The likely causes for the explosion of an ammonium sulfate nitrate silo in Oppau 1921 are reviewed on the basis of publications from 1922 - 1926. Ammonium sulfate nitrate is a mixture of 2 salts: ammonium nitrate (explosive) and ammonium sulfate (inert). The explosiveness of the mixed salt is mainly determined by the ratio of these 2 salts. The influence of the mixed salt composition on the sensitivity to detonation impact had been thoroughly investigated before the accident as it was common practice to use small explosive charges to loosen the salt which had solidified during storage in the silo. About 20.000 blasting operations were carried out without any notable incident. Experiments conducted after the accident revealed that several physical parameters (particle size, density, water content, homogeneity of crystal structure) also had an influence. The introduction of a new drying process for ammonium sulfate nitrate (spray process, "Spritzverfahren") caused changes in all these parameters resulting in a salt with increased explosiveness. In addition a dust-like fine fraction with increased ammonium nitrate content was formed which accumulated at the edge of the silo. It can be assumed that on the day of the accident at least one of the explosions for loosening the caked salt was carried out in the area of the fine fraction, thus initiating a detonation of this fine fraction and causing the detonation of further fractions with approximately "normal" nitrate content. This accident is a strong example for the need of Management of Change (MOC) procedures, which are an integral part of today's process safety.
