Nd–Na–F– H ₂ O System: Solubility Measurements at 23 °C and Thermodynamic Modeling

A thermodynamic model has been developed for calculating solubilities and speciation in aqueous Nd–Na–F–H2O systems. The model combines experimental data from the literature for NdF3 and NaF in aqueous environments with new measurements of the solubility of both NdF3 and NaNdF4 as a function of pH at room temperature. For NaNdF4, the solubility data complement the recently reported solid state property data, thus establishing a comprehensive database for this hitherto poorly characterized compound. The solubilities of NdF3 and NaNdF4 have been measured through the dissolution method in HCl (0.1, 0.01 mol·kg–1 H2O) solutions at target pH of ∼1 and ∼2 over an equilibration period of up to 12 weeks at room temperature. The composition of the solution phase was determined through Inductively Coupled Plasma–Optical Emission Spectrometry. X-ray diffraction was performed to determine the remaining equilibrated solid phases. For modeling, the Mixed-Solvent Electrolyte framework has been used, which was previously parametrized and validated for various rare earth systems. The model combines a comprehensive speciation treatment in the aqueous phase with solid-state thermodynamics to reproduce the solubility of solids at varying processes conditions, thus providing a foundation for designing novel processes.