1.1 Types, deposits, and uses of Asbestos, chemical structure

Asbestos is a collective term for a group of silicate fibers. The aforementioned six different Asbestos minerals can be categorized on the basis of their chemical composition (primary cation) and their crystalline and fibrous structure into the groups amphibole or serpentine Asbestos.

Serpentines have a leafy or layered structure. Amphiboles have a chain-like crystalline structure.


Figure 1: Classification of Asbestos Types

· Serpentine Asbestos:

Serpentine Asbestos (chrysotile, as most important type, and antigorite) is leaf structured and consists of fine fibers. The leaves consist of alternating layers of silicate tetrahedron (SiO4), which are held together by hydroxide groups (OH) and magnesium ions. The structure is similar to that of serpentine minerals.

Chrystolite Asbestos:

Chrysotile Asbestos fibers have a very small diameter, are tubular, very soft and bendable. The individual fibrilles have a diameter of 100 - 250 Angstrom (A). Chrysotile originates from the hydrothermal decomposition of ultra basic, primarily olivine-containing rock; particularly severe weathering occurs in subtropical and tropical climates.

Mines are located primarily in Ural (Bazenovo, Asbest, Dzetygara, Kiembaj), in Canada (Quebec, Ontario, British Columbia, New Foundland), as well as in South Africa (Zimbabwe, Botswana, South Africa).

Chrysotile, or white Asbestos, is the most widely used form of Asbestos. In the USA cat 95 % of all Asbestos types used in buildings is white Asbestos. Of the total German use of Asbestos, 96 % is chrysotile (1976).

· Amphibole:

Amphibole Asbestos types have a chain-like crystal structure, which stipulates their fibrous nature. Individual fibers have larger diameters, are straight, firm and hard, but elastic.

Amosite:

Amosite is a long-fibered Asbestos. Fiber length can reach 35 mm. Due to its needle-like structure it is a very dangerous type of Asbestos. On the basis of quantity (ca. 1% of all used types in Germany in 1976), amosite or brown Asbestos is second to only chrysotile as the most common form in buildings. It is primarily used in the manufacturing of light, fire-proof insulation sheets. The important mining areas are in South Africa.

Crocidolite:

The blue Asbestos is the most hazardous type of Asbestos and is primarily applied in pressure resistant pipes made of Asbestos cement. Economically, it is the most important type of amphibole (amphibole represented 3% of all Asbestos used in Germany in 1976, whereby > 90 % was from the manufacturing of pressure resistant Asbestos cement pipes). The diameter of fibers is very small: 0.1 - 0.2 mm; the surface of crocidolite consists of SiO4 - tetrahedrons. Important deposits are in South Africa and in West Australia.

· Mining:

The world production of Asbestos increased steadily until the early 1980's, since the economic value of the fibers stood in the foreground, although the risks of Asbestos have been known at least in part since cat 1930. The total world production of Asbestos peaked around 1976 at approximately 5.2 million tons (Mt). Currently the trend shows a steep decline (1986: 4.1 Mt, 1991: 3.5 Mt., US Bureau of Mines). The main producing and consuming countries were (see also the maps in Annex 1):

Table 1: Percentage of Asbestos Production among the Main Producing and Consuming Countries

Source: own compilation from different sources

Other important producing countries are currently: Brasil, Zimbabwe, China, Greece, India, Swaziland, Columbia and Japan. Meanwhile, the market shows a surplus of supply. The known worldwide supply of Asbestos ores will be sufficient to last far into the next century.

The largest part of Asbestos production is chrysotile (over 90 %), the remainder primarily crocidolite (ca. 4 %). The types amosite, anthopyllite, tremolite and actinolite are quantitatively (together < 2 %) of subordinate importance.

The annual tonnage, location and type of Asbestos at each mining site can be found in Tables 2 through 4 and the respective references.

Some of the most important Asbestos deposits are listed in Table 2.

Table 2: Asbestos Deposits

Asbestos Type

C = Chrysotile
T = Tremolite
An = Anthophyllite
A = Amosite
Cr = Crocidolite

Rock Formation

Se = Serpentine Rock
P = Periodotite
AS = Altered Sedimentaries
H&I = Harzburgite & Iherzolite
Do = Dolomite
AL = Altered Limestone
BI = Banded Ironstone
SP = Serpentinized Periodotite
D = Dunite
AM = Altered Marble
S&C = Altered Schist&Carbonates
US = Ultramafic Serpentinite
U = Ultramafic Rock

Source: Schreier, H.: Asbestos in the Natural Environment, Studies in Environmental Science, Amsterdam 1989

Table 3: Estimated Production Capacity of Asbestos Useable in Industry

Source: Schreier, H.: Asbestos in the Natural Environment, Studies in Environmental Science, Amsterdam 1989

· Processing:

In the western world 75% of Asbestos rock is produced in openpit mining with blasting, and in total 85% of the worldwide Asbestos rock is produced in this manner. Mining production refers to the amount of fiber gained from Asbestos mills. The mass content of fibers in rock lies around 3 - 10%. The crude rock is crushed in the mines with breaking and pan grinding operations and separated into fiber bundles. This is a largely mechanical process, which consists of many steps of screening and air sifting. These procedures are illustrated in Figure 2.

The conditions in the important Asbestos producing countries in Africa are markedly different. In Simbabwe and Swasiland Asbestos production is performed exclusively through deep mining, in the Republic of South Africa to 95% extent. Asbestos production in China is also 40% underground mining. In Canada the percentage of deep mining for Asbestos lies just above 5%, in the former USSR deep mining is not considered to play a significant role.


Figure 2: Illustration of Asbestos Mining Along a Slope and Dry Processing (Source: 1980)

· Further Processing Asbestos Cement

Asbestos cement is a fiber composite material made of primarily chrysotile Asbestos and cement and in some cases other additives, such as quartz, pigments, etc.. The first technical and economic procedures for manufacturing this construction material were developed by Hatscheck (1900).

Currently, even in the developing countries the large-scale technical procedures used are exclusively variations of that introduced by Hatschek, which is according to paper technology.

The manufacturing of Asbestos cement pipes according to Mazza and Mattei (1913) is also based in principle on the suggestions of Hatscheck.

In a continuous procedure, Asbestos, spread open to fine fibers, is mixed with cement and a great deal of water into a thin liquid suspension and then thickened to paper-like sheets as the excess process water is removed via felt cloth. Depending on the type of production, further manufacturing operations are performed with presses for sheets or with wrapping machines for pipes. (A detailed description of the historical and technological development of Asbestos cement is provided by Klos, 1967)

Table 4: Asbestos Percentages and Types in the Most Important Asbestos Cement Forms

Source: UBA - Report 1/80

· Consumption

In 1979, the countries with the highest consumption of Asbestos in percent of the total were:

Table 5:Asbestos Consumption ( 1979)

Source: own compilation

From the maps in Annex [(with 1978-1981 data from the BGR/DIW study), it is apparent that the use of Asbestos materials primarily took place in the industrial nations. From this standpoint it can be argued that Asbestos is not a specific problem of the developing countries. Upon further analysis, however, it becomes clear that the present situation is different from that in 1981. Furthermore, in the study (BGR/DIW) no statements were made regarding how many Asbestos products are exported to the developing countries, leading to health risks there.