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Heap Leach Pad Monitoring To Manage Risks In Gold And Copper Mining

by

G2 Imaging

G2 Imaging installed an electronic leak detection system (ELDST) under a heap leach pad at a gold mine located in Northern Nevada. The system consisted of 147 stainless steel electrodes placed in a rectangular grid within the subgrade of the pad. Two PVC pipes were also installed in the subgrade to introduce measured amounts of salt solution for system calibration.

The ELDST array contained a total of 215 data points providing coverage for approximately 85% of the leach pad area. Monitoring was conducted biweekly using an Iris Instruments Syscal Junior resistivity meter. ELDST software downloaded data from the receiver, and translated the values to produce contour and three-dimensional surface plots of the data.

The plots represented the difference between the current data set and baseline values. Baseline values were collected from the grid prior to the introduction of leachate solution, and during system calibration. The baseline data is adjusted for periodic, seasonal, and yearly fluctuations in resistivity values normally experienced by the system.

Data sets collected during routine monitoring are added to the baseline data after calculation of the current offset. If the offset file shows a large magnitude change from baseline conditions, it will appear as a spike which alerts the operator to the presence and location of the leak.

THE SOLUTION

At the example heap leach pad, a geosynthetic liner was found to be leaking shortly after startup. Up to 5 gallons per minute of leachate solution were produced at a physical leak observation port shortly after the introduction of solution started. ELDST was used to determine the position of the leak, and verify the effectiveness of the remediation activities performed on the liner.

After ore placement and leaching began, the ELDST revealed four separate, significant leaks, each of which alone would have been large enough to initiate facility shutdown. These leaks located at a downstream pipe penetration, a perimeter solution conveyance ditch, and two interior liner rips. The first data sets collected by ELDST located the suspected leak, and the area was excavated by hand. Liner tears and pinholes where located and repaired, and the pad was re-buried.

When the pad was wetted again, solution shows were once again present in the physical leak detection ports. The ELDST also indicated that leakage was still present, adjacent to the previous leaks. The volumes of leakage produced at the leak observation ports were somewhat less than prior to the repairs, but still represented a total volume which was in excess of permit limits.

Once again the pad was excavated, and additional leaks were found and repaired. Response of the ELDST to the gradual reduction of solution in the subgrade was predictable, with the magnitude of the conductivity anomaly decreasing through time. The physical leak detection system showed gradually decreasing flows as the subgrade de-watered. The ELDST continued to be viable for monitoring of the pad over the active life and during the closure period.

SUMMARY

With a history of nine years successful service in the heap leach sector of the mining industry, ELDST has proven to be reliable, cost effective, and accurate for the detection and location of leaks within the sensor network. Over six million square feet of liner are currently monitored by ELDST. It is a tool for monitoring the entire area under a liner, not a point specific method like lysimeters or monitoring wells.