At around 3:50 p.m., acid fumes were emitted from a 10 m³ mixing tank at an industrial detergent formulation plant following an exothermic runaway reaction of a cold 5500 l reagent mixture (60% water, hydrogen peroxide, sulphonic and sulphuric acids). The operator implemented the plant’s internal emergency plan. One employee, affected by acid droplets that had passed through the ventilation duct, was taken to hospital and released that evening. The effervescence of the reaction spilt a portion of the mixture onto the ground, into a sump at the foot of the bund, and onto the facility’s ventilation ducts. The facility was cleaned. The spilt mixture was pumped out and eliminated by an approved route. The material damage was slight: the tank was intact; only the plastic reagent supply hatches were displaced by the excess pressure. The operator issued a press release.
The inquiry confirmed an exothermic temperature greater than 50 °C and short-term production of effervescent foam (less than 2 min.). Half the contents of the mixture were thrown out of the tank. This was the first time that such a large of amount of the formulation (which was not exothermic) had been made in this tank. Until then, only a small amount (1500 l) had been produced over a one-year period because demand was low. Before the accident occurred, production of the mixture had ended at 11:30 a.m. and the batch had been declared satisfactory at 2:30 p.m.
An analysis of the mixture remaining in the tank showed a higher-than-expected acid content, the absence of hydrogen peroxide, and an unusually high iron content. This confirmed an exothermic decomposition of the hydrogen peroxide in water and oxygen following a change in the production operating conditions, which caused the rise in acidity. The change in the operating conditions was caused by iron from degradation of the impeller’s steel shaft (damaged protective polypropylene matrix: 50 cm cracks). Acid amongst the reagents accelerated corrosion of the shaft and leached ferrous compounds into the tank for several hours until the mixture was transferred out for packaging. The dissolved iron subsequently acted as a catalyst for decomposition of the hydrogen peroxide.
The tank had been repaired two weeks before the accident and had held four production batches. The operator checked the impellers of its other mixing tanks and the seals on each process component in the workshop. It changed the acid ventilation system and adopted a procedure for visually checking the impeller shafts of its tanks. It also conducts annual in-house inspections of these shafts.
