Microwave-Induced Phase Transformations of Natural Clay in Air and Humid Media

Abstract

We have studied the effect of microwave processing (power of 700 W, frequency of 2.45 GHz, and duration of 600 s) on the phase and polymorphic transformations of natural clay particles in air and humid media. In iron oxides, hematite (Fe₂O₃) converts into Fe-cordierite (Fe₂Al₃[AlSi₅O₁₈]) and magnetite (FeO ∙ Fe₂O₃). The aluminum oxide γ-Al₂O₃ undergoes a partial polymorphic transformation, and corundum (α-Al₂O₃) crystallizes from an amorphous phase. Silicon oxides also undergo polymorphic transformations. Of the three silica polymorphs present in natural clay, only two, β-quartz and β-cristobalite, persist after microwave processing in air and only β-quartz persists in a humid atmosphere. The working medium has been shown to influence the nature of the phase transformations. Our results demonstrate the possibility of initiating structural changes in disperse clay systems by microwave processing.