Abstract
Abstract : Chromium (Cr), manganese (Mn), cobalt (Co), and molybdenum (Mo) have been reported to occur in higher concentrations than global averages in South African coals. The trace element data pool for South African coals is fairly limited, and research is required to confirm the origin and mode of occurrence of trace elements. The main aim of this study was to investigate the mode of occurrence specifically of Cr, as well as Mn, Co, and Mo in select South African coals and to determine the possible source of Cr. Coals from the Waterberg, Soutpansberg, Witbank, Highveld, and the Nongoma Coalfields were considered. A total of 21 samples were obtained and assessed using coal petrography and chemical analyses for coal characterization, XRD and XRF for mineralogy, ICP-MS for trace elements, and the scanning electron microscope (SEM) equipped with the Energy Dispersive X-ray Spectroscopy (EDX), density fractionation, and sequential leaching to determine modes of occurrence. The Waterberg, Soutpansberg, Witbank, and Highveld Coalfield coal samples have higher Cr concentrations compared to the Nongoma Coalfield samples and global averages for hard coals. Chromium in these coals could have originated from the Bushveld Complex, but the Cr concentrations are far much lower than those of coals formed near an ultramafic source. Chromium from primary sources could have been dissolved and been redistributed during coal formation. Chromium is associated with both the organic and inorganic matter in the selected coal samples. High Cr concentrations were observed in the lighter density fractions, indicating an association of Cr with the organic matter, which is in agreement with literature. A positive correlation coefficient between Cr and kaolinite was determined, which implies Cr could be hosted in kaolinite. From selective leaching, most of the Cr was leached using hydrofluoric acid (HF), indicating an association of Cr with silicates. The Cr that was not leached and remained in the residue could be associated with the organic matter. These apparently conflicting results are in agreement with the literature and imply multiple modes of occurrence. It would have been of interest to determine Cr speciation in these coal samples in order to understand if high Cr contents are of health and environmental concern.
M.Sc. (Geology)