Structure Simulation And Experiment Study Of CaO-SiO₂-Al₂O₃-Li₂O Molten Slag

Li₂O,as a common fluxing agent,plays an important role in adjusting the physical and chemical properties of metallurgical slag.In recent years,Li₂O has been widely used in mold fluxes during continuous casting of some key steel grades.The addition of Li₂O into mold fluxes designed for ultra-low carbon steel and peritectic steel can improve the metallurgical properties of mold fluxes,such as viscosity,melting temperature and crystallization property.Li₂O also can maintain a relative stability of the performance of mold fluxes after absorbing Al₂O₃ inclusions from the molten steel.In order to reduce the reactivity between molten steel and mold flux during the continuous casting of steels with high content of Al and Ti,the content of SiO₂ is much lower than that of convention CaO-SiO₂-based mold flux,thus the mold flux converts from silicate to aluminate system,while adding Li₂O in the mold flux can control the melting and solidification characteristics of mold flux synthetically and coordinately to meet the requirements of continuous casting process.The above influences of Li₂O on these macroscopic properties of molten slag are closely related to the microstructure of slag system.However,there are few studies on the mechanism of the effect of Li₂O on the structure of aluminate and silicate melts up to now,thus it is of great theoretical and practical significance to study the effect of Li₂O on the structure of CaO-SiO₂-Al₂O₃ melt.The current study applied the method of molecular dynamics?MD?simulation to obtain structure information and dynamic transport properties of the slag system of CaO-SiO₂-Al₂O₃-Li₂O,including partial radial distribution function,coordination number,distribution of oxygen and Qⁿ,and bond angle among particles.The influences of Li₂O and basicity on the structure characteristics of melts with high SiO₂content and high Al₂O₃ content were compared.Based on the analysis of slag structure,the relationship between melt structure and measured macroscopic property i.e.viscosity was discussed.At the same time,Raman spectroscopy was employed to analyze the relative content and variation of structural units in some typical slags,which were compared with the prediction of MD simulation.For binary systems of Li₂O-SiO₂ and Li₂O-Al₂O₃,the calculation of MD simulation indicates that the structure of[AlO₄]^5-tetrahedron is less stable than that of[SiO₄]^4-tetrahedron.With the increase of Li₂O content,the initial complex network structure in the melt is depolymerized and the proportion of closely linked Al-O in the network structure decreases,while the length of O-O bond increases significantly.Besides,due to the excess charge of[AlO₄]^5-structure in Li₂O-Al₂O₃ melt,high coordination aluminium(^VAl,^VIAl)and Q⁵ structure units are formed,resulting in complicated structure.In the ternary system of Li₂O-SiO₂-Al₂O₃,the bond angle distribution of O-Si-O is more concentrated than that of O-Al-O.When the value of R?Li₂O/Al₂O₃?equals0.75,the diffusion activation energy of Li⁺is the highest,i.e.E_D=0.58eV,meaning the diffusion of Li⁺is slow.With the increase of R,Li₂O will destroy the bridge oxygen in the structure of Al?Si?-O network,depolymerizing the complex network structure and enhancing the diffusion of Li⁺.In the quaternary system of CaO-SiO₂-Al₂O₃-Li₂O,the bond length of Si-O almost remains unchanged,while that of Al-O increases with the increase of basicity?CaO/SiO₂?for slag system with high SiO₂ content.In addition,the stability of[SiO₄]^4-tetrahedron is higher than that of[AlO₄]^5-tetrahedron.In the slag system with high Al₂O₃ content,on the one hand Li₂O not only plays the role of depolymerizing the microstructure of the melt,on the other hand Li⁺charge compensates the[AlO₄]^5-structure in preference to Ca²⁺.The measurement of Raman spectrum experiments reveals that the ranges of Raman shift of Al-O-Al bending vibration,Si-O-Si bending vibration,Si-O-Al bending vibration,Al-O symmetrical stretching vibration and Si-O symmetrical stretching vibration are 530-587cm^-1,610-645cm^-1,708-737cm^-1,652-780cm^-1,and880-1107cm^-1,respectively.In the slag system with high SiO₂ content,the rise in basicity can decrease the content of Q³ and Q² and increase the content of Q⁰ and Q¹ in the Si-O^-stretching vibration,which makes the structure convert from lamellar to dimer and monomer island type.The depolymerization effect of Li₂O in low basicity slag system is higher than that in high basicity slag system,which is consistent with the results of MD simulation.In the slag system with high Al₂O₃ content,the addition of Li₂O makes the network structure transform from Al-O-Si to Al-O-Al.As Li⁺has a strong charge compensation ability for[AlO₄]^5-network structure in melt,it has a good effect on the stability of[AlO₄]^5-structure.On the basis of MD simulation,the melt viscosity is calculated using Stokes-Einstein and Eying formulas.The calculated results have a good agreement with the measurement,indicating that the macroscopic properties are closely related with the structure of melts.Meanwhile,it is also found that the viscosity of slag system decreases with the increase of Li₂O,and the viscosity-temperature curve becomes more stable,which proves that Li₂O plays an important role in stabilizing the[AlO₄]^5-network structure in the slag system with high Al₂O₃ content.