Rigidity, filter media

Rigid Filter Media Fixed Rigid Media... 

The powders or granules may be formed into a suspension and filtered as a precoat onto a more conventional rigid filter medium to protect the supporting medium from becoming fouled by the solid matter in the feed... 

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. 

The filter medium can be fibrous, such as cloth granular, such as sand a rigid solid, such as a screen or a mat, such as a felt pad. It can be in the shape of a tube, sheet, bed, fluidized bed, or any other desired form. The material can be natural or man-made fibers, granules, cloth, felt, paper, metal, ceramic, glass, or plastic. It is not surprising that filters are manufactured in an infinite variety of types, sizes, shapes, and materials. 

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. 

The mutual effects of the operating variables can be seen in Equation 58.1. When the cake is composed of hard granular particles that make it rigid and incompressible, an increase in pressure results in no deformation of the particles or their interstices, i.e., n = 0. If filter medium resistance is neglected. Equation 58.1 becomes... 

It has been stated that a filter medium is a porous (or at the very least semi-permeable) barrier placed across the flow of a suspension to hold back some or all of the suspended material. If this barrier were to be very thin compared with the diameter of the smallest particle to be filtered (and perforated with even sized holes), then all the filtration would take place on the upstream surface of the medium. Any particle smaller than the pore diameter would be swept through the pores, and any particle larger than that (assuming the particles to be rigid) would remain on the upstream surface. Some of the larger particles, however, would be of a size to settle into the individual pores and block them. The medium surface would gradually fill with pores blocked in this way, until the fluid flow reduced to below an acceptable level. At this point filtration would be stopped and the medium surface would be brushed or scraped clean (although many automatic filters have their surface continuously brushed or scraped). 

Fabrics make up the largest component of filter media materials. They are made from fibres or filaments of natural or synthetic materials, and are characterized by being relatively soft or floppy, lacking the rigidity of dry paper, such that they would normally need some kind of support before they can be used as a filter medium. 

Very high filtration efficiencies are possible with filter pockets, depending on the filter medium used, e.g. up to 99.8% efficiency with DIN 24 185 test dust. Performance also depends on the pockets remaining rigid and inflated , even at reduced or zero air flow. Most pockets are self-supporting by their very construction, particularly if they are mounted vertically others require hooks, loops or wire mesh for support. 

For a given slurry, the maximum filtration rate is determined by the minimum cake thickness which can be removed—the thinner the cake, the less the flow resistance and the higher the rate. The minimum thickness is about 6 mm (0.25 in) for relatively rigid or cohesive cakes of materials such as mineral concentrates or coarse precipitates like gypsum or calcium citrate. Sohds that form friable cakes composed of less cohesive materials such as salts or coal will usually require a cake thickness of 13 mm (0.5 in) or more. Filter cakes composed of fine precipitates such as pigments and magnesium hydroxide, which often produce cakes that crack or adhere to the medium, usually need a thickness of at least 10 mm (0.38 in). 

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