ETD Collection
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Item Acid based recovery of PGMs from spent autocatalytic convertors using AlCl3 and HOCl(2018) Saguru, Collins TatendaThe environmental importance of the platinum group metals (PGMs) for catalytic reduction and oxidation of toxic gases emitted by internal combustion engines has been responsible for the increasing demand for these metals. Consequentially, the majority of PGMs have been reporting to end-of-life autocatalytic convertor scrap, providing an opportunity for recycling, to recover the PGMs. A combination of pyrometallurgy and hydrometallurgy is employed in most of the current global industrial recycling centres. However, an exclusively hydrometallurgical process has some advantages over the current industrial establishments, chief amongst them being the lower energy consumption as a result of eliminating the smelting step. The aim of this study was to investigate the leaching of the PGMs via a purely hydrometallurgical process, using readily available and environmental friendly reagents. Platinum (Pt), palladium (Pd) and rhodium (Rh) were leached into a chloride solution under acidic, oxidizing conditions. AlCl3 and HOCl obtained after Ca(OCl)2 dissolution, were used as the complexing/acidifying and oxidising agent respectively. Maximum leaching efficiencies of 15%, 78% and 86% for Rh, Pt and Pd respectively were obtained from preliminary experiments designed using the central composite methodology. A second battery of experiments using the one factor at a time methodology and compounding on insights obtained from the preliminary set of experiments was conducted. Recoveries of Pt, Pd and Rh were improved to 99%, 99% and 68% under optimised conditions of 100 oC, 1.2 M AlCl3, 0.15 M Ca(OCl)2 and S/L ratio of 3g : 10ml. Solvent extraction of both Pt and Pd was conducted to illustrate the feasibility of co-extracting the 2 PGMs into a tri-iso octyl amine (Alamine 308) in Kerosene with n-Decanol organic phase. Recoveries from pregnant leach liquor of 99.6% and 87% for Pd and Pt were recorded, using a 4 stage batch simulation process for a continuous counter current solvent extraction process. There was negligible co-extraction of other metals except for Fe, which had an extraction into the organic phase of 98 %. Pt and Pd could then be selectively stripped using 0.5 M HCl with 2 different concentrations of thiourea. An exclusively hydrometallurgical process was therefore developed with overall recoveries for Pt, Pd and Rh of 85 %, 97 % and 67 % respectively. A preliminary cost benefit analysis based on operating costs alone was conducted, which revealed that the process could attain an operational profit margin of approximately 9.9 % / oz. (3E).Item Recovery of PGMs from an oxide ore by flotation and leaching(2018) Sefako, Relebohile BasilFroth flotation is the process used in the Platinum Group Metal industry to upgrade the run-of-mine ore for subsequent processes such as smelting and hydrometallurgical PGM refining. The PGM concentrator plants achieve high PGM recoveries (>85%) when treating prestine (unweathered) sulphide ores. However, the depletion of prestine sulphide PGM bearing minerals has triggered interest in exploration of techniques for PGM recovery from near surface oxidised PGM ores. All earlier attempts to process the oxidised PGM ores by conventional flotation methods achieved poor recoveries (typically less than 50 %) hindering the commercial exploitation of these resources. The characterisation of the non-sulphide PGM ore used in this study indicated that the ore is enriched in oxide iron minerals as a result of weathering. In the flotation work, the maximum PGM flotation recoveries achieved using the sulphide co-collector schemes were 55.1% 3E (Pt, Pd and Au). Application of the hydroxamate oxide collector improved the flotation performance to recoveries of 74.7% 3E. The superior PGM recoveries achieved with hydroxamates probably lies in their ability to form complexes with metals such as iron. Hydroxamates co-collectors have been proven to improve recoveries without any adverse effects on performance of primary collectors such as SIBX. In this study the non-sulphide PGM ROM ore was leached directly using different acids. Low PGM extractions were recorded for hydrochloric acid (36.6% Pt and 8.8% Pd) and nitric acid (34.5% Pt and 7.1% Pd). The best leaching results of 48% Pt and 24.5% Pd were obtained using aqua regia solution though it is non-selective. Leaching of ROM ore is generally not preferable as it leads to high reagent consumptions. In this study it was postulated that leaching of low grade flotation concentrate would be preferred. Experiments were conducted to leach the concentrate that had the highest PGM recovery with sulphuric acid in order to target the base metals and further concentrate the PGMs in the residue. The base metal recovery from flotation concentrate using sulphuric acid was only efficient for copper and nickel while poor iron recoveries were achieved.Item Galvanic interactions between minerals during dissolution(1994) Holmes, Paul RichardA quantitative description of galvanic interactions between sulphide minerals based on thermodynamic and kinetic parameters has been developed. The basis for quantitative description involves conducting a voltage balance over the galvanic couple. The contributions to the voltage balance include the galvanic couple cell emf, kinetic descriptions of the anodic and cathodic half reactions, the voltage characteristics 'of mineral-mineral contacts and solution voltage losses. The rates of the anodic and cathodic half' reactions were modelled by the Butler-Volmer equation and ti1ediffusion equation. A potentiostat was used to vary the voltages losses across mineral-mineral contacts. TIle galvanic couples were constructed. as rotating ring disc electrodes and hence electrolyte voltage losses were negligible. Three galvanic couples, copper-platinum, copper-pyrite and galena-pyrite, were electrochemically characterised under different conditions of ferric concentration, electrode rotation rate and temperature. The effect of illumination on the anodic dissolution of galena was investigated. The electrochemical model is in good agreement with experimentally measured galvanic currents. Galvanic interaction is a dynamic function and various models are developed which account for dynamic behaviour in galvanic cells.Item Extraction of platinum, palladium and rhodium from tailings by a metal chloride based leach reagent(1993) Cocksedge, Mark BurdenHigh extractions of the platinum group metals' from a refractory tailings concentrate (PGM Concentration : Pt 81.5ppm, Pd 21.5ppm, Rh 17.5ppm) was achieved after short. low-temperature roasting and subsequent leaching with a zinc chloride based leach reagent containing nitric acid under atmospheric pressure at temperatures from 105-C to 120-C. [Abbreviated Abstract. Open document to view full version]Item Development of a high pressure hydrometallurgical process for the extraction of iron from iron oxide bearing materials(2016) Rolfe, WesleyThe feasibility of extracting iron from iron(III) oxide bearing materials with acetylacetone has been under investigation for many years. This is an alternate, environmentally friendly process for the recovery of iron compared to conventional processes that are energy intensive, have numerous costly process steps and produce large quantities of greenhouse gases. Iron(III) oxide bearing waste materials can be used in this process which reduces its environmental impact as it would not require waste storage. This study investigated the feasibility of reducing the reaction time of the liquid phase extraction of iron from iron ore fines by performing the extraction at elevated pressures and temperatures. It was found that that the extraction under pressure was dependent on temperature, pressure, particle size and solid to liquid ratio. It was found that at high temperatures and long extraction times, an unknown secondary reaction occurs that consumes the desired product, iron(III) acetylacetonate, and inhibits the recovery of these crystals. This results in lower extraction yields. It was found that the side reaction was largely dependent on the temperature of the system and the amount of iron(III) acetylacetonate present. The effects of the side reaction could be limited by lower operating temperatures and reducing the total reaction times. An optimum conversion of iron(III) oxide to iron(III) acetylacetonate of 47.2% was achieved for synthetic iron (III) oxide (> 95 wt% Fe2O3) at a total extraction time of 4 h, 160 °C, 0.025 g:1 mL, operating pressure of 1700 kPa, initial N2 feed pressure of 1010 kPa and 375 rpm stirrer speed. The optimum extraction of iron from iron ore fines (> 93 wt% Fe2O3) to iron(III) acetylacetonate was found to be 20.7% at 4 h, 180 °C, 0.025 g:1 mL and operating pressure of 1900 kPa, initial N2 feed pressure of 1010 kPa and 375 rpm stirrer speed. These are the optimum conditions where the side reaction is limited to improve the recovery and desired reaction conversion capabilities of the process. The operation under pressure yielded lower conversions than that of the atmospheric leaching process developed by Tshofu (acetylacetone water system under reflux). It was also found that it was not possible to reduce the extraction time and achieve comparable extractions when operating at higher temperatures and pressures. The formation of an additional unwanted product would also lead to unnecessary treatment costs in an industrial process. Hence, it was found that pressure leaching as an alternative is not currently viable due to the lower yields and associated high costs. Atmospheric leaching seems to be the most economically feasible option until a better alternative is found.Item Solution density modelling for single and mixed base metal electrolytes at ionic level(2015-01-23) Chagonda, TrevorSolution density modelling is important in hydrometallurgical processes as accurate predictions of single and mixed electrolytes can be used in the design of equipment and their sizing, heat transfer calculations and choosing of materials for construction. This research project entails modeling of electrolyte solutions by extending the Laliberte and Cooper (compound level) model to ionic level where an electrolyte solution is modeled as a mixture of cations, anions and water molecules. This modeling predicts single and mixed electrolyte density as a function of electrolyte temperature in degrees Celsius; water, cation and anion apparent volumes in cubic centimeters; and their respective concentrations in the electrolyte as mass fractions. The model was developed by fitting single electrolyte density data reported in literature using the least squares method in Microsoft Excel®. The following 26 single electrolyte solutions were used in the fitting exercise: Al2(SO4)3, BaCl2, CaCl2, CdSO4, CoCl2, CuSO4, FeCl3, FeSO4, HCl, HCN, HNO3, K2CO3, LiCl, MgSO4, MnCl2, Na2SO3, NaF, NaI, NaOH, (NH4)2SO4, NiCl2, SrCl2, ZnCl2, ZnBr2, (NH4)2C2O4 and KNO2. The above electrolytes attributed to the following ions: Al3+, Ba2+, Ca2+ Cd2+, Co2+, Cu2+, Fe3+, Fe2+, H+1, K+1, Li+1, Mg2+, Mn+2, Na+1, NH4+1, Ni2+, Sr+2, Zn2+, SO42-, Cl-1, CN-1, NO3-1, CO32-, OH-1, SO32-, Br-1, F-1, I-1, C2O4-2 and NO2-1. This translated to a combination of at least 216 single electrolyte solutions which could be feasibly modeled, and a solution with at most 10 anions for mixed electrolytes, which is comparable with practical hydrometallurgical solutions. A database of volumetric parameters was generated comprising a total of 18 cations and 12 anions. The validation of the developed model was done by predicting densities for both single and mixed electrolytes not used in the fitting exercise. The average density error i.e. the difference between experimental and model density for the single electrolyte solutions was 22.62 kg m-3 with a standard deviation of 39.66 kg m-3. For the mixed electrolytes, the average density error was 12.34 kg m-3 with a standard deviation of 24.48 kg m-3. These calculated errors translated to a maximum percentage average error of less than 4% for single electrolyte solutions and maximum average percentage of less than 3% for mixed electrolyte solutions.