Comprehensive Thermodynamic Model of Boron-Containing Brine Systems: Phase Diagram Experiments and Modeling for Quaternary K2B4O7–K2SO4–KCl–H2O System

Boron-containing brines are widely distributed in nature, and within the pH range of salt lakes, B4O72– is one of the primary boron forms. To express the brine properties and phase equilibrium behavior of boron-containing brine systems, experiments and thermodynamic research based on the eNRTL framework were carried out. Tetraborate aqueous systems are focused in this study: (1) the phase diagram data of K2B4O7–K2SO4–H2O and K2B4O7–K2SO4–KCl–H2O systems at 298.15 and 323.15 K were experimentally determined; (2) the heat capacity of the B4O72– ion was determined via simultaneously fitting the thermodynamic property and solubility data of Li2B4O7, Na2B4O7 and K2B4O7 aqueous systems; (3) the liquid parameters (interaction parameters between water–ionic pair and ionic pair–ionic pair) and solid parameters (ΔfGk°, ΔfHk°, ΔCp,k°) for three binary systems (Li2B4O7–H2O, Na2B4O7–H2O, K2B4O7–H2O) and two ternary systems (K2B4O7–K2SO4–H2O, K2B4O7–KCl–H2O) were determined by fitting the multitemperature phase diagram data; and (4) the complete structures of the phase diagram for the ternary and quaternary systems were predicted. The results demonstrate that (1) the models can effectively describe the solution properties and solid–liquid phase equilibrium behavior of tetraborate-containing aqueous systems, (2) the parameters exhibit thermodynamic consistency, and (3) the predicted complete phase diagram is reasonable.