Evolution of structural changes during flash calcination of kaolinite. A 29Si and 27Al nuclear magnetic resonance spectroscopy study

Abstract

Kinetically frozen samples of flash calcined kaolinite (rapidly heated to 1000 °C, maintained at that temperature for a variable residence time and then rapidly cooled) have been produced in a laboratory calciner. Time resolution of the structural changes occurring has been achieved by following ²⁷Al (78.15 MHz) and ²⁹Si (59.58 MHz) magic angle spinning nuclear magnetic resonance (MAS NMR) spectra as a function of residence time. ²⁹Si spectra can be deconvoluted into a minimum of four Gaussian components with shifts ranging from mullite-like (–90 ppm) to Q⁴(–110 ppm) Si environments. ²⁷Al spectra show peaks for four-co-ordinate and six-co-ordinate Al. The derived picture of flash calcination is progressive transformation of kaolinite to a single product which undergoes little further chemical reaction during its short time in the calciner.