Perlite permeability

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. 

Foamed cement slurries have been used to provide a low density cement slurry to reduce permeability damage to highly sensitive formations through reduced fluid loss (29). Glass microspheres have also been used to substantially reduce cement slurry density (30, 31). Other additives which reduce cement slurry density to a lesser extent include bentonite, fly ash, silicates, perlite, gilsonite, diatomaceous earth, and oil emulsions (see citations in reference 29). 

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. 

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. 

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. 

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. 

Filtration through cellulose sheets or lenticular modules. These are permeable boards consisting of cellulose fibers with incorporated granular components (diatomaceous earth, perlite, cation resins, polyethylene fibers, etc.)... 

Deep flat-sheet Alters consist of permeable boards made of plant cellulose fibers combined with granular compounds, such as diatomaceous earth or perlite, and possibly cation resins to increase the electrical charge. 

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 (hatomite 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 Wgher than those of diatomite and perlite. 

On the basis of permeability data the flow through perlite beds is similar to flow through diatomite beds, as indicated in Table 2.20. However, the equivalence of flow rate does not correspond to similar performance in respect of clarification achieved, where diatomite is Ukely to be superior if maximum clarity is required, as inferred in Table 2.21. 

Filter aids can improve the permeability and sometimes porosity of a filter cake, improve filtrate clarity and help to prevent filter medium blinding. They comprise relatively porous particles such as diatomite, perlite and activated carbon and are either filtered as a precoat onto the medium or mixed as body feed with the suspension during a pretreatment stage the latter beneficially improves the porosity of a subsequently formed filter cake. Both the cost of filter aid and the need to remove filter aid from the processed solids can present problems however, the use of filter aids on rotary drum filters and in the filtration of dilute feeds (such as those found in the brewing industries) can bring undoubted benefits. More detail on filter aids is provided in Chapter 2. 

Cellulosie fiber (ground wool pulp) is applied to eover metallie eloths. This filter aid forms a much more compressible cake with good permeability but displays a smaller particle retentivity than diatomite or perlite. The cost of cellulose is higher than that of diatomite or perlite. Thus this and other filter aids are only applied to special cases (such as precoat stabilization or chemical resistance). 

A variety of flake or di-, tri-, and tetrahedral particle shapes are Obtained by this exclusive process. These unique shapes provide numrous functions including high surface area, permeability and reinforcement and have densities among the lowest of any of the mineral fillers. In addition to superior spatial loading, this property affords customers the added bonus of density control in their finished products. Oicalite Perlite fillers are off-white and do not interfere with the coloring of the products into which they are formulated. 

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