Managing salt levels in mine water

Ensuring water balance models remain useful requires managing salt water levels to prevent development of brine or corrosion of circuits.

A water balance model is commonly used in the mining industry to monitor and manage the distribution of water within a mine; however, it is also vital to keep an eye on levels of salt in on-site water, especially as mine water is increasingly re-circulated in the interests of conservation. Water balance development is a core strength across all of SRK’s global offices and water balance methods have been developed for every continent.

Mines use a water balance model to establish losses and determine how much inflow is required, mainly, to replace the evaporation and seepage from tailings facilities and return water dams. As mines comply with increasingly stringent water conservation regulations, they increase the amount of water that is re-treated and retained for use in the process plant.

This trend has generally had a positive impact on the levels of discharge from mines into their surrounding environments. The downside of this process, however, is that the salt level of on-site water rises steadily when it is not diluted by proportionate amounts of clean water from outside sources.  

Under these conditions, certain dangers are introduced to the mining circuit:
• As the plant is designed for a certain minimum water quality, it may not operate optimally if salt levels impair this quality beyond a certain point
• High salt levels will corrode most metal components in a circuit, raising operational and replacement costs
• Steadily increasing salt levels will turn water into a brine that needs specialised removal from the site as a hazardous material

Salt balances developed for the Rustenburg area have seen the total dissolved salts (TDS) increase from about 1000mg/l ten years ago to about 4000mg/l, due mainly to efforts to reuse as much of the water as possible. This increase in TDS does not materially corrode the steel pipes but has been included in future budgeting and long-term replacement of steel infrastructure. Water treatment plants have been installed at Amandelbult mine to remove the salts and the water is used as potable water to reduce the mine’s reliance on the already strained potable water supply to the area.

SRK employs a range of tools to monitor and control salt levels in mine water from a simple spreadsheet-based method that a mine can employ without specialist skills, to purpose-designed computer models that are more complex and costly.

Peter Shepherd:
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