Rate of decarburization of iron-carbon melts: Part II. a mixed-control model

The observed retarding effect of sulfur on the decarburization of Fe-C melts has been interpreted by means of a mixed-control mechanism involving gas-phase mass transfer and dissociative adsorption of CO₂. A mathematical model formulated on the basis of the proposed mechanism gave an excellent fit to the experimental data. The application of the model to the data provided a value of 4.42 x 10⁻³ mole · cm⁻² · s⁻¹ · atm⁻¹ for the dissociative adsorption rate constant for CO₂ on liquid iron at 1973 K; the fraction of surface sites that cannot be occupied by sulfur, even at apparent surface-saturation, was found to be 0.085. The model predicts a residual rate of decarburization at high sulfur concentrations; this prediction is borne out by the experiment. The effect of convective motion within the levitated melt on the rate of decarburization below a characteristic carbon concentration was quantified. The liquid-phase mass transfer was greatly enhanced by the stirring effect of the electromagnetic field. The effective diffusivity of carbon in Fe-C melts under levitation conditions has been found to be 3.24 x 10⁻³ cm² · s⁻¹, a value ten times as large as that under stationary conditions.     

Reduction and decarburization during the annealing of atomized iron-carbon alloy powders

Conclusions General laws have been established governing the kinetics of the reduction and decarburization of an atomized cast iron powder during annealing in the gaseous products of the reaction between the oxygen and carbon in the powder and during subsequent annealing in a hydrogen stream. Reduction and decarburization rate curves have maxima, attained simultaneously; to the ascending branches of the kinetic curves there correspond time stages in which reduction and decarburization are not hindered by carbon diffusion toward the particle surfaces (as a result, at the cementite/oxide and austenite/oxide interfaces reactions predominate which are accompanied by CO evolution); to the descending branches of the kinetic curves there correspond stages during which the carbon concentration gradient in the metallic nuclei of particles decreases, as a result of which the reactions at the austenite/oxide interfaces become less, and those at the oxide/gas interfaces more, vigorous, the reactions accompanied by CO₂ evolution also increasing in intensity; after the material of the particles has been depleted of carbon, an equilibrium is attained in the process taking place in the FeO Fe stage, while the introduction of hydrogen into the reactor increases the rate of reduction, creating a second maximum on the kinetic curve.The reactions between the solid phases in the metallic nuclei of the particles and the oxygen in the scale on the surfaces of these particles have been found to play a dominant role.In the light of the adsorption-catalytic theory it may be concluded that the reduction-decarburization annealing of an atomized Fe-C alloy powder involves the following processes:Translated from Poroshkovaya Metallurgiya, No. 2(230), pp. 5–15, February, 1982.The authors wish to express their gratitute to V. F. Pekach and N. P. Kurganskii for the loan of the testing apparatus and assistance with its operation.     

Rate of decarburization of Fe-Csat melts by H2O at 1523 and 1873 K

The rate of decarburization of carbon-saturated iron by H₂O gas at 1523 and 1873 K was measured as a function of sulfur content. The measurements were made under conditions for which the gas phase mass transfer is negligible or a reasonable correction for its effect can be made. The measured rates are consistent, with the rate-controlling reaction being the dissociation of H₂O on the surface. Sulfur was found to significantly decrease the rate. The rate constant for the dissociation of H₂O on iron is considerably higher than those for the dissociation of CO₂ and N₂ on liquid iron; about 2 to 3 times higher than CO₂ and 300 to 600 times higher than for N₂ at 1873 K. The practical significance of the present results relevant to steel ladle processing and iron bath smelting is briefly discussed.     

Hydrodynamics of gas stirred melts: Part I. Gas/liquid coupling

A hydrodynamic model of submerged gas injection systems and their effects on liquid metal stirring is presented. It is argued that hydrodynamic conditions at the nozzle, tuyere, or plug are not critical to flow recirculation produced in large cylindrical vessels(i.e., furnaces or ladles). An analysis of a buoyancy driven plume generated through gas injection shows that gas voidages are usually quite low (less than 10 pct). By equating the energy supplied by rising bubbles to turbulent energy losses within the bath, it is shown that mean plume velocities can be predicted using the relationship,U _p α (Q ^1/3 L ^1/3)/R^1/3 whereU _p equals mean plume velocity,Q is gas flow rate (at mean height and temperature),L is depth of liquid, andR is radius of the vessel. Associated rates of liquid turnover as a function of vessel dimensions and gas flow rate can also be predicted and these are similarly presented.     

Mathematical modeling of the argon-oxygen decarburization refining process of stainless steel: Part I. Mathematical model of the process

Some available mathematical models for the argon-oxygen decarburization (AOD) stainless steelmaking process have been reviewed. The actual situations of the AOD process, including the competitive oxidation of the elements dissolved in the molten steel and the changes in the bath composition, as well as the nonisothermal nature of the process, have been analyzed. A new mathematical model for the AOD refining process of stainless steel has been proposed and developed. The model is based on the assumption that the blown oxygen oxidizes C, Cr, Si, and Mn in the steel and Fe as a matrix, but the FeO formed is also an oxidant of C, Cr, Si, and Mn in the steel. All the possible oxidation-reduction reactions take place simultaneously and reach a combined equilibrium in competition at the liquid/bubble interfaces. It is also assumed that at high carbon levels, the oxidation rates of elements are primarily related to the supplied oxygen rate, and at low carbon levels, the rate of decarburization is mainly determined by the mass transfer of carbon from the molten steel bulk to the reaction interfaces. It is further assumed that the nonreacting oxygen blown into the bath does not accumulate in the liquid steel and will escape from the bath into the exhaust gas. The model performs the rate calculations of the refining process and the mass and heat balances of the system. Also, the effects of the operating factors, including adding the slag materials, crop ends, and scrap, and alloy agents; the nonisothermal conditions; the changes in the amounts of metal and slag during the refining; and other factors have all been taken into account. []—metal phase; ()—slag phase; {}—gaseous phase; and 〈〉—solid phase     

Mixing phenomena inside the ladle during RH decarburization of steel melts

The RH (Rheinstahl-Heraeus) process is in widespread use to produce ultra-low carbon steel by vacuum treatment. Among other factors, the decarburization rate is effected by mixing phenomena inside the ladle. Thus, supervisory control of the RH treatment requires an adequate and precise representation of the mixing conditions. In this work, mixing is characterised by concepts of chemical reaction engineering. Different sizes and geometries of RH installations are taken into account using similarity criteria. A simple model is derived from the experimental results and detailed numerical flow computations. For this model, a mathematical function of mixing is developed which can be incorporated into supervisory control systems.     

Hot model experiments of the metal bath spraying effect during the decarburization of Fe-C melts through oxygen top blowing

During recent years decarburization has been steadily gaining importance in converter steel metallurgy at the expense of refining reactions for other slag-forming companion elements. Because decarburization is currently a low-slag operation, the phase contact between gas and metal is critical. With the decrease in the amount of slag foam, more attention must be paid to the spraying of iron droplets during oxygen blowing. The experiments were carried out in a hot model reactor with a 50 kg capacity by oxygen top blowing upon Fe-C melts. The resulting spray of iron droplets was collected with the help of a special droplet sampler in the blowing converter. In the metal droplets a pronounced enhanced decarburization was found in comparison to the metal bath. The amount of metal spray was determined with respect to the oxygen blowing pressure, nozzle diameter, and distance between the lance and bath. Depending on the reactor contents, a high circulation rate of the droplets could be observed. At low blowing rates, FeO-slag is formed and sprayed along with the metal.     

The effect of sulfur and of phosphorus on the rate of decarburization of solid iron in hydrogen

The effect of sulfur and phosphorus on the rate of decarburization of solid iron in hydrogen between 1173 and 1413 K has been investigated. Sulfur and phosphorus both retard the rate significantly, indicating a high degree of surface coverage by adsorbed sulfur and phosphorus atoms on solid iron. The rate constant for an alloy containing 0.25 wt pct P is about one-quarter of that for an alloy containing no phosphorus. The rates for Fe-C alloys in H₂-H₂S mixtures containing from 0.26 to 0.0028 vol pct H₂S were measured. As little as 0.0028 pct H₂S reduced the rate constant by about a factor of ten at 1413 K. The results indicate almost complete surface coverage by adsorbed sulfur atoms at sulfur activities as low as 0.004 with respect to Fe-FeS.     

On the flow criteria for suspending solid particles in inductively stirred melts: Part I. newtonian behavior

A mathematical formulation has been developed to represent the behavior of both buoyant and nonbuoyant particles in a fluid which is undergoing turbulent recirculating flow. In the formulation the fluid velocity field is represented by the turbulent Navier-Stokes equations, in conjunction with thek-ɛ model for the turbulent viscosity. Regarding the fluid-particle interactions in calculating the drag, allowance has been made for both the time smoothed and the fluctuating velocity components. The principal findings of the work are that turbulence plays a key role in suspending the particles in the melt, and that for identical density differences, it is much easier to prevent the flotation of a buoyant particle than the sedimentation of a particle which is heavier than the fluid.     

On the flow criteria for suspending solid particles in inductively stirred melts: Part I. newtonian behavior

A mathematical formulation has been developed to represent the behavior of both buoyant and nonbuoyant particles in a fluid which is undergoing turbulent recirculating flow. In the formulation the fluid velocity field is represented by the turbulent Navier-Stokes equations, in conjunction with thek-ɛ model for the turbulent viscosity. Regarding the fluid-particle interactions in calculating the drag, allowance has been made for both the time smoothed and the fluctuating velocity components. The principal findings of the work are that turbulence plays a key role in suspending the particles in the melt, and that for identical density differences, it is much easier to prevent the flotation of a buoyant particle than the sedimentation of a particle which is heavier than the fluid.     

Nucleation and phase selection in undercooled Fe-Cr-Ni melts: Part I. Theoretical analysis of nucleation behavior

The selection of the primary solidifying phase in undercooled stainless steel melts is theoretically analyzed in terms of nucleation theory. Nucleation phenomena are considered using different models for the solid-liquid interface energy. The classical nucleation theory for sharp interfaces and an improved modification, the diffuse interface theory, are applied. The influence of deviations of the nucleus composition from the overall alloy composition is also revealed. A preferred nucleation of the metastable bcc phase in fcc equilibrium solidification-type alloys is predicted. The critical undercooling of metastable crystallization as a function of alloy composition is calculated for an isoplethal section at 69 at. pct Fe of Fe₆₉Cr_31-x Ni _x alloys. The results are summarized in a phase selection diagram predicting the primary solidification mode as a function of undercooling and melt composition.     

A mathematical model for the dynamic behavior of melts subjected to electromagnetic forces: Part I. model development and comparison of predictions with published experimental results

A mathematical model was developed to predict electromagnetically driven flow and (particularly) free surface behavior in melts subject to electromagnetic forces. Such melts appear in electromagnetic casters, induction furnaces, and other metal processing units. The calculations started with Maxwell’s equations and Ohm’s law, which were solved by a novel “modified hybrid technique.” The instantaneous continuity and Navier-Stokes equations (rather than their time-averaged versions) were then solved with electromagrretic forces as input. The calculations allowed for the dynamic behavior of the free surface of the melt, and electromagnetic fields were recomputed as the free surface changed. In this first part of a two-part article, the model predictions are compared with the experimental measurements of induced current, magnetic field, melt velocity, and free surface deformation reported by others.     

Vacuum distillation of liquid metals: Part I. Theory and experimental study

The kinetics of vacuum distilling copper, tin, manganese, and sulfur from melted steel scrap have been measured. The experiments found that 70 to 90 pct of initial copper, 60 to 80 pct of initial tin, 80 to 100 pct of initial managanese, and 20 to 40 pct of initial sulfur can be eliminated in 30 minutes exposure to vacuum. Melt masses were in the range 10 to 60 kg, melt temperatures in the range 1850 to 2050 K, and chamber pressures in the range 3 to 400 pascals. Crucible diameter was 0.2m. Mass transport has been described in terms of Machlin's model for melt phase diffusion, Langmuir's model for evaporation, and convective bulk flow for gas phase mass transport. Two preliminary criteria are shown to demonstrate the suitability of vacuum distillation to any particular system and a third operational criterion is developed to define the range of vacuum required to eliminate gas phase mass transport resistance effectively. W.G. Davenport formerly with McGill University     

Hydrodynamics of gas stirred melts: Part II. Axisymmetric flows

A predictive model of gas stirred melt is presented. Based on the differential approach and following a review of previous models, the importance of natural convection or buoyancy driven phenomena is underscored. Predicted flow patterns are shown to be consistent with laboratory and pilot scale experiments, and with the macroscopic plume model in Part I of this paper.     

Rate of reduction of ore-carbon composites: Part I. Determination of intrinsic rate constants

A process for ironmaking was proposed consisting of the combination of a rotary hearth furnace and a bath smelter employing wood charcoal as reductant and energy source. This article examines reactions in composites of iron oxides and carbon at elevated temperatures in conditions developed to minimize the influence of mass and heat transfer to the overall rates. A combined reaction model considering the steps of carbon oxidation and reduction of the iron oxides was developed allowing the measurement of rate constants for carbon oxidation and wustite reduction to be used in a comprehensive pellet model developed in Part II of the current article. This analysis showed that wustite reduction can have a significant effect on the overall rate of reduction in composites at high temperatures or in the presence of large excess of carbon. Rate constants measured for graphite showed that graphite is as reactive as wood charcoal, possibly due to the catalysis of graphite or its higher temperature dependence. The poisoning of carbon surfaces by CO is less significant than anticipated from works of previous authors.     

The interfacial kinetics of the reaction of CO2 with nickel: Part II. the decarburization of liquid nickel

The kinetics of decarburization of liquid nickel in CO₂-CO mixtures have been studied at 1400 and 1500°C, using the experimental arrangement of the impinging jet. At carbon concentrations above about 1 wt pct, pressures of CO₂ ⪯ 0.1 atm, and for total gas flow-rates above about 40 l/min (STP) impinging on a metal surface of 2.08 cm², it is concluded that the interfacial reaction step controls the rate. Comparison with isotope exchange studies indicates that dissociative chemisorption of CO₂ is the rate determining step. Rate constants, based on the nominal surface area, are 1.2 ×10⁻³ and 1.4 × 10⁻³ mol/cm² · s · atm at 1400 and 1500°C, respectively. on leave of absence from the Homer Research Laboratories of the Bethlehem Steel Co.     

Fluid flow in pachuca (air-agitated) tanks: Part I. Laboratory-scale experimental measurements

Gas-agitated reactors are used in a number of process industries, including the metallurgical industry, where they are known as “Pachuca” tanks. In spite of the fact that it is the circulation (i.e., velocity and turbulent kinetic energy distribution) within these tanks that governs the main process requirements,i.e., mass transfer and particle suspension, very little attention has been paid to the question of fluid flow. In the present study, velicity measurements made in a laboratory-scale Pachuca tank have suggested the importance of the fluid flow pattern in governing the performance of air-agitated tanks and have shed some light on the efficient operation of these tanks. Full-center-column tanks with large tank height-to-diameter ratios have a “near-stagnant zone” in the lower section of the annulus. The stagnant zone is a region of low turbulent kinetic energy and is undesirable, since it costs energy and is likely to provide very little in return in terms of mass transfer. An increase in the draft tube diameter, for a given tank diameter, leads to higher velocity and turbulence levels in the annulus, which, in turn, should promote mass transfer. Free-airlift tanks seem to be more vigorously agitated than full-center-column tanks. The present study shows that operating a full-center-column Pachuca tank with the liquid surface at or below the same level as the draft tube top would be disadvantageous in terms of particle suspension and mass transfer and also illustrates that it is erroneous to correlate the turbulence on the liquid surface with the turbulence level within the tank.