THE EFFECT OF REACTION ATMOSPHERE ON THE EARLY STAGE CARBOTHERMAL REDUCTION OF KAOLINITE : AN XRD, 29SI AND 27AL MAS NMR STUDY

Phase formation in kaolinite heated at 1200°C in eight different reaction atmospheres in the presence and absence of carbon has been studied by solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) and X-ray powder diffraction. Mullite (3Al2O3-2SiO2) and amorphous SiO2 are the principal products in all atmospheres. The amount of mullite formed is generally greater under vacuum and in reducing atmospheres, but the precise effect of the atmosphere is also modified by the presence of carbon. Vacuum and reducing atmospheres generally produce mullites of alumina: silica composition nearer 3∶2 than 2∶1 (estimated from unit cell measurements) and containing a higher proportion of Al*, i.e. tetrahedral aluminium associated with an oxygen defect (estimated by 27AlNMR measurements). The result of most significance to sialon formation is the detection by 29Si NMR of silicon oxynitride formation at 1200°C in systems containing either nitrogen or ammonia, in the presence of carbon. The preferential formation of Si-O-N bonds at such an early stage of the reaction under carbothermal conditions was confirmed by thermodynamic calculations, which also clarify other details of the complex interactions between the aluminosilicate, carbon, and the various gas atmospheres.