Case Study – Copper Mine in Arizona

Heap leach pads don’t always perform as expected. An optimally operating heap is one that wets up uniformly all the way to the liner and drains to a constant, albeit low, moisture. In our experience of mapping leach pads, regardless of their construction, placement, grain size, or type of metal, they are never uniform. Issues such as ponding, preferential flow, phreatic surfaces, and compaction have led to some serious geotechnical and metallurgical consequences. In this geophysical case study for a copper mine in Arizona that we investigated a few years ago, a fine-grained ore was truck dumped for the first 5 lifts. The material had about 25% fines and was easily compacted with the haulage and earth moving equipment, despite ripping after stacking. This led to many of the problems listed above.

HGI has years of experience of copper heap characterization, identifying problems related to adverse hydraulic processes including of ponding, seep formation, compaction, and preferential flow. Once identified, solutions can easily be formulated to mitigate these problems.

The concept of mining geophysics to solve mining related problems is nothing new. However, at HGI believe that we have a winning methodology of applying electrical resistivity for heap characterization to solve highly complex problems. We incorporate as much information as we can into the modeling and interpretation to create a holistic view point of the hydraulic processes that are occurring inside the heap. In this geophysical case study, we used electrical resistivity on a relative new heap with an underliner to investigate ponding and percolation issues.

As the photos show, the method of ore placement was probably not ideal, leading to low percolation rates and hence lower recovery. To investigate the internal hydraulic processes of the heap, 3D electrical resistivity was used for the heap characterization. Electrodes for the survey were placed along the surface in parallel lines over an area of about 10 acres. Once the data were collected, they were modeled and presented as horizontal and vertical slices as shown below. As you can see, the heap is far from uniform in electrical resistivity, leading us to believe the moisture is nonuniform as well. Areas of ponding near the surface and a phreatic surface at depth are shown in the two horizontal slices. A vertical slice shows how the features extend to depth, where it appears that the phreatic surface is building on a compacted region in the deepest part of the leach pad. This leaves very little room for drainage, and irrigation rates at the surface have to be adjusted to account for this problem.

If caught early in the construction of the heap, remedies could include installation of drains to move leachate downward more easily or a change in stacking methodology. The exact drain placement would be crucial and the geophysical data can be used to locate the best spots.