Properties governing flow of solution and air through crushed ore for heap leaching: Part II unsaturated dual-phase flow

Abstract

Heap leaching of low-grade ores is often restricted by the hydrology of the bed, which limits the rate of dissolution of the target mineral. Although much research has been published on the flow of solution and air through sand, little has been published on the hydrology of coarser rocks. Physical and hydraulic testing of rocks and agglomerates is often incorporated as a part of heap leach design, however, much still relies on experiential rules. Over the past decade, the lead author has performed physical and hydraulic testing of a large number of ore samples considered for heap leaching. In reviewing the data, it was found that the hydraulic conductivity function (HCF), relating the hydraulic conductivity to the degree of saturation, can be modelled with traditional capillary models such as Brooks-Corey (BC) and van Genuchten-Mualem (VGM), but only at lower moisture contents. Furthermore, it was found that the HCF can be modelled as a discontinuous J-curve, with the hydraulic conductivity equal to the saturated hydraulic conductivity (Ks) above the air-entry point, rather than a continuous J-curve with a fitted Ks value, as recommended by other authors. Previous authors have also suggested that the discontinuity represents a transition between capillary- and gravity-controlled flow. However, the author(s) propose that the discontinuity is caused rather by the presence of “dead voids”, which fill up at higher moisture contents without generating additional flow channels. The point of discontinuity was found to correspond to the air-entry value obtained independently from the air conductivity curves. The experiential rule of a maximum void saturation of 65% for heap aeration appears to correspond on average with the experimental data.