Mega tailings dam under threatSep 25 2011 11:51
Johannesburg - If you need to travel through Kuma, the township outside Stilfontein, on a windy winter’s day you will probably have to stop periodically, waiting for the blustering wind to abate somewhat, as the dust in the air makes it impossible to continue driving.
The situation is caused by 15 old tailings dams in and around Stilfontein, all relics from the old Buffelsfontein and Hartbeesfontein gold mines, whose surfaces become so dry in the winter months that Kuma residents live in a perpetual uranium-bearing dust cloud. The situation in Stilfontein itself is slightly better, but not much.
The Kareerand mega tailings dam being built by First Uranium 15km outside Stilfontein will resolve the problem, but is in danger of being kyboshed – ironically enough because of opposition from environmental groups, pressure groups and unreasonable demands from landowners.
From an engineering viewpoint the mega tailings dam is a pioneering project that would make a dramatic difference to the quality of air and water in the mine-polluted area.
The techniques being applied to the design and building of the enormous tailings dam, which will hold 450 tonnes of processed mine tailings in the first stage of its lifetime, have never been used elsewhere.
The walls of tailings dams are always built at a 45-degree slope, but the Kareerand Dam’s slope is 15 degrees. This is to reduce the wind resistance on the wall. The final wall is also being constructed with scalloped inlets to retain the rainwater that runs down and thus prevent erosion, said Fraser Alexander engineer Beric Robinson, who designed the tailings dam. He is regarded as a worldwide expert in tailing dam design.
“We try to imitate nature, but managing to do so is not a daydream. The techniques being used are based on comprehensive analysis and years of observing tailings dams,” said Robinson.
“The challenge to the question as to whether the dam will improve current conditions is not scientific. Rather, it is an economic challenge to construct the tailings dam and operate it economically.”
In building the dam, surface soil from the 2km terrain was collected in an outside weir right around the dam. When the dam wall has finally set, this will be pushed up and positioned against the final wall.
Unlike other tailings dams, Kareerand’s drainage system has a discharge point right in the middle of the dam, which feeds water to smaller supplementary dams beside the big dam, for recirculation from those points. The water will then be used in a sophisticated water irrigation system.
This system, which consists of different batteries of sprinklers, will be controlled by a wind-monitoring system so that dust is spread across the tailings dam when the wind speed reaches particular levels.
The old tailings are first fed through a gold and uranium plant which was built in Stilfontein at a cost of about R3.2bn over the past three years. It involves the extraction of 57% gold and 33% uranium from the tailings.
Dumping on the Kareerand mega dam will put an end to the residual leaching of uranium, salts and sulphates into the Vaal River. In time, when the uranium plant is in full production, it will also remove the sulphates from the mine tailings.
The Kareerand tailings dam will not permanently solve the problem, but assumptions developed from scientific models indicate that it will take about 200 years before contaminated water from the Kareerand Dam will reach the Vaal River.
Geological changes and the quality of underground water will be continuously monitored through boreholes around the Kareerand Dam. In future years the findings will be continuously tested in line with projections developed from scientific models to predict the future soil and water quality in the area around the dam. If ecological weathering veers outside these criteria, steps will be taken to rectify the situation.
It took two years of research before a decision was reached to build the mega tailings dam on the current site. Seven different sites were considered in collaboration with the Department of Water Affairs before the decision was taken for the current site some 15km south-east of Stilfontein.
First Uranium preferred a site 2km to the north-east of Stilfontein, but the Department of Water Affairs turned this down because it was, like the other 15 old tailings dams, over a dolomite formation. It's precisely this porous dolomite that results in water containing uranium, heavy metals and sulphates filters down from the tailings dam and into the groundwater and ultimately into the nearby Vaal River.
The choice of terrain pushed up the project costs by R200m. The pipes to convey the tailings along the increase distances cost an additional R154m and the pipes constructed around the tailings dam on the new site cost another R200m-odd.
The total cost of building the dam is around R400m.