Structural and chemical changes in kaolinite caused by flash calcination: Formation of spherical particles

• We present the physical and chemical characteristics of flash metakaolins
• Spherical particles were formed after flash calcinations.
• We observed and analyzed the composition of a spherical particle.
• The mechanism of flash calcination explains the formation of spherical particles

To understand the morphological changes of three commercial kaolins during flash calcination and to compare them with those obtained during traditional heat treatments in the laboratory (an electric furnace at 700 °C for 5 h), this paper presents the physical and chemical characteristics of metakaolins obtained from an industrial flash calciner. In the metakaolin products, kaolinite was not completed dehydroxylated during calcination, and the proportion of untransformed kaolinite was greater in flashed metakaolins than in traditional rotary-calcined metakaolins. Several particle morphologies were discernible in the metakaolins, including spherical particles that were formed in flash calcinations. These spherical particles were cut with a focused ion beam (FIB) and were revealed to contain a vitrified aluminum silicate phase with traces of mullite and various gases. These spherical particles were produced from the direct calcination of several submicron kaolinites near the flame of the calciner.