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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - April 2022)
The rotary kiln-electric furnace (RK-EF) process is a common pyrometallurgical route for ferronickel production from nickel laterite ores. Sulfur is a harmful impurity that deteriorates the mechanical properties of nickel alloys. Due to the low sulfur content of the laterites, it is believed that the majority of the sulfur in crude ferronickel is originated from the process additives such as fuel and reductant in the rotary kiln. Therefore, it is crucial to investigate the effect of the sulfur content of the fuel on the calcine composition. Reducing sulfur absorption from the fuel to the calcine is beneficial to reduce the load of the refinery. This study investigates sulfur deportment in the nickel laterite calcination to obtain a fundamental understanding of the behavior of sulfur present in the rotary kiln fuel.In this work, the main reactions that occur in the calcination stage are identified. The kinetics of the reactions are investigated by combining model-free and model-fitting methods. The sulfurization reactions in the nickel laterite calcination are identified, and the main sulfur-containing compound in the calcine is found to be pyrrhotite (Fe7S8). Using coal with higher sulfur content, employing a more aggressive reducing atmosphere in the furnace, and increasing the gas flow rate result in an increase in the sulfur content of the calcine. Increasing temperature from 600 to 700 °C leads to higher sulfur deportation from the gas phase to the calcine. However, raising the temperature above 700 °C decreases sulfur deportation due to sintering of the particles and recrystallization of the silicate compounds. A comprehensive kinetic analysis on the sulfurization reactions revealed that the sulfurization reaction is diffusion-controlled and has a low activation energy of 1.4-5.3 kJ/mol. Using CaCO3 as a sulfur absorbent leads to 70.8-91% sulfur removal in the calcine. The effect of the processing temperature and time on reducing the sulfur content of the calcine are also investigated. Increasing time from 30 to 120 min results in decreasing sulfur removal from 91 to 78.3%. Raising temperature from 700 to 800 °C promotes sulfur removal; however, sintering of additive particles at above 800 °C reduces sulfur removal.
The global gold industry is under increasing pressure to search for alternative technologies to replace cyanidation due to the inefficiency of cyanide in treating low grade refractory gold ores and increased public scrutiny on the use of cyanide in gold mining. Chloride has been identified as a promising candidate to replace cyanide. The objective of this research was to gain fundamental knowledge on the thermodynamic and kinetic limitations of gold dissolution in ferric chloride media at moderate temperatures. First, thermodynamic calculations were done to determine gold and iron speciation in chloride solution at varying initial ferric concentration, total chloride concentration, solution potential, and type of chloride salts. Then the effects of these variables on the kinetics of gold dissolution in ferric chloride media were studied by batch leaching and electrochemical tests using pure gold as the model mineral. Finally, the efficacy of ferric chloride media for gold leaching from real ores was tested by batch leaching of an oxide ore sample. The speciation calculation showed that gold dissolves as Au(I) species in ferric chloride solution and that the predominant ferric species is FeCl₂+, which was considered to be the main oxidizer. The batch leaching tests showed that the gold extraction increased with the initial ferric concentration up to 0.3 M; increasing total chloride concentration and the solution potential also had positive effects on gold dissolution; the kinetics of gold dissolution was significantly slower in the presence of the divalent salts. Gold extraction was thermodynamically controlled at low free chloride concentrations and low solution potentials. When these two variables were sufficiently high, the dissolution process was under kinetic control. The potentiodynamic tests revealed that the dissolution kinetics was cathodically controlled at ferric concentrations up to 0.3 M with sufficient chloride; while at sufficiently high ferric concentrations, the anodic process controlled the dissolution process, the kinetics of which increased with the total chloride concentrations. Gold was extracted from the oxide ore sample, but the drop in the solution potential led to a low overall extraction. The dissolution kinetics was enhanced by acid curing as pretreatment and increasing leaching temperature.
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.
Master's Student Supervision (2010 - 2021)
Unusual water chemistry coinciding with the appearance of a precipitate were observed in two seepages from a rock storage facility at Mount Polley Mine. This rock storage facility was originally characterized as non-acid generating, meaning that the rock under surface weathering/oxidizing conditions will not produce acid. However, these two seepages were not behaving according to drainage predictions and resembled a product of neutralized acid rock drainage. Due to the localized nature of the affected seepages, the cause of this unusual water chemistry was hypothesized to be the leakage of an external source of acid into the RSF. The possible acid sources were proposed to be an experimental lined heap leach pad and excess stockpiled elemental sulfur that had been sourced to provide the acid required for the heap leach pad. An inspection revealed that the heap leach pad liner appeared to be intact. However, drainage from the sulfur stockpile was insufficiently contained. By using bacteria culture experiments, bacterial DNA extraction, and bulk characterizations of the elemental sulfur, it was concluded that the sulfur had been oxidized, leading to the generation of sulfuric acid that infiltrated the underlying rock. To determine the extent to which the rocks had reacted with the acid generated from the sulfur stockpile, the rocks underneath the sulfur stockpile were sampled. Visual inspection and further laboratory analyses revealed extensive alteration of these rocks under acidic conditions, rendering them similar to leached oxide ore sampled from the experimental heap leach pad. pH-controlled weathering fronts were evident in these rocks, progressing from leached near the contact with the sulfur pile (pH 7). These altered rocks have been re-classified as PAG due to the depletion of carbonates from exposure to sulfuric acid. The findings of this research will help the mine assess the long-term stability of the altered rocks and guide future development of remediation strategies.
Fluid flow is a critical process involved in the valuable metals extraction from low grade ore in heap and dump leaching as well as the release of harmful substances from waste rock piles. The mechanisms by which fluids move through the porous media depend on the fluid properties and the intrinsic properties of the porous media, with permeability being one critical factor. Particle size distribution is a key factor that affects permeability by forming pores of different structure and size. The objective of this research was to assess the particle size distribution in heterogeneous packed ore/rock beds and quantify the effect of particle size distribution on porosity. In the studied mine site, the particle size distribution in the dump leach pad was determined by analyzing aerial images of multiple dump faces taken by a drone. Particles spanned a wide range in size from less than 2 cm in diameter to larger than 2 m in diameter, with a P80 to be 2 m. The spatial segregation of fine particles and coarse particles along the dump faces was observed, which may contribute to the formation of preferential flow.The effect of particle size distribution on porosity was quantified by two methods: the bulk density and CT-imaging techniques. Porosities under three particle sorting conditions were studied: narrow-sized particles, poorly sorted particles and well sorted particles. For narrow-sized particles, the porosity measured by the bulk density method decreased as the particle size was increased up to 0.151 mm after which the porosity remained constant in the range tested. The influence of the particle size on the porosity for the well sorted particles was similar to that of the narrow-sized particles from both of the methods. For poorly sorted particles, in both methods, porosity decreased as the fraction of the fine particles added was increased to a certain value, after which the porosity started to increase as the fraction of fine particles was further increased. The results have important implications for metal extraction from run of mine ores using dump leaching and release of contaminants from waste rock piles by influencing fluid flow properties.