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.

Keywords

Pyrophyllite / Dehydroxylated pyrophyllite / Mullite / Cristobalite / Chemical analysis / Thermal treatment / Crystallization / Microstructure / Ceramics / Refractory materials

Author Contributions

P.J.S.-S.: Conceptualization, Investigation, Methodology, Funding acquisition, Formal analysis, Data curation, Supervision, Writing—original draft, Writing—review & editing. E.G.: Conceptualization, Investigation, Writing—original draft, Writing—review & editing, Supervision, Formal analysis, Visualization, Validation. V.G.-G.: Investigation, Methodology, Writing—review & editing. J.A.S.-G.: Software, Data curation, Writing—review & editing. L.P.-V.: Writing—review & editing, Supervision, Data curation. S.M.-M.: Investigation, Formal analysis, Writing—review & editing. All authors have read and agreed this original paper submitted for revision and publication in High Temperature Materials.

Ethics Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The readers are informed about the accessibility of research data linked to this paper writing to the corresponding authors.

Funding

This work has been carried out as part of the projects financially supported by Research Groups TEP 204 and AGR 107 through Andalusian Regional Government.

Declaration of Competing Interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

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