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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 surfacepassivating 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.

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CO 2 gas injection is known as one of the most popular enhanced oil recovery techniques for light and medium oil reservoirs, therefore providing an acceptable mass transfer mechanism for CO 2 -oil systems seems necessary. In this study, interfacial mass transfer coefficient has been evaluated for CO 2 -normal heptane and CO 2 -normal hexadecane systems using equilibrium and dynamic interfacial tension data, which have been measured using the pendant drop method. Interface mass transfer coefficient has been calculated as a function of temperature and pressure in the range of 313-393 K and 1.7-8.6 MPa, respectively. The results showed that the interfacial resistance is a parameter that can control the mass transfer process for some CO 2 -normal alkane systems, and cannot be neglected. Additionally, it was found that interface mass transfer coefficient increased with pressure. However, the variation of this parameter with temperature did not show a clear trend and it was strongly dependent on the variation of diffusivity and solubility of CO 2 in the liquid phase.

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