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Factors and challenges affecting coal recovery by opencast pillar mining in the Witbank coalfield
Ngwenyama, Philani Larrance; De Graaf, Wolter Willem; Preis, Eugene Pieter
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.
Description:
This paper
was first presented at the New technology and
innovation in the Minerals Industry Colloquium’,
9–10 June 2016, Emperors Palace, Johannesburg,
South Africa.