Modelisation des equilibres thermodynamiques impliquant le fer dans la cryolithe lors de l'electrolyse de l'aluminium a l'aide d'anodes inertes.

摘要

Since the end of the 20th century, the aluminium industry is seriously interested in replacing consumable carbon anodes in the Hall-Heroult process by the so-called inert anodes. Nickel ferrite (NiFe 2O4) is a promising material for the replacement of the traditional carbon anodes. However, the use of such anodes can cause iron and nickel to be dissolved in the cryolitic bath and therefore in the final product, which is not desirable. The dissolution rate of inert anodes in the electrolyte has to be minimized. Modeling the thermodynamic and phase equilibria behavior of inert anodes in contact with cryolite is useful to understand the dissolution processes. The mandate of this project, included in the project of "a virtual laboratory for the aluminium industry" (CRSNG, Rio Tinto Alcan, Alcoa, Hydro Aluminium) is to add the iron to an already existing database for the cryolitic bath.;Parameters for the liquid phase and solid solutions models have been optimized using available experimental data. Experimental techniques have been examined and only reliable data have been selected. Parameters for the liquid phase have been evaluated from experimental data for the following binary fluoride systems: Fe-F (FeF2-FeF3), NaF-FeF 2, NaF-FeF3, CaF2-FeF3. The binary systems FeF2-AlF3 and CaF2-FeF2 are estimations based on chemically similar systems and the FeF3-AlF 3 liquid solution is assumed to be ideal. Parameters for the liquid phase have also been evaluated in the two ternary fluorides systems NaF-FeF 2-AlF3 and NaF-FeF3-AlF3 (based on available data for the isoplete sections Na3AlF6-FeF 2 and Na3AlF6-FeF3). The new evaluated solid solutions for the fluoride systems are: high-temperature cubic cryolite ((Na+,Va)8(Na+)4(AlF 63-,FeF63-)1(AlF 63-(AlF4-),FeF6 3-(FeF4-))3), low-temperature cryolite (Na3(Al,Fe3+)F6), chiolite (Na 5(Fe3+,Al)3F14), AlF3-FeF 3 ((Al,Fe3+)F3), weberite (Na2(Al,Fe 3+)(Fe2+,Mg)F7) and CaAlF5-CaFeF 5 (Ca(Al,Fe3+)F5).;Parameters for the liquid phase have also been evaluated for the two binary oxide systems Na2O-FeO and Na2O-Fe2O 3 based on selected experimental data. The wustite solid solution (FeO-[Na 2O,Fe2O3]) from the FToxid FactSage(TM) database has been modified to take in account Na solubility. The corundum (Al 2O3-Fe2O3) and the spinel ((Al, Fe 2+, Fe3+)(Al,Fe2+,Fe3+) 2O4) solid solutions have been directly taken from the FactSage(TM) database FToxid.;Multi-component reciprocal systems NaF-FeF2-AlF3-Na 2O-FeO-Al2O3 and NaF-FeF3-AlF 3-Na2O-Fe2O3-Al2O3 have been optimized for NaF-AlF3 rich melts based on iron solubility data as a function of the cryolitic ratio (CR) and the alumina (Al2O3) content at diluted iron concentrations. The system NaF-CaF2-FeF3-AlF3-Na2O-Fe 2O3-Al2O3 has been estimated as well for low CaF2 content (1 wt%) at diluted iron concentrations.;The global system under consideration is NaF-AlF3-Al 2O3-CaF2-FeO-Fe2O3. The thermodynamical study has to take in account the following ions Na +, Ca2+, Al3+, Fe2+, Fe3+ // F-, O2-, Va- (anionic vacancy). The two cations Fe2+ and Fe3+ have been added to the existing model for the Na+, Ca 2+, Al3+ // F-, O2-, Va- system optimized by Chartrand and Pelton [1]. Thermodynamic properties of new pure compounds have been evaluated. The quasichemical model in the pair approximation has been used in order to describe the cryolitic bath (liquid) behavior. New solid solutions have been modeled using the Compound Energy Formalism (CEF).;Thermodynamic calculations show that iron added to the cryolitic bath as oxides (FeO or Fe2O3) seems to dissolve preferentially as fluorides and its solubility decreases with increasing alumina content. In oxidizing conditions (Fe3+), the maximum solubility of iron is obtained for a CR value of about three and iron solubility decreases for higher or lower CR values. In reducing conditions (Fe2+), the minimum solubility of iron is obtained for a CR value of about three and increases for higher or lower CR values. These tendencies are well reproduced by the new model.