An improved method for treating manganese-containing materials, such as seafloor manganese nodules, by leaching with aqueous HNO3 and NO gas, and more particularly to methods for recovering valuable constituents from such nodules, especially manganese, cobalt, nickel, iron, and copper. It also provides a method to leach manganese material to release the titanium, vanadium, cerium, molybdenum and other metals from the manganese oxides and to make them available to be recovered.