At around 2.30 pm in a unit specialised in the synthesis of active species for the pharmaceutical industry, 600 kg of hydrogen chloride (HCl) was released into the atmosphere forming a 20 m² cloud visible from outside the site (westerly wind). The accident occurred while transferring acyl chloride (production residues) into a tanker truck. The acyl chloride was stored pending its incineration at an authorised waste disposal centre. The rapid hydrolysis of the chlorides 4-butyroyle and butyroyl, an exothermic reaction, increased the pressure in the tanker being loaded causing its safety valve (calibrated at 0.5 bar) to open.

The internal contingency plan was put into action. The safely service set up two “peacock” water curtains to neutralise the acid fumes. The firemen arrived at the site around 2.40 pm and installed a 3rd water curtain. As the dome of the tank was hot to the touch, a spray nozzle was used to cool down the tank. Measurements taken 2 m from the tank indicated an HCl concentration of 3 ppm (threshold limit value for the workers: 5 ppm). The gas drainage line was rerouted to a “cubitainer” filled with water, and constantly replenished. This system appeared to be efficient as the releases of HCl was limited to just a few fumes. The incident was finally brought under control around 3.30 pm. Monitoring of the tank and the “cubitainer” was continued until 10/31, at which time the contents of the tank was transferred into 6 “cubitainers” stored in a covered but open building pending new solutions to remove these substances. The polluted water was recovered in a 5,000 m³ catchpit

The incident also highlighted the existence of a leaking valve on the catchpit (approx. 30 m³/h) that led to releases into the Garonne River. The operator offered two 2 hypotheses regarding this incident: the presence of water in the poorly drained and poorly dried tank or water combined with the HCl in the residues. Compatibility tests of these products had not been conducted. Preventive measures were proposed: bulk mixtures of containing acid chlorides is prohibited (residues must first be treated in an appropriate installation), strict control of each tank to be transported prior to loading to ensure water is not present, drying certificate, determination of the compatibility between various residues.