Recent developments in the science and technology of in situ solvent leaching of tailings for reprocessing, rehabilitation and closure

Tailings storage facilities pose environmental hazards and their failure can cause contaminated materials to be released, affecting people and ecosystems. Conversely, tailings are significant resources of unrecovered metals, especially in older facilities produced using less efficient mineral processing. The already finely ground material makes remining and reprocessing of tailings appealing, but challenges include the risk of damaging the structural integrity of the tailings, handling costs, and the potential for release of contaminants. Here we examine in situ solvent leaching as a potential alternative to conventional excavation due to some recent developments in the underpinning science and technology. An overall successful, economic and safe in situ process requires four key components: A solvent that is effective, safe and cheap. Fluid flow should be feasible, can be monitored and ideally controlled, and solvent and metals can be recovered. It should have neutral or, ideally, positive impacts on the ecosystem, including microbiota, flora and sustainable land use pathways. The necessary social licence to operate must be obtained, especially locally. These components are illustrated through a case study on a Cu-rich tailings facility in the Philippines. This demonstrates how solvent design involves assessing not just efficiency, but also environmental safety including the impacts on the microbiota and plant growth of solvent treatment. Near-continuous electrical resistivity tomography (ERT) monitoring is shown to be able to image solvent flow and reaction, while electrokinetics is being developed to enhance leaching, direct flow and recover metals. Field demonstration of solvent leaching and ERT monitoring is achieved through ~1 m3 instrumented mesocosm experiments based on the tailings and open to the natural environment. Social licence to operate is being developed through long-term engagement with local stakeholders to understand needs and aspirations.