Fundamentals of size separation

Screening and classification are used to separate particles based on differences in size. Industrially, these processes are done on a continuous basis to produce two (or more) product streams of varying degrees of fineness. Screening involves the separation of particles based on the probability of passage through a series of apertures of uniform size. Variables such as the number of presentations per second, size of the particle relative to aperture size, and retention time on the screen all are important in determining the probability of passage through the screen. In classification, the separations are determined by the movement of particles in a fluid, typically water or air. In this case, the probability of a particle reporting to the coarse or fine stream depends on the relative effect of gravity and fluid drag. This paper describes the factors affecting both screening and classification, the types of devices used in the separations, and the effects of staging to improve process performance.