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–H 2 O systems. The model combines experimental data from the literature for NdF 3 and NaF in aqueous environments with new measurements of the solubility of both NdF 3 and NaNdF 4 as a function of pH at room temperature. For NaNdF 4, 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 NdF 3 and NaNdF 4 have been measured through the dissolution method in HCl (0.1, 0.01 mol·kg –1 H 2 O) 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.