Edouard Asselin

Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Mar 2019)
Corrosion behavior of B206 aluminum-copper casting alloy in seawater environment : electrochemical and microstructural studies (2018)

Aluminum-copper casting alloys have relatively high strength and hardness, fatigue and creep resistances and good machinability, all of each are dependent on the copper content of the alloy. The Al-Cu casting alloy (4.2-5.0 wt% Cu), known as B206, is a potential candidate material for use in marine applications where good mechanical properties and high strength to weight ratio is desired. These properties are ideal for components of tidal-based energy generating systems. However, corrosion continues to be an issue. This dissertation presents and discusses the results of several electrochemical and microstructural investigations conducted on B206, contributing to a further understanding of the fundamental corrosion processes. Applications of this research are strongest within the marine industry field, yet are extendable to other infrastructural and engineering applications such as aerospace and military.Results of this work elucidate the mechanism of localized corrosion of B206 alloy in seawater. Focused ion beam (FIB) used to determine the subsurface microstructure at local attack sites within the corroded area reveals that localized corrosion is propagated where continuous particles are buried beneath the surface. Propagating away from the initiation sites, corrosion develops preferentially along the grain boundary network beneath the alloy surface. Retrogression and re-aging (RRA) of the alloy to modify the grain structure and render uniform the distribution of the second phase is revealed not to have a substantial effect on the corrosion susceptibility of the alloy. However, Electrochemical Impedance Spectroscopy (EIS) and Mott-Schottky tests support the feasibility of implementing anodizing and possibly anodic protection systems for B206 in specific service environments. EIS was also used to determine the effect of cathodic protection (CP) on coated B206 and reveals that its corrosion resistance with CP is superior to the situation without CP and, therefore, that the coating is compatible with CP. Due to its use in the as-cast state, the effect of casting porosity on the corrosion of B206 was investigated using a pencil electrode method. Results reveal that the corrosion can be attributed to the local chemistry inside the pores (conductivity and potential at the bottom of pores).

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Iron precipitation and associated metal loss from simulated process solutions (2017)

Ferric iron precipitation is an important integral step of the hydrometallurgical processes. The precipitation product is often amorphous and leads to a significant amount of valuable metal loss to the residue along with residue disposal issues.Characterization of the leach residue samples from CESL and Vale medium temperature (150 °C) hydrometallurgical processes revealed that the precipitation of iron in the form of amorphous iron oxide phases results in approximately 2-4 times higher metal loss compared to the crystalline phases.To address the issue, simulated process solutions were used to study the effect of process parameters and their relative importance on batch precipitation conditions with the aim of obtaining a stable iron oxide phase i.e. hematite, while minimising associated metal loss to the precipitation product. It was found that the factors: initial ferric, H2SO4 and seed concentrations play an important role in the iron precipitation step. Mathematical models were developed for the iron precipitation and metal loss to the precipitates using statistical data analysis techniques.Results from this study show that the presence of low ferric or high acid concentrations and moderate amounts of seed are required to minimize metal loss to the precipitation product with moderate to high levels of iron precipitation. The supersaturation and the nucleation to growth ratios were found to determine the final product quality i.e. the particle size and associated metal loss.The presence of various anions or cations was also found to play an important role on the iron precipitation rate and product quality/nature. For example, the presence of chloride in the solution accelerated precipitation kinetics. The sulphate salts of the metals such as Mg and Cu increased the extent of precipitation, while aluminum sulphate decreased the extent of precipitation. Presence of the sodium ion in the system accelerated the precipitation kinetics but changed the nature of the product to sodium jarosite. The presence of low levels of arsenic (As:Fe ≤ 0.08) in the system were found to severely retard the precipitation rate. Adsorption of sulphate and incorporation of OH‒ into the hematite structure were responsible to produce a poorly crystalline hematite product.

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High temperature and high pressure corrosion of titanium in hydrometallurgical applications (2015)

The corrosion characteristics of titanium (ASTM Grade 2) in copper pressure leaching environments are determined from room temperature and pressure up to high temperatures and pressures (230 °C, 430 psi). Anodic oxidation and controlled chemical oxidation methods are used to improve the corrosion resistance of Ti. Electrochemical and mass loss measurements are performed to evaluate the corrosion resistance of pre-oxidized titanium, compared to that of titanium with no prior oxidation, to generate a best practices guide for the hydrometallurgical industry. The results at low temperature showed that H₂SO₄ solution is very corrosive for Ti with a freshly polished surface. The corrosion rates (CRs) of Ti are obtained using mass loss and electrochemical measurements in H₂SO₄ with Cl-, Cu²⁺ and Fe³⁺ additions up to 175 °C. It is found that the CRs of Ti are unaffected by the presence of Cl− ions in H₂SO₄ solutions. CRs obtained from mass loss and electrochemical measurements confirm that Cu²⁺ and Fe³⁺ ions are good corrosion inhibitors for Ti. Iso-corrosion diagrams, with 0.1, 0.5 and 1 mm yr−1 lines for Ti in 3-50 wt.% H₂SO₄ solutions with Cu²⁺ and Fe³⁺ additions from room temperature to 175 °C are constructed from immersion test data. The effects of temperature (100-230 °C) and SO₄²− concentration (0-0.5 mol L−¹) on the pitting corrosion of Ti are studied in neutral Cl− containing solutions using cyclic potentiodynamic polarization and linear-sweep thermammetry measurements. A metastable pitting temperature threshold (MPTT) is defined for Ti as a function of sulfate to chloride mole ratio using linear-sweep thermammetry measurements.iiiAnodic oxide films (AOFs) are potentiostatically formed on Ti in 0.5 M H₂SO₄ solutions at various anodizing voltages (up to 80 V) at 25 °C. A new method is developed to fabricate chemically oxidized films (COFs) with high corrosion resistance by controlled chemical oxidation with H₂O₂ solutions at 90 °C. The corrosion behavior of the as grown AOFs and COFs is investigated in copper sulfide leaching solutions. It is confirmed that chemical oxidation with 2 M H₂O₂/0.1 M HCl solution leads to the best improvement of the corrosion resistance of Ti.

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Speciation of the sulfuric acid-ferric sulfate-ferrous sulfate-water system and its application to chalcopyrite leaching kinetics up to 150 ºC (2015)

Hydrometallurgical oxidation methods are increasingly being considered for the extraction of Cu from chalcopyrite. However, the kinetics of cathodic ferric ion reduction are poorly understood. This thesis investigates the kinetics of cathodic ferric ion reduction on chalcopyrite and its influence on the leaching process in acidic iron sulfate solution, with an emphasis first placed on the development of a speciation model for the H₂SO₄-Fe₂(SO₄)₃-FeSO₄-H₂O system from 25°C-150°C. Speciation results show that most Fe(III) is distributed as complexes or precipitates and the free Fe³+ accounts for only a minor percentage (up to 5.2% of total ferric) whereas a large amount of Fe(II) exists in the form of free Fe²+. The Nernst equation was used to study the redox potential of Fe³+/Fe²+ couple. The speciation model explains the change of redox potential with temperature for all nominal Fe³+/Fe²+ ratios. This model was validated by reliable prediction of measured redox potential, comparison of previously published results of ferric solubility, together with an analysis of the calculated pH and ionic strength.A novel expression was also developed to predict the redox potential of the ferric/ferrous couple. It seems that the redox potential can be easily and accurately determined only based on the variables of temperature and nominal Fe³+/Fe²+ ratio. The calculated free Fe³+ concentration allowed for a detailed investigation of the reduction kinetics of ferric ion on chalcopyrite by cathodic potentiodynamic polarization. The exchange current densities of the Fe³+/Fe²+ couple are on the order of 10-⁷–10-⁵ A/cm² in the range of 25–150°C, substantially less than that on platinum. Calculated rate constants can be well described by the Arrhenius equation. The transfer coefficient increases linearly with temperature (rather than being constant).The importance of the cathodic ferric ion reduction reaction on the overall leaching process is progressively increased when increasing the nominal Fe³+/Fe²+ ratio and temperature. Leaching is under anodic control when the nominal Fe³+/Fe²+ ratio is around 1:1, whereas at higher nominal Fe³+/Fe²+ ratios and temperatures it is under mixed control.These findings provide the basis for mechanistic analyses and attendant optimization studies of industrial leaching processes of chalcopyrite and other sulfide minerals.

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Particulate fouling by iron oxide at elevated temperatures : surface chemistry, interfacial electrochemistry and sensor development (2014)

Particulate fouling as a result of corrosion product sedimentation is known to be a significant issue in the heat exchanger of nuclear power plants and thus the development of monitoring technologies for the detection of fouling is important. Since electrochemical processes are usually very sensitive to water chemistry, they provide an opportunity for the development of accurate and online sensors. The main aim of this work is to develop an electrochemical sensor to detect particulate fouling at various temperatures and pressures up to 200 ºC. In order to develop such an electrochemical sensor, knowledge of the interfacial chemistry and electrochemistry of both the suspended particles and the sensing probe are required. Potentiometric titration was used to measure the pH of zero charge (PHZC) of magnetite and hematite (both known particulate foulants) from 25 ºC to 200 ºC. Electrochemical impedance spectroscopy (EIS) was used to measure the minimum differential capacitance of a glassy carbon electrode (GC) as a function of electrode potential i.e. the potential of zero charge (PZC). The obtained results clarified the oxide particle-electrode interaction since a GC electrode was used as a detector probe. A sensor for particulate fouling detection was then investigated and a new experimental method for the detection of magnetite particles at temperatures up to 200 ºC was developed. An electromagnetic GC electrode was employed to collect the magnetite particles from the suspension solution and it was observed that changes in double-layer capacitance could be used to detect deposition at different conditions. Finally, the impact of particulate fouling on water chemistry was studied. A novel electrochemical method was employed to accurately measure the kinetics of H2O2 decomposition on the surface of magnetite at temperatures up to 200 ºC. This work provides an experimental methodology for the prediction of failures due to particulate fouling processes in heat exchangers by providing a means to estimate the extent of fouling, the interactions between colloidal foulants and their corresponding impact on water chemistry.

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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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Master's Student Supervision (2010-2017)
Modeling the kinetics of the zinc pressure leaching process - oxidative sphalerite leaching in sulphuric acid media (2017)

In the field of hydrometallurgy, the industrial uptake of leaching models has been overlooked partially due to the lack of universal models. A model developed for one plant cannot easily be transferred for the application of a different plant without redesigning the leaching kinetics in the code. The Multiple Convolution Integral (MCI)-based model developed in this thesis has the ability to be universally applied by user-controlled inputs. Chemical reactions can be selected while the modeling software calculates the mass and energy balances. Residence times, operating conditions, and the rate-limiting reagents can also be defined to calculate a precise fraction reacted (leach extent) for sulphide minerals. The ability of the using the MCI model for predicting sphalerite leaching is examined in comparison to hydrometallurgical plant data collected from a Canadian pressure leach operation. The results are promising, showing that the model can predict plant Zn extraction data to within an error of 1.5 %. The model is further verified through bench scale pressure leaching experiments where 94 % of the zinc is extracted within 90 minutes using a concentrate sample from the same industrial plant. The effect of temperature is analyzed and the activation energy is calculated to be 40.8 kJ/mol. Interesting discoveries with respect to the reagent concentrations and their effect on the overall fraction reacted are also explored from the model results. In addition, the limitations of the MCI model are explained along with suggestions for improvement.

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Reduction of limonitic laterites by ferrous sulphate in ammoniacal solutions (2016)

Leaching of a limonitic laterite (1.49% Ni, 0.33 % Co, 2.29% Mn, 51.8% Fe) using ferrous sulphate as reductant was studied in ammoniacal solutions. Studied parameters included temperature, agitation, ferrous sulphate concentration, solids content, ammonia concentration and a comparison between ammonium sulphate and chloride. Tests were performed in a batch cell with temperatures between 20 and 80ºC at atmospheric pressure. Leaching kinetics for nickel were different to cobalt and manganese. Nickel was favoured by temperature in ammonium sulphate reaching 66% extraction in 3 hours, whereas in ammonium chloride it showed slower but steady kinetics with an extraction of 75% in 24 hours. Cobalt extraction in ammonium sulphate was low at high temperature and solids content; however, at low solids content (4% w/w) and high temperature (80°C) cobalt extraction reached 80% in 4 hours. With 11% solids at the same temperature, extraction decreased to 20%. In presence of ammonium chloride, temperature had a positive impact on cobalt reaching 80% in 8 hours at 23% w/w solids. Cobalt appeared to re-precipitate in presence of ammonium sulphate. Manganese behaved similarly to cobalt, however, in presence of ammonium sulphate, it only reached 60% extraction after 8 hours at 80°C at low solids. In presence of ammonium chloride, manganese reached 65% in 3 hours while increasing solids was not detrimental. Manganese suffered adsorption and/or co-precipitation in ammonium sulphate and chloride. This decrease in extraction exhibited by cobalt and manganese was attributed to co-precipitation and/or adsorption in iron oxides/hydroxides. Nickel extraction appeared aided by ferrous sulphate. Cobalt and manganese improved in presence of higher ammonia/ammonium, agreeing with the observation that cobalt and manganese are more prone to co-precipitation/adsorption; for which they prefer lower ferrous sulphate and higher ammonia/ammonium in order to stabilize their ammines. Feed and solid residue analyses using chemical assays, x-ray diffraction, scanning electron microscopy and mineralogy, provided better understanding of limonite reduction. The main phase in the residue was magnetite. Differences in behaviour between nickel and cobalt/manganese suggest a two-stage process, and chloride solutions seem promising due to better stability of cobalt and manganese.

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The corrosion behaviour of aluminium alloy B206 in seawater (2016)

Aluminium alloy B206 is one of the strongest and toughest alloys in the cast aluminium family. Although it is light and has excellent low cycle fatigue strength, AA B206 has been known to perform adversely due to its poor corrosion resistance. Thus corrosion has been identified as one of the major issues that jeopardizes the long-term use and performance of B206. The corrosion behaviour of B206 in seawater is studied through immersion testing and electrochemical techniques such as Potentiodynmaic Polarization, Potentiostatic Polarization, Cyclic Potentiodynmic Polarization and Linear Sweep Thermmametry in two different solutions, namely natural seawater and simulated seawater, at various temperatures. Techniques like Optical Microscopy, Energy Dispersive X-ray Spectroscopy and Scanning Electron Microscopy have been used to investigate the microstructure and surface morphology before and after the electrochemical tests. Heat treatment has been performed on the as-received samples using RRA and T7 heat treatment techniques to compare the corrosion behaviour of the former with the latter using electrochemical techniques and image analysis. Lastly, hardness tests have been performed on various heat treated and as-cast samples to establish a comparison in mechanical properties. This study shows that the extent of B206 corrosion depends on the oxidizing nature of the seawater environment i.e. low or high redox potential rather than on the temperature of the seawater. Natural seawater is more aggressive than simulated seawater. Also, heat treatment improved the corrosion resistance as compared to as-cast B206 which was determined by the values of corrosion current density and surface analysis. Furthermore, heat treatment has led to better mechanical properties as determined by hardness tests.

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High temperature and high pressure cobalt cementation onto zinc dust (2015)

Trace amounts of electrolyte cobalt during zinc electrowinning (EW) significantly decrease the current efficiency of the EW process by accelerating the parasitic hydrogen evolution reaction. The maximum tolerable level of cobalt in zinc EW can be as low as 0.1- 0.3 mg/L. The typical method to remove cobalt from zinc electrolyte, which is based on cementation onto zinc dust at approximately 85°C, is not an efficient process. It suffers from long retention times (2-3 hours) and high consumption of reagents; especially zinc dust. The aim of the present research was to study cobalt cementation at high temperature and high pressure (HT/HP) to accelerate the rate of cobalt removal and reduce the consumption of the reagents (zinc dust and activators). Experimental variables included temperature (85-150°C), pressure (0-100 psig), zinc dust dosage, zinc dust particle size, and activators (copper and antimony).Based on this research, the following results were obtained:1. Increasing temperature had a significant effect on the rate of cobalt removal. The optimum temperature was found to be 125°C - temperature at which the target level of cobalt (0.1 mg/L) could be met in 20 min.2. At 125°C and in the presence of 2.5 mg/L Sb and 45 mg/L Cu, 3.5 g/L zinc dust was found as the optimum zinc dust addition to lower cobalt concentration from the initial level of 15 mg/L to below 0.1 mg/L.3. Smaller zinc particles showed better cobalt removal results, but the cement redissolution was also more severe with these particles.4. The role of Sb in the activation system was more important than Cu. However, the best result in terms of the rate and extent of cobalt removal was achieved when both of the activators were added to the solution together.5. As expected, increasing the overhead pressure of N₂ (tested at 85°C) did not alter the cobalt removal profile greatly. Also, the effect of increasing the partial pressure of H₂ (tested at 125°C, and above the amount generated in situ by the reaction) on cobalt removal was negligible.

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Metal recovery from medium temperature pressure leach residues (2015)

Residues of the CESL Cu and Vale Ni sulphide medium temperature pressure leach contain elevated levels of the interest Cu and Ni metals. Metal losses are disproportionately found within a poorly formed amorphous and semi-crystalline iron oxide phase analogous to ferrihydrite. This phase results from incomplete ferric hydrolysis and precipitation that occurs simultaneously with the leaching of the sulphide minerals. An investigation was conducted by ageing the residues at 95°C in water to assess the extraction of lost Cu and Ni and the transformation of ferrihydrite to the more crystalline goethite and hematite species. It was found that 22% of the Cu could be removed from the CESL residue in 24 hours. Repeated ageing was only capable of extracting 38% of the Cu. Extractions of 22% of the Ni and 6% of the Cu were achieved in 24 hours from the Vale residue. Recovery of the lost Cu and Ni was mainly attributed to the washing of entrained and adsorbed Cu and Ni sulphates evenly distributed throughout the residues. A small amount of Cu and Ni was gained from ferrihydrite dissolution during the ageing process. Further investigations by SEM-EDS revealed the presence of unleached Cu and Ni sulphides that also contribute notable metal losses. EMP analysis showed that all hematite precipitated in the Vale residue contained Cu, Ni and SO₄²−. The average hematite particle consisted of 66.1% Fe, 0.85% Ni, 0.73% Cu and 1.19% SO₄²− for an equivalent of 30% Ni and 25% Cu losses to the residue. QXRD analysis of the aged residue showed little evidence of ferrihydrite transformation though ageing stabilised the residues by reducing the amount of X-ray amorphous material present. Improved stability was confirmed by a reduction in mass loss following treatment by a sequential extraction procedure utilising acidified hydroxylamine hydrochloride. This treatment dissolves the ferrihydrite present in the residues and is used as a proxy to assess residue stability and losses of Cu and Ni to the non-crystalline iron oxide phase.

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Synthesis and solubility of arsenic tri-sulfide and sodium arsenic oxy-sulfide complexes in alkaline sulfide solutions (2014)

Alkaline sulfide leaching (ASL) at approximately 100 ºC has been used to selectively extract arsenic and antimony from enargite and tetrahedrite concentrates. Sodium thio-arsenate has been postulated to crystallize from alkaline sulfide leaching solutions upon cooling. However, literature data on the solubility of sodium thio-arsenate as well as proof of its crystallization from ASL solutions is scant. In this thesis, the solubility of leach-produced and synthetic sodium thio-arsenate is studied.To determine arsenic solubility in ASL solutions, sodium thio-arsenate and sodium arsenic oxide sulfide complexes are synthesized by various means and characterized by EDX, QXRD, and ICP. The synthesis of amorphous As₂S₃, sodium arsenic oxy-sulfide complexes, and sodium thio-arsenate is first presented. For amorphous As₂S₃ synthesis, the effect of concentration of sodium sulfide (0.1 M) and hydrochloric acid (1 M), temperature (40 ~ 60 ºC), and aging time (48 hours) was optimized. The solubility of synthetic sodium arsenic oxy-sulfide complexes and sodium thio-arsenate in ASL solutions increases significantly as temperature is increased to 95 ºC. More importantly, the solubility of sodium thio-arsenate at certain temperatures is significantly affected by the concentration of sodium hydroxide and sulfide in solution. Due to the common ion effect, if NaOH and HS- concentrations are very high, the solubility of sodium thio-arsenate decreases. Enargite leaching tests were done to characterize the precipitate that occurred upon cooling and to verify the arsenic saturation point, which should be between 38.5 ~ 58 g/L (0.51 M ~ 0.78 M) As depending on the NaOH and HS- concentration. Comparison with solubility experiments of pure sodium thio-arsenate shows that arsenic solubility in ASL solutions is supersaturated. However, direct comparison of saturation in ASL solutions and the solubility as obtained by the synthetic solutions/crystallites prepared here is not possible given the complex nature of the ASL crystallites that appear not to contain the often discussed “sodium thio-arsenate”.

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Cobalt adsorption and/or co-precipitation onto ferric oxyhydroxide (2011)

The Caron process was developed in the 1920s. In this process, the ore is dried and milled, then roasted in a reducing atmosphere to convert nickel and cobalt to their metallic form. Nickel and cobalt are then leached in ammoniacal solution but some losses occur due to adsorption and co-precipitation with iron oxide or hydroxide precipitates. The effects of various parameters on iron precipitation in the Caron process leach were studied. The parameters studied include pH, temperature, contact time with iron and its precipitates and initial ferrous concentration. The adsorption of otherwise soluble Co onto freshly precipitated ferric hydroxide was measured at four different temperatures between 20ºC and 35ºC. The percentage of cobalt adsorbed onto iron precipitates was directly related to the initial iron concentration but the effect of temperature was less clear. The amount of cobalt adsorbed depended on the pH. Solution contact time with iron precipitates had a very small effect on cobalt adsorption (2-3% difference). The highest adsorption of cobalt (99%) was obtained with an initial concentration of 0.2 M ferrous sulfate at pH 7 after 2 hours of contact time. The main iron precipitate was ferrihydrite. X-ray analysis revealed the characteristic “2 line” ferrihydrite (Fe₅HO₈4H₂O) with no goethite being observed in any of the precipitates. TEM revealed an amorphous structure, which is also indicative of ferrihydrite. SEM showed a preponderance of poorly resolved precipitates, which did not appear to be crystalline. Traces of cobalt were measured in the ferrihydrite particulate by EDX.

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Electrochemical dissolution and passivation behavior of iron in ammoniacal caron leaching solution (2011)

The electrochemical passivation of pure Fe in ammoniacal solution was investigated to determine the stability of both Fe-oxides and Fe-ammines during anodic polarization. The potentiodynamic experiments were done in 6M total NH₃[3M NH₄OH and 1.5M (NH₄4)₂CO₃] solution. Different experimental parameters such as temperature (15°, 25°, 35°, 45° and 55°C) and pH (6, 8, 10 and 12) were used. Pure oxygen and argon gas was sparged during the experiment to oxygenate or de-oxygenate the solution with no stirring in either case. Polarization plots show that both active anodic dissolution and passive regions are present for pure iron in ammoniacal solution. It also shows that as the temperature increased the dissolution rate increased in both anodic and passive regimes. At the same time, the active region for iron dissolution is present across a wider potential range. For pH 10 the highest dissolution rate is around 0.025A/cm² or 260 g/m²hr¹ at 55°C and passivation of iron generally occurs at ca. -0.36 V (SHE) irrespective of the temperature. The peak anodic dissolution rate (0.2 A.cm₋² or 2080 g/m²hr¹) surprisingly occurs at pH 6. Potentiostatic experiments were done at different fixed potentials at pH 9 and 25°C. The highest current density was registered at -0.6 V. The peak dissolution current observed for the potentiostatic tests is roughly two orders of magnitude lower than that observed in the potentiodynamic test for pH 9 and at 25°C. Solution and morphological analyses were done by ICP and SEM, respectively for pH 6 and 9 solutions. The current efficiency for pH 6 is far lower than at pH 9 which implies that the current registered at pH 6 is used for the formation of a product film. Speciation calculations indicate that this film may be siderite (FeCO₃) at low potential. From XPS analyses, it is believed that the passive layer formed at higher potentials (more than 0.40V) is Fe₂O₃. Speciation diagrams point to the stability of iron tetra-ammines at pH 10. It was shown that metastable Eh-pH diagrams for Fe/NH₃/CO₃/H₂O system can be generated through potentiodynamic measurements aimed at active and passive behavior of iron.

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The alkaline sodium sulphide leaching of enargite (2011)

Leaching of enargite samples containing approximately 12 % As, 0.5 % Sb and 38 % Cu was studied in alkaline sodium sulphide solutions. Samples were leached in the presence of sodium hydroxide and sodium sulphide, which is expected to hydrolyse and form sodium hydrosulphide. Kinetic parameters studied included temperature, particle size, reagent concentration and stoichiometry in high pulp density tests. Leaching behaviour of arsenic and antimony was very similar; it was enhanced as temperature and reagent dosage was increased and/or particle size decreased. Copper, iron, zinc, and silver were not extracted during the leaching procedure. Through chemical analysis, X-Ray Diffraction and Scanning Electron Microscopy leach solutions and residues were characterised. Arsenic and antimony were efficiently removed, leaving copper-sulphur compounds such as digenite, bornite and sodium copper sulphide (NaCu₅S₃). Some of the leaching results differ from those found in the literature, especially in regards to the nature of the solid residue and the leaching reaction given.Removal of arsenic from solution was analysed by acidification and crystallization. Acidification removed arsenic and antimony from solution to produce a mixture of oxides and sulphides; however, sulphide was removed from solution most likely as hydrogen sulphide, which would need to be scrubbed in a sodium hydroxide solution. Finally acid consumptions over arsenic plus antimony ratios were too large for a practical application. Crystallization on the other hand is a simpler alternative. The main requirement is to have high concentrations of arsenic and antimony in solution. In this case part of the arsenic and antimony would be recirculated to the leaching stage. Other aspects included behaviour of chalcopyrite and pyrite in alkaline solutions and the possibility of producing sulphide ions in situ. Unfortunately no considerable amounts of aqueous sulphide were produced. Also, the behaviour of arsenic and antimony (III) in sodium sulphide alkaline solutions was analysed using arsenic and antimony trioxide. These results are in an early phase of study and could be a relevant topic for further research. In both cases a black precipitate formed containing elemental antimony and oxides. However, no crystallization of thio-compounds seemed to have occurred.

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