Enhanced Metal Recovery

Enhanced metal recovery is a subsurface leaching program to help draw down the remaining metal inventory in heap leach piles where mineral resource is leached with a reagent.  Large heap leach operations cover many hectares and contain millions of metric tons of low-grade ore.  The size fraction and types of ore, plus the method of ore placement on the leach pad can lead to discrete areas of compaction and densification.  Leaching around these compacted zones will cause channeling of solution, with the potential of many dry or chemically unaltered zones remaining within the heap.  These zones may contain significant quantities of underleached passive inventory that present recovery and forecasting challenges.


HGI’s Enhanced metal recovery technology helps draw down remaining metal inventory in heap leach piles.


Unfortunately, few operational techniques exist to truly improve recovery of the passive inventory.   Secondary recovery methods, i.e., processes used to rework the heap after cessation of the initial leaching cycle, have typically involved recontouring of side slopes, reripping, and managing solution and reagent delivery strictly at the surface.  These methods indirectly promote an enhancement of recovery, as the solution is diverted from existing surficial channels in hopes of reaching the more distant underleached ore.  The key to significantly reducing metal inventory in a heap leach pad, however, is to promote direct contact of the lixiviant to the location of the underleached zones, which cannot be conducted from surface work alone.

An enhanced metal recovery technology that promotes the direct contact between lixiviant and ore within a heap is Hydro-Jex.  The Hydro-Jex technology stimulates metal production by pushing solution deep within a heap from a centralized well.  The process mechanically changes the heap by pumping solution at pressures that exceed lithostatic, similar to the enhanced recovery methodologies used in the oil and gas industry.  The mechanical changes open up new solution pathways, thereby promoting a more direct means of lixiviant contact to the locations of remaining metal inventory that were missed by the initial surface leaching cycle.  The enhanced metal recovery technology also includes a post-injection rinse treatment that promotes additional solution delivery over time, allowing greater recovery of metal.


Geotection Resistivity Monitoring System deployed on heap leach.


HGI has conducted resistivity monitoring on a number of heaps that have undergone enhanced metal recovery with direct injection.  We have developed new resistivity monitoring systems dedicated to capturing the highly dynamic nature of the solution movement.  Below is an example of monitoring an injection over an 8 hour period (and subsequent 10 hour rest) using a set of surface and borehole electrodes.  Data were captured on a 15-minute cycle to produce a highly resolved subsurface reagent plume.  The six volumetric snapshots reveal a strong downward movement in the direction of lowest overburden stresses (towards the slope).


Injection monitoring into a heap leach pile. The figure represents solution movement from a well (red) over a 18 hour window.  Electrodes are on the surface (black dots) and in boreholes.