Toward sustainable recovery of iron and rare metals from red mud via electrothermal shock-induced metastable phase engineering

Abstract

Red mud (RM), a major by-product of alumina production, represents an abundant yet underutilized secondary resource for iron and rare metals (e.g., Sc and Ga). Its valorization, however, is hindered by recovery processes that are energy intensive, chemically demanding, and environmentally costly. Herein, a reagent-free flash Joule heating (FJH), an electrothermal shock reaching >3000 °C within seconds, was proposed as a new method for the modification of RM. Compared with conventional roasting, FJH reduced carbon emissions and energy consumption by >77% per kg of RM. Following FJH, the extraction efficiency of Fe, Sc, and Ga with relatively dilute acid (i.e., 0.5 M HCl) was significantly increased by 266, 60.6, and 71.6%, respectively, thereby achieving synergistic recovery of major and high-value metals and advancing more comprehensive RM valorization. Multiscale characterization, model mineral tests, and theoretical calculations collectively showed that FJH with tunable energy density drove a preferential sub-second transformation of α-Fe₂O₃ to metastable γ-Fe₂O₃, a phase evolution unattainable under conventional roasting. This transformation weakened the confinement of Ga and Sc within Fe-bearing phases, thereby unlocking their extraction towards recovery. This study elucidated the mechanistic connection between FJH-induced major phase transformation and enhanced metal extraction from RM, offering a green, rapid, and mechanistically guided route for more comprehensive metal recovery from RM.