At a chemical facility producing a hormone for herbicides, an explosion accompanied by the splashing of viscous material occurred at 8:45 pm on a column used to distil dichloroethane (DCE) / methyl nitrochlorobenzene (NBE). A temperature increase at the base of the column, which had ceased 25 min earlier following difficulties encountered during drainage operations, attracted the attention of technicians at 9:10 pm. The explosion took place at the time this crew was working on the column; 5 technicians were injured, 2 seriously (one of whom would succumb as a direct result of injuries sustained). The column equipment was completely destroyed. The surrounding environment was not contaminated. The return of an acidic solution between 2 tanks by the siphon effect and its concentration in the column caused the NBE to decompose. The unit was subsequently modified.