| Aluminum is the most abundant metal element on the earth.The annual production of aluminum is the second place and only less than steel and occupies the first place in nonferrous metals.Hall-Héroult aluminum electrolysis method has been adopted in industrial aluminum production since its invention.With the depletion of energy,low temperature aluminum electrolysis gradually applied in aluminum industry because of its advantages such as lower energy consumption,higher current efficiency and more economy.In order to explore and predict the properties of molten cryolite-alumina electrolyte in aluminum electrolysis,scholars began to focus on the study of the micro-structure of molten salts in the system.Raman spectroscopy was chosen as predominant experimental method due to its potentials in application under high temperature,while X-ray diffraction?or scattering?being used for judging crystal structure.Raman wavenumber and scattering activity of cryolite-alumina crystal structure were calculated by density functional theory.Thus the crystallographic data were correlated with Raman vibration modes,which laid a foundation for the evolution of cryolite-alumina crystal structure by in situ high temperature Raman study.Because of the high volatility of fluoroaluminate,we had designed a crucible for the measurement.Compared with the traditional sample pool,the crucible avoids the effect of the volatilization of the sample on the composition of the molten salts,thus improving the accuracy of the experiment.A series of in situ Raman spectra with good signal-to-noise ratio were obtained by using in situ high temperature Raman spectrometer with low wavelength laser and anti-volatilization crucible.In situ Raman spectroscopy data and density functional theory calculations were used to explain the structure evolution of temperature dependent materials.Quantum chemistry ab initio calculation was used to interpret the obtained molten Raman spectra.In this paper,based on in situ Raman spectroscopy and simulation methods,the temperature dependent structure evolution and molecular vibration modes of cryolite-alumina system were systematically studied.The content of clusters in the molten state of cryolite-alumina system was quantitatively described.The main conclusions are as follows:The molten Raman spectra tended to be consistent when Na3AlF6-Al2O3 system was heated in the air.The absorption of oxygen in the air made the Raman spectra of the system concordant.A series of typical Al-O-F clusters with characteristic structure were designed to calculate their Raman spectra.Comparing with the molten Raman spectra,the species of the main ions in Na3AlF6-Al2O3 system after heating in the air is[Al3O3F6]3-clusters which proves that the system will be disturbed by oxygen when the system was exposed to air.In the current research,the Na3AlF6-K3AlF6-Al2O3 system which used for low temperature aluminum electrolysis has gradually started to replace the traditional Na3AlF6-Al2O3 system.Compared with sodium cryolite system,potassium cryolite system has more advantages in the acquisition of experimental Raman spectra and application.In order to remove oxygen interference,the potassium system was used to conduct heating experiments under inert atmosphere.KF-AlF3 system is the basic system of K3AlF6-Al2O3 system.In the molten KF-AlF3 system,there are four kinds of ion clusters,[AlF4]-,[AlF5]2-,[AlF6]3-and[Al2F7]-with the main Raman vibrational frequencies of 621.9,554.6,520.4 and446.6,682.8 cm-1,of which 446.6 and 682.8 cm-1 are belonging to[Al2F7]-ion clusters.When KF:AlF3?CR?=1,the main anion cluster in molten system is[AlF4]-.When CR=2 and 3,the main anion cluster is[AlF6]3-.With the increase of CR ratio,the content of[AlF4]-decreases,while the content of[AlF6]3-increases,and a small amount of[AlF5]2-and[Al2F7]-exist in the system.In K3AlF6-Al2O3 system,[AlF5]2-,[AlF6]3-,[Al2OF6]2-and[Al2O2F4]2-are conformed in molten states,and their main Raman vibration frequencies are 554.6,520.4,467.7 and 412.8 cm-1.With the increase of the content of Al2O3,the content of[AlF6]3-is raised up,but the content of[Al2OF6]2-is fallen down.Meanwhile,[AlF5]2-and[Al2O2F4]2-ion clusters exist in a small amount in the system.The most important part of quantitative analysis,Raman scattering cross section,is the key to quantitative analysis of molten Raman spectroscopy.A series of characteristic anion cluster models were designed,and the normalized scattering cross sections were obtained by ab initio calculation of quantum chemistry.Combined with the deconvoluted Raman spectra of the melts,the quantitative analysis was carried out,and the change trend of anion cluster content in the molten system with the change of dopant concentration was obtained.The types of ion clusters in cryolite-alumina system were explored and revealed by in situ high temperature Raman spectroscopy combined with first-principles simulation.The content of ion clusters in cryolite-alumina system was quantitatively analyzed.It was also proved that the method of in situ high temperature Raman spectroscopy combined with first-principles simulation could not only be used for qualitative analysis but also provide feasibility for quantitative research.The use of anti-volatilization crucible makes it possible to measure Raman spectra of molten salts at high temperatures,and makes it possible to study the molten structure of fluoroaluminates.In this paper,the experimental Raman spectroscopy was effectively interpreted by combining structure with spectroscopy method,and the evolution of temperature-dependent structure of fluoroaluminate was studied,which provided a basis for the research of properties of electrolyte in aluminum industry.The understanding of microstructure of molten salts was impressed,which provided a bridge between microstructure and macroscopic properties of molten salts. |
PDF Full Text Request