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The Oppau explosion happened on September 21, 1921 when a tower silo storing 4500 tonnes of a mixtue of ammonium sulfate and ammonium nitrate fertilizer exploded at a BASF plant in Oppau, now part of Ludwigshafen, Germany, killing 500-600 persons and injuring about 2000 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 was able to be produced without overseas resources, using the Haber-Bosch process.
Compared to ammonium sulfate, ammonium nitrate is strongly hygroscopic, so the mixture of ammonium sulfate and nitrate clogged together under the pressure of its own weight, turning it into a plaster-like substance in the 20m high 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. The procedure was tried experimentally and was considered safe. Nothing extraordinary 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. A theory is that the mixture changed and a higher concentration of ammonium nitrate was present.
Scale of the explosion
The explosion was estimated be equivalent to about 1-2 kilotonnes of TNT and was heard as a loud bang in Munich, more than 300km away. The pressure wave ripped roofs off up to 25km away and destroyed windows even farther away. In Heidelberg (30km from Oppau), traffic was stopped by the mass of broken glass on the streets.
About 80 percent of all buildings in Oppau were destroyed, leaving 6500 homeless. At ground zero a 90m by 125m crater, 19m deep, was created.
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