At the start of filtration in a mud cake filter some solid particles enter the pores of the medium and are immobilized, but soon others begin to collect on the septum surface. After this brief initial period the cake of solids does the filtration, not the septum; a visible mud cake of appreciable thickness builds up on the surface and must be periodically removed. Except as noted under bag filters for gas cleaning, cake filters are used almost entirely for liquid-solid separations. As with other filters they may operate with above-atmospheric pressure upstream from the filter medium or with vacuum applied downstream. Either type can be continuous or discontinuous, but because of the difficulty of discharging the solids against a positive pressure, most pressure filters are discontinuous.
DISCONTINUOUS PRESSURE FILTERS
Pressure filters can apply a large pressure differential across the septum to give economically rapid filtration with viscous liquids or fine solids. The most common types of pressure filters are filter presses and shell-and-leaf filters.
A filter press contains a set of plates designed to provide a series of chambers or compartments in which solids may collect. The plates are covered with a filter medium such as canvas. Slurry is admitted to each compartment under pressure; liquor passes through the canvas and out a discharge pipe, leaving a wet mud cake of solids behind.
The plates of a filter press may be square or circular, vertical or horizontal. Most commonly the compartments for solids are formed by recesses in the faces of molded polypropylene plates. In other designs, they are formed as in the plate-and·frame press shown in Fig. 1, in which square plates 6 to 78 in. (150 mm to 2m) on a side alternate with open frames. The plates are ! to 2 in. (6 to 50 mm) thick, the frames ¼ to 8 in. (6 to 200 mm) thick. Plates and frames sit vertically in a metal rack, with cloth covering the face of each plate, and are squeezed tightly together by a screw or a hydraulic ram. Slurry enters at one end of the assembly of plates and frames. It passes through a channel running lengthwise through one corner of the assembly. Auxiliary channels carry slurry from the main inlet channel into each frame. Here the solids are deposited on the cloth-covered faces of the plates. Liquor passes through the cloth, down grooves or corrugations in the plate faces, and out of the press.
Thorough washing in a filter press may take several hours, for the wash liquid tends to follow the easiest paths and to bypass tightly packed parts of the cake. If the cake is less dense in some parts than in others, as is usually the case, much of the wash liquid will be ineffective. If washing must be exceedingly good, it may be best to reslurry a partly washed cake with a large volume of wash liquid and refilter it or to use a shell-and-leaf filter, which permits more effective washing than a plate-and-frame press.