Flash calcination of kaolinite: mechanistic information from thermogravimetry

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Abstract
Kinetically frozen samples of flash-calcined kaolinite (rapidly heated to high temperature, maintained at that temperature for a variable residence time and then rapidly cooled) have been produced in a laboratory calciner. The degrees of dehydroxylation (β) of calcines relative to the parent kaolinite have been determined using thermogravimetry (TG) and found to increase with calciner temperature, heating rate and residence time in the calciner. Variations in the forms of the TG curves for the subsequent dehydroxylation of calcines as a function of β provide information concerning the sequential loss of hydroxyls from kaolinite during flash calcination. This is postulated to occur in two waves: interlamellar hydroxyls (75%) are preferentially removed, with a secondary loss of intralamellar hydroxyls (25%) becoming dominant as loss of interlamellar hydroxyls approaches completion.