Use of Filter Aids

When the cycle consists of filtration, washing and dewatering, the tests are considered principally in the same manner. The economic aspects of cloth selection should be considered after complete determination of cloth characteristics. 

Filter aids and/or 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, inconpressible. The most common filter aids applied as an admix to the suspension, are 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 excessive. Further, it facilitates the removal of 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 conpressibility, 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 fim. 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. 

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Use of filter aids is a technique frequently applied for filtrations in which problems of slow filtration rate, rapid medium blinding, or un-satisfactoiy filtrate clarity arise. Filter aids are granular or fibrous solids capable of forming a highly permeable filter cake in which veiy fine solids or slimy, deformable floes may be trapped. Application of filter aids may allow the use of a much more permeable filter medium than the clarification would require to produce filtrate of the same quahty by depth filtration. 

Continuous Cake Filters Continuous cake filters are apphcable when cake formation is fairly rapid, as in situations in which slurry flow is greater than about 5 L/min (1 to 2 gal/min), shiny concentration is greater than 1 percent, and particles are greater than 0.5 [Lm in diameter. Liquid viscosity below 0.1 Pa s (100 cP) is usually required for maintaining rapid liquid flow through the cake. Some designs of continuous filters can compromise some of these guidelines by sacrificial use of filter aid when the cake is not the desired product. 

Sedimentation tests are of value particularly for rapid evaluation of the effects of aging, flocculants, vibration, and any other variables that conceivably could affect a rate of filtration. The results may suggest what kinds of equipment to exclude from further consideration and what kind is likely to be worth investigating. For instance, if sedimentation is very rapid, vertical leaves are excluded, and top feed drums or horizontal belts are indicated or it may be indicated that the slurry should be preconcentrated in a thickener before going to filtration. If the settling is very slow, the use of filter aids may be required, etc. Figure 11.1 illustrates typical sedimentation behavior. Figure 11.2 summarizes an experimental routine. 

Filtration. Filtration can include filter presses, rotary drum vacuum filters (RDVF), belt filters, and variations on synthetic membrane filtration equipment, such as filter cartridges, pancake filters, or plate and frame filter presses. These processes typically operate in a batch mode when the filter chamber is filled up or the vacuum drum cake is exhausted, a new batch must be started. This type of filtration is also called dead-end filtration because the only fluid flow is through the membrane itself. Due to the small size of cells and their compressible nature, typical cell cakes have low permeability and filter aids, such as diatomaceous earths, perlite, or other mined materials are added to overcome this limitation. Moreover, the presence of high solids and viscous polymeric fermentation byproducts can limit filtration fluxes without the use of filter aids. 

Vinegar clarification is accomplished by filtration, usually with the use of filter aids such as diatomaceous earth or bentonite. After clarification, vinegar is bottled, sealed tightly,... 

Choosing the filter medium to be used in a pressure filter is a very important consideration. The medium should be selected primarily for its ability to retain the contaminant to be separated, and to have an acceptable life in the filter environment. Filter media are manufactured from cotton, synthetic polymers, glass, cellulose, metal, carbon, refractories, and other porous or perforated solids, and sand and other particulate solids. The filter s effectiveness may be enhanced by the use of filter aids. 

Filtration Equipment Good filtration equipment and proper operation are essential if the filtration process is to be effective and economical. Modem-day bleaching earths should not, except in rare cases, require the use of filter aids such as diatomaceous earth. Rather, the clay is used as the precoat. According to Veldkamp (123), the two main objectives in precoat filtration are as follows ... 

Briefly, the use of membrane filtration by breweries has been slowed down by the difficulties in filtering this very special beverage made from natural ingredients. To overcome these problems intensive research and resources have been spent, mostly because the market is promising. The bottom line is that membrane filtration produces a high-quahty filtered product without the use of filter aids that need to be disposed of, membrane processes are easy to automate, and their operation is more continuous-like as compared to the filter-aid methods. 

It is axiomatic in the use of filter aids that the ability of the filter aid to remove small particles of suspended matter decreases as the particle size, and thus the flow rate, increases. Conversely, as filter aid particle size, and therefore the flow rate, decreases, the ability of the filter aid to remove small particles of suspended matter increases. The extent to which this takes place will depend very much on the type and particle size distribution of the undissolved solids being removed. 

For amorphous materials, sludges or other poor filtering products, improved filtration characteristics and/or filtrate clarity are enhanced with the use of filter aids. Slurry additives such as diatomaceous silica or perlite (pulverized rock), are employed to aid filtration. Diatomite is a sedimentary rock containing skeletons of unicellular plant organisms (diatoms). These... 

In identifying the filtration requirements, it is recognised that, in some cases, the filter fabric may require additional assistance, for example, by way of filter aids, body feeds, or even filter papers. The use of filter aids, of which there are many types, is designed to precoat the fabric with a layer of powder, such as diatomaceous earth. This is carried out in order (1) to protect it from blinding (2) to assist in the collection of particularly fine particles or (3) to enable more efficient cake release. In special circumstances, filter papers may also be used for similar reasons, especially where absolute clarity is essential. Body feeds, on the other hand, are added to the slurry to be filtered in order to enable the formation of a more porons cake than would otherwise be the case, thereby enhancing the rate of filtration flow. 

Fully automatic operation is essential to take complete advantage of this mode of cake discharge. Design changes involving a reduction in vessel size, even for dry cake discharge, reduced overall costs. These units are also usefiil in liquor clarifications invoking the use of filter aids, as a pre-coat and/or as a body feed. 

The water treatment process, which may not be necessary in some plants and not connected with the chlor-alkali operation in others, may remove suspended solids, dissolved solids (primarily hardness elements), color bodies, and other oiganics. Suspended solids may be removed directly by sedimentation or with the aid of coagulants. Dissolved impurities are usually removed by precipitation with chemical treating agents. The solids generated are removed by sedimentation and filtration, often with the use of filter aids. All these additives increase the volume of waste. Control measures are quite similar to those used with similar process wastes. 

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