Crystallization kinetics of sodium sulphate in a salting out MSMPR crystallizer system Na₂SO₄/H₂S0₄/H₂O/MeOH /

The growth and nucleation rates of sodium sulfate crystals salted out from their solution in a 38% w/w sulfuric acid solvent by the addition of an 80:20 w/w methanol : water solution, were determined from measurements of the steady-state crystal size distribution (CSD) generated in a continuous mixed-suspension, mixed-product-removal (MSMPR) salting out crystallizer, and the effect of the supersaturation, the crystal suspension density and the temperature on these rates was investigated. The effect of the Cr+ + + impurity was also briefly studied. For the pure system, the power-law kinetic rate equations of crystal growth (G = KGSg ) and crystal nucleation (B = KBSb ) were fitted to the experimental data and the fitting parameters were determined at each of 25, 30 and 35°C. In addition, the nucleation and growth rate data were fitted to the relative kinetic equation (B = KbGi) to provide a basis for comparison between i and b/g. Secondary nucleation effects were investigated by testing, in parallel with the primary nucleation rate equation B = KBSb, the rate equation B = KNMTJSu which includes a nucleation dependence on the crystal suspension density. The relative kinetic equation B = KnMJTGv was also tested for comparison (v compared with u/g). No evidence of secondary nucleation could be shown. The fitted estimates of j were non-positive at all temperatures and statistically non-different from zero at 35 and 30°C, just approaching significance at 25°C with a negative point estimate for j. Primary nucleation was shown to be the dominant nucleating mechanism for this system. The growth rate kinetic order g was determined to be essentially unity (G = KGS0.97 ) and was statistically non-changing with temperature. The nucleation rate kinetic order b was similarly temperature independent and was determined as 2.1 (B = KBS2.1). The kinetic rate constants KG and KB were both a function of temperature and were fitted to an Arrhenius type temperature dependence to give KG = 43960 exp(-24600/RT) and KB = 0.064 exp(70870/RT) where R is in kJ/kmole-K. The activation energy for growth was positive at 24600 kJ/kmole. The nucleation activation energy was negative and larger in magnitude at -70870 kJ/kmole. In the presence of the chromium impurity, the kinetic parameters KG , g, KB , Kb, and i were determined at three levels of the concentration of the impurity in the crystallizer feed. An increase in the impurity concentration was shown to cause an increase in the growth rate constant KG but had no significant effect on the nucleation kinetics. The result was a decrease in the relative rate constant and therefore a larger crystal size. The kinetics of crystallization as determined for this system would indicate that a high temperature, high crystal suspension density and long residence time are conditions which are favourable for the production of a large sodium sulfate crystal.

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