Zeolite synthesis was studied using two silica rich filtration residues (FR 1 and FR 2) as Si-source and sodium aluminate in a direct synthesis at 60˚C at strong alkaline conditions (8 M-16 M NaOH). In addition to these one-pot syntheses, a two-step process was investigated. Here, an alkaline digestion of FR at 60˚C was followed by gel precipitation with sodium aluminate and gel crystallization under usual conditions of 80˚C-90˚C. The results show that the substitution of chemical reagent sodium silicate by a waste material like FR as Si-source is possible but requires fine tuning of the reaction conditions as zeolite crystallization is a process under kinetic control. The solubility behaviour and impurities of the inserted filtration residues strongly influenced the course of reaction. Thus zeolites like hydrosodalite or intermediate zeolite between cancrinite and sodalite, or zeolite NaA or Z-21 in cocrystallization with hydrosodalite could be observed in the one pot syntheses already in a short time interval between 1-4 h depending on the alkalinity. The two step process yield to zeolites NaA and NaX in very good quality. The reaction process of FR in both reaction methods was characterized by chemical analyses, X-ray powder diffraction, Fourier transform infrared spectroscopy as well as scanning electron microscopy. Surface area and water content of selected products were further characterized by the BET-method and by thermogravimetry. Summing up the results, we can show that zeolite formation from filtration residues is possible by several reaction procedures as model cases for a re-use of industrial waste materials. Beside the importance for environmental protection, the reactions are of interest for zeolite chemistry as the re-use of FR is possible under economically conditions of low energy consumption at 60˚C and short reaction periods.

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Abstract. This paper is a case study of complete substitution of sodium-metasilicate in zeolite Na-A synthesis by an electrostatic filter ash (FA) arising in high amounts during silane waste incineration process. The silicate abundant FA is a suitable material for reinsertion in zeolite chemistry. This is shown in the presented investigation by the development of a model reaction process at low temperatures (50-60°C) and short times (1.5-4 h). The experiments were performed under addition of NaAlO 2 and variation of the alkalinity and the reaction period. Characterization and fine tuning of the process was mainly

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Zeolite formation was investigated under low temperature conditions between 40˚C -25˚C under insertion of a Si-rich filtration residue (FR) from waste water cleaning of the silane production. Syntheses were performed under open system conditions without stirring. The application of templates was excluded but gel formation was optimized by addition of citric acid. Beside a minimal consumption of process energy during synthesis under very low temperatures even FR was inserted without an expansive pre-treatment like calcination. Disordered intergrowths of FAU-EMT intermediates were obtained in each case, even at a temperature of only 25˚C. The reaction products always occurred in two sequences, one at the bottom and another at the top of the reaction bin. A better crystallinity was detected for the latter products and the Si/Al ratio of these FAU-EMT members was close to 1.0 like in zeolite LSX. In contrast chemical analyses revealed an enrichment of Ca-and Mgimpurities from FR in the bottom products.