An improved method for treating manganese-containing materials, such as seafloor manganese nodules, by leaching such materials with aqueous HNO3 and polymerized nitric oxide (N2O3)x, 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-containing material to release any titanium, vanadium, cerium, molybdenum and other metals from the manganese oxides and to make them available to be recovered, as well as providing a method of producing a fertilizer grade nitrate product.