Firefighters spent 20 minutes tackling a burning roof and scaffolding planks just after midnight last Wednesday.
The fire, at the top of a store tank, was started by a small piece of metal bolt which lay undetected when contractors cleaned up after a day's work.
The slag metal was heated up to an extremely high temperature when the workmen removed bolts so they could replace pipework.
"The hot metal sat there and there was a fire later," said a spokesman for Brunner Mond.
"Checks are carried out at the end of each day and the area was was very carefully damped down. But it seems that on this occasion a piece of hot metal was left there."
Leading firefighter Eric Davies said there was no danger of the fire spreading to other parts of the plant.
"There was no danger of it going anywhere else.
"Nothing could have dropped through the planks. Even if it had it would have fallen on to a wooden floor.
"Putting the fire out wasn't a problem.
"It was quite straight-forward. With it being scaffolding, we didn't have to use our ladders.
"It took us much longer to make sure there weren't any other bits of metal."
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