In the cyanhydric acid (HCN) synthesis workshop at a chemical plant manufacturing adiponitrile, a purification column was being cleaned by introducing air to bleed the cyanhydric acid contained inside, with the residual gaseous mix then being burned in a boiler. During this cleaning step, a separator tank exploded subsequent to backfire in a pipe during the handling operation to burn residual gaseous effluent. The gas alarm was tripped following an HCN release. All nearby municipalities were informed. 300 employees remained confined onsite for 2 hours; internal fire-fighters equipped with self-breathing apparatuses eliminated the HCN vapours within 20 min using water curtains. The quantity of HCN emitted into the atmosphere was evaluated at 25 kg. According to the operator, HCN concentrations would not have exceeded allowable limits beyond a zone immediately adjacent to the workshop.

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