Mechanical activation of kaolinite for sodalite synthesis under mild conditions

Abstract

This study investigated the effects of mechanical activation on the transformation of kaolinite to sodalite under mild conditions. The sodalite prepared via alkaline leaching of kaolinite under different milling times, alkaline concentrations, and leaching times was studied. The prepared zeolites were physically, chemically, and morphologically characterized. The results show that ball milling can significantly reduce the particle size and increase the specific surface area of kaolinite. Highly crystalline sodalite (Na₈Al₆Si₆O₂₄(OH)₂) can be prepared from kaolinite with ball milling for 2 h after alkaline leaching at 6 M NaOH for 24 h. Ball milling destroyed the crystal lattice of kaolinite, generating structural defects and dislocations, destabilizing the framework of kaolinite and increasing its reactivity. Furthermore, mechanically activated kaolinite demonstrated a hybrid transformation under alkaline leaching, involving both direct transformation (kaolinite → sodalite) and a two-step transformation (kaolinite → zeolite A → sodalite), depending on the dehydroxylation degree. The complete dehydroxylation of kaolinite induced a two-step transformation under alkaline leaching, as the thermally activated kaolinite did. Kaolinite was directly transformed into sodalite under alkaline conditions (≥ 4 M NaOH), exhibiting heterogeneous nucleation on the surface of kaolinite. Interestingly, the conversion of zeolite A into sodalite required a higher alkaline concentration for the thermal activation of kaolinite under atmospheric pressure, reflecting the advantages of mechanical activation. Mechanical activation can reduce the alkaline concentration required for the preparation of sodalite from kaolinite despite the conversion route, indicating that the preparation of sodalite can be completed under mild conditions.