A flotation column has been developed Incorporating the use of fine air bubbles (less than 100 microns) to remove ash-forming minerals from micronized coal. The microbubble generator used In this work has been characterized and found to yield a very narrow size distribution. Microbubble column flotation tests have been conducted to study a series of operating variables such as time, bubble size, feed rate, feed point, feed percent solids, column height, bubble number concentration, make-up water addition and countercurrent wash water addition. The results show that i) fine air bubbles are Inherently better suited for floating small particles; ii) both ash and recovery rates Increase with Increasing feed rate, distance of the feed point from the tailings port, feed percent solids and bubble number concentration; iii) taller columns result In Improved recovery and ash rejection; and iv) the countercurrent wash water addition minimizes the entrainment of mineral matter to the froth product. Proper control of these parameters makes It possible to produce super clean coal (< 2% ash). / M.S.

In recent years, a great deal of interest has been shown in developing methods for preparing super—clean coal containing less than 2% ash and 0.5% sulfur. New techniques for recovering fine coal, such as micro—bubble flotation, can achieve the desired result provided mineral matter is sufficiently liberated. To achieve sufficient liberation, however, it is often necessary to grind to a mean particle size finer than 10 microns. Since conventional ball mills are highly inefficient in this fine size range, the stirred ball mill has been proposed as a more suitable means for ultrafine grinding. / Master of Science

In order to demonstrate the ability of microbubble flotation to superclean coal to ash levels of less than 2%, several Eastern U. S. coals have been tested. The results show that the process is capable of producing superclean coal with improved recovery as compared to the conventional flotation process.To further improve and understand the microbubble flotation process, electrokinetic studies of the hydrocarbon oils used in flotation as collectors have been conducted. Also, the effect of oil emulsifiers on the zeta potential of oil droplets has been studied. In general, oil droplets are negatively charged and negative zeta potential is reduced with the addition of nonionic and cationic surfactants. On the other hand, the negative charge is increased with the addition of an anionic reagent. It has also been shown that the negative zeta potential of oil droplets increases with increasing hydrocarbon chain length.The effects of different collectors on induction time and flotation have been determined by conducting microflotation and induction time experiments using an Elkhorn seam coal sample. The results show that industrial oils combined with the coal have the shortest induction times and, therefore, the highest flotation yields as compared to pure hydrocarbon oils. It has also been shown that oil emulsifiers tend to increase flotation yield and reduce particle/bubble induction time. / Master of Science

The Chemical and Electrochemical Coal Cleaning (CECC) process, designed to remove mineral matter from coal, has been investigated by treating coal samples in acidified slurries. Various coals, characterized by different maceral structures and mineral matter contents, were subjected to several experimental procedures under mild conditions. Substantial amounts of mineral matter (up to 70%) could be extracted from coals which were resistant to physical cleaning, while 22% of sulfur could be removed from pyritic coals. The operating conditions of the CECC were studied in order to determine their influence on the process efficiency. Analyses conducted on solids and leachates resulting from the tests demonstrated that different mechanisms were achieving demineralization by the CECC. Between 50% and 95% of the feed mineral matter was removed by dissolution, whereas the balance could be ascribed to liberation. The CECC process is suitable for cleaning middlings, as well as for further extracting mineral matter from physically clean coals, especially from pyritic vitrinite and fusinite type coals. / Master of Science

A new method for demineralizing coal has been developed which is based on the osmotic pressures that occur when electrical double layers overlap. In this technique, coal is exposed to ferric ions in an acidic medium which causes the coal to lose electrons and become positively charged, thereby establishing ionic double layers in the vicinity of its surface. Inside the pores and crevices in which mineral matter is entrapped, the ionic double layers overlap and reduce the chemical potential of water, creating an osmotic pressure. The build-up of such pressure pushes the mineral matter out of the crevices, resulting in mineral liberation. Since the process, which is termed electro catalytically induced liberation (EIL), relies on surface-chemical reactions, the energy consumption is substantially lower than in conventional liberation processes based on comminution. Tests on several different seams of coal from varying geological locations have indicated that the process may be used to remove over 70% of the mineral matter present in coal. Mass balance studies conducted on a Wyodak coal indicate that approximately 90% of the ash removed is by the EIL mechanism, while the balance may be attributed to acid dissolution and the loss of material during handling. Scanning electron micrographs of the coal samples taken before and after treatment show morphological changes consistent with the proposed EIL mechanism. The technique has been used successfully to clean bituminous coals, low-rank coals and preparation plant refuse, and to further reduce the ash content of coals pre-cleaned by other means. A theoretical model has been developed to calculate the osmotic pressure that occurs inside a typical coal crevice during the EIL treatment. The changes in the aqueous chemical potential are calculated using semi-empirical equations derived from solution theory, while partial molar volume changes are accounted for in the final calculation of the osmotic pressure. The model indicates that pressures on the order of 4-7 atmospheres can develop inside crevices with walls 100-1000Å apart. These values are numerically consistent with those predicted by other models developed using different approaches. / Ph. D.

Thesis (Ph.D.)--University of Wollongong, 2005. / Typescript. Includes bibliographical references: leaf 252-266.

As a consequence of new coal mining practices, coal preparation plants have been inundated with increased loads of coal and clay particles in their wastewaters. Traditionally, the industry has employed primary sedimentation as the fundamental treatment scheme for these sediment-laden blackwaters. This study was undertaken to determine the effects of a combination of coagulant addition and rapid mixing on the settleability of these particles. After initial testing, aluminum sulfate and two low molecular weight cationic polymers, Cyanamid Magnifloc 513C and Cyanamid 515C, were chosen as primary coagulants for use in this work. An artificial wastewater was prepared from finely powdered (62 to <38 microns) raw coal samples and tap water, after initial tests indicated that typical frothing and/or collector agents had no demonstrable effect on coagulant function. Initially determined optimum coagulant dosages, as well as flocculation and sedimentation times, were kept constant while rapid mix intensities were varied at G values of 330 sec⁻¹, 700 sec⁻¹, 2000 sec⁻¹ and 7000 sec⁻¹ for each sample. Using a combination of residual turbidity and particle size analyses to determine the effectiveness of each rapid mix intensity, it was discovered that only the highest mixing intensities and durations (G(t) values) caused floe disintegration due to overmixing. At the lower G(t) matrices floe formation and settleability was consistently good. When aluminum sulfate was used as a coagulant, the wastewater was tested at a high pH of 8.1 to 8.3 and a low pH of 5.5 to observe floe behavior under different conditions of coagulant mechanism. The test results were similar for both pH values except at the lowest mixing intensities where the high pH samples settled well, resulting in low residual turbidities, but the low pH samples had relatively high turbidities. / Master of Science

MESMEG / Department of Mining and Environmental Geology / The Mushithe coal occurrence is situated approximately 16 km west of the Tshikondeni coal mine in the Tshipise-Pafuri sub-basin of the Soutpansberg coalfield in South Africa. The Soutpansberg Coalfield has received comparatively less attention compared to other more notable South African coalfields. There is as a consequence very limited information in the public domain applicable to the geology as well as quality of coal in this coalfield, as a result there is no known study focused exclusively on the Mushithe coal occurrence. The aim of the study was to conduct detailed geological mapping of the Mushithe Deposit so as to ascertain the geological environment and petrological characteristics of rocks within the area. Further work involved coal sampling and analysis in order to establish coal quality and its physical and chemical characteristics. Samples were collected using geological field mapping and channel sampling. Nine coal samples were collected from the coalbed and host rock, exposed along the Mbodi River, during geological field mapping using channel sampling. Furthermore, 92 rock samples were collected during geological field mapping of which 10 representative samples were selected for further analysis. X-ray fluorescence spectrometry was conducted on all selected samples. Proximate analysis and ultimate analyses, and calorimetry tests were undertaken on coal samples. Three samples were selected based on calorific value for maceral identification, mineral analyses and vitrinite reflectance using petrographic study. Detailed geological mapping of the area around the Mushithe coal occurrence showed the geological setting of coal in this area. The following lithologies were identified in the study area: sandstone, mudstone, ironstone, calcrete, shale, quartzite, quartz vein with a general strike direction to the north-east. The host rocks including coal were intruded by dolerite dykes and this resulted in the devolatilization of coal. The current study concluded that the Mushithe coal was formed in a wet swampy environment. This has been confirmed based on tissue preservation index (1.69) and gelification index (2.35). Coal rank ranged from bituminous Rank C- B according to United Nations Economic Commission for Europe Coal Classification (UNECE) and samples were characterised by high ash (27.90%), high moisture (10.47%) and low sulphur (0.24%). Furthermore, coal was graded below grade D based on classification for use by ESKOM which consider any calorific value below 24.5 MJ/kg to be in this category. The coal is vitrinite rich (77.75 vol%) and low in Inertinite (22.25 vol%) and devoid of Liptinite and pseudovitrinite, thus it is of good coking quality. Geochemical analysis revealed that the coal was enriched in TiO2 and Fe2O3 which was corroborated by the mineral matter which was mostly clay and pyrite. Comparatively, coal quality analysis revealed the calorific value of 14.26 MJ/kg and vitrinite reflectance between 0.94 %ROV to 1 %ROV which was less than that of the Tshikondeni Deposit but greater than that at Waterberg coalfield. The study recommends further detailed exploration of coal in the area, applying such techniques such as geophysical exploration and borehole drilling leading to resource evaluation. Further studies are recommended to provide a better interpretation of the depositional environment of coal at Mushithe as well as the effect of devolatilaziton by a dolerite dyke. / NRF