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the settling or surfacing of particles in the dispersed phase, such as solid particles, liquid droplets, or gas bubbles, in a liquid or gaseous dispersion medium as a result of a gravitational field or centrifugal force. Sedimentation occurs if the directional motion of the particles under the effect of gravity or centrifugal force predominates over the random thermal motion (seeBROWNIAN MOVEMENT and DIFFUSION).
The rate of sedimentation depends on the mass, size, and shape of the particles, the viscosity and density of the medium, and the acceleration caused by the force field. For fine spherical partices that do not interact with each other, the sedimentation rate can be determined by Stokes’ law. Sedimentation in dispersed systems—especially in a gaseous dispersed medium—is often accompanied by an increase in the size of the settling particles as a result of coagulation or coalescence.
Sedimentation in nature causes the formation of sedimentary rocks, the clarification of water in large bodies, and the release of liquid drops and solid particles from the atmosphere.
In industry, sedimentation is used in the separation of powders into fractions and the isolation of various products in chemical engineering. (See alsoELUTRIATION and SETTLING.)
The separation of a dilute suspension of solid particles into a supernatant liquid and a concentrated slurry. If the purpose of the process is to concentrate the solids, it is termed thickening; and if the goal is the removal of the solid particles to produce clear liquid, it is called clarification. Thickening is the common operation for separating fine solids from slurries. Examples are magnesia, alumina red mud, copper middlings and concentrates, china clay (kaolin), coal tailings, phosphate slimes, and pulp-mill and other industrial wastes. Clarification is prominent in the treatment of municipal water supplies.
The driving force for separation is the difference in density between the solid and the liquid. Ordinarily, sedimentation is effected by the force of gravity, and the liquid is water or an aqueous solution. For a given density difference, the solid settling process proceeds more rapidly for larger-sized particles. For fine particles or small density differences, gravity settling may be too slow to be practical; then centrifugal force rather than gravity can be used. Further, when centrifugal force is inadequate, the more positive method of filtration may be employed. All those methods of separating solids and liquids belong to the generic group of mechanical separations. See Centrifugation, Clarification, Filtration
Particles too minute to settle at practical rates may form flocs by the addition of agents such as sodium silicate, alum, lime, and alumina. Because the agglomerated particles act like a single large particle, they settle at a feasible rate and leave a clear liquid behind.