Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/883641
Bill Langer is a consulting research geologist who spent 41 years with the U.S. Geological Survey before starting his own business. He can be reached at Bill_Langer@hotmail.com CARVED IN STONE 32 AGGREGATES MANAGER / October 2017 M y maternal grandfather was a mechanic who worked in the coal mines in Pennsylvania. My only memories of him start after he retired, but I heard him complain about how dusty it was in the mines. Coal dust not only presents concerns for the workers' health, but also is of concern for coal mine explosions. Grandpa worked in the anthracite region. At the time, the U.S. Bureau of Mines (Coal Dust Explosions, 1911) had already determined that — although anthracite dust may ignite when suspended in the air — "there seems to be little danger of this under ordinary mining conditions." Why this is so is beyond the scope of this article, but suffice it to say that, by definition, anthracite is not volatile enough to serve as a fuel for an explosion. It is a completely different story in under- ground bituminous coal mines where the risk of widespread explosion disasters is quite substantial unless proper precautions are taken. You may recall that there are three necessary elements that must occur simultaneously to cause a fire; fuel, heat, and oxygen (the three sides of the fire triangle). When you remove any one of these elements, a fire cannot occur. For an explosion to occur, two additional elements, suspension and confinement, also must occur simultaneously with the three fire elements. A technique referred to as rock dusting can help disrupt the explosion pentagon. Rock dust (pulverized aggregate) has been used in the U.S. and many major coal producing countries as a primary means of defense against coal dust explosions in underground coal mines. Rock dust can be a variety of light colored rock types, but commonly is pulverized limestone or dolomite. There are specific limits on the particle size, amount of combustible material, and silica in the rock dust. In addition, when rock dust is wetted and dried, it should not form a cake that cannot be dispersed into separate particles by a light blast of air. The proper application and maintenance of rock dust that covers specified areas of mines commonly can remove one or more sides of the explosion pentagon. In a low-velocity explosion, the heavier rock dust covering the lighter coal dust serves as an isolator that prevents the coal dust underneath from becoming airborne, thus removing the suspension side of the explosion pentagon. However, a high-velocity explosion can quickly reach supersonic speed and generate a shock wave that suspends coal and rock dusts in the mine air. The airborne rock dust absorbs the heat from the hot air and lowers its temperature, thus acting as a heat sink, reducing the heat side of the explosion pentagon. If the temperature of the explosion exceeds 1,472 degrees F, the main component of limestone rock dust, CaCO 3 , almost in- stantaneously will be converted into lime (CaO) in an endothermic (heat adsorbing) chemical reaction. The thermal energy con- sumed in the limestone-to-lime conversion can greatly reduce the heat side of the explosion pentagon and, thus, greatly reduce the intensity of the coal dust explosion. Mine dust production is only a small portion of the overall crushed stone production. During 2014, about 816,813 short tons of crushed stone, worth about $19,700,000, were produced for mine dusting or acid water treatment. (The two uses are grouped to prevent disclosing proprietary data.) You can bet that not many coal miners complain when they see that kind of dust at their work site. AM The use of rock dust makes coal mining safer. Rock Dusting