Cake appearance, 342 Figs

As shown in Fig. 8-8, the appearance of crystals suggests that the crystal growth rate in this operation was fast. Furthermore, the dried cake appears to be lumpy and less crystalline. 

A plate-and-frame filter press consists of alternate solid plates and hollow frames in a sandwich arrangement. The open frames are covered by the filter medium (e.g., the filter cloth), and the slurry enters through the frames and deposits the cake on the filter medium. The operation is batch, in that the filter must be disassembled when the cake fills the frame space, then cleaned and reassembled, after which the entire process is repeated. A schematic of a plate-and-frame press is shown in Fig. 13-7. In the arrangement shown, all of the frames are in parallel and the total filter area (which appears in the equations) is... 

Formulation of pharmaceutical suspensions to minimise caking can be achieved by the production of flocculated systems. A flocculate, or floe, is a cluster of particles held together in a loose open stmemre a suspension consisting of particles in this state is termed flocculated (Fig. 7.27). There are various states of flocculation and deflocculation. Unfortunately flocculated systems clear rapidly and the preparation often appears unsightly, so a partially deflocculated formulation is the ideal pharmaceutical. The viscosity of a suspension is obviously affected by flocculation. 

Recessed-plate filter pre.s.s. This press is similar to the plate-and-frame press in appearance but consists only of plates (Fig. 18-126). Both faces of each plate are hollowed to form a chamber for cake accumulation between adjacent plates. This design has the advantage of about half as many joints as a plate-and-frame press, making a tight closure more certain. Figure 18-127 shows some of the features of one type of recessed-plate filter which has a gasket to further minimize leaks. Air can be introduced behind the doth on both sides of each plate to assist cake removal. 

B. Polishing Filters. Figure 11.7, showing the brine polishing filters, is quite similar to Figs. 11.3b and 11.6. The simple level control option does not appear here. Maintaining a constant flow is more important with vertical precoated elements than it is with the equipment discussed previously. Variations in flow can allow the cake to fall from the filter elements. 

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