The role of alkaline activation in the structural transformations of aluminosiliceous industrial wastes towards zeolite production

Abstract

The zeolitization of two selected aluminosiliceous industrial wastes, a fly ash and an exhausted Fluid Catalytic Cracking (FCC) catalyst, was studied. Solid products enriched in zeolite A were successfully synthesized by using a simple procedure consisting of waste activation by alkaline fusion followed by a hydrothermal treatment in mild conditions. During activation, it was observed that different crystalline NaAlSiO2 polymorphs could be generated, their crystallization being dependent on the operational variables used for pretreatment (temperature and time). So, the different intermediate NaAlSiO2 polymorphs obtained at temperatures of 800 °C, 830 °C and 850 °C and with calcination times of 0 h, 0.5 h, 1 h, 2 h, 4 h and 12 h were identified and their percentages were estimated by X-ray diffraction and SEM analysis. After pretreatment, two main crystalline phases, called nepheline and low-carnegieite, were formed in the activated samples, their percentage ratio depending on the activation conditions. When the treated samples were subjected to hydrothermal treatment, the nepheline phase was almost completely dissolved during the aging of the reaction mixture at ambient temperature. On the contrary, the levels of low-carnegieite in the solid phase remained insoluble after aging. During the hydrothermal treatment at 90 °C, a gradual decrease of the low-carnegieite content and a concomitant increase in zeolite A content was noted, indicating a topotactic transformation along the crystallization step. The highest zeolite A conversions were obtained by applying an alkaline fusion treatment at 800 °C for 12 h for fly ash (74%), and at 800 °C for 4 h for the exhausted catalyst (79%).