Effect of silver (Ag) catalyst in bioleaching of enargite (Cu3AsS4) concentrate was studied using mixed cultures of moderately thermophilic acidophilic microorganisms at 45 °C. Addition of Ag2S enabled selective Cu dissolution from enargite while suppressing pyrite oxidation: At the highest Ag2S 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, Cu2S; trisilver arsenic sulfide, Ag3AsS4): Addition of Ag2S as Ag catalyst thermodynamically and microbiologically contributed to lowering solution redox potentials during bioleaching, consequently satisfying Eox (Cu2S) < Eh < Ec (Ag+) to enhance enargite dissolution via formation of chalcocite intermediate. Formation of trisilver arsenic sulfide and its intermediate layer (Cu,Ag)3AsS4 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 Ag2S.
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