A small steel process tank was filled with multiple metal baskets of tantalum capacitors cooked overnight in hot nitric acid. The hot nitric acid was used to remove some of the epoxy resin encapsulant from a tantalum anode. The nitric acid was drained each morning after the capacitors were cooked. The baskets of cooked capacitors were removed and rinsed in water. Mostly tantalum anodes remained. Employee #1 was killed and five other employees were seriously injured as a residue, containing picric acid, exploded, when Employee #1 placed a basket back into the drained tank, presumably to remove more encapsulant. Picric acid (trinitrophenol), and possibly other unstable nitrated compounds, formed in a nitration reaction between nitric acid and the bisphenol moiety of the epoxy resin. These nitrated compounds precipitated out of the nitric acid solution. Over time, as the spent nitric acid was drained from the tank after each batch, the precipitated nitrated compounds would accumulate on the inside surface of the tank. The spent nitric acid was typically used for multiple batches further concentrating unstable nitrated compounds on the tank's surface. Placement of the basket was likely the source of ignition. The explosion was estimated to be of a magnitude similar to an explosion involving more than 5 pounds of trinitrotoluene. The five seriously injured employees were treated for burns and bruises at a local hospital.
Source:OSHA.gov
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