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Asbestos based filters & their manufacture, applications, & identification: this article describes the use of asbestos in the produduction of a very wide range of filtering devices and materials. Asbestos-based filters were very widely used in many industries including the production of wine, in medicine, even in heart surgery and sewage treatment for the filtration of very fine materials from liquids and gases. Both chrysotile and amphibole asbestos fibers were used in fiber and powder form for filtration processes.
Page top photo: a cutaway view of an asbestos-based filter used for filtering water, fuel and industrial oils. Arrows show the direction of liquid flow. This articles series about the manufacture & use of asbestos-containing products includes detailed information on the production methods, asbestos content, and the identity and use of asbestos-containing materials.
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Introduction to Asbestos Filters
The following text is Adapted from Rosato (1959) p. 185-193  © 2013 InspectApedia.com
Long asbestos fibers as well as short asbestos fibers are used for filtering requirements. Chrysotile fibers are generally used but where more resistance to acids is required, amphibole varieties are sometimes employed. Chrysotile has been used for filtering wines, fruit juices, beers, pharmaceuticals, sugar, blood plasma, and other liquids or gases. For these applications, asbestos is generally recleaned, acid washed, or purified to eliminate any soluble material that might be associated with the fiber. In some cases, the asbestos is used in conjunction with other fibers or fillers such as cellulose and diatomaceous earth.
The surface per unit weight of asbestos is ten times as great as that of organic and inorganic fibers. Because of this characteristic, filtering through asbestos makes it possible to retain much finer precipitates.
A quality which asbestos fibers possess when made into felt or cloth is their attraction for sludge and dust. This characteristic has been demonstrated in fur cleaning establishments. The cleaning rooms for furs have been lined with asbestos curtains or asbestos felt. The rough surfaces catch and retain lint better than any other known agent.
The same factor applies to linings of rooms where seeds are treated or fumigated. Asbestos felt is used to accumulate dust and foreign matter. Some sewage treatments use asbestos to catch the sludge. It has been reported that when asbestos is suspended in water, it produces a large surface to which microorganisms within the water can attach themselves.
In the chemical industry asbestos is used for filtering acids, leaking solutions and other corrosive and caustic fluids. It is also applicable in the sugar mills and refineries for filtering juices. In the electrochemical industry, the need exists for asbestos in diaphragms in electrolytic processes.
Asbestos powder has been used in new surgical heart aid programs to prevent serious heart damage. A report to the American Medical Association lists the use of dusting asbestos powder over the surface of the heart to stimulate new channels of blood fl ow. *
All wine-producing countries use asbestos for clarifying fluids principally in order to improve their appearance. Asbestos loose-filtering beds allow as much clear wine to pass through in the course of half a day as the old cellulose filters did in a week.
Other general applications of asbestos filters include their use in the filtration of bacteria and penicillin, the isolation of the first virus, in cigarette filters, and in cigarette tobacco to aid in the retention of the ash.
Type of Asbestos-Based Filters
A great variety of filters have been developed for the chemical and metallurgical industries. These filters vary in construction; they are made of wire, fabric cloth, paper, and loose asbestos. The types of equipment which incorporate asbestos for specific applications include gravity filtration,
Coronary Artery Disease," J of the Am Med Assoc, No. 18, 162, 1603- 1606 (1958).
filter presses, leaf pressure filters, cloth filters, loose fiber, and paper filters.
Asbestos filter discs are popular and generally consist of a dozen layers of filter fibers. The fibers on the top or rough side of the disc are relatively long and loosely matted. Each succeeding layer contains shorter and more closely packed fibers. Thus, the top side loose layers catch the coarse particles of foreign matter in a liquid passing through the disc.
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Each additional layer stops finer particles as they penetrate into the disc so that the final result is a solution completely clarified and free of all foreign particles.
A cutaway view of a preformed "Disc-Pak" cartridge element is shown in Figure 11.1. The filtering elements used in this filter are made up primarily of asbestos and cellulose fibers. The liquid to be filtered fills the entire "Disc-Pak" filter cylinder from the top, bottom and sides. The liquid filters through the asbestos base discs. The filtered liquid leaves the discs and passes through the common center outlet.
The gravity asbestos-filtration type of filter is the simplest of all filters. It is the type in which the liquid is directed to flow through the filtering medium under the action of gravity. Another basically simple filtering medium employs the use of asbestos bags for dust collection. This filter is used in highly specialized fields which involve natural dust, gas, temperatures, moisture and acid conditions. Asbestos dust collecting bags are generally used where high temperatures and fire hazards exist.
Asbestos-coated filter presses, filters, and suction apparatus are employed to filter bulk quantities of acids, lyes and other corrosive fluids. The coating consists primarily of asbestos fabric; it varies in its properties with the character of' the material to be filtered. The long fiber high grade asbestos serves to filter liquids which would destroy any other filtering agent such as concentrated hot acids and alkalies. The filtering funnel is lined with long fiber, soft asbestos, and frequently pretreated with acid.
Air Filtration Using Asbestos
Filtration of fine particles from air is of great importance inasmuch as the suspended particles in the air affect human breathing. Most airborne matter is in the size range of 0.2 to 1 u. A micron is 1/1000 mm or 0.00004 in. Nature has provided man with screening and absorbent surfaces in the nasal passages, but small particles may be carried deep into the lungs and there retained.
In industrial areas, aerosols are discharged into the air in the form Qf smoke and fumes. Where the terrain and meteorological conditions hinder the dispersion of the finely divided matter, aggravated conditions may occur such as the smog in the Los Angeles area. While the attack on this problem must emphasize the reduction in the discharge of finely divided matter into the air, filters have their place in protecting individuals and in purifying the air drawn into buildings by fans.
Toxic substances may be used in war to produce aerosols having a lethal effect. Various types of aerosol clouds, having a sufficient concentration of lethal material to cause casualties, can be formed over considerable areas. Therefore, a layer of filter material to screen out particulate matter is an important component of a gas mask. The advent of the Atomic Age has sharply emphasized the need for highly efficient filters in order to prevent the pollution of the air by radioactive particles. The potentialities for damage by these particles are distinctly greater than the contaminants commonly encountered in the past.
There are many types of air washers, electric precipitators, and wet and dry filters that will remove a high percentage of the total dust load in the air. They show a high efficiency for separation of particles from a micron and larger, and occasionally for smaller particles of high density. On an over-all basis they may show impressively high efficiency in tests on dusts of mixed sizes. However, they are quite ineffective for removing particles in the size range from 1 u diameter to 0.2 to 0.1 u, as determined by the actual particle count before and after the filtering.
Asbestos fibers play an important role in making these efficient filters available. The particulate matter that occurs in ordinary air and that is not removed by conventional air cleaning apparatus may be either liquid or solid. The con- centration of such particles, in the size range from 0.2 to 1 u is of the order of 5 million to 30 million per Cu ft.
In 1941 it was shown that a mixture of cotton fibers and asbestos fibers, when dispersed in water, could produce a small handsheet of filter paper having superior properties. Using commercial papermaking equipment, attempts were then made to produce similar paper in quantity. These attempts were unsuccessful, because the existing equipment did not give adequate dispersion of the fine asbestos fibers essential to the formation of a highly retentive sheet.
A fundamental difficulty is that all mechanically strong papers have high resistance to the passage of air. The usual paper-making process involves hydration and swelling of the cellulose fiber and a subsequent cementing together of the fibers to form the sheet. This process imparts strength, but it is very detrimental to porosity qualities.
To obtain even moderate strength in a sheet that offers low resistance to the passage of air, requires a special formulation and most careful handling in the paper mill. In addition, cellulose fibers are usually 10 to 30 u in diameter; therefore, they can serve only as the lattice or framework on which to suspend the finer fibers essential for the removal of small particles.
The investigations leading to a successful paper have been carried on for a number of years, and have included laboratory study and many paper mill runs. Fibers of many different types were tested, singly and in combination, and with the addition of resins, binders, and surface-active agents.
An important step towards a solution of the problem was the discovery that caustic treatment of a domestic kraft pulp would wrinkle the fibers so that they would interlock to give a paper having adequate strength and an open structure. This material could then replace the imported cellulose fiber that had been employed in Chemical Corps' paper filter products.
Of major importance was the discovery that certain types of asbestos, when combined with cellulose, would yield a sheet meeting all basic requirements. These types are known as blue Bolivian and blue African asbestos. The asbestos must be separated into fibers in a range of sizes from 1 to 2 4 u in diameter down to small fractions of a micron. When cut to short lengths, these fine fibers blend with the cellulose into a fine network free from clumps or voids, forming a sheet that will readily pass air and yet retain nearly all of the extremely fine suspended material, 99.95 per cent or more by actual particle count.
The desired combination of properties is obtained with a sheet consisting of approximately 85 per cent paper pulp and 15 per cent asbestos. Although many attempts have been made, it has not yet proved feasible to substitute domestic asbestos for the imported types. For example, chrysotile asbestos from Canada or Arizona can be separated into much finer filaments, but these fibers agglomerate and give a random uneven formation in the sheet, resulting in lowered filtering efficiency.
Filters for operation at 500°F and higher are now being produced. They have a steel or aluminum frame, aluminum foil separators, and a filter sheet of all mineral fiber. The sheet consists of approximately 80 per cent glass fibers 3 i in diameter; the remainder is asbestos with a small part of resin binder. The sheet is produced with commercial papermaking machinery. At every stage during the formation of the sheet, conditions must be carefully controlled in order to obtain a product having the desired properties. During a run, grab samples are taken off the machine and their filtering efficiency determined in a standard smoke tester. If necessary, more asbestos slurry is added into the feed stock.
Examples of filters with asbestos include those manufac- tured by Flanders Filters, Inc., Riverhead, New York and The Cambridge Filter Corporation, Syracuse, New York. Flanders Filters manufactures filters which are used by the AEC, to remove radioactive particles that are submicron in size.
Applications of the high efficiency filters have already been made in the critical areas of hospitals and research laboratories, in the manufacture of optical instruments and photographic film, in the pharmaceutical industry, in the chemical process industries, and for the protection of fine mechanisms during manufacture and use. They are widely used at AEC installations in order to prevent the discharge of radioactive particles. These filters might find application in the ventilating air intakes of public shelters and in individual protective masks used for civil defense.
High Temperature Asbestos-Based Filters
Asbestos fibers of very fine diameter are mixed with ceramic fibers of coarser diameter to form a filter paper. This is done on paper-making machinery. Because of the relatively large diameter of the ceramic fibers, these fibers cannot be used by themselves inasmuch as the resulting filter efficiency would be too low. Filter paper made with this combination of fibers is usable in temperatures as high as 1,800°F, when contained in a cast ceramic frame and separated by ceramic paper separators. Although the asbestos changes form at approximately 700° to 900°F, filter efficiency is not affected by the change in form.
The Flanders filter separators, made with 4-lb asbestos paper, are used where operating temperatures do not exceed 700°F. While other materials are also used for these conditions, asbestos is a relatively inexpensive way of providing noncombustible material in an air system which might be subject to fire. Separators are made by corrugating the paper between corrugating rolls. The paper runs through a steam box prior to corrugation.
A new filter medium for all series of Absolute ® Filters has been developed. It is manufactured by The Cambridge Filter Corporation, Syracuse, New York, pioneers in the high efficiency air filter field. This new medium, consisting of a combination of selected glass and asbestos fibers, gives the filters an important increase in efficiency as well as greater resistance to fire, temperature, and humidity.
The incorporation of asbestos fibers improves temperature resistance even in comparison to the all glass media sometimes employed prior to the development of the new filters. The use of inorganic fibers only insures the complete moisture and mildew resistance of the new filter medium. Filters to withstand 1,000°F and 100 per cent relative humidity are available.
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