The technologies implemented to help mines meet increasingly stringent water conservation regulations have led to a different contamination challenge that needs managing, writes Laura Cornish.Mining operations’ water usage and processes have evolved considerably over the past two decades. While it was common practise to use and consume clean water for all activities on-site with little thought given for downstream water pollution discharge impacts, such methodologies today are prohibited. “The 1990s saw new trends emerge aimed at reusing different qualities of water in various elements within a closed mining circuit, recycling water and reducing wastewater discharge. Capturing and reusing water within the plant process has evolved to become common practise today – placing less stress on the environment and reducing municipal clean water consumption. Despite this, the necessity for water balance modelling and managing the distribution of water within a mine has and continues to play a more important role within an operations life cycle now than ever before,” SRK partner andprincipal hydrologist Peter Shepherd points out. An evolving challenge “The downside to process water recycling is the significant rise in salt levels if the water is not diluted with clean water at regular intervals,” Shepherd explains. While salt is not considered a toxic substance, it can cause major environmental and process dilemmas. Water recycled over a 20-year period raises salt levels substantially; in some instances, SRK has measured the total dissolved salt (TDS) levels in process water dams of greater than 8 000 mg/ℓ . “To put it in perspective, the maximum allowable drinking water (Class II –SANS 241 2006) is 2400 mg/ℓ,” Shepherd notes. “Water with high concentrations of salts is unpleasant tasting and the long term use may pose a health threat to consumers. .” Salt balances developed for the Rustenburg area have seen the TDS increase from about 1 000 mg/L to about 4 000 mg/L – a result largely caused by the mines’ efforts to reuse as much water as possible. Certain mines in the area have already started budgeting for steel infrastructure replacement as a result of the corrosion water with elevated TDS can cause. Steadily increasing salt levels mean that eventually the water will need to be treated. “The impact of discharging high TDS level water into the environment is significantly more severe than less salty water but the environment is only one factor that mining houses need to consider. The impact of recycled water with a high TDS concentration can drastically affect mining process circuits as well. Plants are designed for a certain minimum water quality and will not operate optimally if salt levels rise beyond a certain point. High salt levels will also corrode most metal components in a circuit, raising operational and replacement costs.
The solutionShepherd says that treating water with a high TDS concentration is an option, but should be the last resort as the extra capital and running costs of treating heavy salt water will only add further to a mine’s overall costs – unnecessarily. “There are alternative, cost-effective, rational methods to combat this issue. Splitting types of water use based on process requirements and utilising the right qualities for the right processes will enable mines to treat less water – and only the most highly polluted,” Shepherd outlines. This will further enable mines to stop using their stormwater dams for process water storage, which pose further environmental hazards should they overflow. Considering higher rainfall is predicted to substantially increase the amount of contaminated water a mine spills into the environment, the list of reasons to revise water reuse strategies is endless. Fortunately, because water has become such an essential operational component, mines are embracing this challenge and acknowledging the need to adapt their processes accordingly, Shepherd declares. And many are turning to Shepherd, his team and SRK to find solutions to what could soon become a significant mining problem. Water balance development is a core strength and strategic business focus for consulting engineers and scientists, SRK. The company employs a range of tools to monitor and control salt levels in mine water, from a simple spreadsheet-based method that mines can employ without specialist skills to purpose-designed computer models that are more complex. Acid mine drainage (AMD) in South Africa’s mature coal fields. Shepherd acknowledges that while South Africa’s gold AMD crisis is being addressed and solutions implemented, other sources of AMD should be evaluated and prevention measures planned for. South Africa’s primary Emalahleni coal fields will ramp down over the next 20 to 50 years as the new Waterberg coal field region develops, leaving a host of defunct underground and opencast mines, as well as waste facilities. “The coal mining industry needs to come together and understand the entire coal basin as a unit to ensure that the environment is preserved when mining ends, and underground water levels are maintained at a stable level,” Shepherd mentions. Rehabilitation of these old mining areas needs to be done in such a way that surface water does not unnecessarily impact the groundwater.