A novel bio-oxidation and two-step thiourea leaching method applied to a refractory gold concentrate

Biooxidation-thiourea (TU) leaching is considered to be a more environmentally friendly and efficient method than cyanidation to recover gold from refractory sulfide gold-bearing ores/concentrates. However, high consumption of the TU reagent and additional oxidants hinders its commercial adoption. In this work, single TU leaching and novel two-step TU leaching processes were designed and performed after the biooxidation of refractory sulfide gold-bearing concentrate from Axi, China. The novel method contained an additional six-hour bioprocess and a six-hour lixiviant leaching. During the two-step TU leaching, the microbes remaining on the residue from biooxidation regenerated Fe3 + as oxidant. Compared with the highest gold recovery rate of the single TU leaching (92.2 ± 1.54%), which had additional Fe3 + and Na2SO3 supplied after prewashing, the rate reached 95.0 ± 0.73% in the two-step TU leaching process without additional oxidant and reductant. A microbial community analysis by 16S rRNA gene amplicon sequencing indicated that the genera Acidithiobacillus and Leptospirillum greatly contributed to the regeneration of ferric ions during the two-step TU leaching process and further improved the recovery of gold. As a non-cyanide method, combining the two-step TU leaching process with biooxidation takes full advantage of the microorganisms and is worthy for industrial application.