In a chemical plant, a violent explosion took place during the batch production of chlorobenzorex from an aqueous solution of sodium borohydride contained in a feed tank on a movable truck. The workshop was devastated: roof ripped out, light walls displaced, solvent pipes ruptured by the projections. The feed tank was destroyed but the remaining process equipment was not damaged. Outside the workshop but near a door, a technician was projected against the containers by the shock wave. He sustained injuries on the ribs and was hospitalised for a month.

The sodium borohydride solution (44 kg of powder for 130 kg of solution) was prepared 2 hours before production in a closed 630 litre agitated feed tank. Since dissolving was problematic, the operating mode was recently modified to include prior heating of the solution by hot water surface runoff (45- 55°C). The reactor was placed under nitrogen circulation (N2) at 2h00 and then connected to the feed tank whose heating was stopped. At 2.15 pm, the feed tank’s N2 inlet was connected to the supply hose. At 2.20 pm, N2 supply to the feed tank started with the partial opening of the supply valves of the reactor and the valves at the bottom of the feed tank (at a flow rate of 100 l/h – P(N2) = 2.8 bar). At 2.40 pm, the N2 hose supplying the feed tank was abruptly torn out letting the gas escape though the opening created. The superfluous staff was evacuated from the workshop and an emergency team installed a valve on the N2 inlet to stop the leak. The team tired to decompress the feed tank in the reactor by opening the reactor valve to the maximum and then opening the feed tank valve that was difficult to access. At this moment, the feed tank’s manhole gave in a few seconds before it exploded.

Post-accident investigations revealed thermal decomposition of sodium borohydride triggered by an excessively high storage temperature (50 °C due to hot water surface runoff). Maintaining the temperature at 50 °C stabilised the decomposition rate in such a way that the small quantities of hydrogen (H2) formed could escape via the leaking feed tank. When the heating was stopped, the system evolved in an independent way: the temperature and pressure increased under the effect of heat resulting from the decomposition causing a thermal explosion. The inspection hole opened under a pressure of 15 to 20 bar followed by the rupture of the feed tank. The depressurisation of H2 in the form of a shock wave was responsible for the damage caused in the workshop.

Lastly, poor knowledge of the properties of hydride and absence of a critical analysis of the process were also responsible for the accident. Modification procedures and taking into account the various productions incidents would have certainly led to the missing critical analysis.