a. The general name for coal particles of small size. In experimental mine testing, particles that will not pass through a 20-mesh screen-- 1/32-in-square (0.8-mm-square) openings--are not considered as coal dust. Rice, 2
b. In 1964, a series of laboratory tests were made with a spark source on aluminum powder and cornstarch (both dusts presenting a more severe explosion hazard than coal dust). It was found that particles passing a U.S. Standard No. 40 sieve (particles less than 0.016 in or 0.4 mm) contributed to an explosion in the laboratory bomb. The 0.016-in particle diameter was recommended as the definition for dust in surface industry. Thus, two definitions of dust exist. For coal mines, dust consists of particles passing a U.S. Standard No. 20 sieve (particles less than 850 mu m), and for surface industries, dust consists of particles passing a No. 40 sieve (particles less than 425 mu m). The use of two definitions is not incongruous since the potential igniting sources in a coal mine can be much more severe than those in surface industries. MSHA, 1
c. The dust produced by the breakage and crushing of coal underground and at coal preparation plants. It is usually intermixed with a varying proportion of stone dust. Coal dust in mines presents two main dangers: explosion hazard and pneumoconiosis hazard. The explosibility of a coal dust cloud depends upon its fineness, purity, and volatile content. The dust particles believed to be harmful from the pneumoconiosis aspect are those of 5 mu m and under. In mines, the most common explosive dust encountered is bituminous coal dust. The U.S. Bureau of Mines has established that coal dust in the absence of gas can explode and that explosions can occur in any shape of mine opening. See also: dust-free conditions
Source:
Dictionary of Mining, Mineral, and Related Terms












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