Compactibility of cakes and sediments is a key factor in

The theory of colloidal filtration in slip casting of ceramics as developed by Aksay and Schilling for incompressible cakes has been extended to compressible materials. The rate of cake deposition in the mold depends upon the capillary pressure and permeability of the mold. Assuming that the capillary pressure is inversely proportional to an average diameter typifying the mold and that the permeability is directly proportional to the square of the diameter, an optimum diameter exists for production of maximum pressure drop across the cake and maximum rate of deposition.

This paper addresses the question of the relative amounts of liquid removed during filtration and expression as a function of cake compressibility. As liquid flows through a porous cake, the accumulative drag collapses the particulate structure, thereby increasing the solid content and displacing the liquid. The use of pump pressure in filtration represents the simplest hydraulic deliquoring process. In expression operations, pistons, membranes, rollers, or belts are employed to squeeze the particulate cake after filtration is complete.Filtration followed by expression at constant pressures ranging from 0.6 to 24.7 MPa (6.1-243.6 atm) was investigated theoretically and experimentally. During filtration and the early stages of expression, the flow rate and average liquid content of highly compressible attapulgite were virtually unaffected by increasing pressure in the range investigated. F. M. Tiller Department of Chemical EngineeringUniversity of Houston Houston, TX 77004 C. S. YehNelson Industries P 0. Box 428 Stoughton, WI 53589 IntroductionReduction of liquid content is important to trucking costs and the landfill characteristics of waste materials. Decreased energy requirements for drying and improved incineration result from lowered moisture fractions. The energy required to express liquid from cakes is negligible compared to the heat required for drying. Consequently, it is desirable to remove the maximum feasible amount by mechanical pressing. Many filters are equipped with membranes for expression. Significant questions include what pressures should be used during filtration and expression to effect the best results. Calculation of the process time required to reach a given average porosity is necessary to design and cycle optimization.With the advent of tighter controls on the liquid content of wet cakes, solid-liquid separation engineers turned to the use of high filtration pressures. Unexpectedly, increasing pressure failed to increase flow rates sufficiently or to decrease the average liquid content of highly compressible cakes. Filters were then equipped with impermeable membranes to squeeze the cakes. Although expression leads to lower liquid content, the times required for treatment of highly resistant cakes can be substantial.After a cake is produced by filtration, the liquid is then expressed by the use of mechanical pressure. The final cake

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