Filter aids perlite

Another method, which is even more successful in preventing binding of the septum, is the use of a precoat.53 Before filtration is begun a coating of 2-6 in (5-15 cm) of diatomaceous earth or perlite filter aid is deposited on the surface of the septum. During filtration operations the scraper is set so that it slowly removes the precoat and, of course, with it the materials that would have plugged the filter. Since the precoat causes a considerable pressure drop, the rate of filtration is slowed down. Flow rates may vary from 2 to 50 gal/hr/ft2 (0.025-0.60 m/hr). The precoat material costs around 3 or 40/lb and is used at the rate of 10- 15 lb/1,000 gal of feed (1,200 to 1,800 kg/m3). 

An example involving the addition of a perlite filter aid to a clay suspension with a volumetric concentration of q)j = 0.058 wUl be discussed. Constant rate filtration (Camacho 1975) with different fractions of perlite was used to obtain cake resistance and cake solidosity as functions of Ap. The specific problem to be analyzed is a constant pressure filtration at 200 kPa (29 psi) of this clay suspension. For constant pressure and constant rate filtration with minimal medium resistance, maximum cycle flux is reached when the filtration time equals the time required to dump the cake, clean the filter, and start the next cycle (Rushton et al. 1996). hi this example, it is assumed that the dead time between cycles is 15 minutes. With t (filtration) = 15 minutes, a cake thickness of L = 0.014 m results when the cycle rate is maximized for this highly resistant, moderately compactible clay. 

Table 5.4 Particle size analysis of e qtanded perlite filter aids wt% [KalaJfiltoghi, 1994] ...

Milled perlite (filter aid) - Some expanded perlite is milled to -100 mesh and sold as a filteraid, especially for use in rotary precoat filtration. Careful milling is required, because oversize particles will inhibit the formation of filtercake while excessive fines will slow the filtration rate. 

Note Bulk density of Perlite filter aid is one half that of DE. When Perlite is used, the above guidelines should be adjusted on an equivalent volume. 

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. 

Addition of filter aids (diatomaceous earths or perlites) to increase the porosity and to reduce the compressibility of cakes... 

Then steam was applied and the temperature was raised to boiling. 200 pounds of coarse filter aid (perlite) was added and the mixture filtered through a string discharge vacuum filter. The cake was washed with 200 gallons of hot water on the filter. 

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. 

PRECOAT FILTRATION - DIATOMACEOUS EARTH FILTRATION. The mOSt economical method of filtering fruit juices with a high suspended solids content is with precoat filters. In these, the filter sheet is created by precoating a liquid-permeable filtering element with the filter aid. In addition to diatomaceous earth, perlite and cellulose are also used as filter aids in fruit juice production. 

Refined filter aids (diatomaceous panded perlite) earth ex- 80-90... 

Filtration—After an adsorbent has selectively captured the impurities, it must be removed from the oil before it becomes a catalyst for color development or other undesirable reactions. Filtration, the separation method most often used for spent bleaching media removal, is the process of passing a fluid through a permeable filter material to separate particles from the fluid. Examples of the filtration materials used are filter paper, filter cloth, filter screen, and membranes. Filter aid, such as diatomite, perlite, or cellulose, are usually used in conjunction with the permeable filters for surface protection. Traditionally, either plate and frame or pressure-leaf filters have been used for spent bleaching media removal. Currently, self-cleaning, closed filters that operate on an automated cycle are available. 

Cold filtration may be performed before or after deodorization. Oil from the bleacher or the deodorizer is cooled to 12-15°C, and after settling for 12 hours, it is cold-filtrated with the addition of filter aid (perlite or diatoma) to prevent clogging caused by the wax. 

Dicalite. TM for a group of products made from either diatomite or perlite, used in filters and filter-aids. 

It is beyond the scope of this entry to review the basic principles governing filtration. However, it is interesting to note that filtration produces a more concentrated and dewatered cell sludge (20-35% w/v) or cell solids (>40% w/v) than settling. A variety of filter media, membranes, and equipment are commercially available. In the case where the deposited cake is compressible with low permeability and thereby adds more resistance to filtration, filter aids or precoats often alleviate the problem. Two of the most widely used filter aids are diatomaceous earth and perlite. 

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... 

For particle diameter <2 cm and >5 pm and solid concenttation 1 to 50%, consider settlers, filters, or centrifuges. For particle diameter <300 pm and solid feed concentration 0.01 to 20%, consider thickeners. Section 16.11.5.9. For particle diameter >20 pm and solid feed concentration greater than 50%, consider dryers. Section 16.11.5.5. For particle diameter 0.01 to 150 pm, consider deep bed filter. Section 16.11.5.13, or dissolved air flotation. Section 16.11.5.15. For particle diameter 0.6 to 40 pm and solids concenttation <0.1%, consider homogeneous separation via ultraflltration, Section 16.11.4.22. For particle diameters from 0.8 to 20 pm, consider using a filter aid to precoat on the filter medium. For example, use diatomous earth or perlite. A fine filter... 

Commonly used filter aids include diatomaceous earth, perlite, asbestos, cellulose, agriculture fibers, etc. 

Cellulose is widely used for filtration of products that cannot tolerate silica. The filterability of cellulose is not as good as DE or perlite, but cellulose can be incinerated. Calcined rice hull ash and fibers from used newspapers are newly available filter aids. They are used for wastewater sludge dewatering as well as other applications. 

Calculations involved in the analysis include (1) concentration of slurry after addition of perlite (Pj (2) time required to produce a cake of 0.014 m t (3) number of cycles with filter aid addition / (volume fraction of filter aids in the total cake solids) to get the same amount of product solids without filter aid addition when/= 0 (4) volume/unit area of filtrate produced in 15 min (5) the cycle rate based on liquid and (6) cycle rate based on product clay solids. The calculated results are shown in Table 22.15. 

Column 6 contains the filtrate volume for time of 15 min. These values are frequently used to obtain the optimum amount of filter aid. They indicate that the maximum flow rate results from adding perlite when/= 0.33. However, the cycle rates in colnmns 8 and 9 show that the optimum dose of filter aid is/= 0.2. The example clearly shows that the choice of a filter aid and the value of / should be based on a cycle analysis and not on v versns t data. 

FILTER AIDS. Slimy or very fine solids that form a dense, impermeable cake quickly plug any filter medium that k fine enough to retain them. Practical filtration of such materials requires that the porosity of the cake be increased to permit passage of the liquor at a reasonable rate. This is done by adding a filter aid, such as diatomaceous silica, perlite, purified wood cellulose, or other inert porous solid, to the slurry before filtration. The filter aid may subsequently be separated from the filter cake by dissolving away the solids or by burning out the filter aid. If the solids have no value, they and the filter aid are discarded together. 

Dead-ended filtration of fermentation broth is complicated by the problem of low porosity and compressibility of the accumulated solids, which results in gradually decreasing permeability during the filtration cycle. With compressible solids, an increase in the differential pressure across the membrane can actually lead to a reduced permeation rate. This problem can be reduced by the use of fitter aids added to the broth and onto the filter paper as a precoat. The two most widely used filter aids are the diatomaceous earths and the perlites. 

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perlite, particles of "puffed lava that are principally aluminum alkali silicate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination of carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

Furthermore, perlite has been reported to be a suitable support for titanium dioxide nanomaterials [516-520]. Basically an amorphous alumina silicate (see Table 13), Perlite is an industrial mineral and a commercial product, useful for its light weight after processing. Due to its low density and relatively low price, many commercial applications for perlite have been developed including construction and manufacturing fields, horticultural aggregates, filter aid and fillers [519]. 

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