Abstract
M.Sc.
The study site for this project is New Clydesdale Colliery situated in the Witbank Coalfield, South Africa. The Witbank Coalfield is located within the northern part of the Karoo Basin where the major coal deposits in South Africa are located. Optimum ash fusion temperatures derived from coal in this region are approximately 1400°C, and higher, although temperatures above 1300°C are also acceptable. In certain coal seams the ash fusion temperatures drop well below this optimum temperature, leading at times to problems in the user industry. Ash fusion temperature which is the
temperature at which the mineral matter in coal begins to soften, flow and fuse, is an important aspect relating to coal utilization but is little understood and under-studied in South Africa. The objective of this thesis is to attempt to determine what factors, mineralogical or inorganic chemical, influence variations in the ash fusion temperatures in certain coal samples. An understanding of this problem will have a bearing on the ultimate assessment of a coal for utilization purposes, in general, and ash deposition prediction in future boiler plants in particular. This study is somewhat pioneering as no detailed studies have been undertaken or published previously. Samples were collected at four sites from New Clydesdale Colliery. The sample sites were selected based on pre-existing knowledge of the ash fusion temperatures obtained from company analytical sheets. The sites were therefore representative of low and high ash fusion temperature coal. The first two sites were located in the opencast area where the No. 2 seam is actively being mined. The other two sites were located underground where mining has ceased. Conventional analyses conducted on the samples include proximate analyses, calorific values, percentage sulphur, and ash fusion temperatures. Further analyses performed included ash analyses (composition of ash), X-ray diffraction, scanning electron microscopy and petrographic studies.
It was found that although complex relationships exist between ash fusion temperature and the various geological and coal parameters, positive results were obtained. These indicate a possible relationship between low ash fusion temperatures and i) finer size fractions, ii) an increase in iron content present in the form of pyrite and iii) the form in which pyrite is present, namely cleats. High ash fusion temperatures tend to correlate with i) the absence of pyrite or ii) pyrite present as framboids.