The fire at the Consumers' Co-operative Refineries facility on the north edge of Regina began shortly after 2 p.m. Witnesses say it sent flames soaring into the air and a plume of thick, black smoke could be seen from several kilometres away.
"We've had an unfortunate accident on our property," said safety manager Gilbert Ledressay.
"We've had an explosion and a fire. The situation is under control and there's no immediate danger to the people on site and/or the community right now."
Several ambulances and fire trucks lined the road at the Co-op, which is a wholly owned subsidiary of Federated Co-operatives Ltd.
A spokesman for the Regina health region said five workers were treated and released at Pasqua hospital, and another eight were taken to the Regina General.
Of those, four were released. She said the condition of the four remaining in hospital was not available.
Two workers were treated on site. Other workers who were forced to flee the plant could be seen standing nearby, but did not want to comment on what happened.
Officials are trying to determine how the blaze started.
"What happened was there was a loss of containment. Right now we suspect it was a pipe ... but we haven't verified that yet," said Ledressay.
"And so hydrogen and diesel would exhaust out of there and it found an ignition source."
The blast and fire produced large plumes of thick, black smoke for a while before it was brought under control within about an hour by the plant's 20 firefighters.
Crews at the plant deliberately let the blaze burn, Ledressay said.
"Why you saw it burning for so long is because on a gas release, you don't want to put it out. You don't want it to re-ignite and explode again, so you isolate the source and you cool areas around it and let it burn itself out."
There was a large workforce in the plant at the time. Ledressay said there were up to 450 permanent staff and about 1,000 contract employees in the affected area. All have been accounted for.
Ledressay said that part of the plant is about 40 years old, but was refurbished with new pipes about five years ago.
Company vice-president Vic Huard said the fire will cut the plant's production of diesel fuel by about 25 per cent.
The plant is in an industrial area of Regina, but Huard still sought to reassure people that the fire didn't release any harmful chemicals into the environment.
"There's no toxic cloud involved. We can state that without fear of contradiction."
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