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Centrifugal separations are of two broad kinds, those using filtration and those operating by sedimentation. The latter are covered in Section 7, while centrifugal filters are the subject of this part of Section 3. Also broadly classifiable into two types, centrifugal filters, which are essentially devices for recovering solids from liquid suspensions, are defined by the way in which solids move within, and are then discharged from, the centrifuge.
All filtering centrifuges consist of a rotating basket, cylindrical or conical in shape, out of the open end of which the separated solids are discharged. The basket is supported at the other end on the drive shaft, coming from a fixed or variable speed motor. The walls of the basket are made from a porous filter medium, usually woven wire mesh, perforated plate or welded wedge-wire screen, with filtrate passing through the basket from the inside out into a surrounding casing, leaving the solids behind as a cake on the filter medium.
The fixed bed filtering centrifuge, as its name implies, lays the separated solids down as a cake that stays in place on the walls of a cylindrical basket during the filtration, washing and dewatering stages of the cycle. The centrifugal force, which
varies with the rotational speed of the basket, enables very effective cake formation and processing. Once the cake is dewatered, it is removed from the basket manually, semi-automatically or completely automatically. The removal of the cake
may need the machine to be stopped or slowed down, although some automatic machines can discharge at full bowl speed. The fixed cake centrifugal filters are therefore batch machines, although some of the completely automatic centrifuges have a very short cycle and can appear to be almost fully continuous in operation.
In the moving bed centrifuge the solid particles quickly separate from suspension in the feed zone, and, once at the wall of the basket, move along the basket in a direction effectively parallel to that of the axis of rotation, until they reach the open end of the basket, from which they are discharged into a collecting ring around the outside of the basket. Filtration, washing and dewatering all take place as the cake moves through the basket, each stage taking a relatively short time because of the high centrifugal force. Baffles in the casing around the basket enable filtrate to be kept separate from wash liquors. Movement of the cake is caused by mechanical devices in a cylindrical basket machine, or by the component of the centrifugal force in the axial direction for conical basket centrifuges. This movement may create continuous cake flow, and hence a continuously discharging centrifuge, or very short cycle semi-continuous movement that is almost continuous.
Most centrifugal filters are quite complex machines, with that complexity very much a consequence of the need to be able to handle, and especially discharge, the separated solids, as near continuously as possible, while causing the minimum of damage to the solid particles, especially where these are crystals.