Electrochemical behavior of the dissolution of gold-silver alloys in cyanide solutions
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INTRODUCTION
M A N Y investigations on the dissolution behavior of gold and silver in alkaline cyanide solutions have been carfled out by researchers with different approachesY -51 Mechanisms involved in the dissolution of these individual noble metals have been documented, c63m but the dissolution behavior of their alloys has not been well understood. Generally speaking, the dissolution of gold and silver in alkaline cyanide solutions includes anodic and cathodic reactions, as shown by the following equations. Anodic reactions: 4Au + 8CN- = 4Au(CN) 2- + 4e 4Ag + 8CN- = 4Ag(CN)2- + 4e
[l] [2]
Cathodic reaction: 02 + 2 H 2 0 + 4e = 4 O H -
[3]
A number of researchers have studied the anodic behavior of gold in aqueous cyanide solutions, t91There were three peaks observed on anodic current-potential curves, namely, those at about -0.6, -0.1, and 0.4 V vs SCE. [5,'0-121 Kirk et al.VU proposed the following reaction sequence. Step 1.
Au + CN- = AuCN~ds-
[4]
Step 2.
AuCNads- = AuCN~ds + e
[5]
Step 3.
AuCN,~s- + CN- = Au(CN)2-
[6]
It was suggested that the anodic dissolution rate was controlled by step 2 in the region of - 0 . 6 to +0.15 V vs SCE and by step 3 in the region of +0.2 to +0.38 V vs SCE. The formation of films interrupted the dissolution processes
XIAOWEI SUN, Graduate Student, Department of Metallurgical and Materials Engineering at Michigan Tech. University. Y. CHARLES GUAN, Postdoctoral Fellow and KENNETH N. HAN, Distinguished Professor of Metallurgical Engineering and Dean of College of Chemical, Physical and Material Sciences and Engineering, are with the Department of Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701-3995. Manuscript submitted November 4, 1994. METALLURGICALAND MATERIALSTRANSACTIONSB
and caused the current drop. All of the results obtained by these investigators indicated that the passivation of gold surface occurred during the dissolution process, and they well explained why the gold behaved abnormally in cyanide solutions. Hiskey and Sanchezt~31investigated the mechanistic and kinetic aspects of the silver dissolution in cyanide solutions. They indicated that the anodic oxidation of silver at various cyanide concentrations followed a kinetic model involving coupled mass transfer and charge transfer; furthermore, the charge transfer rate constants for silver measured during this study and those of gold measured previously by Kudryk and KelloggV4] were in good agreement. There were a number of studies on the cathodic reduction of oxygen on gold and silver. Two different mechanisms have been proposed. One is a four-electron-transfer step in which oxygen is reduced to hydroxyl directly, as shown in Eq. 3. The other is two consecutive 2e steps in which hydroperoxide is produced according to tS,lsa61 02 + H20 + 2e = HO2- + O H -
[7]
Then, HOe is reduced to hydroxyl, as shown by the following equation: HO2- + H 2 0 + 2e = 3OH-
[8]
The first step includes some reduction to water since the overall reaction involved more than two e
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