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Cyanide leaching is the most common method used for production of silver from gold/silver ores. Silver (and gold) can be recovered from pregnant leach solutions (PLSs) using Merrill-Crowe process (zinc cementation) and activated carbon adsorption. Due to their inherent limitations and paucity of selectivity, development of alternative processes for selective recovery of silver from PLSs is of great interest. In the current study, selective recovery of silver by precipitation from cyanide solutions using an environmentally friendly organic reagent i.e. Trimercapto- s -triazine (C 3 N 3 S 3 ; TMT) was investigated. The results have shown that precipitation of silver by TMT is a rapid process in that 87.9% of silver could be precipitated at the initial period of 5 min. Response surface methodology (i.e. central composite design) was adopted to examine the main and interaction effects of [TMT]/[Ag] ratio (0.09–36.52), [CN]/[Ag] ratio (3 − 10) and initial concentration of silver (1–40 mg/L Ag) on the precipitation of silver in five levels. The statistical analysis of the experimental data revealed that ratio of [TMT]/[Ag] (i.e. concentration of TMT) was the key parameter exerting a profound effect on the precipitation of silver. However, effects of [CN]/[Ag] ratio and Ag concentration appeared to be statistically insignificant. Complete precipitation of silver was found to be readily achieved under suitable conditions. The precipitation process was proved to be highly selective for silver over gold and copper as also confirmed by the tests on real pregnant leach solutions. It was found that the presence of TMT had no discernible effect on adsorption of gold by activated carbon, which was observed to even improve from copper containing solutions. Silver-TMT precipitates were characterised using SEM/EDS, FT-IR and other techniques. The current findings demonstrate that this precipitation process can be exploited as a potent alternative for selective recovery of silver from cyanide leach solutions of silver and gold/silver ores or purified/concentrated solutions ahead of gold electro-winning. [ABSTRACT FROM AUTHOR]

Production of gold from copper-rich gold ores has received particular attention due to some technical and economic challenges associated with the ready dissolution of most copper minerals during cyanide leaching. This leads to excessive consumption of cyanide by forming copper cyanide complexes, which adversely affect downstream recovery and cyanide detoxification processes. In this study, a novel method is proposed to precipitate copper from cyanide solutions using sodium dimethyldithiocarbamate (C 3 H 6 NNaS 2 ; SDDC). The influence of various parameters (i.e., reaction period (0–360 min.), concentration of SDDC (165–992 mM), [CN−]/[Cu] ratio (3–8), pH (10–12.5) and temperature (20–60 °C) on the precipitation of copper were investigated using synthetic copper-cyanide solutions. Increasing reaction time, concentration of SDDC and temperature were found to significantly improve the precipitation of copper. Increasing the ratio of [CN−]/[Cu] adversely affected the precipitation of copper with no precipitation being occurred at ratios of ≥6. Strong alkaline conditions appeared to have a slightly negative impact on the process. In the light of these findings, the response surface methodology (i.e., central composite design; CCD) was adopted to reveal the influence and interactions of [SDDC]/[Cu] ratio (21.0–126), initial concentration of copper (50–950 mg/L) and reaction period (15–360 min.) on the precipitation of copper. The analysis of the data confirmed that statistically significant parameters affecting the process were reaction time and [SDDC]/[Cu] ratio. It was also found that initial copper concentration did not significantly influence the process. Tests on real cyanide leach solutions derived from gold ores/concentrates demonstrated that the SDDC could selectively precipitate copper (as well as zinc and silver) from gold-bearing solutions. The activation energy (i.e., ≈67 kJ/mol) calculated by Arrhenius equation has indicated that the precipitation of copper is a chemically controlled reaction. Detailed characterisation of Cu-DDC precipitates was performed using SEM-EDS, FT-IR and particle size analysis. The precipitates (d 80 = 16 μm) are thin, rod-like particles in brownish colour. The recovery of copper from Cu-DDC precipitates using acid solutions (HCl, H 2 SO 4 and HNO 3) was also investigated. Complete recovery of copper was achieved using HCl and HNO 3 solutions at high concentrations. The findings have suggested that SDDC is an effective reagent for selective precipitation of copper from cyanide leach solutions. • Sodium dimethyldithiocarbamate (SDDC) precipitates copper from cyanide solutions. • Time and [SDDC]/[Cu] ratio are the most influential parameters on Cu precipitation. • Copper is selectively precipitated over gold. • Mineral acids can be used to recover copper from Cu-DDC precipitate. [ABSTRACT FROM AUTHOR]