Study on the Process and Mechanism of Preparing Lanthanum Carbonate from Rare Earth Chloride Solution

To address the issues of high wastewater treatment costs and the lack of recycling associated with conventional precipitants such as oxalic acid and ammonium bicarbonate in rare earth precipitation processes, this study proposes a novel gradient alkali conversion-carbonation method based on a green process coupling "rare earth chloride alkali conversion-carbonation with sodium chloride electrolysis." The primary scientific objective is to elucidate the crystallization mechanism and to achieve controlled preparation of high-quality lanthanum carbonate. By gradient-controlling the addition sequence and rate of alkali liquor and CO₂, lanthanum carbonate tetrahydrate was successfully synthesized. Characterization by XRD, SEM, ICP, and laser particle size analysis indicates that the product prepared by the gradient alkali conversion-carbonation method exhibits a single phase with high crystallinity, as evidenced by sharp and clear XRD diffraction peaks. Furthermore, the median particle size of the product obtained via this method is relatively large, reaching approximately 10 μm, while the particle size distribution Span value remains around 1.0. Mechanistic studies suggest that this method effectively regulates the crystallization process by precisely controlling the introduction and slow dissolution of the La(OH)₃ precursor, thereby reducing the supersaturation of the system during carbonation and facilitating the dissolution-reprecipitation of La³⁺. This work provides a theoretical basis for the preparation of high-quality rare earth carbonates and a process reference for the green recycling route.