Investigation of Solid–Liquid Equilibria on the System Na+, K+//Cl–, SO42––H2O and Na+, K+// SO42––H2O at 313.15 K

Solid–liquid equilibria on the reciprocal quaternary system (Na+, K+//Cl–, SO42––H2O) and its ternary system (Na+, K+// SO42––H2O) at 313.15 K are studied with the method of isothermal solution saturation. The solubilities and physical properties (nD, η, ρ) of saturated solutions in the two systems are determined. The phase diagram of the ternary system at 313.15 K includes two invariant points, three invariant curves, and three fields of crystallization of single salts corresponding to K2SO4, Na2SO4·3K2SO4, Na2SO4. The two invariant points are cosaturated with two salts, that is (K2SO4 + Na2SO4·3K2SO4) and (Na2SO4 + Na2SO4·3K2SO4), respectively. The reciprocal quaternary system at 313.15 K is divided into seven univariant curves, three invariant points cosaturated with three-salts and five crystallization fields. A double salt Na2SO4·3K2SO4 can be proven to exist in the reciprocal quaternary system (Na+, K+//Cl–, SO42––H2O) and the ternary system (Na+, K+// SO42––H2O) under phase equilibrium conditions at 313.15 K. On the basis of the phase diagram, the crystallization field of GLa and K2SO4 is bigger than that of the other three salts, whereas the KCl crystallization field is the smallest. Therefore, K2SO4 can crystallize out more easily from this mixing equilibrium solution except for Na2SO4·3K2SO4. The experimental results showed that both systems are a complex eutectic type, and no solid solutions are found at the investigated temperature. All the physical properties (nD, η, ρ) of the equilibrium quaternary system and ternary system change regularly with concentration change of the liquid phase. The measured data and phase equilibrium diagrams can be used for a new technology to design and optimize the separation process of K2SO4 (KH2PO4), and provide fundamental data support for chemical industry development.