As a result of the incident, 18 personnel ended up decontaminated and transported to local medical facilities for evaluation, including seven first responders.
Catalyst Refiners was in the process of decommissioning its facility, which the company wanted completed by June 5. At the time of the incident, the facility ceased its silver reclamation process, and workers removed most of the process equipment from the site.
Wastewater flowed into a receiving tank (hereafter referred to as the “incident tank”), which pumped to a neutralization tank. In the neutralization tank, the pH ended up controlled using sodium hydroxide, and after workers adjusted the pH, the wastewater flowed to a clarifier. The clarifier separated solid silver precipitate, and the remaining water went to an off-site wastewater treatment area.
Silver Recovery Chemicals
As it turns out, Catalyst Refiners uses two chemicals to recover silver from its wastewater: M-2000A and A-50. M-2000A is a proprietary sodium trithiocarbonatea solution used as a metal precipitant, and A-50 is a proprietary calcium chloride solution used as a coagulant. Chemical precipitationc see wide use for removing metals such as silver from wastewater. Both M-2000A and A-50 normally ended dup added to the clarifier.
Nitric acid, which had been used in Catalyst Refiners’ reclamation process, was kept in a storage tank at the facility. At the time of the incident, Catalyst Refiners was disposing of the remaining nitric acid by pumping it into the effluent wastewater pretreatment process.
Also, hydrogen sulfide (H2S) is a colorless, highly toxic gas that can cause damage to the eyes and the respiratory system. Exposure to concentrations as low as 100 parts per million (ppm) is considered immediately dangerous to life or health (IDLH) [3]. Concentrations over 1,000 ppm can cause nearly instant death. Hydrogen sulfide is heavier than air and tends to travel near the ground during a release.
Incident tank, side view (left) and elevated view (right).
Source: CSB
During the process of decommissioning the facility, Catalyst Refiners used the incident tank and its effluent wastewater pretreatment equipment for the disposal of A-50, M-2000A, and nitric acid.
About a week prior to the incident, employees used water to dilute the 67-weight percent nitric acid that was in the nitric acid storage tank outside the facility. According to Catalyst Refiners workers, the nitric acid storage tank level showed 11 percent full before the employees added water.
At the relief shift supervisor’s instruction, the employees added the water until the level in the nitric acid storage tank reached 22 percent full.
Day of the Incident
On the morning of April 22, the day of the incident, the relief shift supervisor instructed two employees to use a 1.5-inch double diaphragm pump to move the diluted nitric acid from the nitric acid storage tank into two empty 275-gallon totes on the site.
The employees placed a hose in the top of the nitric acid storage tank and used the pump to fill the two totes with the diluted nitric acid. After the totes were full, a third employee used a forklift to move the totes and the pump into the production building, where they would transfer the diluted nitric acid into the incident tank.
To dispose of some other chemicals that remained on site, the relief shift supervisor instructed two employees to use the pump to transfer the chemicals into the incident tank.
The relief shift supervisor told the employees the order in which to pump the chemicals into the incident tank.
First, the two employees emptied one and a half 55-gallon drums of A-50 coagulant into the incident tank, and then they moved the suction hose and transferred almost a full 275-gallon tote of M-2000A into the incident tank.
Layout of remaining chemicals for disposal, the incident tank, and the pump used to transfer the chemicals into the tank in the position found by the CSB following the incident.
Source: CSB
After completing the transfer of the M-2000A, the employees moved the suction hose for the pump into the tote of diluted nitric acid.
The employees turned the pump on, and after approximately five inches of the nitric acid in the tote added to the incident tank, they noticed the materials inside the incident tank were reacting and creating a fog over the tank.
Worker Passes Out
After noticing this reaction, one of the employees, who was operating the pump, unplugged the air hose to stop pumping diluted nitric acid into the incident tank. This employee then went outside to get some fresh air, passed out, and, when he regained consciousness, notified the production supervisor of the reaction and the resulting vapors caused him to lose consciousness.
At that time, the production supervisor notified the relief shift supervisor, who oversaw the disposal of the chemicals, there were issues regarding the mixing of the chemicals. The relief shift supervisor then entered the production floor at 9:23 a.m. and walked toward the area of the incident tank. Shortly thereafter, the relief shift supervisor lost consciousness.
According to surveillance video of the facility, two other employees (Employee A and Employee B) proceeded toward the area of the incident tank. At 9:28 a.m., once in the area, Employee A was observed waving their arms and then collapsed. Employee B was seen on video pulling the unconscious relief shift supervisor away from the incident tank area when they also were overcome by the released vapors and collapsed. Although the relief shift supervisor survived, Employees A and B, who were not a part of the job of disposing of the chemicals, eventually died from their injuries.
Other employees working on the production floor saw Employees A and B collapse and notified a large group of employees that three men, including the relief shift supervisor, had collapsed on the production floor.
One of these employees pulled the plant fire alarm at 9:30 a.m., while other employees worked together to pull the three collapsed employees out of the facility and to the front gate, where emergency responders met them.
Incident Victims
Two employees died and four others suffered serious injuries as a result of this incident.
In the ensuing alarm, a shelter in place ended up issued for one mile surrounding the facility at 10:04 a.m., and later lifted at 2:23 p.m.
During a post-incident site visit, the CSB found four full-face respirators in various locations throughout the facility, at least three ended up equipped with 3M 2091 particulate filters (P100 rating), which do not provide protection against hazardous gases, including hydrogen sulfide.
Workers told the CSB that once the facility transitioned from normal production to decommissioning, the company did not require them to wear respirators in the facility. Catalyst Refiners also did not provide, nor did they require the use of, personal gas monitors by employees or contractors.
“This tragic incident underscores the need to have clear procedures and safely manage the serious chemical hazards that can arise during facility decommissioning,” said CSB Chairperson Steve Owens. “Although the CSB’s investigation is still ongoing, we remain concerned that Catalyst Refiners did not require employees to wear respirators during decommissioning and also never provided or required workers to use personal gas monitors that might have alerted them to the presence of the highly toxic hydrogen sulfide gas.”
The CSB is continuing to gather facts and analyze several key areas, including:
- Analysis of onsite chemicals
- Decommissioning hazard analysis
- Use of personal protective equipment
- Corporate governance and oversight
Click here for more information on the CSB update.