Hydrothermal synthesis of analcime and hydroxycancrinite from K-feldspar in Na2SiO3 solution: characterization and reaction mechanism

Abstract

Hydroxycancrinite and analcime were generated in the reaction process of K-feldspar and various concentration solutions of Na₂SiO₃. The effect of the concentration of Na₂SiO₃ on the formation of the product samples and the reaction mechanism were investigated. The X-ray diffraction (XRD) results indicate that there exists hydroxycancrinite (Na₈Al₆Si₆O₂₄(OH)₂·2H₂O) with a hexagonal structure at a Na₂SiO₃ concentration of 2.85–3.9 mol kg⁻¹ and analcime (NaAlSi₂O₆·H₂O) with a cubic structure at 2.34–2.85 mol kg⁻¹ in the reaction of K-feldspar–Na₂SiO₃–H₂O system. Rietveld refinements were utilized to investigate the crystallographic parameters of hydroxycancrinite based on the XRD results, showing that Na1 atoms existed in the form of [Na·H₂O]⁺ cluster and water molecule (O8) are located in the center of those six-membered rings and slightly displace from the plane of the six-membered ring, as well as Na2 atoms and larger anionic group OH⁻(O7) are seated in the larger channel. Fourier transformation infrared spectrometry (FTIR) results suggest that the existence of the –OH and water molecules makes the absorption bands at 3569 and 3451 cm⁻¹ broaden and overlap with each other as the concentration of Na₂SiO₃ decreases. The sharp bands at 1634 cm⁻¹ disappear gradually with the decrease of Na₂SiO₃ concentration owing to the difference of product. It turns out that the reaction process of K-feldspar and Na₂SiO₃ as well as the synthesis of analcime and hydroxycancrinite is an interface coupled dissolution–reprecipitation process mainly in the presence of Na⁺, [Al(OH)₄]⁻, [H₂SiO₄]²⁻ and SiO₃²⁻ and followed by the repolymerization process of the silicate framework.