‘The Oppau explosion occurred on September 21, 1921 when a tower silo storing 4,500 tonnes of a mixture of ammonium sulfate and ammonium nitrate fertilizer 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.
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 20 m 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. [..]’