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an apparatus or device for thickening. Depending on the operating principle, distinctions are made between gravitational, inertial, and filter thickeners.
Gravitational thickeners are based on the sedimentation of solid particles in a liquid of lower density. Mechanical continuous thickeners, also known as radial settling tanks, are widely used. They consist of a cylindrical tank, on the axis of which a central well is installed for inlet of the suspension. The solid particles that settle on the conical floor of the tank are displaced by rakes rotating about the central shaft toward a discharge in the center of the floor. The thickened suspension is removed from the discharge with a pump. The upper layer of the liquid overflows across the edge of the tank into a launder that circles the periphery; the liquid is then removed by gravity flow. Mechanical continuous thickeners are used in the chemical industry and in ore concentration, hydrometallurgy, and water purification. While offering a high degree of thickening, these thickeners have the disadvantage of requiring a large tank size. To compensate for this, thickeners with two or three levels mounted on a common vertical shaft have been constructed. There are also gravitational thickeners of smaller size; this type is based on the removal of the particles from horizontal or inclined streams of pulp at certain velocities. Here, the pulp continuously enters a receiver and then flows down an inclined launder divided longitudinally by walls into a series of narrower launders. The walls create a laminar flow, as a result of which the particles drop to lower layers of the stream by the action of gravity. At the launder outlet, the stream is divided by a horizontal plane into a lower, thickened portion of pulp and an upper, clear portion. Thickeners of this type are inferior to mechanical continuous thickeners in the degree of thickening achieved, but they offer the advantage of having no moving parts. They are used in the concentration of ores for thickening pulp containing fine abrasive particles.
The liquid cyclone is the most common of the inertial thickeners and is used for thickening pulp containing sand and smalls.
In filter thickeners, thickening results from the removal of a portion of the liquid through a filtration medium. Thickeners of this type can be used for thickening systems with solid particles of any density, including particles whose density is equal to or less than that of the liquid, for example, cellulose and paper pulp.
V. V. BERDUS