Application of Bacterial Bioleaching for Gold Recovery from Refractory Pyritic Flotation Tailings

Abstract

The recovery of gold from sulphide-bearing flotation tailings is challenging because gold may remain finely disseminated or associated with sulphide minerals, limiting its accessibility to conventional leaching. This study investigated a combined bio-oxidation and thiourea-leaching route to improve gold extraction from flotation tailings. The flotation tailings were characterised by particle-size analysis, X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS), and Au deportment analysis. Bio-oxidation was investigated using shake-flask cultures and five mechanically stirred reactor experiments. Changes in pH, oxidation-reduction potential (Eh), and dissolved Fe, S, and Zn were monitored to evaluate the treatment. The results were consistent with oxidation and dissolution of sulphide-bearing phases, although no simple relationship was observed between bulk Fe or S dissolution and subsequent Au extraction. Following bio-oxidation, the residues were subjected to 6 h thiourea leaching under acidic conditions. Direct leaching of untreated flotation tailings was used as a reference, and the influence of ferric-ion addition was examined in paired tests. Au was not detected in the pregnant leach solution samples; therefore, extraction was evaluated from initial and final solid-phase Au grades and is reported as apparent Au extraction. The untreated flotation tailings, containing 0.81 ppm Au, produced an apparent Au extraction of 18.5%. Bio-oxidised residues from Reactor Experiments 1, 2, and 3 yielded 24.7%, 22.2%, and 27.2%, respectively, with Experiment 3 providing the highest extraction. Ferric-ion addition increased apparent extraction from 13.6% to 19.8% in the paired Experiment 4 tests, while the pyrite-rich concentrate yielded 22.9%. Overall, bio-oxidation produced a modest improvement in apparent Au extraction under selected conditions, but the maximum extraction remained below 30%. The absence of quantifiable Au in the liquid phase prevented establishment of a complete Au mass balance, indicating that further optimisation of the bio-oxidation and thiourea-leaching conditions and improved analytical sensitivity are required.