Analysis of an explosion accident of nitrogen trichloride in a waste liquid containing ammonium ion and platinum black

•An explosion accident happened, and it was assumed to be caused by nitrogen trichloride (NCl3).•Generation of NCl3 is found to be accelerated by platinum black.•Platinum black worked as an explosion trigger of nitrogen trichloride.•Conditions of reducing a probability of such accidents have been extensively investigated. Five liters of sodium hypochlorite aqueous solution (12mass%) was poured into 300L of liquid waste containing ammonium ion of about 1.8mol/L in a 500 L tank in a plant area; then, two minutes later the solution exploded with a flash on March 30th, 2005. The tank cover, the fluorescent lamp and the air duct were broken by the blast wave. Thus, we have conducted 40 runs of laboratory-scale explosion tests under various conditions (solution concentrations of (NH4)2SO4 and NaClO, temperatures, Pt catalysts, pH, etc.) to investigate the causes for such an explosion. When solutions of ammonium sulfate and sodium hypochlorite are mixed in the presence of platinum black, explosions result. This is ascribable to the formation of explosive nitrogen trichloride (NCl3). In the case where it is necessary to mix these 2 solutions (ammonium sulfate and sodium hypochlorite) in the presence of platinum black, the following conditions would reduce a probability of explosion; the initial concentration of NH4+ should be less than 3mol/L and the pH should be higher than 6. The hypochlorite solution (in 1/10 in volume) to be added at room temperature is recommended to be less than 0.6mol/L.