Iron Minerals in Coal, Weathered Coal and Coal Ash - SEM and Mössbauer Results

The aim of the present investigation was to identify and quantify the iron mineral phases present in South African coal from various coal fields and in coal ash, after industrial and laboratory combustion processes, and to determine the changes that occur in these phases during weathering. Iron in coal is mainly associated with sulphur in the minerals pyrite and jarosite, whilst other iron-bearing minerals such as illite and ankerite also occur, but also occurs as a trace element in kaolinite, a major clay mineral present in coal. The amounts of these minerals vary considerably in coals from diverse origins and thus coal samples from six coal-producing areas in South Africa were studied by means of Mössbauer spectroscopy and SEM analyses. With the aid of Mössbauer spectroscopy, the iron-bearing minerals were identified in the coal, coal ash and weathered coal, whereas in the SEM analyses, apart from these minerals, the non-iron-bearing minerals were identified and found to be mainly quartz, clay minerals and carbonates. Differences in mineral composition were found between the coals from the different regions. Ash samples, obtained from the Lethabo electricity power plant, South Africa, were investigated and laboratory simulations were performed to obtain a comparable analysis of the industrial ash samples. At the high temperatures (≈1400°C) of combustion in the power plant, fly ash and agglomerates are produced and the Mössbauer spectra resulted in two poorly developed doublets, typical of glass. In the laboratory simulation, carried out at temperatures ranging from 200° to 1200°C it was clearly observed how the pyrite changed to hematite and finally was taken up in the glass in addition to the hematite that formed. The high amount of calcium present, identified by SEM analyses, resulted in the agglomeration occurring of the fly ash. The weathering products were also identified using the same techniques and it was noticed that the pyrite changed to a sulphate when the wet coal was exposed to air drying.