Quantitative evaluation of minerals in coal deposits in the Witbank and Highveld Coalfields, and the potential impact on acid mine drainage
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Date
2003-05
Authors
Pinetown, Kaydy Lavern
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
A mineralogical and geochemical study on the coal and coal-bearing successions of
the Witbank and Highveld Coalfields in the Mpumalanga Province of South Africa
was proposed in order to, firstly, investigate the quantitative distribution of minerals
in the lithological units, and secondly, to correlate this data with the potential of the
units to contribute to acid mine drainage conditions in the region.
X-ray diffraction and X-ray fluorescence techniques were used to analyse the
samples from the study area. Samples from the No.1, No.2, No.4 and No.5coal
seams were collected from several mines in the Witbank Coalfield, while samples
from the No.4 and No.5coal seams were collected from borehole material obtained
from the Highveld Coalfield. The inorganic components make up approximately 8.00
to 35.00 wt% of a coal sample. Si02
concentrations varied between 0.00 and 35.00
wt% of a sample, AI203 between 0.50 and 16.00 wt%, Fe203 between 0.03 and
10.00 wt%, and S between 0.15 and 8.00 wt%. Minor concentrations of CaO (0.00 to
8.00 wt%) and MgO (0.00 to 1.00 wt%) were present. P205 occurred in
concentrations of 0.00 to 3.50 wt% and K20 was in the order of 0.00 to 1.30 wt%.
Na20 values were the lowest varying between 0.00 and 0.45 wt%. The only
difference in chemistry between Witbank and Highveld coals was a slight increase in
Na20 (0.00 to 0.51 wt%) in the Highveld coals. These results were confirmed by the XRD investigations. The mineral components in
the XRD patterns were semi-quantitatively evaluated in terms of dominant (>40% of
the mineral fraction), major (10-40%), minor (2-10%), accessory (1-2%) and rare
« 1%) constituents. The mineral fraction in the coals was dominated by quartz and
kaolinite, with major to minor and trace amounts of calcite, dolomite and pyrite, as
well as accessory phosphates phases.
XRF and XRD results for the coal-bearing units were also in good agreement. Higher
K20 and Na20 concentrations were obtained in the sandstones in comparison to the
siltstone and carbonaceous shale samples, and were supported by the presence of
feldspars and clays such as illite in XRD interpretations. A normative program designed for Australian coals and sedimentary rocks, called Sednorm, was used to
calculate normative mineralogical compositions from the geochemical results. Good \
correlations were obtained for comparisons made between the chemical
composition, mineralogical interpretations and normative results for the coal and
sediment samples.
Acid-base accounting was used to investigate the potential of the coal and coalbearing
units to produce acid mine drainage conditions. The acid and neutralising
potentials are largely dependant on the abundance and availability of minerals such
as pyrite and calcite respectively. According to the screening criteria proposed by
Usher et al. (2001), averages for Neutralising Potential Ratio (NPR) suggest that all
the coal and coal-bearing units, excluding the unit between No. 1 and No. 2 coal
seams, are potentially acid generating. The latter lithological unit is considered to be
inconclusive. The average Net Neutralising Potential (NNP) values suggest that the
NO.5 coal seam, NO.4 Upper coal seam, and between NO.4 and NO.2 coal seams
are potentially acid generating. This is a result of the weathering of carbonates in
these lithological units. The other units could become either acidic or neutral. In theory it is possible to calculate the AP from the analysed S by multiplying the S
value by 31.25. Assuming that all sulphide-S is available for oxidation, then the total
S analysed could be used to predict the AP for samples on which no acid-base
determinations has been carried out. Similarly, the excellent correlation between the
NP and CaO, and between the NP and combined CaO and MgO, confirms that these
chemical components are largely responsible for NP values. It is then also possible
to predict the NP by using the CaO and MgO concentrations for samples for which
no AP or NP data is available.
The application of ABA in this study offered a major contribution to understanding the
complexities governing water-rock interactions. Results provided a preview of
situations that might arise regarding groundwater quality in a certain area, but also
offers ample time to decide on appropriate prevention or remediation programs. The
potential for these lithological units to contribute to the deterioration of groundwater
is evident.
Description
Keywords
Coal mines and mining -- South Africa -- Witbank, Acid mine drainage, Water quality -- South Africa -- Witbank, Dissertation (M.Sc. (Geology))--University of the Free State, 2003