In a chemical plant, 60 kg of hydrogen cyanide (HCN) were released into the air for 15 min when restarting the hydrocyanic acid (HCN)/acetone cyanohydrin (CA) workshop after a triennial shutdown. Several HCN detectors in the unit sounded the alarm, the establishment’s internal emergency plan was activated, the unit was switched over to safety mode and staff were confined to the premises for 2 hours. The operator contained the gas emission by cooling the CA storage tank in question, in particular using the spray ring fitted on the tank. HCN measurements taken in the air at the site boundary were negative. The internal emergency plan was lifted 3 hours after activation. One technician was poisoned slightly and the 18 people present in the control room underwent a medical examination in the site’s sick bay. Operating losses related to late startup of the workshop were assessed at 0.4 million euros.

Thermal decomposition of the acetone cyanohydrin in the storage tank, due to the presence of sulphuric acid (H2SO4) and water (H2O), caused the emission of hydrogen cyanide and the pressure buildup in the tank, with the discharge occurring at the tank’s fire damper. The presence of H2SO4, which is a stabiliser for CA, at the bottom of the tank prevented the manual isolation valves of the H2SO4 injection chamber connected to the storage tank from closing for around ten minutes. The technician, aware of this, had considered the presence of H2SO4 in the tank as a risk due to its stabilising properties for CA. A leak test carried out on the water installations before restarting also explained the presence of water in the CA feed line. The exothermicity of the H2SO4 + H2O + CA reaction caused the increase in temperature of the medium and decomposition of the CA.

Following the accident, the internal emergency plan was revised to improve the communication to the neighbouring populations in the event of incidents, the safety report was revised to integrate the feedback from this event and the handwheel valve between the tank and H2SO4 chamber was replaced with an On/Off valve. An inventory was carried out on manual valves with the same configuration before any replacement. Furthermore, a study was carried out to decide whether the H2SO4 chamber was needed on this tank. Finally, staff training on toxic risks was improved.