Numerical modelling of fluid flow and desulphurisation kinetics in an argon-stirred ladle furnace

A full-scale, three-dimensional, transient CFD modelling approach capable of predicting the three-phase fluid-flow characteristics and desulphurisation behaviour in an argon-stirred ladle was developed. The model can accurately predict the molten steel flow and slag eye behaviour. The predicted sulphur content in ladle as a function of time agrees well with the experimental data. The effects of the initial sulphur content, the gas flow rate and the slag layer thickness on the desulphurisation efficiency were also investigated. The predicted results show that the desulphurisation efficiency improves with the increase of the initial sulphur content, the gas flow rate and the slag layer thickness. Higher gas flow rate can improve the slag–steel interaction, which, in turn, helps improving the desulphurisation rate. The thinner the slag layer, the larger the slag eyes and the smaller the interfacial area between the slag and steel phases. The consequence is the decrease in the desulphurisation rate.     

Surface Properties of Manganese – Tin Liquid Alloys

We used the sessile drop method to study the temperature and concentration dependences of the density and surface tension of melts in the manganese – tin system. The density polytherms are linear dependences which are consistent with those calculated by the additivity rule for the specific volumes of the pure components. The temperature dependences of the surface tension are also linear and their temperature coefficient changes from negative values for tin-rich melts to positive values for melts with high manganese content. The surface tension isotherm is satisfactorily described by the Popel-Pavlov equation. We calculated the composition of the surface layer of the studied melts as a function of the composition of the bulk phase. We have shown that the model for the surface layer of melts in the Mn – Sn system at 1300°C is close to monolayer.     

New insights into influencing variables of water atomisation of iron

Abstract

Trace amounts of surfactants have an acute influence on measured surface tension of melts and may influence viscosity. A water atomisation experiment was performed to investigate if variations of these elements could affect quality. Effects of water pressure, melt superheat, and sulphur content, iron scrap oxygen content, and aluminium content were studied. Responses studied were particle size distribution, apparent density, flow, powder chemistry, morphology, green density, and dimensional change. A large sulphur addition reduced the particle size, as a result of a reduction of surface tension, but the largest effect came from changing water pressure. Higher water pressures also resulted in powders with lower apparent density, lower flowrate, and reduced swelling during sintering. An empirical water atomisation model is proposed. Aluminium additions reduced the powder size standard deviation and increased the carbon content of the powder. A reduced powder size standard deviation was seen also for melts with raised superheating.     

Effect of carbon and sulphur on the surface tension of molten iron

Surface tensions of Fe‐4%C‐S alloys were measured at 1623 and 1823 K using the sessile drop technique. Thermodynamic models based on Butler's equation for surface tension of liquid alloys have been compared with experimental results. Calculated values are found to be in good agreement with those of the experimental data of the system. At the same sulphur activity, the effect of carbon on the surface tension of Fe‐C‐S alloys was found to extrude only when the sulphur content was less than 0.005 %.     

Volume and Surface Properties of a Bismuth-Containing Separating Nickel Melt

The influence of a bismuth impurity on the properties of solid and liquid alloys in the concentration range that obeys Henry’s law is considered. The structural and physicochemical properties, specifically, the density and the surface tension, of real melts are studied on relatively pure metals. The changes in the properties of the melts are estimated from changes in the temperature dependences of the density and the surface tension upon heating and cooling and in the concentration dependences of these parameters at a constant temperature. These dependences exhibit a correlation between the volume and surface properties of the melts: the density and the surface tension increase or decrease simultaneously. The introduction of bismuth in the nickel melt is accompanied by the appearance of a relatively strong compression effect (i.e., a decrease in the melt volume). At a certain bismuth content in the melt, the compression effect weakens because of the appearance of an excess phase or its associates and melt separation.     

Thermodynamic and kinetic aspects of the desulphurisation reaction in secondary metallurgy

The desulphurisation of liquid steels with a refining ladle top slag is one of the most important processes of secondary metallurgy. But the process control is even now based on empirical rules. Deviations from the sulphur contents aimed at can often be observed in practice. An improvement of the process control supported by knowledge of the thermodynamics and kinetics of the desulphurisation reaction is of great importance, especially for the production of steels with very low sulphur contents. To investigate the removal of sulphur, operational trials were carried out in 185 t steel ladles. In the trials the stirring gas flow rate, the pressure on the bath surface and the steel composition were varied as process parameters. The obtained results show that ladle slags saturated with lime have a high sulphide capacity. The desulphurisation rate of steel melts increases if the specific stirring energy is enhanced by increasing the gas flow rate or reducing the pressure on the bath surface. In the case of production of Al/Si‐killed steel melts, a desulphurisation degree above 90 % can be obtained by a vacuum treatment within 10 minutes.     

Density and surface tension of a concentrated lead melt in nickel

The influence of a lead impurity on the properties of metallic melts in the composition range that obeys Henry’s law is studied. The formation of the structural and physicochemical properties of real concentrated melts can be traced from changes in the temperature and concentration dependences of structure-sensitive properties, namely, density and surface tension. The surface properties of a solution depend on its volume properties and differ from them in enhancement effect. The lead saturation of the nickel melt is found to be accompanied by a compression effect (decrease in the melt volume), which is enhanced to a certain lead concentration. As this concentration is exceeded, the compression effect weakens because of volume separation and the appearance of an excess lead phase. As the lead content in a nickel base increases, the surface tension decreases, a second phase forms, and the melt undergoes separation.     

NdF_3-LiF-Nd_2O_3体系表面张力数学模型的研究

用拉筒法研究了低NdF3 浓度下NdF3-LiF体系的表面张力 .采用二因子二次回归正交设计 ,得出表面张力 (σ)与NdF3 含量 (C)、温度 (t)的回归方程 ,讨论了NdF3含量、温度对表面张力的影响 ,并考察了加入Nd2 O3 对体系表面张力的影响     

The surface tensions and foaming behavior of melts in the system CaO-FeO-SiO2

The surface tensions of melts in the system CaO-FeO-SiO₂ have been measured in the temperature range 1573 to 1708 K using the hollow cylinder technique. The iron oxide content was maintained constant at 30 wt pct and the CaO/SiO₂ wt pct ratio was varied in the range 0.43 to 1.5. Surface tension increases with increasing basicity and with decreasing temperature. The data were used to test published correlations of slag foaming indexes with surface tension and viscosity. Foam life increases with increasing viscosity and with decreasing surface tension. 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.     

Volume and surface properties of nickel melts containing bismuth impurity determined from the density and the surface tension

The physicochemical and the structural properties of the nickel-based melts with a low (impurity) bismuth content, which, like lead, is one the most harmful impurities in cast high-temperature nickel-based superalloys, are studied using the surface tension and the density. It is found that the surface tension increases as the bismuth impurity content varies from zero to 0.05 wt %, which corresponds to transition of excess substance from the surface into the volume.     

Composition and thermodynamics of the surface layer of binary melts based on iron,cobalt, and nickel

The composition, thickness, thermodynamic activities of components, Gibbs energy and excess energy of formation of the surface layer on binary melts of iron, cobalt and nickel-based alloys with different interatomic interactions were evaluated with the aid of data from the literature and the authors measurements of the concentration dependence of the surface tension and molar volume.     

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.     

Formation of metallic phase on reducing-gas injection in multicomponent oxide melt. Part 2. Density and surface properties

The density and surface tension of melts of ferronickel (0–100% Ni) and oxidized nickel ore are measured by the sessile-drop method, as well as the interface tension at their boundary in the temperature range 1550–1750°C. The composition of the nickel ore is as follows: 14.8 wt % Fetot, 7.1 wt % FeO, 13.2 wt % Fe₂O₃, 1.4 wt % CaO, 16.2 wt % MgO, 54.5 wt % SiO₂, 4.8 wt % Al₂O₃, 1.5 wt % NiO, and 1.2 wt % Cr₂O₃. In the given temperature range, the density of the alloys varies from 7700 to 6900 kg/m³; the surface tension from 1770 to 1570 mJ/m²; the interface tension from 1650 to 1450 mJ/m², the density of the oxide melt from 2250 to 1750 kg/m³; and its surface tension from 310 to 290 mJ/m². The results are in good agreement with literature data. Functional relationships of the density, surface tension, and interphase tension with the melt temperature and composition are derived. The dependence of the alloy density on the temperature and nickel content corresponds to a first-order equation. The temperature dependence of the surface tension and interphase tension is similar, whereas the dependence on the nickel content corresponds to a second-order equation. The density and surface tension of the oxide melt depend linearly on the temperature. The results may be used to describe the formation of metallic phase when carbon monoxide is bubbled into oxide melt.     

Applicability of butler’s equation in interpreting the thermodynamic behavior of surfaces and adsorption in Fe-S-O melts

Expressions for various second-order derivatives of surface tension with respect to composition at infinite dilution in terms of the interaction parameters of the surface and those of the bulk phases of dilute ternary melts have been presented. A method of deducing the parameters, which consists of repeated differentiation of Butler’s equations with subsequent application of the appropriate boundary conditions, has been developed. The present investigation calculates the surface tension and adsorption functions of the Fe-S-O melts at 1873 and 1923 K using the modified form of Butler’s equations and the derived values for the surface interaction parameters of the system. The calculated values are found to be in good agreement with those of the experimental data of the system. The present analysis indicates that the energetics of the surface phase are considerably different from those of the bulk phase. The present research investigates a critical compositional range beyond which the surface tension increases with temperature. The observed increase in adsorption of sulfur with consequent desorption of oxygen as a function of temperature above the critical compositional range has been ascribed to the increase of activity ratios of oxygen to sulfur in the surface relative to those in the bulk phase of the system.     

Effect of Exogenous Zirconia Nanophases on the Structural Properties of the Sulfur- and Tin-Containing Nickel Melts

The surface tension and the density of the nickel melts with introduced ZrO₂ nanoparticles are studied by the sessile drop method using a digital camera and computer processing of images. The revealed differently directed effects of nanoparticles on the surface tension in the Ni–Sn and Ni–S systems points to a change in the structure of the melt–gas surface layer. The nanoparticles are shown to affect the adsorption of surfactants, and the surface layer is likely to consist of adsorbed Ni + (ZrO₂–surfactant) ensembles. The ZrO₂ content in a metal is determined using the technique of separate determination of the zirconium content dissolved in a metal and zirconium in the form of ZrO₂. It was found that, at 0.10 wt % ZrO₂ initially present in a metal, 0.021–0.031 wt % ZrO₂ are retained in samples; that is, about 70 rel % ZrO₂ are removed to the interface in the form of ensembles. Auger spectroscopy analysis of the Ni–Sn–ZrO₂ surface film detected 5–10 rel % Zr in the surface layer.     

Reduction of Chromite in Liquid Fe-Cr-C-Si Alloys

The kinetics and the mechanism of the reduction of chromite in Fe-Cr-C-Si alloys were studied in the temperature range of 1534 °C to 1702 °C under an inert argon atmosphere. The rotating cylinder technique was used. The melt consisted of 10 and 20 wt Pct chromium, the carbon content varied from 2.8 wt Pct to saturation, and the silicon content varied from 0 to 2 wt Pct. The rotational speed of the chromite cylinder ranged from 100 to 1000 rpm. The initial chromium to iron ratios of the melts varied between 0.11 and 0.26. In Fe-C melts, the effect of rotational speed on the reduction of chromite was very limited. Carbon saturation (5.4 wt Pct) of the alloy caused the reduction to increase 1.5 times over the reduction observed in the unsaturated (4.87 wt Pct) alloy at a given rotational speed. The addition of chromium to the carbon-saturated Fe-C alloy increased the reduction rate. The addition of silicon to the liquid phase increased the reduction rate drastically. The reduction of chromite in Fe-Cr-C melts is hindered because of the formation of, approximately, a 1.5-mm-thick M₇C₃-type carbide layer around the chromite cylinders. This carbide layer did not form when silicon was present in the melt. It was found that the reduction rate is controlled by the liquid-state mass transfer of oxygen. The calculated apparent activation energies for diffusion were 102.9 and 92.9 kJ/mol of oxygen in the Si-O and C-O systems, respectively.     

Laboratory experiments and process modelling of the melting and dissolution of low‐density ferro‐molybdenum in steel melts

A low‐density sintered ferro‐molybdenum is presented as a new alloying agent. The paper describes laboratory studies about the dissolution in liquid iron of briquettes of this alloy and of classical high density ferro‐molybdenum pieces. It presents further a mathematical model of the melting and dissolution process. During dissolution, an approximately 1mm thick layer infiltrated by melt forms on the particle surface. The infiltrated melt solidifies in the plane where the temperature has reached the eutectic temperature in the system iron‐molybdenum. Internal dissolution of alloy material in the layer is weak, which means that the dissolution proceeds almost exclusively from the outer surface of the briquette. Dissolution rate increases with sinking briquette density. The lower molybdenum content per volume in the briquettes which is proportional to the density has the effect that the liquidus concentration and the liquidus temperature at the solid‐liquid interface decrease in comparison with compact material. This reduces the mass transfer rate and increases the heat transfer rate. The effect is a faster movement of the interface. Below a critical density of approximately 5200 kg/m³ for the alloy considered, the molybdenum concentration on both sides of the interface becomes equal. From this moment, the alloy is liquefied solely by melting of the moving interface. Mass transfer from the interface gets negligible and distribution of the molten alloy into the bulk melt takes place only by the outer mixing process. From the described behaviour it follows, that below the critical density the melting rate is solely determined by heat transfer. Since heat transfer is faster than mass transfer, melting of alloys with reduced density is correspondingly accelerated. The extremely slow dissolution rates of high melting alloys can thus, be overcome by giving the alloy a certain porosity. The mathematical process model describes the phenomena quantitatively.     

Flow and Heat Transfer Performance of Slag and Matte in the Settler of a Copper Flash Smelting Furnace

Simulation of two‐phase flow in a copper flash smelting settler with simultaneous tapping of slag and matte is carried out using Eulerian‐Eulerian two‐fluid model and the flow and heat transfer performance of slag and matte is examined. Detailed velocity vectors, temperature and volume fraction distributions are obtained. The results show that the small copper droplets will be suspended in the slag. When the droplet diameter is large enough, the slag and matte layer are clearly formed and the dispersion layer between the slag and matte layers becomes thinner at larger droplet sizes and its thickness remains nearly unchanged when the copper droplet diameter is larger than about 500 μm.     

<Emphasis Type="Italic">In Situ</Emphasis> Observation of the Formation and Interaction Behavior of the Oxide/Oxysulfide Inclusions on a Liquid Iron Surface

Inclusion formation and its behavior at the early stage of deoxidation and alloying operation has a considerable influence on steel cleanliness. In the present work, steel alloying, such as Mn, Al, FeSi, and FeMn additions to the liquid steel with different oxygen and sulphur content was simulated with a confocal scanning laser microscope combined with a special addition device. The inclusion formation and the inclusion interaction behavior immediately after the alloying and/or deoxidation were observed in situ. The inclusions were characterized based on both the in situ observation and the quenched sample. The effect of the sulphur and oxygen content in liquid iron, as well as that of the deoxidant type on the formation of oxides/oxysulfides is discussed taking consideration of the thermodynamics of the system. The inclusion behavior on the liquid iron surface, i.e., the interaction after its formation, the dissolution during the high temperature iso-thermal holding, and growth during the cooling was investigated. The dissolution of Mn(O,S) inclusions at 1843 K (1570 °C) was found to be driven by Mn diffusion through the inclusion/liquid iron boundary layer, and its growth during cooling was significantly affected by Marangoni flow.     

高炉内铁?焦界面的渗碳润湿行为研究

高炉内铁水渗碳过程是影响冶炼效率及未饱和铁水对炉缸炉衬侵蚀的重要因素。本文通过高温真空润湿性测试装置模拟了高炉炉缸区的铁水渗碳反应,分析了不同碳质量分数（3.8%、4.3%、4.8%）的Fe?C熔体与质量分数为99.9%的石墨基体在高温下界面间的润湿反应,同时利用扫描电镜（SEM）和能谱仪（EDS）研究了渗碳界面的微观形貌及渗碳距离。结果表明:界面接触角随着Fe?C熔体中碳含量的增加而变大;熔化过程中,接触角随着反应时间延长而减小,并最终趋于稳定;4.8%碳质量分数的Fe?C熔体中由于含碳量已至饱和,处于不润湿状态。扫描电镜分析显示,Fe?C熔体与石墨基体的接触界面形成了“球帽状”凹陷,凹陷半径与体积随碳含量的增加而减小。能谱线扫描分析显示,随着Fe?C熔体中初始碳含量的增加,石墨基体中的碳素溶解量减少,渗碳效果变差,良好的润湿性有利于碳的传质。通过计算表面能发现,石墨基体中碳素溶解进入Fe?C熔体后,有效减小了两者之间的表面能,使得表面张力减小,接触角在熔化期间递减。