Attività

A device that transmits part of a received signal and rejects the rest. The signal may be in the form of a beam of light or other radiation or may be an electrical signal. Optical, ultraviolet, and infrared filters are dyed plastic or gelatin, glass or glasslike substances, or confined liquids, all of which absorb incident radiation except for a relatively narrow band of wavelengths. Such filters are used in astronomical photometry, especially in measurements of magnitudes; transmitting bands typically 100 nanometers wide, they are termed broadband filters. Much narrower wavelength bands, of maybe 1 nm, can be obtained with interference and birefringent filters.

Electrical filters are devices whose attenuation varies with frequency. Filters that allow low or high frequencies to pass without serious attenuation are called low-pass and high-pass filters, respectively. A filter that allows only a limited range of frequencies through is a band-pass filter while its converse is a band-stop filter. See also bandwidth.

The term “filter” is also applied to a number of other devices, which are not discussed in the present article. For example, devices that pass or reject acoustic or electromagnetic waves of certain frequencies are called filters, and the term is sometimes applied to devices that separate polymer ions into fractions or remove salts from solutions by means of ion exchangers.

The slurry is fed into the filter tank. The layer of cake is washed after all the slurry has been removed from the tank.

Filter presses are used chiefly for fine slurries. They include plate-and-frame presses, chamber presses, and automatic filter presses.

A plate-and-frame filter press consists of an assembly of alternating vertical plates and frames. Compression of the assembly is accomplished by a hand screw or a hydraulic or electromechanical closing device. The frames form hollow chambers, into which the slurry is fed when the filter is in operation. The filter medium is laid over each plate, whose faces are ribbed to provide drainage. Under pressure, the filtrate passes through the filter media, runs down the ribbed surface of the plates, and passes through the filtrate outlets to an open launder or a closed channel. The cake formed in the chambers is removed by separating the plates.

The chamber filter press operates in a manner similar to that of the plate-and-frame filter press but permits higher pressures.

In automatic chamber filter presses the filter plates are located between two supporting plates and are positioned horizontally at a certain distance from one another. The top of each filter plate is covered with a perforated sheet, over which an endless-belt filter medium is located. When the plates are compressed, chambers are formed between them. The slurry, wash liquid, and compressed air (for drying) are successively fed into the chambers from the corresponding channels. The filtrate passes through the filter medium, and the solid phase remains on it in the form of a cake. Upon completion of the filtration cycle, the plates are separated, a gap opens up between them, and the filter medium is set in motion. The cake is carried out from between the plates and is removed with scrapers. The filter operates automatically and is four to ten times more efficient than the plate-and-frame filter press.

The principal application of the rotating-pan filter (Figure 4) is the dewatering of coarse slurries in, for example, the production of potassium and the preparation of hard coal and ores. The device is a vacuum filter. The filter surface is annular in shape and is divided into trapezoidal segments, each of which constitutes a filter cell. The upper part of each cell is open, and the bottom is inclined toward the center to facilitate the flow of liquid. The cell is covered by a perforated sheet, on which the filter medium is placed.