MECHANICAL SEPARATIONS

 

 

Mechanical separations can be divided into four groups - sedimentation, centrifugal separation, filtration and sieving.

In sedimentation, two immiscible liquids, or a liquid and a solid, differing in density,are separated by allowing them to come to equilibrium under the action of gravity, the heavier material falling with respect to the lighter. This may be a slow process. It is often speeded up by applying centrifugal forces to increase the rate of sedimentation; this is called centrifugal separation. Filtration is the separation of solids from liquids, by causing the mixture to flow through fine pores which are small enough to stop the solid particles but large enough to allow the liquid to pass. Sieving, interposing a barrier through which the larger elements cannot pass, is often used for classification of solid particles.

Mechanical separation of particles from a fluid uses forces acting on these particles. The forces can be direct restraining forces such as in sieving and filtration, or indirect as in impingement filters. They can come from gravitational or centrifugal action, which can be thought of as negative restraining forces, moving the particles relative to the containing fluid. So the separating action depends on the character of the particle being separated and the forces on the particle which cause the separation. The important characteristics of the particles are size, shape and density; and of the fluid are viscosity and density. 

The reactions of the different components to the forces set up relative motion between the fluid and the particles, and between particles of different character. Under these relative motions, particles and fluid accumulate in different regions and can be gathered as: in the filter cake and the filtrate tank in the filter press; in the discharge valve in the base of the cyclone and the air outlet at the top; in the outlet streams of a centrifuge; on the various sized sieves of a sieve set. In the mechanical separations studied, the forces considered are gravity, combinations of gravity with other forces, centrifugal forces, pressure forces in which the fluid is forced away from the particles, and finally total restraint of solid particles where normally the fluid is of little consequence. The velocities of particles moving in a fluid are important for several of these separations.   Good..