Size classification and classifier

In mineral processing plant, Size classifiers separate particles of various sizes, shapes, and specific gravities in fluids (e.g., water or air) under the influence of gravitational or centrifugal forces. In principle, such devices should make a size split based on particle size rather than other properties. Unfortunately, the split is always imperfect. Measures of the performance of size classifiers are similar to those employed for screens, except that the definition of cut size is not as simple. Separations are normally made between about 20 and 325 mesh, although some pneumatic devices size readily to below 95% passing 0.010 mm.

Classification devices attempt to take advantage of the following aspects of particle behavior

1. Smaller particles fall more slowly in fluids than do larger ones.

2. In free vortex motion (i.e., cyclones), centrifugal forces have greater influence on large particles and lesser influence on small particles.

3. Small particles, having less inertia, tend to behave like the suspending medium or fluid.

4. Larger particles require higher conveying velocity for coarse separation.

5. Collision frequency increases with particle size.

To take advantage of these phenomena various mechanical components (such as rakes, spiral

arms, vanes, spindles, and baffles) are used, as are means to regulate the direction of fluid flow.

Size classifiers are distinguished from each other, initially, on whether the fluid employed is air or water. Those that rely on water include nonmechanical classifiers (surface sorters such as cones, and hydraulic classifiers such as the Richards hindered settler), mechanical classifiers (spiral classifiers), and hydrocyclones. This last type of device uses centrifugal forces, another distinguishing feature. Pneumatic (air) classifiers rely on a suitable interplay between the force of gravity and drag forces, and in many devices collision forces and centrifugal forces, to effect size separation in air. Figure 1 categorizes size classifiers.

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