Metastable zone width and the primary nucleation kinetics for cooling crystallization of NaNO3 from NaCl-NaNO3-H2O system

Abstract

For investigating the fractional crystallization process of high-saline wastewater from coal chemical industry, the primary nucleation kinetics of NaNO3 during cooling crystallization was studied. 20 g NaCl/100 g H2O was added to simulate the high-saline environment, which resulted in a ternary NaCl-NaNO3-H2O mixture system. Based on the measured solubility and supersolubility in the temperature range from 288.09 to 311.70 K by using ultrasonic velocity sensor, the effects of stirring rate, cooling rate and presence of seed crystals on the metastable zone width (MSZW) were analyzed. The results showed that increasing the saturation temperature as well as increasing the stirring rate will make the MSZW of NaNO3 narrow. This is mainly attributed to the intensification of mass transfer and diffusion, which promotes the crystal nucleation. By contrast, the change in the size of solute molecular clusters in the solution may lag behind the transformation of the solution state, so that the MSZW of NaNO3 becomes wide when increasing the cooling rate. Meanwhile, because the addition of seed crystals is beneficial to induce the solution to start crystallization at a lower supersaturation, it always leads to the narrowing of MSZW, whereas the dosage has a relatively weak effect in the scope of experimental research. Furthermore, self-consistent Nývlt-like equation and classical 3D nucleation theory were used to study the primary nucleation kinetics of NaNO3 during cooling crystallization process. Corresponding nucleation kinetic equations were regressed based on the experimental data. The activation energies Esat of NaNO3 nucleation process fitted by these two models were 28.97 and 26.08 kJ/mol, respectively.