Solid-Liquid Phase Equilibria of the Ternary System K2B4O7–KHCO3–H2O at 273.2, 288.2, and 323.2 K

The K₂B₄O₇–KHCO₃–H₂O ternary system serves as a prototypical subsystem for brine in the marine Triassic gas fields of the Sichuan Basin. Investigating its multitemperature phase equilibria is vital for brine resource development. In this work, the isothermal dissolution equilibrium method was employed to study the solid–liquid phase equilibria of the K₂B₄O₇–KHCO₃–H₂O ternary system at 273.2, 288.2, and 323.2 K, pinpointing the system’s solid–liquid phase region. The findings show that at the three temperatures, this ternary system possesses a solitary invariant point, two univariant curves, and two solid-phase crystallization regions. As the temperature increasing, the system’s density increases, yet the crystal forms stay constant as KHCO₃ and K₂B₄O₇·4H₂O. Meanwhile, the solubility of K₂B₄O₇ and its salting-out effect on KHCO₃ both rise with temperature. Furthermore, the Pitzer model and its parameters were employed to fit the single-salt parameters and lnKsp of K₂B₄O₇ at 273.2 and 323.2 K, and mixed-ion interaction parameters θ_{HCO3^–,B4O7^2–} and Ψ_{K⁺,HCO3^–,B4O7^2–} at 273.2, 288.2, and 323.2 K. Combining the Pitzer model with the particle swarm optimization algorithm, the solubilities of the K₂B₄O₇–KHCO₃–H₂O ternary system were predicted at three temperatures. The high consistency between experimental and calculated values confirms the excellent applicability of the model parameters.