Relationships between the pourbaix diagram for Ag-S-H 2 O and electrochemical oxidation and reduction of Ag 2 S
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I.
INTRODUCTION
POTENTIAL v s pH or Pourbaix diagrams for numerous metal-water systems are readily available.1 On the other hand, for ternary systems involving metal-sulfur-water, such diagrams have been published only for a limited number of systems such as Pb, Cu, Fe, and Ni. 2'34 The development of such data for other systems is important in that it can provide a reasonable basis for comparison between various mineral systems which, for example, may be necessary for complex concentrates or deposits. The presence of dissolved silver or Ag2S has been noted in several studies in which a clearer understanding of the thermodynamics may be helpful. For example, silver ion additions have been shown to increase or catalyze the extraction of copper from CuFeS2 in ferric sulfate leaching systems. 5 The mechanism by which this occurs was not determined, but during the course of the reaction Ag2S is deposited on the surface of the chalcopyrite. A similar phenomenon was observed by other investigators who noticed the formation of Ag2S or "tarnishing" of chalcopyrite and tennantite when these minerals were contacted with metallic silver in air. 6 Similar reactions may occur in the activation of FeS by Ag + in flotation systems, although these authors 7 did not detect the formation of Ag2S, The deposition of Ag + on CdS surfaces has also been reported. 8 These results are not surprising, since Ag2S is a more stable sulfide than any of those mentioned above. Due to possible application in these systems and recent interest in the hydrometallurgical extraction of precious metals, the Pourbaix diagram for the Ag-S-H20 has been developed. In addition, the utility of the diagram in anticipating reactions for various oxidative or reductive condiG.W. WARREN is Assistant Professor and D.W. PRICE is a Graduate Student in the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. B. DROUVEN, formerly a Graduate Student in the same department, is presently with Preussag AG, Goslar, West Germany. Manuscript submitted September 22, 1982. METALLURGICALTRANSACTIONS B
tions is demonstrated. The diagrams obtained agree well with one derived by Horvath and c o - w o r k e r s . 9'1~ These authors, however, emphasized the corrosion of metals in solutions containing high levels of dissolved H2S; i . e . , strongly reducing conditions, and therefore that portion of the diagram where Ag2S is oxidized was not presented. This omission has been addressed in the diagrams developed here.
II.
DEVELOPMENT OF THE DIAGRAMS
According to Hansen and Anderko, H the only solid compound in the Ag/S system is Ag2S which has two polymorphic modifications below 500 ~ The low temperature form is a-Ag2S (acanthite) and is stable up to 175 ~ The high temperature modification is /3-Ag2S (argentite). Only acanthite has been used in calculations involved here, since this is the most stable form at ambient temperatures. The calculations required to develop the diagrams were based on the approach developed by Po
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