Explosion in a aluminium manufacturing plant

A explosion occurred in the gas and dust treatment system for the aluminium holding furnaces at a metallurgical plant. The explosion caused the failure of the treatment system, the operation of the production lines without gas treatment resulted in the release of chlorine and chlorinated compounds into the atmosphere, as well as dust containing aluminium and zinc particles in various chemical forms.

The INITIATING CAUSE of the explosion was a buildup of dust in the holding furnaces' suction ducts. This metallic dust reacted with the caustic soda at the base of the column, producing hydrogen.
These same hot particles were the IGNTION SOURCE, by providing the activation energy necessary to ignite the hydrogen.
These materials are composed of aluminium and zinc in their metallic form, as well as oxides, hydroxides, and various salts. They react with water, and this reaction is amplified in the presence of caustic soda and at high temperatures. This phenomenon had not been detected in the risk assessment, and therefore they were not considered in the safety provisions of plant operations.

DESCRIPTION OF THE GAS & DUST TREATMENT SYSTEM
The gas treatment system consisted of a hydraulic treatment zone designed to suppress dust. The gas flow passed through a venturi system composed of two water spray nozzles. The gases and fumes then pass through a demister and are subsequently sent by a fan to a scrubbing tower. Within this tower, a caustic soda solution is diffused by a system of four nozzle arrays to remove chlorine compounds present in the fumes. These fumes are then sent to a second demister and then to an electrostatic precipitator.

Following the accident, the plant operator introduced these modifications:
(1) Adoption of a procedure for weekly tests aiming at detecting fouling or clogging of the suction ducts.
(2) Modifications to the automation program, to be able to shut down the gas purification system upon detection of high temperatures.
(3) Installation of a vacuum relief valve upstream of the fan, and of a flow meter upstream of the electrostatic precipitator with an alert threshold.
(4) Installation of a webcam in the room for verification purposes.
(5) Implementation of a lockout/tagout procedure, to prevent access to the room while the system was operating.