At around 12:20 p.m., a multi-gas detector at a hazardous waste treatment facility went off following an exothermic reaction in the transfer room. A chemical engineer and plant site manager saw a large amount of smoke coming from a tank used to collect liquid waste. They cooled the tank’s outer shell using fire-hose reels. The smoke thickened and turned yellow-green. The response teams donned SCBAs. At 12:27 p.m., an explosion ripped open the tank. The operator implemented the internal emergency plan and evacuated the workers. Firefighters took over the response efforts at 12:45 p.m. Measurements taken with a thermal imaging camera showed a temperature inside the tank of 40 °C. The mobile chemical response unit (CMIC) recorded chlorine concentrations of 1.5 ppm in the transfer room and 0.7 ppm in the secondary containment under the room. The chemical reaction was halted. The temperature as well as the chlorine concentrations in the ambient air dropped and the smoke stopped. The firefighters left at around 2:30 p.m.
The chemical engineer, who was near the tank when it exploded, was taken to hospital for minor injuries. After being checked over, he was brought back to the plant at around 4:30 p.m. The 5 m³ of water used to knock down the smoke and the substances spilt from the tank were collected in secondary containments and then pumped to a tank for disposal. The affected areas were cleaned up. The effluent and sludge in the blown-out tank were sampled and analysed. The use of fire-hose reels to cool the tank knocked down potentially chlorine-laden smoke.
The exothermic reaction that led to the explosion was caused by a handling error. A technician had emptied between eight and nine 30 l containers of 25% sodium chlorite (oxidiser) into a 1 m³ tank containing an organic substance (varnish or ink). He finished at around 11.30 a.m. Fifty minutes later, the sodium chlorite reacted violently on contact with the organic substance and released chlorine. Sodium chlorite had not been identified as an oxidising substance during identification tests conducted at the customer’s site and at the waste treatment facility. It had been classified as a liquid mineral base and was not often brought to the facility for treatment. The technician, who was a temporary worker, had just completed a period of accelerated training/coaching due to the successive departures of several qualified chemical engineers. The event occurred while a portion of the plant was on its midday break. In addition, some staff were on holiday because it was the week between Christmas and New Year’s Day. However, the operator said that the plant was not operating at partial capacity.
The prefect issued an emergency order calling for an analysis of the accident’s causes and a review of the safety report (characterisation release scenarios for toxics) and the risk control measures.
The operator plans to install:
- a smoke extraction system (interlocked with detection) to handle emergencies;
- an automatic water-spray smoke abatement system;
- a fixed multi-gas detector with visual and audible alarms in the transfer area.
He also:
- Analysed the reasons why the oxidiser characterisation test was ineffective and changed it: in addition to the litmus paper and potassium iodide paper tests, a peroxide test will be conducted on bases to detect chlorites;
- Wrote instructions for transferring mineral bases: to be done in new, clean tanks only and not tanks that have previously been used. All containers must be opened and emptied by a chemical engineer under the supervision of a qualified chemical engineer;
- Increased the training period for newcomers and instituted a training assessment;
- Instituted monthly emergency-preparedness exercises.
Multiple accidents likely related to incompatible mixtures occurred at the site in 2017 and 2018 (50866, 50867, 51928).
