CHICAGO - Ardent Mills, the largest U.S. flour miller, said operations resumed on Thursday at its mill in Chester, Illinois, that had been closed since Dec. 29 due to near-record flooding on the Mississippi River. The flooding also shuttered the company’s mill in Alton, Illinois, but that mill reopened on Jan. 5. Ardent Mills is operated jointly by Cargill Inc, ConAgra Foods Inc and CHS Inc.