Monday, June 05, 2017

Cambria Corn Mill Dust Explosion Kills Three

Last Wednesday, May 31, everything seemed normal at the Didion corn mill in the small village of Cambria, Wisconsin.  Like most factories of its type, the mill operated 24 hours a day, and late that night only sixteen workers remained as the machinery processed corn into ethanol and other products.  Shortly after 11 P. M., a tremendous explosion sent flames high into the air, knocked out power, and destroyed most of the processing division of the plant.  Three workers were killed, eleven others were hospitalized with injuries, and millions of dollars of damage was done. 

Ever since 1878, when what was then the world's largest grain mill was destroyed by a dust explosion in Minnesota, grain mill operators have known that the fine particles produced by various milling operations can combine with air to produce explosive mixtures.  Unfortunately, the science of dust explosions does not appear to be as complete as the science of gas explosions, for example.  Scientists have studied mixtures of hydrogen and oxygen for flammability, and can predict down to the third decimal place exactly what the mixture limits are for that combination of gases to explode.

Dust is different.  It comes in all sizes, ranging from particles almost too large to stay in the air very long, down to submicron bunches of molecules that take expensive equipment even to detect.  And as one expert interviewed by the Journal Times of Racine, Wisconsin pointed out, dust lying on the floor is relatively harmless, but if somebody walks by and kicks up a dust cloud, and it's a dry day and the person's body accumulates electric charge and then touches a grounded metal object, you can have the fatal combination of enough dust in the air and an ignition source to cause an explosion, whereas five seconds earlier there was no way an explosion could occur. 

This unpredictability may be one reason that dust explosions are relatively common compared to other types of major industrial accidents.  An insurance executive interviewed by the Journal Times said that over 500 dust explosions at grain processing facilities have occurred since 1982, killing more than 180 people in all.  If all 180 had been killed at once, dust explosions would be more prominent on the nation's scope screen of safety concerns, but a typical grain mill is manned by only a few dozen people at most and so the fatality numbers are rarely high enough to garner more than the occasional national headline.

Another problem with preventing such explosions is that they typically do so much damage that the originating cause is often never determined.  When I was about ten years old, I witnessed a demonstration of a dust explosion performed by a fireman who traveled to elementary schools to give fire-safety lessons.  He had a big box inside of which was a small container of ordinary baking flour, and a rubber hose was rigged to the box along with some kind of ignition source—maybe a candle he lit inside the box.  Anyway, when he closed the hinged lid and blew into the tube, the flour hit the candle, flung the lid open, and produced a huge yellow whoosh of flame.  It impressed the heck out of me, but in retrospect it must have been mostly for show, because dust explosions are not a big domestic fire hazard—almost all of them occur in industrial plants. 

Especially when it occurs in a confined area, a dust explosion's pressure wave wrecks everything in sight.  While investigators can sometimes gather some general idea of the sequence of events, it is often impossible to locate even the specific site where the blast originated, let alone to reproduce the conditions that led to the explosion. 

Anything that can reach a dust-air mixture's ignition temperature can cause such an explosion.  Static electricity is a favorite scapegoat, and in facilities where humidity can be raised high enough to eliminate this hazard, it is easy to control.  But for grain processing, too much humidity can be both expensive to produce and detrimental to the product, especially in drying operations, so other means of prevention are employed:  ventilation to keep concentrations of dust below dangerous levels, regular cleaning to prevent dust piles from accumulating and getting kicked up to cause hazardous clouds, and explosion-proof electrical fittings and equipment, which can be very expensive but are needed in certain locations where dust cannot be avoided. 

Records indicate that the Didion mill was cited for a potentially hazardous dust condition back in 2011, but the owners paid a fine and apparently corrected the problem.  The ongoing investigation may or may not find out what caused the explosion.  But since renewable fuels were mandated to be mixed into U. S. automotive fuels in 2005 and 2007 with the passage of the Energy Policy Act of 2005 and the Energy Independence and Security Act of 2007, the U. S. has outstripped Brazil as the world's largest producer of ethanol, and most ethanol made in the U. S. comes from corn.  Hence, there are a large number of corn mills in the U. S. that turn corn into ethanol, such as the Didion plant did until last Wednesday, and the risk continues that dust explosions in these mills will injure or kill workers.

To many residents of rural areas, making ethanol from corn is one of the few bright spots in an otherwise dismal future faced by farm communities, decimated by children moving to cities, drug problems, and other woes.  It's too bad that the process has the inherent and difficult-to-prevent hazard of dust explosions, but hopefully the industry will learn some lessons from this latest catastrophe and improve its track record by good safety and housekeeping practices. 

The safety culture of leading oil refiners is a gold standard that grain mills could aspire to.  Oil refinery operators have learned how to handle millions of gallons of hot, pressurized, flammable products with an exemplary safety record overall, but only at the price of what seems to outsiders to be ridiculously involved and rigorous safety practices.  It may be time for owners of grain mills to look to their more experienced compatriots in the petrochemical and refining industries for guidelines as to how to keep dust from killing their workers and wrecking their plants.

Sources:  I referred to an Associated Press report on the Cambria explosion (not to be confused with the Cambrian explosion of new species over 500 million years ago!) on the ABC News website at, a Racine Journal Times report at, and Wikipedia articles on dust explosions and ethanol fuel in the U. S.

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