(104d) Case Study: Aluminum-Dust Explosion

An explosion occurred in equipment that was being used to pulverize aluminum-alloy ingots, to form a powder with small particle size. In this process, air was recycled through a grinder, cyclone product separator, and blower, with a side stream to extract undesired dust to a dust collector. However, the concentration of dust in the circulating air stream was above the Minimum Explosible Concentration in most of the recycle system. Ignition of this dust/air dispersion occurred as a result of an exothermic oxidation of accumulated combustible dust in the return-air ductwork, accelerated by the relatively high temperature of the circulating air. Because the system was not protected against internal explosion, an access door on the grinder was forcibly ejected, and the door struck the LPG cylinder on a nearby forklift truck. Escaping LPG vapors ? ignited by the dust fireball from the open grinder door ? resulted in a flash fire that caused fatal burn injuries to the forklift driver.

This powder-producing operation initially was a batch process, and it had operated without incident for many years. When a decision was made to convert the batch process to a continuous process, some hazardous changes were introduced that were not recognized. These changes included (1) a much higher rate of dust generation, (2) high concentrations of very fine dust in the recycle stream, and (3) accumulation of heat in the recycle stream. In addition, the installer of the recycle system did not have experience with combustible powders, and no explosion prevention or protection devices or systems were provided.

Among the lessons learned from this incident were: (1) every significant change in a process should be subjected to a Management of Change analysis; (2) producers of hazardous materials (and particularly small-company producers) need to follow Codes and Standards that apply to their processes; (3) code-enforcing authorities should become more familiar with Codes and Standards that apply to dust-explosion hazards; and (4) dust-explosion characteristics need to be included in Material Safety Data Sheets.