Preferential ammoniacal leaching of Co, Ni and cu from cobaltrich ferromanganese crust under reducing conditions was performed using ammonium sulfite and/or ammonium thiosulfate as reducing agents. The effects of reducing agent concentration, initial pH, leaching time, temperature and the combined concentration of ammonium sulfite and ammonium thiosulfate on the extraction of Co, Ni, Cu, Mn and Fe were investigated. The extraction of Co, Ni and Cu with ammonium sulfite was better than that with ammonium thiosulfate. When ammonium sulfite was used as the reducing agent, Mn was precipitated and appeared in the residue as (NH4)2Mn(S03)·H20.
Deep-ocean ferromanganese nodules and cobalt-rich ferromanganese crusts arc now widely recognized as potential resources of various metals such as cobalt, nickel and copper. Many nations and groups in the world have been developing mining technology and metallurgical processes for deep-ocean ferromanganese nodules because of their extensive occurence in the world" s oceans. Most of the processing methods developed so far have been based on hydrometallurgical treatment, such as acid leaching (Han and Fuerstenau, 1975; Kanungo and Jena, 1988a, 1988b; Kanungo and Das, 1988) and ammoniacal leaching under reducing conditions (Okuwaki et aI., 1974; Okuwaki et aI., 1977; Das et aI., 1986; Anand et aI., 1988; Acharya, 1991). The cobalt content of cobalt-rich ferromanganese crusts is higher than that of deep-ocean ferromanganese nodules. Therefore cobalt-rich ferromanganese crusts arc of particular interest as a source of cobalt. Consequently, the properties of cobalt-rich ferromanganese crusts and appropriate processing methods for the recovery of the metals contained have not been established as yet. This paper presents the result of a study on the preferential ammoniacal leaching of cobalt, nickel and copper from cobalt-rich ferromanganese crusts under reducing conditions using anml0nium sulfite and ammonium thiosulfate as reducing agents.