Acid mine water pollution is affecting communities surrounding disused mines in South Africa. Efforts that are being taken to deal with the problems by relevant organisations are agonisingly short. African Mining Brief finds out from Karen King, Senior Associate, WSP, Environment & Energy, Africa, how the problem can be managed effectively.
AMB: What is the scale of the problem of acid mine water pollution in South Africa?
KK: South Africa’s acid mine drainage (AMD) problem is a serious, long-term environmental problem, previously termed, ‘the single most significant threat to South Africa’s environment’. The problem is centred around the Witwatersrand old mining area, and considerable AMD damage has occurred in the Wonderfonteinspruit, Tweelopiespruit, Tudor Dam and Robinson Lake areas. Depending on the area in which the AMD is occurring, the water may contain high levels of salts, sulphate, iron, aluminium, cadmium, cobalt and radioactive elements. Currently millions of litres of AMD are still flowing into streams connected to both the Vaal and Crocodile Rivers and groundwater systems. This can have and has had devastating consequences for communities and the environment.
KK: Apparently, what would it take to address the problem effectively?
AMB: Measures aimed at AMD migration need to be concerned with its transport medium; water. Surface water flowing toward the pollution sources should be diverted, water seepage and groundwater infiltration into the site should be prevented and acid-generating waste placement needs to be controlled. Recognition of the fact that polluted water can be a resource and not a liability is also needed.
There is no short-term solution to the AMD issue. Considering the existing pollution, the AMD needs to be pumped out of the mining areas and treated, but this exercise is often started and not continued, and is often so expensive that it could bankrupt some mines. Awareness of the extent of the problem is good in South Africa, and this needs to continue. Increased pressure needs to be placed on the government to address the issue, and adequate responsibility and funding for AMD treatment need to be assigned to the relevant mining houses and government departments. This is often difficult as the older AMD-generating mined have been closed for many years.
Considering the potential for additional AMD generation, The Inter Ministerial Committee on Acid Mine Drainage (IMC) has been established. The mines need to stop further seepage and polluted water leaving their properties. Many mines have, however, been poorly managed in the past, and environmental regulations under the National Water Act 54 of 1956 were not strict enough to adequately protect the environment previously. While abandoned mines are now controlled under the Water Amendment Act 58 of 1997 and South Africa’s current environmental policies centred on mining and polluted water are far better, these need to be far more strictly enforced. Mine closure plans need to adequately account for AMD prevention solutions and make adequate financial provision for such solutions, and new mines should not be allowed to open without such closure plans. Solutions such as flooding of potential AMD-generating areas before oxidation can occur are being considered.
AMB: As a reputable and extensively experienced EPCM, what would you advise as the sustainable way to address the problem?
KK: The best solution for treating AMD will vary depending on the extent and specific make-up of the AMD, its volume, the geographical area in which the problem occurs, the finances available to treat the problem and where the liability lies.
The most commonly used commercial process for treating AMD is lime neutralisation, which involves lime precipitation in a high-density sludge process, whereby lime is added to AMD to increase the pH to about 9. At this pH, most toxic metals become insoluble and precipitate. Air may be added to oxidize iron and manganese and assist in their precipitation. A number of variations on this process exist. A simpler version of this process, such as lime neutralisation, can be used, and is far less costly to build, but less efficient. This would be more suitable to small flows or less complex AMD.
Using similar chemical principles, calcium silicate neutralisation, carbonate neutralisation and precipitation of metal sulphides can also be used to treat AMD. A South African company has patented an ion-exchange process that treats AMD economically and constructed wetlands have been successfully used to treat AMD in South Africa.
AMB: What do you consider when constructing a facility to treat acid mine water efficiently, from design to commissioning?
KK: One needs to consider the potential AMD decant volumes and specific chemistry, pre-treatment including removal of certain particulates, the best methodology to select for the specific type/types of AMD that the facility will be treating, maintenance of the facility, financing of the facility, the training of the personnel tasked with managing the facility and the water quality monitoring programme for the water pre- and post-treatment. The standard to which the water needs to be treated should also be considered, as this will affect the design and pricing of the facility; this may to a potable, industrial or agricultural standard.
AMB: What are possible causes of the failure of acid mine treatment plants?
KK: Possible causes of failure include planning failures, such as selection of the incorrect methodology to treat the specific AMD problem or inadequate pricing or financing schemes, as well as mechanical failures, such as pipes used for transportation of treated acid mine water to aerators and settling basins becoming clogged, aeration equipment failure as treated mine water can contain calcium sulphate in a supersaturated solution, as well as large quantities of ferrous and ferric hydroxide and some aluminium. Aeration equipment that treats metallic ores may not supply sufficient aeration and the components of the system need to be cleaned frequently. The impellers of an aerator tank pump and the water decant system can become clogged when calcium sulphate levels are high.