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Asbestos mining processes: where and how was asbestos mined around the world. This article describes the processes used to mine and prepare asbestos for industrial use, describing both hand and machine mining and asbestos or preparation.
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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The method of mining, milling, and processing asbestos depends on the type of asbestos being handled as well as such specific physical characteristics as soft versus harsh fiber. Since chrysotile fiber accounts for approximately 95 per cent of all the asbestos used, this discussion will pertain to Canadian chrysotile.
Photo at left illustrates Crocidolite asbestos under the microscope, photo by Daniel Friedman while at McCrone Research Institute. [Click to enlarge any image]
For a satisfactory mining operation, the content of fiber in the rock should range from 3 to 12 per cent. Such variables as availability of labor, transportation facilities, and size of the deposit determine whether or not a low asbestos content mine will be a profitable operation.
Separately at ASBESTOS PRODUCTION QUANTITY we give tables of the quantities of asbestos ore that was produced, organized by country and by asbestos ore type.
Asbestos Ore Mining Methods
Description of Hand-Cobbing Asbestos Mining
The hand cobbing asbestos mining operation involves releasing fiber from the asbestos ore rock by means of manually (by hand) hammering away the rock from the ends of fibers. This type of fiber is not processed through the standard crushing units during the milling operation of the shorter filers. See Figure 1.4 at left
Several processes or systems are used by the mines to process the milled fibers in order to release rock and fiberize bundles of fibers.
Figure 1.4. Single-Fiber Asbestos Vein (hand cobbed).
Description of Asbestos Ore Rock Processing by Machine
Asbestos fiber is found in rock deposits classified as cross fiber, slip fiber, and mass fiber. Asbestos usually occurs as cross fiber in which the fiber runs at relative right angles to the walls of the rock; see Figure 1.5 shown at left.
The asbestos fiber length rarely exceeds 2 or 3 in.
Figure 1.5. Cross-fiber asbestos veins
The average long fiber asbestos for chrysotile is 3/4 in. In some cases a bend occurs in the fiber which results in a fiber that is weak at the point of benching.
In slip fibers, the fibers lie in the direction parallel to the seam or crack in the rock. Generally, this type of fiber is weaker than the cross fiber. It is considered unsuitable for spinning.
In the mass fiber classification, there is a complete absence of any orientation of fiber. The asbestos is distributed at random. This type of formation generally occurs with the amphibole varieties of asbestos. The fibers tend to be soft.
There are two methods of extraction which are similar to those used for iron ore, namely, open pit and underground mining. The open pit method is the more desirable because it is much cheaper and less complicated.
To open a quarry or pit mine, it is first necessary to remove surface matter. After this, standard blasting techniques are used to loosen the asbestos- containing rocks.
Milling / Crushing Methods for Asbestos Production
After the asbestos ore has been mined, the material goes through repetitive crushing operations to separate the asbestos from the rock. Crushing and screening operations occur. These operations are referred to as milling. Special mill equipment is required for these operations.
The methods used in the various mines are basically the same; however, they generally differ in size, depending on the mine capacity and type of fiber.
Figure 1.6. Big chunks of asbestos rocks are dumped into the jaws of a primary crusher to be broken into rock less than 6 in. square. After further crushing, the ore is milled to remove valuable asbestos fiber and the useless rock, or "tailings," is discarded onto a waste pile. (Courtesy Ruberoid Corp.)
In milling, the two major objectives are to separate asbestos from the rock and to prevent reduction in the length of the asbestos fiber. The value of asbestos fiber depends mainly upon fiber length; thus, the prevention of reduction in length is vital.
The primary crusher, usually of the jaw type, has the capacity to handle rocks up to 48 in., weighing as much as a ton or more, and to crush them to approximately 5 to 6 in. in width. See Figure 1.6 at below left.
Crushed rock is carried to trommel screens. These screens are large revolving cylinders with openings of various size. The trommel screens permit only the relatively small rocks, approximately 5 to 6 in. in width, to continue to the secondary stage of crushing.
This type of crusher is generally a gyratory or cone crusher. Crushed rock is dried in tower or rotary driers which vary in temperature from 2000 to 1,000° F.
Figure I.6. Big chunks of asbestos rocks are dumped into the jaws of a primary crusher to be broken into rock less than 6 in. square. After further crushing, the ore is milled to remove valuable asbestos fiber and the useless rock, or "tailings," is discarded onto a waste pile. (Courtesy The Ruberoid Co.)
In order to remove asbestos successfully, the material must be dry. The mined asbestos contains moisture which varies depending U0fl weather conditions and the season of the year. The moisture is removed by means of mechanical driers. The dry fiber can then easily be removed by air suction. The process of separating asbestos from rock during the milling operation takes into account the specific gravity difference of the two materials.
After the drying operation, the rocks which range in size from 2 to 3 in. are fed into a third crushing unit which is H either of the gyratory type or of the cone type. The crushed ore and fiber are fed to heavy shaker or bumper screens which are operated by means of eccentric drives. Powerful air suction hoods remove fiber. The rock passes over the end of the screens. The undersize material then passes through the screen, after which it is fed to a fiberizer for further treatment..
Additional shaking screens are used to separate fibers into long , medium and short categories. Duplex shaking screens are used. The long fibers remain on time top of time screen, the medium fibers in time lower screen and time short fiber passes through both screens.
The asbestos ore rock which was passed over the end of the heavy shaking screens continues to other crushers. The cycle is repeated whereby the crushed rock passes over shaking screens and in turn fiber is removed by air-suction.
After the last crushing cycle, the remaining rock is separated by means of fiberizers. The fiberizers break the rocks by means of impact or direct blow of high speed hammers. The crushed stone is again fed through the shaking screen in order to remove additional fiber. See Figure 1.7 at left.
Figure 1.7. Shaking tables are the key to asbestos fiber separation. Finally crushed ore is passed over a Vibrating SCCefl which forces fibers to lice above the heavier bits of rock.
The milling operations are continuous. After the crushing and suction operations, the asbestos is cleaned, graded and put into bags for shipment.
At the end of the table the vacuum pick-tip captures the loose asbestos, draws it through grading screens and into storage bins for packing. (Courtesy The Ruberoid Co.)
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Web search 01/20/2011, original source: http://epa.gov/asbestos/pubs/verm_questions.html
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