Activity of Ferric Oxide in Steelmaking Slag

Refining reactions in steelmaking primarily involve oxidation of impurity element(s). The oxidation potential of the slag and the activity of oxygen in the metal (h_O) are the major factors controlling these chemical reactions. In turn, the oxidation potential of the slag is influenced strongly by the equilibrium distribution of oxygen between ferrous and ferric oxides. We recently investigated the activity coefficient of FeO in steelmaking slag and the effect of chemical composition thereon. This work is focused on estimation of the activity coefficient of Fe₂O₃.     

Manganese Distribution between FeO‐MnO‐SiO2–CaO–MgO‐Al2O3 Slags and Molten Iron

Pretreatment of high manganese hot metal is suggested to produce hot metal suitable for further processing to steel in conventional LD converter and rich manganese slags satisfy the requirements for the production of silicomanganese alloys. Manganese distribution between slag and iron represents the efficiency of manganese oxidation from hot metal. The present study has been done to investigate the effect of temperature, slag basicity and composition of oxidizer mixture on the distribution coefficient of manganese between slag and iron. Ferrous oxide activity was determined in molten synthetic slag mixtures of FeO‐MnO‐SiO₂–CaO–MgO‐Al₂O₃. The investigated slags had chemical compositions similar to either oxidizer mixture or slags expected to result from the treatment of high manganese hot metal. The technique used to measure the ferrous oxide activity in the investigated slag systems was the well established one of gas‐slag‐metal equilibration in which molten slags contained in armco iron crucibles are exposed to a flowing gas mixture with a known oxygen potential until equilibrium has been attained. After equilibration, the final chemical analysis of the slags gave compositions having a particular ferrous oxide activity corresponding to the oxygen potential of the gas mixture. The determined values of ferrous oxide activity were used to calculate the equilibrium distribution of manganese between slag and iron. Higher manganese distribution between slag and iron was found to be obtained by using oxidizer containing high active iron oxide under acidic slag and relatively low temperature of about 1350°C.     

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.     

Some metallurgical aspects of steelmaking with sponge iron in electric arc furnaces

Test charges containing sponge iron proportions varying from 38.7–95.4 wt% of the metallic input were melted in a UHP electric arc furnace with a design capacity of 70 tons per heat. After melting started, samples of molten metal and the corresponding slag were taken simultaneously at different time intervals and analysed. The time dependence of the chemical composition of the metal and slag as well as the variation of the temperature of the melt with time are given. Thermodynamic calculations using different methods for finding the activity of ferrous oxide show that there is a linear relationship between a_(FeO) and the total ferrous oxide content in the slag. It is also found that the total ferrous oxide content in the slag and the oxygen concentration in the metal vary linearly with the iron proportion from sponge iron in the metallic input. The activity of ferrous oxide decreases with increasing slag basicity due to the formation of calcium ferrites. The relationship between the oxygen concentration and the reciprocal of the carbon content in liquid steel is almost linear. A formula showing the influence of some important factors on the oxygen content in molten steel is given. The effect of sponge iron on the sulphur concentration in the steel is also investigated. The present results indicate that the sulphur content in the steel can be reduced from 0.02 to 0.004 wt% by increasing the iron proportion from sponge iron in the metallic input from 35 to 95 wt %. Using the boundary layer diffusion model, it is found that the rate of decarburization of a steel bath is enhanced by increasing the sponge iron proportion in the metallic input. The activation energy of the decarburization reaction is found to be 56.9 kJ/mol and the mass transfer coefficient of carbon has a value of 0.0161 cm/s at 1 600°C. In a way similar to that used for decarburization, it is also found that the rate of oxidation of manganese dissolved in the bath is enhanced by increasing the sponge iron proportion in the metallic input.     

Lead solubility in FeO x -CaO-SiO2 slags at iron saturation

Experiments have been carried out to determine the equilibria between FeO _x -CaO-SiO₂ slag and lead metal in iron crucibles at temperatures ranging from 1473 to 1573 K. It has been found that the highest lead solubilities are observed in the silica-saturated iron silicate slags, while the lowest solubilities are observed in the CaO-saturated calcium ferrite slags. The activity coefficient of PbO varies from 0.15 to 3, depending on the slag composition. Changes in temperature do not have a significant impact on the activity coefficient. The activity of FeO and pct Fe³⁺/pct Fe²⁺ ratios have been determined as a function of slag composition. These new experimental data have been incorporated into an optimized thermodynamic slag model using the computer package FACT.     

Thermodynamics of MnO, FeO, and phosphorus in steelmaking slags with high MnO contents

The thermodynamics of managanese oxide and iron oxide and the phosphate capacity of CaO-SiO₂-MnO-Fe _t O-P₂O₅-MgO_sat slags with high MnO contents relevant to the smelting of MnO ores in steelmaking were investigated. Previous data were limited to about 5 pct MnO, whereas in this study MnO contents up to 25 pct were studied. The activity of MnO showed positive deviation from ideal behavior and increased with basicity, while that of Fe _t O decreased with basicity. This is reflected in that the manganese distribution ratio at a given Fe _t O content decreases with basicity and high Mn in the metal is favored by high basicity. CaF₂ additions up to 4 pct did not affect the activities of MnO and Fe _t O. The activity coefficient of P₂O₅ decreases and the phosphate capacity increases with basicity. There was not adverse effect of high MnO contents on the dephosphorizing abilities of the slags.     

The effect of CaF<Subscript>2</Subscript> on thermodynamics of CaO-CaF<Subscript>2</Subscript>-SiO<Subscript>2</Subscript>(-MgO) slags

To address the role of CaF₂ in the CaO-CaF₂-SiO₂(-MgO) slag system employed for the production of low-pressure rotor steels, the thermodynamic aspects of the slag were investigated by equilibrating it with liquid iron at 1873 K in CaO or MgO crucibles. Presaturation of slag with an oxide block piece of CaO or MgO in a Pt crucible and application of a carbon paste to the outside of an oxide crucible were designed to prevent crucible failure during the slag-metal experiments. The liquidus isotherm and phase boundary of the preceding slag system were investigated using the slag-metal equilibria. Also, the effect of CaF₂ on the sulfide capacity and the activity coefficient of Fe _t O were of particular interest in controlling the sulfur level and cleanliness of low-pressure rotor steels.     

A Thermodynamic Model of Phosphorus Distribution Ratio between CaO-SiO2-MgO-FeO-Fe2O3-MnO-Al2O3-P2O5 Slags and Molten Steel during a Top–Bottom Combined Blown Converter Steelmaking Process Based on the Ion and Molecule Coexistence Theory

A thermodynamic model for calculating the phosphorus distribution ratio between top–bottom combined blown converter steelmaking slags and molten steel has been developed by coupling with a developed thermodynamic model for calculating mass action concentrations of structural units in the slags, i.e., CaO-SiO₂-MgO-FeO-Fe₂O₃-MnO-Al₂O₃-P₂O₅ slags, based on the ion and molecule coexistence theory (IMCT). Not only the total phosphorus distribution ratio but also the respective phosphorus distribution ratio among four basic oxides as components, i.e., CaO, MgO, FeO, and MnO, in the slags and molten steel can be predicted theoretically by the developed IMCT phosphorus distribution ratio prediction model after knowing the oxygen activity of molten steel at the slag–metal interface or the Fe _t O activity in the slags and the related mass action concentrations of structural units or ion couples in the slags. The calculated mass action concentrations of structural units or ion couples in the slags equilibrated or reacted with molten steel show that the calculated equilibrium mole numbers or mass action concentrations of structural units or ion couples, rather than the mass percentage of components, can present the reaction ability of the components in the slags. The predicted total phosphorus distribution ratio by the developed IMCT model shows a reliable agreement with the measured phosphorus distribution ratio by using the calculated mass action concentrations of iron oxides as presentation of slag oxidation ability. Meanwhile, the developed thermodynamic model for calculating the phosphorus distribution ratio can determine quantitatively the respective dephosphorization contribution ratio of Fe _t O, CaO + Fe _t O, MgO + Fe _t O, and MnO + Fe _t O in the slags. A significant difference of dephosphorization ability among Fe _t O, CaO + Fe _t O, MgO + Fe _t O, and MnO + Fe _t O has been found as approximately 0.0 pct, 99.996 pct, 0.0 pct, and 0.0 pct during a combined blown converter steelmaking process, respectively. There is a great gradient of oxygen activity of molten steel at the slag–metal interface and in a metal bath when carbon content in a metal bath is larger than 0.036 pct. The phosphorus in molten steel beneath the slag–metal interface can be extracted effectively by the comprehensive effect of CaO and Fe _t O in slags to form 3CaO·P₂O₅ and 4CaO·P₂O₅ until the carbon content is less than 0.036 pct during a top–bottom combined blown steelmaking process.     

Reduction of synthetic stainless steel slags by aluminium

One of the most efficient ways to eliminate the harm of chromium oxide in stainless steel slag is to reduce chromium oxide in stainless steel slag using aluminium. In the present work, the Al reduction of synthetic CaO–SiO₂–Al₂O₃–MgO–Fe₂O₃–Cr₂O₃ stainless steelmaking slags at different conditions, including temperature, slag basicity and Al amount was investigated to get optimal conditions for the reduction and the metal–slag separation. It was found that the agglomeration of metal droplets and metal–slag separations were improved by increasing temperature. The reduction degrees of SiO₂, Fe₂O₃ and Cr₂O₃ were enhanced with increasing basicity of slag. The addition of CaF₂ in slag leads to better agglomerations of metal droplets and metal–slag separations. The highest reduction degree of chromium could reach 99% in slag with basicity of 2 at 1873 K.     

Theoretical concept on slag–oxygen sensors to measure oxide activities related to FeO,SiO2 , and CaO contents of steelmaking slags

Abstract

A theoretical concept is presented on the slag–oxygen sensors for in situ measurements of the FeO, SiO₂ , and CaO contents of steelmaking slags in the furnace and in the ladle. The purpose of this disclosure is to stimulate R & D interest in academia and technical institutes to further the development of new measuring devices applicable in steelmaking operations. The slag–oxygen sensor conceived* consists of two dissimilar electrodes such that when immersed in molten slag there will be a difference in oxygen potentials at the slag/ electrode interface between the two electrodes, registering an open circuit cell emf. Examples are given of different types of electrodes for different oxides in the slag; also, equations are derived for the theoretical relation between the oxide activity and sensor emf reading.     

Parameters affecting manganese oxidation in top blown basic oxygen converter

The data obtained from 84 heats carried out in a 90-t top blown basic oxygen converter were used to study the effects of slag composition and temperature on the activity coefficient and activity of manganous oxide in the slag as well as on the manganate capacity and the manganese distribution between slag and metal. In addition, the dependence of manganese activity in the metal on the concentration of maganese and temperature was also investigated. The present study carried out in wide ranges of temperature, 1350–1690°C, and slag basicity expressed as (CaO)/(SiO₂), 1.4–10.6, clarifies the dependence of MnO activity coefficient mainly on temperature. The activity coefficient of MnO increases by decreasing the temperature. On the other hand, activity of MnO increases by increasing MnO concentration and temperature. Both activity coefficient and activity of MnO in the slag slightly increase by increasing the slag basicity. At constant temperature, the activity of Mn in the molten metal varies linearly with Mn concentration and tends also to increase with increasing temperature at constant Mn concentration. The increase in manganese activity by increasing Mn concentration is much steeper at high temperatures. The manganate capacity as well as manganese distribution ratio decrease with increasing temperature at constant basicity and tend also to slightly decrease with increasing slag basicity at constant temperature. Equations describing the parameters affecting activity coefficient and activity of manganous oxide in the slag, manganese activity in the metal, manganate capacity and manganese distribution ratio have been derived.     

Activity of VO1.5 in CaO‐SiO2‐MgO‐Al2O3 Slags at Low Vanadium Contents and Low Oxygen Pressures

In the present work, the gas‐slag equilibration technique was employed for the measurement of the thermodynamic activity of vanadium oxide. The vanadium‐containing slag kept in a platinum crucible was equilibrated with a gas mixture of CO, CO₂ and Ar, with well‐defined oxygen partial pressure at a pre‐determined temperature. The slag sample was quenched and the composition of the final slag was determined by chemical analysis. From the value of the oxygen partial pressure, the thermodynamic activity of VO_1.5 could be calculated using the value for the activity of vanadium in V‐Pt alloy. The measurements were carried out in the temperature range 1823～1923K and the oxygen partial pressures employed were 10^‐3, 10^‐4, 10^‐5 Pa. The present results show that the activity of vanadium(III) oxide in slag exhibits a negative deviation from ideality in the present composition range. With increasing basicity of the slag, the final content of vanadium oxide in the slag was found to show an initial increase followed by a constant content. The activities of vanadium(III) oxide did not exhibit any significant change with increasing temperature. The activity coefficient of vanadium(III) oxide decreased sharply with slag basicity approximately up to a basicity of 1, beyond which it showed a near–constant value. Increase in basicity was found to cause a change in the distribution of vanadium between the slag and the alloy phases even though this effect was less pronounced. From the present results, a mathematical relationship for estimating the vanadium content in slag for a given activity of vanadium in the molten metal phase was developed.     

Distribution of nickel between copper-nickel and alumina saturated iron silicate slags

The solubility of nickel in slag was determined by equilibrating copper-nickel alloys with alumina-saturated iron silicate slags in an alumina crucible at 1573 K. The experiments were carried out under controlled oxygen partial pressures in the range of 10^-10 to 10^-8 atm by use of suitable CO-CO₂ gas mixtures, and at Fe/SiO₂ ratio 1.34. The results showed that nickel dissolves in slag both as Ni²⁺ (nickel oxide) and Ni‡ (nickel metal), and the relation obtained was: (Wt pct Ni in slag) = (ie33-01) The activity coefficient of nickel oxide (γ^dgNio) and distribution coefficient of nickel (A_Ni) is calculated to be 0.375 and 233.3, respectively. γ^dgNio and A_Ni are found to be independent of oxygen partial pressures. The presence of alumina increases the solubility of nickel in slags.     

The Reaction Behavior of Direct Reduced Iron (DRI) in Steelmaking Slags: Effect of DRI Carbon and Preheating Temperature

An experimental study was conducted to quantify the rate of direct reduced iron (DRI) decarburization in a steelmaking slag using the constant volume pressure increase technique. Experiments were conducted by dropping DRI pellets into molten slag at temperatures from 1773 K to 1873 K (1500 °C to 1600 °C). Subsequent experiments were carried out in which the DRI pellets were preheated while the slag temperature remained constant. The effect of the initial carbon content and the preheating temperature of the DRI on the reaction rate was investigated. The decarburization of DRI seems to comprise two stages, a reaction between the FeO and DRI followed by decarburization through the iron oxide of slag. Carbon has a significant effect on the kinetics of both stages, whereas the preheating temperature mainly influences the rate of decarburization between FeO and carbon inside the pellet.     

Role of Heat Transfer in Early Stage Decarburization of DRI in Slag

The gas generation from reactions between direct reduced iron (DRI) pellets and steelmaking slags is known to take place in two stages; (1) the reaction of FeO and carbon within DRI, i.e., pellet internal reaction, followed by (2) the reduction of slag FeO with DRI carbon at the pellet?Cslag interface, if any carbon remains from the first step. To understand the controlling mechanism of the reaction between FeO and C inside DRI, the rate of the gas release and the temperature of pellets suspended in a slag-free atmosphere were quantified. The results were used to determine the apparent thermal conductivity of DRI that showed values of approximately 0.5 to 2 W.m^?1.K^?1 for a temperature range of 573?K to 1273?K (300?°C to 1000?°C). Furthermore, it was found that the experimental gas evolution rates are consistent with the values predicted by a heat?Ctransfer based model, confirming that the FeO-C reaction within pellet is controlled by the rate of heat transfer from the slag to the DRI pellet.     

Influence of Partial Pressure of Sulfur and Oxygen on Distribution of Fe and Mn between Liquid Fe-Mn Oxysulfide and Molten Slag

The authors proposed an innovative process for recovering Mn from steelmaking slag. The process starts with the sulfurization of steelmaking slag to separate P from Mn by the formation of a liquid sulfide phase (matte). Then, the obtained matte is weakly oxidized to make a Mn-rich oxide phase without P. High-purity Fe-Mn alloys can therefore be produced by the reduction of the Mn-rich oxide phase. However, to the authors?? knowledge, the sulfurization of molten slag containing P and Mn has not been sufficiently investigated. It was recently found that P was not distributed to the matte in equilibrium with the molten slag. To gain knowledge of the process??s development, it is important to investigate the influence of the partial pressures of sulfur and oxygen on the equilibrium distribution of Mn and Fe between the matte and the molten slag. In the current work, a mineralogical microstructure analysis of the matte revealed that the existence of the oxysulfide and metal phases was dependent on the partial pressure of sulfur and oxygen. The Mn content of the matte increased with partial pressure of sulfur while the O content of the matte decreased. In contrast, the ratio of Mn/Fe in the matte was constant when the metal phase of the matte was observed at a log $ P_{{{\text{O}}_{2} }} $ below ?11. These results also corresponded to the relationship between the activity coefficient ratio of MnS/FeS and the mole fraction of MnS/FeS in the matte. The ?? _MnS/?? _FeS value decreased exponentially as the mole fraction of MnS/FeS increased.     

Kinetic model for reduction of iron oxide in molten slags by iron-carbon melt

Rate of reduction of iron oxide in iron and steelmaking slags by mass contents of dissolved carbon (>3%) in molten iron depends upon activity of FeO, temperature, mixing of bulk slag and other experimental conditions. A general kinetic model is developed by considering mass transfer of FeO in slag, chemical reaction at gas-metal interface and chemical reaction at gas-slag interface, respectively, as the three rate controlling steps. A critical analysis of the experimental data reported in literature has been done. It is shown that in the case of slags containing mass contents of less than 5% FeO, the reduction of FeO is controlled by mass transfer of FeO in slag plus chemical reaction at gas-metal interface; when slags contain more than 40% FeO, the reduction of FeO is controlled by chemical reaction at gas-metal interface plus chemical reaction at gas-slag interface; at intermediate FeO mass contents (between ～ 5 and 40% FeO), the reduction of FeO is controlled by all three steps, namely, mass transfer of FeO in slag, chemical reaction at gas-metal interface and chemical reaction at gas-slag interface. The temperature dependence of rate constant for the gas-slag reaction is obtained as: In k₂ = –32345.4(&6128)/ T + 19.0(&3.42); σ_lnk2,1/T = &0.3. where k₂ is expressed in mol m^-2 s^-1 bar^-1. The mass transfer coefficient of iron oxide in bulk slag is found to vary in the range 1.5 × 10^-5 to 5.0 × 10^-5 m/s, depending upon the slag composition as well as experimental conditions.     

Kinetics of chromite ore reduction from MgO-CaO-SiO2-FeO-Cr2O3-Al2O3 slag system by carbon dissolved in high carbon ferrochromium alloy bath

Abstract

Kinetic experiments were performed in an induction furnace to investigate the reduction of chromite ore by carbon dissolved in a high carbon ferrochromium alloy melt under conditions of varying Cr₂O₃ concentration, slag basicity, and temperature. The results obtained show that chromite reduction by dissolved carbon in slag systems of the type MgO-CaO-SiO₂-FeO-Cr₂O₃- Al₂O₃ occurs principally by a stagewise process encompassing an intermediate reaction in which the divalent chromium oxide species is involved. During the fast period, Cr₂O₃ reduction is controlled by the diffusion of oxygen species in the slag for which a mass transfer coefficient of 0·003 cm s^-1 was calculated. An activation energy value of 117 kJ mol ^-1 obtained for the reduction of Cr₂O₃ implies the rate controlling step is mass transfer of Cr₂O₃ from the slag to the slag/metal interface, since activation energies for metal phase control are typically <70 kJ mol ^-1. The second period represents a pseudo-equilibrium condition with respect to Cr₂O₃ reduction that is probably under thermodynamic control by a step or mechanism involving the reduction of divalent chromium oxide to chromium.     

The effect of carbon in slag on steel reoxidation and carbon analysis by x-ray photoelectron spectroscopy in the CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO-MnO-Fe<Subscript><Emphasis Type="Italic">t</Emphasis></Subscript>O system

An equilibrium experiment was carried out at 1873 K to investigate the effect of carbon in CaO-SiO₂-Al₂O₃-MgO-MnO-Fe _t O slag systems on their Fe _t O and MnO activity coefficients, representing the slag’s thermodynamic potential for steel reoxidation. The activity coefficients of Fe _t O and MnO showed not only a sharp increment but also a simultaneous slow decrement with increasing carbon content in slag, suggesting opposite roles of the carbon in slag according to its stable forms. X-ray photoelectron spectroscopy (XPS) was introduced to determine the stable forms of carbon in slag. The XPS results proved that carbon dissolves in slag as carbonate, and carbide ions under oxidizing and reducing atmospheres, respectively. The simultaneous consideration of the activity coefficients of Fe _t O and MnO and stable carbon forms showed that carbonate ions increase the activity coefficients of Fe _t O and MnO, but that carbide decreases them. This article suggests an application method of the present results to actual ladle refining processes, in order to enhance steel cleanliness with maintaining (Fe _t O + MnO) in slag to some allowable amount.