Enhanced Aluminium Extraction from Gibbsitic Bauxite by Electroreduction in the Bayer Process

The Bayer process is used to extract alumina from bauxite, resulting in the formation of a highly alkaline solid residue known as bauxite residue (BR). However, such residue contains insufficient iron (<35% Fe) and complex impurity composition for use in blast furnace ironmaking. This study investigates the potential for enhancing the extraction of aluminium (Al) and increasing the concentration of Fe in residue by electrolytical reduction in suspension of BR in a spent Bayer process solution. A maximal current efficiency of 43.7% was obtained during the electroreduction of the coarse fraction of BR. The magnetite-containing residue obtained was further used as an aid in the high-pressure Bayer process leaching of gibbsitic bauxite. Adding the reduced BR increased the Al extraction rate by up to 7.2%. The kinetics of bauxite leaching at 120–160 °C and time interval 0–40 min in the presence of reduced BR were investigated using a shrinking core model (SCM). The results showed that the leaching kinetics of Al correlate well with the intraparticle SCM equation, indicating that the reaction velocity is regulated by the diffusion of the OH− or Al(OH)4− through the product layer. The apparent activation energy of the process at 140–160 °C was found to be 32.2 kJ/mol. Al in the solid residue is closely associated with Fe, i.e., it is enclosed in a solid matrix of iron minerals.