Heap leach pads are continuously irrigated with raffinate solution during their operational life; typically cyanide solution for gold ore, and sulfuric acid for base metal leaching such as copper or nickel.
At the end of the active leach operations, once the volumes or grades of recoverable solution diminish, there remains the problem of long-term draindown of a solution that is unsuitable for discharge to the environment, and therefore requires containment and management. Reducing the period of active management and fast-tracking closure is therefore of clear benefit to the operator.
In a recent study of a leach pad in an arid setting, a multi-disciplinary team from SRK Vancouver and SRK Cardiff developed a complex unsaturated flow model to evaluate the length of the active closure period required to reduce the solution inventory. The earlyperiod draindown volumes were too large to be contained in the existing raffinate ponds. Unable to discharge the water, the best closure option was to re-circulate the solution to the top of the pad – but this time the aim was not to leach gold, but to utilise the arid climate to reduce the solution inventory. As the solution inventory declined, the draindown volume would reduce to a point where it could be managed within passive evaporation cells. The arid climate, which had previously been an operational constraint, would now be a benefit - allowing the leach pad to close with no long-term discharge.
Numerical modelling was used first to assess and then to optimise the closure options while accounting for the inherent uncertainties of climate and heterogeneity of hydraulic behaviour. Geochemical modelling of the recirculated solutions was used to assess the long-term chemistry and potential corrosive and scaling properties.
Using this approach, SRK was able to provide a range of options to shorten the period of active management and achieve zero discharge at closure, as well as provide the client with the basis for assessing which option would be most cost-effective to implement.
