High Pressure Injection – Enhanced Recovery

HGI recommends high pressure injection for leach pads with excessive metal inventories and adverse hydraulic and metallurgical conditions.  We have the experience and expertise to make sure the injection projects run smoothly and safely.

Modern mining techniques for low grade ore deposits of precious and base metals make use of leach pads for metal extraction. Most gold and silver leach pads use sodium cyanide as the reagent with a concentration that generally ranges from 50 to 400 ppm with a pH between 9.5 and 11.5. Free cyanide in solution combines with gold or silver to remove them from the ore. Other metals in the ore can consume the cyanide, including iron, copper, and zinc. Alternatively, low pH conditions from sulfidic ore can convert the cyanide to a gaseous HCN, which cannot leach gold and can be dangerous. It is not uncommon for there to be no free cyanide remaining in the PLS once it reaches the collection pond. In addition to these undesirable metallurgical conditions, adverse hydraulic conditions can direct fluid along preferential paths with narrow drainage channels. Shallow, perched water tables of leach solution can also prevent barren solution from percolating through low permeability regions that develop in the deeper parts of leach pads. The end result, then, is the potential for gold to be left in the leach pad in regions that are not being effectively leached.

Untapped and hard to liberate precious metal inventories in heap leach pads can be considerable. Enhanced recovery with high pressure injection offers mine operators new tools to recover more.

We have seen this scenario play out across many leach pads with remaining gold inventories that can easily reach 500,000 ounces. The choice to go after the lost inventory is simple, but the continued surface irrigation will do little to draw it down. The alternative is to inject, and operators of gold leach pads have traditionally chosen high pressure injections for enhanced metal extraction due to the ability to leave metal cased wells in the leach pads for long periods of time. High pressure injections, as opposed to low pressure, typically operates above 100 psi with flow rates from 800 to 2000 gpm (180 – 450 m3/hr) with the aid of a pumping skid. The photo montage below shows many different skids that we have seen deployed on gold heaps.

hydroGEOPHYSICS, offers a large suite of metallurgical monitoring services for surface and subsurface leaching. By tracking the solution volume and grades, we can create production models for economic analyses of injection projects, and quantify the metal extraction with this innovative technology.

HGI’s role in these high pressure injections include hydrological and metallurgical monitoring, validation, and troubleshooting. Our hydrological monitoring ability includes electrical resistivity tomography and measuring water levels and seep formation should they occur.  We keep a tab on where the solution is going to help assure stability of the slope and solution coverage of the ore. In particular, our ERT methodologies with the state-of-the-art Geotection system can capture the highly dynamic nature of the rapidly moving and growing plume created by the injection. As the graphic below shows, which represents three different injection projects on different mines, the plume could go anywhere, including through previously injected zones that had their permeability significantly altered. Having a complete understanding of the plume’s location and drainage will help to reduce costs and keep safely operating.

In addition to the hydraulic monitoring, we have a suite of services geared towards metallurgical monitoring to help ensure optimized reagent grades, oxygen, pH, and metal extraction.  These data can be used to investigate interim conditions for cyanide consumption or oxygen depletion.  Alternatively, we use the final metal grades and injected volumes to track liberated ounces, which in turn can be used to create production models for economic analyses.  In the examples below, we compare injections into crushed ore and ROM ore.  Within the crushed ore projects, one mine clearly outperforms the other over the same time period.  One reason can be attributed to the total number of wells; the more injection wells, the better.  For the ROM ore projects, which were completed on the same mine but in different heaps, showed Pad 1 producing more ounces.  Pad 1 injection was optimized for pH. Through our monitoring services, we were able to validate and optimize metal extraction, thus reducing costs and maximizing profits.