A mechanism and thermodynamic model for the interaction of silicate minerals with lead-assisted gold electro-oxidation in a dilute cyanide medium
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Gold (Au) oxidation kinetics in cyanide (CN) solutions can be increased by the addition of low concentrations of aqueous lead [Pb(II)]. However, the Pb(II) addition is not always effective in improving Au oxidation kinetics. The present study provides a mechanistic and thermodynamic view of the Pb-assisted Au oxidation process in the presence of silicate minerals and presents evidence that Pb adsorption onto the silicate minerals plays a substantial role in the oxidation kinetics. AuCN(ads) film formation on the Au surface, in an Au-CN-H2O system at a low overpotential range of linear sweep voltammetry (LSV), was confirmed through both electrochemical study and XPS analysis. Once Pb(II) was added into the system, AuCN(ads) film formation was hindered, and the Au oxidation kinetics were significantly enhanced. However, the improving effect of Pb(II) on the Au oxidation process was reduced with the addition of quartz, due to Pb adsorption onto the quartz surface. As demonstrated through UV/Vis analysis and thermodynamic modeling, the cationic and neutral species, PbOH+, PbCN+ and PbO(a), were likely adsorbed onto the quartz surface. At lower CN concentration, the effect of quartz in reducing the Pb-assisted Au oxidation kinetics was more severe, and the Au oxidation was chemically controlled.