Mechanisms of Surface Passivation during Galena Leaching by Hydrogen Peroxide in Acetate and Citrate Solutions at 25–50 °C

Organic solutions are promising lixiviants for fast and environmentally sustainable Pb extraction from ore minerals at low temperatures. However, engineering of novel leaching flowsheets has been hindered by poor understanding of the leaching mechanism, particularly the formation of surface-passivating phases. Here, we studied leaching of galena (PbS), the most abundant Pb ore mineral, in citrate and acetate solutions at 25–50 °C. The results show faster and higher Pb extraction in citrate solutions than in acetate solutions. For example, leaching of 53–106 μm galena particles at 35 °C for 2 h achieved 69.3% Pb extraction in a pH 7 citrate solution but only 30.1% in a pH 3 acetate solution. Investigation of solid residues by SEM, EDS, quantitative powder X-ray diffraction, and Raman spectroscopy proved the formation of a porous yet poorly permeable layer of anglesite during acetate leaching by pseudomorphic replacement of galena and subsequent overgrowth, hindering further leaching after 55.6% Pb extraction. In contrast, in citrate solutions, no anglesite was observed, but the formation of an impermeable thin Pb-oxide layer caused surface passivation after 87.1% Pb extraction. Our experimental results and thermodynamic calculations suggest that Pb-citrate complexes [ e.g., Pb 2 (C 6 H 5 O 7 ) 2 2–, Pb(C 6 H 5 O 7 ) 2 4–, and Pb(C 6 H 5 O 7 ) − ] are far more effective than Pb-acetate complexes [ e.g., Pb(CH 3 COO) + and Pb(CH 3 COO) 2 ] in suppressing the precipitation of anglesite because of the high solubility of Pb-citrate complexes in sulfate-rich solutions. This work provides a scientific basis for developing greener approaches such as in situ leaching and heap leaching for recovering Pb from galena-bearing ores.