Although the technology to treat and purify mine water to acceptable standards already exists, the main challenge for the mining industry and society is affordability, says water and environmental consulting firm GCS regional director and senior hydrogeologist Pieter Labuschagne.
Currently, the operational cost and capital requirement for mine water treatment technology are generally very high. In addition, the waste products generated by conventional reverse osmosis require strict handling and deposition which, in turn, require further capital and resources.
Labuschagne says the duration of required treatment, as well the high electricity costs associated with this, further increases costs. However, he notes that the technology has improved substantially in the past few years and that its electricity use is significantly lower.
Meanwhile, legislative requirements surrounding mine water treatment and purification are time- and cost-intensive, he notes.
“For example, an integrated water-use licence for a small- to medium-sized water treatment project at a mine site can cost as much as R1-million, if all specialists and engineering studies are taken into account. The timeframe for approval can be as long as two years, depending on the complexity of the site.”
The easy implementation of mine water treatment and purification are also hampered by, for example, remote sites in South Africa being prone to vandalism and theft, with inconsistent guidance and leadership from provincial and water catchment authorities also a considerable concern, he tells Mining Weekly.
In some cases, other issues can prevent water treatment from being implemented optimally.
“For example, for low levels of contamination, passive treatment application like wetland systems were tested at various sites. However, this has been restricted, owing to veld fires occurring in winter months, below-freezing temperatures, stagnant systems and water quality fluctuations,” Labuschagne explains.
Apart from suspended solids, the principal contamination from most mines is caused by the oxidation of pyrite-bearing minerals. This results in ferrous iron, sulphate and acidity, which are all water soluble and need to be treated.
Other metals, such as zinc, manganese, aluminium cadmium, arsenic and copper, can be released into the solution.
While significant contamination can be generated during active mining, it tends to peak after mine closure, as accumulated oxidation products are flushed out of the rock mass upon flooding the mine. These then decline over the years to a residual value, which Labuschagne says typically remains “stubbornly higher” than the premining concentrations.
“While the flushing period can be relatively short, the long-term legacy may need treatment for decades to come,” he notes.
Because mine-impacted water and the management thereof continues long after production has stopped, it is essential that a sustainable solution for mine-impacted water management is found.
Various factors, including life-cycle costs, feed-water quality and quantity, target-water quality, waste generation, environmental aspects, implementation risks, regulatory approval aspects and buy-in by interested and affected parties, all influence the selection of the most appropriate technology.
Currently, technology selections are often based on capital costs and existing systems, Labuschagne says.
“People feel more confident in implementing processes that have been tested and are running in similar applications. However, every site is unique,” he notes.
GCS is involved in various projects where mine drainage treatment is tested.
Some of these projects include the use of bioengineered tree plantations, also known as phytoremediation.
This technique, for example, is being used to control recharge into old rehabilitated opencast mine pits and seepage from gold tailings storage facilities.
Another option currently being tested at a gold mine tailings storage facility and coal mines is biological sulphate reduction to develop subsurface bio-barriers to manage sulphate plumes.
GCS was involved in a project for the Water Research Commission where various applications of pit lakes were verified as a sustainable long-term solution for mine closure.Labuschagne notes that, for the pit lake solution to be sustainable, scientific hydrochemical, water balance and hydrogeological assessments are required.
GCS has also been involved in conventional pump and treat facilities, where large well fields were established to intercept contaminated groundwater. This water is then reused in operations and treated after closure.
Labuschagne says that GCS has also been looking into assessing the artificial recharge of treated groundwater back into adjacent aquifer systems to develop a hydraulic barrier to protect adjacent aquifer systems.
“There are also various ways of stabilising mine waste during construction, operations and after closure to minimise contamination seepage,” he explains.
One of these techniques includes waste stream management – where material with a high sulphide content is encapsulated by liners, while waste with less sulphide material is dumped on unlined facilities. Another technique is the co-disposal of waste of different grain sizes to establish less permeable, more stable waste bodies.