Hydrothermal synthesis and thermal decomposition characteristics of bastnaesite with varied rare-earth compositions

Bastnaesite is an important rare-earth mineral and a key precursor for functional materials. Controllable hydrothermal synthesis enables the preparation of composition-defined bastnaesite for investigating their thermal decomposition and roasting behavior. In this work, bastnaesite with different rare-earth compositions were synthesized hydrothermally from RECl 3 ${}_3$ , NaF, and Na 2 ${}_2$ CO 3 ${}_3$ at 100̃140 °C for 12̃72 h. Single-rare-earth bastnaesite samples containing Ce, La, Pr, and Nd were successfully prepared under optimized fluorine-containing conditions. In the cerium-containing system, CeF 3 ${}_3$ and CeO 2 ${}_2$ appeared under overly acidic or alkaline conditions, whereas chloride was more favorable than nitrate for bastnaesite formation because it suppressed Ce(III) oxidation. Rare-earth composition strongly affected the decomposition behavior. Increasing Ce content lowered the onset decomposition temperature from 418.9 °C for La-bastnaesite to 298.4 °C for Ce-bastnaesite, with further decreases in air. Under N 2 ${}_2$ , Ce-bastnaesite underwent multistep decomposition involving decarbonation, formation of CeO 2 − x ${}_{2-x}$ and CeF 3 ${}_3$ , and subsequent generation of cerium oxyfluoride phases, whereas La-, Pr-, and Nd-based bastnaesite was largely insensitive to atmosphere and was converted mainly into the corresponding REOF phases in nitrogen. These results provide reference for understanding the roasting behavior and thermal-treatment window of bastnaesite.