Cobalt extraction in ammoniacal solution: Electrochemical effect of metallic iron
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INTRODUCTION
IN the past, the metallurgical industry has tended to treat cobalt as a poor relation of copper and nickel since it usually occurs in ore deposits as a minor constituent in association with these metals. This view of cobalt has dramatically changed in the last few years largely due to the unstable supplies of cobalt. Most of the world's cobalt is present in low grade, practically untouched, oxide ores: the laterites and oceanfloor manganese deposits which contain typically only about 0.2 pct cobalt. In the past, these oxides have not received much attention because of their low content of valuable metal. However, the picture is rapidly changing as the higher grade sulfide deposits become increasingly depleted. Lz An important commercial scheme for treating these low grade oxide ores is the reductive-roast ammonia leach process. J-~2Cobalt extraction, however, is often found to be poor with this process, ill2 This may be attributed, in part, to coprecipitation of cobalt with iron and manganese as well as its adsorption on the finely divided oxide-hydroxide leach residues. 3`t~ In addition, there are indications that cobalt behavior during ammonia leaching may be parlly related to an electrochemical interaction between cobalt and iron, even though publications involving the relevant electrochemical measurements are scarce. 1618 In the reductive-roast ammonia leach process as applied to ferruginous oxide ores, Cu-, Ni-, and Co-containing oxides are first selectively roasted under reducing conditions to give more leachable metallic phases of Cu, Ni, and Co. There is a certain amount of alloying with iron, however, so that the subsequent leaching step involves the dissolution of Fe-Co-Ni-Cu alloys. Since the dissolution of metallic phases is an electrochemical process, akin to corrosion, ~9'2~ electrochemical techniques such as polarization measurements which are frequently utilized in corrosion research may provide valuable K. OSSEO-ASARE, Associate Professor, J.W. LEE and H.S. KIM, Graduate Students, and H.W. PICKERING, Professor, are all with the Metallurgy Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted December 14, 1982. METALLURGICALTRANSACTIONS B
information on the mechanisms controlling cobalt dissolution in ammonia hydrometallurgical systems. Furthermore, the study of the nature and formation of passive films under well controlled electrochemical conditions may give additional insight into the manner in which metallic iron and cobalt interact during ammonia leaching. The work reported here represents some of the initial results of a long-term study aimed at investigating the electrochemical behavior of iron, cobalt, and iron-cobalt alloys in aqueous ammoniacal solution, in terms of aqueous chemistry, electrochemical, and metallurgical factors. In this paper, a review of the solution chemistry of the FeNH3-H20-CO3 and Co-NHrHzO-CO3 systems is presented with the aid of aqueous stability diagrams, and el
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