rubbing: It yields very fine products from soft, nonabrasive materials. Ex- a file.

· Cutting: It gives a definite particle size and sometimes a definite shape, with few or no fines. Ex- a pair of shears.