Case Study – Injection Monitoring in Mexico

HGI has worked to map and help increase metal production on many gold heaps comprised of crushed ore. Because they are typically over 300 ft (100m) tall, there is a common thread among these heaps: the deeper material becomes compacted, and a perched water table is typically formed above the compacted material, closer to the surface. The perched water table intercepts surface-based irrigation of barren cyanide solution, never allowing residual leaching of the deeper parts of the heap. Thus, mines typically leave behind significant metal inventory in the lowermost parts of the leach pad. Fortunately, there are methods to extract the remaining metal inventory, most commonly through the use of enhanced metal recovery via deep subsurface injection.

HGI has conducted a number of studies on gold leach pads, each having a common thread: a shallow perched water table overlying deep compacted material. Methods are available to extract inventory from the compacted ore.

Projects typically start by understanding the issue. In our metallurgical case study, we used electrical resistivity geophysics and follow-up target drilling to form a conceptual model of the hydraulic and metallurgical processes occurring in the gold heap. The graphic below shows our survey design for mapping the heap and results, where located a perched water table (low resistivity material) on top of compacted ore (high resistivity material). Drilling confirmed the resistivity targets and assay data provided residual inventory values. Although this did not bode well for the operation of the facility, it did provide answers as to why production was low.

Mapping of a crushed gold heap using electrical resistivity

In our experience, it is difficult to correct these undesirable hydraulic and metallurgical conditions solely from the continued application of surface irrigation. For our metallurgical case study, a pilot test was conducted on the gold heap in Mexico with enhanced metal recovery (i.e., injecting the reagent directly to the underleached ore). The mine directed the injections and HGI was responsible for comprehensive monitoring. The solution was applied at high pressures and high flow rates that exceed 200 psi and 1200 gpm, as shown schematically below. The injection methodology had two main effects: a hydraulic effect that broke up tight formations by fluidizing material near the well bore and moving fines away from the injection zone, and a metallurgical effect by delivering reagent to underleached ore.

High pressure Injection schematic for enhancing hydraulic and metallurgical conditions in a heap

Subsurface injections are an efficient means to extract metal from difficult to leach ore. HGI has experience on more than a dozen mines in helping getting an injection project underway. We can offer guidance and help optimize the extraction.

The results of the injection are shown below. The injected plume derived from 3D rendered electrical resistivity data using Geotection <> showed different behavior depending if the injection occurred in the compacted material or in/above the perched water table. In the compacted material, the plume shape was more round and confined to be near the well, with a radius of about 20m. In the water table, the plume was elongated in the direction of the hydraulic gradient. Downstream of the injection, we captured solution in monitoring wells. The metallurgical data showed spikes in free cyanide above and below the water table. The gold grades dropped for short a period of time due to dilution, but quickly rebounded to values greater than before injection due to release.

The end result of our monitoring effort provided recommendations for well separation, injection duration, and cautions against overly diluting the PLS. These recommendations were specific to this site and it is advisable for each pilot project to undergo similar intense monitoring to develop optimal engineering parameter for recovery.

3D electrical resistivity used to monitor an injection into a gold heap