The US Chemical Safety and Hazard Investigation Board (CSB) has identified several process safety failings in its interim assessment of a deadly hydrogen sulfide release during decommissioning at Catalyst Refiners in April.

The tank in which mixed chemicals produced hydrogen sulfide gas

Source: © CSB

Workers were disposing of residual chemicals during decommissioning of the plant, using the company’s wastewater pretreatment facilities

Workers were disposing of residual chemicals using the site’s wastewater pretreatment tank. Workers first added two proprietary chemicals: A-50 (calcium chloride) used as a coagulant and M-2000A (sodium trithiocarbonate, Na2CS3) used to precipitate silver from wastewater. These two solutions were routinely used together in wastewater pretreatment.

The workers proceeded to dispose of residual nitric acid, which reacted with the sodium trithiocarbonate to produce a fog containing hydrogen sulfide over the tank. While the employees performing the disposal attempted to raise the alarm, two other workers attending the incident were eventually overcome by the gas and killed.

The CSB report notes that the safety data sheet for M-2000A describes the solution as incompatible with strong acids, and that Catalyst Refiners had no recorded procedure for disposing of this combination of substances.

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