Sustainable Hydrometallurgy of Energy-Relevant Critical Minerals from Unconventional Sources: Ex-situ Carbon Mineralization and Thermal Swing Salting-Out Process

Abstract

The clean energy transition toward carbon neutrality requires shifting from a ‘fossil fuel–based’ to a ‘metal–based’ energy system. However, limited metal reserves demand the extraction of critical minerals such as lithium (Li), nickel (Ni), cobalt (Co), copper (Cu), and rare earth elements (REEs) from unconventional sources like industrial wastes, tailings, low-grade ores, and brines. These sources pose challenges due to low grades and complex chemistries, leading to higher energy use and CO2 emissions per ton of metal produced, creating a fundamental “metal–energy–CO2” dilemma. This study proposes a new framework that integrates carbon mineralization and resource recovery, reconceptualization of mineral resources by valorizing the waste and gangue materials of unconventional sources as CO2 sequestration media. In this framework, the economic viability of lower-grade deposits and wastes may be enhanced by the added decarbonization value, positioning them as potential future reserves. To address the simultaneous handling of abundant alkaline earth metals and trace critical elements, integrated processes were developed: (i) selective REE recovery during indirect carbon mineralization, (ii) chelation- assisted REE extraction during direct mineralization, and (iii) a thermal swing salting-out (TSSO) process for Li extraction from unconventional brines. The REEs recovery research began with collecting 22 types of industrial wastes and investigation physicochemical properties of slags. Compositional analysis revealed that blast furnace slag (BFS) and electric arc furnace slag (EAFS) contained over 300 ppm REEs and 40 wt% of Ca and Mg contents, and are selected as proper candidates. In sequential extraction and n(H+) regulation experiments, REEs were found encapsulated within host matrices, implying the presence of ‘mineralogical barrier’. However, the modified direct and indirect carbon mineralization effectively liberated REEs without impeding CO2 sequestration. A modified pH- swing using oxalic acid significantly enhanced REE recovery, producing REE-rich solids exceeding 1.9 wt% while sequestering 117 kg CO2 per ton of slag. Chelation-assisted direct carbonation further enabled selective extraction of Mn and heavy REEs over Ca, achieving over 85% extraction efficiency. Surprisingly, REE leachability increased with atomic number, attributed to the slightly higher Kf values of heavy REEs (HREEs) compared to light REEs (LREEs). This study highlights that carbon mineralization can act as ‘REEs liberation methodology’ which can make synergies with REEs recovery process. Next, the Li recovery research was focused on developing novel TSSO process. The objective of TSSO process was extracting Li from unconventional brines by utilizing natural thermal energy sources including diurnal temperature variations and geothermal heat. By using thermoresponsive amine-based solvents (i.e., diisopropylamine (DIPA)) and differences of salt solubility, the TSSO process selectively concentrated Li while rejecting impurities. Application to simulated geothermal brine (consist of Li, Na, K) demonstrated a 2.5-fold increase in Li concentration, elevating it to commercially relevant levels (~500 ppm). In an optimum point, 0% of Ca, 1.1% of Na, 1.1% of K, and 52% of Li were recovered in regenerated aqueous solution. These results highlight TSSO process could be used as thermally driven Li extraction method suitable for resource-constrained environments such as alpine and desert regions. In conclusion, this study highlights that a holistic and integrated approach, rather than sector-specific methods, provides an effective strategy for overcoming the interconnected “Metal–Energy–CO2” dilemma. Future advancements toward carbon neutrality and the energy transition will require the development of integrated process frameworks that bridge mining, metallurgy, waste management, carbon sequestration, energy systems, construction, and steel industries.