Thin-cake filters

Figure 19, Principal components of a staged, thin-cake filter thickener.

Surface Filters. In surface filters (Fig. 2), the goal is to achieve separation on the upstream side of a relatively thin filter medium. The particles to be separated must be larger than the pores in the medium, ie, in strainers, membrane filters, etc, or the particles must approach the pores in large numbers and bridge over the pores, as in cake filters. 

As can be seen, for constant cake thickness doubling the feed concentration doubles the yield. So-called high duty vacuum dmm filters use a unique cake discharge method to allow very thin cakes to be discharged and can therefore be operated at very high speeds up to 25 revolutions per minute. 

No internal piping and no conventional filter valve are needed with single-cell dmm filters where the entire dmm also operates under vacuum. The cake discharge is effected by air blowback from an internal stationary shoe mounted inside the dmm at the point of discharge. There are very close tolerances between the inside surface of the dmm and the shoe in order to minimize the leakage. The inside of the dmm acts as a receiver for the separation of air and filtrate conventional multicompartment dmm filters require a separate external receiver. This type of filter permits operation of the filter with thin cakes so that high dmm speeds, up to 26 rpm, can be used and high capacities can be achieved. Sizes up to 14 m are available. 

Dislodging of Cake by Reverse Flow. Intermittent back-flushing of the filter medium can also be used to control cake growth, leading to filtration through thin cakes in short cycles. Conventional vacuum or pressure filters can be modified to counter the effects of the forces during the back-flush (23,24). 

Normal Pressure. Water or mud blocking. Screen plugging. Minimum filtration rate muds. Thin friable filter cakes. 

Bird-Young Continuous Suction Filter is designed for slow-filtering slurries because of its ability to operate with very thin cakes. 

A special aligning device keeps the medium wrinkle-free and in proper line during its travel. Thin cakes of difficult solids which may be slightly soluble are good applications. When acceptable, a sluice discharge makes cakes as thin as 1.5 to 2 mm (about Vie in) feasible. Several manufacturers offer belt-discharge filters. 

Cake compressibility is the ability of a cake to reduce its volume, i.e., porosity, when stress is applied. The resulting cake will display an increase in hydraulic resistance. This is not necessarily caused by an average change in porosity, as a porosity gradient can occur by the redistribution of the solid material. Rigid granular particles tend to be incompressible and filter well even with thick cakes. Materials that are easily deformed such as amorphous or thixotropic materials will respond well to mechanical pressure or operation with thin cakes. (See Ch. 6 on Cake Compressibility.)... 

FIGURE 22.47 Alex Bagdasarian, KS Cheng and Frank M. Tiller, Bench Scale Filter for Studying Thin-Cake Filtration, Filtration Separation, January/February 1983 Filtration Separation, p. 32, Figure 1, January/February 1983. 

Or Mix 500 parts potato starch with 1500 parts of cold distilled water and 8 parts of pure oxalic acid place this mixture in a suitable vessel on a water-bath, and heat until a small sample tested with iodine solution does not produce the reaction of starch. When this IS found to be the case, immediately remove the vessel from the water-bath, and neutralize the liquid with pure carbonate of lime. After having been left standiug for a couple of days the liquor is filtered, and tho clear filtrate evaporated upon a water-bath until the mass has become quite a paste, which is removed by a spatula, and, haring been made into a thin cake, is placed upon paper aud further dried in a warm place 220 parts of pure dextrine are thus obtained. (5a AT). 29250... 

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