This paper discusses the investigation of an accident caused by the mixing of incompatible chemicals, a strong oxidizing agent with a reducing agent. The exact mixture and sequence of mixing was unknown, but the materials included an aqueous solution of 40% sodium permanganate (NaMnO4) and possibly sodium thiosulfate or sodium metabisulfite. These chemicals were part of a new technology being pilot-tested for remediating contaminated groundwater. The concentrated sodium permanganate solution was injected into the soil and any excess solution was collected above ground. Throughout the operation, excess waste permanganate was transported by hand in five-gallon pails from the injection area to a designated nearby area for neutralization. During one such transfer, a worker was seriously burned when a pail of sodium permanganate solution erupted onto him. The liquid splashed onto and ignited his clothing.
The exact contents of the pail were unknown, but the presence of permanganate was apparent, and witness statements were contradictory on several facts. The multiple witness accounts were therefore used to piece together possible accident scenarios. Two neutralization chemicals, sodium thiosulfate and sodium metabisulfite, were in use at the site. Early hypotheses assumed that mixing of one or the other in solid form with the permanganate solution could cause the observed violent reaction in a delayed fashion. That scenario was evaluated in the course of this investigation by thermodynamic calculations, followed by laboratory compatibility tests, and culminated in full-scale tests. Empirical test results revealed that the kinetics of the neutralization reaction could be masked by mass transfer limitations associated with the dissolution process. This case highlights the dangers of relying solely upon simple metrics for chemical reaction hazard identification. Simple empirical tests, performed cautiously, can reveal important complementary information about reactivity hazards.
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