Characterization and Thermal Study of Raw and Purified Pyrophyllites

ABSTRACT: Pyrophyllite is a 2:1 layered silicate with interest in ceramics, refractories, and several other important applications. In this work, an investigation into the thermal behaviour of several natural and purified pyrophyllite samples, including a pyrophyllite clay, has been conducted. A previous characterization of these samples has been carried out by AA, XRD, thermal analysis by thermo-dilatometry and DTA-TG, surface area, and SEM-EDX. Thus, relevant chemical, mineralogical, thermal, and textural data of these samples have been obtained. As a second step of this investigation, the thermal behaviour of these pyrophyllite samples has been investigated by XRD and SEM after several thermal treatments at 800, 1100 and 1150 °C during 24 h. The formation of dehydroxylated pyrophyllite as a crystalline phase in the samples was established after 1050 °C by XRD, and its permanency above this temperature, with little changes in morphological features, as revealed by SEM. When thermal treatment was progressive at higher temperatures (1300 °C) the following was evidenced by XRD: (a) the formation and crystallization of mullite (3Al₂O₃·2SiO₂), with a progressive destruction of dehydroxylated pyrophyllite, and (b) the formation of cristobalite (SiO₂). This later phase was formed by crystallization of the amorphous silica, detected as a hump by XRD, which is segregated in the solid-state reaction of formation of mullite. This treatment produces a new microstructure with elongated and needle-like crystals of mullite according to SEM observations. All these results have been found of interest for the preparation of ceramic materials, mullite-based ceramics, and refractories using these pyrophyllite samples.