Gauteng Business News

Send  Share  RSS  Twitter  30 Apr 2010

MINING: Innovative Afrox Response to AMD Challenge


Recent Gauteng Business News

Afrox has responded to the much-publicised challenge of Acid Mine Drainage (AMD) with an innovative process making it possible to harness this potentially large source of contaminated surface and underground water for commercial use.

The process is essentially the oxidation of the ferrous iron to ferric iron followed by pH adjustment resulting in the precipitation of the iron as a brown iron hydroxide sludge. Once the water is clarified it can be economically reused in the gold plant leach circuit thereby making a substantial saving on cyanide and Rand Water usage.

In 2007 a West Rand gold mine commenced operation of a water treatment plant to treat AMD water arising from both underground and surface runoff, which is currently stored on the mine with oxygen supply from Afrox.

AMD has long been a problem associated with gold mining operations — since mining commenced in the Witwatersrand in the 1880s. On the surface, AMD-contaminated water pools are usually located on the footprint of tailings dams and rock dumps and can easily be identified by their vivid dark red colourations.

This water is highly acidic and may have a pH as low as 2, making it environmentally unsafe. AMD is formed when water becomes contaminated with pyrite (FeS2), a mineral which is associated with the gold bearing ore in the Witwatesrand gold mines. The pyrite which is exposed to the atmosphere oxidises and in the presence of bacteria the sulphides are converted to sulphuric acid (H2S04).

Surface water runoff from gold mining tailings dams and rock dumps are a major source of AMD. Some of these tailings storage facilities, built in the early gold mining days, are found in areas with underlying dolomite, where the acidic waters result in the formation of sinkholes and contaminate the ground water. Currently, an average of 15 ML (15 million litres) per day of acidic and iron-rich water decants into the environment from the Western Witwatersrand Basin alone.

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