A geometallugical characterisation of the Hwange coalfield: how does coal formation affect coal exploitation?

Abstract

The Hwange Colliery located in northwest Zimbabwe hosts significant coal resources. The mine produces thermal coal for power generation, as well as some of southern Africa’s best coking and blend coking coals. However, since the commissioning of Chaba Opencast and 3 Main Underground Mine in 2005, certain anomalies, which have technological implications, have arisen, and there is need to establish the reasons for these anomalies. Questions posed included: a) Why does the Chaba coal reserve previously classified as coking coal based on cut-offs of a cumulative ash maximum of 15% and cumulative volatile matter minimum of 23.5% fail to produce coking coal in the plant? b) Why do-good coking properties produced in some areas of Hwange Colliery not respond well to conventional coking tests? A compelling need arose to characterise the Hwange coals and estimate the tonnages of the mineable reserves for each of the three coal categories, namely: thermal coal, general purpose coal, and coking coal, thus ensuring Hwange Colliery’s preparedness to meet the coal quality demands for a variety of users. Historical geological data generated by previous exploration programmes and new coal samples representative of the future mining areas and anomalous coals were examined. Coal samples were collected from Hwange Colliery’s Chaba and 3 Main Underground mines and key analyses and tests were conducted on them. These included proximate, ultimate, and total sulphur analyses for basic characterisation of the Hwange coals; petrographic analyses to determine coal rank, maceral composition and mineral matter in the coal. XRF analysis was undertaken to determine the ash composition as well as combustion prediction and coking tests. The geological mapping study revealed that faulting is dominantly listric increasing in intensity westwards and the main coal seam becomes progressively deeper westwards. The footwall of the seam plunges from 850m above sea-level in the northeast to approximately 490m above sea-level in the south-west. Localised topographic highs and lows occur were found within the coalfield. The coal seam is significantly thicker in the pre-coal forming topographically low-lying areas but thins out over topographic highs. In the analytical investigation, proximate and petrographic analyses of the coal samples revealed that a low-ash, vitrinite-rich coal band occurs near or at the base of the seam, and that the overlying coals passing progressively up through the seam become richer in ash and inertinite. Based on this sequence, it is suggested that the maceral profile reflects an initially wet swamp environment in early coal-forming times which was progressively replaced by increasingly drier forest swamps. The low-ash basal horizon is significantly thicker in the topographic depressions (up to 6m) but is thinner (1-2 m) where the seam abuts over or against topographic highs, namely, in locations in excess of 600 m above sea level. The mineral matter comprises silicates, sulphides and carbonates, with clay the dominant mineral, particularly towards the top of the coal seam. Combustion tests reveal that the vitrinite-rich basal samples devolatilise at lower temperatures, have lower peak temperatures, and burn out more rapidly than the inertinite-rich samples found higher in the seam. In terms of rank, the Hwange coals are Bituminous and more specifically Medium Rank B and C. No heat effect from sills and dykes has been observed. No specific regional trend in rank has been observed. With regard to coking attributes, the study revealed that volatile matter is not a good indicator of coking properties. Some Hwange coals yield high volatile matter contents but possess a free swelling index lower than the 3.5, the minimum for most coking coals, while other Hwange coals possess relatively low volatile matter contents and swell to some extent. Proximate analyses alone are, therefore, insufficient to indicate coking capacity in these coals. Of all coals tested, only the lowest part of the seam in the Chaba location yielded a sufficiently high vitrinite content and free swelling index to qualify as a coking coal. The samples higher up the seam, although high in the volatile matter (23.5% and above), have low free swelling indices and vitrinite contents insufficiently high to qualify as coking coal. The reason for the anomalous swelling capacity in coals that showed good swell, but low volatile matter content appears to be associated with the presence of exudatinite in association with reactive semi-fusinite, fusinite and secretinite. Exudatinite, is a high volatile, viscous material that would have emanated from liptinite and vitrinite at specific levels of rank during coalification, and as a consequence of regional heating. Coals that have high volatiles but do not show swelling capacity simply have vitrinite contents that are too low in proportion to provide any degree of swelling and coking properties. Based on a review of the extensive borehole data undertaken throughout the Hwange Mine, and the resource and reserve calculations both undertaken in this thesis, it is estimated that the remaining life of mine for coking coal is 43 years, and for thermal coal as used in the agricultural, cement and brick making industries is 39 years. Assuming the production rates are sustained, Hwange Colliery has the potential to generate in excess of USD 1 277 000 000 per year over the next 43 years