Study On The Effect Of Na₂O And K₂O On The Evaporation Mechanism Of Electroslag Remelting Low Fluorine Slag

Electroslag remelting（ESR）is an important metallurgical technology for further purification of steel and alloys,which can improve the cleanliness of steel and refine the solidification organization.It is widely used in the production of high value-added special steels and high temperature alloys.ESR has become an important technology in the metallurgical industry because of its unique and outstanding advantages such as uniform composition,effective removal of non-metallic inclusions,significant effect of dephosphorization and desulfurization,and good surface quality.In the process of ESR,slag plays a very important role.Most of the current industrial use of medium and high fluorine slag system,in the high-temperature smelting process will produce fluoride evaporation,not only will corrode equipment,endanger person health,and change the chemical composition of the slag,there are explosions,fire,electric shock and other safety hazards.At the same time,the emissions into the air will also pollute the environment and increase the burden on the environment.The low fluorine slag has the advantages of reducing environmental pollution and improving the ESR process.To reduce the influence of the evaporation of fluoride on the composition of slag,it is necessary to study the low fluorine slag.In this paper,CaF₂-CaO-Al₂O₃-Mg O-Ti O₂slag is used as the research object to determine two ideas for developing low-fluorine slag:one is to adjust the slag system composition ratio without changing the slag system.Firstly,the content of CaF₂was changed by fixing CaO/Al₂O₃.Secondly,the evaporation of low fluoride slag system was studied by fixing the content of CaF₂and changing CaO/Al₂O₃.The evaporation of the low-fluorine slag system was studied by adding alkali metal oxides Na₂O and K₂O respectively to the original slag system.Finally,different amounts of Na₂O and K₂O were added with equal total additions of alkali metal oxides to comparatively study the evaporation of low-fluorine slag systems.In this study,the Fact Sage software was used to calculate the evaporating materials,activity and equilibrium constant of the slag.The effects of CaF₂content,CaO/Al₂O₃mass ratio,Na₂O content and K₂O content on the non-isothermal evaporation of CaF₂-CaO-Al₂O₃-Mg O-Ti O₂slag were studied by TG-DSC,and a kinetic model was established to verify the evaporation process.The melting point and structure characteristics of low fluorine slag system were investigated by hemispherical method and Fourier infrared spectroscopy.The evaporation mechanism of slag system was studied by calculation and experimental analysis.The results show that:（1）The gaseous substance evaporated from the CaF₂-CaO-Al₂O₃-Mg O-Ti O₂slag is mainly CaF₂.The calculated vapour pressure in a sequence is CaF₂>Mg F₂>Al F₃>Al OF.Furthermore,with the increase in CaF₂content from 15 wt.%to 30 wt.%,the evaporation of fluoride increases from 0.934%to 1.047%,and the melting temperature drops from 1480°C to 1443°C.With the increase in the CaO/Al₂O₃mass ratio from 0.8to 1.4,the weight loss increases from 0.92%to 1.95%and then decreases to 1.70%,and the melting point drops from 1460°C to 1324°C.The slag evaporation is controlled by the Valensi reaction mechanism,and its mechanism function is G（α）=α+（1-α）ln（1-α）,which belongs to two-dimensional diffusion.（2）In the CaF₂-CaO-Al₂O₃-Mg O-Ti O₂-（Na₂O）slag,CaF₂is the main evaporation substance,followed by Na F.It contains a small amount of Mg F₂and Al OF.Increasing Na₂O from 0 wt.%to 6.6 wt.%reduces the onset temperature of CaF₂evaporation by100°C-150°C,and evaporation of Mg F₂and Na F by 50°C and 100°C,respectively.With rising temperature,fluoride evaporation ratio rises.With an increase in Na₂O content from 0 wt.%to 6.6 wt.%,the evaporation ratio rises from 0.76%to 1.69%.The Anti-Jander mechanism function,which is referred to as three-dimensional diffusion,seems to be in charge of controlling the evaporation.In the associated kinetic mechanism function G（α）=[（1-α）^1/3-1]²,the decrease of activation energy indicates that Na₂O can increase the evaporation rate of the reaction,and promote the evaporation of fluoride in slag.Increased Na₂O encourages Na⁺ion migration,which in turn boosts diffusion.The rate-controlling step are chemical reactions（4-1）and（4-2）and mass transfer in liquid slag.（3）CaF₂,KF and Na F are the main evaporating substances in the CaF₂-CaO-Al₂O₃-Mg O-Ti O₂-（Na₂O-K₂O）slag,and the evaporation of Mg F₂,Al F₃and Al OF is negligible.KF evaporates absolutely in the early stage of the reaction,and CaF₂evaporates in a large proportion in the late reaction period.At 1500°C,the order of vapor pressure is KF>CaF₂.When K₂O and Na₂O are added to the residue sample at the same time,the evaporation ability of KF is stronger than that of CaF₂and Na F.As the K₂O content increases from 0 wt.%to 8.3 wt%,the evaporation increases from 0.76%to1.21%.The evaporation rates of slag containing 5.7 wt.%K₂O and 6.3 wt.%Na₂O are1.48%and 1.32%,respectively.Under the same conditions,K₂O has a greater effect on evaporation than Na₂O.Fourier infrared spectroscopy results show that the addition of K₂O depolymerizes the network structure and K₂O can depolymerize the network structure better than Na₂O.The diffusion coefficient of KF in argon is higher than that of CaF₂and Na F,which confirms the higher evaporation rate of KF.（4）Varying the CaF₂content,the CaO/Al₂O₃mass ratio and the addition of the alkali metals Na₂O and K₂O to the slag system are suitable for the development of electroslag remelting slag systems.While metallurgical properties can be improved,Na₂O evaporates less compared to K₂O and therefore the addition of Na₂O is more suitable for the development of electroslag remelting slag systems.