An electrochemical study on the dissolution of gold and copper from gold/copper alloys
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I.
INTRODUCTION
THERE have been numerous studies IH61 concerning the dissolution behavior of pure gold in cyanide solutions. However, very few investigations have been conducted to study the dissolution behavior of metals from alloys. The current study is part of an ongoing investigation on the understanding of the underlying principles involved in the extraction of metals from scrap. In the previous article, tjJ the dissolution behavior of copper and gold from their pure states and gold/copper alloys in aerated cyanide solutions was studied using rotating discs by monitoring the amount of these metals dissolved in the solution. In general, the overall reaction for the dissolution of the alloy Au,Cum in aerated cyanide solutions may be written as m+n
Au,Cum + (2n + 3m)CN- + - 4
m+n 02 + - -
2
H20
= nAu(CN)2- + mCu(CN)32- + (n + m)OH[1] which can be separated into two electrochemical components representing the oxidation of Au,Cum and the reduction of oxygen, respectively. Aunfu m +
(2n + 3rn)CN-
= nAu(CN)2- + mCu(CN)32- + (n + m) e
[2]
and n+m
--O2
4
n+m
+--H20
2
+ (n + m) e = (n + m)OH-
[3]
Reactions [2] and [3] occur at the alloy-electrolyte interface. At the mixed potential, the oxidation of the alloy and the reduction of oxygen are taking place in the same rate, and the net current is zero. YICHANG GUAN, Postdoctoral Fellow, and KENNETH N. HAN, Professor and Department Head, are with the Department of Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701. Manuscript submitted November 15, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
The anodic and cathodic reactions can be studied separately. A potential difference can be imposed across the alloy-electrolyte interface with the aid of an external voltage source. The study on the anodic reaction can be conducted in oxygen-free cyanide solutions. When the potential difference across the interface is made more positive, the dissolution rate, measured as an electrical current, will keep increasing until it reaches the limiting current, provided sufficient cyanide is present and there is no change occurring in the composition of the alloy surface. The cathodic reaction can be studied in cyanide-free oxygen solutions. When the potential difference across the interface is made more cathodic, the oxygen reduction rate, measured as a current, increases and leads to a limiting current with the decrease of potential, provided sufficient oxygen is present. According to the mixed-potential theory, the dissolution potential and the dissolution rate (current) can be obtained by combining the anodic and cathodic currentpotential curves. It is the purpose of this study to identify and discuss the electrochemical reaction mechanism by which the dissolution process of gold and copper takes place from gold/copper alloys. II.
EXPERIMENTAL
Analytical-grade chemicals were used in this study. Sodium cyanide (NaCN) was used as a lixivant and sodium hydroxide (NaOH) was used as a pH adjustor. All anodic dissolution experime
