Approche thermodynamique de la corrosion des refractaires aluminosiliceux par le bain cryolithique : modelisation thermodynamique du systeme quaternaire reciproque AlF3-NaF-SiF 4-Al2O3-Na2O-SiO2

The main objective of this thesis is the thermodynamic modeling of liquid and solid solutions, the stoichiometric compounds and the gaseous phase of the chemical system which represent the species involved in the corrosion of the refractory lining of the electrolysis cell. This chemical system is the quaternary reciprocal system AlF3-NaF-SiF4-Al 2O3-Na2O-SiO2 with aluminum and carbon. The modeling of the entire reciprocal system has never been realized before and is very challenging due to the nature of the oxyfluoride solution. The thermodynamic modeling is based on Gibbs free energy functions coming from the theory of solutions. The central hypothesis of this project is that, if it is possible to reproduce the strong short-range ordering observed between the ions of the liquid solution, then reasonable results will be obtained for the phase equilibria involved in this chemical system. The thermodynamic model used in this thesis is the Modified Quasichemical Model in the Quadruplet Approximation (MQMQA) which takes into account the short-range ordering between first- and also second-nearest-neighbors, and is the best suited to model the oxyfluoride liquid solution.;The ternary system Al2O3-Na2O-SiO 2 and the ternary reciprocal system AlF3-NaF-Al2O 3-Na2O have been treated jointly due to the fact that compromises were necessary for the optimization of the system common to the two systems, Al2O3-Na2O, in particular for the β-alumina solid solution. The numerous experimental data relative to the thermodynamic properties of the stoichiometric or the solid solutions in the ternary system have been taken into account for the modeling of NaAlSiO4 and NaAlSi 3O8. Furthermore, the cation NaAl4+, used in the modeling to reproduce the charge compensation effect (substitution of Si4+ by Al3+ coupled to Na+ in the SiO2-rich region of the ternary oxide system), is present in both of these systems and the properties of the end-members related to it (NaAlF4 and NaAlO2) have been adjusted to reproduce the thermodynamic properties of the liquid phase. The absence or lack of experimental data in specific composition regions, in particular in the Na2O-rich region in the oxide system or in the oxide-rich region of the reciprocal system, makes the optimization of these two systems difficult.;The modeling of the quaternary reciprocal system Al, Na, Si // F, O has been performed mainly by taking into account the experimental data available in the isoplethal sections NaF-NaAlSiO4, NaF-NaAlSi3O 8, Na3AlF6-NaAlSiO4 and Na3AlF 6-NaAlSi3O8 and by using the model parameters allowing the adjustment of the thermodynamic properties of the liquid solution in the composition region under consideration. The few available data and the conflict among them, undoubtedly related to the experimental difficulties, make the modeling of the quaternary reciprocal system complex. (Abstract shortened by UMI.).;First, the reciprocal system with the most negative Gibbs free energy change for the exchange reaction between the end-members, NaF-SiF4-Na 2O-SiO2, was modeled, allowing thus the validation of the recent modifications of the MQMQA aimed at improving the modeling of the thermodynamic properties of reciprocal solutions presenting a strong short-range order among first-nearest neighbors as well as second-nearest neighbors. In order to model this system, an estimate of the thermodynamic properties of the hypothetical SiF4 liquid was necessary. The experimental data in the binary systems NaF-SiF4 and Na2O-SiO2 are reproduced within the experimental error limits. In the reciprocal system, a group of data in conflict with all others could not be reproduced. The data considered reliable are well reproduced. A unique set of parameters was used to reproduce all the reciprocal sections and extrapolations outside these sections should be satisfactory.