Phase Equilibrium In KHCO₃-NaHCO₃-H₂O System: An Applied Study Of Pitzer's Theory

The study is a part of the project Technology of Extracting K2CO₃ from Insoluble Potassium, the filtered liquid of the process is a NaHCO₃-KHCO₃-H₂O system. The NaHCO₃-KHCO₃-H₂O system has been studied at 35℃and 25℃by evaporation equilibrium method, and at 25℃by dissolution equilibrium method. The study can be referred to the Technology of Extracting K2CO₃ from Insoluble Potassium.After the studying of the NaHCO₃-KHCO₃-H₂O system by evaporation-equilibrium method at 35℃, the data of equilibrium liquid phase and chemical-physical properties have been mensurated,and the phase graph have been drawn in NaHCO₃-KHCO₃-H₂O system. The NaHCO₃·Na₂CO₃·2H₂O and double salt KNaCO₃ have been found during processing of phase equilibrium and the NaHCO₃?Na2CO₃?2H₂O is absolute crystal section, the liquid density , pH value, conductance and refractive index rises with the augment of K/Na and keep invariableness after reaching given proportion some point.After the studying of the NaHCO₃-KHCO₃-H₂O system by evaporation-equilibrium method in 25℃, with the increading of K/Na, the liquid refractive index, density, conductance and pH value of this liquid phase rise steadily until keep invariableness after reaching equilibrium. The double salt KNaCO₃ have not been found during processing of phase equilibrium just like in 35℃. and the thermodynamics model of the Pitzer` theory have been established in this system. The concentration of the liquid phase have been forecasted by pitzer equation and the phase graph has been drawn in the NaHCO₃-KHCO₃-H₂O system. The outcome of calculation by pitzer equation accords with the result of experiment.After studying the phase equilibrium and the chemical-physical propertiesof contents in NaHCO₃-KHCO₃-H₂O system by dissolution-equilibrium method, the following conclusion have been drawn, with the increading of KHCO₃, the liquid refractive index, density, conductance and pH value of this liquid phase rise steadily until maximum when reaching K/Na equilibrium and falls from then on. Correspondingly, by means of XRD according to the selected points, it is can been found that with the adding of KHCO₃, the main solid content is NaHCO₃ at the beginning and it changes to KHCO₃ with the adding of KHCO₃ incessantly till the W_KHCO3/W_NaHCO3≥4.8 in this system.