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I.

INTRODUCTION

THE dissolution of

gold and silver in cyanide solutions is an electrochemical process, as described by Eqs. [1] through [3]: Au + 2CN

= Au(CN)~- + e

[1]

Ag + 2CN

= Ag(CN)~ + e

[2]

02 + 4H + + 4e = 2H20

[3]

In general, the dissolution of a metal in cyanide solution will be affected by many factors such as cyanide and metal concentrations, pH, and the electrochemical potential. For example, according to previous electrochemical investigations, ~-6 the rate of the anodic dissolution of gold increases with increasing potential at low potentials, i.e., the reaction is in an active region. However, when the potential reaches a certain value, the dissolution rate decreases sharply, i.e., passivation occurs. According to passivation theory,7 such a phenomenon is a result of the formation of an oxide or the adsorption of a species on the surface. In order to predict the feasibility of dissolution reactions and to determine which species or compounds may correspond to the observed passivity, a thorough analysis of the aqueous chemistry and thermodynamics is necessary. In this paper, the solution chemistry of the gold- and silver-cyanide systems is analyzed in terms of aqueous stability diagrams. Logarithmic activity-activity diagrams, such as Eh-pH, log[Metal]-pH, and log[Ligand]-pH diagrams make it possible to predict on a thermodynamic basis and for a given element, the equilibrium states of all the possible reactions between the element, its ions, and its solid and gaseous compounds in the presence of water. Therefore, these diagrams have direct significance to aqueous chemical processes in fields such as corrosion science, hydrometallurgy, mineral processing, geochemistry, and environmental pollution control.~ Pourbaix constructed Eh-pH diagrams for a large number of metals, including gold and silver in aqueous medlar Subsequently, in order to investigate the behavior of gold and silver in cyanide solution, a few papers appeared which T. XUE, Graduate Research Assistant, and K. OSSEO-ASARE. Professor, are w~th the Metallurgy Program, Department of Materials Scmnce and Engineering, The Pennsylvama State UmversW, Umverslty Park, PA 16802, Manuscnpt submitted September 29, 1983 METALLURGICALTRANSACTIONS B

dealt with the Eh-pH diagrams for Au-CN-H:O ~~and AgCN_HeOt~ 12 systems. These previous results are helpful in understanding the reactions occurring during the cyanidation process. However, these earlier studies were limited in the number of variables or species considered and in the range of activities or potentials utilized. For example, Deitz and Halpern H considered only the lower potential region (Eh 9.21, CN predominates. In the practical leaching process, the cyanide concentration is kept at 10-2 to 10 -3 kmol m 3 and the pH is maintained at a value of about 10. 32`33From Eqs. [5] and [6], it can be seen that operating in the range pH > 9.21 prevents the evolution of HCN gas. A main characteristic of the Eh-pH diagrams for the gold system is that the stability region of the aurocyanide co