The platinum mining sector in South Africa has made “considerable” progress over the past ten years in its attempts to recycle and reuse water, with 50% to 60% of water pumped into tailings dams being returned to the plant for reuse, indicates SRK Consulting, a firm of consulting engineers and scientists.
This progress ranks the sector as one of the most successful in terms of recycling and reusing water, says principal hydrologist and partner Peter Shepherd.
Before, this figure was about 30%, and the aim is to further increase the water returned to above 60%, he tells Mining Weekly.
Shepherd says this target is being extensively pursued, as mines lose most of their water through tailings dams – either through seepage, evaporation, or water being locked up in the dam.
Mines in water-scarce South Africa are susceptible to factors such as high levels of evaporation and climate variability, says Shepherd, explaining that climate variability is a combination of heavy rainfall in a short period and subsequent long dry spells, requiring mines to implement contingency plans to ensure a consistent water supply during these wet and dry periods. Shepherd notes that 2017 bears this out, with February a significantly wet month, compared with March, in the platinum belt areas.
A balanced water supply can be achieved by creating additional water storage facilities that are used to store excess water during the wet months, after which this water can be used as a buffer in the dry months.
Large mines with concentrators, shafts and pits may require the storage of a buffer water supply of between 500 000 m3 and 1.5-million cubic metres. “Sizing dams accordingly is, therefore, important and makes the field of hydrology exciting,” enthuses Shepherd.
He notes that there has been an increase in the amount of runoff water captured on site by mines in the past, with the initial drive being prompted by legislation, such as the National Water Act, No 36 of 1998. Shepherd points specifically to Regulation 704, which includes conditions pertaining to the separation of ‘clean’ and ‘dirty’ water systems, ensuring that mine infrastructure is located outside floodlines, the choice and use of mine-related material, as well as pollution control.
He expounds that clean water runoff from areas upstream to the mine site has to be diverted away from mining infrastructure, whereas dirty water emanating from the mine site must be collected and reused on site.
Shepherd avers that mines do not want buffer dams that are empty most of the time and fill up with water only every ten years or so. Rather, mines would ideally like to meet their water needs and decrease the associated cost by capturing, reusing and recycling as much water as possible and reducing their reliance on supply from water service providers.
The resultant reduction in demand for municipal water supply by mines will further benefit the communities and other industries that surround these operations by freeing up capacity to meet the water consumption needs of these communities and industries, especially those without the potential to recycle water like mines.
Shepherd avers that SRK has been involved in developing water management methods aimed at improving water reuse in South Africa since before the introduction of the country’s legislative requirements in 1998. The methods it has recommended and features it has installed to collect and treat water, inclusive of groundwater barriers and collection facilities, can be used to reduce the area allocated to tailings storage facilities and decrease evaporation, and also comprise technology for the production of thickened, or drier, tailings.
Shepherd says advanced thickener technology is gaining traction in South Africa, although this is hindered somewhat by its high cost. This technology reduces the amount of water lost to tailings dams, as it focuses on producing drier tailings material before it leaves the plant for storage.
In terms of preventing water loss during tailings storage, owing to seepage, dams may in the future be lined to create a groundwater barrier, while directional drilling is another valuable strategy employed, says Shepherd. By drilling a U-shaped hole, rather than a vertical one along the front of the tailings dam, a greater area is covered, reducing seepage from tailings dams and pollution of the surrounding environment.
To mitigate the amount of water lost to evaporation, it is important to reduce the area of a dam exposed to sunlight by creating deeper dams, or covering dams with plastic covers. Shepherd states that this could result in mines saving between one-million and two-million cubic metres of water a year.
“An innovative solution applied abroad to limit evaporation involves floating solar panels on effluent dams.” However, he expects that the high cost of this innovation, as well as the likelihood of theft, will limit its uptake in South Africa.
Further water management methods advocated by SRK involve mines compartmentalising return water dams, limiting the area exposed to the sun, thus, limiting evaporation and allowing for differing water qualities to be stored in the various compartments, providing the necessary versatility to implement a water hierarchy.
Such a hierarchy, an important strategy for mines, entails allocating the lowest possible water quality required to facilitate water-consuming activities such as plant processes and irrigation. This ensures that treated water of a higher quality is reserved for process requiring potable water.