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UBC Theses and Dissertations

Catalytic leaching of silver with ferricyanide-cyanide solution Xie, Feng


Significantly lower silver extraction than gold has been commonly observed in gold-silver cyanidation plants, especially in heap leaching operations. The different mineralogy of gold and silver in the ores is suspected to be the main reason, e.g., the occurrence of low solubility acanthite may result in low overall silver extraction. Leaching of silver sulfide with ferricyanide and its potential application on gold-silver ores has been investigated with the purpose of increasing the silver extraction. It was found that under the experimental leaching conditions (0.5 g/1 NaCN, 0.5 g/1 Fe as K₃Fe(CN)₆, pH =11.5, N₂, room temperature), the main reaction during leaching of silver sulfide with ferricyanide-cyanide solution is, Ag₂S + 5 CN ̄+ 2Fe(CN)₆³ ̄=2Fe(CN)₆⁴ ̄+CNS ̄ + 2Ag(CN)₂ ̄ indicating that at least 6 kg K₃Fe(CN)₆ would be needed to extracted 1 kg silver from the pure silver sulfide. The dissolution of silver sulfide and gold in ferricyanide-cyanide solution was examined by performing a series of rotating disc tests. The results indicated that the dissolution rate of Ag₂S in the ferricyanide-cyanide system (0.5 g/1Fe as ferricyanide) is 21 μmol m ̄² s ̄¹ (0.78μm/hr), much higher than the value of 13 μmol m ̄² s ̄¹ (0.38 μm/hr) in aerated cyanide solution. The dissolution rate of gold disc in the same leaching system gives a value of 19 μmol m ̄² s ̄¹ (0.74 μm/hr) which is greater than that in aerated cyanide solution (about 5 μmol m ̄² s ⁻¹ , 0.19 μm/hr). The effect of pH, temperature, ferricyanide and cyanide concentration, and rotating speed on the dissolution rate of silver sulfide and gold in catalytic leaching system were examined. The results indicate a typical diffusion-controlled process for both Ag₂S and Au dissolutions in ferricyanide-cyanide solutions with activation energies of 6.7 kJ/mol and 7.8 kJ/mol respectively. The leaching kinetics of mixed sulfide minerals in ferricyanide-cyanide solutions showed that non-silver sulfide minerals decrease silver extraction from synthetic silver sulfide to different extents. A significant decrease on silver extraction is observed in the presence of chalcopyrite and pyrrhotite. It is speculated that most of the available ferricyanide has been consumed by the non-silver sulfide minerals. Catalytic leaching with ferricyanide to improve silver extraction from Veladero and Pierina gold-silver ore has been investigated through tank leaching and column leaching tests. The results of comparison tests show that improvements of Au and Ag extraction are not significant in catalytic leaching over baseline leaching. By analyzing the slurry potential variation, cyanide and alkaline consumptions and sulfide content in the leaching residues, it is believed that non-silver sulfide minerals, such as chalcopyrite, pyrrhotite, or pyrite, may compete for the available ferricyanide with acanthite in the ore and limit the efficiency of ferricyanide.

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