Investigation of the influence of seeding on the crystallization of zeolite A in the membrane-type reactor

The mutual interactions between zeolite A seed crystals, added to a clear aluminosilicate solution (seed-side), and an amorphous aluminosilicate gel (gel-side), were studied by crystallization of zeolite A. The seed-side in a membrane reactor was separated from the gel-side by a membrane (made of nylon-66) having average pore size openings of 0.8 mu m. The membrane allows 'free' passage of soluble species, while keeping the solids separate. An increase of the amount of zeolite formed by growth of the seed and a simultaneous decrease of the amount of zeolite crystallized in the gel-side of the reactor indicate that the reactive species formed by dissolution of the gel pass from the gel-side through the membrane to the seed-side of the reactor. Particulate properties of the zeolite formed in the seed-side of the reactor depend considerably on the reaction time needed for complete transformation of the gel into the zeolite, and thus on the amount of seed added to the seed-side of the reactor as well as on the agglomeration processes that occur simultaneously with the crystallization process. On the other hand, the particulate properties of zeolite A formed in the gel-side of the reactor do not depend on the amount of seed added to the seed-side of the reactor. Hence, it was concluded that the crystallization of zeolite A in the gel-side of the reactor occurs by the growth of nuclei 'released' from the gel dissolved during the crystallization. (C) 1999 Elsevier Science B.V. All rights reserved.