CSB to Investigate Wisconsin Cardboard Mill Explosion

The U.S. Chemical Safety Board announced that it will conduct a full investigation of the storage tank explosion that killed three workers and injured a fourth at the Packaging Corporation of America, a corrugated cardboard mill in Tomahawk, Wis., on July 29.

The incident occurred as workers were performing welding to repair a flange fitting on top of an 80-foot-tall storage tank, which contained a mixture of recycled paper pulp and water. The three workers were standing on a catwalk above the domed, cylindrical tank performing welding when an internal explosion ripped open the tank lid. All three workers died of traumatic injuries, including two who were found on the ground beneath the tank. A fourth, who had been observing the work from a further distance, survived with minor injuries.

CSB investigators examined the accident scene, conducted interviews, and gathered evidence. The investigation is expected to take approximately a year to complete, and will likely involve further work at the site. Among the issues the investigation will examine is whether anaerobic microbes, which grow in the absence of oxygen and feed on organic matter, produced flammable gas to fuel the explosion.

Large storage tanks at paper mills--especially those that are stagnant, poorly mixed, or have long material retention times--are at an elevated risk for the growth of anaerobic bacteria. Recycled process water, known in the industry as "white water," contains nutrients that promote growth of the organisms, producing hydrogen or other flammable gases that can be ignited by hot work or other sources of ignition.

The cause of the explosion remains to be determined. CSB plans to conduct microbiological and chemical testing to understand what fueled the blast.

"The CSB has noted that there have been previous tank explosions at paper mills during hot work and we will be closely examining the tragedy in Wisconsin to determine if there are similarities," said CSB Chairman John Bresland, who announced the investigation.

CSB is an independent federal agency charged with investigating industrial chemical accidents. The Agency's board members are appointed by the president and confirmed by the Senate. CSB investigations look into all aspects of chemical accidents, including physical causes such as equipment failure as well as inadequacies in regulations, industry standards, and safety management systems. The Board does not issue citations or fines but does make safety recommendations.

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