MECHANIC WAS ASSIGNED TO REPLACE AN EXPANSION JOINT ON THE LINE #3 FURNACE WINDBOX RECOUP DUCT IN THE PELLET PLANT. UPON REMOVING THE RUBBER-LIKE JOINT MATERIAL, A LARGE CHUNK OF DUST BUILDUP, HIDDEN BEHIND THE JOINT, FELL AND STRUCK THE MECHANIC'S RT. KNEE. THIS HYPER-EXTENDED HIS RT. FOOT AND ANKLE.