Desilication mechanism and kinetics of synthesized hydroxy-sodalite in high-alkali sodium aluminate solutions

Desilication in the solution with Na2O concentration below 300g/L was extensively studied, such as various desilication agents mostly of calcium salts or compounds proposed in the literature. However, desilication in high-alkali sodium aluminate solution was necessary in the production of sodium aluminate hydrate (SAH), but much difficult as the high silicate solubility in concentrated caustic solution. Therefore, an effective desilication method was investigated with different mechanism in this paper. The hydroxy-sodalite compound was synthesized and found with excellent desilication properties in highly caustic aluminate solution until 470-530g/L Na2O, and the desilication mechanism in concentrated sodium aluminate solution was presented firstly. The crystallization reaction of silicate with aluminate in the sodium aluminate solution during the desilication was proposed. It was observed that the aluminate concentration in the concentrated solution considerably affected the desilication. Furthermore, the kinetics equation was inferred as -d sigma/dt=exp(29.30-11311/T) (sigma)(n), with the obtained apparent activation energy and order dependence of 94.04kJ/mol and 2.47 +/- 0.47. Meanwhile, the equilibrium SiO2 concentrations of the synthesized hydroxy-sodalites in the sodium aluminate solutions with 470-530g/L Na2O at 353-393K were determined