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Electrochemical and surface-enhanced Raman scattering spectroscopy studies were carried out on the dissolution of Au in sodium and ammonium thiosulphate media and in leach solutions containing Cu(II) and ammonia. The anodic dissolution of Au in thiosulphate solution was accelerated by the presence of ammonia and significantly enhanced by the addition of Cu but was inhibited by the presence of adsorbed surface species, with passivation being observed under certain conditions. In solutions containing thiosulphate alone, the inhibiting surface layer on Au was S, with a coverage of one S atom per three surface Au atoms (1/3 ML) in the potential region of leaching, increasing to close to 1 ML at higher potentials. When ammonia was added, the S coverage under the same conditions remained at 1/3 ML, but co-adsorption of a polythionate occurred at higher potentials and resulted in passivation. Ammonia copper polysulphide was formed on the Au surface when Cu was present as well as ammonia, with the surface layer under conventional leaching conditions developing to a composition of 2/3 ML S and 1/3 ML Cu., Electrochemical and surface-enhanced Raman scattering spectroscopy studies were carried out on the dissolution of Au in sodium and ammonium thiosulphate media and in leach solutions containing Cu(II) and ammonia. The anodic dissolution of Au in thiosulphate solution was accelerated by the presence of ammonia and significantly enhanced by the addition of Cu but was inhibited by the presence of adsorbed surface species, with passivation being observed under certain conditions. In solutions containing thiosulphate alone, the inhibiting surface layer on Au was S, with a coverage of one S atom per three surface Au atoms (1/3 ML) in the potential region of leaching, increasing to close to 1 ML at higher potentials. When ammonia was added, the S coverage under the same conditions remained at 1/3 ML, but co-adsorption of a polythionate occurred at higher potentials and resulted in passivation. Ammonia copper polysulphide was formed on the Au surface when Cu was present as well as ammonia, with the surface layer under conventional leaching conditions developing to a composition of 2/3 ML S and 1/3 ML Cu.

The use is discussed of thiourea as an alternative lixiviant to cyanide in Au extraction, particularly for preg-robbing Cu-containing ores. Surface-enhanced Raman scattering was used to study sub-monolayer surface species and oxidation products derived from thiourea during leaching. The results showed that the C-S stretch band shifted to lower wavenumbers when adsorbed on the Au surface. Sorption of thiourea occurred concomitantly with co-adsorption of the chloride or sulphate electrolyte. As oxidation continued, broad bands were observed indicating the formation of a Au2S-like phase. Formamidine disulphide was not observed at the Au electrode and did not appear to have a catalytic role. At high potentials a CN-containing species was formed at the electrode., The use is discussed of thiourea as an alternative lixiviant to cyanide in Au extraction, particularly for preg-robbing Cu-containing ores. Surface-enhanced Raman scattering was used to study sub-monolayer surface species and oxidation products derived from thiourea during leaching. The results showed that the C-S stretch band shifted to lower wavenumbers when adsorbed on the Au surface. Sorption of thiourea occurred concomitantly with co-adsorption of the chloride or sulphate electrolyte. As oxidation continued, broad bands were observed indicating the formation of a Au2S-like phase. Formamidine disulphide was not observed at the Au electrode and did not appear to have a catalytic role. At high potentials a CN-containing species was formed at the electrode.

Two aspects are presented: the development of innovative gold leaching processes and of novel study techniques. Process research has concentrated on the development of new thiosulphate leaching systems that use Fe(III) stabilised by ligands as the oxidant in place of Cu(II)+ammonia. Innovative study techniques include the use of surface- enhanced Raman scattering (SERS) to study the leaching of gold in cyanide solutions. In-situ SERS yields information that cannot be obtained by other methodologies., Two aspects are presented: the development of innovative gold leaching processes and of novel study techniques. Process research has concentrated on the development of new thiosulphate leaching systems that use Fe(III) stabilised by ligands as the oxidant in place of Cu(II)+ammonia. Innovative study techniques include the use of surface- enhanced Raman scattering (SERS) to study the leaching of gold in cyanide solutions. In-situ SERS yields information that cannot be obtained by other methodologies.

Real-time surface-enhanced Raman spectroscopy, used to identify the surface species formed during leaching, showed that chloride ion is adsorbed prior to gold dissolution but that its coverage is significantly diminished when leaching begins. A gold chloride band is again observed whem dissolution is rapid, together with bands characteristic of AuCl4(-) ions in solution. With thiosulphate. a thin gold sulphide layer is formed slowly in the potential region where leaching occurs. At higher potentials, thiosulphate is oxidised to sulphur and sulphate; the rate of oxidation is independent of pH, indicating that the rate-determining step is the first electron transfer from adsorbed thiosulphate ions., Real-time surface-enhanced Raman spectroscopy, used to identify the surface species formed during leaching, showed that chloride ion is adsorbed prior to gold dissolution but that its coverage is significantly diminished when leaching begins. A gold chloride band is again observed whem dissolution is rapid, together with bands characteristic of AuCl4(-) ions in solution. With thiosulphate. a thin gold sulphide layer is formed slowly in the potential region where leaching occurs. At higher potentials, thiosulphate is oxidised to sulphur and sulphate; the rate of oxidation is independent of pH, indicating that the rate-determining step is the first electron transfer from adsorbed thiosulphate ions.

Surface-enhanced Raman scattering (SERS) spectroscopy at surfaces under electrochemical control was used to study the adsorption of thiol collectors. Spectro-electrochemical studies at Au, Ag and Cu electrodes were carried out on ethyl xanthate and three iso-xanthates, O-isopropyl-N-ethylthionocarbamate, 2-mercaptobenzothiazole, diisobutyldithiophosphinate and butylethoxycarbonylthiourea. The results showed that adsorption occurs via charge transfer to form a metal-sulphur bond. The collector molecules do not decompose and, for those for which the reversible potential of the formation of the bulk phase is known, the chemisorption occurs at underpotentials., Surface-enhanced Raman scattering (SERS) spectroscopy at surfaces under electrochemical control was used to study the adsorption of thiol collectors. Spectro-electrochemical studies at Au, Ag and Cu electrodes were carried out on ethyl xanthate and three iso-xanthates, O-isopropyl-N-ethylthionocarbamate, 2-mercaptobenzothiazole, diisobutyldithiophosphinate and butylethoxycarbonylthiourea. The results showed that adsorption occurs via charge transfer to form a metal-sulphur bond. The collector molecules do not decompose and, for those for which the reversible potential of the formation of the bulk phase is known, the chemisorption occurs at underpotentials.

Voltammetry and Raman spectroscopy were used to investigate the interaction of the sulphide mineral collectors diisobutyldithiophosphinate (DlBDTPI) and n-butylethoxycarbonylthiourea (BECTU) with Au, Ag and Cu metal surfaces. DIBDTPI is oxidised to its disulphide on Au whereas metal-collector compounds are formed on Ag and Cu. Surface enhanced Raman scattering (SERS) spectra observed from Ag in the presence of DIBDTPI showed that charge transfer chemisorption takes place over a wide potential range. At high potentials, spectra from AgDIBDTPI were also detected. No SERS spectra were evident with Cu electrodes, but the formation of CuDIBDTPI was confirmed from Raman spectra. Similar results were observed with diethyldithiophosphate. At high potentials on Au, Raman spectra were observed that corresponded to those of the disulphide and AuDIBDTPL. SERS spectra observed with Au under laser illumination, showed that S is formed by photolysis of intermediate DIBDTPI radicals. Raman spectra from the Cu(I) BECTU compound are consistent with the crystal structure of this compound which shows the organic molecule to be bonded to metal atoms through both S and N. SERS spectra showed that the major surface species on Cu was the BECTU compound at potentials greater than -0.4 V. The spectrum at about -0.5 V was indicative of charge transfer chemisorption of BECTD., Voltammetry and Raman spectroscopy were used to investigate the interaction of the sulphide mineral collectors diisobutyldithiophosphinate (DlBDTPI) and n-butylethoxycarbonylthiourea (BECTU) with Au, Ag and Cu metal surfaces. DIBDTPI is oxidised to its disulphide on Au whereas metal-collector compounds are formed on Ag and Cu. Surface enhanced Raman scattering (SERS) spectra observed from Ag in the presence of DIBDTPI showed that charge transfer chemisorption takes place over a wide potential range. At high potentials, spectra from AgDIBDTPI were also detected. No SERS spectra were evident with Cu electrodes, but the formation of CuDIBDTPI was confirmed from Raman spectra. Similar results were observed with diethyldithiophosphate. At high potentials on Au, Raman spectra were observed that corresponded to those of the disulphide and AuDIBDTPL. SERS spectra observed with Au under laser illumination, showed that S is formed by photolysis of intermediate DIBDTPI radicals. Raman spectra from the Cu(I) BECTU compound are consistent with the crystal structure of this compound which shows the organic molecule to be bonded to metal atoms through both S and N. SERS spectra showed that the major surface species on Cu was the BECTU compound at potentials greater than -0.4 V. The spectrum at about -0.5 V was indicative of charge transfer chemisorption of BECTD.

The interaction was investigated of the three collectors with chalcopyrite, chalcocite, pyrite and galena using surface-enhanced Raman scattering spectroscopy (SERS). Chalcopyrite, chalcocite, synthetic CuS and a gold electrode all exhibited SERS bands attributable to 2- mercaptobenzothiazole adsorption following exposure to the flotation reagent in air-saturated 0.05 M sodium tetraborate solution. The spectra were consistent with bonding between the exocyclic S and Cu. The bands were broad and not generally attributable to compound formation. Isopropyl xanthate adsorption was observed on chalcocite and galena. No adsorption of the three collectors was observed on pyrite. The only system in which butylethoxycarbonylthiourea adsorption occurred was chalcopyrite., The interaction was investigated of the three collectors with chalcopyrite, chalcocite, pyrite and galena using surface-enhanced Raman scattering spectroscopy (SERS). Chalcopyrite, chalcocite, synthetic CuS and a gold electrode all exhibited SERS bands attributable to 2- mercaptobenzothiazole adsorption following exposure to the flotation reagent in air-saturated 0.05 M sodium tetraborate solution. The spectra were consistent with bonding between the exocyclic S and Cu. The bands were broad and not generally attributable to compound formation. Isopropyl xanthate adsorption was observed on chalcocite and galena. No adsorption of the three collectors was observed on pyrite. The only system in which butylethoxycarbonylthiourea adsorption occurred was chalcopyrite.