Effects of initial Fe²⁺ concentration and pulp density on the bioleaching of Cu from enargite by Acidianus brierleyi

To maximize Cu recovery and minimize As release during the bioleaching of enargite (Cu₃AsS₄) by Acidianus brierleyi at 70 °C, the initial Fe²⁺ ion concentration and pulp density were investigated in batch tests. A maximum Cu recovery of 91.0 ± 0.5% was obtained with an initial Fe²⁺ ion concentration of 1.8-2.7 g/L and a pulp density of 1.0%. As the immobilization of As depended on the formation of scorodite (FeAsO₄•2H₂O) and cupric arsenate, a more rapid and stable As immobilization was achieved with an initial Fe²⁺ ion concentration of 2.7 g/L. Thus the initial Fe²⁺ concentration of 2.7 g/L and 1.0% pulp density provided for a combination of As precipitation and > 90% Cu recovery. When the initial Fe²⁺ concentration was increased, K-jarosite (KFe₃(SO₄)₂(OH) ₆) precipitated, leading to passivation of the enargite surface. When the initial Fe²⁺ concentration was lowered, the Cu recovery was incomplete due to insufficient oxidation. Elevating the pulp density to 2.0% showed that the increase in Eh significantly lagged behind the exponential phase in the planktonic cell growth curve. Pulp density also clearly affected the makeup of secondary minerals in solid residues after the bioleaching of enargite. There is a tendency for dissolved Fe³⁺ ions to readily precipitate as potassium jarosite at relatively smaller pulp densities and as scorodite at relatively larger pulp densities.