Study On The Effect Of Additives On The Structure And Properties Of Aluminum Electrolytic Molten Salt

Additives are commonly used to improve the physical and chemical properties of electrolyte in order to improve the technical and economic indicators of aluminum electrolysis production. However, it is still not very clear that how additives affect the microstructure, transport and diffusion of ions and physicochemical properties of aluminum electrolyte. In addition, there is great limitation and blindness for the experimental screening of additives. In this dissertation, molecular dynamics simulations were used to reveal the effect mechanism of aluminum fluoride, calcium fluoride and magnesium fluoride on the ion forms and structure, transport and diffusion mechanism, physical and chemical properties of cryolite and cryolite-alumina melts, which will provide guidance for aluminum electrolysis process optimization and improvement and may provide a theoretical basis for the development of efficient and environment-friendly additives. The main findings are drawn as follows.1. The effect of aluminum fluoride on the structure and properties of cryolite and cryolite-alumina melts were studied by molecular dynamics simulations. It is found that the RDFs of different ions in the mixture change regularly in the cryolite melts. With the increasing of AlF3 mole fraction, the interaction between Al3+ and F- is weakened, whereas the interaction of Al3+-Al3+ and F--F- are enhanced, which results in the formation of complex aluminum ionic groups with AlF63- connected by fluorine bridge. The formation of complex aluminum bound groups leads to the result that the diffusion coefficients of ions in the melts increase gradually, the conductivity decreases and the viscosity increases at first and then decreases. With the increasing of temperature, the interaction of Al3+-A13+ and F--F-are weakened and the complex aluminum fluorine ion groups may dissociate. The diffusion coefficients of ions in the melts increase, the viscosity decreases and the conductivity increases. The first and second peak of RDFs between Al3+ and Al3+ are caused by the interaction of Al3+-O2-and Al3+-F- respectively in cryolite-alumina melts. Aluminum-oxygen-fluorine ions generate in the melts and with the increasing of AIF3 mass fraction, the diffusion coefficients of different ions decrease, the viscosity of the melts increases and the conductivity decreases, which is in consistent with the trends of experimental results.2. The study of the effect of calcium fluoride on radial distribution functions, diffusion coefficients, conductivity and viscosity of cryolite and cryolite-alumina melts reveals that with the increasing of CaF2 mass fraction, the first peak values of RDFs for Al3+-F- and F--F- decrease gradually, while the first peak values of Al3+-Al3+ RDFs increase in the cryolite melts. This indicates that the interaction of Al3+-F- and F--F- are weakened whereas the interaction between Al3+ and Al3+ increases. Complex aluminum ionic groups generate in the system and the diffusion coefficients of different ions and the conductivity of the melts decreases, while the viscosity increases. In cryolite-alumina melts the interaction between Al3+-Al3+ caused by oxygen bridge is weakened and the interaction caused by fluorine bridge is enhanced. The generation of aluminum-oxygen-fluorine ions leads to the result that the diffusion coefficients of ions in the melts decrease, the viscosity of the melts increases and the conductivity decreases, which agree well with the experimental results.3. Molecular dynamics simulations were used to study the effect of magnesium fluoride on radial distribution functions, diffusion coefficients, conductivity and viscosity of cryolite and cryolite-alumina melts. It is found that the interaction of Al3+-F- and F--F- are weakened whereas the interaction between Al3+ and Al3+ increases, which leads to the generation of aluminum ion groups in the cryolite melts. The diffusion coefficients of different ions and the conductivity of the melts decreases, while the viscosity increases. With the increasing of magnesium fluoride mass fraction, aluminum-oxygen-fluorine groups generate in cryolite-alumina melts. The diffusion coefficients of ions in the melts decrease, the viscosity increase and the conductivity decreases. The trends of conductivity and viscosity are in consistent with experimental results.4. The effect of different additives on the structure and properties of cryolite-alumina melts were studied and compared. It is found that all the three kinds of additives may lead to the generation of aluminum-oxygen-fluorine groups, which is in favor of aluminum electrolysis. The ability of the three additives to give fluoride ions is CaF2> MgF2> AlF3, which is consistent with that calcium fluoride is the most widely used additive in industrial aluminum electrolysis. When the radius of cation in additive is larger and the charge is smaller, the diffusion coefficient and conductivity of aluminum electrolyte are relatively larger after the additive is added in the molten salt. Therefore, the salt which is in favor of the formation of oxygen-containing aluminum ionic groups and can increase the conductivity of aluminum electrolyte should be selected as an additive to improve the aluminum electrolysis process.