Cake discharge casing

Horizontal Vessel, Horizontal Leaf Filters. These filters consist of a horizontal cylindrical vessel with an opening at one end (Fig. 19). A stack of rectangular horizontal trays is mounted inside the vessel the trays can usuaUy be withdrawn for cake discharge, either individuaUy or in the whole assembly. The latter case requires a suitable carriage. One alternative design aUows the tray assembly to be rotated through 90° so that the cake can faU off into the bottom part, designed in the shape of a hopper and fitted with a screw conveyor. 

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 ... 

A cake discharge mechanism that can be either a scraper, belt, roll and in rare cases a string discharge. Blow is applied only to filters with scraper and roll discharge mechanisms but not to filters with a belt or string discharge,... 

Total submergence is used in the vacuum disc filter thickener (Figure 13.12), in which case the cake discharge, by backwashing with filtrate, occurs as each sector passes through the lowest point of the slurry tank. 

In the cake discharge compartment of the casing, linings of rubber. PTFE. PVC, steel and even Stellite have all been used variously to combat wear or to overcome cake sticking. Flexing of some materials here is encouraged to aid the desticking process. 

In some applications it is found necessary to surround the cake discharge area with a stationary collector, fixed to the casing, to prevent cake sticking to the main casing. The function of the collector is assisted by impellers, which are bolted to the outer surface of the beach and/or the rear hub. The impellers sweep clean the inside of the collector, although they also add to the noise level of the decanter. 

The variations in the casing cake discharge are mainly associated with aids introduced to maintain a smooth cake discharge without sticking. 

Thickened cake discharge can simply be into a hopper which is emptied by a pump actuated by level probes in the hopper. However, modern technology often requires the discharge to be monitored for solids content, if not rate. For this a small stirred buffer tank is used. A sample from this tank is pumped and recycled continuously to provide a continuous sample. The stirred buffer tank is sized to smooth out major fluctuations which can occur in the decanter discharge, due to hold-up in the casing. 

The pressure filter with tubular elements has also been used as a thickener, when the cake, backwashed by intermittent reverse flow, is redispersed by an agitator at the bottom of the vessel and discharged continuously as a slurry. In some cases the filter cake builds up to a critical thickness and then falls away without bio whack. 

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