Effecting a uniform dispersion of liquid, semisolid, or solid ingredients of a mixture by means of mechanical agitation. Low-viscosity liquids and suspensions are mixed with impellers of the turbine or propellor type. The mixing action results both from direct contact of the impeller blades with the liquid and from the turbulence induced by the impeller in the outer portions of the liquid. For this reason the diameter of the impeller need be only from one-fourth to one-half that of the container. For liquids of medium viscosity, revolving paddles of various shapes are used. Thicker mixtures involving volatile solvents are mixed in closed containers (churns) equipped with finlike members mounted on a rotating shaft. For liquids of very high viscosity, helical rotors, sigma blades, and similar devices are necessary. Because turbulence cannot be initiated in such fluids, the blades must fit closely within the walls of the container so as to make contact with every part of the material being mixed. While most industrial mixing of such pasty materials is done batchwise in kneaders, Banbury mixers, etc., continuous mixing of these can also be effectively carried out in horizontal compartments equipped with rotating screws whose pitch and flight are contoured in such a way as to provide both rotary and axial motion. There are a number of ingeniously engineered types of these for mixing plastics, rubber, food products, and similar products. Dry, solid particulates are mixed in rotating cylinders or tumbling barrels.See Impeller; Agitator; Kneading; Muller.
