Crystallization of monosodium aluminate hydrate from a concentrated aluminate solution containing silica

The influence of silica content in concentrated aluminate solution on the crystallization of monosodium aluminate hydrate (MAH) has been investigated systematically. The resultant crystals were found to be round flakes, with a high silica concentration significantly inhibiting the crystallization. The conversion ratio and the average particle size of MAH crystals decreased with increasing SiO2 concentration in solution. The silicate impurity phases which crystallized simultaneously with MAH were identified as NaAlSiO4 and Na2SiO3 from low silica solution, Ca3Al2Si3O12, Ca3Al2(SiO4)2(OH)4, and Na4Ca4(Si6O18) we obtained when there was a high silica concentration. These phases have a much higher silica content than the more common desilication product Ca3Al2(SiO4)n(OH)12–4n from the Bayer process for alumina production where n is usually below 0.8.