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MADISON - The U.S. Chemical Safety Board (CSB) released a Factual Investigative Update into the fatal May 31, 2017, combustible dust explosions at the Didion Milling facility in Cambria, Wisconsin. The explosions killed five of the 19 employees working at the facility on the night of the incident. The other 14 were injured.

Officials with the U.S. Chemical Safety and Hazard Investigation Board held a news conference on April 30 to discuss preliminary findings from their investigation into the deadly blast.

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, according to a CSB news release. The CSB is treating the Didion incident as one or more combustible dust explosions, CSB Chairperson Vanessa Allen Sutherland said in the news conference. 

The report provides a "multi-perspective narrative" of the event through detailed accounts from Didion workers present in the facility before and during the incident. 

"In general, the workers believe the conditions in the mill on the night of the explosion were normal," said CSB Investigator Mary Beth Mulcahy. "Workers believed they were dealing with a manageable problem that they were going to be able to solve or they were unaware of the incidents that were occuring in the mill." 

Mulcahy said workers reported seeing and smelling smoke coming out of the facility and went to investigate. 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.

A superintendent investigating the smoke heard a "huge boom" and then a constant roar. As he looked toward the sound, he saw the filter from the south gap mill air intake line flying in the air and a 3- to 4-foot flame shooting out of the line.

Determining it was best to leave the room rather than try to extinguish the flames, the superintendent yelled to another worker who was with him to get out.  As he ran out, another explosion knocked him down. The superintendent described the chaotic scene.

"You know, the building, as I was running, I was looking behind me and the whole building was just, there were fireballs and stuff. It just kept exploding. Just sounded like thunder, like constant thunder," the superintendent explained in the report. "And I saw concrete and stuff blast past. I just thought I was the only one that made it. I thought the whole place was done, you know."

Didion is reviewing the CSB report, according to an official statement on the press conference, and will continue to assist in efforts to learn what happened at the corn milling facility last May.

"We have been working cooperatively with the CSB as well as industry experts, on ongoing investigations into the incident to determine its cause. Throughout this process, Didion has reiterated our commitment to the safety of our employees. Our teams have worked diligently to ensure that industry safety practices and protocols are in place," the Didion statement said. "Didion pledges to continue to work cooperatively with the CSB and other agencies to maintain and enhance the safety of our team, which is our highest priority. We are also working with industry experts on construction of a new state-of-the-art corn mill, which will feature the latest technology and most effective and safe operational systems available today."

CSB Investigator Mary Beth Mulcahy says the board's probe is continuing and it could be six months before it issues any conclusions.

Sutherland said the CSB will continue to "examine several areas of interest, including the design and construction of the equipment and buildings that might be subjected to dusts explosion hazards." 

The CSB is using computer modeling to determine the amount of dust necessary to cause the extensive damage that occurred at Didion, while also exploring the safe management of combustible dust and explosion prevention.

The Didion explosion is the eighth major combustible dust incident investigated by CSB, Sutherland said. 

"Our agency has been very concerned about this issue for many years," added Sutherland.

In a combustible dust study completed in 2006, combustible dust was the first issue included in the agency's list of critical drivers of safety change, said Sutherland. 

In the 2006 study, CSB identified 281 combustible dust explosions between 1980 and 2005, that killed about 120 workers and injured more than 700, in incidents across more than 40 states, in different industries and involved a variety of different materials, Sutherland said. 

Between 2006 and 2017, CSB has confirmed at least 111 additional combustible dust incidents have occurred, with CSB investigating five incidents since the dust study, including Didion. 

"These five incidents alone have taken the lives of 27 workers and injured 61 others. Dust is an insidious hazard in multiple industries," said Sutherland in the news conference. "Our investigations have focused on understanding the risk associated with dust and appropriate means to manage and mitigate the risk of a dust explosion." 

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