A sustainable approach for bauxite residue utilization: Elucidation of high-efficiency reduction mechanisms driven by renewable biochar

Biomass, as a renewable and low-carbon reducing agent, demonstrates considerable potential in treating industrial by-products such as bauxite residue to recover valuable metals. In this study, Fourier-transform infrared spectroscopy was first employed to determine the suitable pyrolysis temperatures for three types of biomass: wheat bran, corn stalk, and pine sawdust. Subsequent reduction experiments identified wheat bran as the most effective biomass. A systematic and in-depth investigation into the reduction and pyrolysis mechanisms of biomass was then conducted by comparing the reduction performance of wheat bran, wheat bran-derived biochar, and coke, combined with analytical techniques such as thermogravimetric mass spectrometry and specific surface area analysis. The results indicate that wheat bran charcoal significantly enhances the reduction efficiency of bauxite residue compared to raw wheat bran, owing to its suitable specific surface area and unique porous structure containing both micropores and mesopores This structure facilitates the internal diffusion of CO and the release of CO 2 . Kinetic analysis further revealed that the reduction process is jointly controlled by diffusion and chemical reaction steps. The reduction of Fe 2 O 3 in the residue by wheat bran charcoal depends mainly on its fixed carbon rather than the volatile components released during pyrolysis. When the temperature exceeds 670 °C, the Boudouard reaction is markedly activated, continuously generating a substantial amount of highly reactive CO gas, which effectively promotes the stepwise reduction of Fe 2 O 3 to metallic iron. This process remains unaffected by the addition of bauxite residue. However, excessively high temperatures should be avoided to prevent structural deactivation of the biochar and a consequent decline in reactivity.