Union workers at Century Aluminum, who have been locked out of the company’s Hawesville plant for almost a month in a contract dispute, will vote on a new contract offer tomorrow. The Federal Mediation and Conciliation Service says the agreement was reached with the help of federal mediators. Union workers at the plant have been without a contract since April 13. Century Aluminum officials locked out about 560 union members May 12 after union rank and file rejected a third contract offer. Since the lockout, the plant has been kept operating with company employees.