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Asbestos: how was asbestos material classified for purposes of manufacturing asbestos-containing products: classifications of raw or mined asbestos ore, fibers, or dust. Here we describe the standard asbestos product material classes that were based on fiber length and other properties.
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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Grading of asbestos is generally (lone on the basis of length rather than on quality. The qualities winch are important to the fibers are flexibility and fineness. The more flexible and fine fibers are used for spinning and weaving textile products. The shorter milled fibers are used in such varied products as asbestos-cement and tile.
The Canadian Asbestos Producers' have set up what is referred to as the Quebec Standard Asbestos Testing Machine for classifying milled asbestos. This method is based on the mechanical sieving of fibers. There has been a general trend to adopt the Canadian Standards Classification; however, many of the mines in Canada still retain their own methods which are specifically tailored to their customers. The classification divides chrysotile into two main classes— crude and milled asbestos.
The crude asbestos consists of the hand selected crossveining material essentially in its native or unfiberized form. Its length can be determined by combing. Milled asbestos consists of all grades produced by the mechanical treatment of asbestos ore. These two main divisions are subdivided into different groups ranging from group No. 1 to No. 9. Some of these groups are listed in Table 1.1. [Click to see an enlarged, detailed version of this table]
Table 1.1. Classification of Some Types of Chrysotile Asbestos According to Groups
The Quebec Standard Asbestos Testing Machine consists of three stacked screens and a pan which rests on a table. See Figure 1.3. These screens are driven by an eccentric gear so that an elliptical motion is produced on the screens. Each screen measures 24½ in. by 144 in. with each of the boxes being 3 1/2 in. in depth. The top box which is identified as box 1 consists of a wire screen with 1/9 in. openings, box 2 has a 4-mesh wire, box 3 has a 10-mesh wire and box 4 is a closed receptacle which collects all matter filtered through the other three boxes. The motion of the boxes and the time that the fibers are subjected to that motion are fixed.
Figure 1.3 (at left) Quebec Standard Asbestos Testing Machine. (Courtesy Johns Manville Corp.)
The test procedure is such, that it introduces 16 oz. of asbestos on the top tray. After two minutes of motion, the fiber in each tray is weighed. The more fiber retained on the first screen and the less fiber falling into the pan the higher the grade of product. As an example Group No. 3D specifies 10.5-3.9-1.3-0.3.
The greatest increase in the use of fibers has been in the shorter lengths that comprise approximately 50 per cent of Canadian asbestos production. The classification of shorts and floats follows the same principle of the shaker test; however, other factors such as bulk, grit percentage, and viscosity are involved.
The term refuse is a railway freight classification; it does not mean that shorts and floats are byproducts of asbestos milling. As the names imply, floats and shorts are fibers so fine and light that they are collected by air flotation. They are precipitated into float chambers by gravity settling or collected by filtering media.
Shorts and floats have shown a remarkable increase in use for several reasons. Quality improvements have developed new uses; improved finished products have resulted in the shorter fibers supplementing some of the longer fiber applications. Markets have expanded for such relatively new or growing industries as the vinyl tile, plastics, asphalts, and asbestos-cement industries.
Other methods of classifying fibers have different identifications depending upon the type of asbestos, length of fiber, fiberization, and other characteristics. In some cases, the mines add additional letters or numbers to the Quebec Standard Testing Machine nomenclature for these products. Asbestos designations by other methods do not tally with the Quebec. Classification. Other asbestos classification methods are reviewed in Table 1.2 shown below.
Table 1.2 - Classifications of Types of Asbestos Fibers & Materials
[Click to see an enlarged, detailed version of this table or its continuation just below]
Various machines and apparatus are available for obtaining accurate measurements of the length of asbestos. One of these units is identified as the Bauer-McNett Classifier; it is manufactured by the Bauer Brothers Company, Springfield, Ohio. This apparatus utilizes four elliptical tanks, each with a screen of different mesh. The four tanks are arranged in cascade so that the material introduced into the highest tank passes by gravity through the other three, and the discharge from the lowest tank carries the finest material to waste. Each tank has a drain at the bottom which is closed by a stopper.
The screen in each tank holds back fibers greater in length than the size of its opening. The coarsest mesh is in the top tank. Screens of different size mesh are used with the equipment.
Asbestos, Its Industrial Applications - Rosato: Text& Chapter Index 
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Web search 01/20/2011, original source: http://epa.gov/asbestos/pubs/verm_questions.html
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