Removal of Impurities from Refractory Gold Ore using Bio-reduction and Bio-oxidation processes

Gold in refractory gold ore is difficult to be extracted by conventional metallurgical method due to the presence of sulfide minerals with elevated levels of iron (Fe), sulfur (S), and arsenic (As) as impurities, resulting in low gold (Au) recovery. Conventional methods such as cyanide leaching has been proven ineffective for gold extraction from refractory ore due to gold being intricately bound within the sulfide minerals. Consequently, this study explores the application of bioleaching as an alternative to conventional cyanide leaching. Shewanella oneidensis (S. oneidensis) and Acidithiobacillus ferroxidans (A. ferroxidans) serve as bio-reduction and bio-oxidation agents, respectively in the bioleaching process. The composition of minerals in the ore was determined through XRD analysis (Model: Rigaku’s Miniflex 600) and EDX analysis (Model EDX 3). Meanwhile, SEM analysis (Zeiss EVO LS15 SEM) was utilized to examine the morphology structure. The concentrations of impurities (Fe, S, and As) were assessed using a spectrophotometer (Model: DR3900 Hach) meanwhile the Au concentration was determined through ICP-OES (Model: G8015A5110 ICP-OES). Sieved refractory gold ore samples with less than 32 um and varying in weight (0.5 g, 1.0 g, 1.5 g, 2.0 g), underwent bio-reduction and bio-oxidation processes. The results indicated a rougher surface morphology of raw sample as observed through SEM analysis. Furthermore, XRD and EDX results demonstrated a decrease in impurity concentrations, suggesting a potential increase in gold purity. Notably, the bio-reduction process exhibited a superior enhancement in Au concentration with the values of 138.89% compared to biooxidation with the value 122.22%. Thus, the bio-reduction process proved more effective in increasing Au concentration compared to bio-oxidation.