The mass transfer kinetics of niobium solution into liquid steel

Niobium cylinders were immersed into liquid steel and their mass-transfer rates measured under dynamic conditions. The cylinders were suspended from a load cell, and the apparent weight of the cylinder as well as temperatures at various locations in the immersed specimens were measured continuously during immersion and recorded with a data acquisition system. From the weight measurements, the mass-transfer rate was deduced. A steel shell period and free dissolution period were identified. During the steel shell period, a shell of frozen steel wraps the cylinder following its initial immersion. When niobium cylinders were immersed into liquid steel with low superheat, a reaction was detected at the steel shell/niobium interface. This reaction took place during the later stages of the steel shell period. The intermetallic compounds Fe₂Nb and Fe₂Nb₃ were identified as reaction products. The mass transfer which takes place during the free dissolution period was found to be exothermic, and the rate-controlling step was found to be liquid phase diffusion through a mass-boundary layer. The apparent activation energy was estimated to be 172 (±15) kJ/mol. This uncommonly high value of apparent activation energy is best explained on the basis of the macroexothermic reaction which takes place as niobium dissolves into liquid steel. Formerly Postdoctoral Fellow, Department of Metallurgy and Materials Science, University of Toronto     

The Dissolution of titanium in liquid steel

The dissolution kinetics of solid cylinders of titanium in liquid steel has been studied. Two separate dissolution periods were identified: asteel shell period anda free dissolution period. During thesteel shell period a customary shell of frozen steel encased the cylinder following its initial immersion. Premature internal dissolution then began as a result of liquid eutectic forming at the inner steel shell boundary.This phenomenon triggered an exothermic dissolution of the inner surface of the steel shell. The net result was to shorten considerably shell melting times. In the second,or free dissolution period it was found that the surface temperature of the exposed titanium cylinder rose above the bath temperature as a result of continued exothermic dissolution phenomena. This caused the dissolution process to become self-accelerating. A simplified mathematical model of the process has been developed to describe the complex coupled heat and mass transfer phenomena involved.     

The Exothermic Dissolution of 50 wt.% Ferro-Silicon in Molten Steel

Abstract

The kinetics of dissolution of solid cylinders of 50 wt.% ferro-silicon in liquid steel has been studied. It is shown that the customary frozen shell of steel encases the ferro-alloy cylinder following its initial immersion. Premature internal melting of the cylinder then begins as a result of liquid eutectic of Fe₂Si composition forming at the inner steel shell boundary. This phenomenon triggers an exothermic dissolution and erosion of the inner steel shell. The outer boundary of the steel shell concurrently melts back as a result of convective heat transfer from the steel bath. The net result is considerably shortened shell dissolution times in comparison with more conventional ferro-alloy/steel systems.

A simplified mathematical model of the process has been developed to describe the coupled heat and mass transfer phenomena involved, and this is shown to provide a good quantitative representation of the processes studied.

Résumé

La cinétique de dissolution de cylindres de ferro-silicium 50% solide dans l'acier liquide a été étudiée. Il est montré que l'habituelle coquille d'acier solidifié enferme le cylindre de ferro-alliage des les premiers instants de son immersion. Une fusion interne prématurée du cylindre se produit alors par suite de la formation d'un liquide eutectique de composition Fe₂Si sur la face interne de la coquille d'acier. Ce phenomène déclanche une dissolution exothermique et une érosion de l'interieur de la coquille d'acier. La face externe de la coquille d'acier refond simultanement par suite du transfert convectif de chaleur depuis le bain. Il en resulte des temps de dissolution de la coquille considérablement plus courts que ceux obtenus avec systémes ferro-alliage/acier plus classiques.

Un modèle mathématique simplifié du procédé a été développé pour décrire les phénomènes de transfert de masse et de chaleur couplés qui se produisent et il semble fournir une bonne représentation quantitative des processus étudiés.     

Interfacial Reactions During Titanium Dissolution in Liquid Iron: A Combined Experimental and Modeling Approach

The dissolution behavior of Ti in liquid Fe has been investigated experimentally and theoretically within the framework of inclusion formation in steel. In the experimental study, Ti cylinders have been immersed into liquid Fe and, subsequently, water-quenched. Macroscopic observation of quenched samples shows the initial solidification of an Fe shell around the Ti. Microstructural analysis of the Fe-Ti interfacial area with a scanning electron microscope equipped with an energy dispersive X-ray spectrometer reveals that a reaction zone develops in a three-step process: formation of a first liquid eutectic layer rich in Ti, formation of a second eutectic layer rich in Fe, and then mixing of both layers. The reaction zone grows in thickness up to 40 pct of the original sample radius and dissolves both parts of the Ti sample and the Fe shell. A simplified, one-dimensional, implicit finite volume model has been used to describe these phenomena theoretically. Good qualitative agreement is achieved between experiment and model. The model has been used to estimate the influence of original addition size, preheating, convection, and superheating on the required melt-back time.     

Mathematical Simulation of Process Parameters in the case of Rapid Solidification of Steel

Rapid solidification of steel was investigated experimentally under laboratory conditions by immersion of cold copper rods in steel baths. For a better understanding of the process parameters during rapid solidification, an explicit finite difference model was employed. In the calculation, a coefficient of heat transfer between a frozen steel shell and solid copper of α = 40 [KW/m²·K] is assumed in good agreement with experimental data derived from temperature measurements. The solidification parameters such as local time of solidification (LST), local time of the superheat reduction (LShRT), local solidification and cooling rates (LSR, LCR) and local heat flux density of solidification and superheat reduction (LSHFD, LShRHFD) can be calculated using the developed model, in dependence on the processing conditions. This influence of processing parameters, such as steel bath superheat, steel bath material, immersed body material, initial temperature of immersed body and immersed body geometry, were the subject of intensive investigations.     

Deoxidatlon rate of copper droplet levitated in Ar-H2 gas stream

Levitated copper droplets, 5 mm in diameter with initial oxygen contents of 0.036 to 1.9 wt pct, were deoxidized at about 1970 K in an Ar-H₂ gas stream. The Ar-H₂ gas mixture having hydrogen partial pressure less than 4 kPa was introduced into a silica reaction tube of 11-mm ID at gas flow rates up to 2 x 10^-4 Nm³s^-1. The effects of initial oxygen content of the droplets, hydrogen partial pressure, and gas flow rate on the deoxidation process were examined. A mixed control model for the deoxidation rate involving both gas and liquid film mass-transfer resistances was combined with a thermodynamic relationship for the dissolved species in molten copper. The value of 2 × 10^-4 m 73x00D7; s^-1 was assigned to the liquid film mass-transfer coefficient of dissolved oxygen throughout all experimental conditions. Under the experimental conditions of low initial oxygen content and high hydrogen partial pressure, the liquid film mass-transfer resistance was significant. When a droplet of high initial oxygen content was deoxidized, transition phenomena from gas to liquid film mass-transfer control were noticed in the later stage of reaction. It was deduced from the present model that the accumulation of dissolved hydrogen was indispensable to these phenomena. Formerly Student     

Dissolution of solid antimony in molten bismuth under static conditions

The dissolution of solid antimony in molten bismuth was studied under static and isothermal conditions using the reaction couple and immersion methods between 623 and 773 K. The dissolution of antimony under the antimony upper position, using reaction couple method, was governed by diffusion, and the diffusion coefficient of antimony in molten bismuth was obtained as follows:D _L = 2.08 X 10-⁸exp(-ll kJ mol^-1/RT), (m²/s). The dissolution of antimony under the immersed condition was governed by the natural convection resulting from the density difference in the melt, and the dissolving antimony was distributed uniformly by natural convection in molten bismuth. The apparent activation energy for the dissolution of solid antimony in molten bismuth was the same as that for the diffusion of antimony in the melt.     

Mass transfer between solid and liquid in vessels agitated by bubble plume

Mass transfer from solid benzoic acid cylinders to a gas-stirred aqueous bath has been investigated both theoretically and experimentally. Two typical gas injection configurations, the conventional central injection and the C.A.S. (C omposition A djustment by S ealed Argon Bubbling) were employed and the rates of dissolution of the acid compacts at various locations in the bath were measured at different gas-flow rates. These demonstrated that the mass-transfer rates are the highest in the two-phase region, while elsewhere in the bath, these were found to be practically identical. Furthermore, mass-transfer rates at the corresponding locations were found to be relatively greater for the conventional central injection than those for the C.A.S. configuration. Distribution of velocity and turbulence intensity in the vessel were computed theoretically using a previously reported calculation procedure. Based on these, values of various relevant dimensionless numbers were estimated so as to assess the adequacy and effectiveness of heat and mass-transfer correlations reported in the literature. These, however, did not fit with the present experimental observations. For this reason, a new correlation has, therefore, been proposed and it is shown that the experimental data can be described reasonably well by the equation Sh = 0.73 (Re_loc,r)^0.57 (T_i)^0.32 (Sc)^0.3. This correlation also embodies more plausible definitions of Reynolds number (Re_loc,r) and the turbulence intensity (Ti) in contrast to those reported in literature, since it has been derived using the local resultant mean velocity and the local fluctuating velocity in the fluid.     

Nonaqueous Phase Liquid Pool Dissolution as a Function of Average Pore Water Velocity

Bench-scale reactor experiments were performed to study the dissolution of a binary naphthalene-in-nonane mixture nonaqueous phase liquid (NAPL) pool over a wide range of average pore water velocities, vx (≈0.1–60 m/day). Experimental NAPL pool dissolution flux values were determined using a steady-state mass balance approach. The experimental flux data were compared to model predictions made assuming either local equilibrium or mass-transfer limited conditions. The local equilibrium model could describe the trends in the average effluent concentration and dissolution flux with 0.110?m/day. Data determined to be under mass-transfer limited conditions were fit to the nonequilibrium model to estimate values for an overall mass-transfer coefficient. The calculated overall mass-transfer coefficients had an average value of 0.407 m/day and showed no correlation with vx, probably due to mass-transfer resistance becoming dominated by the diffusional resistance in the NAPL. These results suggest that the nonequilibrium approach is better suited for describing high velocity (vx>10?m/day) dissolution of multicomponent NAPL pools, and that flushing of groundwater at very high velocities may not be an effective approach for enhancing NAPL-pool dissolution flux.     

Mixing and solid-liquid mass-transfer rates in a creusot-loire uddeholm vessel: A water model case study

The process of mixing and solid-liquid mass transfer in a one-fifth scale water model of a 100-ton Creusot-Loire Uddeholm (CLU) converter was investigated. The modified Froude number was used to relate gas flow rates between the model and its protoype. The influences of gas flow rate between 0.010 and 0.018 m³/s and bath height from 0.50 to 0.70 m on mixing time were examined. The results indicated that mixing time decreased with increasing gas flow rate and increased with increasing bath height. The mixing time results were evaluated in terms of specific energy input and the following correlation was proposed for estimating mixing times in the model CLU converter: T _mix=1.08Q ^−1.05 W ^0.35, where Q (m³/s) is the gas flow rate and W (tons) is the model bath weight. Solid-liquid mass-transfer rates from benzoic acid specimens immersed in the gas-agitated liquid phase were assessed by a weight loss measurement technique. The calculated mass-transfer coefficients were highest at the bath surface reaching a value of 6.40 × 10⁻⁵ m/s in the sprout region. Mass-transfer coefficients and turbulence parameters decreased with depth, reaching minimum values at the bottom of the vessel.     

Kinetics of scrap melting in liquid steel

The melting rate of steel bars with various sizes, shapes, and initial temperatures in a 70 kg liquid steel bath (1650 °C) was measured to investigate the kinetics involved in steel scrap melting. Our measurements revealed that a solidified shell was formed around the original bar immediately after it was immersed into the liquid steel. This shell and an associated interfacial gap generated between it and the original bar were found to be critical to the melting kinetics. We also found that the total melting time decreased linearly with increasing initial bar temperature. The melting process was simulated using a two-dimensional phase-field model that considered heat convection with a constant heat-transfer coefficient. Our simulations were in good agreement with our experiments and showed that the heat conduction associated with the interfacial gap was one of the most important physical aspects controlling the melting of steel scrap.     

Dissolution of reactive strontium-containing alloys in liquid aluminum and A356 melts

The dissolutions of commercial purity strontium and a 90 pct Sr-10 pct Al alloy in liquid aluminum and A356 alloys has been investigated. The dissolutions of these alloys was found to be accompanied by the formation of various intermetallic compounds, the type of which depends on the chemistry and temperature of the melt. Additions at low melt temperatures resulted in the exothermic formation of those intermetallics that have the lowest strontium contents, as seen in the relevant phase diagram,i.e., Al₄Sr in liquid aluminum and SrAl₂Si₂ in liquid A356. Due to low reaction rates at these temperatures, these intermetallics formed as dispersed particles that could easily dissolve in the melt, yielding high recoveries. At high melt temperatures, the associated chemical reactions yielded, as products, the higher strontium intermetallics, which formed with little or no exothermicity. These compounds were observed to be scarcely soluble in the melt, resulting in low recoveries. The dissolution time of these alloys were found to show good agreement with calculated values based on a two-stage model comprising an initial exothermic reaction period and a subsequent free dissolution period. In general, the high-strontium alloys were determined to be efficient at low melt temperatures of 675°C to 700°C. These reactive alloys were observed to form thick surface scales in air, which, in the case of commerical purity strontium, proved to be detrimental to dissolutions because they formed a barrier between the solid and the liquid.     

Desulfurization kinetics of molten copper by gas bubbling

Molten copper with 0.74 wt pct sulfur content was desulfurized at 1523 K by bubbling Ar-O₂ gas through a submerged nozzle. The reaction rate was significantly influenced not only by the oxygen partial pressure but also by the gas flow rate. Little evolution of SO₂ gas was observed in the initial 10 seconds of the oxidation; however, this was followed by a period of high evolution rate of SO₂ gas. The partial pressure of SO₂ gas decreased with further progress of the desulfurization. The effect of the immersion depth of the submerged nozzle was negligible. The overall reaction is decomposed to two elementary reactions: the desulfurization and the dissolution rate of oxygen. The assumptions were made that these reactions are at equilibrium and that the reaction rates are controlled by mass transfer rates within and around the gas bubble. The time variations of sulfur and oxygen contents in the melt and the SO₂ partial pressure in the off-gas under various bubbling conditions were well explained by the mathematical model combined with the reported thermodynamic data of these reactions. Based on the present model, it was anticipated that the oxidation rate around a single gas bubble was mainly determined by the rate of gas-phase mass transfer, but all oxygen gas blown into the melt was virtually consumed to the desulfurization and dissolution reactions before it escaped from the melt surface.     

Thermodynamics and kinetics of coal gasification in a liquid iron bath

The thermodynamics and kinetics of the Molten-Iron-Pure-Gas (MIP) process used for coal gasification have been analyzed. In the MIP process, oxygen, fine-grained coal, and fluxes are injected into a liquid iron bath to produce a high temperature gas consisting of CO and H₂ plus a liquid basic slag. The sulfur is transferred from the coal to this slag. Computer calculations bearing in mind test conditions were used to determine equilibrium conditions as well as mass and energy balances; these indicated that the MIP process is technically feasible. The kinetics of the gasification process have been investigated by analyzing and assessing the basic reactions for a bottom-blowing MIP reactor. A comparison of all relevant reactions reveals that the dissolution of carbon in iron is the rate-determining step of the process. The bath turbulence induced by the injected gas and by the product gas results in intense mixing and dispersion of the reactants and their intermediate products. These phenomena create extremely large mass-transfer surfaces and extend the retention time of the reactants in the liquid iron bath. This results in high conversion rates relative to the volume of the MIP reactor.     

Rate of dissolution of solid nickel in liquid tin under static conditions

The dissolution kinetics of solid nickel in liquid tin have been investigated under static conditions. The cylindrical nickel specimens were immersed in liquid tin over the temperature range of 551 to 803 K in the reaction time range of 0.9 to 6.0 ks. A natural convection model for mass transfer and the dissolution rate equation derived by considering intermetallic compound layer formation were used to interpret the experimental dissolution data. A larger dissolution at the upper part of specimen causing natural convection and an intermetallic layer formation with a linear relationship at solid/liquid interface occurred. Below 628 K, the dissolution rate appears to be controlled by chemical reaction of an intermetallic compound layer. At mid-range temperatures (of 681 K), the dissolution process was governed by a mixed control mechanism involving diffusion in liquid tin and chemical reaction of the intermetallic compound layer. At temperatures above 735 K, the rate seems to be controlled by diffusion across a concentration boundary layer in liquid tin. The formation of an intermetallic compound layer did not interfere with the dissolution.     

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.     

Heating and melting of wustite pellets in liquid slag

The melting behaviour of the metallized, porous wustite pellets immersed in liquid slag, as well as the influences of the metallization ratio, pre-heating temperature of the pellet and slag temperature were examined in this work by means of an X-ray imaging system. The internal structure of the pellets after having been immersed in slag was checked by optical microscope and EPMA. The adoption of digital image processing improved the image analysis dramatically and, as a result, some important phenomena, such as solid slag shell forming and melting, slag penetration, wustite component dissolution as well as the influence of experimental conditions, were quantified.     

Phase Evolution During the Liquid-Phase Bonding of Zirconium and Austenitic Stainless Steel with Zinc Insertion

Liquid-phase bonding experiments were performed at 1073?K (800?°C) between ZIRCALOY-2 and type 316 austenitic stainless steel by inserting zinc as an interlayer. The evolution of the microstructure at the interface was studied and the formation of various phases was detected. On the zirconium side, the very rapid formation of Zn₃Zr was detected, whereas on the steel side, an unexpectedly large amount of the base austenitic steel was observed to react with liquid Zn. The reacted iron solidified into a nickel-poor ferritic phase containing around 10?mol?pct?zinc, which grew into the austenite accompanied by a formation of a zinc-rich phase containing nickel. The reaction stopped when the zinc-rich phase reached saturation with a nickel content between 20 and 25?mol?pct. Thermodynamic calculations showed that the addition of nickel to liquid zinc greatly decreases the free energy of the liquid phase, thus enabling a large stability range for the ferrite?+?liquid zone and reducing the stability range of the austenite. The primary equilibrium between the austenite and the liquid phase is thus metastable, and thus, the austenite transforms into ferrite and a high-nickel-content liquid. The transformation front then progresses until ternary equilibrium is reached between austenite, ferrite, and the zinc-rich phase.     

Experimental Evaluation of the Dissolution Rates of Ti and FeTi70 in Liquid Fe

During secondary steelmaking, improving alloy yield and engineering inclusion content require understanding and quantification of the alloy distribution in the melt. When additions are dropped in the melt, a steel shell solidifies around them. After this shell has melted, the alloy is spread in the melt. The influence of process parameters on the duration of the shell period for Ti and FeTi70 additions has been experimentally evaluated. For Ti, the melt temperature and the initial addition size were varied and for FeTi70, only the melt temperature was varied. By continuously measuring the apparent weight of submerged samples with a load cell, the shell period and the amount of molten alloy within the shell were determined. The shell period increases at lower superheats and for larger sample sizes. For a certain size of Ti additions, the molten content within the shell increases with increasing shell period. The importance of this period, relative to the total dissolution time, increases at lower superheats. All investigated FeTi70 samples may melt completely within the shell. While the shell period lasts longer for FeTi70 than for the corresponding Ti samples, this fast internal melting yields a net reduction in total dissolution time.