Ab Initio Molecular Dynamics Study On Aluminum Electrolyte Molten Salt System Containing Ca/Mg Impurities

Raw materials from complicated sources,such as alumina,cause the Ca/Mg impurity ions in aluminum reduction cells to continuously enrich,affecting the operating stability and current efficiency of these cells.Ca/Mg concentration management and parameter adjustment in manufacturing practice,however,lack molecular-level theoretical guidance.In this paper,the microscopic behavior of an acidic aluminum electrolyte containing Ca/Mg was investigated using an ab initio molecular dynamics calculation method coupled with a tree structure recursive search algorithm on the CP2K platform.The goal was to understand the influence mechanism of Ca/Mg ions and to provide theoretical guidance for the control of Ca/Mg ion concentration in electrolyte systems.The outcomes of the primary research are as follows:（1）To investigate the mechanism of influence of Ca/Mg ions in the melt,ab initio molecular dynamics models of 2.2NaF-AlF₃ molten salt systems with varying CaF₂/MgF₂ concentrations were developed.The results indicate that the distinction between Mg-F and Ca-F interactions influences the average coordination number of Al-F.Ca-containing systems have an average coordination number of 5.04～5.22,which is slightly larger than that of the Mg-containing system（4.98～5.06）.In molten salt,aluminum fluorine complex ions such as[AlF₄]^-,[AlF₅]^2-,[AlF₆]^3-,[Al₂F₉]^3-,[Al₂F₁₀]^4-,[Al₃F₁₅]^6-and[Al₃F₁₄]^5-coexist,with[AlF₅]^2-being the predominant complex ion.The struggle forF^-between Mg²⁺and Al³⁺will increase the amount of free fluorine,but not the cross-linking between aluminum fluoride complex ions.When the CaF₂ mass percentage exceeds 5%,the cross-linking degree of[AlF₆]^3-and[AlF₅]^2-rises.By lowering the Al/F molar ratio in the right way,the cross-linking of complex ions in molten salt can be lessened,and Al³⁺can move through the salt more easily.（2）The ab initio molecular dynamics model of the nAl₂O₃-CaF₂/MgF₂-2.2NaF-AlF₃ molten salt system was established to study the effect of Al₂O₃concentration on Ca/Mg electrolyte.The results show that with the increase of Al₂O₃ concentration,the terminal fluorine in the molten salt system decreases first and then continues to increase,and the free fluorine increases first and then decreases.The reason is that the competition of oxygen ions makes theF^-on the complex ions gradually removed,and when the concentration of Al₂O₃ is too high,the average coordination number of complex ions is too small to reduce the free fluorine.In general,the addition of Al₂O₃ makes the crosslinking of complex ions in the melt complicated and large-scale.（3）The interaction of various ion pairs and the microstructure of the KF-LiF-CaF₂-MgF₂-Al₂O₃-NaF-AlF₃ molten salt system were investigated.The system’s ion interactions were Al-O>Al-F>Mg-F>Ca-F>Li-F>Na-F>K-F,according to the data.In the melt,terminal fluorine is the predominant form of fluorine ion,while bridging fluorine is less abundant.