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Asbestos Of the several zirconium minerals known only two, viz., zircon and baddeleyite are of commercial importance.
The name asbestos came from ancient times from a Greek word meaning unquenchable in reference to its resistance to fire and heat. For many centuries, small cloths woven from asbestos were a luxury item, for handling of hot items in kitchens and foundries.

"Asbestos" refers to a small number of minerals that are formed of flexible fibers, and have the useful physical property of being very heat resistant. Because asbestos forms as flexible fibers, it is woven to make fabrics for heat-resistant and insulating materials.

Chrysotile asbestos, the fibrous variety of the mineral serpentine, is by far the most important type of asbestos. It forms in metamorphic rock, that is, rock that has been altered by intense heat and pressure. Another asbestos mineral is called crocidolite. Crocidolite is a dark blue variety of the mineral riebeckite. Crocidolite occurs in metamorphic rock. Only about 4% of the asbestos consumed is crocidolite. Other, less important asbestos minerals in occasional use are amosite, anthophyllite asbestos, tremolite asbestos, and actinolite asbestos.

Asbestos has a very high melting point. This, together with the flexible nature of the fibers, helps to determine its usefulness.

Since the discovery in the mid-Twentieth Century that asbestos can cause a fatal lung condition, the mining and use of asbestos has decreased, and has become closely regulated in all countries.

Uses

Asbestos fibers have been used in many different applications in our industrialized society. The most common uses of asbestos have been: In the United States, commercial use of asbestos began in the early 1900's and peaked in the period from World War II into the 1970's. Under the Clean Air Act of 1970 the Environmental Protection Agency (EPA) began regulating many asbestos-containing materials which, by EPA definition, are materials with more than 1- percent asbestos.

Properties

Generally, asbestos fibers are long, thin, strong, fireproof, flexible, and resistant to the action of many chemicals. Chrysotile, the most common asbestos type, is usually white or off white, with long wavy fibers. In some mines, the fibers are exceedingly long and flexible, making this chrysotile excellent for weaving into fire and heat resistant cloth. Shorter chrysotile fibers were more apt to be used as binders and strengtheners in plastics, cement or insulation.

Amosite is usually brown or tan, with much straighter and fibers than chrysotile. Amosite was the second most commonly used type of asbestos, comprising approximately 5%of the asbestos placed in buildings and factories.

Crocidolite is a fairly rare form of asbestos, unique because of its very obvious blue color. Crocidolite fibers appear long and straight, much like Amosite. The remaining three asbestos types were not extensively used in commercial products in the United States.

Physical characteristics of Asbestos

Asbestos has a host of physical properties that make it almost a superstar in the world of industrial chemistry. Its tensile strength surpasses that of steel. It has tremendous thermal stability, thermal and electrical resistance and is non-flammable. It can be subdivided into fine fibers that are strong enough and flexible enough to be spun into material that is a flame retardant, chemically inert thermal and electrical insulator. Note that asbestos binds with better insulating materials to create the ultimate construction materials.

Asbestos fibers have no detectible odor or taste. They are all solids that do not move through soil and are insoluble in water. Its color will vary according to type, and metallic composition. Crocidolite, which has iron and sodium as its only metallic elements, is the most colorful, adorned in a range of colors including shades of lavender, blue and green. In general, asbestos-containing iron may display a green color ranging from a hint of green to solid green depending upon the amount of iron present.

Tremolite contains no iron, but is part of a continuous mineral series with actinolite, in which iron and magnesium can freely substitute with each other. As a result, some specimens of tremolite may show a hint of pale green. Chrysotile and tremolite, which in pure form contain no iron, tend to be white, together with actinolite and anthophyllite are grouped together as "white asbestos" and classified as UN2590 (under the United Nations chemical ID numbering system). Amosite and crocidolite are classified as UN2212. Amosite and crocidolite have been used extensively for commercial use, and are considered to be extremely hazardous. Chrysotile is more flexible and has been considered to be less hazardous than either amosite or crocidolite. Until now, anthophyllite, actinolite and tremolite have been lumped with the "lesser evil" chrysotile under the UN Identification numbering system. Their occurrence in industry has been less extensive. Tremolite has been used in laboratories for filtering chemicals. Actinolite is used for industrial asbestos. There is not much reported use of anthophyllite. All three of these amphiboles also have non-asbestos forms associated with them in nature.

Unique structure

Asbestos can be subdivided into two major classifications of minerals: amphiboles and serpentines. All but one form, chrysotile, are amphiboles. Chrysotile is a serpentine. Both amphibole asbestos and serpentine asbestos are fibrous, but they have very different forms. The amphiboles are double-chain silicates also called inosilicates. The basic structural unit is (Si4O11)-6 with side groups that are responsible for the overall amphibole structure. Amphiboles are distinguished from one another by the amount and positioning of metal atoms including: sodium, calcium, manganese, magnesium, iron(II), iron(III) and aluminum. There is a complete solid solution between Na and Ca end members and between Mg and Fe end members.

The serpentine group of minerals has the formula Mg3Si2O5(OH)4 . Serpentine structure is a bending sheet. There are only three known serpentines. Two are massive and fine grained (not asbestos form). Chrysotile is the only one in which the sheets are continuous and bend to form continuous tubes, which give the mineral the fibrous habit related to asbestos. Chrysotile is very flexible and less likely to be "friable" than the amphiboles. Friability of asbestos is generally defined as the ability to easily be turned into a dust with finger pressure. It is this friability that releases asbestos fibers into the atmosphere and results in health problems.

Health Hazards of Asbestos

There are three principal diseases which have been shown in laboratory experiments to be caused by exposure to one or more of the asbestos minerals. These are: The history of asbestos as a carcinogen is an interesting saga-- one that is clearly not complete. Although there seems to be no question that the residency of large amounts of asbestos fibers within the lung and pleura for long periods of time can cause lung cancer, asbestosis and mesothelioma, the exact mechanism of disease production and the relative potencies of the different asbestos minerals are subjects of current study and lots of debate. To illustrate the problem, following the discovery of the laboratory connection between cancer in rats to exposure of large quantities of asbestos, the next step was to search for a correlation between human exposure to asbestos and cancer. Such a study was made of individuals in the small mining community of Thetford, Quebec in the early 1970's.This community was/is essentially built around a mine and mill devoted to the production of chrysotile asbestos. By studying the mortality rate of male workers since the asbestos mine opened, it was determined that an unusually high percentage of cancer-caused deaths occurred in this community relative to the rest of Canada. This study was responsible for the beginning of the awareness in the public sector that asbestos causes cancer. The study was, however, scientifically flawed. It failed to report or consider the fact that nearly all (85-90%) of the blue-collar workers at Thetford Mines also were heavy smokers. The media and the more radical environmentalists used this flawed study to promote the ban on all asbestos.

More recent studies of cancer mortality rates among workers in the asbestos mining and milling professions paint a less-disastrous picture. Owing to the fact that the potential hazard is lung cancer, and that most workers exposed to asbestos also have tended to be heavy smokers, it is not currently possible to statistically prove a relationship between asbestos exposure and cancer in a general (i.e., normal-smoking, non-asbestos profession) population. Several important indications are, however, available: As one example, McDonald and others (1980) found that for men exposed to chrysotile dust averaging 20 fibers/cm3, the total mortality was less than expected in a population of workers not exposed to asbestos. Exposures to 20 fibers/cm3 are about an order of magnitude greater than those currently experienced in asbestos mines and mills; thus, chrysotile miners working a lifetime under the present dust levels should not be expected to suffer any measurable excess cancer. It turns out that about 95% of the asbestos in the current US market is chrysotile. Given these data, has our recent concern about asbestos in schools and homes really been justified?

At the present time it seems that many of the benefits obtainable from asbestos may be retained with minimal health risk through utilization of the common chrysotile form of asbestos, provided that dust emissions are controlled. It appears that instead of treating all asbestos minerals as equally potent carcinogens, each mineral should be examined on its own merits with regard to its usefulness to society and its potential to cause disease.

Present Uses

Although asbestos fibre has been known to man since antiquity, it was not until the end of the nineteenth century that it came into industrial and commercial use. Its excellent insulating properties, durability and fire retardance have resulted in its use in a variety of industrial applications.

Asbestos-cement pipe has been used in water distribution systems around the world for more than 70 years. Pressure pipe is used primarily for the distribution of potable water, sewer force mains and industrial effluent process piping. Non-pressure pipe is used for sanitary and storm drainage systems, casings for electric cables or duct work.

Asbestos-cement construction products include flat and corrugated sheets used for commercial and residential buildings, roofing shingles and siding.

Asbestos fibres are combined with a resin or woven with metallic fibre to produce temperature resistant and durable friction linings. They can also be mixed with various additives, then molded to produce disk and drum brake linings and clutch facings.

There are close to 3000 known product applications for asbestos, though today its use is limited primarily to high density products in which the asbestos fibre is locked into a cementitious or resinous matrix. Modern-day products include asbestos-cement (pipe, roofing tiles and sheet), friction materials, gaskets, specialty papers and additives, with asbestos-cement accounting for approximately 85% of chrysotile fibre consumption.

1995 World Production (metric tonnes)

Commonwealth of Independent States 1,000,000
Canada 510,800
China 250,000
Brazil 180,000
Zimbabwe 145,000
South Africa 100,000
Greece 50,000
Swaziland 30,000
India 25,000
United States 9,000
Colombia 5,000
Romania 3,000
Yugoslavia 1,000

Canada

The majority of the 500,000 metric tons of chrysotile asbestos produced annually in Québec is exported. Canada uses some 6,000 tons of chrysotile asbestos every year. Friction products, composite materials and asbestos textiles are the main domestic market outlets. Unlike most countries, Canada no longer manufactures chrysotile-cement products. In the residential sector, the North American market has turned to wood because of the availability of this raw material. On the infrastructure side, which includes networks of water mains made of asbestos-cement pipe, the work is for all intents and purposes complete. Though no longer manufactured in Québec, 2,000 to 3,000 tons of asbestos cement are imported and used on construction sites each year.

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