Phthalic acid-assisted rare earth leaching process from ionic rare earth ore: leaching behavior, kinetics, and complexation mechanism

The extraction of rare earth (RE) elements from ion-adsorption type RE ores relies on in-situ leaching technology. However, the leaching process faces technical challenges such as low leaching efficiency and limited recovery rates. In this study, MgSO4 was used as the leaching agent, with phthalic acid (PA) introduced as a complexation-assisted additive to improve RE leaching efficiency. The addition of 0.15% PA increased the RE leaching efficiency by 12.4% compared to using MgSO4 alone. Leaching kinetics results indicate that as PA concentration increases, the RE leaching process exhibits two-stage behavior, with the leaching process governed by the internal diffusion model. Spectrophotometric analysis revealed that the PA primarily forms 1:1 and 1:2 coordination compounds with RE3+, thereby reducing the concentration of free RE3+ ions in solution. Quantum chemical calculations further revealed that the order of binding energies for the reactions between the four RE ions and hydrogen phthalate ion (PA-) is: Y3+ > Pr3+ > Nd3+ > La3+. The computational results are consistent with the electrostatic potential distribution of RE3+, indicating that PA- interacts more strongly with RE3+ ions possessing higher charge density. In aqueous solution, the carboxyl groups of PA release protons, and the resulting oxygen atoms serve as negatively charged sites that coordinate with electron-deficient RE3+ ions to form stable RE carboxylate complexes. This interaction promotes the desorption of RE ions from clay mineral surfaces, thereby enhancing RE leaching efficiency.