A group of laboratory and production operations whereby the components of a polyphase mixture are separated by mechanical methods into two or more fractions of different mechanical characteristics. The separated fractions may be homogeneous or heterogeneous, particulate or nonparticulate. Mechanical separations are differentiated from two other classes of separations of major importance: those that depend on separation at the molecular level on the basis of thermodynamic or physicochemical properties and by means of such mass-transfer processes as evaporation, dissolution, precipitation, absorption, and intraphase or interphase diffusion; and those that depend on chemical reaction of a component and subsequent separation of the product by mass-transfer or mechanical methods.