Rare earth element recovery in hard-rock acid mine drainage and mine waste: A case study in Idaho Springs, Colorado

Abstract

While the geochemistry of hard-rock generated acid mine drainage (AMD) and waste rock has been extensively studied for environmental and toxicological impacts, their critical metal contents have only recently begun to be characterized. We investigated rare earth element (REE) geochemistry in AMD and waste rock in the Idaho Springs gold district, Colorado. Specifically, we found that two AMD inputs to the Argo Tunnel Wastewater Treatment Plant (ATWTP) contain high dissolved REE concentrations (0.7 and 1.3 mg*L−1), and that REEs could be concentrated using a two-stage pH adjustment. The first stage precipitated iron and removed thorium by adsorption, while the second stage precipitated aluminum, which removed REEs by adsorption. Solids generated were 2670 and 2800 mg*kg−1 for each of the REE-bearing AMD sources, and at 90% REE recovery, the ATWTP could produce nearly 1000 kg of REEs each year with an economically favorable MREE enrichment. REE leaching in waste rock was also investigated, but results indicate that it is unlikely to be a significant source of REEs. More work is needed to characterize both the critical metal content (including other metals besides REEs) and economic feasibility of extraction for mine waste and AMD in hard-rock mining environments.