Investigation of Solid–Liquid Equilibria on the Na+//Cl–, NO3–, SO42––H2O System and the Na+//NO3–, SO42––H2O System at 313.15 K

Abstract

Equilibria on the Na+//Cl–, NO3–, SO42––H2O system and the Na+//NO3–, SO42––H2O system at 313.15 K are investigated used the isothermal solution saturation method. The composition and physicochemical properties (refractive index nD, viscosity η, density ρ, and pH) of saturated solutions are determined. The phase diagram of the Na+//NO3–, SO42––H2O system includes two invariant points, three invariant curves, and three crystallization fields of single salts, Na2SO4, Na2SO4·NaNO3·H2O, NaNO3. Invariant points are cosaturated with (Na2SO4 + Na2SO4·NaNO3·H2O) salts and (NaNO3 + Na2SO4·NaNO3·H2O) salts, respectively. The Na+//Cl–, NO3–, SO42––H2O system consists of five univariant curves, two invariant points cosaturated with three salts, and four crystallized regions. Double salt Na2SO4·NaNO3·H2O can be verified to exist in the Na+//Cl–, NO3–, SO42––H2O and Na+//NO3–, SO42––H2O systems under equilibrium conditions at 313.15 K. In addition, comparing with reported phase diagrams in different temperatures, crystallized regions of Na2SO4·NaNO3·H2O decrease with increasing temperature and disappear at 373.15 K. The crystallized region of sodium sulfate widens, while both crystallized regions of NaNO3 and NaCl decrease with increasing temperature. All of the physicochemical properties (nD, η, ρ, pH) of equilibrium solutions alter with a composition change of the liquid phase. Phase equilibria of Na+//C1–, SO42–, NO3––H2O at 313.15 K were also investigated by the through and intermediate translation techniques. All data and results from our work are important to design and optimize the crystallization of high-saline wastewater in the coal chemical industry and could be used to solve inorganic salt separation from the system containing the above-mentioned salts.