Study on metastable zone width and primary nucleation kinetics for cooling crystallization of KNO3
For investigating the cooling crystallization process of KNO3 in a ternary KNO3-NaNO3-H2O mixture system,the solubility of KNO3 was measured by static equilibrium method in the temperature range from 293.15 K to 341.38 K.In order to present the relationship between solubility and temperature,the experimental data was fitted by the Apelblat equation and Van't Hoff equation,and the dissolution enthalpy and dissolution entropy of KNO3 were calculated.Then,the effects of NaNO3 content,stirring rate,and cooling rate on the width of the metastable zone(MSZW)of KNO3 in the temperature range from 298.03 K to 332.30 K were analyzed by using an ultrasonic velocity sensor.The results showed that the existence of NaNO3 would make the MSZW of KNO3 narrow,but the effect was limited,and the MSZW became narrow with the in-crease of stirring rate and wide with the increase of cooling rate.Based on the measured solubility and dissolution enthalpy,the primary nucleation kinetics of KNO3 were investigated.The nucleation order m obtained using the self-consistent Nývlt-like equation was less than 3.Instantaneous nucleation was the dominant nucleation mechanism of KNO3,and the nucleation rate showed a tendency to increase firstly and then decrease with increasing saturation temperature.In the classical three-di-mensional nucleation theory,nucleation was more likely to occur,and when the saturation temperature reached a certain level,the solid-liquid interface energy γ tended to be stable.Furthermore,the high-purity KNO3 product was obtained by cooling crystallization with agricultural potassium nitrate as raw material.When the saturated KNO3 solution was cooled from 343.15 K to 298.15 K,the stirring rate was 300 r/min,the cooling rate was 25 K/h,and the seed crystal was added,the Na+content in the solution was only 0.001 1%.
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