Sorting techniques may be categorized loosely as: manual, dimensional, dynamic, electric and magnetic, optical, and "exotic."
Manual sorting from conveyors is still the most common method, employed at nearly all composting plant sand some incinerators. Mechanical methods have been developed recently by adapting proved techniques from the mineral processing industry. Dimensional separation, perhaps the simplest sorting operation, is accomplished typically by passing refuse over a series of vibrating screens.
Dynamic sorting techniques rely on combinations of characteristics that affect the motion of refuse particles (size, specific gravity, inertia, shape, and elasticity). The devices include the stoner, fluidized bed separator, Osborne dry separator, air classification unit, vortex classifier, ballistic separator, secator, and inclined conveyor. A number of other techniques (flotation, heavy medium separation, hydropulping) and devices (jigs, Wilfley tables, spiral classifiers) require an aqueous slurry or pulp and are not generally applicable to solid waste processing.
The air classification unit consists of zig-zag shaped columns and cyclone separators and relies on differences in specific gravity and aerodynamic characteristics of materials. A stream of air tiowing up through the columns carries lighter particles to the top, where they are removed by cyclones, while heavier particles fall to the bottom.
