The Solid-State Conversion of Kaolin to KAlSiO₄ Minerals: The Effects of Time and Temperature

Abstract

Abstract In recent years KAlSiO 4 polymorphs have become minerals of interest from an industrial point of view; they have various applications in technological and medical fields. The costs of synthesis processes are often significant and so, in the present study, an attempt was made to develop a new synthesis protocol using a widely available and inexpensive, natural starting material. The KAlSiO 4 polymorphs synthesized here were kalsilite and KAlSiO 4 -01 — 01 refers to the high-temperature polymorph of KAlSiO 4 (Cook et al. , 1997; Gregorkiewitz et al. , 2008; Kremenovic et al. , 2013). KAlSiO 4 polymorphs were synthesized using kaolin; the effects of time and temperature on the synthesis process were investigated. A solid-state synthesis protocol was developed which required the mixing of the calcined kaolin with K 2 CO 3 in stoichiometric proportions at temperatures of 700 and 800°C at atmospheric pressure. Crystallization of kalsilite at 700°C was demonstrated while that of KAlSiO 4 -01 was revealed at 800°C. Synthetic kaliophilite H2 was found in both of the experiments as a metastable phase. The products of synthesis were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma optical emission spectrometry (ICP-OES), infrared spectroscopy (IR), and 29 Si magic-angle spinning solid-state nuclear magnetic resonance spectroscopy ( 29 Si MAS NMR). Calculation of cell parameters (through Rietveld refinement) and the density and specific surface area of the phases synthesized was also achieved. The amount of amorphous phase in the synthesis powders was estimated by means of quantitative phase analysis using the combined Rietveld and reference intensity ratio methods. In particular, the results of the spectroscopic, chemical, and morphological characterizations are in agreement with the data available for these minerals in the literature, thus confirming the effectiveness of the experimental protocol. The quantitative phase analysis (QPA) also indicated the high purity of the powders synthesized, thus allowing for industrial applications.