The roles of lead ions in restoring the floatability of pyrite depressed by free cyanide

•Lead ions significantly counteracted the depression of free cyanide on pyrite flotation.•The adsorption of lead ions on pyrite surface was enhanced by the adsorbed cyanide.•Lead ions counteracted free cyanide by covering iron cyanide and thiocyanate on pyrite.•Lead ions preferentially adsorbed and accumulated on the top of thiocyanate.•A higher amount of lead ions is required to fully activate cyanide adsorbed-pyrite. Due to the drawbacks of copper ions used to activate pyrite flotation in gold processing plants where free cyanide occurs in process water and the benefits of lead ions in gold cyanidation, this study investigated the counteraction of lead ions on the negative effects of free cyanide on pyrite flotation in an alkaline condition. Flotation results indicate that 50 ppm (6.8 × 10-5 mol/L) free cyanide sufficiently depressed pyrite flotation, while 9.5 × 10-4 mol/L lead ions substantially activated pyrite flotation. It was interesting to find that 9.5 × 10-4 mol/L lead ions significantly counteracted the depression of 50 ppm free cyanide on pyrite flotation but did not completely restore the flotation of pyrite activated by the same amount of lead ions. Cryogenic X-ray photoelectron spectroscopy was then applied to determine the surface species responsible for pyrite flotation in different conditions. Iron cyanide and thiocyanate formed on pyrite surface were attributed to the depression of free cyanide on pyrite flotation, while lead species, in particular lead hydroxides, adsorbed on pyrite surface were attributed to the activation of lead ions on pyrite flotation. The adsorption of lead ions on pyrite surface was enhanced by the adsorbed cyanide species and lead ions counteracted free cyanide in depressing pyrite flotation through the covering of lead species on the top of iron cyanide and thiocyanate. However, lead species preferentially adsorbed and accumulated on the top of thiocyanate, limiting the capability of lead ions in counteracting free cyanide.