Oppau explosion

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 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.

This apparently suicidal procedure was in fact common practice; it was well known that ammonium nitrate was explosive - it had been used extensively as such during World War I - but tests conducted in 1919 had seemed to indicate that mixtures of ammonium sulphate and nitrate containing less than 60% nitrate were unlikely to explode, on which grounds the material handled by the plant, nominally a 50/50 mixture, was considered stable enough to store in 50,000-tonne lots - more than ten times the amount involved in the disaster. Indeed, 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. However, it is known from work more recent than the abovementioned 1919 tests that the "less than 60% nitrate = safe" criterion is inaccurate; in mixtures containing 50% nitrate, any explosion of the mixture is confined to a small volume around the initiating charge, but increasing the proportion of nitrate to 55-60% significantly enhances the explosive properties and creates a mixture whose detonation is sufficiently powerful to initiate detonation in a surrounding mixture of a lower nitrate concentration which would normally be considered minimally explosive. Changes in humidity and density also significantly affect the explosive properties.

...
Wikipedia