Balancing recovery efficiency and environmental impact in microwave-assisted leaching of rare earth elements from red mud

Abstract

Red mud, a hazardous byproduct of alumina production, represents a critical secondary resource for rare earth elements (REEs) but poses significant extraction challenges due to its complex mineralogy and high iron content. Conventional REEs recovery from red mud relies on unselective leaching with sulfuric acid, which suffers from complex post-leaching treatments to reduce the impurities. This study introduces an environmentally balanced microwave-assisted leaching (MWAL) process employing formic acid, a green alternative, to selectively recover REEs in just 5 min (Nd, Pr, Y) while minimizing the co-dissolution of Fe and Ti. Under optimized conditions (600 W, 5 min, 1 % solid-to-liquid ratio), the process achieved exceptional recovery of 100 % Nd, 91.65 % Pr, and 60.21 % Y, with <4 % Fe leaching – significantly enhancing selectivity compared to conventional sulfuric acid treatment, and organic acids like malic acid and acetic acid. Microstructural investigations using Field Emission Scanning Electron Microscopy (FE-SEM) and Brunauer-Emmett-Teller (BET) analyses revealed that microwave irradiation induces particle fragmentation and increases porosity and surface area, thereby enhancing acid accessibility. This porous residue (activated red mud) can be repurposed as an adsorbent, turning the process into a zero-waste system. Thermodynamic simulations (using PHREEQC software) further identified pH and temperature as critical parameters for mineral stability, with acidic conditions (pH < 3) maximizing the dissolution rate. The efficient internal heating through microwave irradiation combined with REEs complexation by formic acid, renders this approach a low-impact and effective way to recover REEs from red mud.