Optimization of acid leaching for rare earth element recovery from red mud using the taguchi method

Red mud, generated in large quantities as a by-product of the Bayer process, represents a promising secondary resource for rare earth elements (REEs) such as La, Ce, Nd, and Pr. In this study, REE recovery from red mud by mineral acid leaching was systematically investigated, and key process parameters were optimized using the Taguchi experimental design method. X-ray diffraction analysis showed that the red mud matrix is dominated by iron-bearing and aluminosilicate phases controlling REE dissolution during acid leaching. Four operational parameters – acid concentration, liquid-to-solid (L/S) ratio, leaching time, and temperature – were evaluated using an L16 orthogonal array. Leaching experiments were conducted using HNO₃ and H₂SO₄ and compared with previously reported HCl-based systems. Statistical analysis based on signal-to-noise (S/N) ratios and analysis of variance (ANOVA) indicated that the L/S ratio is the dominant factor controlling overall REE recovery, whereas temperature strongly influences Ce dissolution. Under optimised nitric acid conditions (3, 40 mL/g, 8 h, 95°C), REE recoveries of 74–87% were achieved, while, sulphuric acid yielded lower recoveries (17–57%). The results highlight the critical role of acid type in REE extraction efficiency and process operability, providing guidance for scalable REE recovery from red mud.