Selective dissolution of rare earth elements with maleic acid from unneutralized and CO2-neutralized bauxite residues: A Box-Behnken experimental design approach

• Maleic acid was used for REE dissolution from bauxite residue of Bayer process. • CO2-neiutralized residue showed higher selectivity of REE dissolution and iron. • Higher REE dissolution rate was reached with unnetralized residue. • The acid concentration and temperature were identified as key dissolution factors. • 2nd order quadratic model provided a better statistical fit for the neutralized system. This study aims to develop a process for the selective dissolution of rare-earth elements (REEs) from unneutralized and CO 2 -neutralized bauxite residues, using maleic acid as a leaching agent. A Box–Behnken design was used to evaluate the influence of the liquid-to-solid ratio (20, 60, 100), temperature (25, 50, 75 °C) and maleic acid concentration (0.25, 0.625, 1 mol L -1 ). Analyses focused on the median points of the design showed slightly higher REE-dissolution rates in unneutralized residues, but accompanied by noticeable iron solubilisation. Neutralisation moderately reduced iron dissolution, thereby improving process selectivity in favour of REEs, a key parameter for optimising yield. Analysis of variance (ANOVA, α = 5 %) identified maleic acid concentration, followed by temperatur e , as the most influential factors on yield and selectivity, with conditional effects. Second-order quadratic models fit the data from neutralised residues better, suggesting enhanced compatibility due to physicochemical changes induced by neutralisation.