An experimental study of the solubility and speciation of the Rare Earth Elements (III) in fluoride- and chloride-bearing aqueous solutions at temperatures up to 300 °C

The solubility of REE(III) fluoride solids was determined in fluoride- and chloride-bearing solutions at 150, 200 and 250 °C and saturated water vapor pressure. These experimental data, together with experimental data from previously published studies, were used to evaluate formation constants for chloride- and fluoride-bearing aqueous species of the entire REE(III) group at temperatures up to 300 °C. The data show that the stability of these species differs significantly from that predicted theoretically. For example, contrary to the theoretical predictions, LREEF2+ species are more stable than HREEF2+ species at elevated temperature. The behavior of the chloride-bearing species is similar. Parameters for the Helgeson–Kirkham–Flowers (HKF) equation of state were determined for REEF2+, REECl2+ and REECl2+ complexes using these experimental data and permit calculation of formation constants of these species at conditions not investigated experimentally. These data now permit the mobility of all REE in fluoride- and chloride-bearing hydrothermal systems to be reliably evaluated at intermediate temperatures and pressures.