The kinetics and mechanism of kaolin powder sintering I. The dilatometric CRH study of sinter-crystallization of mullite and cristobalite

The densification of kaolin with high content of middle ordered kaolinite was investigated by non-isothermal thermodilatometric analysis (TDA) under load. The compacted powder was heated with constant heating rate (CRH) from 0.75 to 10 °C min− 1. The sintering reaction of kaolin upon the temperature interval, where sinter-crystallization of mullite and cristobalite takes place (1090–1250 °C), is significantly affected by the heating rate. The dTDA results show that the process is sensitive to heating rate and the formation of both phases can be studied separately if heating rate higher than 4 °C min− 1 is used. The mechanism of involved formation of mullite and cristobalite is changed at heating rates higher than 6 °C min− 1. The values of apparent activation energies of mullite and cristobalite formations were determined by Kissinger equation and by several equations of model-free integral type of p(y)-isoconversional methods. The mechanism and thermodynamics of the process were evaluated through Kissinger and Eyring laws, respectively.