CHARACTERIZATION OF UNSATURATED FLOW IN HEAP LEACH PILES

Over the past 40-50 years, heap leaching has become the dominant method for extraction of gold from low grade hydrothermal ore deposits, such as those found in the Carlin Trend of north-central Nevada. The chemical reactions that extract the gold are well understood, as is flow of the reactive liquid (lixiviant) within individual pieces of ore (~1 - 3 cm diameter). Conversely, we know much less about how the lixiviant moves through the leach pile itself, which may cover several acres and be up to 100 m in thickness. Flow through leach piles is a complex unsaturated flow problem that involves highly heterogeneous media. We expect that some portions of the pile will experience rapid flow along narrow pathways, while in other areas, flow is blocked by accumulations of fine material. Both of these scenarios are undesirable from an economic standpoint.

Here we present an intermediate-scale experiment designed to better understand flow processes within leach piles. The first step is to consider flow through a relatively homogenous system. As an analog for ore, crushed sandstone with a nominal diameter of 2 cm is washed to remove fine materials and surface contaminants. After drying, the crushed sandstone is packed into an 20 cm diameter column, 100 cm in height. A steady supply of water is applied to the top of the column from a point source located at the center. Fluid is at 6 ml/minute, a rate comparable to that used on leach piles operating in the western US. The bottom of the column is segmented into nine basins of equal area. Outflow from each basin is measured as a function of time in order to identify heterogeneous flow paths within the column. Flow measurements are supported with tracer experiments in which a conservative tracer (NaCl) is used to simulate the migration of gold-bearing lixiviant.