Microwave pretreatment followed by acid-free solid-chloride leaching of red mud for multi-element recovery

초록

Despite growing interest in red mud valorization, most existing recovery methods still rely on acid-intensive hydrometallurgical processes, whereas acid-free routes based on microwave-induced solid-state reactions remain scarcely explored. This study investigated microwave pretreatment followed by acid-free solid-chloride leaching (MPSCL) as a sustainable approach for multi-element recovery and residue valorization from bauxite residue (red mud) using solid chloride additives (NH4Cl, FeCl3, and NaCl). The red mud-chloride mixtures were irradiated at 1.1 kW for 5, 10, and 15 min to examine the effects of additive type and ratio on temperature evolution, chlorination-reduction reactions, and subsequent leaching behavior. High NH4Cl loading (2:1 wt%) resulted in rapid heating to 674 degrees C, accompanied by in-situ formation of HCl and NH4 gases that reacted with metal oxides to form water-soluble chlorides. Under these conditions, Cu was completely recovered (100%), whereas Mn, Co, and Ni achieved recoveries of 57.4%, 27.6%, and 30.8%, respectively. Rare-earth elements (REE: Ce, La, Nd, and Sm) also exhibited enhanced recoveries (10.6-14.0%). Mineralogical analyses confirmed the decomposition of cancrinite-type aluminosilicates and the formation of maghemite and magnetite, which improved metal accessibility and leaching reactivity. The selective removal of Na and Ca produced an Fe-Al-enriched magnetic residue (similar to 60 wt% Fe, similar to 35 wt% Al), enabling potential magnetic separation and dual valorization. These findings demonstrate that integrating microwave-induced solid-state chlorination with subsequent acid-free leaching offers a sustainable, waste-minimizing strategy for simultaneous metal recovery and residue recycling from red mud.

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