At around 10:00 a.m., a fire broke out in the filter system of the dust collector installed in a machine cleaning room used to crush metal at a metal powder manufacturing plant. The dust collector was connected to a cleaning system to which a crusher was connected for cleaning. The crusher had been running under vacuum for one hour. Smoke, accompanied by an odour, was seen coming from outside the mechanical cleaning room. Two maintenance technicians discovered that the fire was coming the dust collection tank in the mechanical cleaning room.

Several factors caused the filter cassettes in the dust collector to catch fire. Firstly, they were made of polypropylene, which is combustible. Secondly, operating the crusher under vacuum meant that the ambient air (i.e., oxygen) in the room was drawn into the dust collector. Lastly, significant overheating of the dust inside the crusher, which was then drawn into the dust collector, created the ignition source. Consisting of cobalt and magnesium, this fine powder was flammable and had oxidised little as the crusher had recently been disassembled. At the time of the accident, more powder than usual was being drawn into the system. The heat and the fact that the doping solution had not been analysed for one month led to a doping solution with an excess concentration of magnesium. The powder became sticky and fouled the dust collector.

Since the accident, the operator plans to:

• replace the filter system with one made of non-combustible materials;
• pre-treat the dust-laden air using a cyclone separator, scrubber, or a suspended particles trap to reduce the amount of dust in the filters;
• install heat detectors fitted with quick cut-off and isolation and CO2 injection systems.

The operator also reminded plant operators of the manufacturing procedures to be followed.