A Study of Anodic Dissolution of Gold in Aqueous Alkaline Cyanide

Abstract

The rate of dissolution of gold in aqueous alkaline cyanide solutions was studied as a function of potential within the range --0.9 to +0.4V vs. SCE. The dissolution rate was found to exhibit three maxima at --0.66, +0.04, and +0.38V vs. SCE. These maxima corresponded to three current peaks found in anodic potential sweep measurements. Data from weight loss measurements were used for the determination of the stoichiometry of the electrochemical dissolution reaction and showed that for the region --0.9 to +0.6V vs. SCE, n ~ 0.85 ~ 0.01; for the region --0'.1 to +0.15V vs. SCE, n ~ 0.95 0.01; for the region +0.2 to +0.38V 'vs. SCE, n = 1.05 • 0.06. The dependence of the dissolution rate on the concentration of potassium hydroxide and potassium cyanide was determined in the two more anodic regions. At +0.38V vs. SCE the dissolution was directly proportional to the cyanide concentration and independent of hydroxide concentration. At +0,04~ r vs. SCE the dissolution rate was approximately linear with cyanide for low concentrations (<0.1M) and decreased with increased hydroxide concentration. At moderate hydroxide concentrations (0.1M) the dissolution rate decreased for cyanide concentrations greater than 0.2M. Several of the mechanisms suggested in the literature to account for the reactions in these two reglons were shown to be incorrect in some way; instead, the following sequence was proposed Au

Electrochemical dissolution of roasted gold ore in cyanide solutions

“…3c) at the end of the experiment after 16 h. It can be deduced that due to the decrease in most active potentials, less active (most cathodic) potentials on the surface of pure gold electrode were increased. This result suggests that the surface of the gold became passive by the time due to the formation of oxide and/or insoluble cyanide films (Kirk et al, 1978;Nicol, 1980). This was consistent with the potentiodynamic polarization results where pure gold showed a certain passive peak and the current density was sharply decreased to around zero.…”

“…Gold showed three passive peaks at around −0.1, 0.4, and 0.7 V with current densities of 0.2, 2.3, and 1.8 mA/cm 2 , respectively. AuOH ads , AuCN ads , [Au(III)OH(CN 3 ) − ] ads , and Au(OH) 3 products are believed to be responsible for the formation of these passive peaks (Kirk et al, 1978). Furthermore, Bas et al (in press) characterized gold surface by XPS after polarization tests and found Au 1+ and Au 3+ corrosion products as responsible for the passivation of gold surface.…”

“…Principally agitation does not promote the peak formation since the presence of intermediates at the interface becomes more limited. Second peak, known as a less anodic gold dissolution peak, could be passivated due to the oxygen adsorption (Dorin and Woods, 1991), gold oxide formation (Nicol, 1980), and by the formation of gold (I) hydroxide according to Au + OH − → AuOH ads + e − (Kirk et al, 1978), or by the conversion of Au(I) to Au (III) was 2CN − + [Au(I)OHCN − ] ads → [Au(III)OH(CN 3 ) − ] ads + 2e − (Cathro and Koch, 1964). Moreover, the adsorption of cyanide ions was found to initiate the passivation of second peak (Poskus and Agafonovas, 1995).…”

“…Mrkusic and Paynter (1970) identified the passivation as the film formation of dissolved species from calcine while Nicol (1980) reported that passivation of gold mainly depends on the presence of impurities in the solution. It was suggested that gold oxides and cyanide films are responsible for the passivation of the surface of gold (Kirk et al, 1978). The different types of dissolved minerals found in the ore may have a positive and/or negative effect on the rate of gold dissolution (Jeffrey and Ritchie, 2001;Nicol, 1980).…”

A study of the electrochemical dissolution and passivation phenomenon of roasted gold ore in cyanide solutions

“…Passivation phenomenon of gold has been known since 1907 and assumed that the passivity is due to the formation of a surface film on the surface of gold (Cathro, 1961). Passivation of gold has been linked to dissolved species, presence of impurities, gold oxide and hydroxide films, and insoluble sodium aurocyanide film (Kirk et al, 1978;Nicol, 1980;Mrkusic and Paynter, 1970).…”