FILTRATION. In many industrial processes it is neces sary to separate finely divided solid materials from liquids. The process of filtration consists in passing the liquid through a porous medium, which retains the solid particles. In some cases the re covery of the solid material is the main object; in others the filtration is done for the sake of purifying the liquid. Many types of filter are in use, differing in respect to (I) the various porous materials which are available for filtration, and (2) the construc tion of the apparatus in which these are applied.
Filter Materials.--The following are the main types of ma terial which are in use: I. Sheets of woven or felted material. These comprise porous filter paper, cotton, woollen or linen cloth, felt and woven metal.
2. Unwoven fibrous material, such as cotton wool, linen fibre, cellulose pulp, metal fibres, sponge.
3. Granular or powdered materials, such as gravel, sand, earth, coke, sawdust, cork.
4. Porous plates of stone, porcelain, carbon, silica, etc.
The choice of material depends upon (a) the fineness of the substances to be filtered, (b) the chemical nature of the liquid, (c) convenience in collecting the solid material after filtration.
Laboratory Filtration.—Porous paper is generally used as the filter medium, being resistant to most of the liquids which require filtration. The commonest method of filtration is to fold a circu lar filter paper twice at right angles, and open it out to a cone with three thicknesses of paper at one side and one at the other. This is placed in a conical glass funnel, and the liquid is poured into it. In some cases, the filter funnel is provided with a hot-water jacket to keep the liquid warm during filtration. Filter paper is also manufactured in the form of thimbles, for extractions with solvents, such as alcohol and ether, in special apparatus. The Buchner funnel is made of porcelain and has a flat, perforated bottom on which a circular filter paper is placed. This form of filter has the advantage that the filtration may be assisted by the application of suction below the filter. The Gooch crucible is a porcelain cup with a perforated bottom. A thin layer of asbestos serves as the filter, and the crucible can be weighed, after drying, to ascertain the weight of solid material collected. Filters are now made for laboratory use in which the porous material consists of a fritted mass of hard glass, which has been ground and sieved to a definite degree of fineness. This is made into discs, which are fused into various types of glass apparatus, and are resistant to almost all chemical reagents.
Filtration of Drinking Water.—Domestic filters are usually supplied in the form of earthenware vessels with filter beds of charcoal ; in some cases filters of paper, asbestos or stone are used. It was originally thought that charcoal filters would unfailingly remove micro-organisms, but the action was subsequently found to be less complete than had been believed. Indeed, if the filter material be not removed at intervals, it may become the seat of organic growths. Large-scale filtration of town supplies of water is generally effected in sand filters.
Industrial Filtration.---When possible, it is advantageous to allow the solid precipitate to settle in the liquor for some time, and to run off as much as possible of the clear liquid. The sedi ment may then be stirred up with water, allowed to settle, and the liquid again decanted, so as to minimise the bulk of liquor to be filtered, and to facilitate the subsequent washing. The design of industrial filters varies according to the requirements. The chief considerations are that the filter shall present a maximum available surface without occupying too much factory space, that it shall withstand the required pressures, that it shall not be easily clogged, but shall be readily cleaned and easily controlled at all points. In some cases filters are designed to work continu ously. Simplicity and economy of construction often outweigh all other considerations ; thus for many purposes it is sufficient to filter through a bed of sand, or through a simple sheet of cloth resting on a flat, perforated support. Suspended bags of cloth are also frequently used, and have the advantage that the bag can be squeezed or wrung out after filtration to remove as much liquor as possible from the solid material.
Vacuum and Pressure Filters.—As in laboratory filtration, suc tion is frequently applied to accelerate the flow of liquid. A filter cloth is spread on a perforated plate of earthenware or other ma terial, and the receiver into which the filtrate flows is connected with a vacuum pump. Conversely, pressure may be applied to the surface of the liquid in the filter. In place of cloth filters, porous plates are sometimes used; these may be either flat plates or cylindrical "candles" of porous material, presenting an increased filter area. Leaf filters may be used either for suction or pressure. A typical form of leaf consists of a rectangular frame of perfo rated pipes to which is attached a stiff, corrugated coating of coco nut matting or other material, and the whole enveloped by a cloth bag. The liquid passes from the exterior to the interior of the leaf, and is assisted either by external pressure or by internal suction. A filtering unit consists of a number of parallel leaves, thus pre senting a large filtering surface.
Filter presses are somewhat similar in principle. In one form, recessed plates are supported on a framework, so that they can be firmly pressed together by a screw press. Sheets of cloth are placed between the plates, and when the filter press is as sembled, it forms a series of narrow partitions separated by cloth filters. Channels are provided so that the sludge to be filtered can be forced under pressure between the cloth sheets, and the filtered liquid is collected from taps at the bottom of the plates. After filtration and washing, if necessary, the plates are separated, and the cake of solid material is removed. In place of recessed plates, some filter presses have alternate plates and distance frames.
In centrifuges the filtration is assisted by the rapid rotation of a circular pan. The periphery of the pan is perforated, and covered on its inner side with a filter cloth, supported by a basket of metal. The rapid rotation (up to about i,000 revs. per min.) whizzes the liquid through the filter, with a force several hundred times greater than in gravity filtration.
Continuous Filters.—In the above-mentioned filters, the solid material has to be collected at intervals, and this necessitates in terrupting the filtering operation. Continuous filtration can be effected by rotary filters. A hollow drum is supported in such a way that it can be rotated on a horizontal axis. The periphery of the drum is perforated and is covered with a filter cloth, and the drum dips into the liquid to be filtered. By suction, the liquid is drawn through the filter cloth to the interior of the drum, and the solid remains on the outside. The drum rotates slowly, and a scraper is fixed in such a position that it scrapes off the solid ma terial continuously. In some cases, rotating discs are similarly used. A recent advance in filtration methods is the stream-line filter, which permits of the separation of extremely finely divided solid matter, which would pass through ordinary filter cloths. The liquid is forced through the edge of a pack of sheets of paper pressed tightly together. The channels between the sheets are so minute that the finest precipitates are retained. The filter is made in various forms to permit of continuous filtration, etc.
Coagulation as an aid to Filtration.—The greatest difficulties in filtration are presented by slimy substances, which clog the filter. Application of increased pressure only accentuates the difficulty, and considerable importance attaches to methods whereby the slimy material may be caused to clot together and settle out. This can be effected by the addition of kieselguhr, fuller's earth, clay, talc, silica-gel and various carbonates. Another procedure is to coat the filter with a layer of finely divided kieselguhr, boneblack, sawdust or other finely porous material, which retains the slime and prevents it from penetrating into the filter material.
(R. C. F.)