Extraction of K2CO3 from Low Concentration [K+] Solutions with the Aid of CO2: A Study on the Metastable Phase Equilibrium of K2CO3-Na2CO3-H2O Ternary System

Food grade (K2CO3 wt.%>99.0%, GB/T 25588-2010) potassium carbonate (K2CO3) has been extracted from low concentration [K+] solutions of K2CO3-Na2CO3-H2O, which refers to a subsystem of brine water in the Lop Nor Lake in West China. Procedures of the isothermal evaporation, crystallization, CO2 acidification, filtration, and calcination were employed to prepare the K2CO3. This research focuses on the phase study of metastable equilibria between K2CO3-Na2CO3-H2O and KHCO3-NaHCO3-H2O. The solubility, density, conductivity, and pH values were determined. Phase diagrams were plotted at temperatures of 298.2 and 313.2 K. At 298.2 K the ternary system of K2CO3-Na2CO3-H2O was found to have double salts in form of solid solutions Na2CO3·K2CO3·(6–12)H2O, which would hinder the process of getting pure K2CO3 by the means of isothermal evaporation. In this protocol the high-pressure CO2 was charged to the ternary carbonate solution, and the K2CO3-Na2CO3-H2O was moved to bicarbonate system KHCO3-NaHCO3-H2O. This quaternary system is of a simple co-saturation type of diagram, in which a solid solution (KHCO3·NaHCO3) was found to be greatly affected by temperatures. This finding affords efficient separation of KHCO3 from the KHCO3-NaHCO3-H2O solutions.