The gas treatment system consists of a wet dust suppression system wherein a flow of gas passes through a venturi system consisting of two water-spray nozzles. The gases and fumes then pass through a mist eliminator before being sucked into a scrubber by a fan. Inside the scrubber, four spray bars spray a sodium-hydroxide solution to remove chlorinated compounds from the fumes. The fumes are then sent to a second mist eliminator and then to an electrostatic precipitator.

At about 3:10 a.m., an explosion occurred in the gas and dust treatment system of the aluminium holding furnaces of a metallurgical plant. The operator had an assessment performed to determine the causes of the accident. After the explosion, the production lines were operated without any gas treatment, resulting in chlorine and chlorinated compounds as well as dust containing various chemical forms of aluminium and zinc particles to be emitted to the atmosphere.

The explosion was caused by fouling of the holding furnaces’ suction ducts. Metal dust came off, reacted with the sodium hydroxide at the bottom of the column, and produced hydrogen. As the dust was hot, it provided the activation energy needed to ignite the hydrogen. The dust consisted of aluminium and zinc in the form of metal, oxide, hydroxide, and various salts. These forms react with water and the resulting reaction is amplified by sodium hydroxide soda and high ambient temperatures. The reaction had not been identified in the risk analysis.

After the accident, the operator instituted a weekly procedure to test for fouling or clogging of the suction ducts. The PLC program was modified to make the gas cleaning system shut off if high temperatures are detected. A vacuum breaker valve was fitted upstream of the fan and a flow meter with an alert threshold was fitted upstream of the electrostatic precipitator. A webcam was installed in the room to dispel any doubts. The room is now locked to prevent access to it when the facility is operating.