Filter aid

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 rnicrofidtrations to tnaintain high flowrates 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 ctystalline silica grades suit a wide range of process, water and wastewater applications. 

Addition of another powdered solid, termed a filter aid, to the system can produce a significant improvemaat to a filtration operation. The filter aid may be used in two ways, either separately or in conjunction. The first of these methods is to precoat the filter medium with a layer of fiher ail cake. Precoat aids must filter quickfy without bleeding or penetrating throu the cloth and must give a uniform thickness with a reproducible filtering surfiice. The susp ntion is then fihered onto the precoat by surface and depth filtration mechanisms. Thus surface properties are inq ortant in the choice of aid. 

The second approach involves the admixture of the filter aid powder to the suspension. Such material is called body aid . The object is to open iq tire pores of the fiher cake and provide ster filtration. The size distribution may be made coarser. The object of retreating a slurry prior to a s aration process is to change tire properties of the suspension so that a desired inq rovement resuhs to the separation process. It is useful to conrider the parameters that mi t be altered. 

Fflter Aid Materials and their Properties. A successM filter aid must have a high permeability vriiich can be obtained through a combination of particle rize and shape distribution and, in some cases, because fire particle itself is porous, ch makes a significant contribution to the overall permeability. Ihe surfiice properties of the filter aid may be a major factor in the successfiil applications of a filter aid in some processes. Other fiictors such as chemical reristance and compatability widi the product specification are also important. Some of the major filter aid materials that are available commercially are described and discussed below. 

In the brewing process diatomite is used as precoat and body aid on candle filters and horizontal leaf filters. Hermia Brocheton [1993] give a conparison of these methods and cite typical precoat dosage as 1.5 kg/rn and body feed dosage as 1 kg/rn with a cake average specific resistance of lO m/kg. Other aspects of diatomite filtration are discussed comprehensively in a historical review by Cummins [1973]. 

Asbestos is a fibrous mineral with conqilex surface properties which make it particularly successfiil in many applications such as the removal of protein hazes fi om beverages. It was often used in mixtures with other filter aids made to form eets, but contains soluble iuqturities of Fe, Mg and Ca. It is rarely used now because of health hazard problems. 

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Films, anodic oxide Films, passivating Films, plastic Film theory Film wrappers Filter Filter aid Filter aids Filter fabrics Filtering centrifuges Filter media Filters... 

Filtration through an acidic filter aid or siUca removes the last traces of soap from the oil. The finished oil is heated under vacuum to remove small amounts of fatty acid methyl esters. 

Pretreatment of Suspensions. Another important aspect of soHd—Hquid separation is conditioning or pretreatment of the feed suspension to alter some important property of the suspension and improve the performance of a separator that follows. A conditioning effect is obtained using several processes such as coagulation and docculation, addition of inert filter aids, crystalliza tion, freezing, temperature or pH adjustment, thermal treatment, and aging. The first two operations are considered in more detail due to their importance and wide use. 

Addition of Inert Filter Aids. FUtet aids ate rigid, porous, and highly permeable powders added to feed suspensions to extend the appheabUity of surface filtration. Very dilute or very fine and slimy suspensions ate too difficult to filter by cake filtration due to fast pressure build-up and medium blinding addition of filter aids can alleviate such problems. Filter aids can be used in either or both of two modes of operation, ie, to form a precoat which then acts as a filter medium on a coarse support material called a septum, or to be mixed with the feed suspension as body feed to increase the permeabihty of the resulting cake. 

In the precoat mode, filter aids allow filtration of very fine or compressible soHds from suspensions of 5% or lower soHds concentration on a rotary dmm precoat filter. This modification of the rotary dmm vacuum filter uses an advancing knife continuously to skim off the separated soHds and the... 

In the precoat and body feed mode, filter aids allow appHcation of surface filtration to clarification of Hquids, ie, filtration of very dilute suspensions of less than 0.1% by volume, such as those normally treated by deep bed filters or centrifugal clarifiers. Filter aids are used in this mode with pressure filters. A precoat is first formed by passing a suspension of the filter aid through the filter. This is followed by filtration of the feed Hquid, which may have the filter aid mixed with it as body feed in order to improve the permeabiUty of the resulting cake. The proportion of the filter aid to be added as body feed is of the same order as the amount of contaminant soHds in the feed Hquid this limits the appHcation of such systems to low concentrations. Recovery and regeneration of filter aids from the cakes normally is not practiced except in a few very large installations where it might become economical. 

Materials suitable as filter aids include diatomaceous earth, expanded perilitic rock, asbestos, ceUulose, nonactivated carbon, ashes, ground chalk, or mixtures of those materials. The amount of body feed is subject to optimisa tion, and the criterion for the optimisa tion depends on the purpose of the filtration. Maximum yield of filtrate per unit mass of filter aid is probably most common but longest cycle, fastest flow, or maximum utilisation of cake space are other criteria that requite a different rate of body feed addition. The tests to be carried out for such optimisation normally use laboratory or pilot-scale filters, and must include variation of the filtration parameters such as pressure or cake thickness in the optimisation. 

The scraper or knife discharge consists of a blade that removes the cake from the dmm by direct contact with the filter cake. It is normally used for granular materials with cake thickness greater than about 6 mm. In order not to damage the filter cloth, a safety distance of 1 to 3 mm between the blade and the cloth must be observed. If the residual layer is made not of filter aid but of the product, there is danger of its blocking by fine particles and by successive consoHdation by the scraper blade. 

In units designed to use a precoat filter aid, the dmm can be evacuated over the fiiU 360° and fitted with an advancing knife system that continuously shaves off the deposited soHds together with a thin layer of the precoat. The precoat has to be renewed periodically. 

The filter usually has an endless cloth, traveling intermittently between the plates via roUers, to peel off cakes. Unfortunately, if the cloth is damaged anywhere, the whole cloth must be replaced, which is a difficult process. Each time the filter cloth zigzags through the filter, the filtering direction is reversed this tends to keep the cloth clean. Most of these filters incorporate membranes for mechanical expression, and cakes sometimes stick to the membranes and remain in the chamber after discharge. Some vertical filters are available with a separate cloth for each frame. The cloths maybe disposable and such filters are designed to operate with or without filter aids. 

Results of test work with this filter, producing cakes of 1 mm thickness using a 3 mm clearance, have been pubUshed (33,34). The cake formed on the medium was generally stable, giving high filtration rates over long periods of time, and the precoat type cake did not blind with time. There was no evidence of any size selectivity of the process the only exception was conventional filter aids which were preferentially picked up by the rotating fluid. This... 

Clarified lime juice, made by mixing juice with filter aid prior to passing through a filter press, is the one clarified citms juice that is a significant article of commerce. The pasteurized bottled juice is popular for drink mixes, punch bases, and fountain drinks (13). 

Flocculants and surfactants (qv) are used frequently as filter aids, particularly when slimes are present or when the particles to be filtered are very fine and difficult to filter. Low molecular weight polymers are more commonly used. These form small, dense floes which provide higher cake porosity. 

Liquid Dosage Forms. Simple aqueous solutions, symps, elixirs, and tinctures are prepared by dissolution of solutes in the appropriate solvent systems. Adjunct formulation ingredients include certified dyes, flavors, sweeteners, and antimicrobial preservatives. These solutions are filtered under pressure, often using selected filtering aid materials. The products are stored in large tanks, ready for filling into containers. QuaUty control analysis is then performed. 

In diatomaceous-earth filtration, the powdered filter aid is built upon a relatively loose septum to screen out suspended soHds. The filter becomes clogged, and pressure losses become excessive backwashing is then necessary. The smallest removable particle is 0.5—1 p.m (see Diatomite). 

Batch-stirred vessels are most often used in treating material with powdered activated carbon (72). The type of carbon, contact time, and amount of carbon vary with the desired degree of purification. The efficiency of activated carbon may be improved by applying continuous, countercurrent carbon—Hquid flow with multiple stages (Fig. 3). Carbon is separated from the Hquid at each stage by settling or filtration. Filter aids such as diatomaceous earth are sometimes used to improve filtration. 

Alcoholic Solutions or Extracts. AlcohoHc extracts are prepared by dissolving the flavor-beating body ia a solution of alcohol and water. They may require filtration usiag filter aids to remove any iasoluble precipitates or oils that may form. AlcohoHc extracts are clear solutions and are used ia beverages that do not require a haze or cloudiness. 

Oregon production of filter aids is from a deposit near Ontario, Oregon, and of pet Utter from near Christmas Valley. An extensive deposit near Terrebonne, Oregon, was operated from 1936 to 1961. Four different companies operate at least five deposits in Nevada. Of the several comparatively large deposits in Washington, only one, near Quincy, is being operated on a commercial scale (7). 

Property Natural Filter aids Calcined Flux-calcined Fillers, all types... 

There are two principal ways in which finished diatomite products are used in manufacturing plants either as a filter aid (see Filtration), where the diatomite is expendable, or as a filler, where the diatomite becomes a component and remains as part of the manufactured product. As of 1990, the use of diatomite products was 71% filtration, 15% fillers, and 14% other (7). 

Filtration. Diatomite is used as a filter aid for appHcations with difficult-to-filter soflds to improve permeabiUty of the filter cake, to prevent the blinding of filter elements, and where high clarity is required such as in the poHsh filtration of wine (qv) or beer (qv) before bottling. It is also used in sugar (qv) refining, water treatment, and in the production of fmit juices (qv) and industrial chemicals. 

Celite Filter Aids for Maximum Clarity atEowest Cost, Internal Pubhcation No. EA-84, CELITE Corp., Lompoc, Calif., 1984. 

Net consumption of materials should be used for catalysts, solvents, filter aids, etc., that may have a recoveiy value. Current prices of chemicals are pubhshed in various trade journals. However, quotations from suppliers should be used whenever possible. 

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