Following the explosion at the Hayes Lemmerz International Plant, the US Chemical Safety Board investigation report investigated six possible sources of ignition discussed below.
The first possible cause of the ignition was a thermite reaction of aluminium and iron oxide at a high temperature or appropriate velocity, which can release enough energy to ignite most flammable mixtures. The reaction may have been provoked by a large aluminium particle colliding with rusty iron or steel or by a piece of rusty metal hitting a coating of aluminium powder on a hard surface. It was discovered that steel objects such as screws were found in the drop box of the dust extraction system. The interior of the dust ducts were investigated and light coatings of aluminium were found inside, this revelation and the fact that the drop box was not emptied regularly, lead to the theory that a thermit reaction caused by a rusty piece of metal hitting a surface inside the drop box or duct caused the spark that set of the explosion.
Another potential cause of the spark was an impact spark of one of the steel objects fallen into the feed colliding with the duct or drop box. If the spark caused by the collision landed on aluminium dust, it could have resulted in a smouldering nest.
Significant amounts of burning embers in the aluminium melting furnace, due to periodic fluxing and drossing to remove impurities from the molten metal were not out of the ordinary for the operators. They were a result of hydrocarbon deposits from cutting oil and other combustibles which accumulated inside the fume hood. If these deposits broke of and fell into the molten aluminium, they could easily ignite and release embers. Those embers could have easily been extracted by the dust system in at least two openings and landed in the drop box where they could ignite the aluminium dust, making this another possible source of the spark.
A common source of ignition sparks documented for dust explosions is electrostatic discharge. It is common for aluminium particles to generate static discharge during transfer, however for an explosion to occur this needs to be coupled with a dust cloud. Aluminium is a conductive metal and it is likely that this charge would dissipate before such cloud could occur, hence it was not considered as a probable cause of the ignition.
Another common cause of ignition sparks regarding dust explosions is faulty electrical equipment or wire. However, no electrical equipment was present in the dust system, eliminating this option.
Aluminium powders self-ignite at 426.67°C, which is below the melting point of the metal (660°C). If dust accumulated in the vortex box and fell onto the molten aluminium, that material would immediately ignite. This material could then enter the cyclone and could be pulled by the dust duct. However, this version of events is unlikely due to the chip feed pipe being completely filed with chips to the point they were falling out. This would prevent a flame from passing through the cyclone into the dust collector. Hence, a hot surface was not deemed as a probable cause of the ignition spark.
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