GB972833A - Hardened molecular sieve agglomerates and manufacture thereof - Google Patents

Abstract

An agglomerate formed of a zeolitic molecular sieve and a clay mineral binder is hardened by impregnating in the hydrated state with an aqueous solution of an alkali metal silicate having a solid content of 3-35% by weight, and firing the impregnated agglomerate at at least 343 DEG C. but below the temperature at which the crystalline zeolotic molecular sieve loses its structural stability. The sieve may be zeolite A, T, X, or Y, or the natural minerals erionite, chabazite, analcite, faujasite, clinoptilolite, or mordenite, and the silicate may be sodium, lithium, potassium, rubidium or caesium silicate. In the case of sodium zeolite A used for removing moisture from a refrigerant fluid such as di-fluoromonochloromethane, the adsorptive capacity of the sieve for the fluid is reduced to substantially zero while the absorptive capacity for water is retained. The clay may be attapulgite, kaolin, sepiolite, polygarskite, kaolinite, a plastic ball clay, bentonite, montmorillonite, illite, chlorite, or an attapulgite-, kaolin-, or bentonite-type clay. The weight ratio of alkali metal oxide: SiO2 in the silicate solution may be 0.27-0.63: 1, while the weight ratio (on a substantially water-free basis) of zeolite: clay in the agglomerate may be 9: 1 to 3: 1. The agglomerate is preferably in its unfired form, but, if not, it should be restored to a fully hydrated state before impregnation. The agglomerates may be soaked in the silicate solution either in batch or continuous operation, and the soaking may be hot or cold, temperatures below 60 DEG C. being preferred. The immersion time may vary from a few minutes to several days according to conditions, longer times effecting a deeper hardening. Immersion may be followed by a brief rinse to remove surface solution to prevent adhesion of the agglomerates. The agglomerates may be fired immediately or given a preliminary drying to reduce the moisture content to 15-22% by weight. Already-fired agglomerates require a temperature of at least 343 DEG C. to set the silicate, while impregnated unfired agglomerates require firing in the range 565-660 DEG C. In a comparative procedure, fully wet 1/8 inch diameter agglomerates of 80 parts by weight sodium zeolite A and 20 parts by weight of attapulgus clay are allowed to dry in air for 24 hours, and are then dried at 100 DEG C. for 3 hours, and are fired at 625 DEG C. for 15 minutes in a forced air purge. In Example I, the wet agglomerates of the comparative procedure are soaked in a sodium silicate solution for 16 hours, drained, rinsed with warm water, and are dried and fired. In Example II, the fired product of the comparative procedure is rehydrated in a steam-heated oven, soaked in a sodium silicate solution for 7 hours, and further treated as in Example I. Example III is similar to Example II, except that the rehydration is effected in air for 48 hours and in water for 2 minutes. In Example IV, a potassium silicate solution is used. A table shows the crushing strength of a number of agglomerates subjected to differing treatments. Specifications 777,232, 777,233, 909,266, 912,936 and 972,831 are referred to.