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Formation mechanisms of surface passivating phases and their impact on the leaching kinetics of sphalerite (ZnS) and galena (PbS)

Nikkhou, Fatemeh (2021) Formation mechanisms of surface passivating phases and their impact on the leaching kinetics of sphalerite (ZnS) and galena (PbS). PhD thesis, Murdoch University.

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Sphalerite (ZnS) and galena (PbS) are the primary sources of zinc and lead, respectively. They often coexist in ore deposits. Extraction of zinc and lead is traditionally achieved by high-temperature roasting and smelting processes in which gaseous SO2 is produced, requiring expensive treatment. Hydrometallurgical leaching such as heap leaching and in situ leaching are environmentally more sustainable due to higher energy efficiency and no release of SO2, but slow leaching rate is the main challenge restricting their wide applications. The leaching rate is significantly affected by the formation of surface passivating phases, but the mechanism is not clearly known.

The objectives of this thesis are to understand the formation mechanisms of surface passivating layers and their impact on the leaching kinetics of sulfide minerals. The thesis has five main research chapters, focusing on (1) leaching of sphalerite in ferric sulfate, ferric chloride, and ferric nitrate and investigating surface passivation mechanisms in each case; (2) leaching of sphalerite in ferric methanesulfonate as a green leaching medium with less impact on the environment; (3) leaching of galena in ferric chloride, ferric perchlorate, and ferric nitrate and studying the formation mechanisms, porosity evolution, and sample textural characterization of the product phases; (4) leaching of galena in acetate-based and citrate-based solutions in the presence of hydrogen peroxide for providing a green and sustainable option for Pb extraction at lower temperatures (25-50 °C); and (5) developing a flow-through cell for in situ PXRD investigations of minerals leaching that provides more accurate insights into the leaching mechanisms.

Leaching residues were analyzed by SEM, EDS, PXRD, EPMA, FIB-SEM-tomography, synchrotron in situ PXRD, and Raman spectroscopy, and leach liquors were characterized by ICP-MS, ICP-OES, and AAS. The experimental results, together with thermodynamic modeling, confirmed that coupled dissolution-reprecipitation mineral replacement reaction was the responsible mechanism for the formation of surface phases and that type of anion significantly affected leaching.

For sphalerite, surface passivation was caused by hydrated Zn-Fe sulfate phases during ferric sulfate leaching and by elemental sulfur during ferric chloride leaching. The presence of these product layers made the effective contact between the mineral surface and leaching solution difficult. Thus, leaching rate became strongly dependent on species diffusion through the product layer. In contrast, no passivation occurred during ferric nitrate and ferric methanesulfonate leaching due to the formation of porous/permeable sulfur, and leaching rate was controlled by surface chemical reaction throughout the process. In addition, the composition of primary sphalerite was an important factor in the oxidation rate of sphalerite, so that the presence of 13.94 wt.% Fe in sphalerite solid solution enhanced the rate of Zn extraction by a factor of 3.1.

For galena, lead chloride and elemental sulfur were the major phases causing surface passivation during leaching in ferric chloride and ferric perchlorate solutions, respectively. Passivation by lead chloride occurred during chloride leaching of galena immediately after the start of the reaction despite no solubility limitation of lead chloride; this was due to the local supersaturation of lead chloride in the vicinity of the particle, leading to significant passivation. On the contrary, no passivation occurred during ferric nitrate leaching, and the process was controlled by surface chemical reaction alone. Moving forward, response surface methodology (RSM) was employed to optimize the leaching of galena with hydrogen peroxide in acetate-based and citrate-based solutions. Galena leaching in citrate solutions showed promising results at pH 7 and 25°C. However, the formation of a thin lead-oxide layer caused passivation in the later stages of the process. To eliminate passivation, the size fraction needed to be chosen in the range of <38 µm. In acetate solutions, the formation of lead sulfate around the particle and alongside the cleavage plane significantly inhibited lead extraction.

For better investigating minerals leaching, a flow-through cell was developed for in situ synchrotron PXRD experiments at the Australian Synchrotron. The designed flow-through cell provides the opportunity to further investigate the mechanisms of minerals leaching in time-resolved conditions.

Overall, the outcomes of this thesis provide some scientific basis for developing greener approaches such as in situ leaching and heap leaching for recovering Zn and Pb from their sulfide ore minerals.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Centre for Water, Energy and Waste
Harry Butler Institute
Chemistry and Physics
Supervisor(s): Xia, Fang and Deditius, Artur
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