Classification Theory

Fitch, B. ; Roberts, E. J.
Organization: Society for Mining, Metallurgy & Exploration
Pages: 10
Publication Date: Jan 1, 1985
Definitions and Terminology Classification is defined within this section as the separation of solid particles into size or weight fractions or classes by differential settling through a fluid. It is usually thought of as a size separation, but is actually made on the basis of particle settling rate, and so is affected by any variations in particle density and shape. The fluid may be liquid (wet classification) or gas (dry or air classification). This section will treat only wet classification. There are two basic types of classification; pool and hydraulic. In pool classification a suspension of feed particles is fed into and out of a pool of some kind at such a rate that only part of them¬ the coarser and faster-settling part-has time to settle out. The remain¬der overflows as fines. In hydraulic classification the feed suspension is introduced into or above one or more columns or pockets through which water is rising at a controlled velocity. Coarse particles subside through the pockets and are removed through spigots. Those which settle slower than the rising velocity in any column or pocket are (at least ideally) prevented from subsiding and so must overflow. Hydraulic classifiers or sizers are capable of making far sharper sepa¬rations than pool classifiers. The force causing settling may be gravity, or alternatively may be from centrifugal acceleration as developed in a centrifuge or hydra cyclone. Two things are needed to characterize the separation produced by classification: first, the size, D„ or equivalently the screen mesh at which it is made; second the sharpness of separation. Separation size may be defined in various ways. In closed-circuit grinding it is commonly specified by some point in the overflow screen analysis: For example, 5% on 65 mesh. In other applications, notably
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