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Fatal accident,due to decomposition of a diazonium salt

Due to an undetected manufacturing defect the welded mount of a glass-lined vessel in one of the dye stuff plants in Leverkusen broke and the vessel fell one story into the basement. The vessel contained approx. 5 cbm of a suspension of diazotized tribromoaniline in sulphuric acid.

During work to make the vessel safe, a decomposition reaction occurred, leading to an explosion in the vessel. An employee was seriously injured and subsequently died.

A lengthy and detailed examination to clarify the cause of the accident has now shown that the explosion was due to the decomposition of an unforeseeable accumulation of 2,4,6 tribromophenyl diazonium bromide. This product turned out in subsequent test to be extremely sensitive to mechanical influences. A previously unknown fact about this product is that in wet form friction or impact can cause it to explode.

The diazotization reaction produces mainly tribromophenyl diazonium hydrogen sulphate, together with tribromophenyl diazonium bromide, which is formed as a by-product with bromide ions derived from the bromination reaction. This mixture of diazonium salts is known to be mechanically stable, i.e. it is not decomposed by either impact or friction.

This diazonium salt was suctioned out of the vessel and immediately subjected to a chemical reaction after the vessel had fallen. The vessel was exposed to water coming from ruptured pipelines and was also rinsed continuously with water during this operation. An unforeseeable accumulation of tribromophenyl diazonium bromide was formed during rinsing because this salt is far less readily soluble than the corresponding hydrogen sulphate salt.

During the final emptying of the vessel, mechanical forces on the remaining diazonium bromide resulted in a reaction and explosion.

The diazotization reaction has been performed frequently in the past without problems because the diazotization suspension is coupled immediately. Moreover, the suspension demonstrably has no sensitivity to mechanical influences, even in the presence of bromide. This unfortunate and tragic chain of events was started by the vessel falling.

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