David Dixon


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Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Electrochemical and Raman investigation of pyrite and chalcopyrite oxidation (2012)

No abstract available.

The surface chemistry of chalcopyrite during electrochemical dissolution (2012)

Hydrometallurgy may be an alternative to the currently practiced smelting process forcopper extraction from chalcopyrite (CuFeS₂). However, the low temperaturehydrometallurgical processes for chalcopyrite continue to face challenges, mostly relating totheir slow dissolution rates or high sulfuric acid production. The slow dissolution rate of themineral is strongly linked to the formation of the passive film on its surface. However,despite 40 years of research on this topic, there is still not a complete agreement betweenresearchers about the composition and stability of chalcopyrite’s passive film in sulfuric acidsolutions. In this work, the nature of chalcopyrite’s passive film and its stability were studiedby application of a variety of electrochemical techniques. Additionally, the electrochemicalresults of the chalcopyrite study were compared to those obtained for a pyrrhotite electrode(Fe₁₋xS), as pyrrhotite electrochemistry represents a simplified case of the chalcopyritesystem. X-ray photoelectron spectroscopy (XPS) was used to analyze the composition of theproduct layers formed on the surface.It is shown that the chalcopyrite electrode is passive for potentials up to 0.90 VSHE.Above this potential, transpassive dissolution occurs. Results of XPS studies have suggestedthat a metal-deficient sulfide film (Cu₁₋xFe₁₋yS₂₋z) is the most plausible copper and ironcontaining sulfide phase which passivates the surface of chalcopyrite. In addition, an outerlayer of iron oxyhydroxide (FeOOH) forms on the passive film. FeOOH forms via oxidationof the passive film’s ferrous sulfide phases. The thickness of the sulfide passive film wascalculated to be approximately 6.7 nm. It is demonstrated that the transpassive dissolution ofchalcopyrite is significantly linked to oxidation of sulfur (from sulfide in the passive film toelemental sulfur and maybe sulfur species with higher oxidation states, e.g. thiosulfate). Noelemental sulfur or polysulfide species were detected on the surface for potentials below 0.90VSHE.

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Electrochemical and leaching studies of enargite and chalcopyrite (2011)

The oxidative behavior of chalcopyrite and enargite in acidic ferric solutions was studied using surface characterization methods, leaching experiments and electrochemical techniques with massive electrodes and single fine particles. Results demonstrate that chalcopyrite oxidation displays classical active passive behavior, as often observed in passivating metals. Values predicted electrochemically on massive samples for the passivation potential Epp are in excellent agreement with leaching experiments in batch reactors. A transpassive regime was observed to appear after the passive regime and total dissolution of chalcopyrite was observed at potentials higher than 1.2 V vs. SCE. Passivated surfaces at low potentials between 300 and 550 mV showed non-stoichiometric chalcopyrite compounds and some isolated areas covered by sulfur. Passivated particles of chalcopyrite were reactivated with the addition of pyrite. At high potentials > 600 mV vs. SCE a dense sulfur layer was detected on particle surfaces and is assumed to be responsible for passivation at these potentials.Electrochemical studies of fine particles of enargite also showed active-passive behavior. The anodic active dissolution of enargite began at 300 mV and became passive at 700 mV vs. SCE. A compact sulfur layer on the surface of enargite particles was detected at potentials higher than 700 mV and caused passivation. Based on these electrochemical studies, enhancement of enargite leaching by addition of pyrite was proposed and validated. Leaching tests in batch reactors demonstrated that enargite can be dissolved effectively at atmospheric conditions producing elemental sulfur. Total extraction of copper was achieved within 24 h with a pyrite-to-enargite mass ratio of 4:1. Solid residues consisted entirely of porous elemental sulfur and all arsenic was found in the solution phase, predominantly as As(III). The implementation of this process at an industrial scale to leach chalcopyrite enargite concentrates will be significant, since there is no process operating at moderate temperatures and atmospheric conditions that is able to efficiently leach high-arsenic copper concentrates.

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Development and testing of a 2D axisymmetric water flow and solute transport model for heap leaching (2009)

Heap leaching is an industrially relevant process for extracting metal values from marginal ores and/or pretreating ores for subsequent extraction of valuable metals. Its advantages include low capital and operating costs, rapid construction times, and relatively low environmental impact.The movement of water and solutes through a heap of ore has serious implications for efficiency of heap leach operations. Solutes (whether introduced into or generated within a heap) move with the water and tend to mix with the bulk solution by the mechanisms of advection and dispersion. Though it is well known that ore-water characteristics influence water flow and solute transport properties during heap leaching, these properties have not been adequately incorporated into the design and operation strategies of heap leaching. In this work, a mathematical model and the results of special laboratory experiments are used to study the reaction and/or transport of solutes during heap leaching. The model is 2D axisymmetric, comprising transport of solutes by advection and dispersion in a bed of broken ore under a point source of solution representing a single drip emitter. The ore hydraulic parameters characterize the water retention and permeability properties, while the transport and chemical parameters affect the distribution of water and solutes, and the rates and extents of reactions of solutes in the heap. The close agreement between the observed data and the predicted results lends credence to the modeling approach and its solution methodology. It was observed that, within the limited water content range relevant to safe and successful operation of a heap leach facility, dispersivity increased with increasing irrigation flux and hence water content. The model captured the trends in water and lixiviant distribution, and solute transport in heaps, clearly demonstrating the importance of incorporating ore hydraulic parameters in modeling heap leaching. It was also found that transport of solutes is slower than leaching and hence the former is the rate limiting step in the process. The model can be used to simulate various process enhancement strategies for efficient operation and optimal design, as well as economic evaluations and potential environmental impact assessment of heap leaching operations.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Fundamental study of the chemical stability and oxidation of thiourea in acidic ferric media (2020)

The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.

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Assisted dissolution of copper sulfides in ferric sulfate solutions: thiourea and its derivatives (2018)

The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.

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Effect of thiourea on the dissolution of chalcopyrite in acidic ferric solutions at different temperatures (2018)

The effect of thiourea on the electrochemical dissolution of a chalcopyrite electrode and in leaching of a chalcopyrite concentrate in acidic-ferric sulfate solution at mild temperatures (50°C, 65°C, and 80°C) was investigated in the present study. The investigation was conducted in two phases: the first was the electrochemical experimentation to test a broad set of experimental conditions; and the second was the batch leaching of the chalcopyrite concentrate, once the working conditions were narrowed after the electrochemical experiments. The effect of thiourea on the dissolution current density was found to be dependent on the temperature and acid concentration, resulting in higher values at 80°C and 0.3 M H₂SO₄ than at 50°C and 65°C. As for the leaching of the chalcopyrite concentrate at 80°C, the copper extraction is increased by 15% utilizing a 30 mM concentration of thiourea, and 20% at 100 mM concentration, both compared to 0 mM thiourea in solution.

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Kinetic study of the dissolution of chalcopyrite in the presence of a catalyst under acidic sulfate medium (2018)

The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.

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Application of ammonium-citrate-thiosulfate leaching on awaruite-bearing serpentinite ores (2016)

Nickel is an important alloying element of stainless steel and non-ferrous alloys. Awaruite as an alternative nickel source is gaining attention at the depletion of sulfide nickel and the high cost of laterite nickel, which are traditional nickel resources. In view of the absence of a nickel recovery method from awaruite resources, UBC hydrometallurgy laboratory developed an in-house hydrometallurgical technology of atmospheric leaching in ammonium-citrate-thiosulfate solution. Ammonium selectively dissolves nickel in the presence of oxygen while citrate acts as a ferric chelator and thiosulfate a depassivation agent of nickel-iron alloy. In order to prepare the technology for pilot testing, in-depth batch leaching tests as well as preliminary solvent extraction tests were conducted and reported in this study. The leaching tests on an alternative source had an optimized result of ~85% nickel extraction. The nickel leaching reactions were believed under surface reaction control as indicated by the estimated activation energy of 54.4kJ/mol through kinetic model fitting of leaching test data. However, the leaching tests with high pulp density (>10%) and recycled liquor simulating industrial settings demonstrated hindered nickel extraction due to iron accumulation. Specifically, a targeted comparison test showed that in order to retain 80% nickel extraction at 30% pulp density, simultaneous increases to four times the original ammonia concentration and six times the original citrate concentration are required in comparison with the 10% pulp density baseline test. Therefore, this technology needs to be improved on iron removal and economical leach reagents recycle for commercialization. In the solvent extraction study, ACORGA K2000 as an ideal reagent candidate based on hydroxyoxime was tested on synthetic PLS containing 1 g/L Nickel (II), 0 – 0.8 g/L Fe (III), 0.1 – 0.5 M citrate and 1.5 – 6 M total ammonia. Hydroxyoxime has been proven effective in separating base metals from ammoniacal media. Results revealed that complete separation of nickel from iron was achieved at 5 v/o reagent solution concentration. Nickel can also be readily eluted with diluted sulfuric acid at pH = 1. The extraction kinetics was found to be relatively slow with 30 min of mixing/settling time required to reach equilibrium.

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Optimization of Magnesium Removal from Hydrometallurgical Leach Liquors by Struvite Formation (2016)

Nickel is a vital metal which occurs in two types of ore: laterites and sulfides. Historically sulfide ores have been the primary source of nickel however due to their limited resources and the vast occurrence of laterites, laterite processing has gained a great deal of attention. Nickel in laterite ores is accompanied by impurities such as magnesium and iron, due to their similarity in ionic radii and their capability in replacing one another in crystal lattices. Magnesium compounds are highly soluble, therefore, magnesium impurity in nickel processing leads to the production of non-recyclable process water which results in high water consumption and negative environmental impact due to the wastewater being discarded to the environment. An approach has been derived at the University of British Columbia which involves the removal of magnesium compounds from nickel plant wastewater streams as struvite. Struvite, MgNH₄PO₄·6H₂O, is a valuable fertilizer which provides three important nutrients of magnesium, phosphorus and nitrogen to plants. This study confirms the ability of the proposed flowsheet to successfully remove magnesium as struvite from nickel laterite plants from both aspects of the magnesium removal efficiency and the produced struvite purity; with the experiments conducted at the base conditions having magnesium removal efficiency of above 95 percent and the produced struvite purity being above 97 percent. This study focuses on the optimal conditions leading to the maximum magnesium removal and therefore struvite precipitation. Struvite precipitation is affected by many parameters. The most influential factors affecting struvite crystallization are mixing intensity, seeding and the seeding technique, pH, temperature, crystal retention time, magnesium to phosphate ratio, supersaturation level and the impurities present in the system. This study verifies that each of these parameters affect struvite precipitation differently. For instance, variations in supersaturation level and pH have greater effect on struvite precipitation then variations in temperature and mixing intensity. Additionally, this study confirms that the optimum condition for struvite precipitation is the same as the existing conditions of the effluent solution such as ambient temperature and pH; therefore, no further adjustment is required; however, solution seeding with struvite powder does improve the magnesium removal efficiency.

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Studies on the curing and leaching kinetics of mixed copper ores (2013)

Heap leaching is a metal extraction process from low grade ores where crushed ore is stacked on an impermeable pad and irrigated from the top with a solution of chemical reagents. An enriched solution containing the targeted metal is collected at the bottom. This technique involves complex chemical/electrochemical reactions and transport processes. Among the main features of this method of extraction include low capital and operative cost, modularity, and relatively high inventory of solutions. The need to optimize a heap operation has led to research studies in order to understand and interpret the chemistry and transport involved in a heap leach. These scientific investigations are focused on mathematical expressions of the reactions and transport phenomena of the minerals and reagents from the particle scale to the bulk scale. However, it was envisaged that pretreatment of these minerals are not accounted for in existing mathematical models of heap leaching. Sulfuric acid curing is a pretreatment to accelerate the extraction kinetics of copper ores and is widely used in copper operations. The curing process involves the addition of a highly concentrated sulfuric acid to the copper ore during agglomeration. Then, chemical reactions already begin prior to irrigation of the heap, transforming the initial copper species into new copper species which are easier to solubilize once the leach solution is provided to top of the heap.The present study aims to provide a means for the systematic integration of the curing pretreatment and the subsequent leaching process. The numerical implementation of the model is done using the Matlab programming language. The focus of this curing and leaching model is to represent the leaching kinetics of each mineral species, which involves solution of a system of ordinary differential equations. The numerical parameters of the proposed curing and leaching kinetic model were found from a set of laboratory experiments. Additionally, novel methods for determining the optimum agglomeration moisture, the optimum sulfuric acid dose for acid curing, and the relevant solute transport parameters were employed. The resulting model can be applied for design, scale-up, and optimization of a new or existing commercial heap leach operation.

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Atmospheric leaching of enargite in iron sulfate solutions catalyzied by activated carbon (2012)

Traditionally, smelting has been the primary method of treatment for copper sulfide concentrates. In modern smelters the environmental problem of sulfur dioxide emission has been addressed effectively, but pyrometallurgical treatment of concentrates containing elevated levels of arsenic is still difficult and costly. Therefore, arsenic is considered a penalty element for smelters. However, the depletion of “clean” (non-arsenical) copper deposits and the increasing demand for copper will make the treatment of copper-arsenic sulfides such as enargite and tennantite unavoidable. Thereby a viable processing method is required. Hydrometallurgical treatment of enargite using atmospheric leaching promises a comparatively simple method for managing arsenic by co-precipitating it with iron in the form of scorodite. The major challenge involved with this option is the slow rate of enargite leaching.A novel treatment for enargite-rich copper concentrates through atmospheric ferric leaching catalyzed by activated carbon is presented in this study. Enargite concentrates from three different sources in Chile and Peru and one enargite mineral sample from the United States were used in the leaching experiments. Batch leaching tests were conducted in sealed, jacketed, glass stirred-tank reactors. The results showed that enargite leaching was up to 6 times faster in the presence of activated carbon, making it possible to achieve virtually complete copper extraction within 24 hours. SEM studies revealed changes in the morphology of the passive layer on enargite particles which is formed as a product of leaching. The laboratory-scale tests indicated that desirable leaching kinetics could be maintained after recycling activated carbon particles multiple times to new leaching tests and also at a carbon:enargite concentrate mass ratio as low as 0.25. Activated carbon loss was reduced from 26 % to 5 % of the initial carbon mass by decreasing the impeller speed from 1200 rpm to 800 rpm, while the leaching performance remained similar. The effects of concentrate grind size, solution redox potential and initial total iron concentration on copper extraction have also been studied. The presented method promises a commercially attractive route to treat enargite concentrates.

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