Explosion d'Oppau

The Oppau explosion occurred on September 21, 1921, when approximately 4,500 metric tons of a mixture of ammonium sulfate and ammonium nitrate fertilizer stored in a tower silo exploded at a BASF plant in Oppau, now part of Ludwigshafen, Germany, killing 500–600 people and injuring about 2,000 more. The plant began producing ammonium sulfate in 1911, but during World War I when Germany was unable to obtain the necessary sulfur, it began to produce ammonium nitrate as well. Ammonia could be produced without overseas resources, using the Haber process, and the plant was the first of its kind to do so in the world. Compared to ammonium sulfate, ammonium nitrate is strongly hygroscopic, thus the mixture of ammonium sulfate and nitrate compresses under its own weight, turning it into a plaster-like substance in the silo. The workers needed to use pickaxes to get it out, a problematic situation because they could not enter the silo and risk being buried in collapsing fertilizer. To ease their work, small charges of dynamite were used to loosen the mixture. This highly dangerous procedure was in fact common practice. It was well known that ammonium nitrate was explosive, having been used extensively for this purpose during World War I, but tests conducted in 1919 had suggested that mixtures of ammonium sulfate and nitrate containing less than 60% nitrate would not explode. On these grounds, the material handled by the plant, nominally a 50/50 mixture, was considered stable enough to be stored in 50,000-metric ton lots, more than ten times the amount involved in the disaster. Indeed, nothing extraordinary had happened during an estimated 20,000 firings, until the fateful explosion on September 21. As all involved died in the explosion, the causes are not clear.