Phase equilibria in the system Na+, K+//SO42−-(CH2OH)2-H2O and Na+, K+//Cl−, SO42−-(CH2OH)2-H2O at 328.15 K

Abstract

An experimental study on phase equilibria at 328.15K in the system (Na+, K+//Cl−, SO42−-(CH2OH)2-H2O) and its subsystem (Na+, K+//SO42−-(CH2OH)2-H2O) was carried out by the method of isothermal solution saturation. In these systems, the initial mass fraction of ethylene glycol is 30% in the salt-free binary solvent (water+ethylene glycol). The solubilities, densities, viscosities, and refractive indices of saturated solutions have been measured for the two systems. According to the experimental results, solubility curves and physical properties (ρ, η, nD) curves are plotted. In the subsystem (Na+, K+//SO42−-(CH2OH)2-H2O), there are two invariant points, three invariant curves, and three fields of crystallization corresponding to K2SO4, Na2SO4.3K2SO4, Na2SO4. In the system (Na+, K+//Cl−, SO42−-(CH2OH)2-H2O), there are five crystallization fields, seven univariant curves, and three eutonic points corresponding to three salts, that is (K2SO4+Na2SO4.3K2SO4+KCl), (Na2SO4+Na2SO4.3K2SO4+KCl) and (Na2SO4+NaCl+KCl), respectively. No solvate phases with ethylene glycol are observed and no solid solutions are found. All the physical properties (ρ, η, nD) of both systems change regularly with concentration change of the liquid phase. In comparison with the phase diagram of the system (Na+, K+//Cl−, SO42−-(CH2OH)2-H2O) at 328.15K and this quaternary system (Na+, K+//Cl−, SO42−-H2O) at 323.15K, the size of the salt crystallization fields changes significantly. The experimental results reveal that the presence of ethylene glycol significantly reduces the solubility of the salts in the aqueous solution. These changes between phase diagrams at different solvents and temperatures will be very useful for extracting the salts. According to this work, the measured values and phase equilibrium diagrams can be used for a new technology to design and optimize the production of K2SO4 by the conversion process with industrial KCl and mirabilite aqueous mixtures, and provide fundamental data support for chemical industry development.