Atlantic Richfield said it sold an aluminum plant.

The Los Angeles-based oil company sold its Columbia Falls, Mont., plant to an investor group headed by former Arco Vice President Brack W. Duker for an undisclosed amount. The sale, which is scheduled to close Sept. 17, is part of the company’s previously announced plan to divest its metals businesses. The plant employs 1,000 and has a 180,000-ton capacity.