Leaching and recovery of gold from low grade raw materials in cyanide-free media

Due to the toxicity and low efficiency of the dominant cyanide technology in the treatment of refractory gold ores, there is a growing demand for environmentally friendly and techno-economically efficient gold extraction methods. At the same time, high grade gold ores are depleting fast, whereas low grade gold ores and residues are currently underutilized. Therefore, there is a need for a cyanide-free leaching process that can be used to extract gold from low grade raw materials. This study investigates gold extraction from flotation tailings and gold ore by chloride and glycine leaching, as it has been suggested that these leaching media are among the most potential leaching methods in the route towards cyanide replacement. Chloride leaching can provide fast dissolution kinetics and has the capability for simultaneous oxidation of sulfides and gold. Additionally, the impact of biological pretreatment on gold recovery in chloride leaching was studied as this can potentially provide a supportive pretreatment process specifically for sulfide minerals. It is environmentally friendly and can be used for oxidation of sulfidic minerals from tailings, with no necessity for high temperature, pressure, or gaseous emissions. Glycine is a more recent non-toxic innovation and has been claimed to provide good leaching efficiency at low reagent cost. Besides the leaching processes, the applicability of the electrodeposition-redox replacement (EDRR) method for gold recovery from chloride and glycine solution was investigated and the results are compared with the conventional carbon adsorption process. The results of this research suggest that biological pretreatment support subsequent chloride leaching, providing improved gold extraction for flotation tailings, with up to 63% extraction. In the direct cupric chloride leaching of flotation tailings, an increase in the cupric and sodium chloride concentration also improved the final gold extraction. However, the increase in ferric ion concentration enhanced gold dissolution only up to 30 g/L. Beyond this concentration, a negative effect was observed. For flotation tailings, ferric chloride leaching was shown to provide the highest gold extraction (67%). For the investigated gold ore too, the highest gold extraction was achieved in ferric chloride leaching (93%), followed by cupric chloride (91%), glycine (90%), and cyanide (80%). The optimal conditions in glycine media were 1.25 M glycine concentration, pH = 12, and T = 60 °C. EDRR was applied for the first time for gold recovery from chloride and glycine solutions. A recovery of 9.3% was achieved from chloride pregnant leach solution (PLS), whereas for glycine solutions, 88% gold could be recovered from synthetic glycine solution, but only 35% from real PLS. This suggests lower gold recovery by the EDRR parameters used compared to state-of-the-art carbon recovery, which resulted in 100% gold recovery from glycine media.