ABSTRACT: Penneability and porosity measurements, and microscopic examination, of rock samples obtained from an in situ copper mining operation were carried out before and after several cycles of leaching. The results showed that the penneability and porosity increased initially, but slowed down with continued leaching. Also, the amount of copper recovered decreased gradually. Topographic imaging of the specimens showed an increase in the number of fractures and fracture aperture as leaching progressed. These observations are attributed to dissolution of copper minerals present along the fractures first, resulting in increased porosity and permeability. This phase is followed by stable conditions due to depletion of soluble material along the fractures, and possibly, matrix flow. The field observation showed that the permeability remained constant initially, increasing dramatically after about eight months, and finally, decreasing significantly. The final decrease in permeability is attributed to precipitation of sulfates and/or migration of fines produced, clogging up the fractures.
BACKGROUND
In situ mining is defined as the removal of minerals by dissolving them from the solid matrix, without excavation from undisturbed ore deposits. An inexpensive and readily available liquid, tenned lixiviant, is injected into the mineralized rock mass through injection wells. The liquid travels through the rock at a slow pace, dissolving the desired minerals/metals as it does so, and is recovered through recovery wells. The pregnant leach solution is processed, and the desired metal is recovered. The entire process is shown schematically in Figure 1.
As near surface deposits get depleted and deep underground mining continues to lose ground due to the high costs of mining, transportation and processing large volumes of rock, in situ mining is becoming an increasingly attractive method for extraction of minerals from low grade ore deposits. The technique offers several advantages over conventional mining. The process requires less capital investment (Pugiliese, 1989). The start-up time for production is reduced considerably. The method eliminates the process of extraction, crushing and milling, thereby reducing the production costs.
The safety is enhanced since miners are not exposed to the hazards of underground mining. Environmental disruption is significantly less than conventional mining because the process only removes the target metals from the ground. Finally, the site is easily reclaimed to its original condition by treating the ground water, capping the wells and dismantling the processing plant. The drawbacks include low flowrates of leach solution within the orebody (Ahlness et al, 1983) and solution loss.
The flow of solution in the orebody depends on the design parameters such as injection rate and pressure, as well as flow characteristics of the rock like porosity and permeability of the rock matrix, permeability, size and density of the fractures. The flow parameters vary continuously over the life of an orebody, depending on mineral dissolution, precipitation, clay hydration and physical sedimentation. One of the prerequisites to the success of this method is, therefore, a good understanding of the physical and chemical processes involved in the overall recovery process.