| The metallurgical process of steel is accompanied by various interfacial phenomena,such as the transfer of elements at the phase boundary and the physicochemical reactions occurring at the interface,all of which affect the smelting efficiency,making interfacial phenomena and interfacial properties a fundamental part of the steel production process.In the context of "double carbon",further exploration of the smelting efficiency improvement link is inevitably based on a deeper understanding of the structure and properties of the gas-slag-metal interface.However,due to the smelting process with high temperature,closed "black box"characteristics,the lack of advanced in-situ research means,the current metallurgical interface phenomenon research mainly to observe the interface morphology and determine the surface tension of the melt,slag-metal interfacial tension,etc.,failed to the slag interface structure,element distribution and interfacial oxygen potential and other interface key parameters to study.In this thesis,a slag system containing CrOx,MnO and FeO is used as the object of study,and an innovative X-ray photoelectron spectroscopy(XPS)etching technique is used to characterise the elemental distribution and interfacial structure at the gas-slag and slag-metal interfaces,to obtain the gas-slag-metal interfacial structure and elemental distribution,and to establish a new method to directly calculate the interfacial oxygen potential and boundary layer thickness.In addition,Ab initio molecular dynamics(AIMD)based on first principles was introduced into the study of slag structure and interface,and the influence of metal cations on the interfacial structure was analysed from the microscopic perspectives of elemental valence,charge,bonding and migration processes.The pathways of sulfur migration are explained.The main contents are as follows:Firstly,the effect of metal oxides on the slag bulk structure was investigated using a combination of high temperature quenching experiments and classical molecular dynamics.The results show that FeO,MnO and CrOx have different effects on the structure of the slag.At low basicity(B=0.5),the metal oxides lead to a significant decrease in the ratio of Q3/Q2 in the structural unit(0.9-0.6)on the one hand,while FeO has the lowest ratio,followed by MnO and CrOx,and on the other hand to a decrease in bridged oxygen and an increase in non-bridged oxygen,where the bridged oxygen in FeO decreases more and the free oxygen in CrOx is less;under high basicity(B=1)conditions,the metal oxides lead to a slight increase in the ratio of Q3/Q2(0.2～0.3),while the ratio of FeO is the highest,followed by MnO and CrOx,while the metal oxides have less effect on the bridged oxygen,nonbridgedoxygen and free oxygen.In summary,under low basicity conditions,due to the high number of Si-O and Al-O units,FeO and MnO mainly take on the role of destroying the structure,with FeO being more destructive.CrOx has a valence change,and on the one hand,as a metal cation it can destroy the structure,and on the other hand,it promotes coordination,leading to structural aggregation;under high basicity conditions,due to the low number of Si-O and Al-O units,the metal oxides have less influence on the structure.Secondly,the state,bonding mode and migration mechanism of sulfur in the molten MnO-SiO2 and CaO-SiO2 native structures were comparatively investigated using the AIMD method.The results show that manganese and calcium have different effects on the state of sulfur incorporation and migration mechanisms.In MnO-SiO2,sulfur rapidly forms a stable Mn-S-Mn structure and the sulfur valence state is in the range of-0.8 to-0.4 e.However,in the CaO-SiO2 system,the sulfur atoms do not undergo rapid bonding transitions and tend to be stable around the calcium atoms and the sulfur valence state is in the range of-1.4 to-1.3 e.This is due to the uneven distribution of charge due to the off-core electrons of manganese,which affects the interconversion of the oxygen structure and leads to a decrease in bridged and an increase in non-bridged oxygen during the sulfur reaction.Whereas there is no charge influence outside the calcium nucleus,the non-bridged oxygen is consumed in the exchange reaction of sulfur,leading to a decrease in non-bridged oxygen and an increase in bridged oxygen.Again,direct characterisation of the gas-slag interface structure and elemental distribution of Cr and Mn-containing slags was achieved using gas-slag equilibrium experiments and XPS etching techniques.The results show that there is a gradient trend in the interfacial structure and elemental distribution,with the interfacial structure mainly consisting of bridged and non-bridged oxygen,with non-bridged oxygen increasing and bridged oxygen decreasing with depth;within the boundary layer,the calcium content gradually increases and the silicon and oxygen content gradually decreases,with sulfur and metal cations aggregating within the interfacial boundary layer.The interfacial structure and elemental distribution indicate that the interface tends to be a high oxygen potential silica-oxygen structural unit,which gradually decondenses with depth and tends to the native structure.The gas-slag interface oxygen potential for this slag system was obtained by quantifying the elemental and FactSage equilibrium calculations for the Mn-containing slag interface on the order of 10-19 to 10-18 atm.Based on the etching time and rate,a boundary layer thickness calculation method was established,and the boundary layer thicknesses were 117.90 nm,102.1 8 nm and 94.32 nm for equilibrium reaction times of 1 h,3 h and 6 h,respectively.Then,high temperature equilibrium experiments and XPS etching techniques were used to study the interfacial structure,elemental distribution and interfacial interaction mechanism between the manganese-containing slag and metallic ferrofluid.The results show that the slag-metal interfacial structure is similar to that of the gas-slag interface,consisting mainly of bridged and non-bridged oxygen,with the interfacial bridged oxygen content increasing from 35%to 75%and the interfacial non-bridged oxygen content decreasing from 65%to 25%as the alkalinity increases from 0.5 to 1.2.The relative contents of silicon and oxygen decrease progressively with increasing depth,while the relative contents of calcium,manganese and sulfur increase with increasing depth of interfacial etching.At the same time,Mn is present in the positive divalent form,sulfur in the negative divalent form and Fe in both Fe-O and Fe-S bonds.Based on the results two interfacial chemical reactions are proposed:one is the redox reaction of Fe with Mn/Si in the slag.The other is an exchange reaction between sulfur and oxygen.Finally,the migration process of sulfur at the gas-slag-metal interface was investigated using classical molecular dynamics and AIMD methods to obtain the migration pathways of sulfur.The results show that at the gas-slag interface,S2 molecules in the gas phase are absorbed by the slag interface,while the manganese termination layer is more likely to absorb S2 molecules compared to the oxygen termination layer.The gradual decrease in the valence of sulfur at the interface is due to the conversion of sulfur from the gaseous S2 molecule to the Mn-S bond.In the slag-metal interface,sulfur first migrates from the metal liquid to the slag-gold interface and aggregates at the interface,mainly forming Mn-S bonds.When the slag comes into contact with the metal liquid,the valence of Fe at the interface increases and the valence of Mn and S decreases,proving that in addition to the desulfurization reaction,an oxide reduction reaction of metallic Fe and Mn([Fe]+(Mn2+)=[Mn]+(Fe2+))will also occur at the slag-metal interface.The combination of experimental and computational results shows that sulfur will eventually stabilise in the slag,and that the interface is the most critical location in the migration process,with its interfacial structure and the distribution of interfacial elements having an impact on the migration of sulfur. |
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