Twenty-five kilos of chlorine gas (Cl2) were released at 2:50 p.m. from the sulphuric acid and bleach storage area at a chlorochemical plant. The network of chlorine detectors raised the alarm at 2:55 p.m. The operator initiated its internal emergency plan and alerted the nearby plants. The release was located on the sump of the sulphuric acid tanks’ retaining basin. The internal firefighters were able to abate the Cl2 vapours with fire hoses equipped with flat fan spray nozzles. The contents of the tank were sent to the bleach digester. A maximum of 12 ppm of Cl2 was measured at 10 m from the sump. The internal emergency plan was lifted at 4:10 p.m.

The inquiry conducted by the operator showed that, when the bleach tank next to the acid tanks was being filled, 2 m³ of bleach overflowed and part of it penetrated the wall separating the bleach tank’s retaining basin from that of the acid tanks due to a breach in the wall’s seal. The bleach then flowed by gravity to the sump of the acid tanks’ retaining basin where it reacted violently with the dripping sulphuric acid (an incompatible mixture produces Cl2) that had accumulated there following repeated tanker truck unloading operations. The bleach tank overflowed because the transfer operator had stopped the transfer only when the alarm on the “continuous” high-level measurement sensor triggered at 85% full, whereas the “on/off” vibrating level sensor had already triggered a high level alarm at 83% full.

The operator immediately drained the bleach retaining basin and sealed the separation wall (using a butyl membrane on the acid side and epoxy paint on the bleach side), before completely resealing the retaining basin of the 2 acid tanks, with the installation of acid-resistant tiling. The high-level alarm of the continuous level measurement sensor was reset to 83% full. Slaving of the shutdown of the bleach transfer pumps upon the high-level detection of the “on/off” level sensor of the bleach tank was implemented.