The electrochemical oxidation of chalcopyrite in ammoniacal solutions
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I.
INTRODUCTION
INVESTIGATIONS on hydrometallurgical processes for the recovery of metal values from sulfide concentrates and low grade ores have indicated that corrosion mechanisms are often involved in the dissolution process. Various lixiviants have been studied: sulfuric acid, ferric sulfate, ferric chloride, and ammonia to name only a few. In the case of ammoniacal solutions much of the recent work has been directed toward chalcocite dissolution or reduction, ~,2 covellite oxidation, 3,4,5 or toward an understanding of the heterogeneous equilibria in metal-ammonia-water systems. 6 In an electrochemical investigation similar to the present study, Filmer et al. 5 have proposed that the electrochemical oxidation of CuS occurs through the formation of a thin layer of a copper polysulfide, CurS (y < 1). Initially the rate of dissolution decreased sharply according to a parabolic relationship and then leveled off at a constant rate. This behavior was explained5 in terms of a growing film which reached a constant thickness at the point where a linear rate relationship was obtained. This type of corrosion behavior is frequently found in the corrosion of metals both in aqueous solutions and in hot gases] Similarly, the formation of a defect intermediate layer, such as CurS, has been proposed for metal oxides on metals 7 as well as for metal sulfides on chalcopyrite. 8 Results of the present investigation also indicate that a thin film of copper deficient CuFeS2 quickly forms on the surface and that the rate is linear except for the first few minutes of dissolution. Several investigators have previously addressed the oxidation of chalcopyrite in ammoniacal solutions. 9 15The ammonia, oxidation leaching of chalcopyrite with oxygen is G.W. WARREN is Assistant Professor," Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. M.E. WADSWORTH is Dean, College of Mines and Mineral Industries, University of Utah, Salt Lake City, UT 84112-1183. Manuscript submitted August 22, 1983. METALLURGICALTRANSACTIONS B
worthy of special attention since at least two commercial ventures have been based upon this approach. "'16 It has been shown that chalcopyrite dissolves according to the overall reaction9.m CuFeS2 + 4NH3 + 17 4 O 2 + 2OH- = Cu(NH3)4~z 1
[1]
+ ~Fe203 + 2 S 0 } + H20
The presence of iron in the system involves the added complication of the formation of a hematite phase adjacent to or on the mineral surface. This can be considered beneficial in that a separation between copper and iron is achieved. It is possible, however, that the formation of iron oxide may passivate the surface contributing to the lower rates of dissolution observed for CuFeS2 compared with Cu2S or CuS. Beckstead and Miller] for example, report roughly 20 pct extraction in three hours at 75 ~ for 120/xm particles in an ammoniacal solution of pH 10.2. By comparison, for three hours reaction time under similar leaching conditions, over 60 pet reaction of CuS has been reported 3'4 and over 90 pc
