Optimization of microbiological redox potential control in chalcopyrite bioleaching

Chalcopyrite (CuFeS₂) is considered one of the most promising future copper resources, yet its relatively recalcitrant property to chemical and microbiological oxidation requires further technological improvement to enhance its dissolution. Our studies so far demonstrated utility of the “microbiological solution redox potential control” in chalcopyrite bioleaching to enhance chalcopyrite dissolution: Solution redox potentials were microbially controlled based on the Fe(III)/Fe(II) ratio by using “weak” to “strong” Fe-oxidizers to target different redox potential levels during the bioleaching. As a result, improved Cu recovery was achieved at 0 <E_normal <1, especially by utilizing the “weak” Fe-oxidizing bacterium, Sulfobacillus sp. strain YTF1 in mixed culture with Acidithiobacillus caldus strain KU (S-oxidizer) (Masaki et al., submitted). In order to optimize the process, this study further investigated the effect of various parameters (e.g., pH, initial Cu²⁺ concentration, and pulp density) on “microbially redox-controlled” chalcopyrite bioleaching reaction. Lowering the initial pHs (from 2.0 to 1.75, 1.5 and 1.25) lead to greater abiotic acid chalcopyrite dissolution, but a rapid pH increase in sterile controls (to 2.5-3.0) suppressed the final Cu recovery. Whilst in inoculated cultures, the greatest Cu recovery of 65% was obtained at pH 1.75 by day 70 (cf. 25% in sterile controls) by maintaining the E_normal level at around 0.43. Since the positive effect of starting at pH 1.75 (compared to pH 2.0) was likely attributed to the presence of acid-dissolved Cu²⁺ ions at the beginning of the bioleaching reaction, the next experiment intentionally added different concentrations of Cu²⁺ ions (0, 5, 10, 20 mM) before starting the bioleaching tests. The effect of Cu²⁺ addition will be presented and discussed at the conference.