CSB issues update on investigation of two NOx releases at Austin Powder

The CSB yesterday released an update on its investigations into hazardous nitrogen oxide (NOx) gas releases at two facilities owned by Austin Powder.

Nitrogen oxides are highly-hazardous chemicals capable of causing serious respiratory injuries and environmental harm.

The first incident occurred Nov. 24, 2024, at the U.S. Nitrogen facility in Midway, Tennessee - which Austin Powder owns - and on June 11, 2025, at Austin Powder’s Red Diamond explosives manufacturing facility in McArthur, Ohio.

“These incidents underscore the serious hazards that can occur with nitric acid processes,” said Steve Owens, CSB chairperson. “We are concerned that hazardous nitrogen oxides gas was released at two Austin Powder facilities in the span of less than seven months.”

The CSB’s investigations into these incidents are ongoing. The agency is reviewing equipment performance, operating procedures, safeguards, alarms, and emergency response actions. Final findings and safety recommendations will be issued in the CSB’s final investigation report.

Red Diamond Facility - McArthur, Ohio

Austin Powder’s Red Diamond explosives manufacturing facility in Ohio manufactures several products used in the blasting and mining industries.

On June 11, over 3,900 pounds of NOx gas was released from the Red Diamond explosives manufacturing facility. The release occurred through an emergency pressure relief valve and a process vent associated with the facility’s nitric acid storage and recovery operations.

A large, yellow-reddish-brownish-colored plume was seen coming from the facility. The visible emissions led to the evacuation of residents in the nearby town of Zaleski and prompted the FAA to restrict airspace for a 30-mile radius around the facility. The release lasted for over three hours.

Cooling system shut down, temperature climbed 100 degrees above normal

CSB Investigators found temperatures in a nitric acid storage tank increased dramatically after cooling systems had been shut down for an extended period.

During normal operation, the tank’s bulk liquid temperature is below 50 degrees Fahrenheit. With the chilled water system off, the temperature steadily increased during the week preceding the incident.

By the morning of June 10, approximately 24 hours before the release began, the storage tank temperature had increased to over 80 degrees, more than 30 degrees greater than normal. On the morning of the incident, the temperature increased to over 150 degrees, more than 100 degrees above normal.

Shortly after 7:54 a.m. on the morning of the incident, maintenance personnel discovered that an emergency pressure relief valve on the tank was repeatedly opening and closing approximately every 30 to 60 seconds, discharging NOx gas. By 8:19 a.m., the excess nitric acid tank’s emergency pressure relief valve was open continuously and remained open until approximately 10:19 a.m.

CEO acknowledged the concern and disruption caused by release

John Rathbun, Austin Powder president & CEO, issued a letter to Zaleski residents following the release, which reads in part:

“The entire Austin Powder family recognizes the concerns and inconveniences associated with this incident, particularly to those of you who had to evacuate your homes out of caution,” Rathbun said. “While the nitric acid remained fully contained, we fully recognize the seriousness of what occurred and the inconvenience it caused our neighbors.”

“When this incident happened, our facility took action to stabilize the situation and notified the proper authorities. Our built-in safety systems functioned as intended, and we have worked closely with the Ohio EPA and local first responders. All information indicates no contamination of drinking water or soil.”

Rathbun said the plant would reopen after comprehensive inspections and safety evaluations, final clearance from appropriate regulatory agencies, and once the company was fully confident in the safety of the site for both its employees and the surrounding community.

“We are also conducting an internal investigation and, where appropriate, we will be implementing additional safety reviews,” he added.

Rathbun encouraged residents who were displaced by the evacuation to contact the company to discuss reimbursement for hotel stays, gas, or other reasonable costs.

U.S. Nitrogen Facility - Midway, Tennessee

The U.S. Nitrogen facility in Tennessee produces nitric acid that is used at Austin Powder’s Red Diamond explosives manufacturing facility in Ohio, in addition to other substances.

On Nov. 24, 2024, two NOx releases occurred during multiple attempts to start up the nitric acid unit after it had been shut down for several days for maintenance.

State Department of Environment and Conservation issued $11,000 civil penalty

The releases, which occurred at approximately 6:47 a.m. and 8:42 a.m., resulted in the estimated release of 910 pounds of NOx gas. The unit was shut down after visible emissions were observed. Like the release in Ohio, a large yellow-reddish-brownish-colored plume of NOx gas was emitted.

The Tennessee Department of Environment and Conservation (TCED) has issued a Notice of Violation and an order to U.S. Nitrogen, seeking a $11,000 civil penalty and asserting that the company failed to follow its standard operating procedure for starting up the nitric acid plant. U.S. Nitrogen appealed TDEC’s Order July 30.

No injuries were reported during either NOx release.