Document EvaeayDZvdRV8VB5gJYDm4z6x

TABLE 3. - Roclc-dusting in West Virginia R. I. 3U65 Rock dust used per ton of Year Rock dust coal mined in used, rock-dusted 193S tons mines, pounds 32,495.00 -- 1937 40,775*72 i93S , 42,69s.83 1.297 1.562 1939^/ 12,587.62 1.596 1/ January to March. Mines using rock dust 133 162 179 16S Rock dust used per mine, tons 244 245 239 Commercial mines 5I+2 549 536 - It is common for interior surfaces of coal mines in many parts of the United States to "sweat" during the summer months as moisture in the air is deposited on the roof, ribs, and timber. In some mines the rock dust on the ribs and roof becomes "slimy" mud while in others it may be dry or moist. This absorption of moisture virtually destroys the effect of the rock dust. If coal dust, usually difficult to wet, is mixed with rock dust the entire mixture becomes wet; a thin layer of coal dust settling later on the surface of the wet rock dust is likely to become wet also, but the top of a thick layer of coal dust may remain dry. On drying, some dusts remain caked and will not disperse readily, if at all, while othersvirtually regain their original dispersibility. Sampling. - Passageways should be sampled at regular intervals to give an accurate cross section of the condition of the mine with respect to dust. Analysis of dust sample. - A comparatively simple fire analysis may be used to determine the total combustible matter, except when limestone is used. The Bureau of Mines modification of the Taffand volumeter may be used to determine the combustible content of any kind of rock dust and is accurate within practical limits. Extent of rock-dusting. - The Bureau of Mines recommends that all coal mines except anthracite be rock-dusted in every part and that rockdust barriers be used as a secondary precaution but not in lieu of general dusting. TYPES OP DISTRIBUTORS Mechanical rock-dust distributors were assembled as early as 1911, in which year W. J. Murray, of the Victor American Puel Co., developed, a rock-dust distributor employing a fan, hopper, and flexible hose. This machine was used in the Delagua mine in Colorado. In 1914 the Bureau of Mines assembled a fan, nozzle, hopper, and rubber hose and mounted them on a truck, which was used principally for demonstration purposes. 7239 - 7-