The role of bioleaching microorganisms in saline water leaching of chalcopyrite concentrate

In order to tackle the dual challenge of utilizing highly refractory chalcopyrite (CuFeS2) while saving scarce freshwater resources, this study aimed to systematically understand the individual role of chemical lixiviant and bioleaching microorganisms in the complex Fe3+-Cu2+-SO42--Cl- chalcopyrite leaching system. In general freshwater bioleaching conditions, the E-h level sharply increased, and the "high-E-h-bioleaching" became the major leaching driving force. In this case, the lowest Cu yield was obtained. The chalcopyrite leaching reaction responded differently to different salinity levels. At a low salinity of 0.5% NaCl, chemical Cl--leaching effect resulted in a higher Cu yield than the fresh-water "high-E-h-bioleaching" system. The growth of tested microbes was observed at 0.5% NaCl, but partial deactivation of microbial Fe-oxidation suppressed the E-h level. Under this condition, synergism between the chemical Cl--leaching effect and the "low-E-h-bioleaching" effect was found. At a high salinity of 2% NaCl, on the other hand, no active cell growth was observed, and thus pre-grown cells were used to mimic the presence of Cl--tolerant cells. Chemical Cl--leaching readily proceeded at 2% NaCl at low E-h, but quickly ceased upon the depletion of H+. The presence of bioleaching cells somewhat slowed down the speed of chemical Cl--leaching, but the acid depletion was alleviated by microbial acid generation. Chemical Cl--leaching, which favors low E-h condition, was the main driving force for chalcopyrite leaching at 2% NaCl. Therefore, the activity of Cl--tolerant S-oxidizer alone, rather than mixed Fe- and S-oxidizing consortium, was shown to play a critical role in maximizing the chalcopyrite dissolution.