*Electronic Theses and Dissertations (Masters)

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    Upgrading Semi-Soft Coking Coal by Hydrothermal Treatment: Caking and structural properties
    (University of the Witwatersrand, Johannesburg, 2024-02) Ndumo, Jabulile; Bada, Samson
    Based on the current challenges faced by the metallurgical industry in South Africa in importing quality reductants, there is an urgent need to investigate a new approach to enhance the semi-soft coal available in the country. Importing prime coking coal has increased the steel price, resulting in many downstream operations involving steel closing down in the country. With a surplus of semi-soft coking coal in South Africa, this research sought to look into this kind of abundant coal to enhance its property as a reductant for blast furnace applications. For this reason, a study was conducted on two Southern African coals, Grootegeluk (semi-soft coking coal) and Moatize (higher quality coal). Both coals were individually hydrothermally treated and then blended at different ratios to further upgrade their metallurgical properties. The as-received Moatize coal showed properties that were more of prime coking coal with high total carbon content (76.50%), a crucible swelling index of nine, a maximum dilatation of 59% and volatile matter of 20.39%. It was a highly vitrinite coal with a vitrinite reflectance of 1.28%, a higher micropore volume than mesopore volume and a very low maximum fluidity of 24 dial divisions per minute (ddpm). According to the initial test, the Grootegeluk coal sample had a crucible swelling index of 5.5, a high volatile matter of 35.02% and a low vitrinite reflectance of 0.72%. In addition, the sample had a maximum dilatation of -10%, a maximum fluidity of 3ddpm and a higher mesopore volume than the Moatize coal. Hydrothermal treatment was conducted on the coal samples at numerous temperatures (100ºC to 200ºC), at various residence times (30 to 90 minutes) and at different coal masses (300 to 600grams (g)). According to the results, the optimal hydrothermal conditions were 200ºC, 90 minutes and 600g. Another hydrothermal treatment was performed at a higher temperature and residence time of 280ºC and 180 minutes. The same sample mass of 600g was used and the result showed no further improvement. The coal samples were then blended at various Grootegeluk/Moatize ratios (15% to 50% Grootegeluk), and further hydrothermal treatment tests were carried out based on the optimum conditions achieved. Both the hydrothermal test and the blending of the coal led to a coal with volatile matter ranging from 21.46% to 23.79%, which is a required specification for metallurgical application. The total carbon of the enhanced coal blend also ranged from 68.8% to 82.10%, with total sulphur below 1%. The mesopore-micropore ratio of the treated blend was higher than the individual coal samples, which is what is expected of a metallurgical coal. Based on these findings, coke was produced and analysed to identify a coke capable of withstanding blast furnace conditions. Using the particle reactivity index (PRI), proximate analysis and the pore size distribution, 90-(50% Grootegeluk+50% Moatize)-C product was identified as the coke with the least PRI and high fixed carbon. Further investigation showed that the blending and hydrothermal treatment affected the coal’s physiochemical, rheological and micro-molecular properties. The study has established that metallurgical properties of the locally mined semi-soft coking can be enhanced solely and when mixed with a hard coal. Even though the 90-(50%GG+50%M)-C did not meet the overall specifications required for use in the blast furnace, it was identified as a suitable reductant for other metallurgical applications.
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    Characterisation and surface finish evaluation of Direct Energy Deposited AlCoCrCuFeNi High Entropy Alloys
    (University of the Witwatersrand, Johannesburg, 2024-01) Modikwe, Thembisile Patience; Mathe-Maleboho, Ntombi; Maledi, Nthabiseng
    This study focused on the use of direct energy deposited (DED) techniques for the fabrication of AlCrCoCuFeNi high entropy alloy (HEAs) samples. HEAs have become a ground-breaking research field that provides solutions to complex problems in the aerospace industry. The industry requires improvement in the application of structural materials that are well-functioning at a low cost for example turbine blades. The fabrication of HEAs via DED commonly produces poor surface finish Ra in the range of 5 μm - 20 μm due to the layer-by-layer deposition method, as a result, it fails the industrial application requirements where the usual range of roughness tolerance required in the industry ranges from Ra is 0.8 μm < Ra < 1.6 μm thus, the need to deploy post-processing methods. This study focused on electropolishing (EP) and centrifugal barrel finishing (CBF) of AlCrCoCuFeNi-HEA samples. The polishing was performed using 80% methanol and 20% per-chloric acid solution used as the electrolyte. The samples were polished for 30 and 60 seconds in a Struers LectroPol-5 electrolytic polishing and etching device. The surface removal at 1200W for 30 sec on sample a was 50.29%, 58.65% for sample b, and 75.48% for sample c. The surface removal at 1400W for 60 sec on sample d is 63.25%, 45% for e, and 49.19% for f. The samples were polished for 7 and 14 hours in a CB320-CBF. During the period of 14 hours, a surface removal where the proportion of material removed for sample a was 55.37%, sample b was 43.13%, and sample c was 32.2% at a laser power of 1200W. After 7 hours of polishing, sample d achieved a surface removal of 86.02%, sample e achieved a surface removal of 43.18%, and sample f achieved a surface removal of 90% at a power of 1400W. Oxidation tests were conducted in static air at 1000˚C for 200h. The presence of FCC, BCC, and Fe2O3 oxide scales resulted in a noticeable increase in mass, with Fe2O3 scales being the most prevalent.
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    Selection of a technique to separate carbon dioxide from methane for recovery of natural gas at Lake Kivu
    (University of the Witwatersrand, Johannesburg, 2024-02) Ntini, Hermann Ekini; Nkazi, Diakanua; Mukaya, Elie
    Lake Kivu is situated between the Democratic Republic of Congo (DRC) and Rwanda. It is known to contain large amount of dissolved carbon dioxide and methane. It is termed a killer lake due to the toxic nature of these gases, which could emerge on the surface during a catastrophic eruption and cause massive devastation in this region. Extracting these toxic gases proves to be crucial to avoid natural disasters and to afford economic benefits in the form of electricity generation or energy export.
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    Wear Reduction and Media Density Optimization for the Single Stage Pipe Densifier at Sishen Iron Ore’s Beneficiation Plants
    (University of the Witwatersrand, Johannesburg, 0202-02) Botha, Simone; Kabezya, Kitungwa
    The depleting high-grade iron ore mining supply at Sishen Mine in the Northern Cape, South Africa, has given rise to its beneficiation plants operating at higher media densities to upgrade lower-grade ore. In this study, densification was numerically modelled using an MPPIC model and experimentally tested using a 200-mm diameter centrifugal densifier from two local suppliers – Multotec and HMA. Shear stress, wear rate, separation efficiency and media losses were measured at increasing operating densities and differing vortex finder sizes. Optimum operating conditions were established. It was found that a feed density of 3.60 t/m3 and a shear stress of 9.70 e-3 N/m3 at the inlet using a vortex finder diameter size of 30 mm exhibited favourable performance in terms of media densification and downstream recovery. The practical significance of this is proven in terms of wear rate and its predictability to provide a consistent overflow of below 1.20 t/m3 media to the recovery circuit. Furthermore, information about ideal operating conditions in terms of inlet pressure and controls to identify premature failures were established.
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    Investigating the effect of size, density and shape of Iron ore particles on batch jig performance
    (University of the Witwatersrand, Johannesburg, 2024-02) Dzaringa Kisembo, Daniel Elvis; Woollacott, Lorenzo
    The gravity separation method is one of the oldest methods of mineral beneficiation that takes advantage of the difference in the specific gravity of particles that are being separated. The separation occurs in a fluid medium, usually water, and involves floating off lighter material to leave behind denser ones. There are several types of gravity separation techniques, and they vary according to the equipment that is used for the separation or the property of the medium that is being used; the main gravity separation methods that are widely used for the beneficiation of Iron ore are Jigging and Dense Medium Separation (DMS). In this research, the jigging method is selected to investigate the concentration of an Iron ore by using a batch laboratory Jig; the jigging method was preferred for its simplicity and availability, generally Jigging has several advantages, some of which include cost effectiveness and simplicity of operation and its minimum impact on the environment. During the beneficiation of minerals using the jigging method of ore concentration, several feed material characteristics affect the efficiency, such as the particle density, size and shape. The aim of this research was to investigate the effect of these feed properties on jig performance. Tests were conducted on a Hematite ore sample using a batch jig to gain a deeper understanding of how the density, the size and shape of particles affect segregation. The iron ore samples were screened and any extremely small particles were removed, maintaining a particle size range between 2.8 and 10 mm. The results showed that particles were stratified on the basis of their specific gravity, denser particles reported toward the bottom layer of the bed and separated more efficiently. Less denser particles reported more toward the upper layer of the bed and were less efficiently separated. Coarser particles tend to report to the bottom layer of the bed and finer particles to the top product layer. Particles that were flatter and more elongated tended to end up in the bottom layer of the bed more often, while more rounded particles were not as likely to be found in the bottom layer.