Slag-metal Reaction Research Articles

Thermodynamic analysis of inclusions in Ti‐deoxidised steels

The precipitation of non‐metallic inclusions in Ti‐deoxidised steels is analysed with the multiphase equilibrium code CEQCSI which is based on the IRSID slag model. For that purpose, the slag model has been extended to systems containing titanium oxides, using data available in the literature, and validated with industrial results obtained on various steel grades. Applications concerning slag‐metal reactions in ladle metallurgy are described. 2 applications concerning inclusions in Ti‐deoxidised steels are also presented: (a) experimental study of the effect of steel contamination by magnesia refractory lining on the composition of oxide inclusions in a Ti‐deoxidised steel: there is a good correlation between analysed oxide compositions and compositions predicted by the model; (b) evaluation of the optimised steel composition to obtain Al2O3‐TiO2 inclusions during solidification for steels pertaining to the so‐called “metallurgy of oxides in steels”.

溶融スラグによる溶銅中のＰｂの酸化除去速度

Kinetic behavior of lead removal from liquid copper by slag-metal reaction was investigated at 1200 °C on a laboratory scale. Various slags composed of Cu2O, Fe2O3, B2O3 and SiO2 were used, For Cu2O-MxOy (M = B, Si, Fe) binary slags, B2O3 addition was most effective for the lowering of activity coefficient of PbO in slag. As a result, larger removal rate of lead from liquid copper was obtained under the experimental condition of higher contents of B2O3 and oxygen. SiO2 was also an effective slag component for the lead removal, but the reaction rate was retarded by its addition to slags. The kinetic behavior of lead removal by oxidizing slags could be simulated by the reaction model based on the two film theory.

Assessment of P2O5 Activity Coefficients in Molten Slags.

From a critical review of the existing experimental data, the free energy equations are presented. A new estimate is made of the standard free energy of formation of the hypothetical pure liquid P2O5, which is used in calculating the P2O5 activity coefficients from the experimental data of independent studies of slag–metal reactions. It is found that in low P2O5 slags, log( γP2O5) is a linear function of CaO from 0 to 60%, independent of temperature. Whereas, in slags containing P2O5 > 10%, log(γP2O5) is a linear function of CaO content > 40% only and increases with an increasing temperature. A novel concept of a slag–P2O5 emf sensor is presented for experimental evaluation.

Slag Composition Variations Causing Variations in Steel Dephosphorisation and Desulphurisation in Oxygen Steelmaking.

Study of plant analytical data for BOS and OBM processes revealed that the iron oxide content of slag has a decisive effect on the extent of steel dephosphorisation and desulphurisation. The slag/metal distribution ratios (%P)/[%P] and (%S)/[%S] from the plant data are scattered about the calculated equilibrium values for the slag-metal reactions involving the iron oxide content of the slag. The data also revealed consistent interrelations between the concentrations of FeO, CaO and SiO 2 in the slag at the end of oxygen blowing. However, there are variations in the slag composition in the interrelations between the oxide contents: 5 to 6 % FeO and 4 to 5 % CaO or SiO 2 between the low and high values. These unpredictable variations in slag composition from one heat to the next bring about variations in the extent of steel dephosphorisation and desulphurisation. Consequently, the observed distribution ratios (%P)/[%P] and (%S)/[%S] are scattered within a slag composition range, bordered by the equilibrium curves for the low and high iron oxide contents with the corresponding CaO and SiO 2 contents of the slag.

Experimental evidence for electrochemical nature of the reaction between iron oxide in calcia-silica-alumina slag and carbon in liquid iron

The electrochemical nature of the reaction between iron oxide in calcia-silica-alumina slag and carbon in liquid iron has been studied by measuring the kinetics of the slag-metal reaction. A base slag (48 pct CaO-40 pct SiO2-12 pct Al2O3) containing iron oxide (≤8 wt pct FeO t ) was reduced by an Fe-C metal bath (∼4 wt pct C) at 1400 °C. The reaction rate was calculated from measurements of the total inlet gas flow rate and the CO concentration in the outlet gas stream. The slag was “internally short circuited” by dipping an iron plate through the slag layer, and this resulted in an increase in the rate of CO evolution. An external circuit was produced by dipping a graphite rod (shielded from the slag) into the metal bath and a steel or molybdenum rod into the slag layer; the open-circuit voltage and short-circuit current were measured when iron oxide was added to the base slag layer. The reaction rate was enhanced by applying a voltage across the slag layer, and an electric arc cathode was employed in some of these “electrolysis” experiments.

Metal-slag reaction through a solid interfacial layer

Liquid Cu–Ni–Al alloys and two Li 2O–Al 2O 3–SiO 2 liquid slags were contacted for 1–8 min at 1300 and 1450°C. Their interface was deformed by the formation of a lithium aluminate crust due to aluminum oxidation. The interfacial area evolved with time depending on the compositions of the liquids. This led to the formation of structures made of liquid metal sheathed by a solid lithium aluminate layer. These structures were studied from metallographic sections of quenched samples and from X-Ray radiographs in real time. Their formation was attributed to the semi-permeability of the lithium aluminate layer allowing the transfer of aluminum ions to the slag but impeding the reduced silicon from reaching the liquid metal. Indeed, at the end of the slag-metal reaction, the silicon produced by the reduction of silica by aluminum was not found in the Cu–Ni alloy but silicon-rich droplets were found on the slag side of the crust. An explanation of these phenomena is presented.

Selective oxidation of copper from liquid copper-silver alloys

In this work, the oxygen refining of liquid copper-silver alloys with a borosilicate slag was studied. First, a comprehensive thermodynamic analysis was performed using the data available in the literature. The results indicate that since silver oxide is relatively unstable in silicate-based slags, then it should be thermodynamically feasible to oxidize copper from copper-silver alloys with a very low silver loss to the silicate slag. In actual practice, although relatively low copper levels can be achieved in the metal phase, the silver losses to the slag are excessive. Therefore, in the present work, both kinetic and equilibrium experiments were performed on a molten copper-silver alloy containing 12.68 mass pct silver in order to elucidate the mechanism of silver loss to the slag. The kinetic experimental results indicated that copper levels of less than 2 mass pct could be achieved with silver recoveries of about 95 pct after relatively short refining times of 15 minutes. In the equilibrium experiments, the copper contents of the metal were less than 1 mass pct, and these values were in good agreement with those which were calculated from the data of previous researchers. In order to explain the relatively high silver losses to the slag, a model was developed which is based on the transport of silver from the metal phase to the slag phase both in metallic form and as silver oxide in the copper oxide oxidation product. The copper and silver oxides and the metallic copper-silver alloy are all transported into the slag by the oxidizing gas bubbles. It is proposed that once in the slag, the silver oxide is unstable and decomposes into metallic silver which is not easily recovered in the metal phase. Also, the transfer of the copper-silver alloy into the slag, by the gas bubbles, promotes the slag-metal exchange reaction, which again results in the generation of silver particles in the slag.

A New Approach to Model Sulphur Refining in a Gas-stirred Ladle. A Coupled CFD and Thermodynamic Model.

In the present work, a new modelling approach has been put forward to study slag-metal reactions. Sulphur refining in a gas-stirred ladle has been taken as an example. A two-dimensional fluid dynamic model accounting for the steel, slag and argon phases has been incorporated with the thermodynamics of desulphurisation. Comparison of the results of the model calculation with plant data from Ovako Steel indicates that the present approach has great potential. It has opened up the possibility of developing process models based on fundamental equations.

Processing of high carbon Fe3Al based intermetallic alloy

A melting procedure for air induction melting (AIM) of an Fe 3 Al based intermetallic alloy Fe-15.38 wt%Al-1.1 wt%C is described. Use of an appropriate slag cover during AIM results in elimination of hydrogen gas porosity in cast AIM ingots. Criteria for slag selection and slag to metal ratio are discussed. Refining by slag-metal reactions results in significant reduction in impurity levels (S, O, N) during AIM. Consequently, low cost raw materials such as mild steel scrap and commercial aluminium were used for melting the alloy. The AIM ingot exhibited excellent tensile properties. The ductility and hot workability of the ingot may be further improved by subsequent processing through electroslag remelting. It is also argued that the presence of carbon may be necessary to get AIM castings with desirable mechanical properties.

Slag-Metal Reactions in Submerged Arc Welding:An Overview

The requirements placed on welding fluxes and the resulting slag are numerous. For instance the flux and the resulting slag must be able to contribute to arc stability, weld metal alloying, weld metal deoxidation, weld metal protection, ease of slag detach- ability, control of inclusions due to slag entrapment, hence the resulting mechanical properties, control of porosity, facilitating welding in various positions, among others (1, 2, 3). However, these requirements can be better fulfilled only if there is a good understanding of the many reactions occurring simultaneously at the slag metal interface during welding (1). This is because there are extremely complex interconnections between the aforementioned requirements and the reactions occurring at the slag metal interface. The aim of this article, therefore, is to present a simple overview of slag-metal reactions occurring during submerged arc welding.

Viscosities of LF Slags and Their Impact on Ladle Refining.

The traditional approach to modeling ladle processes involvea critical survey of the thermodynamic aspects of the processincluding mass and heat balantes at the macroscopic level. Onthe other hand, the macrokinetics, specific to the reactor haveoften been studied separately, using computational fluiddynamits. Over the years, it has been realized that traditionalmethods have obvious limitations in providing an understandingof heterogeneous processes. In a long range perspective, acomplete description of ladle metallurgy and a fundamentalunderstarrding of the underlying process phenomena require aconsideration of all the individual reactions involved andtheir interrelations. This, in tum, clearly requiressimultaneous solution of thermodynamics, heat and fluid flow.The salient features of the present study are the considerationof fundamental transport equations and establishedthermodynamic relationships so that mathematital models takinginto account both thermodynamics and fluid flow could beachieved. A technique to' incorporate the slag phase in two-phasefluid-flow models for ladle treatment is introduced in thisthesis. This development enabled, for the first time,predictions of fluid flow conditions in the important steelklagregion. Good agreement is shown between predictions andphysical modeling reported in literature, regarding thecriteria for initiation of entrapment of a lighter phase into aheavier phase. Furthermore, a method to combine fluid dynamits andthermodynamics to model slag/metal reactions is introduced andis demonstrated in the case of sulphur refining during gasstirring. By considering the volume of mixing between slag andmetal and the thermodynamic equilibrium in the two-phase zone,the calculations of interfacial area and the mass-transfercoeffcients for different elements are avoided. In this way,the combined effect of fluid-flow and thermodynamic conditionscould be predicted from fundamental transport equations.Comparison between predicted data and plant data demonstratesthat the method could be used to predict the desulphurizationwith an acceptable accuracy. A new method to model ladle processes involving gas-liquidreactions is also introduced in this thesis. In the cases wherethe interfacial reaction and mass transfer in the gas-phase arerapid, the overall mass transfer resistance could be expressedby the mass-transfer in the melt. The hydrogen refinementprocess during vacuum degassing in an ASEA-SKE ladle furnacehas been taken as an example. The predictions have beencompared with data from production scale experiments carriedout at Ovako Steel. Good agreement between the calculatedhydrogen toncentrations and the hydrogen tontent in the sampletaken after vacuum degassing substantiates the feasibility ofthis technique. Key words:Slag-Metal Reactions, Metal-Gas Reactions,Ladle Metallurgy, Sulphur Refining, Hydrogen Refining, VacuumDegassing, Combined Kinetics and Thermodynamics, 3-Phase Model,Gas Stirring, Modeling, CAS-OB, CFD

Mathematical Expression of Slag-Metal Reactions in Steelmaking Process by Quadratic Formalism Based on the Regular Solution Model.

During the past three decades many theoretical treatments have been developed to predict the thermodynamic properties of silicate melts. In this paper, an attempt has been made to describe the slag-metal reactions in steelmaking process quantitatively by the relation of quadratic formalism based on the regular solution model. The approximate validity of the model was quite satisfactory to formulate the reactions of the oxygen distribution, the ferrous-ferric iron equilibrium, the manganese distribution, the phosphorus distribution and the hydroxyl capacity. The prediction of activities of components in FetO-CaO-SiO2 and FetO-MnO-SiO2 ternaries are also in good agreement with the measurements.

溶銅または溶鉄とCaO-Al<SUB>2</SUB>O<SUB>3</SUB>系スラグ間反応速度の電極インピーダンス法による解析

A chemical reaction at the interface between slag and metal can be analyzed as a kind of electrode reaction because of the ionic nature of liquid slag.In the present work, application of an A.C. impedance method to slag-metal reaction in iron- and steel-making process was tried to clarify the individual kinetic factors concerning the electrode reactions at the interface between slag and metal. The total impedance between CaO-Al2O3 slag and Cu-S or Fe-C-S melts was measured in this paper.The results obtained were successfully used to determine the individual factors on kinetics of the electrode reaction between slag and metal, such as solution resistance of slag, interfacial capacitance in electric double-layer, overvoltage of charge transfer, rate constant and so on.

A.C. Impedance Analysis of the Kinetics of Reactions between Molten Cu or Fe and CaO-Al2O3 Slag.

A chemical reaction at the interface between slag and metal can be analyzed as a kind of electrode reaction because of the ionic nature of liquid slag.In the present work, application of an A.C. impedance method to slag-metal reaction in iron- and steel-making process was tried to clarify the individual kinetic factors concerning the electrode reactions at the interface between slag and metal. The total impedance between CaO-Al2O3 slag and Cu-S or Fe-C-S melts was measured in this paper.The results obtained were successfully used to determine the individual factors on kinetics of the electrode reaction between slag and metal, such as solution resistance of slag, interfacial capacitance in electric double-layer, overvoltage of charge transfer, rate constant and so on.

Slag-metal reactions during welding: Part I. Evaluation and reassessment of existing theories

A critical review of current thermodynamic theories of slag-metal reactions is presented. A series of preliminary experiments indicates that the previously proposed droplet theory is incorrect and the primary reactions controlling Mn, Si, and Cr content occur in the weld pool. In addition, these experiments show that the net transfer of oxygen is independent of the transfer of Mn and Si.

Slag-metal reactions during welding: Part II. Theory

A kinetic model is developed to describe the transfer of alloying elements between the slag and the metal during flux-shielded welding. The model accounts for changes in alloy recovery based on the geometry of the resulting weld bead. It also distinguishes compositional differences between single-pass and multiple-pass weld beads. It is further shown that the final weld metal oxygen content is directly related to the weld solidification time as well as the type of flux used.

Slag-metal reactions during welding: Part III. Verification of the Theory

A previously developed kinetic model of alloy transfer (Part II)[1] is tested experimentally for transfer of Mn, Si, Cr, P, S, Ni, Cu, and Mo. The results show very good agreement between theory and experiment. The transfer of carbon and oxygen is also discussed. It is shown that the transfer of oxygen into the weld metal occurs in the zone of droplet reactions, whereas oxygen is lost by formation and separation of inclusions in the solidifying weld pool. Methods of applying this analysis to multipass welds and active fluxes containing ferroalloy additions are also described.

Development of Analysis and Control Method for Hot Metal Dephosphorization Process by Computer Simulation.

In order to apply a coupled reaction model to the simulation of industrial scale hot metal pretreatment process, a new computer program of MACSIM (Mathematical Analysis Codes for Slag-metal Reaction and Injection Metallurgy) has been developed. In this program, the reactions by injected flux, slag on bath surface and top blown oxygen gas can be calculated and most of the parameters were determined based on the experimental results of the previous paper. The following phenomena were clarified by this program and the control of the hot metal dephosphorization process became possible.(1) In the case of hot metal pretreatment by torpedo car, about 50% of top blown oxygen gas had the same effectiveness as Fe oxide for dephosphorization reaction. The remaining oxygen contributed to the heat supply through decarburization and post combustion reactions.(2) When [P] concentration before treatment was high, the efficiency of oxygen and that of lime for dephosphorization were high but a clear dependence on temperature was not found.(3) The efficiency of oxygen and that of lime for dephosphorization mainly depended on CaO/O and hot metal composition ([P], [Si]) although dephosphorization treatment time and flux supply rate differed widely.

Kinetics of Reactions between High Carbon Molten Iron and FeO Containing Slag

The rate of silicon oxidation between Fe-4.4% C-Si alloy and Li2O-CaO-SiO2-FeO slag has been investigated under the condition of mechanical stirring at 1300°C. The initial concentrations of FeO in the slag, (%FeO)0, and of silicon in the metal, [%Si]0, were 2.5-30 and 0.008-0.1, respectively. The rotating speed of stirrer was 50-400rpm. The activity of SiO2, aSiO2, in the slag has also been determined experimentally by measuring the concentration of silicon in carbon saturated iron equilibrated with SiO2 in the Li2O-CaO-SiO2 slag at PCO=1 atm and 1300°C.Under the experimental conditions adopted, the apparent mass transfer coefficient in the metal phase, k'Si, increases with increasing (%FeO)0, while k'Si does not change much with varying [%Si]0. It is found that k'Si increases approximately in proportion to the 1/3 power of the rotating speed of stirrer. However, the effect of mechanical stirring on the mass transfer of FeO in the slag is negligible except at low (%FeO). A mathematical model describing the simultaneous reactions of silicon, carbon and FeO is proposed. The reaction mechanism of the slag-metal reactions is explained with the aid of the mathematical model proposed.

Kinetics of fundamental reactions pertinent to steelmaking process.

This is a review of kinetic studies on fundamental reactions pertinent to steelmaking processes. With citing research results mainly obtained in theauthor's laboratory, description of the paper is made aiming at showing the present state of the knowledge on the selected subjects. Results of studies on the behavior of bubbles and jets at the submerged orifice (nozzle) are reviewed. The mixing characteristics in a gas-stirred molten metal is discussed. Kinetic studies on carbon-oxygen reactions and nitrogen reactions in molten iron are presented. Comments are made on the rate-controlling mechanism of slag-metal reactions. A summary is made of theresults of studies on the rate of mass transfer between molten slag and metal influenced by gas injection. Kinetic problems in scrap melting and the rates of dissolutions of solid iron and Cr2O3 in liquid iron with formation of CO are handled.