Workers at a Wisconsin mill believed that conditions at the mill were “normal” just before a combustible dust explosion that killed five employees.

That’s one of the startling findings of a Factual Investigative Update released by the U.S. Chemical Safety Board (CSB), which is investigating the May 31, 2017 incident at the Didion Milling facility in Cambria, Wisconsin. In addition to the five fatalities, the other 14 employees working at the facility on the night of the incident were injured.

The explosion occurred in Didion’s “dry corn milling” facility, where raw corn is processed to create a variety of corn products. The dry corn milling process – particularly the acts of grinding and separating individual kernels of corn into distinct components – produces corn dust. Corn dust is combustible and is known to be explosive under certain conditions. The CSB’s Factual Investigative Update presents the perspectives of 10 of the 14 survivors as the events unfolded the night of the incident.

The CSB determined:

• Workers believed the conditions at the mill on the night of the incident were “normal,” and up until just moments before the explosions, workers either were unaware of any problems or assumed their troubleshooting efforts would reveal a typical and manageable situation.
• Prior to the explosion workers reported seeing and smelling smoke coming from the facility.
• Several workers entered various mill buildings to locate the source of the smoke, but were unable to immediately find the cause.
• Approximately 15 to 30 minutes before the explosions, workers focused their inspections on a piece of equipment called a gap mill. Workers then observed an air filter blow off of the gap mill’s air intake line, resulting in corn dust filling the air and a three- to four-foot flame shooting from the air intake line.
• Around 11:00 PM, one or more explosions occurred. The explosions caused the complete collapse of four of the nine buildings that make up the Didion facility; the remaining five were severely damaged. 
• Five workers died as a result of the explosions and the collapse of the buildings. The other 14 workers sustained injuries that ranged from minor to life-threatening. 

The CSB’s full technical analysis of the incident and its examination of dust management at Didion are still underway and will be included in its final report. The final report will also explore the conditions that influence the safe management of combustible dust and the challenges associated with dust explosion prevention.
 
The CSB is an independent, non-regulatory federal agency whose mission is to drive chemical safety change through independent investigations to protect people and the environment. The agency’s board members are appointed by the president and confirmed by the Senate. CSB investigations look into all aspects of chemical incidents, including physical causes such as equipment failure as well as inadequacies in regulations, industry standards, and safety management systems. For more information, contact public@csb.gov