0
-
An empty cart
You have no item in your shopping cart
The leaves in a pressure leaf filter hang, or are held, vertically and, while this permits filtration on both sides of the leaf, it also means that the accumulating cake can easily fall off the leaf for lack of support below the cake. This problem is overcome by mounting the filter elements horizontally, when they are normally called plates, with filtration occurring on the upper surface only. More often than not, the plates are circular, and they are supported, equi-spaced, on a central post that acts
as the filtrate off-take from below the filter medium on each plate, as illustrated in Figure 3.64 . This whole array then is housed in a vertical cylindrical pressure vessel. Suspension is fed into the pressure housing, and filtrate flow is through the
plate and out through the central pipe.
As with leaf filters, the pressure plate filter is operated batch-wise, with a given quantity of suspension fed into the filter, until the design thickness of cake is formed. The main filtration is then stopped and the residual vessel contents are filtered through the plate array. A heel of suspension is left below the bottom plate and this is filtered separately through a scavenger plate that hugs the bottom of the vessel as closely as possible (Figure 3.65). This plate remains closed during the main filtration part of the cycle. At the end of the filtration, the main outlet is closed, and the residual suspension is forced out through the scavenger plate, leaving virtually no unfiltered liquid.
A filter that combines features of leaf and plate is the dual disc plate filter illustrated in Figure 3.66 . The filter elements are now two-sided, filtering on top and bottom (with a single disc at the base of the vessel to act as scavenger). This gives
a large filtration area in a smaller floor area, but, of course, suffers from an unsupported cake on the underside of each element. To some extent, this problem is selfcorrecting since, if cake falls off the underside, the flow resistance decreases at that point, so more filtrate flows through it and the cake is redeposited. This type of filter is mainly used for clarifying and polishing duties.