The chemical behaviour of silver in the pressure oxidation of complex sulphide ores and concentrates

In this study, it was found that silver could also react with sulphur to form silver sulphide in an acid media containing ferrous and ferric sulphate. Under deoxygenated condition, the reaction apparently involves the reduction of aqueous silver to silvery white flakes (metallic silver) and the subsequent reaction of metallic silver with sulphur. The reduction of aqueous silver is very rapid under highly reducing conditions. The reaction of metallic silver with sulphur is believed to be the rate-determining step because of the positive effect of sulphur on the rate of reaction. Under the pressure oxidation condition, the presence of oxygen minimises the silver sulphide formation by oxidising ferrous to ferric. Also, any silver sulphide formed is oxidised to aqueous silver. Silver precipitation as silver sulphide, therefore does not occur under the pressure oxidation condition.; The addition of soluble iodide ion to low to medium temperature autoclave oxidation experiments confirmed that silver could be precipitated as silver iodide in the autoclave discharge residue. The reaction of aqueous silver ion and iodide to form silver iodide (AgI) is therefore an effective way of deporting silver into a recoverable form. Although silver iodide has a low solubility product, silver extraction by cyanidation from the autoclave precipitate which is formed at high temperature is not complete, which suggests that some silver ions are still captured by the jarosite phase during iron precipitation. The deportment of some silver ions to the jarosite phase is promoted by the decrease in stability of silver iodide at high temperature and the increase in the stability of jarosite specie at high temperature. Therefore, in the presence of iodide and under the optimal conditions that favour jarosite precipitation, some aqueous silver may react preferentially with hydrolysed ferric and sulphate to form jarosite. The resulting jarosite specie will impact the ability to recover silver from oxidation residues.; Three different complex sulphides were oxidised in the temperature range 110–150°C, and potassium iodide was added to the initial solution to promote AgI in the autoclave residue. High zinc extraction into solution was achieved. The resulting residues were subjected to cyanidation for silver recovery. High recovery of silver (up to 98%) was achieved because its deportment into the lead-hydronium jarosite during the pressure oxidation was minimised, while its deportment into the cyanide-soluble silver iodide was promoted. Silver recovery decreased slightly at high temperatures (140 and 150°C) because of the enhanced stability and kinetics of lead-hydronium jarosite formation, but this could be overcome by increasing acidity and iodide concentration of the autoclave slurry. (Abstract shortened by UMI.)...