Mechanism and kinetics of the growth of zeolite microcrystals - Part 1: Influence of the alkalinity of the system on the growth kinetics of zeolite A microcrystals

The kinetics of the crystal growth of zeolite A were measured during the crystallization of zeolite A at 80 degrees C from an amorphous aluminosilicate precursor (1.03Na(2)O . Al2O3 . 2.38SiO(2) . 1.66H(2)O) dispersed in 1.2 M (system I), 1.4 M (system II), 1.6 M (system III), 1.8 M (system IV) and 2 M (system V) NaOH solutions. Analysis of the experimental results (changes in the concentrations C-Al of aluminum and C-Si of silicon in the liquid phase as well as of the dimension L-m of the largest crystals of zeolite A during crystallization) shows that the growth rate of zeolite A crystals is independent of size, and that the growth is governed by the reaction of monomeric and/or low-molecular aluminate, silicate and aluminosilicate anions from the liquid phase on the surfaces of the growing zeolite crystals. The crystal growth takes place in accordance with the model of Davies and Jones for growth and dissolution, and taking into consideration the particularities of crystallizing zeolite systems, the kinetics of crystal growth of the zeolites can be defined as dL/dt(c) = k(g)[C-Al - C-Al*][C-Si - C-Si*], where C-Al* and C-Si* are the concentrations of aluminum and silicon in the liquid phase which correspond to the solubility of zeolite A at given crystallization conditions, and k(g) is the rate constant of the linear growth of zeolite A crystals. (C) 1999 Elsevier Science B.V. All rights reserved.