3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Thermodynamic and Kinetic Study of the Production of Carbon Nanotube Yarn from Chemical Vapour Deposition Reactor(2019) Mahangani, NdanganeniOvercoming the low production rate of carbon nanotubes (CNTs) could be instrumental to reducing their cost of production and enhancing its wide application. Understanding the kinetics of the production of yarn from CNTs, through kinetic model development could assist in the optimisation of the production and this makes the application of yarn as a replacement to filament in incandescent bulb promising. In this study, kinetic of the production of CNTs, an intermediate in the production of yarn, is presented. Several experiments were conducted using Ferrocene as catalyst in a CVD reactor. The reaction of CH4 on Ferrocene catalyst is via heterogeneous catalysis because methane is in gaseous phase and Ferrocene in solid phase. Langmuir Hinshelwood was used to develop a kinetic model based on these two- phase phenomena. The effect of CH4 concentration was investigated as well. The derived kinetic model fits the experimentally measured data with 95% confidence interval. Good quality CNTs were obtained at methane flow rate of 125 ml/min. The CNTs produced at this flow rate has high purity, low tube diameter and spinnable long array as compared to the one produced at 100 ml/min and 150 ml/min. Yarn was produced at this flow rate (125 ml/min) at a reactor temperature of 900, 950 and 1000oC. Characterisation techniques such as Scanning Electron Microscopy (SEM), Transition Electron Microscopy (TEM), Energy-Dispersive X-ray (EDX), Gas Chromatography (GC) and Raman Spectroscopy were used to analyse the product from experiments. At all studied reactor temperatures CNT yarns were synthesised as observed in SEM images. Four Probe method was used to measure the electrical conductivity of as-produced CNTs and yarn. By using the proposed CH4 flow rate (125 ml/min) the produced CNT yarn was found to be metallic and electrically conductive. The studied electrical conductivity of as-produced CNTs were found to be approximately 2 times higher than their yarn. Yarn produced at reactor temperature of 950oC proved to have high quality and more electrically conductive than those produced at 900 and 1000oC. The thermodynamic properties of CNTs yarn were studied using TGA/DSC equipment. Polynomial models for predicting Specific heat capacities of yarns produced in this study were developed. The results showed that the temperature at which yarn is produced has an effect on a thermodynamic property such as heat capacities, enthalpy and entropy.Item Purification of coal fly ash leach liquor for alumina recovery(2018) Rampou, MohauOver the years, much work has been done on the extraction of alumina from coal fly ash (CFA). Previous works were motivated by the results of the quantitative analysis of CFA that showed a high content of alumina; comparatively ranking second to the primary bauxite ore. Thereafter, research studies have been dedicated to finding efficient routes of recovering alumina from CFA. Amongst recent studies, a 2-step leach process developed by researchers from the University of Witwatersrand gave promising recovery results. The current work is an extension of the 2-step leaching process, where an attempt is made to purify the leach liquor recovered from this leaching process in order to generate a purer aluminum solution by removing iron and titanium, which are considered as the two major impurities. The aim is to obtain an impurity free solution that can be used to generate smelter grade alumina. In this study, solvent extraction using Primene JMT in kerosene was applied to remove the impurities from the leach solution. Optimum parameters tested during the solvent extraction process included contact time, extractant concentration and the organic to aqueous (O/A) ratios. As expected, most of the titanium was extracted in the organic phase and much of the aluminum remained in the aqueous phase. However, the results showed that most of the iron, which occurred in solution as iron (III) and iron (II), also remained in the aqueous phase. This was an unwanted occurrence, as this residual iron would then co-precipitate with the alumina leading to a contaminated product. Two approaches were then applied for a more effective separation of iron from aluminum. The first approach (Route 1) considered the reduction of the iron (III) species into iron (II) prior to solvent extraction for titanium removal. Alumina crystallization then followed and since iron (II) precipitates at higher pH than aluminum, there was reduced contamination of the alumina product. The second approach (Route 2) firstly considered solvent extraction to remove titanium ions and any iron ions. This was followed by the reduction of residual iron (III) to iron (II). In both approaches, aluminum was crystallized out and iron (II) precipitated using (NH4)2SO4. Titanium (IV) was stripped from the organic phase using NH4OH. The results obtained showed a successful purification of the coal fly ash leach liquor for both processes. Route 1’s disadvantage is that it used more metal iron filings for iron (III) reduction to iron (II) than route 2. Route 2’s disadvantage is that due to the 2-stage stripping applied, it used more excess reagents, therefore implying higher operational costs than route 1. As a result, Route 1 was considered for further tests. The final alumina product generated with the route 1 purification process had a 99.4% Al purity. This purity level meets the smelter grade alumina product specifications. The grade of the final product therefore, indicates that the applied process route was successful.Item Beneficiation of Waterberg Coal(1992) Eroglu, BerrinModern methods of mechanised mining and the necessity for the utilization of total reserves have caused the inclusion of more and more impurities in run of mine coal. This fact, together with the limited supply of naturally clean coal fCI gasification, liquefaction and metallurgical purposes, has made some Iorm l){ beneficiation obligatory at many mines not only in South Africa but also in many other countries. One of the South African Coalfields, Waterherg, contains the continent's largest reserves (approximately 46% of South African known reserves). At the Grootegeluk Coal Mine, approximately 15 m tons of coal per annum are mined by opencast methods. The coal is characterised by containing a high proportion of reactive macerals. The Waterberg Coalfield is currently supplying coal for coke manufacture and middlings for power generation. This coal could also be used for other markets, as Waterberg coal is low in oxygen, contains up to 30% volatile matter. Because it contains 90% vitrinite, it is suitable for direct liquefaction, and possibly coal-water mixtures. However the yield of coal suitable for coking or liquefaction (approx 10% ash) is only 12%, with another 24% of 35% ash coal, currently used for power generation. These yields render mining generally uneconomical if making a simple product. The objective of this project is to ascertain whether the yields of washed coal from the Waterberg Coalfield might be increased by using comminution. Thereafter appropriate beneficiation techniques might be employed on different size fractions. Liberation, float and sink, froth flotation and oil agglomeration processes were examined to identify the best way of treating the coal. Work was carried out on the existing clean coal, middlings and discard fractions. The major objective was to optimise the yield of 10-15% ash coal.The results of the experiment indicate that it is possible to obtain low ash coal from middlings, and middlings from discard for power station. The capital and operating costs for improved new plants are calculated by using available factorised data. The results of experiments on both middlings and discards indicate that yields are significantly higher than those currently obtained, but the cost of obtaining such enhanced yields can be too high for normal commercial application.Item Investigations into the effect of size and width to height ratio on the strength of the laboratory sized coal specimens(1996) Canbulat, IsmetThe design of bord and pillar working in South African collieries is based on the pillar strength formula developed by Salamon and Munro1967 and which has been used widely since then for designing pillars. This formula is based on the statistical analysis of 27 collapsed and 98 intact coal pillar cases from collieries located in the Transvaal and the Free state. The main objective of this study is to establish the difference in the strength of the coal material in ditferent seams by means of laboratory testing. In this manner, some 753 coal samples from 10 collieries from 4 seams were tested. The size and width to height ratio effects on strength were analysed. The size effect showed that the difference between the seams was obvious, with a difference of 59,4 per cent between the strongest and weakest coal. The statistical re-analysis showed that the strength of the six blocks from the No 2 seam, Witbank Coalfield occurred in a fairly tight strength range; and that laboratory coal strengths from individual seams or mines could deviate to a significant although relatively small extent from the overall average.