Reduction of FeO in smelting slags by solid carbon: Experimental results

The reduction of CaO-SiO₂-Al₂O₃-FeO slags containing less than 10 wt pct FeO by solid carbonaceous materials such as graphite, coke, and coal char was investigated at reaction temperatures of 1400 °C to 1450 °C. The carbon monoxide evolution rate from the system was measured using stationary and rotating carbon rods, stationary horizontal carbon surfaces, and pinned stationary spheres as the reductants. The measured reaction rate ranged from 3.25 × 10^?7 mol cm^?2 s^?1 at 2.1 pct FeO under static conditions to 3.6 × 10^?6 mol cm^?2 s^?1 at 9.5 pct FeO for a rotating rod experiment. Visualization of the experiment using X-ray fluoroscopy showed that gas evolution from the reduction reaction caused the slag to foam during the experiment and that a gas film formed between the carbon surface and the slag at all times during experimentation. The reaction rate increased with increased slag FeO contents under all experimental conditions; however, this variation was not linear with FeO content. The reaction rate also increased with the rotation speed of the carbon rod at a given FeO content. A small increase in the reaction rate, at a given FeO content, was found when horizontal coke surfaces and coke spheres were used as the reductant as compared to graphite and coal char. The results of these experiments do not fit the traditional mass transfer correlations due to the evolution of gas during the experiment. The experimental results are consistent, however, with the hypothesis that liquid phase mass transfer of iron oxide is a major factor in the rate of reduction of iron oxide from slags by carbonaceous materials. In a second article, the individual rates of the possible limiting steps will be compared and a mixed control model will be used to explain the measured reaction rates.     

Reduction of FeO in EAF Steelmaking Slag by Blends of Metallurgical Coke and End‐of‐Life Tyre

The reduction of FeO‐containing slag by blends of metallurgical coke and end‐of‐life tyres (RT) have been investigated through experiments conducted in a laboratory‐scale horizontal tube furnace. Composite pellets of EAF slag (47.1% FeO) with coke, RT, and blends of coke/RT (in four different proportions) were rapidly heated at 1550°C under high purity argon gas and the off gas was continuously analyzed for CO and CO₂using an online infrared (IR) gas analyzer. The extent of reduction after 10 min, level of carburization and desulfurization, and the total amount of CO₂ emissions were determined for each carbonaceous reductant. The results indicate that the extent of reduction, level of carburization and desulfurization of the reduced metal are significantly improved when coke is blended with RT. Blending of coke with RT resulted in a decrease in direct CO₂ emissions from the reduction reactions.     

Dynamic dissolution of CO2/H2O(g)-gasified coke by slag containing FeO

In top gas recycling-oxygen blast furnace (TGR-BF) and injection coke oven gas-blast furnace (COG-BF), lower coke ratio highlight the importance of coke in the melting and dropping zone. To understand the deterioration of coke in lower blast furnace, the dynamic dissolution of CO₂/H₂O-gasified coke by slag containing FeO and reaction kinetics were analysed. The results showed that the coke deterioration was more serious in CO₂ than in H₂O for the same gasification ratio. To understand the internal variation of coke, pore distribution and microstructure of coke were observed using mercury porosimeter and scanning electron microscope (SEM). For the same gasification ratio, pore structure was more uniform in H₂O than in CO₂. In addition, the kinetics of coke's reaction with FeO in slag was analysed.     

Reaction Kinetics of FeO Containing Molten Slags

The kinetic study of FeO‐containing slag is valuable if we consider slag‐gas and slag‐metal reactions in steelmaking process. In the present work, the reduction rate of Fe_tO‐SiO₂–TiO₂–MO_x (MO_x = CaO, MgO, AlO_1.5, PO_2.5) melts in equilibrium with solid iron by CO gas was measured using the thermobalance system at 1673 K. A rate equation was developed based on the results obtained. The mechanisms of the reaction and the effect of P₂0₅ as a surfactant were discussed. Solid CaO was reacted with FeO‐containing slag at 1573 to 1673 K. The CaO–slag interface was analyzed by SEM and EDX, and the reacted layers were identified. The rate of solid CaO dissolution into a stagnant FeO‐containing slag at hot‐metal temperatures was explained by the FeO diffusion in slag phase.     

Slag–graphite wettability and reaction kinetics Part 1 Kinetics and mechanism of molten FeO reduction reaction

Abstract

The present paper reports results relating to the kinetics and mechanism of FeO reduction by graphite, the data being obtained from experimental investigations into the wettability of graphite by molten slag containing FeO. The rate of FeO reduction was determined by measuring the volume of CO gas formed as a result of the reduction of FeO in experiments conducted in the same sessile drop apparatus. The reduction reaction initiated by direct slag–graphite contact produces CO gas which spreads into the molten slag droplet causing foaming of the slag; further reduction of FeO proceeds mostly via indirect reduction. The rate of reduction was found to depend directly on the initial FeO content. An increase in temperature improves the rate of reaction, which has an activation energy of 112·18 kJ mol^-1. These results indicate that transport of FeO (Fe²⁺, O^2- ) in the liquid slag phase is probably the slowest step.     

Smelting reduction reactions by solid carbon using induction furnace: foaming behaviour and kinetics of FeO reduction in CaO–SiO2–FeO slag

Abstract

Smelting reduction process technology is progressing rapidly, and research to understand the reduction of FeO in molten slag and the associated foaming behaviour has gained importance. The present paper reports experimental data on the reduction of FeO in molten slag generated in a 30 kW capacity induction furnace. The influence of FeO content in the slag and temperature on the foaming and kinetics is discussed. The foaming index, a parameter describing the travel time of gas in the reactor, is shown to decrease with an increase in the superficial gas velocity. The quantitative dependence of the foaming index on slag properties viscosity, surface tension and density has been studied. The data have also been analysed to give an estimation of the activation energy for the reduction reaction. The reduction reaction, initiated by direct slag–graphite contact, produces CO gas, which spreads into the molten slag bath causing foaming of the slag; further reduction of FeO proceeds mostly via indirect reduction. The rate of reduction is found to depend directly on the initial FeO content. An increase in temperature increases the rate of reduction, which has an activation energy of 118 kJ mol^?1 of FeO. The results indicate that transport of FeO in the liquid phase is the rate controlling step. The major findings are in agreement with those reported by earlier investigators.     

Thermodynamic Activities of “FeO” in some Binary “FeO”‐Containing Slags

In the present investigation, experimental measurements of the thermodynamic activities of iron oxide in the Al₂O₃‐“FeO”, CaO‐“FeO” and “FeO”‐SiO₂ systems were performed in the temperature range 1823‐1873 K by using gas equilibration technique. The molten slag, kept in a Pt‐crucible was brought to equilibrium with a gas mixture of known oxygen partial pressure. A part of the Fe from the “FeO” was reduced during the equilibration and got dissolved in the Pt phase. The samples were quenched after the required equilibration time and the slag phase as well as the platinum crucible was subjected to chemical analysis. The activities of “FeO” in the slag were calculated from the experimental data using thermodynamic information on the Fe‐Pt binary metallic system generated and assessed earlier. The experimental results are compared with earlier thermodynamic studies of the slag systems. Reassessment with the KTH slag model is performed and the results are compared with other thermodynamic models, viz. F*A*C*T? and Thermo‐Calc? respectively. The experimental activities predicted by the KTH slag model are in good agreement with the experimental data available in the literature. A general agreement between the various models is also observed.     

A study of the reduction rate of FeO in slag by solid carbon

The reduction reaction of FeO in slag by carbon plays an important role in bath smelting reduction processes. In this study, the rate of this reaction was measured to understand the kinetic behavior of FeO reduction in slag by using the mass spectrometer technique. The present experimental results implied that the rate-determining step would change from the mass transfer of FeO at a low FeO content (<5 wt pct) to the chemical reaction at the gas/carbon interface at a high FeO content (>30 wt pct), while the total reduction rate would increase with an increasing FeO content in the slag. Based on the results of this study and comparisons with thermodynamical data for FeO in slag, the reduction rate of FeO can be expressed by the following equation:

Reduction Behaviour of BOF Slags by Carbon in Iron

Experiments were carried out in a system with BOF slags from industrial operations in order to optimize the conditions of recycling BOF slags produced in the steelmaking process. Reduction reactions of FeO and P₂O₅ proceeded steadily and the FeO reduction rate was almost identical to that of P₂O₅. The reduction reaction of FeO and P₂O₅ in BOF slag at the slag/gas interface is the rate‐controlling step. The reaction rates of FeO and P₂O₅ by dissolved carbon in molten iron are of first order with respect to their respective concentrations. The reduction reactions of FeO and P₂O₅ by dissolved carbon in iron are much closer to the equilibrium state compared with the reduction by solid carbon. It is necessary to control the portion of phosphorus vaporization during reduction treatment in order to obtain efficient operational conditions for BOF slag reduction.     

Kinetics and Mechanism of the Simultaneous Carbothermic Reduction of FeO and MnO from High-Carbon Ferromanganese Slag

The carbothermic reduction of 38.7 pct MnO-12.1 pct CaO-5.4 pct MgO-9.3 pct Al₂O₃-24.1 pct SiO₂-10.4 pct FeO slag in Ar at 1600 °C was studied using the sessile drop wettability technique. Pure graphite, coke, and charcoal were used as the carbon material substrates. The reduction rates were evaluated by sampling at different reduction times and by analyzing the chemical compositions of the reduced slag and the produced metal. The carbothermic FeO reduction from slag is initially fast followed by a much slower reduction rate. However, the rate of the MnO reduction is slow in the fast FeO reduction stage, and it starts to increase significantly during the slow FeO reduction stage. The kinetics of FeO and MnO reduction are affected by the type of carbonaceous materials. Moreover, the rate of the carbon dissolution/transfer into the produced metal phase and the amount of the transferred manganese to the metal phase depend on the type of carbon. Based on the experimental observations and the thermodynamic calculations, a mechanism for MnO reduction was proposed. According to this mechanism, MnO is mainly reduced through a metallothermic reduction by Fe and the rate of MnO reduction is controlled by the rate of the consumption of FeO from the slag, which takes place simultaneously. In contrast, the rate of FeO reduction in the fast initial reduction stage is controlled by the rate of the carbon dissolution/transfer into the metal phase. However, at the second slow FeO reduction stage, it is reduced mainly by the solid carbon.     

The reaction behavior of Fe-C-S droplets in CaO-SiO2-MgO-FeO slags

The rate of reduction of FeO in the slag by carbon in iron droplets (2.9 wt pct C, 0.01 wt pct S) was studied for CaO-SiO₂-MgO slags containing between 3 and 35 wt pct FeO and temperatures ranging from 1643 to 1763 K. The effects of Fe₂O₃ additions to the slag and sulfur variations in the metal on the reaction rate were also studied. It was found that the behavior of the metal droplets in the slag, as observed by X-ray fluoroscopy, changed significantly with FeO content in the slag. Below 10 wt pct FeO, the droplet remained intact while reacting with the slag; however, above this FeO concentration, the droplet became emulsified within the slag. The large increase in surface area of the metal droplet due to emulsification caused the rate of reaction to be one to two orders of magnitude faster than for droplets that did not become emulsified. It was suggested that when the droplet is emulsified, the surface area and reaction kinetics are greatly increased, and the rate becomes controlled by mass transfer of FeO as Fe²⁺ and O²⁻ ions in the slag to the emulsified droplet. At low FeO contents for which the droplet does not emulsify, the rate is controlled by dissociation of CO₂ on the metal. It was also found that a critical temperature exists for a given FeO content at which point the rate of CO evolution increases dramatically. Additions of Fe₂O₃ to the slag and sulfur to the metal caused significant changes to the rate of reaction possibly by affecting the emulsification behavior of the droplet.     

Measurement of FeO activity and solubility of MgO in smelting slags

In bath smelting, the FeO activity of the slag must be known to predict the equilibrium of slag-metal reactions and for effective control of the rate of reduction in the system. Also, knowledge of the solubility of MgO in these slags is useful for reducing refractory consumption. A series of measurements of the FeO activity in simulated bath smelting slags (CaO-SiO₂-Al₂O₃-MgO_sat-FeO) were conducted by the electromotive force (EMF) technique. The influence of the slag composition on the relationship between the FeO activity coefficient and FeO content was studied. It has been found that the measured FeO activity coefficient decreases with increasing FeO content in the slag and increases slightly with increasing slag basicity, which is defined as (CaO + MgO)/(SiO₂ + Al₂O₃) on a mole fraction basis. The measured values of the FeO activity coefficient are in reasonable agreement with previously published data. The solubility of MgO was also measured and found to rang from 16 to 30 pct and decrease with increasing basicity.     

Influence of slag and foam characteristics on reduction of FeO-containing slags by solid carbon

The present study reports experimental results on the reduction of FeO in molten CaO-SiO₂-Al₂O₃-MgO-FeO slags by solid carbon in an extended-arc plasma reactor. The reduction reaction was found to be controlled by mass transport of FeO in liquid slag. The CO gas generated stirs the bath to establish a convective mass transport system. CO also causes foaming. An analysis using dimensionless numbers provides correlations between the rate constant, k, as well as the foaming index, Σ, with some properties of the slag such as viscosity, surface tension, and density. A correlation between k and Σ is also developed using these parameters for slag characteristics.     

Hot metal pretreatment by powder injection of lime‐based reagents

A mathematical model earlier proposed has been improved to predict the kinetics of multicomponent reactions in the hot metal pretreatment through the injection of reactive fluxes. It is assumed that there are two reaction zones along the flux injection operation: a transitory reaction between the rising particles and the bulk metal, and the permanent reaction between the metal and the top slag. A criterion to estimate the fraction of solids which will react with molten iron in a three‐phase jet (gas‐solid‐liquid) was considered; this fraction of solids carries out the transitory reaction. The model also takes into account the thermodynamic changes produced in the metal and slag due to the chemical reactions. Calculated results of the model are in good agreement with experimental results for the desulfurization of hot metal through the injection of CaO‐SiO₂‐CaF₂‐FeO‐Na₂O reagents at 1400 ‐ 1450 °C. Two kinds of hot metal were tested, one with a low carbon mass content of 3 % and the other with a high carbon mass content of 4.5 %.     

Slag foaming in bath smelting

Slag foaming measurements in terms of the foaming index (∑) were conducted on bath smelting-type slags (CaO-SiO₂-FeO, CaO-SiO₂-MgO-Al₂O₃-FeO) at 1773 K. It was found that the slag foam stability decreases with increasing FeO (FeO > 2 pct) content and basicity. For the slag system (CaO-SiO₂-FeO), no stable foam was observed at very low FeO content (<2 pct). As pct FeO increases, the slag foaming index goes through a maximum and then decreases; a similar phenomenon was observed for CaO-SiO₂-NiO slags with respect to the NiO content. The foaming index determined from the normal small-scale experiments (3.8-cm ID diameter) were confirmed on a larger scale (9.2-cm ID diameter), indicating that the foaming index is independent of container size. Measurements were also made for the actual compositions for bath smelting slags. For these slags, the foaming index is higher than those of simple CaO-SiO₂-FeO slags, because MgO and Al₂O₃ may increase their viscosities. The foam index is believed to be a function of the physical properties of the slag. Consequently, a dimensional analysis was performed, and a correlation was developed relating the foaming index to the viscosity, surface tension, and density of the slag. An estimation of slag foaming in actual pilot plant trials was also made from the results of the present study. Good agreement was observed between the predicted and observed foam heights and indicated coke in the slag can reduce the foam height by more than 50 pct. R. Jiang, Formerly Graduate Student, Carnegie Mellon University, is deceased.     

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.     

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.     

Rate of reduction of FeO in slag by Fe-C drops

The rate of reduction of FeO in slags by Fe-C drops plays an important role in several metallurgical processes, including iron bath smelting. In this study, the rate of this reaction was determined by measuring the volume of CO generated as a function of time, and the reaction was observed by X-ray fluoroscopy. The drops entered the slag in a nearly spherical shape, remained as single particles, and for the major portion of the reaction remained suspended in the slag surrounded by a gas halo. The rate was found to decrease with carbon content for alloys with low sulfur contents. The rate decreased significantly with increasing the sulfur content. Based on the results and a comparison of the calculated rates, for the possible rate-controlling mechanisms, a kinetic model was developed. The model is a mixed control model including mass transfer in the slag, mass transfer in the gas halo, and chemical kinetics at the metal interface. At high sulfur contents (>0.01 pct), the rate is primarily controlled by the dissociation of CO₂ on the surface of the iron drop. At very low sulfur, the rate is controlled by the two mass-transfer steps and increases as the gas evolution from the particle increases. Formerly with Carnegie Mellon University     

Theoretical analysis of the interfacial phenomena during the injection of carbon particles into EAF slags

A theoretical analysis of FeO reduction through the injection of carbon fines in electric arc furnace slags, involving the interfacial phenomena at the liquid‐gas‐solid interface, has been performed using basic principles of transport phenomena and physical chemistry of steelmaking. It was found that small angle contacts between slag and carbon favour FeO reduction. Moreover, FeO in basic slags are more prone to be reduced because the interfacial liquid‐gas interface has more free reaction places. In acid slags FeO reduction is difficult because the gas‐liquid interface is partially filled by polymeric silicates. When the particle size is smaller than 100 μm the influence of slag basicity is considerably decreased. Practical applications of these results can be found in electric arc furnace shops aiming at the mastering of slag foaming practices and energy saving.     

Reduction Behaviour of BOF type Slags by Solid Carbon

Experiments were carried out on a system with artificially prepared slags in a graphite crucible, in order to examine the possibility of recycling BOF slags produced in the steelmaking process. More than 80% of FeO and P₂O₅ was reduced within 20 minutes and the FeO reduction rate was greater than that of P₂O₅. P₂O₅ reduction began after more than 60% of FeO was reduced. Increasing slag basicity enhanced the reduction of FeO and P₂O₅. Temperature also improved slag reduction. The overall reduction rate was controlled by the chemical reaction at the slag/carbon interface. The reduction rates of FeO and P₂O₅ were second and first order with respect to their respective contents. Most of the reduced phosphorus is believed to vaporize in the form of P₂ gas.