(52bd) PYROPHORIC METAL DUSTS:  SECONDARY EXPLOSION HAZARDS

Several types of liquid chemicals are known to ignite spontaneously upon exposure to air, and this includes metal alkyls such as trimethyl aluminum and Grignard reagents. However, the pyrophoric behavior of sub-micron particles of many metals is not as well recognized. This latter group of pyrophoric metals includes aluminum, magnesium, and zirconium. Additional metal dusts that can exhibit pyrophoric behavior are iron, titanium, and tin, but only in the form of very small particle sizes, of the order of 0.03 micron, as a “fume” or “smoke”.

Sub-micron metal particles can be produced by several types of mechanical operations, such as grinding, milling, crushing, pulverizing, and machining, and also by screening, if the processes are not well contained. The escape of sub-micron particles can result in “lofting” of the particles and deposition on elevated surfaces. Similar lofting behavior can result from the use of compressed air to clean equipment and other nearer-floor surfaces. Physical shock of a structure or a primary explosion can result in the dislodging of such dust accumulations, and ignition of the descending dust can result in a devasting secondary explosion. The presence of sub-micron dusts in equipment can create a primary-explosion hazard because of the extremely-low minimum ignition energy of such dusts and the high probability of a dense cloud within equipment as a result of the very low settling velocities.

This paper compiles the available data for metal dusts that are capable of pyrophoric behavior, including the effects of mechanical-processing temperatures on the hazardous particle sizes. Also shown are an extensive bibliography and graphs that show (1) hazardous particle-size distributions and (2) a method for evaluating the explosion hazards of metal [and organic] dusts, to aid in determining the types of systems, devices, and procedures that are needed for explosion prevention.