Unsaturated Flow in Heap Leach Materials

Heap leach practice is considered one of the economical approaches to recover low grade high volume gold/silver and copper mineral deposits. Most of the column studies in heap leach materials were focused on the recoveries and chemistry of the metals, little or no studies were undertaken on the unsaturated flow behavior in heap leach materials. Column testing is typically used during metallurgical evaluations where column sizes may vary in diameter from 10 cm to 1 m.  However, similar tests are not done to evaluate unsaturated flow conditions. Although there are many publications about theoretical aspects and laboratory column tests of unsaturated flow in soils, only a few studies have been done with coarse materials in columns.

Two heap leach materials, crushed (finer) supplied by Mine-1 and run of mine (coarser) supplied by Mine-2 were placed in two large cylindrical columns, 1.3 m diameter and 2 m tall with the bottom of the columns cone shaped filled with drain rock to enhance the outflow. Each column had four instrumentation ports at four different depths. Large column tests can provide useful insights in the behavior of unsaturated flow in coarse materials. Evidence of preferential flow is a reality in heap leach facilities and probably a bigger concern at higher flux application rates. Variability in water content and matric suction measurements can be a result of heterogeneity of materials surrounding the instruments as well as potential preferential flows.

Instrumentation can provide useful trends and are definitely not suitable to provide absolute values for regulatory enforcement. Comparing measured and modeled results require accurate and representative hydraulic parameters. High initial head conditions were considered for the dry material. The relative behavior of the water content and discharge flow rate curves, when flux application stops, provides useful insights in the longer term behavior following heap closure. The experience gained in investigating unsaturated flow in large columns provides insights in the unsaturated flow behavior of heap leach materials as well as the use of instruments in heap leach facilities.

Unsaturated flow in coarse materials is complex and much is left to be learned about the characterization of materials and the detailed flow conditions. Preferential flow is a reality in heap leach facilities and maybe a bigger concern at higher flux application rates. The drain down behavior for both water content and flow rate is exponential in crushed and run-of-mine materials and seems to follow the same general pattern under different flux rates.  The implication is that much can be learned from the detailed monitoring of drain down during operations that can be applied at the time of closure.

The use of instrumentation in heap leach facilities to monitor changes in water content and matric suction must be carefully considered as the results may not provide absolute values for these parameters and may result in a false sense of what “reality” is in the heaps.  Heterogeneity of materials surrounding the instruments as well as potential preferential flows can result in the measurement of variable water content and matric suction. Instrumentation can provide useful trends and it is not realistic to expect more than that.  They are definitely not suitable to provide absolute values for regulatory enforcement.

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