Revealing the Effect of Anions on the Formation and Transformation of Zeolite LTA in Caustic solutions: AnIn SituSynchrotron PXRD Study

Abstract

Zeolite LTA is one of the largest industrial zeolites in both volume and market value. Natural resources or manufacturing wastes such as clay minerals, fly ash, and lithium slag are now used as raw materials for the economically and environmentally friendly synthesis of zeolites. However, soluble impurities such as inorganic anions (CO32–, SO42–, and PO43–) in these feeds have significant effects on the quality of zeolite LTA products, because these impurities enhance or inhibit the formation of other unwanted phases (amorphous zeolites or sodalite) via mechanisms that remain poorly investigated. Here we use in situ synchrotron powder X-ray diffraction to study the effects of different inorganic salts (Na2CO3, Na2SO4, and Na3PO4) on the kinetics of the sequential phase transformations in zeolite LTA during 2.5 h of reaction at 90 °C in both weakly and highly alkaline solutions. The results show that different salts have no effects on the amorphous phase formation stage but do affect the zeolite or sodalite crystallization stage. The addition of Na2CO3 increases the dissolution rate of zeolite, but it barely promotes the crystallization rate of sodalite. The addition of Na3PO4 inhibits sodalite crystallization, whereas the addition of Na2SO4 is most effective in facilitating sodalite formation and improving final sodalite particle size. Also, SO42–-bearing sodalite has a larger unit cell than other types of sodalites. These outcomes suggest that both CO32– and SO42– have negative effects on stabilizing zeolite LTA phases but PO43– has positive effects on these products.