THE CONTRACTOR WAS PERFORMING SCHEDULED HOUSEKEEPING ACTIVITIES BY REMOVING SPILLAGE AROUND THE #6 CONVEYOR BELT HLE. THE CAB OF THE VACUUM TRUCK WAS FOUND TO BE SMOKING AND SOON WAS ON FIRE. THE CHICAGO CITY FIRE DEPT RESPONDED AND THE FIRE WAS EXTINGUISHED BY 10:15 PM. THE CAUSE OF THE FIRE IS UNKNOWN AND IS BEING INVESTIGATED.