Here’s What Happened:
Several construction employees working near a collection pit were overcome with hydrogen sulfide gas. The gas was released nearby when sodium hydrosulfide was accidentally mixed with sulfuric acid. Three workers collapsed almost immediately and three others tried to rescue them. Two of those rescuers also collapsed. In all, ten workers were exposed to the toxic gas. Two died and eight others were injured.
How Did This Happen ?
Fifteen trucks of sodium hydrosulfide (NaSH) had been delivered to the facility in the previous 24 hours. Each truck is believed to have spilled about 5 gallons (19 liters) in to the collection pit. Construction employees were working in the area and needed to enter the collection pit. Operations drained the pit into what was believed to be the wastewater system. Instead, the pit drained in to a sewer line where sulfuric acid was present in quantities sufficient to react with the NaSH. As soon as the two chemicals mixed, they reacted and toxic hydrogen sulfide gas was released. The toxic gas escaped from the sewer system through the seal of a fiberglass manhole cover near the workers.
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