Zeolite synthesis in unstirred batch reactors II. Effect of non-uniform pre-mixing on the crystallization of zeolites A and X

The effects of deliberate non-uniform pre-mixing of the aluminate and silicate solutions in the unstirred batch syntheses of zeolites A and X were studied. Poorly mixed hydrogels gave rise to impurity phases, with the amount of impurity decreasing with improved pre-mixing. Experiments were carried out in reactors specially designed for the growth of large zeolite crystals under microgravity conditions, and in a parallel study using high-density polyethylene bottles. Both studies evidenced the appearance of impurities if the hydrogel was not well mixed prior to heating. No consistent effect was observed on the particle size distributions of the zeolite products. These results underlined the importance of proper reactor design for unstirred syntheses, and the usefulness of a non-invasive diagnostic technique for mixing effectiveness, such as nuclear magnetic resonance (NMR) imaging. The applicability of NMR imaging to the systems studied here was demonstrated in Part I of this series.