Towards sustainable valorization of bauxite residue: Thermodynamic analysis, comprehensive characterization, and response surface methodology of H2 reduced products for simultaneous metal recovery

Bauxite residue (BR) is a valuable polymetallic resource, but its utilization is hindered by challenges such as high alkalinity, fine particle size, and complex mineral composition. Hydrogen reduction is a promising approach for recovering metals from BR, fostering a sustainable circular economy. Successful metal recovery necessitates comprehensive material characterization. This study investigates BR reduction from 400 to 700 °C, varying time (30–120 min), and NaOH addition (10–25 wt %) under different H2 environments to maximize magnetite (Fe3O4) and water-soluble aluminate (NaAlO2) formation in reduced pellets to extract valuables (such as Fe, Al, Ti, and REEs including Sc). The reduced products are analyzed using different analytical techniques. H2 reduction conditions and parameter interactions are explored using response surface methodology (RSM). The findings reveal that NaAlO2 formation increases until 700 °C, and complete hematite to magnetite conversion occurs at 500 °C. Metal recovery can be hindered by the formation of sodium iron oxide, wüstite, sodium aluminum silicate with the higher H2 flow rate (≥45 L/h (or H2 amount ≥450 L/(h.kg)) under 5 vol % H2 + 95 vol % N2 environment or with 20 L/h (or H2 amount 200 L/(h.kg)) under 100 vol % H2 concentration), >600 °C, and ≥25 wt % NaOH. These trends are aligned with thermodynamic calculations. The study advances the understanding of H2 reduction of BR for sustainable processes and polymetallic resource recovery.