A chlorine release occurred at approximately 8 p.m. in a chemical plant. Feeling ill, local residents contacted the emergency services, who in turn alerted the factory. The shift technicians were able to locate the odour at the iron purification reactor in the zinc chloride workshop. Zinc oxide was introduced into the reactor which stopped the release of chlorine at 8:25 p.m. The reactor remained shut down for 48 h, while the neutralisation reaction was maintained.

The release was attributed to the accidental introduction of 1.8 t of 40% sodium chlorate (NaClO3) into the reactor before its initial filling phase with residual acids. The NaClO3 reacted with the ferrous iron of the acids and then decomposed to chlorine and chlorine dioxide in the presence of the HCl from the acids when the reactor’s temperature increased.

The presence of NaClO3 could have been caused by an unexpected start of the NaClO3 transfer pump. Furthermore, on the day of the accident, the pump’s discharge valve remained open after filling the NaClO3 tank, although the procedure dictated that it be closed. Finally, the distance between the reactor and the NaClO3 tank did not allow a technician near the reactor to hear the transfer pump in operation.

The operator implemented the following corrective measures:

• installation of a pushbutton on the actuator of the NaClO3 transfer pump, requiring the technician to apply continuous pressure on it;
• systematic testing to ensure the absence of NaClO3 before starting the reactor heating process;
• installation of a key-operated emergency stop button on the NaClO3 transfer pump with an associated procedure to formally authorise the pump to be started;
• launch of a feasibility study to replace NaClO3 with hydrogen peroxide.