Vintage: Oppau ammonium sulphate and ammonium nitrate fertiliser explosion in 1921, Germany
The Oppau explosion occurred on 21 September 1921, when approximately 4 500 tons of a mixture of ammonium sulphate and ammonium nitrate fertiliser stored in a tower silo exploded at a BASF plant in Oppau, located along the river Rhine, now part of Ludwigshafen in Germany, killing 500 to 600 people and injuring about 2 000 more. Two explosions, half a second apart, occurred at 7h32am on 21 September 1921 at Silo 110 of the plant, forming a crater 90 by 125m wide and 19m deep. In these explosions 10 percent of the 4 500 tons of fertiliser stored in the silo detonated. The explosions were heard as two loud bangs in north-eastern France and in Munich, more than 300km away and are estimated to have contained an energy of one to two kilotons TNT equivalent. About 80 percent of all buildings in Oppau were destroyed, leaving 6 500 homeless. The pressure wave caused great damage in Mannheim, located just across the Rhine, ripped roofs off up to 25km away and destroyed windows farther away, including all the medieval stained-glass windows of Worms cathedral, 15km to the north. In Heidelberg, 30km from Oppau, traffic was stopped by the mass of broken glass on the streets, a tram was derailed and some roofs were destroyed. Five hundred bodies were recovered within the first 48 hours, with the final death toll recorded being in excess of 560 people.
The damage to property was valued in 1922 at 321 million marks, estimated by The New York Times at the time to be equivalent to seven million US dollars (since Germany suffered heavy hyperinflation in 1919 to 1924, given amounts and exchange rates were not very descriptive).
The funeral was attended by German President Friedrich Ebert and Prime Minister Hugo Lerchenfeld and saw crowds of 70 000 people at the cemetery in Ludwigshafen.
The plant began producing ammonium sulphate in 1911 but during World War I when Germany was unable to obtain the necessary sulphur, 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 sulphate, ammonium nitrate is strongly hygroscopic, so the mixture of ammonium sulphate and nitrate compacted under its own weight, turning it into a plaster-like substance in the 20-metre-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 fertiliser. To ease their work, small charges of dynamite were used to loosen the mixture.
This seemingly suicidal 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 sulphate and nitrate containing less than 60 percent 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-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 21 September 1921.
As all involved died in the explosion, the causes are not clear. However, according to modern sources and contrary to the above-mentioned 1919 tests, the “less than 60 percent nitrate = safe” criterion is inaccurate; in mixtures containing 50 percent nitrate, any explosion of the mixture is confined to a small volume around the initiating charge, but increasing the proportion of nitrate to 55 to 60 percent greatly increases 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, density, particle size in the mixture and homogeneity of crystal structure also affect the explosive properties.
A few months before the incident, the manufacturing process had been changed in such a way as to lower the humidity level of the mixture from three to four percent to two percent and also to lower the apparent density. Both these factors rendered the substance more likely to explode. There is also evidence that the lot in question was not of uniform composition and contained pockets of up to several dozen tons of mixture enriched in ammonium nitrate. It has therefore been proposed that one of the charges had been placed in or near such a pocket, exploding with sufficient violence to set off some of the surrounding lower-nitrate mixture.
Two months earlier, at Kriewald, then part of Germany, 19 people had died when 30 tons of ammonium nitrate detonated under similar circumstances. It is not clear why this warning was not heeded.