Document MQO6EDv8099vJXNdQJw7xOVL

V t Amsterdam, herapeutics, Transmitted * C Thomas, ledicine, 5th 3, Inc., Acton, , a division of Jniversity 4 i Vol. 3 ASBESTOS 267 ASBESTOS Asbestos is a generic term describing a variety of naturally formed hydrated sil icates that, upon mechanical processing, separate into mineral fibers. There are two fundamental varieties of asbestos: serpentine and the amphiboles. Serpentine asbestos is known as chrysotile and the amphiboles include five species identified as anthophyllite, amosite, crocidolite, actinolite, and tremolite. Each of these varieties of as bestos differ from each other chemically as illustrated in Table 1. Asbestos fibers are unique minerals combining unusual physical and chemical properties which make them useful in the manufacture of a wide variety of residential and industrial products. Of mineral origin, asbestos does not bum, does not rot, and, dependent on variety, possesses extremely high tensile strength as well as resistance to acids, bases, and heat. Similarly, when processed into long, thin fibers, asbestos is sufficiently soft and flexible to be woven into fire-resistant fabrics. Historical records show that asbestos has been known for more than 2000 years. Applications of this noncombustible fiber are mentioned by Plutarch and Pliny, particularly with reference to asbestos textiles used for cremation cloths, oil lamp wicks, etc. The asbestos industry per se had its inception in the 18th century in the Russian Ural mountains and by the mid-19th century both Italian chrysotile and tremolite varieties were mined and processed into commercial products. At the same time as bestos was discovered and mined on a commercial scale at Thetford Asbestos in Quebec, Canada. To this day, these Canadian and Russian locations are the major producers of chrysotile asbestos. The amphibole asbestos industry is of more recent origin. Blue asbestos, crocid olite, was discovered in South Africa about 1803 to 1806, but it was not until 1893 that this variety was commercially exploited. Production, sale, and use of the amosite species from the Transvaal followed in the early 20th century (1). Origin and Occurrence Through the years, the origin of asbestos has been the subject of extensive geo logical research. Serpentine asbestos occurs under widely differing geological conditions from the amphiboles. Similarly, the modes of occurrence or the manner in which the fibers are physically imbedded in the host rock also differ widely. The current opinion is that chrysotile fiber resulted from two separate metamorphic reactions in ultrabasic Table t Asbestos Species CAS Registry Number Variety chrysotile" anthophyllite amosite" actinolite tremolite crocidolite" [12007-29-5] [1 7068- 78-9) [12172-73-51 [13768-00-8] [14567-73-8] [12001-28-41 serpentine amphibole amphibole amphibole amphibole amphibole 0 Asbestos species of major commercial significance. Chemical composition 3MgO.2SiO1.2H3O 7MgO.8SiOj.HjO UFeO.3MgO.16SiOj.2HjO 2CaO.4MgO.FeO.8SiOj.HjO 2CaO.5MgO.8SiOj.HjO Na20.Fe20a.3Fe0.8 S1O2.H2O MTC 000325 268 ASBESTOS rocks of igneous origin. The initial hydrothermal reaction altered the olivines and pyroxenes to serpentine. At a subsequent point the serpentine was redissolved and the mineral-rich solutions flowed into cracks and crevices in the host rock where chrysotile fiber was reprecipitated. In this reprecipitation process, asbestos fiber was usually deposited in a cross-vein mode of occurrence; ie, the fiber is arranged perpendicular to the wall rock as illustrated in Figure 1. In some cases, chrysotile was either deposited or affected by earth movements such that the fibers lie principally parallel to the wall rock as illustrated in Figure 2. This mode of occurrence is referred to as slip fiber. The third and unusual mode of occurrence of chrysotile is referred to as a massive or agglomerated form wherein the fibers have been deposited as platelets having no specific fiber orientation. This unique formation has been found and commercially mined in the New Idria serpentine deposits of California (2) and at Stragari, Yugo slavia. In these cases the asbestos content of the ore is abnormally high, but the fiber length is very short as compared to commercially useful cross-vein or slip fiber deposits. This mode of occurrence is illustrated in Figure 3. The origin of the amphibole varieties of asbestos are not as clearly defined as those of chrysotile. The two commercially significant amphibole fibers, crocidolite and amosite, occur in metamorphosed sedimentary strata known as banded ironstones. These sedimentary formations vary considerably in composition which accounts for the compositional variations of the associated amphibole fibers (I). Figure 1. Cross-vein chrysotile (Courtesy of Johns-Manville Research Center). MTC 000326 olivines and ssolved and rock where a cross-vein is illustrated movements in Figure 2. as a massive ts having no ommercially agari, Yugobut the fiber her deposits. ined as those cidolite and 1 ironstones. accounts for j i I ; i ' i j i s i * ter). i Vol. 3 ASBESTOS 269 Figure 2. Slip fiber chrysotile (Courtesy of Johns-Manville Research Center). Crocidolite and amosite species of asbestos occur in cross-vein modes and the anthophyllite and tremolite species often occur in slip modes although both of the latter can also occur in a massive mode wherein the fibers are nonoriented. Actinolite, a noncommercial species of amphibole asbestos, is most often found as brittle, acicular, or bladed forms in a massive mode. In its generic connotation, asbestos is a mineral found all over the world. In most cases, however, the asbestos species most easily located are those having limited or no commercial utility such as actinolite, anthophyllite, or tremolite. In 1977, Canada and Russia are the major producers of chrysotile asbestos, and Africa, China, and the United States provide modest quantities. The major producing area of the important amphiboles, crocidolite, and amosite is Africa. In fact, the only deposits of amosite known to date are in the Eastern Transvaal. Chrysotile Asbestos Crystal Structure. The mineral species associated with the serpentine group, serpentine [12168-92-2], chrysotile, lizaidite [12161-84-1], and antigorite [12135-86-3], although differing structurally, are compositionally almost identical. All have the approximate chemical composition of Mg3(Si205)(0H)4. The crystal structure of chrysotile is layered or sheeted similarly to the kaoiinite group [1318-74-7] (3-4). It is based on an infinite silica sheet (SijOs),, in which all the silica tetrahedra point one way (see Silica). On one side of the sheet structure, and joining the silica tetrahedra, is a layer of brucite, Mg(OH)2, in which two out of every three hydroxyls are replaced by oxygens at the apices of the tetrahedra. The result is a layered structure illustrated in Figure 4 (1,5). MTC 000327 270 ASBESTOS Figure 3. New Idria (Coalinga) chrysotile platelets (Courtesy of Johns-Manville Research Center). Figure 4. Fundamental sheet of a chrysotile structure. Mismatches and strains between the layers cause the structure to curve and form cylinders or fibers (6-7). Individual chrysotile fibers have ultimate diameters of 0.02-0.03 (tm. X-ray and electron microscope studies have confirmed this cylindrical form and diameter range. In fact, the first electron microphotographs indicated a tubular structure as illustrated in Figure 5 (8-10). This characteristic appearance has now become one of the more definitive identification techniques for chrysotile. MTC 000328 Vol. 3 r . T> 1 ! 3 Research I ASBESTOS 271 ; and form .meters of :ylindrical idicated a irance has >otile. Electron microphotographs have shown most chrysotile fibers with a hollow cy lindrical form and a single magnesia-silica sheet rather than the earlier double-layer concept (11-12). The lattice planes have a multispiral arrangement as suggested by earlier x-ray studies (13). MTC 000329 272 ASBESTOS Chemical and Surface Properties. Chrysotile asbestos is a naturally formed mineral. The chemical compositions vary somewhat, depending on deposit location, from the idealized composition of Mg3(Si2Os)(OH)4. Chemical analyses of chrysotile range approximately as follows: SiO^ 37-44%; MgO, 39-44%; FeO, 0-6.0%; Fe203,0.1-5.0%; AJ203,0.2--1.5%; CaO, trace-5.0%; H20,12.0-15.0%. Variations in chemical analyses may be due to either associated mineral impurities or to isomorphic substitutions in the crystal lattice. Common mineral impurities found in commercial grades of chry sotile from various locations include magnetite, chromite, brucite, calcite, dolomite, and awaruite. Within the chrysotile lattice, nickel and iron can occur as minor iso morphic substitutions for magnesium (14-17). Chrysotile, a hydrated silicate, is subject to thermal decomposition at elevated temperatures. This thermal decomposition is a two-stage reaction consisting first of a dehydroxylation phase and then a structure phase change. Dehydroxylation or the loss of water occurs at 600-780C. At 800-850C the anhydride breaks down to forsterite and silica. These reactions are irreversible and are illustrated by the typical differential thermal analysis shown in Figure 6. Structural changes in chrysotile can occur under conditions of intense grinding. As a result of these effects the structure can become amorphous and no longer iden tifiable by either x-ray diffraction or electron microscopy. These structural changes apparently occur because of localized temperature surges in the fibrils with accom panying dehydroxylation as they absorb the tremendous impact energies, eg, extensive dry ball milling results in an amorphous mass and wet ball milling results in short fibrils that retain their crystallinity (16). Because of its hydroxyl outer layer, chrysotile is readily attacked by acid and will, ultimately, completely dissolve the magnesium component, leaving essentially a fi brous, but fragile, silica structure. Similarly, because of its alkaline surface, chrysotile is not readily attacked by caustic solutions except under conditions of extreme alkali concentration and elevated temperatures (18-19). Dispersions of chrysotile fiber in carbon dioxide-free distilled water exhibit al- Fiffure 6. Typical differential thermal analysis of chrysotile. MTC 000330 mineral rom the e range 1-5.0%; inalyses tions in of chryilomite, nor isoi subject sition is :ructure )-850C versible ure 6. rinding. ;er idenchanges i accomxtensive <rt fibrils and will, ally a fiirysotile ne alkali Jiibit al i " | i ! 1 j . } I / j ! ] 1 i 1 ^ ] !, Vol. 3 ASBESTOS 273 kaline properties. Such suspensions will reach a pH of 10.33 as with magnesium hy droxide suspensions tested under the same conditions. Solubility product constants for chrysotile fibers range from 1.0 X 10-u to 3 X 10~12, indicative of the magnesium hydroxide outer layer (20-22). The electrokinetic behavior of chrysotile is also related to its surface character istics. Normally, below its isoelectric point of approximately pH 11-12, chrysotile exhibits a positive charge. Above this pH range, it demonstrates a negative charge. Exceptions to this general behavior have been noted with chrysotile fibers from certain locations (23). Because of its very small fiber diameter, its high specific surface area and its relatively reactive surface, chrysotile is a selectively adsorptive material. Commercial grades of chrysotile adsorb as much as 2-3 wt % moisture from saturated air. Ad sorption studies of a variety of organic compounds from both vapor and liquid media show that chrysotile has a greater affinity for polar molecules. Heats of adsorption have been measured ranging from 38 kJ/g (9 kcal/g) for hexane to 67 kJ/g (16 kcal/g) for water (5,24-27). Chrysotile also adsorbs iodine from solutions in a maimer similar to magnesium hydroxide or brucite. This adsorption characteristic is often used as a staining technique for the detection of chrysotile asbestos (28). Physical Properties. The common physical properties of chrysotile asbestos are given in Table 2. Asbestos fibers are used in composite materials (qv) to provide reinforcement. Tensile strength of the fiber is, therefore, an important and highly significant physical property. Unfortunately, because of their extremely fine diameter and the complicating factor of the effect of sample length on strength determinations, it is extremely difficult to measure the tensile strengths of asbestos with precision. Most recent information indicates typical chrysotiles have tensile strengths in the order of 3727 MPa (5.4 X 10 psi) which exceeds corresponding values for steel piano wire and fiber glass. A comparison of typical strength values for the different asbestos varieties is given in Table 3. Since all these measured values are far less than the theoretical value of over 10,000 MPa (1.45 X106 psi) attributable to silicate chain structures, the values given should be considered as relative for the different varieties rather than specific (1,29-31). Physical strengths of asbestos are adversely affected by elevated temperatures. Table 4 shows decreasing tensile strength as dehydroxylation takes place (1). What visually appears to be a single fiber in commercial grades of asbestos is in actuality a bundle of a large number of individual fibrils. These bundles can be sub divided into a multitude of finer bundles, but only with special processing can a large portion of fiber mass be divided to its ultimate fibril diameter. The specific surface areas of commercial asbestos fibers vary with the extent of mechanical defibrillation. Surface areas by nitrogen adsorption tests on samples teased by hand from chrysotile crude are 4-12 m2/g; however, when aggressively milled or fiberized, surface areas of 30-50 m2/g result (32). Chrysotile asbestos can be separated into smaller diameter fibrils (higher specific area) more readily under wet processing conditions than under dry mechanical milling. For this reason many asbestos product manufacturing pro cesses utilize wet opening techniques to provide improved fiber reinforcing efficien cies. The term harshness in the asbestos industry refers to the fiber's brittleness, flexibility, form, and modulus of elasticity. Commercial grades of chrysotile are usually MTC 000331 275 MTC 000333 276 ASBESTOS Table 3. Tensile Strength of Asbestos* chrysotile, Arizona, USA chrysotile, Thetford, Canada crocidolite, Koegas, Cape Province, Africa crocidolite, Koegas, Cape Province, Africa crocidolite, Pomfret, Cape Province, Africa crocidolite, Pomfret, Cape Province, Africa crocidolite, Cochabambo, Bolivia amosite, Penge, Transvaal, Africa amosite, Penge, Transvaal, Africa anthophyllite, Paakilla. Finland * See ref. 1. Tensile strength, Young's modulus, MPa (psi X 103) GPa (psi X 10) 3780 (548) 3640(528) 2840 (413) 3090(448) 4660(675) 3550 (515) 1440(209) 2580 (374) 1980 (287) 2450 (356) 145 (21.1) 146 (21.2) 147 (21.3) 151 (21.9) 169 (24.5) 175 (25.3) 170 (24.6) 143 (20.8) 143 (20.8) 156 (22.6) Table 4. Effect of Temperature on Tensile Strength of Asbestos* Temperature, "C 200 320 430 550 670 period of heat soak Percentage of original tensile strength Chrysotile Crocidolite Amosite 91.6 73.3 59.5 32.0 3 min 100 100 70 58 32 32 20 15 4h 4h 0 See ref. 1. classed as soft or nonharsh. Commonly, they are silky, of fine diameter, and extremely flexible. Contrary to chrysotile, the commercial amphiboles are harsh fibers. They are relatively stiff, brittle, coarser in diameter, and rodlike in appearance under the microscope. These differing physical characteristics account for the different operating characteristics exhibited in the manufacture of various asbestos-containing products. For example, soft chrysotiles can be more readily spun into textiles than the amphi boles but they have poorer drainage properties when used in wet manufacturing processes. However, several fiber treatments have been developed to improve the filtration or drainage properties of chrysotile and thereby increase the production rate of wet machines manufacturing asbestos-cement products (33-37). Amphibole Asbestos Crystal Structure. The structure of all the amphiboles consists of two chains or ribbons based on SUOn units separated by a band of cations. Seven cations form the basal unit. Two hydroxyl groups are attached to the central cation in each unit cell. These hydroxyls, unlike the chrysotile structure, are contained entirely within the amphibole structure. The final structure is composed of stacks of these sandwich ribbons as illustrated in Figure 7 (1,5). The bonding between these ribbons is rather weak and the crystals are easily cleaved parallel to the ribbons along cleavage line A-A. If the cleavage is very facile, the result is an asbestiform mineral (1,5). Amphiboles MTC 000334 xlulus, < 10) .1) .2) .3) 9) t.5) ..3) '.6) >8) 1.8) 1.6) wite 30 58 32 15 tremely s. They ider the derating roducts. amphiicturing :ove the :ion rate it i it i hains or init cell. thin the andwich is rather line A-A. phiboles Vol. 3 ASBESTOS 277 can also occur in nonfibrous forms (38) which may result because of structural disorder (39). The dominant cations are Mg2+, Fe2+, Fe3+, Na+, and Ca2+ (see Table 1). Minor isomorphic substitutions of Al3+, Ti4+, K+, and Li+ also occur. Because of the wide compositional range, the amphiboles are often assigned to three generic series; ie, the anthophyllite-cummingtonite [2 7499-08-0] series, the calcic amphiboles and the soda amphiboles. Chemical and Surface Properties. The chemical compositions of the amphibole fibers are more complex and variable than chrysotile. Typical compositions are shown in Table 5. Like chrysotile, the amphibole asbestos fibers dehydroxylate and decompose at elevated temperatures. The presence of large quantities of iron (particularly ferrous iron) makes the decomposition or thermal analysis determinations particularly complex and very dependent on the composition of the atmosphere. Table 6 is an Table 5. Typical Chemical Compositkxu of Amphibole Asbestos Crocidolite, % Amosite, % Anthophyllite, % Si02 MgO FeO Fe2C>3 AI2O3 CaO k2o NajO HoO 49-53 0-3 13-20 17-20 0-0.2 0.3-2.7 0-0.4 4-8.5 2.S-4.5 49-53 1-7 34-44 0-0.4 trace 2.5-4.S 56-58 28-34 3-12 0.5-1.5 1-6 Actinolite, % 51-52 15-20 5-15 0-3 1.5-3 10-12 0-0.5 0.5-1.5 1.5-2.5 Tremolite, % 55-60 21-26 0-4 0-0.5 0-2.5 11-13 0-0.6 0-1.5 0.5-2.5 MTC 000335 278 ASBESTOS Table 6. Decomposition Reactions of Amphiboles Under Neutral Conditions* Amphibole variety Dehydroxylation DTA peak, *C Structural breakdown, *C Decomposition products crocidolite amosite anthophyllite actinolite 610 780 950 1040 800 600-900 950 1040 Na-Fe pyroxene, magnetite, silica Fe-Mg pyroxene, silica Mg-Fe pyroxene, magnetite, silica CaMgFe pyroxene, silica " See ref. 1, oversimplification of the thermal decomposition reactions of the amphibole fibers (1,40). Compared to chrysotile, the amphibole fibers are relatively acid resistant. How ever, under boiling conditions and high acid concentrations the amphiboles can exhibit weight losses of approximately 2-20%. Relative order of acid resistance is: tremolite > anthophyllite > crocidolite > actinolite > amosite > chrysotile Amphibole fibers have a negative charge as contrasted to chrysotile's usual positive charge. The magnitude of the charge exhibited by the amphiboles is substantially lower than chrysotile's. Physical Properties. See Table 2 for the more common physical properties and characteristics of the various amphibole fibers. In general, amphibole fibers are harsh, springy, and brittle as compared to the chrysotile variety. These physical properties make the amphiboles fast draining and bulky when used in manufacturing pro cesses. As illustrated in Table 3, the tensile strengths of amphibole asbestos fibers differ widely. The typical tensile strengths of asbestos fibers have the order. crocidolite > chrysotile > amosite > anthophyllite > tremolite > actinolite As shown in Table 4, the amphiboles also lose their strength with increasing temper ature. Amphibole fibers do not divide into fibrils as fine in diameter or as symmetrical as the chrysotile variety. Ultimate diameters of amphiboles have been reported to be about 0.1 fim (1) and the surface areas of amphibole asbestos are considerably smaller than chrysotile. Fully fiberized commercial grades of crocidolite, for example, have surface areas by gas adsorption of 3-15 m2/g compared to the 30-50 m2/g values of chrysotile (2,41). MTC 000336 Vol. 3 ASBESTOS 279 Milling >n Imbedded asbestos fibers are removed from the ore by a repeated series of crushing, fiberizing, screening, aspirating, and grading operations (milling). A typical, e. greatly simplified asbestos mill is shown in Figure 8. The ore is crushed, dried, and fiberized in a variety of impact mills. The short fiber and granular material is removed le, by screening the fiberized mass. The oversized fractions are stratified on a screen where the spherical, granular material of high density seeks the screen's surface and the fluffy, le, low-density fiber rises to the top of the bed. At the end of the screen, the fiber is sep arated from the rock by an aspirating hood. The coarse granular fractions that still contain veins of chrysotUe fiber, are refiberized and rescreened to recover shorter fibers. ene. Fibers recovered from these primary screening operations are rescreened to remove entrapped granular material and classified into grades by fiber length. i The recovery of milled asbestos fiber from ore is fairly low. In general, a 5% re covery is typical. The chrysotile mines in California are notable exceptions where 50% ale fibers recoveries are common from agglomerated ores (Fig. 3). The fibers, however, are very short and are normally sold as reinforcing fillers (qv). mt. Howin exhibit Conventional asbestos milling uses large quantities of air both for separating the fibers as they are freed from the ore and for dust control. Approximately 130 m3/s (275,000 cfm) are used to process one metric ton of ore (42). Preconcentration of ore includes selective grinding, screening, and magnetic techniques (43-45). Hot gases to collector and stack" 1 positive ally lower irties and are harsh, )roperties iring pro- aers differ g temper- mmetrical rted to be ly smaller ciple, have ; values of Fiberizer Figure 8. Schematic of a typical asbestos milling flowline. MTC 000337 280 ASBESTOS Production and Usage In 1977,80% of the world's chrysotile was produced by Canada and Russia. Most of the Canadian production is from Quebec and the balance from Newfoundland, British Columbia, and the Yukon Territory. Russian production is predominantly in the Bazhenovo District in the Central Urals, the Dzhetygara area northwest of Kazakhstan, and the Aktourak deposits near the Yenisei River. Crocidolite and Amosite are produced in significant quantities only in South Africa. Main producing areas are at Bosrand, Cornheim, Ouplaas, Owendale, the Kuruman area in the Cape Province, and the Lydenburg District in the Transvaal (46). Production statistics vary widely with the source of information. Table 7 sum marizes ranges given by various authorities (47-53). The largest use of asbestos is in asbestos-cement for products such as pipes, ducts, and flat and corrugated sheets. Pipe products find use in water supply, sewage disposal, and irrigation systems. Asbestos-cement sheets are used in a wide variety of con struction applications. Other uses of asbestos include fire-resistant textiles, friction materials (see Brake linings), underlayment and roofing papers, and floor tiles. Table 8 shows the uses of asbestos in the United States (the largest consumer) and the world. The United States usage patterns differ considerably from the rest of the world (54-56). Standards and Test Methods Gassification. Canadian chrysotile crudes are classified as follows: crude # 1: crude # 2: crude run of mine: 1.9 cm staple and longer 0.95 cm to 1.9 cm staple unsorted crudes Table 7. 1976 Asbestos Production Type and location Chrysotile Canada USSR South Africa Rhodesia (estimate) China (estimate) Europe USA South America Australia others Total chrysotile Amphiboles (South Africa) crocidolite amosite anthophyllite Total amphiboles Total asbestos Thousands of metric tons 1537 2285 111 239 150 321 104 69 66 45 4927 178 79 2 259 5/86 MTC 000338 ssia. Most oundland, Dminantly -thwest of ' in South mdale, the Transvaal )le 7 sumpes, ducts, e disposal, ty of cons, friction lies. Table the world, the world 3 Of ins Vol. 3 Table 8. Asbestos Usage by Product tine Estimated Percent of Production Product line United States consumption, % asbestos-cement asbestos papers floor tile friction materials roof coatings textiles plastics miscellaneous 23-30 26-38 13-21 6-8 4-7 1-2 1-2 12-20 ASBESTOS 281 World consumption, % 65-70 7-8 4-7 2-3 ' 2-3 1-4 trace-1 13-15 Milled Canadian fibers sold from Quebec are classified by a dry screen technique known as the Quebec Standard Asbestos Test This test method grades fibers roughly by fiber or staple length. Minimum test values are guaranteed for each grade and a numerical classification system has been established for fibers ranging from Group 3, the longest grade, to Group 7, the shortest grade. Chrysotile fibers produced outside Quebec are graded or controlled by screening test methods differing from the Quebec Screen Test; however, these basically identify grade by staple length. Test Methods. The major properties of concern are length, granular content, degree of openness or effective surface area, drainage or filtration rate, color, ab sorption, electrical resistivity, bulk density, and strength-giving properties (57). Fiber length is most reliably measured by wet screening techniques. Fiber openness is commonly measured by air permeability surface area methods. A variety of filtration tests are used to estimate a fiber's operating performance in asbestoscement wet machines or paper machines. Granular content is often determined by a dry screening test where the fibrous material is aspirated from the granular impurity remaining on the various screen meshes. The most complex measurement is the determination of the strength-giving property of a fiber for use in asbestos-cement products. In this case, small pressed sheets are made from a slurry of fiber and cement. After curing, the sheet strength is determined and the reinforcing effectiveness or value of the fiber is calculated. Health and Safety Factors The inhalation of excessive quantities of free asbestos fibers over prolonged pe riods of time can increase the risk of developing certain diseases of the lung within 20 or 30 years. The three diseases associated with the inhalation of asbestos are: asbestosis, a nonmalignant fibrotic lung condition; bronchogenic (lung) carcinoma; and mesothelioma, a rare cancer of the lining of the chest or abdominal cavities (53). Reduction of asbestos dust exposure is at present the only known method of preventing disease among asbestos industry workmen. When dust levels are low, risk to employees and the incidence of asbestos-related disease drop sharply. Cigarette smoking greatly increases the risk of developing bronchogenic cancer among persons encountering heavy asbestos exposure. Nonsmoking asbestos workers show no greater incidence of bronchogenic cancer than the average nonsmoker. MTC 000339 :I 282 ASBESTOS There are governmental regulations (OSHA) that describe the allowable airborne fiber levels in work areas. Extensive dust control together with corrective work prac tices are used to implement these regulations and medical examinations are regularly provided to assure worker protection (see Air pollution control methods; Industrial hygiene and toxicology). BIBLIOGRAPHY "Asbestos" in ECT 1st ed., Voi. 2, pp. 134-142, by M. S. Badollet, Johns-Manville Research Center; "As bestos" in ECT 2nd ed., VoL 2, pp. 734-747 by M. S. Badollet, Consultant 1. A. A. Hodgson, "Fibrous Silicates," The Royal Institute of Chemistry, London, Lecture, Series No. 41965. 2. F. A. Mumpton, H. W. Jaffe, and C. S. Thompson, Am. Mineral. 50,1893 (1965). 3. B. E. Warren and W. L. Bragg, Krist. 76,201 (1930). 4. B. E. Warren and K. W. Hearing, Phys. Reo. 59,925 (1941). 5. S. 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CIM Bull. 68, (July 1975). 45. E. Martinez, Trans. Soc. Min. Eng. 255, (1974). 46. H. H. Gossling, 2nd Industrial Minerals International Conference, Munich, Ger., 1976. 47. Asbestos 58, (Dec 1976). 48. Asbestos 58, (Mar. 1977). 49. Asbestos 59, (Nov. 1977). 50. Mineral Industries of the USSR, U.S. Bureau of Mines, MP2, July 1977. 51. Asbestos-1917, U.S. Bureau of Mines, MCP6, Sept. 1977. 52. Mineral Trade Notes, 74(6), (June 1977). 53. World Mining Magazine 30, June 25,1977. 54. Ind. Min. (June 1975). 55. B. Lincoln, Ind. Min., (Oct. 1975). 56. N. W. Hendry, unpublished information, Johns-Manville Corporation, Denver, Colo. 57. Chrysotile Asbestos Test Manual, 3rd etL, Asbestos Textile Institute and the Quebec Asbestos Mining Association, 1974. 58. Chem. Week, 16 (May 3,1978). William C. Streib Johns-Manville Corporation ASBESTOS-CEMENT PRODUCTS. See Cement. ASBESTOSIS. See Industrial hygiene and toxicology. ASCORBIC ACID. See Vitamins. ASPARAGINE. See Amino acids. ASPARTIC ACID. See Amino acids. k . ii MTC 000341 For Reference Not to be taken from this room KIRK-OTHMER ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY THIRD EDITION VOLUME 3 ANTIBIOTICS (PHENAZINES) TO BLEACHING AGENTS A W1LEY-INTERSCIENCE PUBLICATION John Wiley & Sons NEW YORK CHICHESTER BRISBANE . TORONTO MTC 000342 Copyright 9 1978 by John Wiley & Sons, Inc. All rights reserved. Published simultaneously in Canada. Reproduction or translation of any part of this work beyond that permitted by Sections 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to the Permissions Department, John Wiley St Sons, Inc. library of Congress Cataloging in Publication Data: Main entry under title: Encyclopedia of chemical technology. At head of title: Kirk-Othmer. "A Wiley-Interscience publication." Includes bibliographies. 1. Chemistry, Technical--Dictionaries. I. Kirk, Raymond Eller, 1890-1957. II. Othmer, Donald Frederick, 1904- III. Grayson, Martin. IV. Eckroth, David. V. Title: Kirk-Othmer encyclopedia of chemical tech nology. TP9.E685 1978 660'.03 ISBN 0-471-02039-7 77-15820 Printed in the United States of America MTC 000343