mechanical classification[mi′kan·ə·kəl ‚klas·ə·fə′kā·shən]
A sorting operation in which mixtures of particles of mixed sizes, and often of different specific gravities, are separated into fractions by the action of a stream of fluid. Water is ordinarily used as the sorting fluid, but other liquids or air or other gases may be used (see illustration).
The main objective of classification is to separate the particles according to size. This function is identical to that of screening, but classification is applicable to smaller particles, especially those that are undersize. For small particles it is more economical than screening. In classification the oversize and undersize are called sands and slimes, respectively.
Material also may be mechanically classified by specific gravity, a method that separates substances differing in chemical composition. This is called hydraulic separation. Such classification is based on the fact that, in a fluid, particles of the same specific gravity but of different size or shape settle at different constant speeds. Large, heavy, round particles settle faster than small, light, needlelike ones. If the particles also differ in specific gravity, the speed of settling is further affected. This is the basis for the separation of particles by kind rather than by size alone. See Flotation, Mechanical separation techniques, Unit operations