At around 6 p.m., red fumes were seen emanating from the industrial effluent treatment facility at a surface treatment company. These nitrous vapours entered the treatment facility and were drawn into the facility’s two intake systems. They are then treated by the surface treatment unit’s scrubbing towers. The release of heat triggered the sprinkler system, resulting in 6 m³ of water flowing into the retention basin. The fire alarm also went off. The fire brigade set up a safety airlock and evacuated 45 people. Production operations at the site were stopped. Nitric, formic, hydrochloric and hydrocyanic acid measurements, taken 80 m downwind, proved to be negative. Nitrous vapours damaged the control cabinet on the neutralization chain’s vapour extraction system.

The release of nitrous vapours came from a mixture of incompatible products in a used bath recovery tank that was to be removed. On the day of the event, 450 litres of concentrated nitric acid and 150 litres of formic acid were emptied into the recovery tank. A few hours later, the same technicians manually empty 725 litres of an expired product into the same tank without knowing its composition. This product contained sodium hypochlorite, which reacted with the acids already present in the tank. The reaction was catalysed by high concentrations of metal (nickel) compounds from the disposal of the expired product. As a result, the operator highlighted several organisational dysfunctions:

• unrestricted access to the discharge outlet of the baths’ recovery tank. No physical device prevents access to this outlet;
• no formal procedure for the disposal of expired products in pure form and no product safety data sheet;
• a lack of awareness on the part of these technicians regarding the risks associated with the mixing of incompatible products, since they made the decision to release a product without having sufficient information to assess the risk accurately.

The operator took several measures following the accident:

• isolation of expired products in a locked storage facility;
• restricted access to areas where corrosive effluents are discharged into recovery tanks at the site;
• a specific procedure was set up related to the treatment of expired products;
• implementation of a system to monitor the acid bath recovery tank;
• implementation of a system to have product spills validated by a chemist.