There are two distinct methods for separation of particles based on size: screening and classification.
The size distribution of the particles must be controlled for a number of reasons at various stages of a mineral processing plant:
To enable undersized material to bypass the crushing or grinding circuit and to retain oversized particles for further size reduction,
To provide an optimum particle size material for efficient processing in the downstream separation and concentration systems, and
To prepare product that meets particle size specifications required for the market place.


In its simplest configuration, a screen is a hard perforated surface with a matrix of fixed dimension apertures. The material is presented to the screen surface so that material finer than the apertures falls through the screen and the oversize is conveyed to the discharge end of the screen. Screening is generally difficult below 0.5 mm.

Industrial Sizing

There are two methods of industrial sizing.

Screening is generally carried out on relatively coarse material, as the efficiency
decreases rapidly with fineness. Screening is generally limited to materials above about 250 microns in size, finer sizing normally being undertaken by classification. Industnal sizing is used in closed circuit with a crusher or a ball mill. For the large lump sizes coarse grizzlies made of rails or trommel (revolving) screens made of punched plate may be used. For finer material, screens are usually made of woven metal wire. The material that passes through the openings (apertures) of a particular screen is known as the undersize and material that remains on the screen is the oversize.

Classification: Classification is defined as a method of separating mixtures of mineral particles into two or more products according to their settling velocities in water, in air or in other fluids. Industrial classification may be carried out in different types of classifiers and these classifiers are; hydraulic classifiers, mechanical classifiers and cyclones. Basically they all work according to the principle that the particles are suspended in water which has a slight upward movement relative to the particles. Particles below a certain size and density are carried away with the water-flow, whereas the coarser and heavier particles will settle. In classification mostly we use wet cyclones(hydrocyclones) in which rapid spinning of the pulp centrifuges the solid particles.