Asbestos

ASBESTOS, asbestos ores (from the Greek word "asbestos" (unquenchable, indestructible) * EN: asbestos, asbestus, earth flax, mountain flax; DE: Asbest; FR: asbeste, aminate; ES: asbesto; RU: асбест), is the group of the fibrous minerals, which have the ability to split into the thinnest flexible fibers. According to the chemical composition, the asbestos minerals belong to the class of the hydrous silicates of magnesium, iron, partially calcium and sodium. According to the mineralogy, features, and crystal structure, they are sub-divided into the chrysotile asbestos and amphibole asbestos.

The greatest distribution within the nature and practical usage (95% of the production) has the chrysotile asbestos, the mineral of the group of serpentine. Under the electronic microscope, the cross sections of the elementary fibers have the tubular structure with the outer diameter of 26 nanometres, inner diameter of 13 nanometres, and thickness of the wall of 6.5 nanometres. The colour is golden-yellow, green to black, white in the fluffed state. The hardness on the mineralogical scale is 2-2.5. The density is 2500 kilograms per cubic metre; the temperature of the melting point is approximately 1500 degrees Celsius. It poorly conducts heat and electricity, and dissolves within the acids. The strength of the undeformed fibers is 3-3.3 gigapascals, the length is from the fractions of the millimetre to 50 millimetres, sometimes more. They use it for the manufacturing of the fireproof textile products, filters, heat insulation, fire-retardant paints, fillers for the plastics and asbestos-cement.

The deposits of the chrysotile asbestos are genetically related mainly with the ultrabasic rocks, and form themselves during the process of their serpentinization under the impact of the hydrothermal solutions, which are associated with the granitoid intrusions. In general, the asbestos deposits have the zonal structure, with which there are observed from the centre to the periphery the gradual change of the character of the asbestos-bearing property, and the decrease of the thickness of the veins, and of the length of the asbestos fibers. The largest deposits of the chrysotile asbestos within the USSR are the Bazhenovskoe, Krasnouralskoe (Middle Urals), Dzhetygarinskoe and Kiembaevskoe (South Urals), Aktovrakskoe, Sayanskoe, Ilchirskoe (Sayan mountains), Molodezhnoe at the north of the Transbaikalia (Buryat ASSR). Abroad, the large deposits are known within the Canada (Quebec, British Columbia, Newfoundland), Zimbabwe (Zvishavane and Mashava), USA, Italy, Yugoslavia, France, Japan, Brazil, Australia, China, on the Cyprus island. The contact-metasomatic deposits of the chrysotile asbestos within the sedimentary magnesian-carbonatic rocks are small, but may be of the industrial interest as the source of the non-ferruginous asbestos. Within the USSR, the deposits of this type are known within the Krasnoyarsk territory (Aspagashskoe, Bis-Tag), Uzbekistan (Sary Ceku), and Kyrgyzstan.

The amphibole asbestos is represented by the minerals of the groups of the amphiboles; unlike the chrysotile asbestos, they are poorly soluble or insoluble within the acids. There have the greatest importance of them the minerals crocidolite, anthophyllite, amosite, rezhikite and rhodusite, the limited usage the minerals tremolite, and actinolite.

The amosite is the fibrous variety of the rhombic amphibole, namely, gedrite, hydrous ferromagnesian silicate of the complex and non-permanent composition, with great content of alumina; the temperature of the melting point is 1100-1200 degrees Celsius. The strength of the undeformed fibers is approximately 3 gigapascals, the length of the fibers is 100-175 millimetres.

The rezhikite (magnesian arfvedsonite) and rhodusite are the alkaline amphibole asbestos of the complex composition; both varieties have great resistance to the acids, and great sorption capability; the temperature of the melting point is 1100-1200 degrees Celsius. The length of the fibers of rezhikite is 2-3 centimetres, sometimes up to 30 centimetres, of the rhodusite is 2-7 millimetres.

The amphibole asbestos has the great acid-resistance and heat resistance. It is used for the manufacturing of various products, which are working within the corrosive environments, and others. The deposits belong to the type of the hydrothermal-metasomatic ones, which have formed themselves under the conditions of moderate and great depths, and are confined to the rocks of various composition. The deposits of the crocidolite and amosite within the south Africa (Zimbabwe, South Africa) have emerged during the process of metamorphism, and are embedded among the ferruginous quartzites and jaspillites. The deposits of the crocidolite are known also within the Western Australia. Within the USSR, small clusters of the crocidolite and amosite have been uncovered within the ferruginous hornfelses within the Krivoy Rog city. The deposits of the anthophyllite are confined to the massifs of the metamorphosed ultramafic rocks: within the USSR at the Urals (Sysertskoe) and within Kazakhstan (Bugetysaiskoe); abroad, within Finland. The industrial deposits of tremolite are known within the Italy, France, and other countries.

The rocks, which are containing asbestos, which may be isolated during the mechanical processing at the technological lines of the industrial scale, are called the asbestos ores. There are no unified requirements for the asbestos ores: there are established the technical conditions at the deposits, which are being developed, depending on the technological schemes of beneficiation at the plants, and on the required range of the products.

The quality of the ores is appraised according to the content of the asbestos, and according to the composition of the fiber, which is determined by the mechanical sieve analysis. Within the USSR, they refer to industrial asbestos the fiber, which is longer than 0.5 millimetres, and abroad, which is longer than 0.25 millimetres. The ores are usually extracted by the open pit method. At the beneficiation plants, asbestos is extracted by the sequential crushing of the ore, and by sucking the fiber away by the flow of the air, with subsequent operations of the repeated cleaning and classification. Within the USSR, they sub-divide the marketable grades of chrysotile asbestos into 4 groups: hard (is produced from the selected ore of the manual sorting within the open pit mines), intermediate (from the ore of the selective excavation, which has been previously beneficiated within the workshops of the crushing-screening complex), semi-hard and soft (from the ores of mechanized extraction). Within Canada, they sub-divide the asbestos into 8 groups according to the sieve analysis of average samples. The classification of asbestos according to the grades, marks, and groups, is determined by the character of its usage within the textile, slate-cardboard-paper, and asbestos-cement products. The determination of the grade of amphibole asbestos is based on the same principles, as for the chrysotile asbestos, that is according to the length of the fiber, and according to the fields of usage.

The worldwide (without the socialistic countries) reserves of asbestos of all mineral types are estimated at approximately 77 million tonnes, including the chrysotile 64.7; crocidolite 3.7; amosite 7.4 million tonnes (1980). There have the explored reserves (1980, million tonnes) the Canada (37), South Africa (15), Zimbabwe (7), USA (4), Brazil (3.5), Australia (1.9), Greece (1.5), Italy (1), Colombia (0.38), and other countries. The industrial deposits of crocidolite are concentrated within the South Africa (59%) and Australia (39%); amosite deposits are known only within the South Africa, anthophyllite within Finland; the non-significant deposits are within the USA. In the reserves of asbestos, USSR holds the 1st place, Canada holds the 2nd one. The provision of the industry with the explored reserves of asbestos at the current volumes of consumption is estimated at 50-60 years.

See also the "Asbestos industry" article.