Solid–Liquid Phase Equilibria of Quaternary System K2B4O7–KHCO3–Na2B4O7–NaHCO3–H2O at 288.2 and 308.2 K

Abstract

Investigation of the relevant multitemperature phase equilibrium data is of guiding importance for the recycling and utilization of rich-boron brine resources. In this work, the solid–liquid phase equilibria of the K2B4O7–KHCO3–Na2B4O7–NaHCO3–H2O quaternary system at 288.2 and 308.2 K were investigated using the isothermal dissolution equilibrium method. The results demonstrate that the system exhibits Type I hydrate behavior at both temperatures, with each phase diagram featuring two invariant points. The equilibrium solid phases at invariant points are NaHCO3 + Na2B4O7·10H2O + K2B4O7·4H2O and NaHCO3 + K2B4O7·4H2O + KHCO3, respectively. Each phase diagram consists of five solubility equilibrium curves and four crystallization regions corresponding to the solid phases: Na2B4O7·10H2O, K2B4O7·4H2O, KHCO3, and NaHCO3. Comparative analysis of the crystallization regions at the two temperatures reveals that the region for Na2B4O7·10H2O shrinks with increasing temperature, whereas those for NaHCO3, K2B4O7·4H2O, and KHCO3 expand. At both temperatures, the crystallization region of Na2B4O7·10H2O is the largest, followed by that of NaHCO3, while KHCO3 has the smallest region. This trend indicates that KHCO3 has the highest solubility and is therefore the most difficult to crystallize and separate from the saturated solution.