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III. Filter Aids :


PIALITE Perlite Filter Aids


Perlite in its natural state is a dense, glassy rock formed by volcanic action. When crushed and treated under proper conditions it “pops” like popcorn, expanding to 20 or more times its original volume.


Perlite is an amorphous mineral consisting of fused sodium potassium aluminum silicate. PIALITE perlite filter aids are processed in the temperature range of 1500°F to 2000°F (800°C to 1100°C) which eliminates organic matter. They have extremely low solubility's in mineral and organic acids at both low and high temperatures. Solubility's in strong alkalis varies according to temperature and time of contact. PIALITE perlite filter aids do not impart taste or odor to liquids being filtered.


A major advantage of PIALITE perlite filter aids is that most weight loss per unit of volume than standard diatomite filter aids. Thus, pound for pound, PIALITE perlite filter aid often give up to 20% greater filtering capacity resulting in comparable savings in filter aid usage especially on rotary vacuum precoat filters. Two other outstanding advantages of PIALITE perlite filter aids are: (1) high efficiency in filtering high-solids-content fluids; (2) less cake cracking on rotary vacuum precoat filters.

 


INDUSTRIAL FILTRATION WITH PIALITE PERLITE FILTER AIDS


FILTRATION. The separation of solids from liquids is one of the most widely used operations in industrial processing. The wanted material may be either the liquid or the solids, depending on the finished product. This technical paper is concerned primarily with filtration where the liquid is the desired material and substantially 100% removal of impurities/unwanted solids is required. In many systems where the content is low, but the filtration resistance is high, this can be done most efficiently and economically only with the use of filter aids. In this process, the filter septum (cloth, screen, porous stone or metal) serves principally as a support for the cake. The filter aid forms a porous layer on the septum and thus the filter aid is the filtering medium that traps the solids being removed and prevents them from “blinding” the septum.


A filter aid is a finely divided material which when added to the liquor/slurry to be filtered helps control flow and solids removal. It does not interfere chemically with the liquid being filtered, and is not readily compacted nor compressed by the exertion pressure. Filter aid filtration is mechanical, not a chemical action. It is “straining” on a micro scale. Irregular particles interlace and overlay in a fashion that leaves 85% to 90% voids or open spaces. These voids form billions of microscopically fine interstices between the filter aid particles. The size of these opening is so minute that unwanted solids (even in the submicron range) are “strained” from the liquid. The vast number of the openings compensates for their small size resulting in fast flow rates and brilliant clarity.

 


CHARACTERISTICS OF A GOOD FILTER AID


Good filter aids are light in weight, chemically inert, and form high-porosity filter cakes to maintain free flow of the liquid. Specifically an efficient filter aid must:
1. Form a Very Porous Cake
2. Have Low Surface Area
3. Have Correct Particle Size
4. Deviate From Uniform Sizing
5. Be Available in a Number of Grades


Form a Very Porous Cake. Structure of the particles must be such that they will not pack together closely, but will form cakes that are 85% to 90% pores. This not only permits high initial liquid flow but also provides pore spaces to trap and contain the filterable solids and leave a high percentage of channels remaining open for flow.


Have Low Surface Area. Resistance to flow is created by the viscous shear of the liquid over the surface area of the filter aid particles. Therefore, the smaller the surface area, the higher the flow.


Have Correct Particle Size Distribution. Because fine particles having higher surface area cut down flow, and coarse particles give poor clarity, the narrowest possible range of particle sizes (within a given grade of a series of filter aids) is best.


Deviate From Uniform Sizing. Particle size distribution must be “tailored” to include enough very coarse particles to permit precoating on coarse wire and synthetic screens. The percentage of coarse particles necessary for precoating is small and does not adversely affect flow rate or clarity.


Available in a Number of Grades. Grade of filter aid must range from fine to coarse to suit various solids removal requirements. A great many materials have been tested as filter aids. Those made from diatomite, perlite, and cellulose have been most successful and are widely used in industrial filtration.

 

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