Solid-liquid phase equilibria of quaternary systems Na+//Cl−, CO32−, SO42− - H2O and Na+//Cl−, CO32−, NO3− - H2O at T = 313.15 K

Abstract

The solid-liquid equilibrium of quaternary systems (Na+//Cl−, CO32−, NO3− -H2O) and (Na+//Cl−, CO32−, SO42− -H2O) at 313.15 K was studied by an isothermal solution saturation method and the solubility of salts and equilibrium solid phases in the two systems was determined. The solid phases formed in the systems studied were determined by the Schreinemaker wet residue method. Based on the measured data, dry-salt phase diagrams and the relevant water-phase diagrams were plotted in the two systems. The stable phase diagram of Na+//Cl−, CO32−, NO3− -H2O at 313.15 K is characterized by one quaternary invariant point (Q), three univariant curves, and three crystallographic forms (corresponding to NaCl, NaNO3 and Na2CO3·H2O). The crystallization region of NaNO3 (I) is the smallest, and its solubility is the greatest, which indicates that NaNO3 has a strong salt-out effect on Na2CO3·H2O and NaCl. The composition of the solution at equilibrium corresponding to invariant point is w(NaCl) = 9.76 wt%, w(NaNO3) = 32.79 wt%, w(Na2CO3) = 6.10 wt%. Therefore, it can be considered that there is no double salt and solid solution in the quaternary system, which belongs to the simple quaternary system. For the quaternary systems (Na+//Cl−, CO32−, SO42− -H2O) at 313.15 K, experimental results indicate that there are four crystallographic forms (NaCl, Na2CO3·H2O, Na2SO4 and Bur (2Na2SO4 Na2CO3)), five univariant curves, and two invariant points corresponding to three salts, that is (NaCl + Na2SO4 + 2Na2SO4 Na2CO3) and (NaCl + Na2CO3·H2O + 2Na2SO4 Na2CO3), respectively. The experimental results show that this system is a complex eutectic type, and no solid solutions are found at the investigated temperature. All the physical properties (ρ, η, nD) of the equilibrium two quaternary systems change regularly with concentration change of the liquid phase. The phase equilibria in the quinary Na+//C1−, SO42−, CO32−, NO3−-H2O system at 313.15 K were also studied using the through and intermediate translation techniques together. All data and results obtained in this paper are of great significance to the design and optimization of the fractional crystallization process of the high-saline wastewater in coal chemical industry.