Dephosphorization and Desulphurization Pretreatment of Liquid Iron using CaF2‐free Fluxes containing BOF Slags

In order to examine the possibility of recycling BOF slags as dephosphorization and desulphurization fluxes, experiments were performed in a system with liquid iron and artificially prepared fluxes comprising BOF slags, lime and/or sinter ore in a Al₂O₃ crucible at 1623 and 1673K. The phosphorus and sulphur content in liquid iron were expressed as a function of reaction time in the form of exponential decay of first order. CaF₂‐free fluxes comprising BOF slags, lime and/or sinter ore showed very high capacities of dephosphorization and desulphurization, which means that BOF slags could largely be recycled in the hot metal pretreatment processes.     

Influence of Manganese Oxide and Silica on the Morphological Structure of Hematite Compacts

Dry compacts of pure Fe₂O₃ and Fe₂O₃ doped with either (2–6 mass%) MnO₂, (2.5–7.5 mass%) SiO₂ or with both (2–6% MnO₂ + 7.5% SiO₂) were indurated at 1373 K for 6 hours and physically and chemically characterized. The fired compacts were isothermally reduced with pure CO gas at 1073–1373 K. The O₂‐weight loss was continuously recorded as a function of time using TGA technique. The external volume of pure and doped compacts was measured at different reduction conditions and the volume change was calculated. The structural changes accompanying the reduction process were visually and microscopically examined and the different phases were identified by X‐ray diffraction analysis. After firing, manganese ferrite (MnFe₂O₄) phase was identified in MnO₂‐doped compacts. In pure Fe₂O₃ compacts, the external volume of compacts was increased with reduction temperature, showing a maximum swelling value at 1198 K. Catastrophic swelling was observed in MnO₂‐doped Fe₂O₃ compacts, the volume change increased with MnO₂ content showing catastrophic swelling in compacts containing 6%MnO₂ at 1248 K. The catastrophic swelling was attributed to the formation of dense metallic iron whiskers and plates in a highly porous structure. Unlike in MnO₂‐doped samples, no considerable volume changes were detected in SiO₂‐doped Fe₂O₃ and (MnO₂ + SiO₂)‐doped Fe₂O₃ compacts where the presence of silica greatly hindered the swelling phenomenon at all reduction temperatures.     

Thermodynamic behaviours of manganese and phosphorus between CaO‐MgOsat‐SiO2‐Al2O3‐FetO‐MnO‐P2O5 ladle slag and liquid iron

The thermodynamic equilibria of manganese and phosphorus between liquid iron and CaO‐MgO_Sat‐SiO₂‐Fe_tO‐MnO‐P₂O₅‐Al₂O₃ (0–33%) ladle slag have been investigated at 1873 K from the viewpoint of Mn and P yields for the production of high‐strength steels. The equilibrium distribution ratios of Mn and P were found to increase with increasing Fe_tO content; however, these ratios vary with basicity, but they do this the other way round. The addition of alumina into slag at a fixed basicity and Fe_tO content decreases both the equilibrium manganese and phosphorus distributions. The equilibrium distribution ratios were discussed in terms of the variation of activity coefficients of Fe_tO, MnO and PO_2.5, according to the slag basicity and Al₂O₃ content. The quantitative contributions of basicity and (%Fe_tO + %MnO) on L_Mn and L_P were empirically determined and their usefulness was discussed with the aid of plant data: To improve Mn and P yields in the practical RH operation, it is strongly recommended that Fe‐Mn and Fe‐P alloys be added after Al deoxidation treatment inducing relatively high Al₂O₃ in slag and maintaining low Fe_tO content. In addition, a ladle slag composition for the targeted Mn and P contents in liquid iron was substantially estimated using the empirical relationships.     

The kinetics of dephosphorization of carbon-saturated iron using an oxidizing slag

The kinetics of dephosphorization of carbon-saturated iron by oxidizing slags were studied at 1330 °C. Nine slag compositions were investigated in the systems CaO-Fe₂O₃-SiO₂-CaF₂ and CaO-Fe₂O₃-SiO₂-CaCl₂. Increasing Fe₂O₃ up to 50 pct was found to increase the rate and extent of dephosphorization, whereas further increases were found to decrease the rate and extent of dephosphorization. This was explained in terms of two competing effects on the driving force, where increased levels of iron oxide increase the oxygen potential for dephosphorization, hence the driving force, but simultaneously dilute the basic components in the slag, lowering the driving force for dephosphorization. CaF₂ and CaCl₂ were found to decrease the rate and extent of dephosphorization at levels higher than 12 pct. The rate of dephosphorization was found to be first order with respect to phosphorous in the metal and was controlled by mass transport in the slag. The oxygen potential at the slag/metal interface was controlled by the FeO activity in the slag. When the kinetic results were analyzed to take account of different driving forces, Fe₂O₃, CaF₂ and CaCl₂ were all found to increase the mass transfer coefficient of phosphorous in the slag, and a quantitative relationship has been demonstrated between these mass transfer coefficients and the slag viscosity for each system studied.     

Thermodynamic assessment of CaO‐SiO2‐Al2O3‐MgO‐Cr2O3‐MnO‐FetO slags for refining chromium‐containing steels

The slag system of CaO‐SiO₂‐Al₂O₃‐MgO‐Cr₂O₃‐MnO‐Fe_tO relevant to refining chromium‐containing steels such as bearing steel is thermodynamically assessed at 1873 K. The activity coefficient of Fe_tO shows an initially rapid increment followed by a gradual reduction according to Cr₂O₃ content at a constant basicity, and decreases with increasing slag basicity. γ_MnO is decreased abruptly by increasing Cr₂O₃ content and thereafter, maintains a nearly constant level. From the standpoint of inclusion control, the Cr₂O₃ presence in ladle refining slags is thermodynamically harmful in that it minimizes the inclusion level by inducing the increment of γ_FetO even though Cr₂O₃ exists in extremely small amounts. However, it is beneficial in that it diminishes AI reoxidation by decreasing γ_MnO. The presence of carbon in slag decreases γ_FetO and γ_MnO, which turns out to be favourable for the reduction of Al reoxidation. The thermodynamic equilibria of chromium and manganese are quantified in terms of Fe_tO and Cr₂O₃ content as well as slag basicity by using multiple regression analysis. L_Cr and L_Mn are increased by the presence of Cr₂O₃, indicating a low recovery efficiency of Cr and Mn in the treatment of ferroalloy addition. In determining L_S values, Cr₂O₃ is not so important as the basicity of slags.     

Iron redox equilibria in BOF slag equilibrated with ambient air

The Fe²⁺/Fe³⁺ ratio in the CaO‐MgO‐Fe₂O₃‐FeO‐SiO₂ based slag was measured under the condition of equilibrium with the ambient air at 1873 K as a fundamental study for precise slag coating control in BOF operation. The CaO/SiO₂ mass ratios of the main slag were 1, 1.5, 2, 3 and 4, and total iron mass content was in the range of 10 to 35 %. Moreover, mass contents of 1 to 13 % of MnO and 2 to 12 % of Al₂O₃ were added to the melt to evaluate their effects on the Fe²⁺/Fe³⁺ ratio. The effect of slag composition on the Fe²⁺/Fe³⁺ ratio was discussed and quantified into a form of formula. As the basicity in slag increases, the Fe²⁺/Fe³⁺ ratio decreases. The effect of iron oxide mass content is observed to be dependent on the basicity of slag. An increase in iron oxide mass content makes the Fe²⁺/Fe³⁺ ratio higher for basic slag but lower for acidic slag. It is revealed that the redox reaction of iron oxide in steelmaking slag under the ambient air is controlled by the complex anion formation reaction of iron oxide, and that the iron oxide in basic slag exists in the form of 2 or more kinds of complex anion controlling the oxygen anion content. Both Al₂O₃ and P₂O₅ increase the Fe²⁺/Fe³⁺ ratio by diluting the basic oxides as SiO₂ does, while manganese oxide lowers the Fe²⁺/Fe³⁺ ratio enormously down to nearly zero. The Fe²⁺/Fe³⁺ ratio can be described as a function of slag composition, X = (%CaO) + 0.38(%Fe₂O₃ + %FeO)+3.2(%MnO), in the equation of log(Fe²⁺/Fe³⁺) = ‐0.00107X² + 0.0721X ‐ 1.982.     

Enrichment of Fe‐Containing Phases and Recovery of Iron and Its Oxides by Magnetic Separation from BOF Slags

Steel slag normally contains a large amount of iron and its oxides. Therefore, it is a potential renewable resource in case of inadequate iron ore supply. To recover the metals from steel slag, two types of BOF slags were remelted at 1873 K. The liquid slags were cooled using four types of cooling conditions, namely, water granulation, splashing, air cooling, and furnace cooling, to investigate the influence of cooling rate on mineral components, especially the enrichment behavior of Fe‐containing minerals. Subsequently, wet magnetic separation was conducted to examine the relations between iron recovery ratio and cooling conditions. The results show that the slags under the four cooling conditions mainly contained dicalcium silicate, RO phase, Fe_tO, 2CaO(Fe,Al)₂O₃, and calcium ferrite. However, tricalcium silicate, 12CaO·7Al₂O₃, M‐A spinel, and free CaO and MgO were occasionally observed. The amount of glass matrix decreased, the Fe‐containing minerals increased, and the minerals more fully crystallized when the cooling rate of the liquid slag was decreased. From granulation to furnace cooling of the slags, the iron content in the recovered concentrate and the iron recovery ratio both increased. This result is in agreement with the findings on phase transformation through SEM analysis.     

Direct Reduction of Mixtures of Manganese Ore and Iron Ore

This paper presents recent results of direct reduction investigation of different combination of blends of manganese ore, iron ore and coal at the Department of Ferrous Metallurgy (IEHK) of RWTH Aachen University. A mixture of iron and manganese ore in a ratio of 75/25 is a good raw material for steelmaking of high Mn‐alloyed grades. The experimental studies consisting of reduction of (a) fine material and (b) agglomerated material (briquettes) were carried out in the range of 1273 to 1673 K. The behaviour of combined reduction of manganese ore and iron ore and the employment in the direct reduction on a coal and gas basis for production of steels with high Mn content were investigated. It was found that a high metallization degree for Mn can be reached at 1273 K with the reduction of manganese ore by hydrogen‐containing gas. Addition of carbon monoxide to the reducing gas retarded the reduction process. The addition of coal to manganese ore and iron ore blends increased the degree of reduction. The results of carbothermic reduction of briquettes consisting of a mixture of manganese ore and iron ore combined with coal as reducing agent show that a high temperature, a low Mn/Fe ratio and a high Fe₂O₃ content have a favourable effect on the degree of reduction. In order to obtain a high degree of metallization, the temperature should be higher than 1473 K. The reduction of briquettes at higher temperatures (up 1573 K) has shown a molten phase and the separation of slag and metal.     

Isothermal reduction behaviour of MnO2 doped Fe2O3 compacts with H2 at 1073–1373 K

Abstract

Pure Fe₂O₃ and Fe₂O₃ doped with 2, 4, and 6 mass% of MnO₂ (>99%) compacts annealed at 1473 K for 6 h were isothermally reduced with H₂ at 1073–1373 K. The O₂ weight loss resulted from the reduction of compacts was continuously recorded as a function of time using thermogravimetric analysis (TGA). High pressure mercury porosimeter, optical and scanning electron microscopes, X-ray phase analysis and vibrating sample magnetometer were used to characterise both the annealed and reduced samples. In MnO₂ containing samples, manganese ferrite (MnFe₂O₄) was identified. The rate of reduction of pure and doped compacts increased with temperature and decreased with the increase in MnO₂ content. Unlike in pure compacts, the reduction of MnO₂ containing samples was not completed and stopped at different extents depending on MnO₂ (mass%). At initial reduction stages, the decrease in the rate was due to the presence of poorly reducible manganese ferrite (MnFe₂O₄) phase which was partially reduced to iron manganese oxide (FeO_0.899, MnO_0.101) at the final stages. The reduction mechanism was predicted from the correlation between the reduction kinetics and the structure of partially reduced samples at different temperatures. The reduction of pure and doped samples was controlled by a combined effect of interfacial chemical reaction and gaseous diffusion mechanism at their initial stages. At final stages, the interfacial chemical reaction was the rate controlling mechanism.     

Possibility of Selective Oxidation of Vanadium from Iron and Phosphorus in Fe‐V‐P Melt

Experiments on a vanadium recovery method from vanadium containing BOF‐slag using both a Tamman furnace (3 kg scale) and an induction furnace (150 kg scale) were conducted. The vanadium was extracted into the slag phase by bubbling oxidation gas into a metal bath consisting mainly of V (1–10 mass%), Si (less than 1 mass%) and P (about 1 mass%). The first experiments revealed that the slag formed during oxidation reaction had considerably high phosphate capacity. High phosphorus content would rule out the possibility of using the slag as a raw material for the production of ferrovanadium of high quality. In order to reduce the P‐content in the slag, addition of slag former to reduce phosphate capacity was necessary. A suitable slag system (having the initial composition 40 mass% Al₂O₃ ‐ 25 mass% CaO ‐ 35 mass% SiO₂) and a suitable atmosphere, by using CO₂, that enhanced the oxidation of vanadium, but limit the oxidation of iron and phosphorus was found. However, more efforts should be put forward, e.g. study of the phase diagram, the viscosity of the slag and even oxide activities to gain more insight into the slag formed by selective oxidation.     

Influence of Mechanical Activation on Acid Leaching Dephosphorization of High-phosphorus Iron Ore Concentrates

High pressure roll grinding (HPRG)and ball milling were compared to investigate the influence of me-chanical activation on the acid leaching dephosphorization of a high-phosphorus iron ore concentrate,which was man-ufactured through magnetizing roasting-magnetic separation of high-phosphorus oolitic iron ores.The results indica-ted that when high-phosphorus iron ore concentrates containing 54·92 mass% iron and 0·76 mass% phosphorus were directly processed through acid leaching,iron ore concentrates containing 55·74 mass% iron and 0·33 mass%phosphorus with an iron recovery of 84·64% and dephosphorization of 63·79% were obtained.When high-phosphor-us iron ore concentrates activated by ball milling were processed by acid leaching,iron ore concentrates containing 56·03 mass% iron and 0·21 mass% phosphorus with an iron recovery of 85·65% and dephosphorization of 77·49%were obtained.Meanwhile,when high-phosphorus iron ore concentrates activated by HPRG were processed by acid leaching,iron ore concentrates containing 58·02 mass% iron and 0·10 mass% phosphorus were obtained,with the iron recovery reaching 88·42% and the dephosphorization rate reaching 88·99%.Mechanistic studies demonstrated that ball milling can reduce the particle size,demonstrating a prominent reunion phenomenon.In contrast,HPRG pretreatment contributes to the formation of more cracks within the particles and selective dissociation of iron and P bearing minerals,which can provide the favorable kinetic conditions to accelerate the solid-liquid reaction rate.As such,the crystal structure is destroyed and the surface energy of mineral particles is strengthened by mechanical ac-tivation,further strengthening the dephosphorization.     

Evaluation of Desulphurization Flux in CaO‐Al2O3‐BaO‐CeO2‐MgO System

The flux of the CaO‐Al₂0₃‐BaO‐CeO₂‐MgO system as a desulphurization flux containing no fluorine for the secondary metallurgy process was evaluated in this study. The flux composition was designed using the eutectic compositions of the binary systems. The melting and desulphurization abilities of the fluxes were evaluated by measuring their liquidus temperatures and the distribution ratios of sulphur between the fluxes and the carbon‐saturated iron or stainless steel. The lowest liquidus temperature of 1325°C was obtained by adding 5.7 mass% MgO to the 80mass%A‐20mass%B flux. (A: 12CaO‐7Al₂O₃, B: BaCeO₃+12mass%Al₂O₃). The distribution ratios of sulphur and sulphide capacities of the fluxes in this study were higher than those of the commercial product of calcium aluminate flux. This means that the CaO‐Al₂O₃‐BaO‐CeO₂‐MgO fluxes developed in this study have higher desulphurization and melting abilities compared with the commercial product of calcium aluminate flux.     

MgO solubility in BOF slag equilibrated with ambient air

The MgO solubility in the CaO‐MgO‐Fe₂O₃‐FeO‐SiO₂‐(MnO)‐(Al₂O₃) slag was measured under the condition of equilibrium with the ambient air at 1873 K as a fundamental study for precise slag coating control in BOF operation. The CaO/SiO₂ mass ratios of the main slag were 1, 1.5, 2, 3 and 4, and total iron content was in the range of 10 to 35 %. Moreover, 1 to 13 % of MnO and 2 to 12 % of Al₂O₃ were added to the melt to evaluate their effects on the MgO solubility. The effect of slag composition on the MgO solubility was discussed and quantified by means of a newly developed formula. As the basicity in slag increases, the MgO solubility decreases. The effect of iron oxide content is observed to be dependent on the basicity of slag. An increase in iron oxide content makes the MgO solubility higher for basic slag but lower for acidic slag. It is revealed that the MgO solubility in steelmaking slag is controlled by the complex anion formation reaction of iron oxide. Both Al₂O₃ and P₂O₅ increase the MgO solubility by diluting the basic oxides as SiO₂ does, while manganese oxide affects the MgO solubility in a similar manner as iron oxide. The MgO solubility can be described as a function of slag composition, X = (%CaO) + 0.45(%Fe₂O₃+ %FeO) + 0.55(%MnO), in the equation of (%MgO) = 0.00816X^2‐1.404X + 62.31. Based on the results, the guidance for addition of MgO‐containing material could be suggested for best slag coating practice.     

Simulation of primary-slag melting behavior in the cohesive zone of a blast furnace, considering the effect of Al2O3, Fe t O, and basicity in the sinter ore

The alumina content in the iron ore imported to Japan is increasing year by year, and some problems in blast furnace operation, due to the use of the high-alumina-containing sinter, have already been reported. In order to clarify the mechanism of the harmful effect of alumina on the blast furnace operation, the behavior of the primary melt, which is formed in the sinter at the cohesive zone of the blast furnace, has been simulated by dripping slag through an iron or oxide funnel. The effects of basicity, Al₂O₃, and Fe _t O contents in the five slag systems on the dripping temperature and weight of slag remaining on the funnel have been discussed. It was found that the eutectic melt formed in the sinter would play an important role in the dripping behavior of the slag in the blast furnace through the fine poreosity of the reduced iron and ore particles. Al₂O₃ increased the weight of the slag remaining on the funnel, and its effect became very significant in the acidic and low-Fe _t O-containing slag. It was estimated that the increase of the weight of the slag remaining on the funnel by Al₂O₃ in the ore could result in a harmful effect on the permeability resistance and an indirect reduction rate of the sinter in the blast furnace.     

Phosphorus distribution between carbon-saturated iron at 1350 °C and lime-based slags containing Na2O and CaF2

The Corex process is capable of consistently producing hot metal with very low silicon contents (<0.1 pct), and as a consequence, its hot metal is ideally suited for the external removal of phosphorus. Various studies have shown that small additions of Na₂O significantly enhance the ability of lime-based slags to dephosphorize liquid iron. Additions of fluxes (such as CaF₂) may be required to ensure that the slags remain fluid during treatment. The aim of the present investigation was to study the dephosphorization capabilities of lime-based slags from the CaF₂-CaO-Na₂O-SiO₂ system. Phosphorus containing slag and carbon-saturated iron was equilibrated in carbon crucibles at 1350 °C under a carbon monoxide atmosphere. It was confirmed that additions of Na₂O increase the phosphate capacity of silicate and lime-based slags considerably. Additions of CaF₂ to Na₂O containing lime-based slags increase the activity coefficient of P₂O₅ and, therefore, decrease the phosphate capacity thereof. These slags have high phosphate capacities and low melting points, yielding them suitable as effective reagents for dephosphorization, and even desulphurization, of hot metal at relatively low temperatures. However, CaF₂ additions to these slags should be limited.     

LF炉精炼渣用于铁水预处理脱磷剂的可行性分析

基于对铁水脱磷热力学条件及常用铁水脱磷剂组成的分析,对比LF炉精炼渣和脱磷剂的成分和理化性质,认为LF炉精炼渣经过部分处理和合理配料后,可以作为铁水预处理的脱磷剂使用;LF炉精炼渣可替代脱磷剂中的固定剂和部分助溶剂,应用于铁水预处理能够满足生产要求,减少造渣材料消耗和炉渣外排,达到清洁生产的目的。     

Effect of Metal Additions to the Reduction of Iron Oxide Composite Pellets with Hydrogen at Moderate Temperatures

The hydrogen reduction behavior of iron oxide composite pellets containing Ni, Fe, and Mn from 973 K to 1173 K was compared with iron oxide and Al₂O₃ containing reference composite pellets to determine the effect of metallic species on the kinetics of iron oxide reduction. The Mn and Ni containing pellets showed slightly faster initial reduction rates compared to the Fe and Al₂O₃ containing pellets. The effect of the metal phases was found to be more significant at lower temperatures when chemical reaction at the interface is a slower and more controlling factor. From the SEM of partially reduced pellets, a wide intermediate region between an O rich unreacted core and an Fe rich outer shell was observed. Although an initially short topochemical receding interface controlled region exists, the mixed control between the topochemical receding interface and pore diffusion was prevalent. For Fe₂O₃/Mn composite pellets, the thermodynamic stability of the MnO is higher and Mn can act as a reductant for iron oxide. Thus, the overall metallization of the Fe₂O₃/Mn composite pellets decreased compared to the other Fe₂O₃/metal composite pellets. From the temperature dependence of the iron‐oxide/metal composite pellets, the apparent activation energy was calculated to be approximately between 15 to 20 kJ/mol, which is typical of a mixed control reduction mechanism of gas diffusion and interface reaction.     

Reduction of MnO from molten slags with liquid steel of high carbon content

This work estimated the reduction of MnO in slags of the CaO‐SiO₂‐FeO‐CaF₂‐MnO system and liquid steel with the initial composition (mass contents) 0.75 %Mn, 0.16 % Si and 0.5 to 2.0 % C, as an alternative to introducing Mn to the steel melt. The slag basicities (CaO/SiO₂) In the experiments were 2 and 3. MnO was obtained from manganese ore. The experiments were carried out in an open 10 kg induction furnace using Al₂O₃‐based refractory at 1873 K. The oxygen potential was measured throughout the experiments with a galvanic cell (ZrO₂‐solid electrolyte with a Cr/Cr₂O₃ reference electrode). The MnO reaction mechanism was analysed in terms of the slag basicity, the silicon and the initial carbon contents in the melt. The rate and the degree of MnO reduction were found to increase with the increasing of initial carbon content; however, the effect of slag basicity was less important. A kinetic analysis of the process was performed using a coupled reaction model.     

X-ray observation of phosphorus vaporization from steelmaking slag and suppression method of phosphorus reversion in liquid iron

The behavior of phosphorus in CaO-SiO₂-Al₂O₃-MgO-P₂O₅-Fe _t O slag systems during solid Al and carbon deoxidation was investigated at 1873 K. Furthermore, an X-ray apparatus (vertical resistancetube furnace equipped with an X-ray inspector system, which enhances the real-time observation of the generated gas in slag) was introduced to confirm the possibility of the vaporizing phenomena of phosphorus from slag. The X-ray-assisted observations proved that phosphorus gas is produced from P₂O₅-containing slag during the deoxidation process and can be applied to suppress the reversion of phosphorus into the liquid iron by removing it into the air. The results of experiments between slag and metal showed that Al decreased the Fe _t O and P₂O₅ contents in slag simultaneously because of the strong reducing power, but carbon decreased only the Fe _t O content in slag to a certain extent without reducing the P₂O₅. For the prevention of the phosphorus-content increment in liquid iron during the deoxidation process, it was ascertained that the Fe _t O content, the absorption site of phosphorus gas, should be decreased to some extent at the time of phosphorus generation. It could be proposed that it is the two-step deoxidation process that decreases the Fe _t O content by carbon while maintaining the P₂O₅ content in the slag at a nearly constant level and then decreases the remaining P₂O₅ content quickly by vaporizing as a gas phase by Al, without considerable reversion of the phosphorus in hot metal.     

Thermodynamics of manganese distribution between liquid iron and the CaO–Al2O3–SiO2–FeO–MnO system

The thermodynamics of distribution of constituents between liquid iron and the CaO–Al₂O₃–SiO₂–FeO–MnO system at 1600°C was studied using electrochemical indication of the equilibrium partial pressure of oxygen in both phases. The results show that oxidation potential of the Fe(l)–CaO–Al₂O₃–SiO₂–FeO–MnO system, expressed in terms of log p(O₂), is directly proportional to log (x(MnO) · x(FeO)/w| Mn |). Manganese distribution coefficient, L'_mn, in intersection CaO/Al₂O₃ = 1 decreases with increasing slag basicity expressed in terms of activity a(CaO) or 1/γ(MnO). Experimentally determined equilibrium constant K_Mn/Fe is equal to 2.7 for 1600°C. The number of exchanged electrons between Fe-O-Mn-Si electrode and the slag approaches the theoretical value.