Ab Initio Molecular Dynamics Simulation Study on the Molten Structure of Cryolite to the Cryolitic Bath, Na₃AlF₆–AlF₃–Al₂O₃–CaF₂

This study investigates the structural properties of cryolitic melts using ab initio molecular dynamics (AIMD) simulations. Aluminum production relies on the electrolysis of alumina in a molten cryolite bath, which primarily consists of cryolite, aluminum fluoride, alumina, and calcium fluoride and operates at temperatures between 1213 and 1243 Κ. Despite significant advancements, the local structure and speciation within these melts remain incompletely understood. This research employs computational techniques to examine the atomic structure and charge distribution in cryolitic melts, with a particular focus on aluminum atom interactions and the role of bridging anions. AIMD simulations were performed by using the CP2K software package. The Perdew-Burke-Ernzerhof (PBE) approximation was applied for the exchange-correlation functional, and Goedecker-Teter-Hutter (GTH) pseudopotentials were used to model core electrons. The study investigated systems with varying amounts of AlF₃, Al₂O₃, and CaF₂ in molten cryolite, maintaining temperatures slightly above the liquidus point. Structural analysis was conducted using radial distribution functions (RDFs) to determine bond distances and coordination numbers, while electronic distribution was analyzed through Mulliken population analysis. Key findings include the dominance of the AlF₅ ^2- complex in molten cryolite, which is in agreement with previous studies. The addition of alumina influences the formation of oxyfluoroaluminate species, with Al₂OF₆ ^2- and Al₂O₂F₄ ^2- being prevalent at low and high alumina concentrations, respectively. Calcium fluoride impacts the melt's structure by increasing the presence of AlF₅ ^2- and altering molecular conformation due to the strong anionic nature of calcium. The electronic structure analysis revealed minor changes in the average charge of atoms but an overall increase in the anionic character of the melt with the addition of O^2- and Ca²⁺. This study provides valuable insights into the atomic and electronic behavior of cryolitic melts, contributing to a deeper understanding of these complex molten systems and supporting the optimization of aluminum production processes.