A large aluminum plant in Kentucky is temporarily halting production and laying off more than 600 workers due to rising energy costs.

Century Aluminum has announced plans to shut down its Hawesville smelter for 9 to 12 months starting in August. The plant, with about 628 workers, is the second-largest employer in Hancock County, the Owensboro Messenger-Inquirer reported.

The county’s Judge-executive, Johnny Roberts, told the newspaper the company gave him no assurances the plant would reopen after that. Roberts said the company told him that its power costs have risen dramatically this year.

Century Aluminum said the Hawesville plant is its largest “U.S. smelter and the largest producer of high purity primary aluminum in North America.”

The plant along the Ohio River had recently increased production and in April announced an addition of 60 jobs.

“We celebrated that,” said Andy Meserve, president of United Steelworkers Local 9423. “I guess if it hadn’t been for the cost of power, we would still be rocking and rolling.”

Employees learned of the pending layoffs on Wednesday.