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.

What is classification?

Classification is a method of separating mixtures of minerals into two or more products on the basis of the velocity with which the grains fall through a fluid medium. In mineral processing, this is usually water, and wet classification is generally applied to mineral particles which are considered too fine to be sorted efficiently by screening. Since the velocity of particles in a fluid medium is dependent not only on the size, but also on the specific gravity and shape of the particles, the principles of classification are important in mineral separations utilising gravity concentrators. Classifiers also strongly influence the performance of grinding circuits.

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.