Silver-catalyzed bioleaching of enargite concentrate using moderately thermophilic microorganisms

Effect of silver (Ag) catalyst in bioleaching of enargite (Cu_3AsS_4) concentrate was studied using mixed cultures of moderately thermophilic acidophilic microorganisms at 45ºC. Addition of Ag_2S enabled selective Cu dissolution from enargite while suppressing pyrite oxidation: At the highest Ag_2S concentration of 0.04%, Cu recovery reached 96% while Fe dissolution was suppressed to reach only 29% by day 72. Overall results from thermodynamic calculation, liquid/solid analyses and kinetic study suggested that Ag-catalyzed bioleaching of enargite concentrate proceeds via formation of at least two types of secondary products (chalcocite, Cu_2S; trisilver arsenic sulfide, Ag_3AsS_4): Addition of Ag_2S as Ag catalyst thermodynamically and microbiologically contributed to lowering solution redox potentials during bioleaching, consequently satisfying Eox (Cu_2S) < E_h < E_c (Ag+) to enhance enargite dissolution via formation of chalcocite intermediate. Formation of trisilver arsenic sulfide and its intermediate layer (Cu,Ag)_3AsS_4 indicated that Cu ion in the enargite lattice is gradually substituted with Ag. Such secondary products did not impose a rate-limiting step, since the Ag-catalyzed bioleaching was shown to be controlled by a chemical surface reaction, rather than diffusion through product film which was the case in the absence of Ag_2S.