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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]

We apply the SAFT-VR+DE equation of state to the study of experimental mixed-solvent electrolyte solutions. In the non-primitive model based SAFT-VR+DE approach (Zhao et al., JCP 2007, 126 , 244503) the ions are considered fully dissociated within the solvent that is explicitly treated within the model and the theoretical framework. Typically in the study of electrolyte systems the simpler primitive model is used, which requires knowledge of the experimental dielectric constant. With the non-primitive model the dielectric constant is calculated as part of the theory, which is a particularly attractive feature in the study of mixed-solvent electrolyte systems as data for the experimental dielectric constant of such systems is more scarce. Here for the first time as far as the authors are aware, a non-primitive based equation of state has been used for the study of mixed-solvent electrolytes. The solvents considered (water, methanol and ethanol) are modeled using the SAFT-VR+D approach (Zhao et al., JCP 2007, 127 , 084514; Zhao et al., JCP 2006, 125 , 104504) in which the contribution of the dipole to the thermodynamics and structure of the fluid are explicitly accounted for. The theory is found to accurately predict the vapor-liquid equilibrium, as well as dielectric properties of the salt free alcohol-water mixtures both at room and elevated temperatures. Ternary mixtures of salt/water/alcohol are then studied using the SAFT-VR+DE parameters for the salts determined in earlier work (Das et al., AIChE Journal 2015, 61 , 3053-3072) and a cation-alcohol unlike dispersive energy parameter obtained by fitting to mean ionic activity coefficient data at room temperature and pressure. Thus, with only one adjustable parameter, a predictive SAFT-VR+DE equation to study mixed-solvent electrolyte systems is developed. The SAFT-VR+DE predictions are found to be in good quantitative agreement with mean ionic activity coefficient data for several mixed-solvent electrolyte systems over a wide range of molalities and different solvent ratios. The model is parameterized to allow the molecular level interactions between different cations and the surrounding solvent molecules to be explored. [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]