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• A thermodynamic model has been developed for rare earth elements in the presence of hydroxide, acetate, citrate and oxalate ions. • Speciation and solubility are simultaneously reproduced. • Effects of pH, temperature and complexation on solubility are accurately reproduced. • Effect of crystallinity on REE hydroxide solubility is modeled for limiting cases of crystalline and amorphous hydroxides. A thermodynamic model has been developed for calculating speciation and phase equilibria in aqueous solutions of rare earth elements (REEs) with hydroxide, acetate, citrate, and oxalate anions. The computational framework is based on the Mixed-Solvent Electrolyte (MSE) framework, which has been previously used to establish a systematic treatment of binary and multicomponent systems containing REE sulfates and chlorides up to solid–liquid saturation at temperatures up to 300 °C. The model has been parametrized by performing a multi-property analysis of critically evaluated speciation and solubility data. For rare earth hydroxides, a comprehensive model has been developed for fourteen REEs (i.e., for yttrium and all lanthanides except promethium). The effect of the crystallinity of rare earth hydroxides on their solubility has been analyzed. Model parameters have been determined for two limiting cases, i.e., for crystalline and amorphous hydroxides, while recognizing that the hydroxides may span a range of degrees of crystallinity. For the organic ligands, the model has been established for neodymium with additional analysis for other selected REEs. The model accurately reproduces the effects of pH, temperature, and complexation with organic ligands on the behavior of rare earth elements in aqueous solutions. When coupled with previously developed models for inorganic rare earth salts, it provides a thermodynamic tool for the design and optimization of separation processes for the production and recycling of REEs and for predicting the properties of REEs in geological and biological settings. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • MSE model reproduces properties of multicomponent systems containing REE sulfates. • Phase behavior and caloric properties are predicted for REE 2 (SO 4) 3 and Na 2 REE 2 (SO 4) 4. • SLE phase diagrams elucidate the solubility of both stable and metastable phases. • Regularities in solubilities are identified as a function of the REE cation radius. • REE 2 Na 2 (SO 4) 4 show a V-shaped solubility pattern with a local minimum for Pr. Abstract A thermodynamic model has been developed for calculating the phase behavior and caloric properties of rare earth elements in multicomponent sulfate solutions over wide ranges of temperature (up to 300 °C) at concentrations extending to solid–liquid saturation. The model has been constructed using the previously developed Mixed-Solvent Electrolyte (MSE) framework, which combines the standard-state thermochemical properties of aqueous and solid species with an ion-interaction formulation for the excess Gibbs energy. The model accurately reproduces the properties of binary aqueous rare earth sulfate solutions and multicomponent systems that include sulfuric acid and/or sodium sulfate. Solid-liquid phase diagrams have been constructed to elucidate the solubility of both stable and metastable solid phases. Regularities in the solubility behavior have been identified as a function of the cation radius for rare earth sulfates and double sodium-rare earth sulfate salts. The solubility of rare earth sulfates at ambient conditions shows an s-shape pattern with a local maximum at Pr, followed by a plateau between Sm and Tb and then an increase starting from Tb. On the other hand, the double salts REE 2 Na 2 (SO 4) 4 show a solubility minimum at Pr, followed by a strong increase in solubility for heavier rare earths. For the double sulfate salts for which no experimental data are available, solubilities have been estimated using an analysis of trends in thermochemical properties across the lanthanide series. The model is part of a systematic effort to develop a comprehensive treatment of the properties of rare earth salt solutions to facilitate the design and optimization of processes for the recovery of rare earth elements. [ABSTRACT FROM AUTHOR]

A model has been developed for calculating thermodynamic properties and phase equilibria in binary and multicomponent aqueous systems containing rare earth element chlorides. Here, the rare earth elements encompass the lanthanides except promethium plus yttrium. The model is based on a comprehensive database of solid-liquid equilibria, osmotic and activity coefficients, enthalpies of dilution and heat capacities of solutions. The computational framework relies on the previously developed Mixed-Solvent Electrolyte (MSE) model. The model accurately reproduces the properties of binary rare earth chloride – water systems and of ternary mixtures that additionally include HCl, NaCl, or CaCl 2 . Solid-liquid phase diagrams have been obtained to provide a convenient summary of the solubility of stable and metastable hydrated solid phases. Analysis of the stability of solid hydrates reveals systematic trends within the rare earth series. At ambient and elevated temperatures, the calculations show an increase in the stability range of rare earth chloride hexahydrates with decreasing cation radius, accompanied by a gradual disappearance of the heptahydrate. At low temperatures, the stable solid phases transition from the decahydrate at higher cation radii to octahydrate in an intermediate range and a combination of pentadeca- and nonahydrates at lower radii. The calculated standard-state enthalpies of formation of the hydrates agree with independently obtained values from calorimetric measurements, thus verifying the internal consistency of the model. [ABSTRACT FROM AUTHOR]