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 overgrinding in a mill by separating the grinding zone output into fine and coarse fractions. Many types of classifier are available, which can be categorized according to their operating principles. A distinction must be made between gas cleaning equipment, in which the aim is the removal of all solids from the gas stream, and classifiers in which a partition of the particle size distribution is sought. Prasher (1987) identifies the following categories: a) screens, b) cross-flow systems, c) elutriation, d) inertia systems, e) centrifugal systems without moving parts, f) centrifugal systems with rotating walls, and g) mechanical rotor systems. A classification process may combine these alternative principles, sometimes within a single separator, to achieve a desired result.
These contain apertures which are uniformly-sized and spaced, and which may have circular, square or rectangular shapes. Particles which are smaller than the aperture in at least two dimensions pass through, and larger ones are retained on the surface. The screen is shaken or vibrated to assist motion of particles to the surface, and continuous screens are often tilted to further aid particle bed motion along the screen 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.
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, 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.
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 cut is low at each stage (zig-zag), but a required cut size is generally achievable even at high velocities.
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 travelling 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.
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 to the top of the unit. The gas flows in a spiralling fashion towards the bottom end where it experiences a flow reversal and passes up as a central core. In the cone classifier, the central core of gas actually flows in a reverse spiral up the wall of a central feed. 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.
The Larox classifier is another high capacity system, shown in Figure 1. The particles are dispersed by the feed falling across an inlet gas; the coarsest particles fall through the gas stream and into an outlet chute, and are thereby separated. Classification of the remainder occurs in a horizontal cyclone. There are three adjustable flights (A, B and C) to be positioned to give the best cut.
Spiral classifiers, such as the Alpine Mikroplex design for separation in the superfine region, were developed to partially overcome undesirable boundary layer effects associated with spinning fluids at stationary walls (Rumpf and Leschonski (1967)). Air is introduced tangentially at the periphery into a flat cylindrical space and moves along spiral flow lines into the center, from where it is drawn off. The fines follow the flow while the coarse particles spin round at the circumference; in some designs, this recirculating coarse stream is reclassified by passage of the incoming air through it. The coarse fraction leaves through a slit at the periphery (as in the Walther Classifier) or is removed using a screw extractor (as in the Alpine Mikroplex Classifier). The cut size theoretically has a stable circular trajectory in the classifying zone, but (in common with most other classifiers) separation is poorer with higher solids loadings.
To extend effective separation over a wider range of operating parameters, many classifiers are designed with a mechanical rotor built into them. The rotor has several effects: 1) large particles are deflected back into the classifier, thereby reducing the proportion of coarse particles in the fine product, 2) it aids recirculation of the air stream in some classifier types, and 3) the generation of a forced vortex keeps large particles at the periphery, but fines follow a helical trajectory to the center where they pass out with the exiting air.
Etkin, B., Haasz, A. A., Raimondo, S., and D'Eleuterio, G. M. T. (1980) Air Classification of Thin Flakes, Inst. Chem. Eng. Symp. Series 59, Dublin, 5:3/1-5:3/23.
Gonnell, H. W. (1928) Ein Windsichtverfahren zur Bestimmung der Kornzusammensetzung staubförmiger Stoffe, Z. VDI, 72, 945-950.
Prasher, C. L. (1987) Crushing and Grinding Process Handbook, Wiley, Chichester.
Rumpf, H. and Leschonski, K. (1967) Prinzipien und neuere Verfahren der Windsichtung. Chem. Ing. Techn., 39, 1231-1241.
Wessel, J. (1962) Schwerkraft-Windsichter, Aufbereitungstechnik, 3, 222-230.