Acid Mine Drainage Potential Prediction Through Geochemical Assessment For A Small-Scale Coal Mine Project In South Africa
Yibas, Bisrat (1,2); Zhao, Baojin (2)
1: Council for Geoscience, South Africa280 Pretoria Road, Silverton, Pretoria; 2: Department of Environmental Sciences, College of Agriculture and Environmental Sciences, University of South Africa, PO Box 392, UNISA 003, South Africa
The outcome of an assessment of the potential for acid mine drainage from small-scale coal mining and the discard generated therefrom and its usefulness to formulate management and rehabilitation of the site from an environmental perspective is presented herein.
Detailed geological, topographical and morphological investigation, sampling of the coal seams, the interburden and overburden rocks, mineralogical and geochemical characterisation, and study of the surrounding water bodies have been undertaken. The geochemical characterization includes acid-base accounting (ABA), chemical analyses (XRF, ICP-MS), mineralogy (XRD and microscopy), and kinetic geochemical modelling (geochemist's Workbench).
The small-scale coal mining envisaged to exploit 2 coal seams by opencast method from a sedimentary succession known as the Vryheid Formation which at this locality consists of thin carbonaceous shale beds at the base with subordinate sandstone layers and three coal seams. The two lower coal seams that are economically significant are separated by thin carbonaceous shale and subordinate sandstone beds. A succession of carbonaceous shale, clay and weathered sandstone from bottom to top represents the overburden material.
The ABA tests showed that the coal seams have an average net neutralising potential (NNP) of -55.86 kg/t CaCO3 and 0 NP/AP ratio and 1.79 wt % of total sulphur. The overburden and interburden materials have NNP of -4.96 kg/t CaCO3, NP/AP ratio of 0.04 and 0.16 wt % of total sulphur. Thus, the ABA data suggests that the coal seams are potentially acid generation whereas the overburden and interburden materials have no significant acidity or alkalinity. This is supported by the mineralogical data, which indicates that the coal seams in average contain 5 wt % of pyrite whereas the roof materials are devoid of sulphides in the materials.
The kinetic geochemical modelling showed that the coal seams and the coal discard will generate acid mine drainage with respect to pH, SO4=, and total dissolved solids in the first 5 to 10 years. The kinetic geochemical modelling has also shown that the rehabilitation option will result in marked reduction of AMD generation compared to the base case scenario.
For the recommended rehabilitation option to work, the mine waste should be stockpiled separately and then filled back into the mine pit starting with the potentially acid generating material and covering it with the roof material and then the topsoil. In this case oxidation of pyrite will be greatly inhibited and production of acid mine drainage will be significantly reduced.