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COLLECTIVE EFFORT
Water Sewage & Effluent [ September 2008]
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Gold-mining houses are cooperating with
government to find a big-picture solution for a potential
environmental catastrophe.
Mining houses Mintails, Harmony and DRDGold have joined forces
in addressing an historic legacy and future environmental
concern in the Witwatersrand Basin: the issue of acid mine
drainage (AMD). The physical formation of AMD is well known to
industry: underground voids ą formed as a result of mining ą
become filled with water through natural processes. The water
comes into contact with the exposed ore body, which consists of
iron pyrite (FeS).
When the pyrite is exposed to water and air, a sulphuric-acid
solution is formed which dissolves the associated heavy metals
inherent to rock structures. This contaminated sulphuric
solution is known as AMD.
From an operational perspective, active mining companies usually
pump the contaminated underground water to surface as it
restricts access to reserves and poses safety hazards in the
event of flooding. Pumping underground water to surface is an
operational necessity. Once pumped to surface, a component of
the water is treated internally to be reused as process water
and the remainder is treated according to Department of Water
Affairs & Forestry (DWAF) effluent-discharge policies prior to
release. The water is, therefore, also an environmental
liability.
Addressing an unwanted legacy
Understanding the physical phenomenon of AMD and the very basic
operational aspects behind it, one must also understand that
once mining operations cease, the continuous infiltration of
fissure water does not, and it is exactly this issue that was
cause for concern in the Western Basin of the Witwatersrand
around Krugersdorp and Randfontein.
Five mining houses used to mine in the Western Basin of the
Witwatersrand. The mining operations were Lancaster Outcrops and
East Chamdor (now liquidated), as well as existing gold
producers, Mintails, Harmony and DRDGold.
In 1998, many of the underground activities in the Western Basin
came to a halt. Pumps were shut down, underground cavities
flooded and, in 2004, the acidic water started to decant on
surface at Harmony’s Black Reef incline. This could have been
catastrophic from an environmental perspective as the topography
of the area would have led to the contaminated water running
into Tweelopiesspruit, which runs through the Krugersdorp Nature
Reserve and into the Cradle of Humankind ą posing a grave threat
to the 3,5-million-year-old heritage site.
In 2005, DWAF issued a directive to the mining houses in the
Western Basin. It stated that the contaminated water in the area
needed to be treated in a sustainable manner.
Developing a basin blueprint
The Witwatersrand Basin comprises four basins: the Far Western
Basin reaching over the Carletonville area; the Western Basin in
the Krugersdorp-Randfontein region; the Central Basin running
from Krugersdorp to Germiston; and the Eastern Basin in the
vicinity of Germiston-Springs. Although the problem initially
arose in the Western Basin, the mines in all four basins of the
Witwatersrand face the same conundrum. They all need to pump
water to the surface while operating and they all share the same
environmental obligation to apply for closure when they shut
down their operations.
These mines run over very long distances. Many of them are
linked with each other underground but they belong to different
owners. It, therefore, complicates the matter for government
from a closing strategy perspective.
As a result, DWAF requested a “regional closure strategy”
stipulating that an overall solution needs to be found for the
dilemma of AMD in the entire Witwatersrand region.
Adding to this, the project required a critical mass in order to
be economically viable. “You need to produce a certain amount of
water to keep your unit costs low enough to make the project
financially feasible,” states Schoeman. The combination of all
four basins’ contaminated water into one treatment plant will,
therefore, provide a regional closure strategy to gold mining
houses and government but will also increase critical mass and,
therefore, the economic viability of the project.
The first phase of the project will produce around 75 Ml/day of
water. The expansion phase will include the treatment of 200
Ml/day of water, incorporating all four basins. When all the
mines in the entire Witwatersrand basin finally close down (and
no longer use the water for processing), they have the potential
to produce around 300 Ml/day.
Scope of the WUC - water-treatment plant
1. The Western Utilities Corporation (WUC) water-treatment plant
is in development phase and running pilot operations at
Harmony’s No 8 shaft near Randfontein in the Western Basin of
the Witwatersrand.
2. The water plant is expected to reach its commercialisation
phase in December 2010; treating 75 Ml/day of acid mine drainage
(AMD) in the Central Basin of the Witwatersrand (capex:
R1,2-billion).
3. The project plan has been developed to include all the basins
of the Witwatersrand in order to provide a regional closure
solution to government for AMD, as well as making the project
economically viable. Expansion is expected around 2014 and will
treat some 200 Ml/day of AMD (capex: approximately
Ż240-million).
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The legal and environmental issues
Adam Gunn, a director at Routledge Modise Eversheds, says
this is a “legally and environmentally” interesting issue.
The only environmental argument that could be raised is that
the water is being transferred from one catchment to another
but the extent to which the flow of water in South Africa
has been changed seems to mitigate this argument, he adds.
“Legally, the issue is also interesting because, since the
introduction of the National Water Act in 1998, all water in
South Africa has been under the control of the state.”
However this piece of legislation has not improved the
quality of water in South Africa’s aquifers hence
private-sector initiatives seem to hold considerable
potential and solutions to complicated environmental-legacy
issues. Gunn believes there is also a possibility, that,
should demand arise, some of this water could be treated to
potable standard and supplied to communities in the area. |
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15 Ml/day AMD - contaminated water
As dolomite formulations were disrupted, the water that
naturally filtered through them began to subside. Water
draining into the voids mixed with the exposed ore (pyrite)
and combined to form acid mine drainage (AMD) - a sulphuric-acid
solution. Reservoirs of contaminated AMD that collected in
the voids have begun discharging into nearby rivers at a
rate of 15 Ml/day.
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