Thermodynamics Research Center / ThermoML | Journal of Chemical and Engineering Data

ABSTRACT

Fractional crystallization is considered as a critical technology for solving the problem of high-saline wastewater in modern coal chemical industry. This study mainly concentrated on the topic of phase equilibrium of the Na+//Cl-, NO3-, SO42--H2O, which is important fundamental data for the design and control of the fractional crystallization process. The stable phase equilibrium of the quaternary system and its ternary sub-systems at 333.15 K were measured with its focus on the formation of double salt. Different from the simple phase equilibrium of the ternary sub-systems at 353.15 K, there is double salt generated in the Na+//NO3-, SO42--H2O system at 333.15 K. The double salt was determined to be NaNO3*Na2SO4*H2O by combining the wet residual composition analysis and XRD test. Based on this characteristic, the quaternary Na+//Cl-, NO3-, SO42--H2O system totally has two invariant points and five univariant curves, which generate four fields corresponding to the crystallization of Na2SO4, NaCl, NaNO3 and the double salt. By comparing the areas of the crystallization fields, the order of the solubilities of single salts in the multi-component system is Na2SO4 less than NaCl less than NaNO3. Meanwhile, the crystallization region of the double salt is relatively small, which indicates that the double salt is going to be disappeared soon when increasing the temperature further. Related results provide important basis for the design and optimization of the fractional crystallization. It is especially meaningful for the crystallization control to avoid the formation of double salt.