Over the past few years there has been a renewed interest in in-situ recovery methods for metals (ISR). The concept of ISR is by no means new and has been used successfully in the past, especially in the recovery of uranium. CSIRO Waterford in Perth has been leading ISR research efforts in Australia. Along with international collaborators the industry is working towards more extensive application of ISR methods for commodities such as copper. This article is a brief overview of ISR methods and practical considerations for its application.
I attended the recent ISR Symposium at the CSIRO Waterford research facility in Perth in June 2017. The program involved presentations from ISR veterans from the CSIRO, University of Western Australia, Curtin University, Heathgate Resources, and Excelsior Mining Corp. The key topics covered in the symposium included the economic drivers, social license to operate, and a wide range of technical aspects of ISR operations.
In-Situ Recovery is the process of recovering metal reserves by passing reactive fluids (lixiviant) through the ore deposit using a series of injection and extraction wells. The lixiviant moves through natural or artificially created fluid flow pathways, interacts with the ore minerals to scavenge the metals of interest, and returns those metals to surface for extraction from their dissolved form in the lixiviant. The example in figure 1 is from the Beverley uranium mine in South Australia, operated by Heathgate Resources.
In terms of Australian copper ISR prospects Terramin and their partner Environmental Copper Recovery (ECR) are working towards developing the historic Kapunda copper resource to extract an estimated 119 000 tons of copper (Barossa Herald). The remaining copper in the Kapunda resource, left in the ground because of inefficient mining and extraction methods, is unlikely to be economically viable using conventional mining techniques. Modern ISR methods and favourable economic factors have made the Kapunda resource an attractive exploration target and the operators are moving in a positive direction towards developing the site.
A continual theme of the ISR symposium was the social license to operate. Operating companies have recognised that without local community support ISR operations have limited chances to succeed, and they have therefore placed an emphasis on their community engagement efforts. Heathgate Resources highlighted the importance of disclosing critical science issues early in the social engagement process to demonstrate their trust in the community. Continual community involvement and science communication is key to the long term success of an ISR operation, especially in the light of the negative public opinion of shale gas operations and a general failure to communicate clearly, effectively, and honestly with the public. Using the Beverley mine as an example (figure 1) it is critical to highlight and demonstrate the impermeability of the confining clay formations above and below the ore horizon. These natural features help control lixiviant flow, clarify the optimum locations for surrounding monitoring wells, and provide an opportunity to efficiently identify and control spurious flow behaviour.
From an economic standpoint the argument for ISR is similar to what the oil and gas industry has posed for their interest in shale gas reserves, i.e. the easy-to-recover resources have already been found and exploited, often not very efficiently, modern target reserves are becoming increasingly complicated, and conventional mining techniques are often too expensive with unacceptable levels of environmental impact. ISR presents a more affordable alternative to conventional mining techniques with much lower early-stage capital outlay, and it delivers only a fraction of the environmental concern. Practical considerations often render ISR as the only option for resource extraction. Excelsior Mining explained how an interstate highway and railway line located on top of their underground resource at Gunnison, Arizona, USA, left them with no other choice but to use ISR. Having now run the operation for several years they confirmed a more favourable rate of return, even when commodity prices are low.
The environmental considerations and risks of ISR are fairly clear. It does not require a large surface footprint for moving and dumping enormous volumes of ore and waste material, which makes it an extraction method with remarkably low environmental impact. However, a key aspect is to demonstrate hydrogeological control over the injected lixiviant. This necessitates a firm understanding of the local and regional groundwater systems, and the need for a series of monitoring wells, especially in the direction of groundwater resources intended for human or agricultural use.
In more complicated deposits the mineral characteristics are key factors in ISR efficiency. The presence of lixiviant-neutralising minerals such as calcite and dolomite can significantly increase the volume of lixiviant required to offset the loss in recovery, which in turn drives up the cost of the operation. Additionally, dynamic fluid-rock interactions, which are driven by mineralogy and lixiviant chemistry, may have detrimental effects on the fluid flow efficiency and recovery. It is therefore critical to consider carefully the nature of the resource mineralogy and plan the operation accordingly. The challenges of fines migration are well-known in the oil and gas industry where the mobilisation of clay minerals such as kaolinite and illite often alter the fluid flow behaviour of a reservoir by bridging the narrow connecting channels (throats) between pores.