Acidic pit lake
Naturally oxidised sulphide minerals
Acid rock drainage
Acid Rock Drainage (ARD) is a notable environmental concern in the mining industry. ARD occurs when sulphide-bearing rocks are exposed to air and water. Mining activity disturbs these rocks and exposes the sulphide minerals in them to oxygen (O2) in the air and water (H2O), which react and form reaction products. These reaction products are easily dissolved and can form an acidic solution (H2SO4) characteristic of ARD. Acid Rock Drainage is strongly associated with the presence of heavy metals, and hence the environmental risks are significant should ARD not be contained.
ARD can be managed either by stopping the oxidation process from occurring (breaking the contact between sulphides and oxygen) or by treating the acidic water once it has been formed. At Geita Gold Mine (GGM) in Tanzania, a combination of both management techniques is being used.
For the former, potentially acid forming material is encapsulated within non-acid forming material. Cut off drains and containment dams are used to prevent the acidic water exiting the mine lease area. In some cases where testing shows leachate and run-off to have unacceptable levels of contaminants, water is also treated before being discharged from site.
Rebecca Stephen, Environmental Superintendent at Geita Mine says, GGM is always striving towards good environmental practices, and addressing Acid Rock Drainage is one of them.
The mining activities at Geita involve a series of procedures to ensure that ore and waste materials are mined and transported to their final destinations in a controlled manner. ARD management at GGM is mostly done through proper identification and handling of potentially acid-forming waste material from all five operating pits. The procedure for ARD management involves:
However, even with the existing ARD management practices, ARD has occurred from reactive PAF material at Kukuluma and Matandani areas at Geita, (located approximately 20 kilometres from the plant site) inside the pits and at the run of mine pad (ore transfer station). Acidic water seepage from the main waste rock dump has added to this challenge even though it is contained within that area.
Acidic pit lakes currently exist in these two pits which are very close to each other, with an estimated amount of about 500,000m3 of water at Kukuluma and 40,000m3 of water at Matandani. The area is temporarily closed due to low recovery of ore material, explains Stephen.
Other factors which can exacerbate the problem, adds Joseph Chihota, Environmental Manager: Africa Region, are the over-reliance on the visual identification of NAF and PAF, and a high turnover of employees in the environmental department, sometimes resulting in relaxed monitoring of processes.
Chihota adds that non-reliance on visual identification of NAF and PAF but rather on laboratory tests is preferable. Field tests using hydrogen peroxide to supplement visual methodology of NAF and PAF will also be effective. In addition, further investigation using kinetic tests will be used to establish the most effective cover and waste sampling methodology that will minimise longer term assets.
Stephen explains that the way forward with the ARD challenge has began with the construction of containment bunds to direct storm water around the dumps into settling ponds, where quality can be monitored, and treated if need be, before the water is released back into the environment.
Increased capping of waste dumps with compacted laterite material will minimise infiltration of water through the dump, thus minimising ARD formation.
A viable permanent solution for acidic pit lakes is yet to be found, concludes Stephen.
ANGLOGOLD ASHANTI Report to Society 2008