Kinetics of mass transfer during nitrogen injection in industrial Fe—Cr—Ni—Mo-melts

Experiments on the injection of nitrogen into Fe—Cr—Ni—Mo alloys in 30 t VOD-Iadles have been carried out. The nitrogen contents obtained by nitrogen injection were between 0.05 and 0.17% for an initial nitrogen content of about 0.02%. Calculations of mass transfer associated with a rising nitrogen bubble indicate very high mass transfer rates for different nitrogen activities. The high efficiency of nitrogen gas measured in industrial practice agrees with these calculations. The specific interface area increases proportionally with the gas flow rate. According to reports in literature, nitrogen mass transfer can be described either as a 1st or 2nd order reaction. Both possibilities have been examined and the experimental results evaluated accordingly. In order to make possible a comparison with literature rate constants were first determined using comparable methods, i.e. on the assumption that the concentration difference is the driving force for mass transfer. The rate constants depend on the composition of the alloy. Assuming spherical cap bubbles with an equivalent diameter of 2.5 cm, the mass transfer coefficient was estimated to be about 5 to 8.10^?3cm s^?1, using a 1st order reaction model; the apparent activation energy then has a value of about 100 kJ/Mol. If a 2nd order reaction model is assumed, the apparent activation energy has a value of about 125 kJ/Mol. Using the activity difference as the driving force for mass transfer, it was found that the kinetics of mass transfer in multi component alloys could be treated in a homogeneous manner. The mass transfer coefficient calculated in this way for a 1st order reaction lies between about 26 to 37.10^?3cms^?1 and the actual activation energy has a value of about 25 kJ/Mol. The evaluation of the experimental results using a second order reaction model gives an equally good correlation between the rate constants and the gas flow rate as that obtained using a 1st order reaction model. In this case the actual activation energies determined are slightly higher - 40 kJ/Mol. However, the activation energies in both cases (i.e. with or without regard to the nitrogen activity) do not lie in the range of activation energies associated with a process controlled by interface reactions. Due to the low concentrations, it is not possible to make a definite statement about the order of the reaction. However, the activation energies determined fit better to a 1st order reaction model. Relations have been derived which, assuming a 1 st order reaction, and taking into account the composition of the alloy, enable prior calculation of suitable parameters for nitrogen injections in industrial melts. This allows a more precise process control.     

RH精炼过程增氮行为探讨

为了在RH精炼过程对氮的质量分数进行稳定控制,通过热力学计算分析钢水成分、RH真空度等对理论平衡氮质量分数的影响,分析RH提升气体从吹入上升管到进入真空室过程中压力的变化对平衡氮质量分数的影响。从动力学角度分析气相中的传质阻力、界面化学反应阻力、液相中的传质阻力共同作用于钢液的增氮、脱氮过程。提出RH处理过程的3种增氮途径及其对应的平衡氮质量分数。当钢种要求氮质量分数大于真空度下的理论平衡氮质量分数时,RH处理过程存在增氮、脱氮共存的状态。在氮质量分数变化过程中,当脱氮速度等于增氮速度时,钢液中氮质量分数达到动态平衡,不再发生变化。在真空度为5 kPa的条件下,RH钢液中氮质量分数达到动态平衡,不再发生变化时对应的氮质量分数为0.010 0%。     

The rate of absorption of hydrogen into iron and of nitrogen into Fe-Cr and Fe-Ni-Cr alloys containing sulfur

The rate of absorption of hydrogen into liquid iron and of nitrogen into liquid Fe-Cr alloys containing various levels of sulfur was measured by using a constant-volume Sieverts apparatus employing a sensitive pressure transducer. The rate for the absorption of hydrogen was measured by using H2 containing a small amount of H₂S(<0.2 pct) such that the activity of sulfur on the surface of the melt was the same as in the bulk metal. The hydrogen-absorption rate for Fe-S melts containing up to 0.72 pet sulfur was virtually independent of sulfur content and controlled by liquid-phase mass transfer. The liquidphase mass-transfer coefficient for hydrogen in liquid iron, calculated from the results, was comparable to that for nitrogen transfer in liquid iron. The rate of absorption of nitrogen into Fe-Cr melts with low-sulfur contents was controlled by liquid-phase mass transfer. For melts containing significant amounts of sulfur it was controlled by both mass transfer and the chemical rate of the dissociation of nitrogen on the surface in series. Equations were developed to calculate the chemical rate from the measured rate, correcting for mass transfer. The chemical rate decreased with increasing sulfur content as expected, because sulfur is strongly adsorbed on the surface and increased with chromium content at constant sulfur activity, possibly because available Cr sites promote nitrogen dissociation. Formerly with United States Steel Corporation, Monroeville, PA     

Rate of nitrogen desorption from liquid iron-carbon and iron-chromium alloys with argon

The rate of nitrogen desorption from inductively stirred liquid iron, iron-carbon, and iron-chromium alloys with argon carrier gas has been measured by the sampling method for a wide range of nitrogen, carbon, and chromium contents mainly at 1600 °C. The results obtained by the present work and other data of previous investigators are used to clarify the reaction mechanism of nitrogen desorption from liquid iron. The rate of nitrogen desorption from liquid iron and iron alloys is second order with respect to nitrogen content in the metal under the present condition, and mutual relationships among interfacial chemical reaction, liquid-phase mass transfer, and gas-phase mass transfer are elucidated. The effects of oxygen and sulfur on the rate of nitrogen desorption are given byk _' ^{c ’} = 3.15ƒ_N ² [1/(1 + 300a₀ + 130a_s)]. Carbon dissolved in iron increases the rate of nitrogen desorption, and chromium decreases it. The effects of these alloying elements can be explained by the change of the nitrogen activity in the metal. This paper is based on a presentation made at the G. R. Fitterer Symposium on Nitrogen in Metals and Alloys held at the 114th annual AIME meeting in New York, February 24–28, 1985, under the auspices of the ASM-MSD Thermodynamic Activity Committee.     

The kinetics of nitrogen absorption and desorption from a plasma arc by molten iron

A plasma torch and refractory-lined furnace with a 10 kg capacity were used to study the kinetics of nitrogen absorption and desorption in molten iron. In this study, melts containing both oxygen and sulfur were used. In accord with earlier studies, a limiting rate constant of 0.020 cm/s-pct was observed at high oxygen and/or sulfur contents. At lower oxygen and/or sulfur contents, the measured desorption rates are smaller than most of the reported values and appear to be limited by mixed melt, mass transfer chemical control. Absorption of nitrogen from the plasma arc is limited by mass transfer in the melt. The dominant form of convection in the vicinity of jet impingement is surface tension driven flow. The reaction N(g)=N(pct) appears to be responsible for the enhanced nitrogen content of the melt. The nitrogen content of a melt in equilibrium with the atomic nitrogen content of an Ar-5 pct N₂ plasma jet was determined to be 0.30 wt pct or thirty times the equilibrium value. T. B. KING, formerly Professor of Metallurgy at the Massachusetts Institute of Technology, Cambridge, MA, is deceased. This paper is based on a presentation made in the T.B. King Memorial Symposium on “Physical Chemistry in Metals Processing” presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS.     

The rate of absorption of nitrogen into liquid iron containing oxygen and sulfur

The rate of nitrogen absorption into and desorption from liquid iron containing sulfur and/or oxygen was measured by employing a constant-volume technique with a highly sensitive pressure transducer. Critical evaluation of the results demonstrated conclusively that the chemical rate at high oxygen or sulfur contents is second order with respect to nitrogen content in the metal and probably controlled by the dissociation of the nitrogen molecule on the surface. The effect of sulfur on the rate is complex because of the influence of 1) liquid-phase mass transfer at low sulfur contents, 2) the chemical rate on vacant iron sites at intermediate sulfur contents, and 3) the rate on the adsorbed sulfur layer or the limiting rate at high sulfur contents. However, at intermediate concentrations the limiting case for the adsorption isotherm for sulfur is adhered to and the rate is inversely proportional to the sulfur concentration. For Fe-O melts the rate is inversely proportional to the oxygen content except at low oxygen levels where mass transfer affects the rate. The rate for Fe-S-O melts can be calculated reasonably well from the results for the Fe-S and Fe-0 alloys, assuming that oxygen does not effect the adsorption of sulfur andvice versa and that there is nearly complete coverage of the surface with oxygen and sulfur atoms.     

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     

Experimental and theoretical study of the adsorption behavior and mass transfer kinetics of propranolol enantiomers on cellulase protein as the selector

The thermodynamics and mass transfer kinetics of the retention of the R and S enantiomers of propranolol were investigated on a system comprising an acetic acid buffer solution as the mobile phase and the protein cellobiohydrolase I immobilized on silica as the stationary phase. The bi-Langmuir isotherm model fitted best to each set of single-component isotherm data. The monolayer capacity of the nonchiral type of adsorption sites was 22.9 mM. For the chiral type of sites, it was 0.24 mM for the R enantiomer and 0.64 mM for the S enantiomer. Peak tailing was observed, even at very low concentrations allowing operation of the low-capacity chiral sites under linear conditions. This tailing can be explained on the basis of heterogeneous mass transfer kinetics. At higher concentrations, which are often used in analytical applications, the isotherms on the chiral sites no longer have a linear behavior, and peak tailing is consequently more pronounced. Under those conditions, peak tailing originates from the combined effect of heterogeneous thermodynamics and heterogeneous mass transfer kinetics. These complex phenomena are explained and modeled using the transport-dispersive model with a solid film linear driving force model modified to account for heterogeneous mass transfer kinetics. The rate coefficient of the mass transfer kinetics was found to be concentration dependent.     

Rate of nitrogen desorption from liquid iron-carbon and iron-chromium alloys with argon

The rate of nitrogen desorption from inductively stirred liquid iron, iron-carbon, and iron-chromium alloys with argon carrier gas has been measured by the sampling method for a wide range of nitrogen, carbon, and chromium contents mainly at 1600 °C. The results obtained by the present work and other data of previous investigators are used to clarify the reaction mechanism of nitrogen desorption from liquid iron. The rate of nitrogen desorption from liquid iron and iron alloys is second order with respect to nitrogen content in the metal under the present condition, and mutual relationships among interfacial chemical reaction, liquid-phase mass transfer, and gas-phase mass transfer are elucidated. The effects of oxygen and sulfur on the rate of nitrogen desorption are given byk _' ^{c ’} = 3.15?_N ² [1/(1 + 300a₀ + 130a_s)]. Carbon dissolved in iron increases the rate of nitrogen desorption, and chromium decreases it. The effects of these alloying elements can be explained by the change of the nitrogen activity in the metal.     

Heat transfer and fluid flow phenomena in electroslag refining

A mathematical formulation has been developed to represent the electromagnetic force field, fluid flow and heat transfer in ESR units. In the formulation, allowance has been made for both electromagnetically driven flows and natural convection; furthermore, in considering heat transfer the effect of the moving droplets has been taken into account. The computed results have shown that the electromagnetic force field appears to be the more important driving force for fluid motion, although natural convection does affect the circulation pattern. The movement of the liquid droplets through the slag plays an important role in transporting thermal energy from the slag to the molten metal pool, although the droplets are unlikely to contribute appreciably to slag-metal mass transfer The for-formulation presented here enables the prediction of thermal and fluid flow phenomena in ESR units and may be used to calculate the electrode melting rates from first principles. While a detailed comparison has not yet been made between the predictions based on the model and actual plant scale measurements, it is thought that the theoretical predictions are consistent with the plant-scale data that are available.     

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.     

Strategies for nitrogen control during the production of interstitial-free steel

Interstitial-free (IF) steel is a micro-alloyed steel that contains significantly low concentrations of interstitial elements such as carbon and nitrogen. In this context, nitrogen control in liquid steel is a major issue, particularly in cases where the nitrogen content in hot metal or other charged material is high. This study describes the practice of IF steel production at No.3 steelmaking plant of WISCO using hot metal containing high nitrogen due to hot metal desulphurisation treatment with nitrogen as the carrier gas. By adapting new technology for nitrogen removal in a combined blown converter and an Ruhrstahl Heraeus vessel as well as controlling nitrogen pick-up during transfer operations, final product can be generated with an average nitrogen content of about 18?ppm and with some values as low as 10?ppm.     

Mathematical model for nitrogen control in oxygen steelmaking

A mathematical model was developed to quantify the effects of different operational parameters on the nitrogen content of steel produced during oxygen steelmaking. The model predicts nitrogen removal by the CO produced during decarburization and how the final nitrogen content is affected by different process variables. These variables include the type of coolants used (scrap, direct reduced iron (DRI), etc.), the sulfur content of the metal, combined gas blowing practices, and the nitrogen content in the hot metal, scrap and oxygen blown. The model is a mixed control model that incorporates mass transfer and chemical kinetics. It requires a single parameter that reflects the surface area and mass-transfer coefficient that is determined from the rate of decarburization. The model also computes the rate of decarburization and the change in surface active elements, such as sulfur and oxygen, that affect the rate of the nitrogen reaction. Nitrogenization of steel in the converter is also predicted with the model. The computed results are in good agreement with plant data and observations.     

The kinetics of the nitrogen reaction with liquid iron-chromium alloys

The isotope exchange technique was employed to study the interfacial reaction kinetics of nitrogen with liquid iron-chromium and iron-chromium-sulfur solutions. Chromium was found to increase the rate of the nitrogen exchange reaction. The increase in the rate occurs, at least in part, through promotion of N₂ dissociation on the chromium surface sites. Although mass transport effects in the liquid are eliminated through the use of the isotope exchange technique, it was found that a correction for gas phase mass transfer was required for the determination of the interfacial reaction rate constant due to the faster exchange rates encountered in liquid iron solutions containing chromium. Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA     

Activity of nitrogen fixation and nitrogen assimilation enzymes in soybean plants inoculated with Bradyrhizobium japonicum strains

In studying the symbiotic associations of 8 Bradyrhizobium japonicum strains with two soybean varieties (NS-10 and NS-21), we determined the activity of nitrogen fixing (nitrogenase, NG) and nitrogen assimilation (glutamine synthetase, GS) and glutamate dehydrogenase (GDH) enzymes and the content of soluble proteins. The number of nodules was not in correlation with dry matter mass and nitrogen content. Dry matter mass and nitrogen content in the nodules of the soybean varieties in association with Bradyrhizobium strains 1, 1a, 2b, 2/1, 1003 and 8k turned out to be significantly higher than mass and nitrogen content of those in association with B. japonicum strains 17z and 2k. The results obtained indicate that the activity of the enzymes NG and GS in plants correlates with nitrogen fixation for both varieties on average, while the activity of GDH enzyme is conditioned by the specificity of variety and the Bradyrhizobium japonicum strains.     

Variations of nitrogen with silicon content of electrical steels

The nitrogen content of commercial heats of electrical steels has been observed to increase with decreasing silicon content prior to teeming despite a constant nitrogen content prior to addition of alloying silicon to molten steel. This behavior can be qualitatively described by a nitrogen transfer model which predicts: the rate of absorption of nitrogen by the metal is greater at lower silicon contents; a final nitrogen level increasing with decreasing silicon; the thermodynamic activity of nitrogen is approximately constant regardless of silicon content at teem time; the nitrogen level increases with longer holding times; and ladle practice will affect nitrogen levels.     

Heat transfer and fluid flow phenomena in electroslag refining

A mathematical formulation has been developed to represent the electromagnetic force field, fluid flow and heat transfer in ESR units. In the formulation, allowance has been made for both electromagnetically driven flows and natural convection; furthermore, in considering heat transfer, the effect of the moving droplets has been taken into account. The computed results have shown that the electromagnetic force field appears to be the more important driving force for fluid motion, although natural convection does affect the circulation pattern. The movement of the liquid droplets through the slag plays an import-ant role in transporting thermal energy from the slag to the molten metal pool, although the droplets are unlikely to contribute appreciably to slag-metal mass transfer. The for-formulation presented here enables the prediction of thermal and fluid flow phenomena in ESR units and may be used to calculate the electrode melting rates from first principles. While a detailed comparison has not yet been made between the predictions based on the model and actual plant scale measurements, it is thought that the theoretical predictions are consistent with the plant-scale data that are available. Presently on leave from Institute of Chemical Engineering and Technology, Punjab University, Lahore-1, Pakistan.     

The partial charge of the nitrogen atom in peptide bonds

A majority of the standard texts dealing with proteins portray the peptide link as a mixture of two resonance forms, in one of which the nitrogen atom has a positive charge. As a consequence, it is often believed that the nitrogen atom has a net positive charge. This is in apparent contradiction with the partial negative charge on the nitrogen that is used in force fields for molecular modeling. However, charges on resonance forms are best regarded as formal rather than actual charges and current evidence clearly favors a net negative charge for the nitrogen atom. In the course of the discussion, new ideas about the electronic structure of amides and the peptide bond are presented.     

Dissolution Condensation Mechanism of Stress Corrosion Cracking in Liquid Metals: Driving Force and Crack Kinetics

Stress corrosion cracking (SCC) in aqueous solution is driven by exothermic reactions of metal oxidation. This stimulus, as well as classical mechanisms of SCC, does not apply to SCC in liquid metals (LMs). In the framework of the dissolution-condensation mechanism (DCM), we analyzed the driving force and crack kinetics for this nonelectrochemical mode of SCC that is loosely called “liquid metal embrittlement” (LME). According to DCM, a stress-induced increase in chemical potential at the crack tip acts as the driving force for out-of-the-tip diffusion mass transfer that is fast because diffusion in LMs is very fast and surface energy at the solid-liquid interface is small. In this article, we review two versions of DCM mechanism, discuss the major physics behind them, and develop DCM further. The refined mechanism is applied then to the experimental data on crack velocity V vs stress intensity factor, the activation energy of LME, and alloying effects. It is concluded that DCM provides a good conceptual framework for analysis of a unified kinetic mechanism of LME and may also contribute to SCC in aqueous solutions.     

Fabrication of ultrahigh nitrogen austenitic steels by nitrogen gas absorption into solid solution

For the purpose of fabricating ultrahigh nitrogen austenitic steels (>1 mass pct N), the phenomenon of nitrogen absorption into solid solution was thermodynamically analyzed and applied to Fe-Cr-Mn system ternary alloy. During the annealing of the steel in a nitrogen gas atmosphere of 0.1 MPa at 1473 K (nitrogen absorption treatment), the nitrogen content of the steel was increased with the absorption of nitrogen gas from the material surface and then saturated when the system reached a state of equilibrium. Effect of the steel composition on an equilibrium nitrogen content was formulated taking account of interactions among Cr, Mn, and N atoms, and the condition for fabrication of ultrahigh nitrogen austenitic steels was clarified. The nitrogen addition to ultrahigh content markedly increased proof stress and tensile stress of the austenitic steels without losing moderate ductility. For example, Fe-24Cr-10Mn-1.43N (mass pct) alloy has 830 MPa in 0.2 pct proof stress, 2.2 GPa in true tensile stress, and 75 pct in total elongation. As a result of tensile tests for various nitrogen-bearing austenitic steels, it was found that the proof stress is increased in proportion to (atomic fraction of nitrogen)^2/3.