Research makes inroads in dealing with acid mine drainage

National research and development initiator, the Water Research Commission (WRC), is funding and undertaking a number of projects to provide solutions for dealing with acid mine drainage (AMD) in South Africa.

AMD mainly manifests after a mine has closed its operations, although AMD cab also be produced during operation. AMD is produced by the oxidation of sulphide minerals in mine orebodies, such as pyrite, which are exposed in a mine or are present in dust in underground shafts and tunnels. Because of its acidity, AMD dissolves rock material, and may contain a range of toxic metals. Water can thus become saline when AMD is neutralised through its reactions with rocks. Mixing with other water resources contaminates underground or surface waters.

Neighbouring mines are often hydrologically interconnected by underground tunnels, with the result that when mine operations are curtailed or a mine closes down, the recovering water table causes water to flow between mines and exit onto the surface at the mine shaft with the lowest elevation. When the water reaches the surface, it pollutes surface waters, which is detrimental to long-term environment stability. AMD and its effects are often not visible while mines are actively operating because the acid water is treated by mines as part of normal operations to keep the mine dry. The problem of AMD becomes more evident when mines close down, or stop operating.

WRC research manager Meiring du Plessis says that AMD is a significant problem worldwide and, in attempts to manage the long-term environmental hazards of AMD, the WRC has over the years dedicated a number of research projects dealing with this and other mine related problems, covering aspects of regulation, technology and management solutions.

A number of projects aimed at analysing different methods of preventing AMD are ongoing and some have been recently completed. Outputs from this research portfolio are already making an impact in this problem area, he says.

In continuation of ongoing research, two new projects will be initiated this year. One of these will assess how water quality and quantity at the Waterberg coalfields will be affected by the mining methods used. The project will evaluate a number of scenarios of mining methods for guidance to the mines on the best mining methods to reduce negative impacts on mine water quality and quantity.

The second project will investigate the management of sulphidic waste rock and tailings through accelerated oxidation and selective handling as a means to deal with AMD during the life-of-mine.

An example of management options aimed at preventing AMD, is to use soil covers to reduce the ingress of water and air into the waste dumps, thereby reducing the quantity and improving the quality of water in gold tailings and coal discard dumps. While research proved this approach to be effective on a small and experimental scale, its long-term effectiveness cannot yet be ascertained owing to a number of constraints.

These constraints include natural erosion, which reduces cover thickness and decreases effectiveness, as well as steep side slopes which exacerbate the problem by increasing the difficulty to maintain soil covers.

Additionally, ongoing studies consider the water behaviour and surface activities of tailings and rock dumps. A first order decision support system (DSS) that was developed for the sustainable design, operations and closure of metalliferous tailings disposal facilities is now being refined. Knowledge gaps identified during the development of the first order DSS, and new technologies to provide guidance to mines and national regulators on the options for planning, maintaining and closing tailings facilities are being assessed.

In the Mpumalanga coalfields, an ongoing project is modelling the impacts that generated AMD will have on a wide scale disturbance of land and on the natural hydrology, as well as on the quality and quantity of surface and ground water resources. The study is also investigating the cost-effectiveness of implementing management alternatives (such as soil covers) aimed at reducing AMD formation. Preliminary results indicate that treating the AMD polluted water to potable standards is often the more cost-effective option.

One of the costliest components associated with the treatment of AMD is the disposal of the brine generated through reverse osmosis and other treatment processes. A number of methods to deal with brine are currently under investigation. One such method uses freezing to obtain fresh water from brine, which is then separated into a salty solution and ice crystals made up of clean water.

Du Plessis says that an advantage of this technology is that seven times less energy is required to freeze than to evaporate water. Present indications show that, while still significant, cost savings in energy use will not be as pronounced.

A special application of this concept, called eutectic freeze crystallisation, involves cooling the solution to the specific temperature where water simultaneously turns into ice and salt precipitate from the brine, so that both water and salt can be recovered at the same time. This research by the University of Cape Town is being conducted in partnership with the Delft University in Holland.

Brine generated from AMD, can also be used in aquaculture for fish, shrimp and beta-carotene production. Also investigated is wind-aided intensified evaporation that results in accelerated evaporation from storage ponds and a reduced volume of brine. The Dewvaporation process uses a humidification/dehumidification cycle to concentrate brine and condensate pure water in a heat exchange tower consisting of evaporating and dew forming sides. As the temperature of air rising on the evaporation side increases, the temperature evaporates water from brine running down the side of the heat exchanger, and the brine becomes more concentrated. As the air sinks down on the dew forming side it is cooled by the heat exchanger and clean water condensates.

Other treatment options that have been researched and that are in use today are the neutralisation of AMD with limestone (rather than the more costly lime), removing sulphates and neutralising AMD with the Rhodes BioSure process, using sewage as an energy source, and its low maintenance counterpart, the patented degrading packed bed reactor technology. It has also been demonstrated that neutralised AMD consisting mainly of calcium and sulphate, can be used for the irrigation of crops.

The WRC has investigated two complementary approaches for developing long-term strategies, which are geochemical modelling and acid-base accounting methods, to predict whether AMD would be formed at specific locations.

Geochemical modelling, required for developing long-term strategies for AMD, uses computer programmes to predict how different constituents of orebodies will react chemically with one another and to link that with water flow paths to predict concentrations and quantities of different chemical components over time. For example, while AMD formed early during the life cycle of a mine may be neutralised owing to the availability of neutralising rock, the rock's neutralising effects can weaken over the long term, causing acidic solutions to become more prominent.

Acid-base accounting methods, on the other hand, simulate the long-term reactions rocks would undergo using accelerated laboratory tests. Some are designed to provide only the long-term end result, whereas others also simulate expected change over time.

Research by the WRC on "arguably the biggest environmental problem created by our mining industry, is thus providing solutions which will help to reduce the impact AMD has on our precious water resources," says du Plessis.

The WRC, which operates in terms of the Water Research Act, has a mandate to support water research and development, as well as to build a sustainable water research capacity in South Africa. The WRC serves as the country's water knowledge hub for the creation, dissemination and application of water-centered knowledge, focusing on water resource management, water-linked ecosystems, water use and waste management, as well as water use in agriculture.