Influence of Organic Impurities on Fractional Crystallization of NaCl and Na2SO4 from High-Salinity Coal Chemical Wastewater: Thermodynamics and Nucleation Kinetics Analysis

Abstract

It is a valid path to realize the zero discharge of coal chemical wastewater by using the fractional crystallization method to recycle the miscellaneous salt in high-salinity wastewater. In this study, the thermodynamics and nucleation kinetics of sodium chloride (NaCl) and sodium sulfate (Na2SO4) crystallization in coal chemical wastewater were systematically studied. Through analyses of solubility, metastable zone width, and induction period, it was found that the impurity dimethoxymethane would increase the solid–liquid interface energy and critical crystal size during the nucleation of Na2SO4. Ternary phase diagrams of the pseudo-ternary Na2SO4-NaCl-H2O systems in simulated wastewater were plotted in the temperature range of 303.15 to 333.15 K, indicating that a co-ionization effect existed between NaCl and Na2SO4, and NaCl had a strong salting out effect on Na2SO4. Finally, the nucleation rate and growth rate of Na2SO4 crystals under simulated wastewater conditions were determined by the intermittent dynamic method, and the crystallization kinetic models of Na2SO4 were established. The crystallization nucleation of Na2SO4 crystals was found to be secondary nucleation controlled by surface reactions. The basic theoretical research of crystallization in this study is expected to fundamentally promote the application of fractional crystallization to realize the resource utilization of high-salinity wastewater in the coal chemical industry.