The Justice Department yesterday announced a unique settlement in an antitrust case that requires two aluminum companies to operate jointly a production facility involved in the dispute.

The agreement clears the way for the acquisition of Atlantic Richfield Co.'s aluminum division by Canada's Alcan Aluminum Ltd., the world's largest aluminum company, at a price of between $600 million and $1 billion. Arco and Richfield have never announced an exact price for the transaction.

"The use of a production joint venture as a means of settling this type of case is, I believe, an innovation," said J. Paul McGrath, assistant attorney general in charge of the antitrust division.

Under the settlement, which is to be in effect for 10 years, Alcan and Arco will share an Arco aluminum-rolling mill in Logan County, Ky. Arco will get a 60 percent share of the mill's production and Alcan will get a 40 percent share. An independent management company will be set up to run the plant, with representatives on its board from the two companies.

Arco and Alcan will split the costs of operating the mill 60-40, but in essence will run it as if it were two mills, with each company determining its own product mix and marketing those products. The settlement only affects the Logan County facility; Alcan is free to acquire Arco's other aluminum operations as originally planned.

The Logan County plant is not yet open, and its use as the centerpiece of the settlement represents another unusual twist: the first successful use by the Justice Department in years of the concept of "potential competition," in which the the department argues that a merger could remove from the marketplace a competitor that has not yet appeared, but has the potential to do so.

The chief product to be produced by the Logan County plant will be aluminum-can body stock, a business that Alcan is already in but that Arco has not yet entered -- but would have once the Logan County plant was ready. "Based on the new plant's capabilities and Arco's plans for its utilization, it was clear that Arco would have become a significant new entrant into the can-body-stock market except for this acquisition," McGrath said. According to Justice Department figures, the plant would have made Arco the nation's fifth-largest producer of can stock, behind fourth-place Alcan.

"The parties have claimed that Alcan's participation in this joint venture will enable them to bring the plant up faster and operate it more efficiently than would be the case if Arco alone were operating it," McGrath said. "The joint venture agreement specifically provides for both parties to contribute their technology and expertise to the operation of the new mill. If this does increase the plant's efficiency, the joint venture will permit the parties and, ultimately, consumers to realize this benefit while still preserving Arco as an independent entrant."

Arco will be allowed during the period of the agreement to sell its interest in the plant to any company outside the aluminum industry. Arco also will be allowed to expand the plant's capacity, either alone or in conjunction with Alcan.

Best-known as an oil company, Los Angeles-based Arco obtained the aluminum operations when it acquired Anaconda Aluminum Co. in 1977. It announced plans to sell most of its aluminum holdings to Alcan in January, and the Justice Depertment challenged the sale in June on grounds that it violated the Clayton Antitrust Act. Yesterday's settlement was filed as a proposed consent decree with a civil antitrust suit in federal district court in Louisville, Ky. It is subject to 60 days of public comment and approval by the court.

Arco said yesterday that once it completes the sale to Alcan, it plans to put its remaining metals operations up for sale.