Tuning acid extraction of magnesium and calcium from platinum group metal tailings for CO2 conversion and storage†

Avoiding the worst impacts of climate change requires reducing greenhouse gas emissions and removing atmospheric CO₂ with permanent storage. The global shift to low- and zero-emission energy sources demands increased metal mining, resulting in substantial mine tailings. Mineral carbonation offers a method to store CO₂ in alkaline-rich mine tailings, addressing both waste and excess atmospheric CO₂. This study explores the use of a pH-swing process to optimize the extraction of calcium and magnesium from plagioclase feldspar-rich platinum group metal (PGM) mine tailings from the Stillwater Mine in Nye, Montana. Various organic (citric, acetic, oxalic) and mineral (hydrochloric, sulfuric) acids were tested at different concentrations, solid/liquid ratios, and dissolution times. Organic acids, particularly citric and oxalic, were selective for magnesium and calcium, respectively, with citric acid extracting 44% of available magnesium in 72 hours. Sulfuric acid proved most effective in extracting both metals but may be impractical due to corrosion-resistant equipment costs. Carbonation of synthetic leachate indicated precipitation yields above 90% at pressures between 5 and 9 bar, producing carbonate products under 3 μm. Additionally, comparing in situ and ex situ base addition methods suggests that pH pre-swing, i.e., before carbonation is comparable to adding base during the reaction. This study advances the understanding of divalent cation extraction from plagioclase feldspar-rich PGM mine tailings but highlights the need for further research to develop an economic process.