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. As 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.

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 Jiltrations, 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

33.8

Aluminum

7.2

Potassium

3.5

Sodium

3.4

Iron

0.6

Calcium

0.6

Magnesium

0.2

Trace

0.2

Oxygen (by difference)

47.5

Net Total

97.0

Bound water

3.0

Total

100.0

Table 5.Physical Properties of Perlite.

Property

Characteristic

Color

White

Refractive index

1.5

Free moisture, maximum

0.5 %

pH of water slurry

6.5-8.0

Specific gravity

2.2-2.4

Bulk density, normal

2-15 lb/ft3

Mesh Sizes (normal)

4-40 and finer mesh

Property

Characteristic

Softening Point

1600-2000° F

Fusion Point

2300-2450° F

Specific Heat

0.2BTU/lb-°F

Thermal Conductivity

0.27-0.41 BTU.in/h.ft2 °F

Clay - The use of clay based flocculating agent(s) in conjunction with a strong metal precipitator has proven successful in many wastewater treatment applications where the objectives are aimed at metals removal. Clay based flocculants cleans the wastewater and in some cases replaces multistage conventional treatment system and saves the traditional operational difficulties of treatment with several chemicals such as metal hydroxide precipitation, coagulant, flocculants and other methods. Commercial clay-based flocculants usually consist of bentonite and other proprietary ingredients. These products are in a granulated form to minimize any dust exposure. Betonite mixtures are often used for removing many wastewater constituents including, but not limited to heavy metals, oil and grease, pigments, and phosphates. It encapsulates the metals and other wastewater constituents in a clay barrier, producing an easily disposable waste sludge and treated water. Industrial wastewater from rinses and cleaning operation can be mixed with the flocculant at an average of < 1 % (by weight) dosage rate in a mixing tank. Even though the process of flocculation takes few minutes, a rapid multi-stage complex chemistry starts working. These hidden reaction stages, simulate the

Note that filter aid selection must be based on planned laboratory tests. Guidelines for selection may only be applied in the broadest sense, since there is almost an infinite number of combinations of filter media, filter aids, and suspensions that will produce varying degrees of separation. The hydrodynamics of any filtration process are highly complex; filtration is essentially a multiphase system in which interaction takes place between solids from the suspension, filter aid, and filter medium, and a liquid phase. Experiments are mandatory in most operations not only in proper filter aid selection but in defining the method of application. Some general guidelines can be applied to such studies: the filter aid must have the minimum hydraulic resistance and provide the desired rate of separation; an insufficient amount of filter aid leads to a reduction in filtrate quality — excess amounts result in losses is filtration rate; and it is necessary to account for the method of application and characteristics of filter aids.

phenomena of attraction, coagulation, precipitation, and separation of metals, oil, grease, and pigment due to a strong affinity of muti-layer positively charged of the bentonite crystal structure and other blended additives in each package. Depending on the constituent of the effluent, blended specialty products are formulated such that in addition to metal removal, it has added -value chemistries that have strong tendencies to remove chlorinated solvents as well as oil & grease -all in one chemical package system. After addition of the flocculant, the fully reacted mass is a bonded and complex formula that strongly encapsulates wastewater contaminants. These bonds are generally categorized as weak Van der Waals, as well as strong electrostatic forces. The clay has also tendency to entrap and agglomerate the surrounding suspended solids very efficiently. During this stage, some Pozzolanic reactions also occur, in which, a cementatious particles settles down to bottom of the reaction vessel. It is very interesting that the entire microencapsulation process occurs in few minutes as long as the granulated clays are fed into the system, leaving clear solution for reuse, recycle, or discharge. The solidified and flocculated waste sludge is often classified as non-leachable and in many occasions, depending the waste stream, it may pass the TCLP and STLC tests. If the test passes, it confirms that contaminant is surrounded by a barrier of clay particles and is not accessible to external leaching solutions or processes. Cellulose fiber is applied to cover metallic cloths. The fibers form a highly compressed cake with good permeability for liquids, but a smaller retention ability for solid particles than that of diatomite or perlite. The use of cellulose is recommended only in cakes where its specific properties are required. These properties include a lack of ashes and good resistance to alkalies. The cost of cellulose is higher than those of diatomite and Perlite.

Sawdust may be employed in cases where the suspension particles consist of a valuable product that may be roasted. For example, titanium dioxide is manufactured by calcining a mixture of sawdust and metal titanium acid. The mixture is obtained as a filter cake after separating the corresponding suspension

Charcoal is not only employed in activated form for decoloring and adsorbing dissolved admixtures but also in its unactivated form as a filter aid. It can be used in suspensions consisting of aggressive liquids (e.g., strong acids and alkalies). As with sawdust, it can be used to separate solids that may be roasted. On combustion, the charcoal leaves a residue of roughly 2 percent ash. Particles of charcoal are porous and form cakes of high density but that have a lesser retention ability than

Fly ash has a number of industrial filtering applications but primarily is applied to dewatering sewage sludge. The precoat is built up to 2-in. thick from a 60 percent solid slurry. On untreated sludges, filtration rates of 25 lb/ft2-hr are obtainable. This rate can be doubled with treated sludges. The sludge is reduced from a liquid to a semidry state. Fly ash may also be used as a precoat in the treatment of

Polymeric flocculants are high molecular weight organic chains with ionic or other functional groups incorporated at intervals along the chains. Because these compounds have characteristics of both polymers and electrolytes, they are frequently called poly electrolytes. They may be of natural or synthetic origin. All synthetic polyelectrolytes can be classified on the basis of the type of charge on the polymer chain. Thus, polymers possessing negative charges are called anionic while those carrying positive charges are cationic. Certain compounds carry no electrical charge and are called nonionic polyelectrolytes. Because of the great variety of monomers available as starting material and the additional variety that can be obtained by varying the molecular weight, charge density, and ionizable groups, it is not surprising that a great assortment of polyelectrolytes are available to the wastewater plant operator. Extensive use of any specific polymer as a flocculant is of necessity determined by the size, density, and ionic charge of the colloids to be coagulated. As other factors need to be considered (such as the coagulants used, pH of the system, techniques and equipment for dissolution of the poly electrolyte, and so on), it is mandatory that extensive jar testing be performed to determine the specific polymer that will perform its function most efficiently. These results should be verified by plant-scale testing. Types of polymers vary widely in characteristics. Manufacturers should be consulted for properties, availability, and cost of the polymer being considered. Dry polymer and water must be blended and mixed to obtain a recommended solution for efficient action. Solution concentrations vary from fractions of a percent up. Preparation of the stock solution involves wetting of the dry material and usually an aging period prior to application. Solutions can be very viscous, and close attention should be paid to piping size and length and pump selections. Metered solution is usually diluted just prior to injection to the process to obtain better dispersion at the point of application. Two types of systems are frequently combined to feed polymers. The solution preparation system includes a manual or automatic blending system with the polymer dispensed by hand or by a dry feeder to a wetting jet and then to a mixing-aging tank at a controlled ratio. The aged polymer is transported to a holding tank where metering pumps or rotodip feeders dispense the polymer to the process. It is generally advisable to keep the holding or storage time of polymer solutions to a minimum, one to three days or less, to prevent deterioration of the product. Selection must be made after determination of the polymer; however, type 316 stainless steel or plastics are generally used. The solution preparation system may be an automatic batching system that fills the holding tank with aged polymer as required by level probes.

Ultra High Molecular Weight Flocculants - Water soluble polymers with an average molecular weight of > 106 g/mol are generally considered to be high molecular weight flocculants. In recent years there has been a trend for flocculants to be manufactured with ever increasing molecular weight. This new generation of flocculants are referred to as 'ultra high molecular weight'. The use of flocculants with ultra high molecular weights can lead to stronger floes compared to lower molecular weight alternatives. The floe strength is of particular importance where high degrees of mixing energy prevail at the flocculant dosing point of a given dewatering application. Typically, the process operator has no control over this mixing shear, and consequently the choice of flocculant is paramount for ensuring that effective flocculation and subsequent solid/liquid separation occurs. A further advantage of high molecular weight flocculants is their propensity for dose efficient performance. The nature of high molecular weight flocculants, with long chain lengths, increase the likelihood of effective inter-particle polymer bridges compared to lower molecular weight, shorter-chain-length analogues. This dose efficient performance is desirable as a means of minimizing the cost of chemical pre-treatments that aid solid/liquid separation unit operations.

Sewage Sludge Pre-treatment Flocculants - Sewage sludge must be dewatered to facilitate economic disposal. Dewatered sludge decreases the cost of transportation to landfill, or if the sludge is to be incinerated the removal of water combustion. Ultra high molecular weight cationic flocculants are increasingly used for pre-treatment of sewage sludge prior to dewatering because they are particularly effective in the thickening of surplus activated sludges and centrifuge applications. Centrifuges exert a very large shearing force on floes as they enter the centrifuge bowl. The resilience of floes formed with ultra high molecular weight flocculants, readily explain the usefulness of such flocculants for centrifuge applications. It can be observed that increasing molecular weight has improved the dose-efficiency of the flocculant, making the product more cost-effective.

Red Mud Flocculation - In the Bayer process, crushed bauxite is digested in concentrated sodium hydroxide in order to dissolve and extract aluminium. An insoluble residue known as red mud is produced, which is composed primarily of iron oxides, quartz, sodium aluminosilicates, calcium carbonate, calcium aluminate and titanium dioxide. To recover the aluminium, the solid red mud must be removed from the liquor, This is typically achieved by a series of sedimentation tanks referred to as the 'wash train'. The sediment from the primary sedimentation is washed in order to recover entrained aluminium rich liquor. This washing process is repeated several times. The use of ultra high molecular weight anionic flocculants promote a desirable sedimentation rate at an economic dose, and can lead to a more compact sediment than that obtained using lower molecular weight flocculants. The more compact the sediment, the greater the liquor recovery, and therefore the higher the aluminium recovery.

Drainage and Retention Aids - Paper formation is critical for both aesthetic and practical reasons i.e. visual appearance and paper strength, respectively. Increased mixing, which improves the distribution of fibre and filler within the paper has been highlighted as a means of achieving a superior formation. Drainage and retention aids are added to induce the rapid and efficient release of water and to capture and retain the filler within the paper. Over the last decade, the throughput rate on paper machines has increased significantly, while the length of the paper machine, specifically the drainage area, has decreased. Due to the requirements and changes described above, the use of highly soluble, ultra high molecular weight flocculants as drainage and retention aids has increased. A more robust, shear stable floe is produced which withstands the increased mixing intensities and dewatering forces and leads to a better drainage and retention performance compared to lower molecular weight analogues.

Crosslinked Flocculants - The accepted theory of flocculation dictates the need for a water-soluble polymer, which is as linear as possible. However, controlled levels of polymer crosslinking or branching can provide unexpected benefits to enhance solid/liquid separation. The mechanism by which crosslinked flocculants operate is not well documented. It is possible that the crosslinked polymer lies on the surface of a solid particle and, by virtue of its structure, a proportion of the polymer charge is available to interact with adjacent particles, i.e. the crosslinked polymer cannot fully adsorb all its charge to one particle. One could perceive the charge remains available even after a transient bond has been broken. Thus, flocculation and re-flocculation mechanisms may explain the unique dewatering characteristics engendered by crosslinked flocculants. In recent times, the trend has been for manufacturers to develop commercially available flocculants that contain higher levels of crosslinking.

Sewage Sludge Pre-treatmentFlocculants - Highly crosslinked cationic flocculants have gained popular use in the pre-treatment of sewage sludge prior to dewatering by gravity belt thickening and centrifugation. In the laboratory the maximum filtrate volume (after a drainage time of 5 seconds) increases with the degree of flocculant crosslinking. This rapid rate of filtration observed in the laboratory translates into increased throughput rates when highly crosslinked flocculants are used on full-scale plants. As the degree of flocculant crosslinking increases, so does the flocculant dose requirement to achieve the desirable enhanced performance. A similar increase in dose requirement is necessary in order to gain performance benefits of highly crosslinked flocculants, when used to pretreat sludge prior to centrifugation. Advantages of highly crosslinked flocculants for centrifugation are increased throughput rates, increased percentage solid content of centrifuge cakes and cleaner centrâtes.

Encapsulated Flocculants - A recent innovation in commercially available flocculants is a counter ionic system, where one charged moiety is encapsulated and suspended in the counter charged product. This facilitates a mechanism of delayed release of the encapsulated product.

Coal Slurries - Two component treatments prove to be effective on a commercial scale for coal slurry flocculation. The use of encapsulated flocculant suspended in a counter charged flocculant provides the robustness of traditional dual component systems, but with additional performance advantages, which include reduction in filter cake moisture content and an increased throughput rate. Figure 5 illustrates the typical filter cake moisture content obtained by a conventional treatment system compared to the encapsulated treatment system using coal tailings as the substrate.

Microparticulate Systems - Although dual combination treatment systems comprising microparticles and flocculants have been used in the paper industry since the 1980s, it is only recently that there has been a general trend for paper mills to switch from conventional single component systems to dual systems.

Microparticle/Flocculant - In the past few years the use of microparticles in conjunction with ultra high molecular weight flocculants as drainage and retention aids has grown. The more compact, shear stable floes produced by the cationic flocculant followed by the anionic microparticle give technical improvements in both paper formation and dewatering and retention of fibre and fines within the paper. In general terms, the former is regarded as a more powerful drainage and retention system compared to the latter.

Low Molecular Weight Flocculants - Contrary to the trend to increase molecular weight, which has resulted in the commercial availability of ultra high molecular weight (HMW) flocculants, there are a number of applications where low molecular weight (LMW) flocculants are gaining considerable credibility.

SOME USEFUL FORMULAS WHEN DEALING WITH CHEMICAL ADDITIVES

To calculate the Lbs. of chemical per gal. of solution:

Strength of Chemical (as %) x Specific Gravity x Lbs of Water

To calculate mL per min. of water flow in a treatment plant:

(GPD of water x 3785)/1440

To calculate the make-up requirements in gallons for a rectangular tank: Length of tank (in ft) x Width x Depth x 7.47

To calculate the make-up requirements in gallons for a round tank or clarifier: 3.1417 x R2 x Depth (in ft) x 7.47 Where R is the tank radius in ft.

Fermentation Broth Pre-treatment - Flocculants can be used at several stages of the fermentation product purification process, e.g. cell-broth separation, cell debris flocculation and protein precipitation. Such processes are becoming increasingly more relevant with the commercialization of new products using biotechnology routes. The trend in flocculant pre-treatment of whole cells in complex fermentation media is to use low molecular weight flocculants. Herein low molecular weight flocculants lead to more compact centrifuge cakes. This is advantageous whether the fermentation product to be recovered is extra-cellular, intracellular or the cellular material itself.

If the fermentation product is extracellular, the benefits come from the minimisation of product entrainment into the cake. Alternatively, if the product is intracellular or the cellular material itself, a compact cake minimizes the contamination that would result from entrainment of the fermentation medium in the cake.

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  • klaudia
    What are the filter aids give examples?
    7 months ago

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