SYNTHETIC ZEOLITES FORMED FROM EXPANDED PERLITE - TYPE, FORMATION CONDITIONS AND PROPERTIES

The starting material used was expanded perlite with a grain size < 40 mum (74.5 wt.% SiO2; 12.5 wt.% Al2O3). This material is a waste product obtained during the production of expanded perlite. The experiments were carried out with KOH solutions, mixtures of KOH and NaOH solutions (1:1) as well as NaOH solutions in the concentration range 0.5 N to 6.0 N at temperatures of between 100-degrees and 140-degrees-C and with reaction periods of 2 hours to 13 days in closed system. In the experiments with KOH containing solutions zeolite ZK-19 (phillipsite), W (merlinoite), G (chabazite) and F (edingtonite) formed. Without addition of aluminium high percentages of zeolite ZK-19 (80-100 wt.%) and zeolite W(90-100 wt.%) were obtained. The addition of aluminium rendered possibly the formation of 90 to 100 wt.% of zeolite G and 85 to 100 wt.% of zeolite F, respectively. In the experiments with NaOH solutions analcime, zeolite Na-Pc (gismondine), zeolite HS (sodalite hydrate) and zeolite A formed. High percentages of zeolite Na-Pc (90-100 wt.%), zeolite HS (up to 100 wt.%) and analcime (up to 100 wt.%) were synthesized without addition of aluminium. The formation of high percentages of zeolite A (95-100 wt.%), however, needs the addition of aluminium, NaCl and seed crystals. The temperature stability of the zeolites decreases in the following sequence: K-F > K-W greater-than-or-equal-to K-ZK-19 greater-than-or-equal-to (Na), K-W greater-than-or-equal-to Na, K-F greater-than-or-equal-to G(Si-poor) greater-than-or-equal-to (Na), K-ZK-19 much greater than Na-Pc congruent-to G(Si-poor). Zeolite A has a very good temperature stability up to temperatures of approximately 550-degrees-C similar to that of zeolite K-W. At higher temperatures, however, its stability is very poor. The NH4+-exchange capacities (meq/g) of the different zeolites amount to the following values: ZK-19:2.8 - 3.2; W:3.0 - 3.2; G:2.3 - 3.6; A:3.1 - 3.2; Na-Pc:3.5 - 3.6; F:3.9 - 4.8.