Suspension thickening, membrane

The driving force for filtration in pressure filters is usually the liquid pressure developed by pumping or by the force of gas pressure in the suspension feed vessel. Alternatively, or in addition, the liquid may be squeezed through and out of the cake by the mechanical action of an inflatable membrane, a piston or a porous medium pressed on top of the cake. Pressure filtration is, therefore, defined here as any means of surface filtration where the liquid is driven through the medium by either hydraulic or mechanical pressure, greater than atmospheric. The solids are deposited on top of the filter medium (as in all surface filters), with the possible exception of some cartridge filters which also use a certain amount of depth filtration. In this chapter, the suspension is assumed to approach the medium at 90° and this excludes the so-called dynamic fUter/thickeners or cross-flow filters (also driven by pressure) which are dealt with in a separate chapter (11). 

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