Recent experimental determinations of the solubility product of NdPO₄ and stability constants for Nd-chloride and Nd-hydroxide complexes provide a basis to model quantitatively, the control of phosphate phases on REE concentrations in hydrothermal fluids. Data on the solubility of Nd phosphates and the stabilities of Nd complexes at elevated temperatures and pressures are of critical importance to safe nuclear waste disposal, geochronology, and geochemical exploration for REE deposits. A thermodynamic model has been constructed for calculation of the solubility of NdPO₄ and the speciation of Nd in the system NdPO₄-NaCl-HCl-H₂O as a function of m_NaCl, T, and pH.

Calculations were carried out for temperatures from 25º to 300ºC, pH from 2 to 9, and m_NaCl from 0.1 to 5.0 moles/Kg H₂O. The model takes account of the species: H⁺, OH^-_, Na⁺, Cl^-, NaClº, HClº, NaOHº, H₃PO₄º, H₂PO₄^-, HPO₄^2-, PO₄^3-, Nd³⁺, NdCl²⁺, NdCl₂⁺, NdCl₃⁰, NdOH²⁺, Nd(OH)₂⁺, and Nd(OH)₃º. We assumed stoichiometric dissolution, i.e., ΣNd=ΣP in the calculations. Activity coefficients for charged species were calculated using the Helgeson b-dot equation.

At all temperatures, calculated Nd concentrations are highest at the lowest pH and they attain a minimum at near neutral pH. For a given pH, calculated Nd concentrations increase with increasing chloride concentration, and the rate of this increase is greater at higher temperatures. In general, calculated Nd concentrations decrease with increasing temperature. Our results, combined with data in the literature, suggest that the solubilities of REE phosphates are retrograde (i.e., decrease with increasing temperature) up to at least 200 °C. At temperatures above 200 °C, Nd concentration increases with increasing temperature up to 300 °C (the highest temperature investigated). This increase is stronger between 1.0 and 5.0 m than between 0.1 and 1.0 m chloride concentration. This is a result of the increased stability of Nd-chloride complexation with increasing temperature, which more than compensates for the decrease in the solubility product with increasing temperature.