The dissolution of rare earth oxides in molten fluorides is a critical step in the preparation of the corresponding rare earth metals by oxide-fluoride electrolysis. However, quantitatively understanding the nature of dissolution, especially in the case of molten salts, is usually difficult to be achieved by postmortem characterization. In this paper, the dissolution behavior of Nd₂O₃ particles in molten fluorides was studied via in situ observation with confocal scanning laser microscopy. Combining direct observation with thermodynamic analyses on the oxide dissolution, the rate-limiting step(s) and the effects of parameters like temperature, salt type, and composition on the dissolution rate are identified. This study provides a methodology to estimate the dissolution kinetics of rare earth oxides in molten fluorides during their primary and secondary processing.

The analytical and numerical description of the effective dissolution kinetics of spherical particles into a solvent is often difficult in chemical and metallurgical engineering. The crucial first step is to identify the dissolution mechanisms, and subsequently, relevant kinetics parameters can be calculated. In this article, three frequently used approximations, i.e., the invariant-field (IF) (Laplace), reverse-growth (RG), and invariant-size (IS) (stationary-interface) approximations, are systematically discussed and compared with numerical simulation results. The relative errors of the dissolution curves and total dissolution time of the three approximations to the numerical simulations are calculated. The results reveal the appropriate application ranges of the approximations for given precision levels. With further experimental validation, this research provides a methodology to properly assess dissolution kinetics and adequately estimate effective diffusion coefficients and activation energy under the experimental uncertainties.