Formation mechanisms of surface passivating phases and their impact on the kinetics of galena leaching in ferric chloride, ferric perchlorate, and ferric nitrate solutions

Abstract

Hydrometallurgical leaching of Pb from galena is an environmentally friendly alternative to the traditional pyrometallurgical processing. Leaching produces secondary phases passivating the surface of galena and has a negative impact on the leaching rate and Pb extraction, yet a comprehensive understanding of surface passivation is lacking. Here we studied galena leaching in FeCl3, Fe(ClO4)3, and Fe(NO3)3 solutions, using the ex situ leaching-quenching-characterization method complemented by in situ synchrotron powder X-ray diffraction. We examined the effect of salt concentration (0.42 and 0.84 M), temperature (35–90 °C), time (up to 1440 h), solid-weight-to-fluid-volume ratio (0.5–50 g·L−1), and particle size (up to 355 μm) on the mechanisms and kinetics of galena leaching. The results show that leaching in FeCl3 was least effective due to precipitation of a cotunnite (PbCl2) layer on galena surface, and a layer of elemental sulfur on cotunnite, forming a core/mantle/shell structure (i.e., galena/cotunnite/sulfur), which restricted Pb extraction to less than 9%. Sulfur encapsulated PbCl2 and limited its dissolution even when the bulk solution remained undersaturated with respect to PbCl2. Leaching in Fe(ClO4)3 and Fe(NO3)3 solutions was more effective than in FeCl3, achieving 100% Pb extraction. Leaching in Fe(NO3)3 was fastest among the three iron salts. Porous elemental sulfur was the only secondary phase formed using Fe(NO3)3 and Fe(ClO4)3 and the leaching reactions follow the interface-coupled dissolution-reprecipitation mineral replacement mechanism. The effect of passivation was observed when the elemental sulfur layer reached ~20 μm thick. The activation energies calculated by the modified “time-to-a-given-fraction” method were 5.4–67.2 kJ·mol−1 for leaching with Fe(ClO4)3 and 11.4–56.8 kJ·mol−1 with Fe(NO3)3, indicating that leaching was controlled by surface reactions and phase boundary diffusion. This study provides new insights into the mechanisms controlling surface passivation due to the mineralogical and textural compositions of the passivating layers and the resulting leaching kinetics. It provides scientific bases for the development of heap and in situ leaching protocols for Pb extraction from galena ores.