Kinetics of iron oxide reduction from CaO-MgO-FeOn-SiO2 slags by silicon dissolved in liquid iron

At steelmaking temperatures, the kinetics of slag-metal reactions is usually determined by mass transfer. This occurs in two ways: normal mass transfer which is induced by stirring, and mass transfer by interfacial convection induced by interfacially active elements like oxygen and sulphur. In the present work, mass transfer during the reduction of iron oxide from a basic slag by silicon dissolved in liquid iron was studied under defined conditions of gas stirring by argon in MgO crucibles with 1500 g iron and 250 g slag. The variations of the FeO content in the slag and the silicon content in the iron during the reaction were measured by sampling. Trials were carried out with stirring gas flow rates between 1 and 20.4 l/h(STP). The experimental data were evaluated with the multi-component transport model in order to determine the mass transfer coefficients of the reaction components. Simultaneously, the coefficients of normal mass transfer were calculated with the boundary layer theory of liquid-liquid mass transfer for non-turbulent flow conditions. The measured mass transfer coefficients were by a factor 2.5 larger than the theoretically calculated. The difference indicates the presence of mass transfer by interfacial convection. Mass transfer by interfacial convection is superimposed to normal mass transfer.     

Kinetics of mass transfer of manganese and silicon between liquid iron and slags

The kinetics of mass transfer of Mn and Si between liquid iron and slags were investigated in laboratory experiments at 1600°C in MgO crucibles with 1500 g iron and 250 g slag. Three different slags consisting of CaO-MgO-MnO-SiO₂, MgO-MnO-SiO₂ and MgO-MnO-Al₂O₃-SiO₂ were used. The concentration-vs.-time curves, experimentally measured under defined flow conditions generated by gas stirring, were evaluated by application of a multi-component transport model in order to obtain the mass transfer coefficients. The numerical values of the thus determined measured mass transfer coefficients were compared with values calculated by a theory of mass transfer at liquid-liquid interfaces. The measured and theoretical values were in good agreement with each other in the case of reduction of MnO from the slag by Si in the metal, provided that the measurements had been carried out below a critical stirring intensity, above which metal droplets were emulsified in the slag. Experiments, where sulphur was dissolved in the metal melt and where the sulphur contents were systematically varied, showed no changes of the mass transfer coefficient in comparison to sulphur-free melts. The experimental mass transfer coefficients for the reduction of silica from the slag by manganese in the metal were smaller than those calculated by the mentioned mass transfer theory. This could be explained by inhibition of surface renewal under the influence of solid reaction products precipitated at the interface.     

Sulfur Transfer at Slag/Metal Interface—Impact of Oxygen Potential

In the present work, the interfacial movement resulting from sulfur mass transfer at the slag/metal interface was monitored by X-ray sessile drop method in dynamic mode at temperature 1873 K (1600 °C) under nonequilibrium conditions. The experiments were carried out with pure iron and CaO-SiO₂-Al₂O₃-FeO slag (alumina saturated at the experimental temperature) contained in alumina crucibles with well-controlled partial pressures of oxygen and sulfur. The impact of oxygen potential on the droplet oscillation as sulfur from the gas phase reaches the metal drop through the intermediate slag phase was monitored. The interfacial velocity was investigated. It was found that the increases of interfacial velocity and the maximum oscillation time were mainly attributed to the partial pressure of oxygen increases. The experiment results were explained by previous ab initio calculations. The thermo-physical and thermo-chemical properties of slag were also found to influence interfacial velocity.     

Steel dissolution in quiescent and gas stirred Fe/C melts

The dissolution rates of commercial black iron rods in iron/carbon melts under isothermal conditions were measured. The effect of melt carbon content, temperature, natural convection, and gas stirred forced convection conditions were investigated. The experimental data under natural convection conditions (no external stirring) were fitted with a dimensionless correlation for vertical cylinders: Sh = 0.13(Gr . Sc)^0.34, representing mass transport control dominated by turbulent natural convection. Under bottom injection gas stirring conditions, it was found that the kinetic power input had little effect on the rod dissolution rates which were controlled by the total gas flow rate. Derived mass transport coefficients under gas stirring conditions were found to have the following dependence on the gas injection rates:k _m ∝Q ^0.21, wherek _m = mass transport coefficient andQ = gas flow rate. A comparison of the experimental results with previously measured mass transfer coefficients under forced convection conditions gave a plume velocity flow rate dependence ofU ∝Q ^0.3. A general discussion of gas stirring fluid dynamics and resulting mass transport effects is presented.     

Some Investigations into the Dynamic Mass Transfer at the Slag–Metal Interface Using Sulfur: Concept of Interfacial Velocity

In the current work, dynamic studies of mass transfer of sulfur from the gas phase to the metal phase of pure iron through CaO-SiO₂-Al₂O₃-FeO quaternary slag were carried out. X-ray videos were taken that were later processed to identify the oscillation of the metal drop occurring during the mass transfer. It was observed that the metal drop had hybrid oscillations. Each of these oscillations could be identified as composed of a symmetric and an asymmetric element, which was attributed to the changes in the shape of the droplet. The latter (asymmetric part) could be identified by the deviation of the left and right contact angles from the stable configuration. The symmetric oscillations were traced to the surface movement of sulfur at the interface, which created an instantaneous area change at the slag–metal interface. This area change was due to the combined effect of Marangoni flow and interface dilatation. The velocity of sulfur at the interface was calculated from the area change and had a maximum order of magnitude as 10⁻⁴ m/s. It was also observed that the interfacial velocity increased with increase in temperature.     

The Rate of the Phosphorous Reaction Between Liquid Iron and Slag

In the current study, the rates of dephosphorization and rephosphorization of liquid iron with simulated steelmaking slags were investigated at 1873 K (1600° C). The experiments were conducted in an induction furnace with supplemental heating to maintain a consistent temperature within both the metal and slag phases. An integrated form of the rate equation was used to evaluate the results, assuming mass transfer in both the slag and metal was rate controlling. The results of the current and previous studies indicate that the mass transfer parameter, the slag-metal surface area, and the overall mass transfer coefficient (A*k ₀), decreased as the reaction proceeded. It is proposed that initially when the rate and oxygen flux are high, the interfacial energy decreases, and the interfacial fluid velocity increases causing disruption of the slag metal interface. The consequent increases in interfacial area and interfacial fluid flow cause A*k ₀ to be high initially and then decrease as the oxygen flux decreases.     

Interfacial phenomena in some slag-metal reactions

In the present work, the change of the interfacial tension at the slag-metal interface for sulfur transfer between molten iron, slag, and gas phases was monitored by X-ray sessile drop method in dynamic mode in the temperature range of 1830 to 1891 K. The experiments were carried out with pure iron samples immersed partly or fully in the slag phase. The slag consisted of 30 wt pct CaO, 50 wt pct Al₂O₃, and 20 wt pct SiO₂ (alumina saturated at the experimental temperatures) with additions of FeO. Metal and slag samples contained in alumina crucibles were exposed to a CO-CO₂-SO₂-Ar gas mixture with defined oxygen and sulfur partial pressures, and the change of the shape of the metal drop was determined as a function of time. The equipment and the technique were calibrated by measurements of the surface tensions of the pure Cu, Ni, and Fe containing two different amounts of dissolved oxygen. A theoretical model was developed to determine the sulfur content of the metal as a function of time on the basis of sulfur diffusion in the slag and metal phases as well as surface tension-induced flow on the metal drop surface. Attempts were made to compute the interfacial tensions on the basis of force balance. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.     

Mass Transfer in Slag Refining of Silicon with Mechanical Stirring: Transient Interfacial Phenomena

Experiments have been carried out to study the rates of mass transfer between liquid silicon and CaO-SiO₂ slag with impeller stirring at 1823 K (1550 °C). The occurrence of transient interfacial phenomena related to the mass transfer of calcium has been observed; the evidence suggests that the reduction of calcium oxide at the interface leads to a rapid, temporary drop in the apparent interfacial tension. At low apparent interfacial tension, mechanical agitation facilitates the dispersion of metal into the slag phase, which dramatically increases the interfacial area; here, it has been estimated to increase by at least one order of magnitude. As the reaction rate slows down, the apparent interfacial tension increases and the metal recoalesces. The incidental transfer of calcium very likely promotes the transfer of boron by increasing the interfacial area. Mechanical mixing appears to be an extremely effective means to increase the reaction rate of boron extraction and could feasibly be implemented in the industrial slag refining of silicon to improve reaction rates.     

Physical modeling of liquid/liquid mass transfer in gas stirred ladles

Several of the metallurgical reactions occurring in gas stirred steel ladles are controlled by liquid phase mass transfer between the metal and slag. In order to calculate the rate of these reactions, information about the two phase mass transfer parameter is necessary. The mass transfer between two immiscible liquids, oil and water simulating slag and steel, respectively, was measured in a scale model of a ladle. The mass transferred species was thymol which has an equilibrium partition ratio between oil and water similar to that for sulfur between slag and metal. The mass transfer rate was measured as a function of gas flow rate, tuyere position and size, method of injection, oil viscosity, and oil/water volume ratio. In addition, mixing times in the presence of the oil layer and mass transfer coefficient for the dissolution of solid benzoic acid rods were measured. The results show that there are three gas flow rate regimes in which the dependence of mass transfer on gas flow rate is different. At a critical gas flow rate, the oil layer breaks into droplets which are entrained into the water, resulting in an increase in the two phase interfacial area. This critical gas flow rate was found to be a function of tuyere position, oil volume, densities of two phases, and interfacial tension. Two phase mass transfer for a lance and a tuyere was found to be the same for the same stirring energy in low energy regions regardless of lance depth. Mass transfer is faster for a center tuyere as compared to an offcenter tuyere, but mixing times are smaller for the offcenter tuyere. From the results obtained, the optimum stirring conditions for metallurgical reactions are qualitatively discussed. SEON-HYO Kim, formerly Graduate Student in the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University.     

Dynamics of the hot metal dephosphorization with Na2O slags

Dephosphorization reaction of hot metal by Na₂CO₃ has been studied experimentally to determine the reaction mechanism and thermodynamics. Most of the experiments were carried out at 1300 °C using Fe-C_sat.-Si-P-S alloys. The results indicate that the CO₂ gas released from Na₂CO₃ is important in the dephosphorization reaction as an oxidizer and increasing mass transfer by stirring the slag and metal. As the initial Si content in hot metal is increased, the degree of dephosphorization decreases significantly and the rephosphorization takes place earlier. The primary reason for the rephosphorization is that the activity of PO_2.5 increases in the slag because of the evaporation of Na₂O from the slag. The loss of Na₂O increases the activity coefficient of PO_2.5 and decreases the slag volume. At the later stage of Na₂CO₃ treatment, the reactions reach equilibrium with respect to phosphorus and sulfur, and the oxygen potential,P _o2, at the slag-metal interface is determined by the C-CO equilibrium (a _c=1 and 1 atm CO). The presence of sulfur in the metal increases the rate of the dephosphorization because of the electrochemical nature of the reaction; sulfur transfer to the slag accepts the electrons from phosphorus transfer.     

移动电磁场对铁水预处理脱硫速度的影响

 为探讨移动电磁场对冶金反应速度的影响作用,本研究对电磁搅拌下的铁水预处理脱硫进行了实验研究。将CaO-10 mass%CaF2脱硫剂加至铁水的自由表面或用载气带入铁水内部,并采用不同的搅拌方式和强度进行了电磁搅拌下的实验,得到了铁水中硫浓度随时间的变化曲线。研究发现电磁搅拌通过改变渣金界面状态及边界层厚度从而提高铁水脱硫速度,并得到了不同搅拌强度和不同搅拌方式的容量传质系数。最后提出一种环保和易操作的电磁搅拌下铁水预处理脱硫工艺。     

Interfacial phenomena and mass transfer in extractive metallurgy

Interfacial phenomena play an important role in pyrometallurgical processes and knowledge of them and of physical properties involved is necessary for understanding the mechanisms and the kinetics of such reactions. A large number of measurements, performed at Irsid under equilibrium conditions, are presented in this review. Several experimental techniques were used and more particularly: – Sessile drop method for measurement of liquid metals surface tension and contact angle liquid metal/solid oxide; – measurement of the contact angle between a liquid slag drop and its liquid metal substrate from which the interfacial tension can be derived; – direct determination of the interfacial tension from X-ray pictures of metal drops immersed in the slag. The systems studied consisted, for the metal phase, of binary and ternary Fe alloys containing C, Mn, Si, O, S and, for the slag phase, binary and ternary mixtures made from CaO, SiO₂, Al₂O₃, MnO, iron oxides, CaF₂ and Na₂O. A strong effect of O and S potentials was observed. For non-equilibrium conditions, however, the dynamic interfacial tension between liquid metal and slag decreases sharply when an intense mass transfer occurs through the interface. The potential consideration of interfacial turbulence phenomena (Marangoni effect) in metallurgical reactions is also discussed.     

Effect of the bubble size and chemical reactions on slag foaming

Slag foams have been investigated with smaller bubbles than those used in the previous studies.^[5,6,7] The bubbles were generated by argon gas injection with the nozzle of multiple small orifices and by the slag/metal interfacial reaction of FeO in the slag with carbon in the liquid iron. The foam stability in terms of the foam index for a bath-smelting type of slag (CaO-SiO₂-Al₂O₃-FeO) was determined for different bubble sizes. The average diameter of bubbles in the foam was measured by an X-ray video technique. When the foam was generated by the slag/metal interfacial reaction at 1450 °C, it was found that the average bubble diameter varied from less than 1 to more than 5 mm as a function of the sulfur activity in the carbon-saturated liquid iron. The foam index was found to be inversely proportional to the average bubble diameter. A general correlation is obtained by dimensional analysis in order to predict the foam index from the physical properties of the liquid slag and the average size of the gas bubbles in the foam. Formerly Graduate Student and Research Associate, Department of Materials Science and Engineering, Carnegie Mellon University.     

The rate of decarburization of liquid iron by CO2 and H2

The rate of decarburization of liquid iron in CO-CO₂ mixtures and hydrogen at 1800 K has been investigated. The effect of sulfur on the rate in CO-CO₂ was also determined. Two experimental techniques were employed, one with the gas flow parallel to the surface of the melt, the other with gas flow perpendicular to it. The rate of decarburization in both CO-CO₂ mixtures and hydrogen at high carbon contents is controlled primarily by diffusionsion in the gas film boundary layer near the surface of the liquid. The presence of 0.3 wt pct sulfur reduced the rate of decarburization in CO-CO₂ by about 10 pct indicating that a slow chemical reaction on the surface is effecting the rate slightly when the surface is covered with sulfur atoms. The rate of decarburization at low carbon contents in CO-CO₂ is controlled primarily by carbon diffusion in the metal. The mass transfer relationships for the experimental geometries employed were investigated by measuring the rate of oxidation of graphite in CO-CO₂ mixtures. Previous work in which it was concluded that a chemical reaction was controlling the rate were re-examined and it was concluded that gas phase mass transfer was in fact controlling the rate of the reaction.     

Interfacial phenomena between fluxes for continuous casting and liquid stainless steel

Abstract

Casting powders melt on the surface of the liquid metal forming a liquid slag layer. Samples taken during casting revealed convective flows in the flux layer and mass exchange with the liquid metal. It is demonstrated that concentrations of certain elements are considerably higher at the phase boundary than in the bulk of the metal and slag phase. Disturbances of interfacial tension produced by mass and charge transfer evidently cause strong shearing forces which act in parallel with the phase boundary. These forces induce convective movements in the flow boundary layer. Convective flows next to the interface between two liquids have been studied in laboratory experiments using various liquids. The results show that the movement velocity of volume elements next to the interface (due to disturbances of interfacial tension) are dependent on liquid layer thickness and on liquid properties. A new dimensionless number describing this manner of convective flow and suitable for evaluation of experimental results is introduced. Its contribution to the total mass transfer will be shown. A dimensionless function describing the relation between convective flows in the slag layer and mass transport is theoretically developed. Coefficients of this function for Ti transfer into the flux layer have been determined empirically.     

Interfacial Convection and Mass Transport in Thin Liquid Layer

Investigations into the phenomenology of convection flows next to the interface between two liquids have been carried out in laboratory experiments using various liquids. Then convection flows have been observed in industrial tests during continuous casting. The results show that the motional velocity of volume elements next to the interface due to disturbances of interfacial tension (produced by mass and charge transfer) depends on liquid layer thickness and on liquid properties. A new dimensionless number is introduced to describe this manner of convective flow; it is also suitable for evaluation of experimental results. Furthermore, a dimensionless function is theoretically developed to describe the relation between convective flows near the interface in the slag layer and mass transport. Casting powders melt on the surface of the liquid metal forming a liquid slag layer. Samples taken during casting have revealed convective flows in the flux layer and mass exchange with the liquid metal. Coefficients of the developed dimensionless function have been determined empirically for Ti‐transport from the interface into the flux layer.     

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     

Kinetics of reduction of ferric iron in Fe2O3-CaO-SiO2-Al2O3 slags under argon, CO-CO2, or H2-H2O

The rates of reduction of ferric iron in Fe₂O₃-CaO-SiO₂-Al₂O₃ slags containing 3 to 21 wt pct Fe₂O₃ under impinging argon, CO-CO₂, or H₂-H₂O have been studied at 1370 °C under conditions of enhanced mass transfer in the slag using a rotating alumina disc just in contact with the slag surface. For a 6 wt pct Fe slag at a stirring speed of 900 rpm the observed reduction rates by 50 pct H₂-H₂O were a factor of 2 to 3 times higher than those by 50 pct CO-CO₂ and more than one order of magnitude higher than those under pure argon. The observed rates were analyzed to determine the rate-controlling mechanisms for the present conditions. Analysis of the rate data suggests that the rates under 50 pct H₂-H₂O are predominantly controlled by the slag mass transfer. The derived values of the mass-transfer coefficient followed a square-root dependence on the stirring speed for a given slag and, at a given stirring speed, a linear function of the total iron content of the slags. The rates of oxygen evolution during reduction under pure argon were shown to be consistent with a rate-controlling mechanism involving a fast chemical reaction at the interface and relatively slow mass transfer in the gaseous and the slag phases. The rates of reduction by CO-CO₂ (pCO=0.02 to 0.82 atm) were found to be likely of a mixed control by the slag mass transfer and the interfacial reaction. A significant contribution of oxygen evolution to the overall rates was observed for more-oxidized slags and for experiments with relatively low values of pCO. Assuming a parallel reaction mechanism, the estimated net reduction rates due to CO were found to be of the first order in pCO, with the first-order rate constants being approximately a linear function of the ferric concentration. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS. The original symposium appeared in the October 2000 Vol. 31B issue.     

The nitrogen reaction between carbon saturated Iron and Na2O-SiO2 slag: Part II. Kinetics

The kinetics of the nitrogen reaction between carbon saturated iron and Na₂O-SiO₂ slags and between Na₂O-SiO₂ slags and an inert gas phase were investigated at 1200 °C. For the nitrogen transfer from the iron alloy to slag, the overall mass transfer coefficient of nitrogen was calculated to be 2.2 × 10⁻⁴ cm/sec. For nitrogen transfer from Na₂O-SiO₂ slag to argon gas, it is shown that the rate controlling process is mass transfer in the slag phase, and the mass transfer coefficient of nitrogen is 9 × 10⁻⁴ cm/sec. Experiments were also conducted to demonstrate nitrogen removal from hot metal by Na₂CO₃ treatment at 1200 °C. In these experiments, 325 grams of Na₂CO₃ was added to the 6.5 kg of Fe-C-N(-Si) alloy. When the metal contained silicon, nitrogen was transferred from the iron alloy to slag after the silicon was oxidized. When the iron alloy contains no silicon, nitrogen removal was faster. In both cases the nitrogen reversion occurred because of the decrease of slag volume and slag basicity. Furthermore, the presence of silicon in the metal retarded nitrogen reversion. is on leave of absence from the Department of Metallurgical Engineering and Materials Science, Faculty of Engineering, The University of Tokyo Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University