Microstructural transformation and recovery of rare earth elements during hydrochloric acid leaching of apatite concentrate en en Hydrochloric acid concentration plays a pivotal role in governing the dissolution behavior of apatite and the recovery of rare earth elements (REEs) during hydrometallurgical processing. In this study, the influence of hydrochloric acid concentration on the leaching of cerium (Ce), lanthanum (La), and neodymium (Nd) from an apatite concentrate derived from iron ore processing waste was comprehensively investigated by correlating microstructural evolution with leaching performance. Response Surface Methodology (RSM) coupled with central composite design (CCD) was employed to evaluate the combined effects of hydrochloric acid concentration and leaching temperature on the recovery of cerium, lanthanum, and neodymium. Throughout the optimization study, the solid-to-liquid ratio (1:9) and leaching time (60 min) were maintained constant to isolate the influence of the selected variables and improve the reliability of the statistical model. Under the optimum conditions of 37 wt% HCl, a solid-to-liquid ratio of 1:9, a leaching time of 60 min, and a temperature of 65 °C, the recoveries of Ce, La, and Nd reached 94.19%, 86.33%, and 76.34%, respectively. SEM analysis of the leaching residues revealed a progressive transition from relatively smooth, well-preserved crystalline particles at low acid concentrations to severely corroded, fragmented, and porous structures at higher acid concentrations. These microstructural transformations provide direct evidence of enhanced apatite decomposition and increased REE liberation, explaining the substantial improvement in extraction efficiency. The strong correlation between acid-induced microstructural evolution and REE recovery demonstrates that hydrochloric acid concentration is the primary driving force governing leaching performance. This study provides new mechanistic insight into acid-assisted apatite dissolution and establishes an optimized, economically attractive strategy for the efficient recovery of valuable REEs from apatite-rich iron ore tailings. 110 126 Erfan Mohammadzadeh Department of Materials Science and Engineering Sharif University of Technology, Azadi Ave, Tehran, P.O. Box 11155-9466 Iran erfan.mohamadzadeh22@sharif.edu Sayed Khatiboleslam Sadrnezhaad Department of Materials Science and Engineering Sharif University of Technology, Azadi Ave, Tehran, P.O. Box 11155-9466 Iran Hossein Yoozbashizadeh Department of Materials Science and Engineering Sharif University of Technology, Azadi Ave, Tehran, P.O. Box 11155-9466 Iran Mahdi Maarefvand Department of Materials Science and Engineering Sharif University of Technology, Azadi Ave, Tehran, P.O. Box 11155-9466 Iran Rare earth elements Leaching Hydrochloric acid Apatite concentrate SEM Vol 6 No 2 Paper 4.pdf [1] A. Ford, D. Huston, J. Cloutier, M. Doublier, A. Schofield, et al., A national-scale mineral potential assessment for carbonatite-related rare earth element mineral systems in Australia, Ore Geol. Rev. 161 (2023) 105658. https://doi.org/10.1016/j.oregeorev.2023.105658. ##[2] Y. Ghorbani, I.M.S.K. Ilankoon, N. Dushyantha, G.T. Nwaila, Rare earth permanent magnets for the green energy transition: Bottlenecks, current developments and cleaner production solutions, Resour. Conserv. Recycl. 212 (2025) 107966.‏ https://doi.org/10.1016/j.resconrec.2024.107966. ##[3] H. Diao, H. Yang, T. Tan, G. Ren, M. You, et al., Navigating the rare earth elements landscape: Challenges, innovations, and sustainability, Miner. 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