Investigation on Electro-slag Refining for Silicon Removal and Mechanism of Interfacil Reaction

Author:Lv Xin Yu

Supervisor:wang jun


Degree Year:2018





The control of Si element content is one of the focuses in the purification of aluminum melt.Many aluminum alloys have strict requirements on the Si content in aluminum alloys,but the existing investigations have less research on removal Si from aluminum alloys.The electro-slag refining process not only efficiently control purity of steel,but also has been believed to control aluminum quality.At present,there are few researches on the aluminum electro-slag refining process.And types of slags available are few.Research on the interface reaction mechanism is scarce.For this purpose,in this work,the applicability of the electro-slag refining process to reduce Si content in Al alloys was particularly studied,and the mechanism underlying the interfacial reactions of reduction Si content in Al alloys through electro-slag refining was explored.Firstly,the slag compositions of Al electro-slag refining were selected,and the effects of slag compositions on the purifying effect and solidification structure of Al alloys were studied.Then the thermodynamic behavior of Al electro-slag refining interfacial reaction was investigated by thermodynamic calculation.Influence of various factors on interfacial reaction were investigated by orthogonal and single factor experiments,and kinetics mechanisms underlying the interfacial reactions were studied by analysis of change of silicon content with the extending of reaction time.Finally,through molecular dynamics and first principles,the microstructure and properties of slags as well as the slag/metal interfacial reactions were investigated.The main results obtained as follows:(1)It was found the slag 15.41%NaCl-49.15%KCl-35.44%LiCl showed efficient purification effect on aluminum alloys,the highest Si reduction ratio of 1050 Al alloy was 58%,the highest Si reduction ratio and Fe reduction ratio were 26%28%and 1026%for 8021 Al alloy.(2)Through analysis of the phases in refined slag and specific experimental conditions,it was found the direct product from the interfacial reactions of Al electro-slag refining was SiCl2.The calculated results of thermodynamics of Si removal reactions in electro-slag refining suggested that,the critical value of silicon content for interfacial reaction is15%.Analysis of the effects of various factors(electric field,slag/metal ratio,LiCl content etc.)on interfacial reaction suggested that,the electric field plays decisive role in interfacial reaction.Analysis of change of silicon content with the extension of reaction time indicated that,kinetic type of interfacial reaction is first order irreversible reaction.(3)Calculated results of slag structure and properties suggested that,the addition of LiCl into binary slag reduced the slag density and viscosity,increased number of freely moving Cl-,and improved the ionic diffusion coefficient in slags.(4)Silicon concentration distribution in Al melt/slag interface model by molecular dynamics simulation suggested that,Si concentration peak did not appear in slag,it only appears in aluminum melt and at the interface.The results indicated that the Si content could not be reduced by the adsorption of slag,but by the interfacial reaction.The effects of intensity of static electric field and polarized state of Al/Si atom on interfacial reaction were investigated by first principles.The calculated results suggested that the stronger the electric field,the stronger the polarization of the Al and Si atoms at the interface,and the easier Si atoms react with Cl-.In summary,an optimal silicon removal slag composition was filetered out,interface reaction mechanism for removing silicon using the slag was revealed:Si reacts with Cl2 to produce SiCl2,the electric field has a decisive influence on interface reaction,and the effect of electric field on interfacial reaction is essentially achieved by the alteration of atomic polarization state in aluminum melt at interface.The research in this paper is of great significance for continuing to optimize the aluminum electro-slag refining process and understanding the interfacial reaction mechanism of aluminum electro-slag refining.