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 preacoat 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 bodyfeed is usually prepared as a liquid slurry.

Starting with a clean filter, the filter must be filled and veted to remove entrapped gases. The precoat is then introduced into the filter, where it is filtered out as thin layer on the medium. The prefilt 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 filteraid 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 prefilt 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 filteraid to liquid

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

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

It is most difficukt to filter liquids containing finely divided, coloidal, gelatinous or non-rigid solids and obtain high flowrates and clarification without the use of a filteraid. The filter cake formed with filteraids is both permeable and rigid. Filter cake compressibility is minimazed by the rigid filteraid particles.

Filteraids 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 filteraid is added directly to the unifiltered liquid tank, there should be adequate agitation to maintain suspension during the filtratration cycle. Proppeller type agitators are satisfactory and convenient. It is poor practice to use recirculation with a centrifugal pump with break down the filteraid particle size as well as change the particle size distribution. This will reduce the filteraid'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 filteraids be added uniformly, both as to quantity and rate.

Filteraid slurry feeding and concentration

When the filteraid 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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