Flammability of metals and other elemental dust clouds

This paper reports results from a U.S. Bureau of Mines study of the flammability of various metals and other elemental dusts dispersed in air. The data are useful for evaluating the explosion hazards in the minerals and metals processing industries. The dusts studied included boron, carbon, magnesium, aluminum, silicon, sulfur, titanium, chromium, iron, nickel, copper, zinc, niobium, molybdenum, tin, hafnium, tantalum, tungsten, and lead. The dusts were chosen to cover a wide range of physical properties—from the more volatile materials such as magnesium, sulfur, and zinc to the highly “refractory” materials such as carbon, niobium, molybdenum, tantalum, and tungsten. These flammability studies were conducted in the Bureau of Mines 20‐L explosibility test chamber, using strong pyrotechnic ignitors. The parameters measured included the minimum explosible concentrations, maximum explosion pressures, and maximum rates of pressure rise. All of the elemental dusts ignited and burned as air‐dispersed dust clouds except the nickel, copper, molybdenum, and lead. In general, the dusts with the highest explosion pressures and rates of pressure rise were also those with the highest calculated, adiabatic flame temperatures and/or the ones that vaporized most easily, but this was not true in all cases. The effect of particle size on flammability was studied for the aluminum and iron dusts. The minimum explosible concentrations were relatively independent of particle size below 30 μm, but the highest explosion pressures and rates of pressure rise were found at the finest sizes tested.