Mechanism of silver-catalyzed bioleaching of enargite concentrate

Silver-catalyzed bioleaching of enargite concentrate with three bacteria (Acidimicrobium ferrooxidans ICP, Sulfobacillus sibiricus N1, Acidithiobacillus caldus KU) and one archaeon (Ferroplasma acidiphilum Y) was conducted in order to elucidate the catalytic mechanism of silver sulfide in enargite bioleaching. Whereas Cu recovery remained relatively low (43%) and Fe dissolved completely without silver sulfide, Cu recovery was greatly enhanced (96%) and Fe dissolution was suppressed (29%) in the presence of 0.04% silver sulfide. In the latter case, 52% of the solubilized As was re-immobilized, in contrast to only 14% As re-immobilization in the former. The silver-catalyzed bioleaching (at 0.04% silver sulfide) proceeded at low redox potentials within the optimal range, which likely promoted enargite dissolution via formation of intermediate Cu₂ S. XAFS analysis revealed that As was mainly immobilized as As(V), which was in agreement with the EPMA results detecting ferric arsenate passivation on some enargite grains. Furthermore, formation of trisilver arsenic sulfide (Ag₃ AsS₄) was detected by XRD and EPMA, covering the surface of enargite particles. An intermediate layer, consisting of (Cu,Ag)₃ AsS₄, was also observed between the enargite grain and trisilver arsenic sulfide layer, implying that Cu in enargite may be gradually substituted by solubilized Ag. Kinetic study suggested that these secondary minerals do not rate-limit the enargite dissolution. The overall mechanism of silver-catalyzed bioleaching of enargite concentrate will be proposed.