A silicon hydride emulsion made at a Waukegan, Illinois, chemical plant where four people were killed in a May 2019 explosion can produce hazardous amounts of flammable hydrogen gas under certain conditions, federal investigators revealed Wednesday (Dec. 18).

AB Specialty Silcones, site of the explosion, lacked the proper vent systems to release any gases generated, an investigatory update issued by the U.S. Chemical Safety Board states.

An enclosed production building at the plant filled with flammable gas during batch production of EM 652, an emulsion combining silane and siloxane polymers. The resulting explosion devastated the plant's emulsions production area, damaged nearby businesses and was heard by witnesses nearly 20 miles away.

"Specifically, the company did not equip these vessels to mitigate all potential process hazards, such as the capability to generate flammable gas," the update reports.

The company already faces $1.5 million in penalties imposed by the Occupational Safety and Health Administration following the same accident.

Complications arose early with the first batch of EM 652 started by the plant's evening shift, the CSB update states. When the first two ingredients of the process were added to an atmospheric tank known as the Thick Phase Tank it unexpectedly began overflowing with foam.

"Workers would open the top of these tanks during the production process to, among other things, perform visual observations," the CSB update states. "These atmospheric tanks had no engineered system to direct flammable gas, including hydrogen, to a safe location."

Surviving witnesses report that tank was making a "very strange sound" with the contents erupting onto the floor. The area became hazy with pale yellow smoke. Meanwhile, the temperature of the tank's contents continued to rise.

There were no flammable gas detectors or hydrogen gas detectors with alarms to warn workers of impending danger. Placement of the main air mover near the EM 652 process further increased the potential explosion danger from flammable gases, the CSB update states.

Within minutes of the release, at least one and perhaps two explosions rocked the facility.

CSB research indicates that silicon hydride compounds such as EM 652 can generate hydrogen when mixed with strong acids or bases; amines; primary or secondary alcohols and water in the presence of acids, bases or catalytic metals.

"When contacting these materials, SiH (silicon hydride compounds) can rapidly generate hydrogen gas and form flammable and explosive mixtures in air," the update states.

Only one of the four fatalities lived long enough to reach a hospital. Among the dead were the operator and shift supervisor struggling to bring the process under control. Search and recovery of the bodies on site took four days and required moving heavy wreckage.

 A fifth employee suffered extensive injuries that required hospitalization. All others were treated and released.

AB Specialty is a manufacturer and worldwide distributor of specialty silicone chemicals used in a wide variety of industries, including personal care, roof coatings, chemical manufacturing, adhesives and sealants. The company does not store chemicals in sufficient amounts for the company's operations to be regulated by either the EPA or OSHA.