Abstract
M.Sc. (Geography)
Coal mined in South Africa for the competitive international market, has to be
selected to meet the many quality specifications of customers. This upgrading
is done by washing the coal in a heavy medium separation plant. Marketable
coal, discard and slurry are produced from this washing.
Discard consists mainly of poor quality coal, carbonaceous shale and waste
rock. Iron pyrite (FeS2) occurs in all of the above in higher concentrations
than in the marketable coal. Both the carbonaceous materials and pyrites
generate heat when oxidizing. If this oxidation is not arrested at an early
stage on a discard dump and the temperature of the dump increases above
BOoC, spontaneous combustion is quite likely.
The South African Council for Scientific and Industrial Research (CSIR) has
estimated that smouldering discard dumps in the Eastern Transvaal highveld
region contribute approximately 400 000 tons of S02 per annum to the
atmospheric pollution in that area. As a result, significant localized acid rain
occurs, Louw (1990).
The oxidation of iron pyrites to sulphuric acid, and the oxidation of other
trace elements, is accelerated under the high temperature conditions
generated by spontaneous combustion. Leaching of these oxidation products
results in local groundwater and surface water contamination.
This study describes different disposal technique and pilot study aimed at
minimising the oxidation within the dumps.
Slurry, which consists of discard and/or coal of less than 1 mm in diameter
is co-deposited with discard in sequential layers of approximately 200 mm
thick. This has resulted in reducing the permeability, porosity and air and
water exchange within the dump. This in turn has led to a reduction in
spontaneous combustion, pollution and costs. A visual increase in stability
of the discard dumps, moisture content and operational ease of placement
were experienced. The saleable value of the dump as a low value heat source
is also preserved.