Lithostratigraphic and Geochemical Characterisation of the Waterberg Coalfield: Implications to Acid Mine Drainage, Limpopo Province, South Africa

Abstract

The study conducted at Sekoko coal mine in the Waterberg coalfield of South Africa aimed to establish the quality of coal and its host rocks, as well as the potential for acid mine water generation. A total of 18 samples were collected from a box-cut and analysed using various methods such as, Scanning electron microscopy, X-ray diffraction spectroscopy, X-ray fluorescence spectroscopy, proximate analysis, Modified Sobek Acid Base Accounting test as well as petrographic study. The major rock types identified in the study area included shale, sandstone and gritstone. Coal was overlain by these rocks. The study revealed significant concentrations of framboidal pyrite in coal, which increases with depth as determined by X-ray Diffraction spectroscopy and scanning electron microscopy. Whole rock geochemistry indicates that Fe2O3 was higher in coal (1.18 wt%) than in host rocks (0.45 wt%) and increased with depth. Average mean of 1.23% a.d. for sulphur content was revealed thus, classifying the coal as medium sulphur coal. Sulphur occurred as pyritic, organic, and as sulphate, in that order of abundance. Using both proximate analysis and calorific value (with mean value of 21.2 Mj/kg) coal was classified as below grade D. From the vitrinite reflectance value (ranging between 0.6 to 0.7), coal was ranked as medium-grade bituminous C. Paste pH and EC results indicated that the Sekoko coal is already undergoing oxidation, and modified Sobek tests indicated that over 72% of the analysed samples have sufficient sulphur to generate long-term acid mine drainage. The potential for acid generation increases with depth (1.09 to 181 kg/t) and was high on the southern side (181 kg/t) than the western side (52 kg/t). Results indicate that there aren’t enough neutralization materials to counter the acid production potential as the neutralisation potential is negative for all samples. The study concluded that the coal from Sekoko coal deposit is enriched with pyrite minerals, hence this causes an increase in the potential for acid mine water generation. Screening of coal prior to utilization is recommended so as to reduce the content of pyrite in coal.