The Effect of Initial Irrigation Conditions on Heap Leaching Efficiency

Ángel D. Briseño Arellano1, Michael Milczarek2, Tzung-mow Yao2, Mark L. Brusseau1,3, Dale Rucker4, Kenneth C. Carroll5

1Department of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Arizona

2Geosystems Analysis Inc., Tucson, AZ

3Department of Soil, Water and Environmental Science, The University of Arizona, Tucson, AZ

4Hydrogeophysics Inc, Tucson, AZ

5Department of Plant and Environmental Sciences, New Mexico State University

Heap leaching is an unsaturated flow metal recovery process, in which mined ore is irrigated with a lixiviant to dissolve metal contained in the ore. The metal is then extracted from solution. Heterogeneities within the stacked ore can lead to uneven wetting and the formation of preferential flow pathways, which reduces solution contact and lowers metal recovery. Many mine operators believe that slow initial irrigation rates help minimize permeability loss and increase metal recovery rates, but it has not been studied in detail. Experiments were conducted with three different initial irrigation rates in large columns (1.5 m high, 0.5 m in diameter) packed with crushed ore samples that are known to have permeability constraints. Columns were monitored to assess changes in physical and hydraulic properties spatially and temporally. Water content was measured capacitance soil moisture sensors at 9 depths; a neutron probe to periodically log every 30 cm from four different directions; and electrical resistivity sensors to create a 2-dimensional tomography profile of water content over time. A non-reactive tracer was used to characterize advective-dispersive transport under unsaturated conditions. A dye solution was introduced at the end of each experiment to map preferential pathways.