Perlite - Pressure Filtration

 

In any batch filtration there is a sequence of events. From the start to the termination is termed the "cycle". For the typical cake filtration involving precoat and body-feed, there are fundamental characteristic events or actions during this cycle.

First, the precoat must be prepared as a liquid slurry or in a dry form for feeding into a liquid. The body feed is usually prepared as a liquid slurry.

Starting with a clean filter, the filter must be filled and vetted to remove entrapped gases. The precoat is then introduced into the filter, where it is filtered out as thin layer on the medium. The preflight is introduced without disturbing the precoat and usually recirculated to a point where satisfactory clarity is established, then recirculation is stopped and the filtrate directed to the filtrate receiving tank.

At this point filtration has begun. This phase continues with an admix of filter aid being introduced to beneficiate the filtration. It continues until it becomes uneconomical because of build up of cake resistance.

At this point there can be numerous variations in the sequence depending upon the desirability of cake washing, drying and the method of cake discharge. One simple sequence would be to drain the liquid in the tank back to the prefiltration tank and sluice the cake to discharge. Other more complex sequences would involve washing the cake, drying, vibrating, or a combination of sluice vibrating for cake removal.

Addition of filter aid to liquid

The choice of the type and grade as well as quantity to be added is vitally important to obtain the highest filtration flow rate consistent with the clarification required.

The filter aid dosage varies with application. Filtration are handled in the dosage range of 0,1 to 1,0% on the liquid weight.

It is most difficult to filter liquids containing finely divided, colloidal, gelatinous or non-rigid solids and obtain high flow rates and clarification without the use of a filter aid. The filter cake formed with filter aids is both permeable and rigid. Filter cake compressibility is minimized by the rigid filter aid particles.

Filter aids can be added directly to the tank of liquid to be filtered, or proportioned by a dry feeder into a mixing tank, or from a slurry tank into the filter inlet. If the filter aid is added directly to the unfiltered liquid tank, there should be adequate agitation to maintain suspension during the filtration cycle. Propeller type agitators are satisfactory and convenient. It is poor practice to use recirculation with a centrifugal pump with break down the filter aid particle size as well as change the particle size distribution. This will reduce the filter aid's effectiveness. Such a practice can also reduce the particle size of the suspended solids, making clarification more difficult. Whatever the feeding method or equipment, the chief requirement is that filter aids be added uniformly, both as to quantity and rate.

Filter aid slurry feeding and concentration

When the filter aid cannot be added directly to the tank of liquor to be filtered as a body-aid, it is best added with a positive displacement type pump immediately ahead of the filter. In this instance, a slurry feed tank of adequate size for the filtration is required. The maximum slurry concentration using water or a liquid having about the same viscosity is about 120 g/l.

 

 

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