One method of sedimentation includes the use of selective barriers. In this technique, solids are removed from the fluid by pumping or drawing the fluid through a filter medium which retains the solids on or within itself. In a centrifugal filter, the same process occurs (limited to liquid-solid suspensions), but the driving force is that of a centrifugal field. The solids are removed from the medium, continuously or intermittently, or are discarded with the medium when it is replaced. In screening, a screen cloth, perforated sheet or plate is used to pass particles smaller than--and retain particles larger than--the screen.
Another sedimentation method involves hydrostatic separators. This type of sedimentation involves retaining liquid while an immiscible liquid, a gas or a solid separates from it. A supernatant liquid or floating solid may be overflowed or skimmed away; a subnatant liquid or settled solid may be pumped. A sedimentation thickener is a continuous hydrostatic settling tank. Sedimentation centrifuges are substantially hydrostatic separators when used as liquid clarifiers or oil-water separators, since density difference is the prime quality that makes separation possible.
When sedimentation uses electricity, it is called electrostatic/electrodynamic sedimentation. In this sedimentation, forces are used in electrostatic precipitators to drive small dust particles or droplets to a collecting electrode, and thus to clean a smoky or foggy gas stream.
Magnetic sedimentation is generally utilized to get rid of iron from ores being destroyed, thereby protecting the crushers; to remove hazardous materials from food; to recover precious metals in the ore; and to increase the concentration of ores. Tramp iron may be removed by a stationary magnet suspended over a conveyor belt or the material may be passed over a magnetic pulley; in both cases, magnetics are attracted by the magnetic field and separated.
Floatation sedimentation operates on the basis of selective moisture and density difference. Chemicals, called promoters or collectors, cause the particles of one fraction of a suspension to become air-avid and water-repellent. Air introduced to the suspension collects as bubbles, while the remaining solids sink and are withdrawn at the bottom.