Locked-out workers at Rio Tinto Alcan's smelter in Quebec will have to continue to limit their numbers on the picket line until at least early April.

Rio Tinto Alcan was able to renew its injunction in Quebec Superior Court on Tuesday limiting the number of picketing workers allowed near the company's aluminum processing plant, about 225 kilometres north of Quebec City.

The injunction allowing 20 picketing workers to demonstrate at a distance of 150 metres from the front door will have to be considered again by the court on April 2.

Justice Gratien Duchesne refused to grant a permanent injunction but he removed the legal requirement of the locked-out workers of having to keep the peace while picketing.

About 800 unionized employees were locked out on New Year's Day and the company was concerned about intimidation against 200 managers tasked with running the plant.

Neither side is at the bargaining table. The union has said the major issue is the company's increasing use of subcontractors to replace workers who are retiring.

The facility is operating at about one-third of its 438,000-tonne capacity. A company spokesperson confirms that 288 of the 432 reduction cells were shut and has estimated that managers can continue to run at these levels for up to a year if necessary.

The company has said plans are in place to ensure that aluminium production throughout the region continues to run safely and efficiently.

The Alma operation hosts one of Rio Tinto Alcan's most important North American aluminum smelters.