Filtration




Filtration

Filtration is the process of separating suspended solid matter from a liquid, by causing the latter to pass through the pores of some substance, called a filter. The liquid which has passed through the filter is called the filtrate. The filter may be paper, cloth, cotton-wool, asbestos, slag- or glass-wool, unglazed earthenware, sand, or other porous material.
Filtration is very frequently employed in chemical technology, and it often presents great difficulties. In most technical operations, cotton cloth is the filtering material, but occasionally woolen or hair cloth is necessary. The cloth may be fastened on a wooden frame in such a way that a shallow bag is formed, into which the turbid liquid is poured. The filtrate, in this ease, is cloudy at first, but soon becomes clear, and then the turbid portion is returned to the filter. Filtration is often retarded by the presence of fine, slimy precipitates, or by the formation of crystals in the interstices of the cloth, from the hot solution. Any attempt to hasten filtration, by scraping or stirring the precipitate on the cloth, will always cause the filtrate to run turbid.
A better form is the "bag-filter," which is a long, narrow hag of twilled cotton, supported by an outside cover of coarse, strong netting, capable of sustaining a considerable weight and hydrostatic pressure. These bags are often five or six feet long, and eight inches or more in diameter. The open end of the bag is tied tightly around a metallic ring or a nipple, by which the whole is suspended, and through which the liquor to be filtered is introduced. "When hot liquids are filtered, the bags are often hung in steam-heated rooms, the temperature being nearly that of the liquid.
In pressure filtration, the liquid is forced through the interstices of the filter by direct atmospheric pressure, the air being exhausted from the receiver; or by hydrostatic pressure, obtained either by means of a high column of the liquid, or by a force pump. By the first method, called suction filtration, the liquid may be forced downward through the filter into a receiver; the precipitate collects on the top of the filter and becomes a part of the filtering layer. This sometimes causes difficulty, for the particles of certain precipitates unite to form an impervious layer. Or the filtrate may be drawn upwards through the filter, which is suspended in the liquid to be filtered; thus clogging does not occur so easily, as a large part of the precipitate settles to the bottom of the vessel and does not come in contact with the filter until most of the liquid has been drawn off.
In technical work, pressure is usually obtained by the filter press (Figs. 8 and 8 a). This is a strong iron frame, in which a number of

east-iron or bronze filter cells are supported. Each cell is made up of two flat metal plates with raised edges, separated by a hollow "distance frame" of the same metal. There is a hole in the centre of each plate, and grooves on each surface leading to an opening at the lower edge of the plate. A filter is made of two pieces of cloth, slightly larger than the plates, sewed together along the margin of a small circular opening cut in the centre of each. One piece of the cloth is passed through the hole in the plate, and then both pieces are spread out smoothly, one on either side of the plate.
Another plate is prepared in the same way, and a distance frame having been placed between them, the cell thus formed is set vertically in the press frame, where it is supported by lugs on each plate and distance frame. 'When the desired number of cells are ready, they are tightly clamped together by means of a heavy screw, which passes through one end of the press frame. Thus a series of cells, lined with filter cloth and connected by a straight channel through the central holes, is formed. A powerful force pump drives the liquid to be filtered into the cells, where it passes from one to the other until they are all filled. The hydrostatic pressure forces the liquid through the filters into the grooves in the plates, along which it flows, and escapes through the openings at the lower side of the plate. The sediment retained by the cloth collects ill the cell and forms a solid cake, which finally fills each

cell completely. The process is then stopped, the cells taken apart, and the cake of sediment having been removed, the cells are returned to the press frame, to' be again put into operation. The filtrate is caught in a trough.
In another form of press, instead of the central opening, there is a hole in the cornel' of each plate and distance frame in such a position that, when placed in the press, the holes form a continuous channel through the corner of the whole series of cells. A small hole drilled on the inside of each distance frame, at right angles to the direction of the channel, admits the liquid into each cell. The filter is a piece of cloth hung over the distance frame in such a way that both sides of the frame are covered. A frame so
covered is put between each pair of grooved plates. Small holes are cut in the cloth to correspond to the channel in the corners of the cells. The method of filtration is the same as in the central feed machines.

The pressure obtained by the force pump may be only a few pounds, or it may rise to several hundred pounds per square inch. The filter press may contain from a dozen to fifty or more cells, and these cells may be as large as four feet in diameter. For many purposes the press is surrounded by coils or jackets, through which steam or refrigerating solutions may be circulated,
according as hot or cold filtration is desired. The filter press is very rapid
in its action and is extensively employed in industrial chemical work. For use with acid or corrosive liquids, the plates and distance frames are often covered with lead or some alloy which is not easily corroded.
The centrifugal machine (Fig. 9) is, to a great extent, replacing the filter press and other filters, especially when crystals are to be removed. This furnishes the most rapid method and leaves the substance almost dry. The centrifugal machine is a cylindrical box or basket (A) of wire gauze or perforated sheet metal, fixed 011 a vertical shaft (B), which rotates at a very high speed. The contents of the box are driven to the outer wall by the centrifugal force, the solid matter being retained by the gauze or screen. The liquid passes through and is caught in a fixed shell (C), surrounding the rotating basket. These machines are of various sizes from 12 to 60 inches diameter, and 8 to 36 inches, depth of basket. Two general
forms are in use: the over-driven type, in which the driving pulley (P) is fixed at the upper end of the shaft, above the basket; and the under-driven type, in which the basket is placed on the upper end of the shaft, and the pulley below. In the over-driven type it is frequently customary to suspend the shaft in flexible bearings. Thus the basket is enabled to adjust itself to any change in the centre of gravity, caused by unequal loading, and runs without vibration.
Sand filters are sometimes used for work on a large scale. These
are made as follows; Into a box having a perforated bottom, is put
a layer of coarse gravel; this is covered with finer pebbles; these
by sand, and a jute or canvas cloth covers the whole. A wooden
or iron grating is added to protect the filter, when the sediment is
shoveled out. The filter is often placed over a receptacle from
which the air may be exhausted, thus affording pressure filtration
if necessary.


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