INCOMPATIBLE REACTION DURING PREPARING ACID CLEANING SOLUTION CAUSES H2S RELEASE

On March 29, 2023, at approximately 5:05 p.m., toxic hydrogen sulfide gas was accidentally released at a paper mill in Kentucky. Exposure to the hydrogen sulfide gas seriously injured one operator and injured two other operators.
At the time of the incident, three operators were tasked with circulating an acid-cleaning solution through process equipment to remove the buildup of solids impairing its performance. This task required an operator to stand directly over a tank and pour solid sulfamic acid powder into its opening.
When these operators added the sulfamic acid powder, the tank should have contained water, but a valve had been left open. This allowed a “weak wash” process stream to enter the tank before the operators added the solid sulfamic acid. The weak wash contained sodium sulfide, which reacted with the sulfamic acid, generating the toxic hydrogen sulfide gas.
Operator 1, who was standing directly over the tank opening, lost consciousness from exposure to the hydrogen sulfide gas that evolved from the tank. Operator 3 was able to call for help over the plant radio system but lost consciousness soon after. Operator 2 was seriously injured after losing consciousness (while trying to help Operator 1), falling to the floor, rolling through a guardrail system, and falling about 11 feet to a lower area of the structure.

Two other Domtar employees heard the distress call and entered the room to help the operators. All three operators regained consciousness. Operator 1 and Operator 3 were able to walk outside without assistance. Emergency responders transported Operator 2 to a hospital for treatment.The company reported that about 25 pounds of hydrogen sulfide were released.
 

Probable Cause
Based on the company's investigation, the CSB determined that the probable cause of the hydrogen sulfide release was the reaction between the added sulfamic acid and the sodium sulfide in the tank. The company's procedures did not indicate that the weak wash valve should be closed during normal operation, which contributed to the incident. Had the weak wash valve remained closed (or more robustly isolated), sodium sulfide could have been kept out of the tank, preventing the reaction that generated the toxic hydrogen sulfide.

Source:CSB.gov