Classification is a process of dividing a particle-laden gas stream into two, ideally at a particular particle size, known as the cut size. An important industrial application of classifiers is to reduce over-grinding in a mill by separating the grinding zone output into fine and coarse fractions.

　　Screens

　　These contain apertures that are uniformly sized and spaced, and which may have circular, square, or rectangular shapes. Particles that are smaller than the aperture in at least two dimensions pass through, and larger ones are retained on the surface. Static (or low-frequency) screens or grizzlies have a different construction. They are comprised of parallel bars or rods with uniformly clear openings, often tapered from feed to discharge ends. The bars may lie horizontally above a bin, or be inclined to provide the feed to a crusher.

　　Cross-flow systems

　　It is in principle possible to winnow out fines from a falling curtain of material of constant density by a cross-current of air. In practice, the humidity of the air (and moisture on the particles) leads to blockage of the narrow ducts necessary to give a thin enough falling curtain for winnowing. It is possible to winnow thin flakes; Etkin et al. (1980) has successfully classified mica particles with an aspect ratio greater than 30.

　　Elutriation

　　Gravity counter-current classifiers (elutriators) have been reviewed by Wessel (1962). A simple example, the Gonnell (1928) classifier, consists of a long vertical cylindrical tube with a conical transition zone located at the bottom end. Air flows up the tube, carrying with it the finer particles. The disadvantage of this and many other gravity counter-current classifiers is the presence of a laminar velocity profile in the gas, a large cone angle leading to flow separation and eddy formation, settling out of fines due to the retarded velocities near the walls, and the noise of vibrators necessary to prevent particle adhesion to the walls. Their advantage lies in the good dispersion of powders achieved in the cylindrical section. In the zig-zag classifier, vortex formation leads to the acceleration of the main flow owing to a reduction of the effective tube cross-section. Fines follow the main gas stream and coarse particles travel to the wall and fall back against the main gas flow. In this design, the sharpness of the cut is low at each stage (zig-zag), but a required cut size is generally achievable even at high velocities.

　　Inertia systems

　　In an inertial classifier, the particle-laden gas stream is turned through 180° by appropriate internal baffling. In order to reach the exit port, the gas passes through a further 180° to continue in the same direction it was traveling before it was diverted. The fines are able to follow, more or less, the same route as the gas. However, the momentum of coarser or denser particles prevents them from following the same trajectory and they fall into a collection zone after the first turn.

　　Centrifugal systems with no moving parts

　　The capacities of these types of classifiers cover a wide range. Generally, higher-capacity machines have a poorer sharpness of cut. Typical high-capacity industrial units are the cone classifier (often built into some types of mills) and the cyclone. The feed is given a high tangential velocity and is introduced near the top of the unit. The gas flows in a spiraling fashion towards the bottom end where it experiences a flow reversal and passes up as a central core. Under the influence of centrifugal force, coarse particles are thrown to the inner wall of the cone or cyclone. Particles less than the cut size are carried up the central vortex and are carried out of the unit by the bulk of the gas flow. The diameter and position of the vortex finder at the top of the unit are critical in the determination of a specified cut size. Further information on cyclones is given in the overview article on Gas-Solid Separation.

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