Mines are recycling more water than ever before. This is a logical response to water stresses (see our article on Water Balances). However, there is a risk associated with the good stewardship of water. Deleterious elements and compounds can concentrate in recycled mine water as it is recycled again and again, and these deleterious elements can decrease your recovery. Levay & Smart, 2001 observed the following:
For instance, in one case study, it was established using the integrated methodology that very high surface coverages of sulphide minerals were occurring due to precipitation of zinc hydroxide (with some copper(II) hydroxide) at pH>8.
Many metallurgists rightfully obsess over their process chemistry – paying close attention to reagents, pH and other critical factors. In a typical flotation system, water is 65-75% of the total mass flowing through the circuit. Therefore, is it not also prudent to know what is in your water?
As mentioned above, many mines are required to consume or re-use mine contact water (Gunson, 2013), (Norgate & Lovel, 2004). This mine contact water is often overflow from a tailing thickener, concentrate filtrate, or water reclaimed from a tailings pond. But it can also be leachate and runoff from acid-generating mine waste, or low-pH water pumped from a pit or underground workings. This water is nearly always mixed together in a holding pond or a tank. But what happens next could be a hidden risk to your process circuit.
How different water streams with different chemistries react is critically important, for example, if the pH increases, one can form precipitates (principal of which are sludge-like iron ferrohydroxides) or one can precipitate calcium sulfate compounds. These nuisance precipitates can scale up a system, slow down processing, reduce recovery, and can cause problems throughout the flowsheet. As mentioned previously, zinc and copper precipitates can cover the surfaces of sulfides rendering them unresponsive to flotation.
The solution to this problem is to understand the chemical impacts of your water feed, water recycling, water consumption and water usage plan. This is done with a detailed and fully integrated water balance and geochemical analysis that quantifies all sources of mine water and defines the mixing ratio between contact water, recycled water, and fresh water. This analysis produces detailed time-variant (seasonally adjusted) mixing ratios between different water sources around the site. Furthermore, it can ascertain the effects of evapo-concentration occurring in holding ponds.
A simple ratio balance between sources of water is not enough. In aqueous chemistry, mixing water with 1000 mg/L of sulfate and 100 mg/L of sulfate in equal quantities does not always equal a solution with 550 mg/L of sulfate. Equilibrium reactions, precipitation reactions, and other chemical interactions make simple mixing calculations inaccurate.
Modern mines require a site-wide water balance that is integrated with a geochemical model to determine the quality of the resultant water created by mixing many different sources of water together. These efforts do not need to be exhaustive studies, and can be completed relatively quickly. Recycled water can be a source of significant process issues and many times water is overlooked as the cause. If you are operating or planning to develop a process plant, are you paying enough attention to your water?
References
Gunson, A. J. (2013). Quantifying, Reducing and Improving Mine Water Use. Vancouver, BC, Canada: University of British Colombia, pHd Thesis.
Levay, G., & Smart, R. a. (2001). The Impact of Water Quality on Flotation Performance. Journal of the South African Institute of Mining and Metallurgy, 70-75.
Norgate, T., & Lovel, R. (2004). Water Use in Metal Production: A Life-Cycle Perspective. Commonwealth Scientific and Industrial Research Organisation, DMR-2505.