Algoma Steel Group Inc. says a structure supporting utilities piping at its coke-making plant collapsed on Saturday.
The company said there were no injuries, but the incident resulted in a coke oven gas main failure generating abnormal flaring and emissions.
It also said effluent entered an adjacent waterway and that containment efforts were underway.
Algoma said coke production was interrupted, but all other departments were operating normally.
It said that it has adequate coke supply on hand and that it did not expect customer shipments to be affected at this time.
Shares in Algoma closed down 52 cents or 4.26 per cent at $11.70 on the Toronto Stock Exchange on Monday.