Diaphragm press cake discharge

Figure 26. Cake discharge from a diaphragm filter press.

Figure 6.3 Schematic diagram of the horizontal diaphragm press cycle (side view of one chamber shown and cake discharge omitted), (a) Filtration via pump (b) filtration via diaphragm (c) compression deliquoring (d) displacement washing (e) gas deliquoring. Plate and frame and recessed plate press cycles are similar but the filtration phase using the diaphragm (b) and the compression deliquoring phase (c) are omitted.

Figure 6.5 Schematic diagram of a tube press cycle, (a) Filling of press using pump (b) filtration via diaphragm (c) displacement washing (d) cake discharge with element withdrawn.

Tuhtdar Presses As the name implies, this press is composed of a candle filter inside a cyhndrical hydrauhc casing (Fig. 18-192). The filter cloth is wrapped around the filter candle, and a diaphragm is attached to the inner side of the outer casing. During the filtration step, the space in between two cylinders is filled with slurry, and pressure filtration is conducted. At the end of the filtration step, the diaphragm is inflated to squeeze the cake around the filter candle. At the end of expression, the bottom of the hydrauhc casing tube is opened and the filter assembly is lowered. Air is then introduced to pulse the cake off the candle. Alter the cake is discharged, the inner filter candle moves back, and the bottom is closed for the next filtration cycle. 

The vertical filter press, like the horizontal press, is strictly speaking a batch operating filter, but is more correctly called a semi-continuous filter, because the cake is discharged from all of the compartments at the same time. When the filtration cycle starts, the plate assembly (Figure 3.73) closes, and slurry is fed to all of the compartments at the same time. Cakes form on top of the filter medium in each compartment. Once filtration is complete, diaphragms are forced hydraulically down on the cakes to dewater them. Then the diaphragms are relaxed, wash water is introduced, and the cakes squeezed once again, followed by a compressed air blast to dry them further. At this point, the plate assanbly opens, and the continuous band of filter medium advances... 

A small group of filters is classed together as variable volume devices, so called because the filtration zone containing the collected solids is compressed once filtration is complete, in order to dewater the filter cake further before discharge. The diaphragm filter press, described in the previous chapter, fits that definition, but is more sensibly included as a variant of the horizontal filter presses, leaving two other closely related designs. 

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