Factors and challenges affecting coal recovery by opencast pillar mining in the Witbank coalfield

Abstract

The depletion of coal reserves in the Witbank coalfield in Mpumalanga Province has resulted in mining companies exploring the possibilities of extracting coal pillars. These are pillars that were left behind for hangingwall support during underground bord-and-pillar operations. Recent studies of in situ pillar mining have found the extraction of the pillars to be feasible during opencast mining due to the high extraction rates of coal, relatively low stripping ratio, safety of the operation, and general environmental requirements. The geological model of an opencast pillar mining operation within the Witbank coalfield has indicated that some 30% of the coal in the no. 2 seam remains in pillars. The no. 4 and no. 1 seams are yet to be mined. Opencast pillar mining requires maximizing coal recovery in order to be competitive in the market, since a portion of the resource has already been extracted. Exposure and recovery of the coal are crucial in reducing coal losses and dilution due to the coal pillars and voids, and challenges experienced during the mining of pillars from surface. The reconciliation process evaluated the overall flow processes, from in situ coal to the mined-out coal. The similarities between opencast pillar mining and conventional opencast mining were studied in terms of the mining sequence, pit layout, and operations. A correlation between the SAMREC Resource and Reserve definitions was conducted through an investigation of coal losses and contamination during mining. The various types of coal losses affecting production volumes were investigated. The dilution of coal was found to be higher in the no. 2 seam due to blasted material filling the voids in the bords. The presence of bord voids is one of the factors that increases the risk of spontaneous combustion. This in turn affects the productivity of the operation, with buffer blasting management and cladding techniques used to reduce the risk of spontaneous combustion.