Thermal decomposition of ammonium hexafluoroaluminate and preparation of aluminum fluoride

The thermal decomposition process of (NH4)3AlF6 was studied by DTA-TGA method and the related thermodynamic data were obtained. The results show that AlF3 is obtained after three-step decomposition reaction of (NH4)3AlF6, and the solid products of the first two decomposition reactions are NH4AlF4 and AlF3(NH4F)0.69, respectively. The three reactions occur at 194.9, 222.5 and 258.4 °C, respectively. Gibbs free energy changes of pertinent materials at the reaction temperatures were calculated. Enthalpy and entropy changes of the three reactions were analyzed by DSC method. Anhydrous aluminum fluoride was prepared. The XRD analysis and mass loss calculation show that AlF3 with high purity can be obtained by heating (NH4)3AlF6 at 400 °C for 3 h.