Bio-Cyanidation of Low-Grade and Refractory Gold Ores

Abstract

Gold is of the precious metals with high demand and limited global resources. Cyanidation is the most common process to extract gold from its ores. However, due to the decrease in the grade of gold deposits and increase in the complexity of its bearing minerals, cyanide consumption has increased and many environmental problems are caused. Alternatively, bio-cyanidation is a recent technology that employs natural microorganisms to produce cyanide from non-toxic ingredients. In this study, the significant parameters of bio-cyanidation process for gold leaching have been identified, tested, and optimized. First, pH was explored and the results showed that an initial pH of 10 with no further adjustment was optimum for growth and cyanide production by Bacillus megaterium (B. megaterium) where the bacteria generated a maximum of 61.9 mg/L bio-CN. This solution could recover over 87% and 43% gold from oxidic and sulfidic sources, respectively. In the following stage, different cyanide measurement techniques were compared for characterization of the bio-CN in the complicated biogenic system and titration against silver nitrate with end point determination based on potentiometry was the most feasible method to characterize bio-CN. Next, the capability of bacteria to remediate cyanide was studied and a distillation process was employed to isolate the biogenic cyanide (bio-CN) before it is converted into non-cyanide species. In the next stage, the optimum medium ingredients for maximum gold recovery from the sulfidic ore was identified as 12.49 g/L tryptone, 5.36 g/L yeast extract, 2.46 g/L NaCl, and 10.13 g/L glycine, which could deliver 45% gold recovery. In addition, corn steep waste, a low-cost medium ingredient, was investigated as a replacement for the original medium and produced similar results. Additionally, the kinetics of leaching was investigated and showed a mixed mechanism of chemical reaction and diffusion through the solid layer were the obstacles to higher recovery from the sulfidic ore. Finally, the effect of amino acids on bacteria growth, bio-CN production, and gold leaching was explored and suggested 2.84 g/L glycine and 3.00 g/L glutamine as the best condition to deliver a high concentration of bio-CN (86 mg/L).