AngloGold?s Ergo operation is the
largest surface retreatment operation in the world. The company was
first listed on the Johannesburg Stock Exchange as the East Rand
Gold and Uranium Company (Ergo) in 1977, and was merged into AngloGold in
1998 on the group?s formation. Using a novel process, Ergo retreats
slimes or tailings (the residue that remains after mined ore has
been processed) that were dumped (or stored and dried) on massive
hill-like structures in years past. It extracts the gold that
remained behind when processing operations were not able to recover
as efficiently as is possible today. Located some 35 km east of
Johannesburg in Gauteng, Ergo has reclaimed from dumps dispersed
over an area of 50 km from west to east and 30 km from north to
south. At full production, Ergo?s treatment facility at Brakpan
treated around 3 million tonnes of tailings per month but today this
operation is winding down.
The Daggafontein tailings dam was established in 1986 and over a
period of 15 years grew to 200 million tonnes of gold tailings.
These tailings came from a number of sources, mostly smaller dumps
scattered across the Springs/Daggafontein area. Once residue from
the smaller dams was consolidated into the Daggafontein tailings
dam, the other areas could be rehabilitated. However, when
deposition of material into the Daggafontein tailings dam stopped in
December 2001, the dam had to be rehabilitated to achieve
environmental closure. This was necessary both in terms of the
company?s policy in respect of rehabilitation and to obtain formal
closure from the Department of Minerals and Energy.
After an extensive risk-based remediation planning process ? which
entailed an investigation into reshaping geometry in combination
with different cover layers ? the Ergo environmental management team
decided to test the recommended geometry and cover layer on a pilot
scale prior to full implementation.
The south-west corner of the dam (see photographs on left) was chosen for
the pilot testing as a result of the considerable erosion
experienced on the wall surface and the 70 m high wall at this
corner. Work here started in July 2002, with reshaping and placing
of the cover layer continuing until May 2003. The seeding of the
indigenous grass cocktail could begin in earnest when the 2003/2004
summer rains arrived. Monitoring and assessment of the system is
still in progress. Despite below average rainfall during the
2002/2003 summer season, the pilot still appears very impressive as
an erosion-resistant cover system.
Says Hennie Geldenhuys, Manager: Projects and Services, who is
responsible for the rehabilitation activities at Ergo, ?Tailings
material by its very nature is easily erodable. It became apparent
that the wall holding the material back would have to be shaped
using proper geometry and scientific modelling to manage erosion. To
further strengthen the structure, a 300 mm cover layer was used to
provide a competent, erosion-resistant surface. This cover layer
consists of ?armouring? material ? a 60% rock and 40% soil mixture.
While the rock provides solid, erosion-resistant protection, the
soil allows for vegetation to grow, further strengthening the
surface, and resulting in an aesthetically pleasing landscape.
?The trial at Daggafontein is groundbreaking, and although initially
more costly, there is little doubt as to the long-term
sustainability of this method.?
While various other methods were tried to stabilise and rehabilitate
the wall surface, these proved inferior when compared with the
current trial. These methods included amelioration of the tailings
material and establishment of vegetation directly in the tailings as
well as a thin soil layer to cover the tailings. Says Hennie, ?There
are cheaper, short-term solutions one could use, but these are
usually dependent on biological systems. If something like a fire or
a sustained drought were to destroy the biological system, the
long-term erosion resistance of the dam would be compromised. This
trial has proved that with the appropriate methodology and
resources, long-term sustainability is possible. This is a first in
the industry, and we are confident that it is the appropriate
solution for this particular tailings dam.?The results of the
pilot work meant the team had enough confidence to extend the system
to other parts of the dam, and good progress has subsequently been
made with the north wall.
?At only 30 m in height, and with a gentle 16 degree gradient, the
north wall required less profiling, and was dubbed the ?easy?
section of the dam. Having completed the north and the more
difficult south-west corner, the team will be in a position to
estimate the resources required to complete the rest of the dam with
reasonable confidence.
?Another important aspect of rehabilitation is stormwater
management. Our ability to control stormwater has a direct bearing
on the sustainability of the rehabilitation work, as uncontrolled
erosion would compromise the long-term rehabilitation success. The
pilot scale rehabilitation work was designed and constructed in such
a way that we could monitor two distinct possibilities for the
management of stormwater run-off.?
The first entailed managed overland flow (flat benches) without
stormwater chutes channelling stormwater down the wall. Stormwater
flows from the slope to a flat bench and then decants to the next
slope. The second involved tilted back benches with stormwater
chutes. Stormwater thus accumulates on the benches, flows towards
stormwater chutes and then is discharged down the wall in the
concrete chutes. The photographs below illustrate the two types
of stormwater management philosophies.
?After extensive work on this issue, it was established that the
more expensive stormwater control system (option 2) is essential for
the long-term integrity of the system. The problem with overland
sheet flow (option 1), is that preferential flow paths started to
develop lower down the wall. Although no serious erosion occurred
during the 2002/2003 rainy season, the drainage pattern suggested
that the cover system may be compromised in the longer term.
Sustainability of the rehabilitation system now needs to be
demonstrated by means of monitoring and modelling. Monitoring will
receive priority attention during 2004.?
The team is aware that although the trial is completed, the majority
of the work still lies ahead. Total rehabilitation is scheduled for
completion in 2006, although seasonal variations may impact on
timing.
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