Effects of Al₂(SO₄)₃ and K₂SiO₃ Impurities on the Crystallization of K₂SO₄ from Aqueous Solution

The results of an investigation of the effect of Al₂(SO₄)₃ and K₂SiO₃ impurities on the crystallization of K₂SO₄ from aqueous solutions are described and discussed. The metastable zone width (MSZW) of K₂SO₄ aqueous solutions, without and with known concentrations of the impurities, saturated at 40 and 60 °C is determined, using ultrasonic sensor by the controlled solution cooling method at predefined cooling rates R. It is observed that the value of the MSZW of the two types of saturated solutions increases with an increase in the cooling rate R and the concentration C_i of the two impurities. The experimental data of the dependence of MSZW on cooling rate and impurity concentration are analyzed by an approach based on the classical 3D nucleation theory. The results showed that: 1) both impurities suppress the formation of 3D nuclei by physically blocking the active sites on their surfaces, 2) Al₂(SO₄)₃ impurity is more effective than K₂SiO₃ impurity, and 3) the purity of the K₂SO₄ crystals is insignificantly affected by these impurities, but the morphology and size of the obtained crystals are influenced strongly by Al₂(SO₄)₃ impurity.