9.4 Spent Heap Leach

Development of a closure plan for a heap leach facility involves consideration of the unsaturated hydraulic properties of the spent ore materials, as this data is key to prediction of the time frame over which residual leach solutions will be released at the base of the pile. This drain down time of the leach solution often determines the length of time that active management of the pile is required. Estimates of this time period can be derived using simulation models of saturated – unsaturated flow. This presents a challenging application of unsaturated flow concepts because of the size of the system to be considered and the heterogeneous nature of the spent ore in the leach pad. Attention is also directed to the long-term release of other potential contaminants of concern from the leached ore in the pile, as this can also determine the long-term management requirements for the heap leach. Soil covers to reduce infiltration of precipitation through the spent heap leach, and to isolate the spent ore from the terrestrial environment, are also considered as part of the closure process. Cover concepts discussed in Section 9.3 are applicable here. The ultimate requirement in closing heap leach facilities is a demonstration that the strategies adopted for closure are protective of the underlying groundwater resource over the long term.