Transformation of heulandite type natural zeolites into synthetic zeolite LTA

Abstract

This study developed an approach to synthesise zeolite LTA from three different natural heulandite class zeolite deposits (Escott, Avoca, and NextSand). The challenge was to determine the impact of pre-activation of clinoptilolite or heulandite materials upon the synthesis of zeolite LTA. Heulandite (Escott; Avoca) was thermally less stable ( ca. 200 o C) than clinoptilolite (NextSand) ( ca. 600 o C); with all samples losing cation exchange capacity as a result of heating at elevated temperature ( ca. 120 to 30 meq/100 g). Attempts to synthesise zeolite LTA from as received natural zeolite under typical industry conditions (80 °C; 2 h) were not successful due to the stability of the natural zeolite framework. In agreement with this discovery, the amount of active alumina and silica was relatively small (<3.5 % for silica; <36.4 % for alumina). Alternatively, activation with either alkali fusion or a combination of heating and fusion at temperatures up to 800 °C typically resulted in formation of >80 % of monomeric silicates and aluminates. Significantly, the zeolite product resultant from hydrothermal synthesis of the activated materials comprised of ca. 85 % crystalline zeolite LTA and non-diffracting material; independent of the activation method or the identity of the natural zeolite. This study revealed that fusion activation of natural zeolites improved the robustness of the synthesis process and opened up new avenues for green zeolite formation. Future studies should focus on reducing the activation costs by lowering activation temperature and sodium hydroxide consumption. • Zeolite LTA synthesised from natural zeolites in a two-step process. • Thermal activation of natural zeolite inadequate to make zeolite LTA. • Alkali fusion required to depolymerise silica and alumina species. • Fusion conditions have relatively small impact on zeolite LTA quality. • Zeolite LTA production independent of natural zeolite identity.