An explosion occurred in the aluminium nitrate (Al(NO3)3) production unit at a chemical plant. The process involved exposing 300 kg of aluminium powder in the upper part of a 6-m³ multipurpose reactor to nitric acid (HNO3) emitted by gravity flow from a 1875-litre dosing device. The Al(NO3)3 formed was dissolved in 2000 litres of water present at the bottom of the reactor. A programmable controller regulated this acid flow rate over roughly 10 hours.

A runaway reaction would have caused a pressure rise in the reactor, followed by an explosion, which projected the manhole cover, 30 min after the acid began to flow. A technician, who sustained a throat injury when struck by shattered glass, was hospitalised and 13 other employees experienced shock. The unit room was damaged: 16 m² of roof were blown off. Traces of nitric acid and nitrous vapours were detected inside the building, but no noteworthy pollution beyond the room was reported.

The technician had noticed the runaway reaction from its outset: temperature rise, activation of the level detector alarm, considerable bubbling, expulsion of some reaction mass into the venting duct. Once the unit had been placed in safety mode, the vent duct was sprinkled with cold water. The technician had respected procedures, but the programmable controller managing HNO3 flow failed to recognise the defect.

The investigation undertaken revealed that the quality of raw materials was to blame: the aluminium powder contained 5% fines (< 36 µm) vs. 0.15% under typical specifications, knowing that the narrower the particle size distribution the more highly reactive the aluminium powder.

This accident was thus caused by a runaway reaction triggering rapid pressure rise in the reactor (between 14 and 18 bar) induced by turbulence with foam generation that blocked the vent. Unit production resumed several months later with a new reactor (9 m³ in size, equipped with a rupture disc and double-shell cooling) and a new operating protocol: aluminium powder (<1% fines) introduced into an acidic environment (pH<4) in order to avoid hydrogen formation; an initial temperature needing to lie between 82° and 85°C (if T<80°C: delayed start-up; if T<78°C: violent reaction due to HNO3 accumulation); acid flow managed by a programmable controller; and a secure reactor stirring step.