Influence of the freeze-drying of hydrogel on the critical processes occurring during crystallization of zeolite A – A new evidence of the gel “memory” effect

An aqueous aluminosilicate hydrogel having oxide molar composition: 4.72Na2O · Al2O3 · 1.93SiO2 · 254.86H2O was treated in two ways: (1) the hydrogel (system SNL) was heated at 80 °C until the entire amount of amorphous aluminosilicate precursor (gel) has been transformed to zeolite A, (2) the hydrogel was frozen in liquid air and freeze-dried; solid liophilizate was poured in an appropriate volume of water, and the obtained suspension (system SL) was treated in the same way as the system SNL. The drastic treatment of hydrogel did not substantially change the distribution of nuclei in the matrix of the amorphous aluminosilicate precursor (gel) established during its precipitation. The consequence is that, in accordance with the principles of the gel “memory” effect, the crystal size distributions of the crystalline end product (zeolite A) obtained from both the systems (SNL and SL) are almost the same. On the other hand, the above mentioned treatment of hydrogel changes the concentrations of aluminium and silicon in the liquid phase and caused that the rate of crystal growth is somewhat higher in system SL than in system SNL. Population balance analysis of the crystallization processes has shown that the difference of the kinetics of crystallization of zeolite A from the systems SNL and SL is in larger extent caused by the small difference in the crystal growth rate and in the smaller extent by the particulate processes determined by the specific number of nuclei NS and distribution of nuclei in the gel matrix.