School of Chemistry (ETDs)

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Browsing School of Chemistry (ETDs) by SDG "SDG-15: Life on land"

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    Characterization, quantification, and recovery of rare earth elements(rees) in South African coal fly ash samples
    (University of the Witwatersrand, Johannesburg, 2024) Michael Rampfumedzi, Tshilidzi; Chimuka, Luke
    Rare earth elements (REEs) are naturally distributed throughout the Earth's crust, typically in low concentrations. They are not typically found in isolation but are rather present in various minerals, often in amounts too minute for cost-effective extraction. Fly ash is among the sources that are deemed economically viable for extracting REEs. The objective of this study was to create environmentally sustainable approaches for measuring and reclaiming rare earth elements (REEs) in coal fly ash (CAF) samples. The study involved analyzing fly ash samples collected from various coal power stations using a range of standard and advanced techniques, including X-ray fluorescence (XRF), X-ray diffraction(XRD), scanning electron microscopy (SEM), and inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICP-OES). The XRF only shows the presence of REEs from all three fly ash samples with a range of 40 to 100 ppm and mineral oxide ranging from 0.1 to 50 %. The XRD results show that fly ash sample is a siliceous-rich sample with abundant minerals such as quartz (SiO2), magnetite (Fe3O4), and mullite (Al4.52Si1.48O9.74). The SEM analysis of the sample confirmed the presence of rare earth minerals, including monazite which is a light atomic mass (LREE), xenotime, a heavy atomic mass (HREE), and perrierite-bearing minerals. The results obtained from the instrumental analysis show that the ICP-MS instrument is the more effective analytical technique for REE analysis in this context as compared to ICP-OES. Using certified reference materials, the results obtained by two acids digestion technique, acids digestion and sodium peroxide fusion in, CGL 111, CGL 124, and AMISO276, were compared to validate whether the methods are reliable. The acid digestion approach demonstrated greater effectiveness in comparison to the sodium peroxide fusion method. The recovery percentage (%) from ICP‒MS showed an excellent percentage yield (80 – 120%) compared to the ICP‒OES instrument (50 –120%). The ICP‒MS data indicate that all fly ash samples have a high concentration of LREEs and a lower concentration of HREEs. Excellent recovery was obtained by ICP‒MS in a developed microwave acid digestion method. The concentration of REEs obtained from ICP - MS and OES in fly ash samples ranged from 50 ppm to 200 ppm for light rare earth elements and 0.5 ppm to 20 ppm for heavy rare earth elements. The total REE ( TREE) concentrations in all fly ash samples range from 400 ppm to 600 ppm
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    Investigation of rhombohedral 𝑩𝒊𝟐𝑶𝟑 as an oxide conducting electrolyte for solid oxide fuel cell applications
    (University of the Witwatersrand, Johannesburg, 2023-09) Kerspuy, Tanner Royele Rowan; Billing, Caren; Erasmus, Rudolph M.; Billing, Dave Gordon
    The synthesis of a bismuth system co-doped with neodymium (Nd3+) and yttrium (Y3+) was at the core of this project. The focus was placed on the synthesis of the rhombohedral phase of bismuth oxide, which has not been observed in pure bismuth oxide. Neodymium was selected as the main dopant (the one used in highest dopant concentration), due to its Shannon ionic radii. Upon doping with Nd3+ as a single dopant, it is observed that a mixture of the rhombohedral and monoclinic phases is obtained, thus noting that the single dopant system using Nd3+ does not stabilise the rhombohedral phase. When using a co-doped system of 15 mol % Nd3+ and 5 mol % Y3+ (15Nd5YSB), it is observed that we are able to obtain a stable phase pure rhombohedral phase, with a total dopant concentration of 20 mol%. The total dopant concentration % ranges selected ranged between 8.5-10 mol %, 20 mol % and 22.5 mol %. The Rietveld refinement of the X-ray diffraction data obtained for both the laboratory and synchrotron-based techniques indicate sample phase purity and phase stability for the samples under investigation. The refinements obtained for the samples indicated that not only one structure model was used to fit the experimental data. The structural models which fit the Rietveld refinements of the experimental data resulted in the observation of pure phase and mixed phase rhombohedral samples being observed. The Nd0.15Y0.05-Bi2O3 (15Nd5YSB)sample resulted in a phase pure rhombohedral structural model. Hereafter all samples will be referred to with the shorthand notation. The thermal analysis techniques are used to indicate the thermal dependence of the samples, this analysis also indicated phase stability across the temperature range of investigation as no phase transitions occurred throughout the heating and cooling cycles, and minimal weight loss is observed. The samples of importance in this study were the 12.5Nd10YSB sample which obtained a conductivity of 2.4511×10-5 S.cm-1 at 500 ℃, and the 15Nd5Y2.5TbSB sample which obtained a conductivity of 2.1725×10-5 S.cm-1 at 500 ℃. The Arrhenius plots obtained indicated stability 3 of these samples across the 200-500 ℃ temperature range with no discontinuities, which suggests no phase transitions, or order-to-disorder transitions. Variable temperature Raman spectroscopy indicated that the behaviour for all the samples analysed using Raman spectroscopy is consistent, however, a deviation was observed for the 15Nd5Y2.5ScSB sample which has a distinctive spot which exhibits different Raman shift behaviour as compared to all other samples. The VT-Raman spectroscopy spectra indicate a distinctive signature Raman peak at ~250 cm-1, which can be concluded to be the Raman peak which is indicative of the rhombohedral 𝐵𝑖2𝑂3, this peak also appears in the low cubic phase % sample after cooling back to room temperature. This assignment of the Raman spectral peak is confirmed through this peak being evident throughout all the spectra obtained and it being consistent throughout all the spectra observed.