1 SPECI ATI ON AND PHASE BEHAVI OR I N MI XED SOLVENT ELECTROLYTE SOLUTI ONS: THERMODYNAMI C MODELI NG P. Wang, A. Anderko, R. D. Springer, and M. M. Lencka OLI Systems, Inc., 108 American Road, Morris Plains, NJ 07950, U.S.A. pwang@olisystems.com A comprehensive mixed-solvent electrolyte (MSE) model has been applied to provide a thermodynamic foundation for crystallization studies. The model can be used to calculate phase equilibria, speciation, and other thermodynamic properties of multicomponent solutions containing electrolytes (salts, acids, or bases) in water, organic, or mixed solvents. The thermodynamic framework has been design to reproduce the properties of both aqueous and mixed-solvent electrolyte systems ranging from dilute solutions to solid saturation or pure-solute limit. The model combines an excess Gibbs energy model with detailed speciation calculations. The excess Gibbs energy model consists of a long-range interaction contribution represented by the Pitzer-Debye- Hückel expression, a short-range term expressed by the UNIQUAC model and a second-virial- coefficient type term for specific ionic interactions. The model accurately represents the solubility behavior of aqueous, non-aqueous and mixed-solvent electrolyte mixtures that are of interest in crystallization. The accuracy of the model has been demonstrated for the CaO - P 2 O 5 - H 2 O, Na – HCO 3 – CO 3 – H 2 O and Na – K – Mg – Ca – Cl – SO 4 – methanol – H 2 O systems. Of particular importance is the model’s capability of reproducing the solubilities in multicomponent systems based on parameters obtained from binary data and its accuracy of predicting the correct solid phases in systems with widely varying solvent and ionic compositions. 1. I ntroduction Electrolyte thermodynamics is of great importance for the simulation and optimization of crystallization processes where multicomponent electrolyte solutions are encountered at high concentrations under diverse conditions of temperature and pressure. Such solutions are challenging for computational models because of their complex chemical behavior and strong nonideality. A characteristic feature of electrolyte solutions is that phase equilibria and other thermodynamic properties are often inextricably linked to speciation equilibria, which may be due to ion pairing, acid-base reactions, complexation and other phenomena. In many industrial processes, speciation-related properties such as pH, oxidation-reduction potential, distribution of complexed or hydrolyzed species, etc., are of primary importance. Thus, self-consistent treatment of speciation and phase equilibria is of utmost importance for realistic simulation of electrolyte systems. Recently, a general, speciation-based thermodynamic model for mixed-solvent electrolyte solutions has been developed [AND02, WAN02]. This model was shown to reproduce simultaneously vapor-liquid equilibria, solid-liquid equilibria, liquid-liquid equilibria, speciation, caloric and volumetric properties of electrolytes in water, organic, or mixed solvents [WAN04, WAN06]. The model is valid for salts from infinite dilution to the fused salt limit and for various completely miscible inorganic systems (such as acid-water mixtures) over a full concentration range [KOS07]. Also, the model is capable of representing phase equilibria in multicomponent inorganic systems containing multiple salts, acids, and bases [WAN04, WAN05, GRU07]. Complex phase behavior such as formation of multiple hydrated salts, double salts, or the presence of eutectic points has been accurately represented. The model is referred to as the mixed-solvent electrolyte (MSE) model because it is equally valid for classical aqueous systems, those with more