Metallurgical Processes for Extraction of Strategic Materials from Bauxite Residue and Ionic Clays

The global demand for strategic materials, such as scandium and rare earth elements (REEs) is increasing with growing adoption of modern and environmentally sustainable technologies, necessitating the adoption of novel resources to ensure supply. Two metallurgical processes are developed towards the goal of providing alternate sources for these materials that are less environmentally damaging and reagent intensive than currently employed methods: bauxite residue and ionic clay. Bauxite residue is a byproduct of aluminum production, and is a potential resource for iron, aluminum and scandium. Carbothermic smelting – acid baking – water leaching is investigated for the recovery of these materials. Carbothermic smelting recovers 99% of iron as crude metallic iron, which can be separated from slag that contains aluminum and scandium. The slag is treated by acid baking – water leaching, to convert scandium and aluminum to soluble sulfate species, allowing recovery. The process is optimized to achieve 99% scandium and 47% aluminum extraction. The production of solid leaching residue is avoided by employing thermal desulfation to remove 84% of residue sulfur content, allowing recycling as smelting flux. Ionic clays are formed by the natural weathering of REE-bearing minerals and the adsorption of the liberated REE ions onto clay surfaces. A desorption–precipitation process is developed to extract REEs from ionic clays. Desorption, employing ammonium sulfate and active pH control, is developed to efficiently extract REEs. Mechanistic testing demonstrates that REE adsorption/desorption occurs according to physical ion exchange adsorption, and surface complexation chemical adsorption, with the relative importance of these mechanisms depending on pH. Using ammonium bicarbonate solution to both raise pH and introduce carbonate ions in the system enables the sequential precipitation and separation of co-extracted impurities, then a mixed REE-carbonate product. The precipitation characteristics of real leachate solutions are tested experimentally and thermodynamic calculations are used to further investigate the precipitation mechanism. These two processes present potential alternatives to traditional sourcing of strategic materials. In addition to demonstrating the potential technological viability and enhancing process efficiency, this work explores and describes the underlying physicochemical mechanisms, allowing this work to inform future efforts to develop processes for responsible strategic materials sourcing.