Selection of Mineral Acids for Extraction of Rare Earth Elements and Critical Metals from Bauxite Ore Using the Taguchi Method

Abstract The recovery of rare earth elements (REEs) and critical metals (CRMs) is increasingly important because of the growing industrial demand and resource scarcity. Bauxite, the main source of aluminum, has significant potential for REEs and CRMs. Therefore, in this study, the recovery of these strategic elements from Doğankuzu bauxite (Seydişehir, Turkey) is investigated systematically and comprehensively using three mineral acids (HCl, H 2 SO 4 , HNO 3 ). Seydişehir bauxite samples were subjected to direct mineral acids leaching to investigate the effects of acid concentration, temperature, solid-to-liquid ratio, and leaching time on the extraction efficiency of valuable metals. Process optimization was performed using the Taguchi method, identifying the optimal conditions for maximizing REE and CRM recoveries while minimizing the reagent consumption for each acid. Although individual optimum conditions varied by element, the common optimum conditions (4 M acid, 95°C, 480 min, solid-to-liquid ratio: 1/30 g/mL) provided a consistent basis for comparison. Under optimized conditions, La, Nd, and Pr achieved recoveries > 97% across all systems. Nitric acid generally exhibited a slightly higher recovery rate than other acids for most REEs. The highest recovery for Sc was achieved by sulfuric acid (82%), followed by hydrochloric acid (76%), whereas nitric acid (66%) was comparatively less effective under optimum conditons. In addition, Y remained below 55%, 54%, and 46% for nitric, sulfuric, and hydrochloric acid, respectively. Solubility of major oxides (Al 2 O 3 , SiO 2 , TiO 2 ) is more limited (< 30%) in all lixiviants because of incomplete dissolution of refractory phases, while Fe 2 O 3 has high solubility (~ 95%) in HCl and H 2 SO 4 . The ANOVA-based contribution analysis supports that temperature is the primary driver of REE recovery across all investigated systems, whereas other operational parameters exert only minor and secondary effects. These findings demonstrate the feasibility of bauxite leaching for REE/CRM extraction and highlight its potential integration into alumina production, providing a dual recovery strategy that enhances resource efficiency and supports sustainable supply chains.