A Brief History
On September 21, 1921, as many as 600 Germans learned of a new way to die when a silo in Oppau, Germany (part of modern-day Ludwigshafen, Germany), exploded with great force, injuring another 2,000 unfortunate workers. As you probably know, silos are tower like cylindrical structures usually used to store foodstuffs such as grain and corn. In this case, the silo in question was at a BASF plant and was used to store ammonium sulfate and ammonium nitrate.
Digging Deeper
You may also be aware that ammonium nitrate can be used as a mixture with fuel oil to create ANFO, a low order but powerful explosive used for cratering and earthmoving. The reason BASF was producing ammonium nitrate alongside ammonium sulfate (for fertilizer) goes back to World War I when sulfur, used to make explosives, was hard to come by since it was used in manufacturing gun powder and explosives. The Oppau plant had been making ammonium sulfate before World War I (for fertilizer) but had to switch to ammonium nitrate during the war because of the lack of sulfur. (Fertilizer being highly important when Germany was blockaded from receiving food shipments and the German people were nearly starving by the end of the war.)
The 20 meter tall silo was used to store both ammonium nitrate and ammonium sulfate, apparently not a good idea. Ammonium nitrate attracts water (hydroscopic) and became somewhat waterlogged, and when mixed with the ammonium sulfate formed a gooey, stiff paste-like substance that often became quite hard and had to be broken up in order to get it out of the silo. Men with picks could not safely be used to break up the fertilizer chemicals for fear of being sucked down into the stuff when the surface crust was broken. Thus, a genius (no doubt) deemed small charges of Dynamite to be the appropriate solution! It seems German chemists had deemed any mixture containing less than 60% nitrates was safe for use with explosives, and the practice of using Dynamite had been used since during World War I. It seems as if only a matter of time would stand between production and tragedy.
Practice at the BASF plant (BASF is a company that dabbles in a myriad of pursuits, from chemicals, agriculture, photography, to clothing and shoes, among others) had been to store the 2 chemicals together in ratio’s of up to 50/50 of each, not to exceed 50% ammonium nitrate. Bulk storage was permitted up to 50,000 metric tonnes, and the silo in question only held 4500 tonnes of the mixture, so the mixture was considered quite safe. Alas, a change in manufacturing process and specifications resulted in a lower water content in the ammonium nitrate and that difference was not taken into account, resulting in a more unstable mixture. The density of the chemicals was also altered, further aggravating the volatility of the stuff. Another dangerous factor leading to the explosion was the non-uniform nature of the mixture. Far from being homogenous the mixture was uneven, so that portions of the silo held chemicals in a dangerous ratio.
While Dynamite had been used around 20,000 times in Germany between during World War I and 1921, there had been no similar tragedies except for a single case in 1919 when a 30 tonne batch exploded killing 19 men. The German engineers and chemists did not take notice of the incident, and no serious revamping of the practice of handling ammonium nitrate combined with ammonium sulfate took place.
When the silo in question exploded, it did so with incredible force, leaving a crater 19 meters deep and an oblong shape 90 meters by 125 meters. The blast reportedly took place in 2 stages and is estimated to have been the equivalent of perhaps 2 kilotons of TNT, the size of a smallish nuclear weapon. Aside from the masses of dead and injured people, about 80% of all the buildings in Oppau were demolished and at least 6500 people were made homeless. Financial loss was estimated at $7 million (US dollars, which would be about $89 million today) and blew roofs off buildings 25 kilometers away! Windows even father away were shattered. While not the largest man-made non-nuclear explosion ever, the Oppau explosion certainly ranks among the biggest. Incredibly, only about 10% of the ammonium nitrate and ammonium sulfate actually exploded! Can you imagine the devastation if the entire silo-full had all ignited?
(Note: The largest man-made non-nuclear explosion may have been the Halifax Explosion of 1917 in which a cargo ship containing munitions blew up with a force estimated at 2.9 Kilotons.)
The military uses ammonium nitrate explosive to create craters in roads and miners use the stuff to create a large “push” in the earth to move soil and rock in quarrying and coal mining. Road builders use the stuff to reduce hills as well, among other uses. Terrorists have famously used ammonium nitrate from fertilizer mixed with diesel or fuel oil to create extremely large bombs, such as the one that blew up the Federal Building in Oklahoma City in 1995, which also used nitromethane as an ingredient.
Industrial accidents are a too common cause of tragedy, and history is replete with incidents even worst than the Oppau Silo Explosion.
Question for students (and subscribers): What industrial accidents were worse than the Oppau incident? What was the worst explosion accident in the United States? Would you work in an environment with explosives? Please let us know in the comments section below this article.
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Historical Evidence
For more information, please see…
Abelshauser, Werner, et al. German Industry and Global Enterprise: BASF: The History of a Company. Cambridge University Press, 2009.
Akhavan, Jacqueline. The Chemistry of Explosives: RSC (RSC Paperbacks). Royal Society of Chemistry, 2011.
The featured image in this article, a photograph of the damage from the Oppau Explosion that occurred on September 21, 1921, from Popular Mechanics Magazine, 1921, is in the public domain in the United States. This applies to U.S. works where the copyright has expired, often because its first publication occurred prior to January 1, 1923. See this page for further explanation.
