Case Study – Secondary Metal Recovery in Nevada

In this metallurgical and geophysical case study, we present a comprehensive monitoring program in support of subsurface injection was conducted on a gold heap leach pad comprised of crushed ore. The injections were part of a secondary metal recovery project, whereby barren cyanide solution was pumped into isolated zones within wells distributed across the pad. The scope of HGI’s monitoring included metallurgical sampling and analysis, hydrologic analysis, and geophysical support to quantify the spatial coverage of solution during the injections.

HGI has years of experience in helping to manage, monitor, and evaluate injections for secondary metal recovery on heap leach pads. We used our expertise on a gold mine in Nevada to optimize the injections in order to increase gold production.

Monitoring a secondary metal recovery project in Nevada

Metallurgical monitoring included taking solution samples from monitoring wells near the injection wells. The areas around the injection well became locally saturated for short periods of time and we used that to our advantage to understand the spatial distribution of gold, silver, free cyanide, pH, HCN, and dissolved oxygen. From these data we made recommendations for increasing the free cyanide, enhancing pH, and sparging with air to help optimize the gold production.

By far, the largest contribution to the project came from real-time, time-lapsed electrical resistivity geophysical monitoring to capture solution propagation, consumption, and drainage during injection. The Geotection <> system was deployed to observe the behavior of the injections and optimize well placement, injection duration, and injection timing. We placed electrodes for the resistivity survey on the surface, in boreholes, and even used the steel cased wells themselves as electrodes. The Geotection system captured a snapshot every 22 minutes, creating a rich dataset from which to understand the highly dynamic nature of these injections.

An important aspect to injection is understanding where the solution goes. We have developed Geotection, a state-of-the-art resistivity system that is capable of resolving hydraulic details to quantify propagation, consumption, and drainage during injection.

The figure below shows some important results from the resistivity survey. On the left is a side view, showing the maximum propagation of 10 separate injection events stacked on one figure. Each event is a 3D rendered plume generated from modeling the field data. The 10 events took place over a six day period. The plumes were asymmetric about the well, heading in many different directions, likely as a result of internal heap structure. The average coverage from all 10 events were calculated to help optimize new well placements. On the right side of the figure is an overhead view of injections on two wells, W1 and W2. The average diameter from the two wells do not overlap and there are missing areas between the wells that received no cyanide at all. Based on these results, a new well, Wx, was placed at a distance that was 90% of the average injection diameter to ensure sufficient plume overlap. This overlap helps to ensure adequate drainage of newly dissolved gold.

Results of injection monitoring using electrical resistivity geophysics