Thermal behavior of ammonium fluorosilicates complexes: Obtaining and kinetic analysis

In this work, a mixture of (NH4)3SiF7/(NH4)2SiF6 powders was obtained as a by-product of the Li extraction process from α-spodumene aluminosilicate with NH4HF2. The thermal behavior of the powders was analyzed by non-isothermal thermogravimetric experiments. The kinetic parameters that describe the processes involved were obtained using mathematical deconvolution, Friedman's method, combined kinetic analysis, and nonlinear regression optimization. The results show that the process occurs in two partially overlapping steps, the thermal decomposition of (NH4)3SiF7 into (NH4)2SiF6 and the subsequent sublimation of (NH4)2SiF6. The apparent activation energies were 72.6 and 79.8 kJ/mol for steps 1 and 2, respectively. The apparent pre-exponential factors were 1.19 × 106 and 2.50 × 105 s-1, respectively. The kinetic models indicated that Step 1 follows an A2 model, while Step 2 follows an F0 model. Finally, the resulting kinetic parameters allowed the reconstruction of the original experimental curves and obtaining predictions of curves recorded under other heating programs. [Display omitted] •(NH4)3SiF7/(NH4)2SiF6 were obtained by fluorination of spodumene with NH4HF2.•The thermal behavior shows two steps: decomposition and subsequent sublimation.•Ea are 72.6 and 79.8 kJ/mol for decomposition and sublimation, respectively.•Each step is studied by multiple combined kinetic methods.•The kinetic triplets obtained are validated by reconstructions and predictions.