3. Electronic Theses and Dissertations (ETDs) - All submissions
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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 Hard, wear resistant Fe-B-C composites produced using spark plasma sintering(2017) Rokebrand, Patrick PierceFe-B-C composites were produced, from boron carbide and iron powders, using spark plasma sintering. This provided information on the effects of rapid sintering on densification, composition and the microstructure of the materials produced. The composition range included a selection high Fe contents (69.3, 78 and 80.9 vol. % Fe-B4C) and high B4C concentrations (1, 3, 5 vol. % Fe-B4C). The properties of the materials were investigated to determine the potential for using relatively cheap Fe and B4C powders to produce hard, wear resistant materials. High Fe-B4C composites were sintered at 900, 1000 and 1100°C at 60 MPa. Densification increased with increasing temperature and at 1100° each composition achieved ≥ 97 % densification. The materials reacted during sintering with the main phases observed being Fe2B and Fe3(B,C) whilst additional phases formed were FeB, C and Fe23(B,C)6.Comparing the phases that were produced to Fe-B-C phase diagrams showed deviations from expected compositions, indicating the non-equilibrium nature of producing the composites using SPS. Although the composites were not at equilibrium, all the B4C reacted and could not be maintained, even with fast heating and cooling rates. The properties of the materials were dependent on both densification and the phases that were present after sintering. Materials containing higher amounts of the Fe2B phase showed higher hardness and fracture toughness results, up to 13.7 GPa and 3.5 MPa.m0.5 respectively for the 69.3 vol. % Fe-B4C. The materials were sensitive to grain and pore growth which negatively affected properties at 1100°C. The transverse rupture strength of 388.3 MPa for 80.9 vol. % Fe-B4C composite was the greatest, and showed evidence of both intergranular and transgranular fracture. The strength was affected by a fine dispersion of porosity at the grain boundaries, throughout the material, and free carbon in the structure was detrimental to the strength of the 69.3 % Fe-B4C. The wear rates were lower using Si3N4 wear balls compared to stainless steel balls, where 69.3 vol. % Fe-B4C showed the best wear rates, 8.9×10-6 mm3/Nm (stainless steel ball) and 1.77×10-6 mm3/Nm (Si3N4 ball), due to the higher Fe2B composition and free carbon acting as a lubricant during sliding. 1, 3 and 5 vol. % Fe-B4C composites were sintered to densities above 97 % of theoretical at 2000°C and 30 MPa. The formation of a transient FeB liquid phase assisted densification. 1 % Fe-B4C attained hardness and fracture toughness up to 33.1 GPa and 5.3 MPa.m0.5 with a strength of 370.5 MPa. Thermal mismatch between the FeB phase and B4C caused high residual stresses at the interface which led to cracking and pull-out of the FeB phase. Residual carbon at the grain boundary interface exacerbated the pull-out effect. Increasing Fe and the subsequent FeB phase had an embrittling effect. The materials suffered severe wear of up to 36.92×10-6 mm3/Nm as a result of the pull-out with the remaining porosity acting as a stress raiser. 20 vol. % of the Fe in each system was substituted with Ti to reduce the presence of residual carbon. Although in some case the properties of the respective compositions improved, residual carbon was still present in the composites.Item Investigation into the microstructure and tensile properties of unalloyed titanium and Ti-6Al-4V alloy produced by powder metallurgy, casting and layered manufacturing(2016) Masikane, Muziwenhlanhla ArnoldABSTRACT Solid titanium (Ti) and Ti-6Al-4V (wt.%) materials were fabricated from powders using spark plasma sintering (SPS), cold isostatic press (CIP) and sinter, layered (rapid) manufacturing, centrifugal and vacuum casing. ASTM Grade 4 Ti, Al and V, 60Al-40V (wt.%) and the pre-alloyed Ti-6Al-4V powders were used as starting materials. The solid Ti and Ti-6Al-4V materials produced by the SPS were compared to the CIP and sinter method on the basis of density, microstructure and chemistry. The materials produced by the CIP and sinter method were also compared to those produced by vacuum casting method on the basis of microstructure, oxygen pick-up, chemistry and room temperature tensile properties. Centrifugal casting was compared to the vacuum casting technique on the basis of microstructural homogeneity. Rapid manufacturing was compared to SPS and CIP and sinter on the basis of microstructural homogeneity, density and tensile properties. The tensile properties of all materials were also compared to their commercial counterparts to investigate the effect of interstitial oxygen. The technology resulting in materials with superior properties was finally identified as most promising for commercial production of Ti-based materials. On the basis of densification, the SPS method appears superior compared to the CIP and sinter and rapid manufacturing method due to the benefit of pressure aided sintering, while the rapid manufacturing method is superior to the CIP and sinter method due to the use of a high power laser resulting in high densification rates. In cases where microstructural homogeneity is the key requirement, the CIP and sinter and rapid manufacturing methods appear superior compared to the SPS method due to longer isothermal holding time and higher sintering temperature and the use of pre-alloyed Ti-6Al-4V powder, respectively. On the basis of oxygen pick-up and additional contamination, the vacuum casting route is inferior due to the tendency of melt-crucible interaction, resulting in the dissociation of ZrO2 and subsequent pick-up of O and Zr. Based on the homogeneity of the microstructure, centrifugal casting is better than vacuum casting. The ductility of vacuum cast Ti was better than that of CIP and sintered Ti, possibly due to limited diffusion of oxygen from the crucible compared to oxygen absorbed from the controlled atmosphere during CIP and sinter. The vacuum casting of the Ti-6Al-4V alloy resulted in dissolution of oxygen and Zr due to melt-crucible interaction. Hence the ductility was worse compared to the alloy produced by CIP and sinter. The rapidly manufactured Ti-6Al-4V specimens exhibited superior ductility and strength compared to all alloys produced by other methods due to the use of high purity starting powder. The tensile properties of these specimens were also comparable to standard requirements. The similarity of the tensile properties of wrought Ti-6Al-4V alloy reported in the literature was an indication of limited oxygen pick-up during rapid manufacturing. Therefore based on low oxygen pick-up, microstructural homogeneity, high density and superior tensile properties, the rapid manufacturing route appears to be the most promising approach for commercial processing of titanium based materials.