After 18 months of often contentious labor talks with the United Steelworkers union at an aluminum smelter in Quebec, Alcoa Corp. has threatened to idle the entire facility if workers don’t sign what it’s calling a “final offer.”

Aluminerie de Bécancour, a joint venture in which Alcoa controls a 75 per cent stake and Rio Tinto Group owns the rest, made a proposal that expires on July 5. If it’s not signed, the already-curtailed production will be totally suspended, Pittsburgh-based Alcoa said Wednesday in a statement.

Union workers were locked out of the facility in January 2018, and some operations continued to be run by managers. At full capacity, the plant known as ABI could produce 413,000 metric tons of aluminum a year, but that had been cut back to about a sixth of that, according to Bloomberg Intelligence. For context, Alcoa has forecast that its global aluminum shipments will hit a range of 2.8 million to 2.9 million metric tons this year.

If a deal is reached, the company estimates the plant would be fully operational within 10 months.

“We’ve reached a critical juncture in this process,” Jean-Francois Cyr, president of Alcoa Canada, said in an emailed ABI statement. “We want to resolve the conflict for the benefit of all parties and work together to restart our smelter and get everyone back to work.”

ABI said its new proposal would provide more funding to the pension, reduce subcontracting and allocate more paid hours for union business than the last offer, which was rejected in March. It said in addition, “the offer allows everyone on lockout to return to work with annual wage increases that total 15.3 per cent over the six-year contract.”

A spokesman for the United Steelworkers didn’t immediately respond to a request for comment.

--With assistance from Joe Deaux