3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Effects of particle size, shape and density on the performance of an air fluidized bed in dry coal benefeciation(2011-10-07) Chikerema, PheneasMost of the remaining coalfields in South Africa are found in arid areas where process water is scarce and given the need to fully exploit all the coal reserves in the country, this presents a great challenge to the coal processing industry. Hence, the need to consider the implementation of dry coal beneficiation methods as the industry cannot continue relying on the conventional wet processing methods such as heavy medium separation. Dry coal beneficiation with an air dense-medium fluidized bed is one of the dry coal processing methods that have proved to be an efficient separation method with separation efficiencies comparable those of the wet heavy medium separation process. Although the applications of the fluidized bed dry coal separator have been done successfully on an industrial scale, the process has been characterized by relatively poor (Ecart Probable Moyen), Ep values owing to complex hydrodynamics of these systems. Hence, the main objectives of this study is to develop a sound understanding of the key process parameters which govern the kinetics of coal and shale separation in an air fluidized bed focusing on the effect of the particle size, shape and density on the performance of the fluidized separator as well as developing a simple rise/settling velocity empirical model which can be used to predict the quality of separation. As part of this study, a (40 x 40x 60) cm air fluidized bed was designed and constructed for the laboratory tests. A relatively uniform and stable average bed density of 1.64 with STDEV < 0.01 g/cm3 was achieved using a mixture of silica and magnetite as the fluidizing media. Different particle size ranges which varied from (+9.5 -16mm), (+16 -22mm), (+22 -31.5mm) and (+37 -53mm) were used for the detailed separation tests. In order to investigate the effect of the particle shape, only three different particle shapes were used namely blockish (+16 -22mm Blk), flat (+16 -22mm FB) and sharp pointed prism particles (+16 – 22mm SR).Different techniques were developed for measuring the rise and settling velocities of the particles in the bed. The Klima and Luckie partition model (1989) was used to analyze the partition data for the different particles and high R2 values ranging from (0.9210 - 0.9992) were recorded. Average Ep iii values as low as 0.05 were recorded for the separation of (+37 -53mm) and (+22 -31.5mm) particles under steady state conditions with minimum fluctuation of the cut density. On the other hand, the separation of the (+16 -22mm) and (+9.5 – 16mm) particles was characterized by relatively high average Ep values of 0.07 and 0.11 respectively. However the continuous fluctuation or shift of the cut density for the (+9.5 -16mm) made it difficult to efficiently separate the particles. Although, particle shape is a difficult parameter to control, the different separation trends that were observed for the (+16 -22mm) particles of different shapes indicate that particle shape has got a significant effect on the separation performance of the particles in the air fluidized bed. A simple empirical model which can be used to predict the rise/settling velocities or respective positions of the different particles in the air fluidized bed was developed based on the Stokes’ law. The proposed empirical model fitted the rise/settling data for the different particle size ranges very well with R2 values varying from 0.8672 to 0.9935. Validation of the empirical model indicate that the model can be used to accurately predict the rise/settling velocities or respective positions for all the other particles sizes ranges except for the (+9.5 – 16mm) particles where a relatively high average % error of (21.37%) was recorded. The (+37 -53mm) and (+22 -31.5mm) particles separated faster and more efficiently than the (+16 -22mm) and (+9.5 -16mm) particles. However, the separation efficiency of the particles can be further improved by using deeper beds (bed height > 40cm) with relatively uniform and stable bed densities. Prescreening of the coal particles into relatively narrow ranges is important in the optimization of dry coal beneficiation using an air fluidized bed since different optimum operating conditions are required for the efficient separation of the different particle size ranges and shapes. The accuracy and the practical applicability of the proposed empirical model can be further improved by carrying out some detailed rise/settling tests using more accurate and precise equipment such as the gamma camera to track the motion of the particles in the fluidized bed as well as measuring the actual bed viscosity and incorporate it in the model.Item The application of dual energy x-ray transmissions sorting to the separation of coal from torbanite(2011-05-17) Strydom, HayleyDual Energy X-Ray Transmission (DE-XRT) Imaging is a multi-sensor technique employed to conduct particle-by-particle sorting. The system makes use of a dual energy x-ray line scan sensor, which generates images of the transmitted x-rays, similar to images generated for suitcase inspection in airport security applications. The dual energy x-ray system allows for rapid approximation of atomic number range, which is utilised to evaluate the mineral and maceral content of a variety of minerals, including coal. The process is independent of particle surface condition, and can thus be utilised as a dry process. A unique application of this technology is in the removal of torbanite from a coal deposit located in Mpumalanga, South Africa. The separation of coal from torbanite has been a problem for the coal industry for a long time. The separation of coal and torbanite by conventional gravity separation techniques is difficult, due to the overlapping densities of torbanite and coal. The commercial value of both commodities is significantly compromised if contaminated with the other, thus impacting negatively on the financial viability of mining such a deposit. Preliminary laboratory DE-XRT testwork results on high quality coal and torbanite products were promising. In order to evaluate the separation of typical Run of Mine (ROM) material on pilot scale, a production scale Mikrosort X-Tract Sorter was purchased. This was the first DE-XRT sorter available in South Africa, and was housed at Mintek in Johannesburg. A 150t sample was provided from a box cut adjacent to the coal deposit under investigation in order to conduct bulk and pilot sorting tests, the focus of which was on obtaining coal products of low ash and torbanite content. Clear distinctions between the coal, torbanite and shale fractions were observed using this technique. The sorter feed (-80mm+20mm) could be upgraded from a CV of 22MJ/kg to 28MJ/kg. Ash content could be reduced from 26% to 10%, which meets export quality standards. Petrographic analysis of the coal product indicated that a high purity coal product (in terms of torbanite and ash content) was attainable (91% by volume) at a mass yield of 42.9% to the coal product, with shale and mixed humic/sapropelic coal as contaminants. Under these conditions, torbanite contamination was marginal. It was demonstrated that shale could be removed from the torbanite product via a second sorting stage. This however was not the primary focus of the study, and was not optimised for this investigation. Two major limitations of the sorting process were identified, viz.; poor liberation and limited sorter feed size range. These impacted on the process as follows:- • The effects of poor liberation on coal quality could be counteracted by adjusting the sorting criteria of the algorithm to reject additional material. This would result in a lower coal product mass yield. In addition, interlocked coal/shale particles would report to the torbanite fraction. • A significant proportion of the ROM feed reported to the -20mm size fraction, and therefore did not fall part of the sorter feed. This resulted in a very low coal mass yield as a proportion of the ROM feed. If this process were to be adopted, means of minimizing fines production during mining and crushing would need to be investigated to improve overall yield to coal product. The capability to process coarse materials (-80mm+20mm) allows for throughputs in excess of 40t/hr. Consequently, this technique may be applied in simpler coal upgrading processes, such as coal deshaling in arid regions.Item Palynology of a coal seam in Karoo deposits of Botswana and correlation with southern African coal-bearing strata(2010-12-02) Barbolini, NatashaA significant amount of palynological work has been done on southern African coal seams in the Ecca Group, but as yet there is little consensus on how these areas relate to each other. This study investigated the palynology of a coal seam from Mmamantswe (Mmamabula area), Botswana, approximately 70 km north-east of Gaborone. A total of 124 samples were taken from two borehole cores and subjected to acid preparation, oxidation and acetolysis. Coal samples were found to be barren of palynomorphs. Fifty carbonaceous mudstones and siltstone samples yielded twenty-two productive samples. A thermal alteration index of 3.0-3.5 was assigned for the sediments. Palynomorph diversity was high, with 64 genera and 90 species present, dominated by trilete and alete spores. This indicates a parent flora of mostly lower order lycopods, sphenophytes and ferns. Non-taeniate bisaccate and monosaccate pollens were scarce, and striates extremely rare (only two species), suggesting an autochthonous origin for the coal swamp. The Mmamantswe core was sub-divided into five microfloral assemblage zones. A transition from monosaccate dominance in the lower part of the core, to equal numbers of monosaccates and non-taeniate bisaccates in the upper part of the core, was seen. As the Mmamantswe palynoflora possesses elements of both the Late Carboniferous glacial floras and the mid-Permian coal floras, it is thought to represent a cross-over assemblage dating to soon after the Permo-Carboniferous boundary (Sakmarian and Early Artinskian). The Mmamantswe assemblage can be correlated with Assemblage Zones II and III of Falcon (1975a); Biozones B and C of MacRae (1988); and Zones 1, 2 and 3 of Anderson (1977) but does not fit well into any existing biozonation. The Mmamantswe palynoflora was most similar to that of Milorgfjella, Dronning Maud Land, Antarctica (Larrson et al. 1990) and the No. 2 Seam, Witbank, South Africa (Falcon 1989). Taphonomic controls on palynomorph preservation suggest that future studies should also attempt to focus on Permian sediments not containing coal, as microfloral assemblages from coal seams tend to be autochthonous, and subject to local climatic influences. Accordingly they are not as useful for inter-basinal correlation across Gondwana.Item The low temperature oxidation of coal: its kinetics and implications for spontaneous combustion(2010-01-19T10:14:14Z) Itay, MItem Characterisation of metallurgical reductants on the basis of reactivity(2010-01-19T09:53:15Z) Kok, HermanItem A simplified model for spontaneous combustion in coal stockpiles(2009-11-09T12:12:19Z) Brooks, Kevin SethItem Characterising coals for coke production and assessing coke: predicting coke quality based on coal petrography, rheology and coke petrography(2008-04-15T08:41:53Z) Jordan, PierreGiven the high costs and general shortage of coking coals on the domestic and international markets, and because the nature and qualities of many of the coking coals available on the markets are themselves mixed products, conventional mechanisms and tried and trusted formulae for manufacturing coke products based on single coals of known qualities can no longer apply. There is therefore an urgent need to develop more effective techniques for evaluating and assessing the properties of individual coals rapidly and reliably and in a manner that could provide useful data for use in modelling the effect of new coal components in a coke blend. Towards this end, the current research has sought to find more accurate coal characterisation techniques at laboratory scale than currently exists in industry at present. Seventeen coking or blend coking coals from widely different sources were selected and cokes were produced from them in as close to full scale conventional conditions as possible. Both coals and cokes were analysed using conventional chemical, physical, petrographic and rheological coking methods. The results indicated that, whilst all coals had acceptable chemical, physical and petrographic properties as evaluated on individual parameters thereby indicating their potential values as prime coking coals, in fact the resultant cokes of some of the coals had properties that disproved this assessment. These anomalies were investigated by integrating all characteristics and statistically evaluating them. The result [outcome] indicated that the series of coals under review fall naturally into three distinct categories according to rank, as determined by the reflectance of vitrinite, and that the coking coals in each rank category were further characterised by parameters specific to that level of rank. In this way more accurate predictions of coke quality were obtained than has been the case to date when using single set evaluations or previously devised formulae. On this basis it was concluded that, when selecting coals for coke making, it is essential to first establish the rank of the coal by vitrinite reflectance and then to apply coke evaluating parameters specific to that level of rank. The formulae developed for this purpose held good for all coals tested, however, it remains to be seen whether this applies universally to an even wider source of coals.