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Doctoral Student Supervision (Jan 2008 - Nov 2020)
Copper mines increasingly face the challenge of processing large amounts of low-grade sulfides of elevated concentrations of impurities. In most cases, the challenge is compounded by water scarcity. A potential strategy to address such challenge is to use seawater with elevated concentrations of chloride for heap leaching of secondary sulfides. To ensure success of such heap leach processes, we comprehensively investigated aqueous chloride solution properties at high ionic strength, kinetics of copper extraction from chalcocite in chloride media, and mechanisms by which various factors influence leaching rate, in both acidified ferric and cupric chloride media. The aqueous chloride solution properties were determined by thermodynamic calculations supported by laboratory ORP (oxidation-reduction potential) measurements. The leaching kinetics was quantified by conducting a series of reactor and column leaching tests under fully-controlled conditions. The mechanisms were uncovered using various surface characterization techniques, including SEM-EDX and XPS. The thermodynamic calculation determined the speciation of iron and copper at increasing chloride concentration up to 3 M, based on which the actual cathodic and anodic reactions responsible for copper extraction were proposed. The kinetics study showed that the leaching reaction slowed down after 70 – 80% of copper was extracted in both ferric and cupric chloride media at ambient temperature. Kinetic models were first developed to satisfactorily describe copper extraction as a function of ORP, chloride concentration, and temperature in reactors, and then scaled up to describe copper leaching performance in columns. The surface characterization results showed that sulfur sequentially transformed from monosulfide to disulfide, and then to polysulfide and elemental sulfur. The slow decomposition of polysulfide was responsible for the slow leaching at high ORPs, whereas a combination of polysulfide decomposition and diffusion barrier by elemental sulfur layer was the reason for the slow dissolution at low ORPs. The effect of chloride concentration on the reaction rate may only manifest itself at low ORPs where the level of the elemental sulfur crystallinity was lower. This body of knowledge would ultimately pinpoint possible options to optimize the leaching performance.
Preventing arsenic release from mine waste materials, i.e., source control, is a preferable option for controlling arsenic discharge to the environment. Designing effective source control strategies requires comprehensive knowledge on the leaching behavior of arsenic from its bearing minerals. To determine the kinetics and mechanisms of arsenic release, we carried out reactor leaching experiments using arsenic trisulfide (As₂S₃) as a model arsenic sulfide mineral. The experimental results show that the arsenic release increased with pH, the dissolved oxygen concentration, and temperature. The speciation analysis indicates that arsenic was present in solution in the form of arsenite (III) and arsenate (V) and that thiosulfate and sulfate were the main soluble sulfur species. A two-step process that involves a series of primary and secondary reactions was proposed to explain the release of different arsenic and sulfur species. The release rates of arsenic and sulfur from crystalline orpiment were always slower than those from amorphous As₂S₃. Kinetic equations were derived from the leaching data to describe the release rate as a function of the leaching parameters for both amorphous As₂S₃and crystalline orpiment. The magnitudes of the reaction orders and the activation energy indicate that the surface chemical reaction is limiting the rate of arsenic release from amorphous As₂S₃. In contrast, both kinetic modelling and the solid surface characterization support that a mixed-control mechanism determines the arsenic release from crystalline orpiment. Namely, the process is controlled by the surface chemical reaction and the diffusion of dissolved oxygen through a product layer on the solid surfaces. The solid surface characterization shows that this product layer is most likely to be an arsenic-deficient phase enriched in elemental sulfur.