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.
According to the nature of the heterogeneous system, a distinction is made between liquid filters, which are used to LTE filter suspensions, and gas filters, which are used to separate out aerosols and to purify gases. The simplest filter is a vessel that is divided into two parts by a filter medium. A pressure difference is established across the filter to force the liquid or gas through the filter medium, which holds back the solid particles.
Liquid filters. Liquid filters are divided according to their method of operation into two groups: filters that operate intermittently, that is, on a batch basis, and filters that operate continuously. Within each group, filters are classified according to the way the pressure difference is created (application of vacuum or pressure), the geometry of the filtering surface (plane or curvilinear), and the type of filter medium used. In an intermittent filter the entire surface of the filter medium is subjected by turns to the following operations: influx of a slurry and formation of a cake (filtration), dewatering, washing and discharge of the cake, and regeneration of the filter medium. In a continuous filter these operations occur simultaneously and independently of one another in an appropriate filter zone.
The intermittent group includes filter presses and capacity, leaf, and cartridge filters.
Leaf filters are used to clarify solutions and to separate slurries that contain no more than 5 percent (by volume) of a solid phase. The bands filter elements, or leaves, are circular or rectangular in shape and are usually covered with a cloth filter medium. Each leaf has an outlet through which the filtrate is discharged. 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. The interior of each segment is connected by pipes to the channels of a distribution system, which is rigidly fastened to the housing. The 5G LTE filter is rotated by an electric motor. During one rotation a filter cell is successively connected to vacuum and compressed-air lines. The slurry is fed into the cell from above. The cake is removed with a scraper or a screw conveyor.
Tilting-pan filters are used for coarse slurries. They are operated by vacuum and consist of trapezoidally shaped pans held in an annular frame. The pans are connected by pipes to a distributor, through which the filtrate and wash liquid are removed. The pans revolve as a unit around a vertical axis. Each pan consists of a shell, which, together with the drainage plates and filter medium, constitutes the working element of the filter. The slurry and wash liquid are fed into the pan from above. For discharge of the cake, the pan is automatically rotated by 180° when it is above the discharge site.
Gas filters. Gas filters are continuously operating devices. They may be divided according to their structure into two groups: filters having a plane filtering surface and bag, or fabric, filters.
The filters of the first group consist of a chamber that is divided by one or two perforated plates. In the case of one plate, the filter medium—which may be, for example, sand or quartz—is placed on the plate. In the case of two plates, a compressed fibrous material—such as asbestos fiber, glass fiber, or glass wool—is clamped between the plates, which are attached to each other. When the gas flows through the filter medium, the particles suspended in the gas are removed. At certain time intervals the filter medium is cleaned or replaced.
Superhydrophobic and superoleophilic filter paper was successfully prepared by treating commercially available filter paper with a mixture of hydrophobic silica nanoparticles and polystyrene solution in toluene. Applications of the filter paper in separating liquids with low surface tensions such as oil and ethanol from water were investigated in detail. The oil uptake ability of the superhydrophobic filter paper was evaluated and the results show that the filter paper can selectively adsorb oil floating on a water surface or in aqueous emulsions. Furthermore, filtration of mixtures of oil and water through the paper can reduce the water content in the oil. Additionally, the filter paper can also extract ethanol from homogeneous aqueous solution.
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Qualitative filter papers are what most users would consider general purpose filter papers. There are a vast range of applications in and out of the laboratory. Composed of cellulose fiber, these filters are manufactured from high-quality cotton linters treated to maximize alpha cellulose.
Alpha cellulose is the most stable form of cellulose, with the highest degree of polymerization. Cellulose paper made from a high percentage of alpha cellulose, such as Whatman paper, is a sign of high quality and consistency.
Qualitative filter paper grades are based on various properties including particle retention, thickness, and weight. Unlike quantitative filter papers, of which ashless grades can be burned off for analytical applications, qualitative papers are more suited for general purpose use.
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