Thermodynamic analysis of the influence of potassium on the thermal behavior of kaolin raw material

The mineralogy and thermal properties of two kaolin clay samples from Agua Blanca (Hidalgo-México) were determined by XRD, SEM-EDS, TGA-DSC techniques. Kaolin clay A contains a higher $Al_2O_3$ and lower impurities $(K_2O, TiO_2, Fe_2O_3)$ amount than kaolin clay B, while the SiO2 amount is similar for both kaolin clays. A theoretical approach was carried out by a thermodynamic analysis considering the chemical composition of both kaolin clay samples with the FactSage 7.3 software. Stability phase diagrams were obtained to different K2O content from 0.1 to 3 wt % and temperatures in the range from 600 to 1600°C based on the chemical composition of the kaolin clay samples. The main mineralogical compounds predicted are andalusite $(Al_2O_3•SiO_2)$, K-Potash feldspar $(K_2O•Al_2O_3•6SiO_2)$, and the $SiO_2$ polymorphs (quartz, tridymite, and cristobalite) with small amounts of ferric-pseudobrookite $(Fe_2O_5Ti)$, and rutile $(TiO_2)$. As $K_2O$ content is increased, the amounts of mullite and tridymite decrease meanwhile the potash feld spar is increased at high temperatures. A liquid phase is formed at around 1350 and 1400°C for the kaolin clay samples A and B, respectively. The viscosity of the melt is increased for the evaluated $K_2O$ additions to 1400, 1500, and 1600°C.