Discharge precoat

Paper pulp and fiber cloths are excellent materials for precoats and filter aids. Paper pulp gives a high rate of flow, is easily discharged and shows little tendency to clog. 

Often special measures are taken to ease cake discharge and enhance filtration. The measures taken include precoating and the addition of body aid. Precoating the plates prior to introducing the feed is done only in the following cases ... 

When the contaminants are gelatinous and sticky it forms a barrier that avoids cloth blinding. Likewise the interface between the precoat and the cloth departs readily so the cake discharges leaving a clean cloth. 

Once the precoating stage is completed the process slurry is pumped into the filter, the forming cake is retained on the plates and the filtrate flows to further processing. When the solids are fine and slow to filter a body-aid is added to the feed slurry in order to enhance cake permeability. However, it should be kept in mind that the addition of body-aid increases the solids concentration in the feed so it occupies additional volume between the plates and increases the amount of cake for disposal. Likewise, for all those applications when the cake is the product, precoat and filter-aid may not be used since they mix and discharge together with the cake. 

The flow scheme for a conventional precoat filter station typically looks like that shown in Figure 9. The doctor blade discharge configuration for this machine is illustrated in Figure 10. 

Figure 10. Doctor blade discharge for precoat filter.

Because these filters (Fig. 12) are made so that the leaves can be rolled from the filter quickly, they can be easily inspected for tears in the septum, leaks around the edges of the screens or around the gaskets in the manifold, improper precoating, and so forth. If the cake is blown dry in the filter before the leaves are removed from the filter shell, the cake will usually remain on the leaves so that it can be discharged either manually or by means of a vibrator. Hydraulic capacity of the leaves is usually not a problem, because their size need not be excessively large for the larger areas. 

However, it is not iways possible to run a pilot-plant test in order to determine the depth of cut. A well-accepted alternative approach makes use of the more sophisticated test leaf illustrated in Fig. 18-110. This test leaf is designed so that the cake and precoat are extruded axially out the open end of the leaf. The top of the retaining wall on this end of the leaf is a machined surface which serves as a support for a sharp discharge knife. This approach permits variable and known depths of cut to be made so that die minimum depth of cut may be determined. Test units are available from Betts Advanc Metal, hompoc, Calif, (805) 735-5130. 

Vertical filters are not convenient for the removal of dry cake, although they can be used in this service if they have a bottom that can be retracted to permit the cake to fall into a bin or hopper below. They are adapted rather to wet-solids discharge, a process that may be assisted by leaf vibration, air or steam sparging of a filter full of water, sluicing from fixed, oscillating, or traveling nozzles, and blowback. They are made by many companies, and they enjoy their widest use for filter-aid precoat filtration. 

Hopper dewaterer Top feed Scroll-discharge Tilting-pan Belt-discharge Continuous vacuum disk Continuous vacuum Precoat Continuous pressure Precoa... 

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