Filter Aids

Filter aids as well as flocculants are employed to improve the filtration characteristics of hard-to-filter suspensions. A filter aid is a finely divided solid material, consisting of hard, strong particles that are, en masse, incompressible. The most common filter aids are applied as an admix to the suspension. These include diatomaceous earth, expanded perlite, Solkafloc, fly ash, or carbon. Filter aids build up a porous, permeable, and rigid lattice structure that retains solid particles and allows the liquid to pass through. These materials are applied in small quantities in clarification or in cases where compressible solids have the potential to foul the filter medium.

Filter aids may be applied in one of two ways. The first method involves the use of a precoat filter aid, which can be applied as a thin layer over the filter before the suspension is pumped to the apparatus. A precoat prevents fine suspension particles from becoming so entangled in the filter medium that its resistance becomes exces-sive. In addition it facilitates the removal of filter cake at the end of the filtration cycle. The second application method involves incorporation of a certain amount of the material with the suspension before introducing it to the filter. The addition of filter aids increases the porosity of the sludge, decreases its compressibility, and reduces the resistance of the cake. In some cases the filter aid displays an adsorption action, which results in particle separation of sizes down to 0.1 ¡i. The adsorption ability of certain filter aids, such as bleached earth and activated charcoals, is manifest by a decoloring of the suspension's liquid phase. This practice is widely used for treating fats and oils. The properties of these additives are determined by the characteristics

FILTER AIDS are fine, chemically inert powders applied in both process and waste rnicrofiltrations to maintain high flow rates while giving brilliant clarity. For difficult separations this long-established technology is the economical way to produce high quality fluids and manageable solid residues. Examples of filter aids are:

DIATOMITE - Manufactured from either marine or fresh water deposits.

PERLITE - Low density, low crystalline silica grades suit a wide range of process, water and wastewater applications.

CELLULOSE - As fibrous precoat aids or where special chemical compatibilities are required cellulosebased additives achieves separations that would otherwise be difficult or impossible.

of their individual components. For any filter aid, size distribution and the optimal dosage are of great importance. Too low a dosage results in poor clarity; too great a dosage will result in the formation of very thick cakes. In general, a good filter aid should form a cake having high porosity (typically 0.85 to 0.9), low surface area, and good particle-size distribution. An acceptable filter aid should have a much lower filtration resistance than the material with which it is being mixed. It should reduce the filtration resistance by 67 percent to 75 percent with the addition of no more than 25 percent by weight of filter aid as a fraction of the total solids. The addition of only a small amount of filter aid (e.g., 5 percent of the sludge solids) can actually cause an increase in the filtration resistance. When the amount of filter aid is so small that the particles do not interact, they form a coherent structure, and resistance may be affected adversely.

Filter aids are evaluated in terms of the rate of filtration and clarity of filtrate. Finely dispersed filter aids are capable of producing clear filtrate; however, they contribute significantly to the specific resistance of the medium. As such, applications must be made in small doses. Filter aids comprised of coarse particles contribute considerably less specific resistance; consequently, a high filtration rate can be achieved with their use. Their disadvantage is that a muddy filtrate is produced.

The optimum filter aid should have maximum pore size and ensure a prespecified filtrate clarity. Desirable properties characteristics for the optimum filter aid include:

1. The additive should provide a thin layer of solids having high porosity (0.85 to 0.90) over the filter medium's external surface. Suspension particles will ideally form a layered cake over the filter aid cake layer. The high porosity of the filter aid layer will ensure a high filtration rate. Porosity is not determined by pore size alone. High porosity is still possible with small size pores.

2. Filter aids should have low specific surface, since hydraulic resistance results from frictional losses incurred as liquid flows past particle surfaces. Specific surface is inversely proportional to particle size. The rate of particle dispersity and the subsequent difference in specific surface determines the deviations in filter aid quality from one material to another. For example, most of the diatomite species have approximately the same porosity; however, the coarser materials experience a smaller hydraulic resistance and have much less specific surface than the finer particle sizes.

3. Filter aids should have a narrow fractional composition. Fine particles increase the hydraulic resistance of the filter aid, whereas coarse particles exhibit poor separation. Desired particle-size distributions are normally prepared by air classification, in which the finer size fractions are removed.

4. In applications where the filter aid layer is to be formed on open-weave synthetic fabric or wire screens, wider size distributions may have to be prepared during operation. Filter aids should have the flexibility to be doped with amounts of coarser sizes. This provides rapid particle bridging and settling of the filter aid layer. For example, diatomite having an average particle size of 8 ^ may be readily applied to a screen with a mesh size of 175 p by simply adding a small quantity of filter aid with sizes that are on the same order but less in size than the mesh openings. Particle sizes typically around 100 p. will readily form bridges over the screen openings and prevent the loss of filter aid in this example. 5. The filter aid should be chemically inert to the liquid phase of the suspension and not decompose or disintegrate in it.

The ability of an admix to be retained on the filter medium depends on both the suspension's concentration and the filtration rate during this initial precoat stage. The same relationships for porosity and the specific resistance of the cake as functions of suspension concentration and filtration rate apply equally to filter aid applications.

Filter aids are added in amounts needed for a suspension to acquire desirable filtering properties and to prepare a homogeneous suspension before the actual filtration process begins. Essentially, filter aids increase the concentration of solids in the feed suspension. This promotes particle bridging and creates a rigid lattice structure for the cake. In addition, they decrease the flow deformation tendency. Irregular or angular-shaped particles tend to have better bridging characteristics than spherical particles. Generally, the weight of filter aid added to the suspension should equal the particle weight in suspension. Typical filter aid additions are in the range of 0.01 percent to 4 percent by weight of suspension; however, the exact amount should be determined from experiments. Excess amounts of filter aid will decrease the filter rate. Operations based on the addition of admixes to their suspensions may be described by the general equations of filtration with cake formation. A plot of filtration time versus filtrate volume on rectangular coordinates results in a nearly parabolic curve passing through the origin. The same plot on logarithmic coordinates, assuming that the medium resistance may be neglected, results in a straight line. This convenient linear relationship allows results obtained from short-time filtration tests to be extrapolated to long-term operating performance (i.e., for several hours of operation). This reduces the need to make frequent, lengthy tests and saves time in the filter selection process. In precoating, the prime objective is to prevent the filter medium from fouling. The volume of initial precoat normally applied should be 25 to 50 times greater than that necessary to fill the filter and connecting lines. This amounts to about 5-10 lb/100 ft2 of filter area, which typically results in a 1/16-in. to 1/8-in. precoat layer over the outer surface of the filter medium. An exception to this rule is in the precoating of continuous rotary drum filters where a 2-in. to 4-in. cake is deposited before filtration. The recommended application method is to mix the precoat material with clear liquor (which may consist of a portion of the filtrate). This mixture should be recycled until all the precoat has been deposited onto the filter medium. The unfiltered liquor follows through immediately without draining off excess filter aid liquor. This operation continues until a predetermined head loss develops, when the filter is shut down for cleaning and a new cycle.

In precoating, regardless of whether the objective is to prevent filter medium clogging or to hold back fines from passing through the medium to contaminate the filtrate, the mechanical function of the precoat is to behave as the actual filter medium. Since it is composed of incompressible, irregularly shaped particles, a high-porosity layer is formed within itself, unless it is impregnated during operation with foreign compressible materials. Ideally, a uniform layer of precoat should be formed on the surface of the filter medium. However, a nonuniform layer of precoat often occurs due to uneven medium resistance or fluctuations in the feed rate of filter aid suspension. Cracks can form on the precoat layer that will allow suspension particles to penetrate into the medium. To prevent cracking, the filter aid may be applied as a compact layer. On a rotating drum filter, for example, this may be accomplished by applying a low concentration of filter aid (2 percent to 4 percent) at the maximum drum rpm. In other filter systems, maintaining a lowpressure difference during the initial stages of precoating and then gradually increasing it with increasing layer thickness until the start of filtration will help to minimize cake cracking. Also, with some filter aids (such as diatomite or perlite), the addition of small amounts of fibrous material will produce a more compact precoat cake. At low-suspension concentrations (typically 0.01 percent), filter aids serve as a medium under conditions of gradual pore blocking. In this case the amount of precoat is 10 to 25 N/m2 of the medium and its thickness is typically 3 to 10 mm. In such cases, the filter aid chosen should have sufficient pore size to allow suspension particle penetration and retention within the precoat layer. Commonly used filter aids include diatomite, perlite, fS" Filler presses used to de water sludge in industrial wastewater treatments usually suffer severely sub-optimal performance as fine particles blind their cloths after a brief period in service. Even sophisticated synthetic fabrics selected for their cake release properties are subject to blinding, and cake adhesion especially by metal hydroxide sludges or flocked oil and grease, /is a result, high labor costs for manual press cleaning are incurred. In some cases the filter press is bypassed, or sludge with high moisture content is trucked away to a special treatment facility, representing a heightened environmental danger during transport and, again, excessive cost. if-A precoat filteraid applied to the filter septum with clean water before the introduction of the sludge protects the cloth from blinding, and permits long filter cycles. Because the filter chambers can now be completely filled, dewatering performance is greatly enhanced. With the precoat acting as a release agent, the well-dried cakes fall readily with minimal manual cleaning of the press.

fir In some cases, the optimization of an existing press by precoating avoids the necessity of buying more filter capacity as a waste treatment plant expands.

Diatomaceous earth, widely-known and long-used as a filteraid in process and waste filtrations, has a high microcrystalline silica content. As well as being a respiratory hazard in the workplace, the silica is being scrutinized in some jurisdictions as a potentially hazardous dust in landfills in which spent filter cakes are deposited.

cellulose, sawdust, charcoal and flysah, as well as an abundant of commercial additives. The most important filter aids from a volume standpoint are the diatomaceous silica type (90 percent or better silica). These are manufactured from the siliceous fossil remains of tiny marine plants known as diatoms. Diatomaceous filter aids are available in various grades. This is possible because the natural product can be modified by calcining and processing, and because filter aids in different size ranges and size distributions have different properties. The filter aids may be classified on the basis of cake permeability to water and water flow rate. Finer grades are the slower-filtering products; however, they provide better clarification than do faster-filtering grades. Thus a fast-filtering aid may not provide the required clarification. However, by changing the physical character of the impurities (e.g., by proper coagulation), the same clarity may be obtained by using the fast-filtering grades. Calcinated diatomaceous additives are characterized by their high retention ability with relatively low hydraulic resistance. Calcining dramatically affects the physical and chemical properties of diatomite, making it heat resistant and practically insoluble in strong acids. Further information is given in the literature. Diatomaceous earth is a natural occurring siliceous sedimentary mineral compound from microscopic skeletal remains of unicellular algae-like plants called diatoms. These plants have been part of the earth's ecology since prehistoric times. Diatoms are basic to the oceanic cycle, and the food for minute animal life which in turn becomes the food for higher forms of marine life. As living plants, diatoms weave microscopic shells from the silica they extract from the water, then as they die, deposits are formed and then fossilized in what are now dried lake and ocean beds. The material is then mined, ground and screened to various grades, for the countless uses in today's products and processes, from toothpaste to cigars, plastics to paprika, filter media in swimming pools to home fish tanks, as well as insect and parasite control in animals and grains. It is a natural (not calcined or flux calcined) compound with many elements which include:

Silicon Dioxide, Si02 = 83.7 % Aluminum Oxide, A1203 = 5.6 %

Semi quantitive spectrographic analysis of other elements: 2ppm

Copper:

Strontium:

Titanium:

Manganese:

Sodium:

Vanadium:

Boron:

Zirconium:

lOOppm

1800ppm

200ppm

2000ppm

500ppm

50ppm

200ppm

Diatomaceous earth's unique combination of physical properties include:

High Porosity: Up to eighty-five percent of the volume of diatomaceous earth is made up of tiny interconnected pores and voids. It is quite literally more air than diatom.

High Absorption: Diatomaceous earth can generally absorb up to 1 times, its own weight in liquid and still exhibit the properties of dry powder.

Particle Structure/High Surface Area: Diatom particles are characterized by their very irregular shapes, generally spiny structures and pitted surface area. They average only 5 to 20 microns in diameter, yet have a surface area several times greater than any other mineral with the same particle size. Diatomaceous earth increases bulk without adding very much weight. These features, it is believed, are what make it an ideal mineral for internal parasite control in animals: It is approved by the USDA up to 2% by weight of total ration for use as an inert carrier or anti-caking agent in animal feed. It is not necessary to use this percent of product on a continual basis. It may be varied to suit individual purposes.

Grain Storage: A rate of seven pounds per ton of grain in barley, buckwheat, corn, wheat, oats, rice, rye, sorghum and mixtures of these grains. It is most effective when grain is treated directly after harvest by coating the outside surface of the gain. This can be done by applying the powder at the elevator or auger when grain is being moved into storage. When used at proper rates, diatomaceous earth has been effective against ants, aphids, bollworm, salt marsh caterpillar, cockroach, comworm, earwig, house fly, fruit fly, lead perforator, leaf hopper, lygus bug, mite, pink boll weevil, red spider mite, slugs, snail, termites, Japanese beetle (grub stage) and many other insects. Diatomaceous earth is a natural grade diatomite. It requires no warning label on the bag or container. However, the continual breathing of any dust should he absolutely avoided.

Perlite and Solka-floc® are finely divided powders manufactured from a volcanic mineral and from wood pulp respectively, which have filtration properties very similar to those of diatomite. Like diatomite, they are inert to a wide range of process liquids. Like diatomite, they are available in a range of particle-size distributions to give the desired clarity and flowrate in different applications. On a cost-of-use basis, they are as economical as, or more economical than, diatomite.

Although less known than diatomite, these products have been in wide use for many years so that there exists a sound body of applications knowledge upon which to base grade selection, dosage, and procedures. Perlite and Solka-floc® have the same availability in bagged, semi-bulk, or bulk formats as diatomite.

Perlite is glass-like volcanic rock, called volcanic glass, consisting of small particles with cracks that retain 2 percent to 4 percent water and gas. Natural perlite is transformed to a filter aid by heating it to its melting temperature (about 1000°C), where it acquires plastic properties and expands due to the emission of steam and gas. Under these conditions its volume increases by a factor of 20. Beads of the material containing a large number of cells are formed. The processed material is then crushed and classified to provide different grades. The porosity of perlite is 0.85 to 0.9 and its volumetric weight is 500 to 1,000 N/m3. Compared to diatomite, perlite has a smaller specific weight and compatible filter applications typically require 30 percent less additive. Perlite is used for filtering glucose solutions, sugar, pharmaceutical substances, natural oils, petroleum products, industrial waters, and beverages. The principal advantage of perlite over diatomite is its relative purity. There is a danger that diatomite may foul filtering liquids with dissolved salts and colloidal clays. Perlite is not a trade name but a generic term for a naturally occurring siliceous rock. The distinguishing feature that sets perlite apart from other volcanic glasses is that when heated to a suitable point in its softening range, it expands from four to twenty times its original volume. This expansion is due to the presence of two to six percent combined water in the crude perlite rock. Then quickly heated to above 1600°F (871° C), the crude rock pops in a manner similar to popcorn as the combined water vaporizes and creates countless tiny bubbles that account for the amazing light weight and other exceptional physical properties of expanded perlite. The expansion process also creates one of perlite's most distinguishing characteristics: its white color. While the crude rock may range from transparent light gray to glossy black, the color of expanded perlite ranges from snowy white to grayish white. Expanded perlite can be manufactured to weigh as little as 2 pounds per cubic foot (32 kg/m3) making it adaptable for numerous applications. Since perlite is a form of natural glass, it is classified as chemically inert and has a pH of approximately 7. Refer to Tables 4 and 5 for some general properties data.

Table 4. Elemental Analysis of Perlite

Component

Weight Percent

Silicon

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