The influence of gas phase resistance on mass transfer to a liquid metal

When a dilute gas is absorbed in a liquid metal in which Sievert’s law is obeyed, the gas phase resistance is likely to be significant. The influence of gas phase resistance will de-pend on the concentration of the transferred species in both the gas and the liquid as well as on the mass transfer coefficients in both the gas and liquid phases. An overall mass transfer coefficient is defined by: n″(X)₁=k_ov√pb -cb and equations are developed relating this to the variables mentioned. Experimental work has been carried out with dilute oxygen jets (p(O₂) = 0.1 and 0.2 atm) blowing onto molten silver, and the results have confirmed that the resistance in the gas phase contributes to the overall resistance. From these results it has been possible to estimate gas phase mass transfer coefficients which range from 1.4 cm/s for a jet momentum of 8000 dyne and lance height of 18.5 cm to 4.8 cm/s for a jet momentum of 56,000 dynes and lance height of 10.5 cm. A. CHATTERJEE, formerly in The John Percy Research Group in Process Metallurgy, Imperial College of Science and Technology, University of London, England     

Drops and bubbles in extractive metallurgy

Drops and bubbles are of great importance to the extractive metallurgist in his attempts to speed up processes by the use of sprays, foams, and jets. In this lecture the ways in which bubbles bring about mass transfer in liquid metals and in slag metal reactions are described. The role of interfacial turbulence is considered together with the effects of bubble size and frequency and the properties of the slag and metal phases. Reactions between drops of metal and flowing gases are analyzed in terms of mass transfer in the reacting phases and of chemical steps at the interface. Recent results obtained on reactions involving metal drops falling through liquids are considered in relation to mass transfer models in which internal circulation plays an important part. The work described reports only one facet of the rapidly developing subject of Process Engineering which ought now to feature prominently in metalurgical education. Dr. F. DENYS RICHARDSON. Professor of Extraction Metallurgy. Department of Metallurgy, Royal School of Mines. Imperial College of Science and Technology, London, England, graduated in chemistry at University College, London, in 1933, and obtained a Ph.D. in physical chemistry in 1936. From 1937 to 1939 he was Commonwealth Fund Fellow at the University of Princeton. From 1946 to 1950 he worked as superintendent chemist at BISRA, building up the work of the chemistry department. He went to Imperial College in 1950 to found the Nuffield Research Group in Extraction Metallurgy and advance the study of chemical metallurgy at high temperatures. He received awards in recognition of his work on the thermodynamic properties of high-temperature systems with special reference to iron- and steelmaking and for his work on high-temperature chemical metallurgy. He was appointed Professor of Extraction Metallurgy at Imperial College in 1957, his objectives there being to establish the department as a research center for chemical and process engineering metallurgy, and to develop a metallurgy course in which these subjects receive as much attention as physical metallurgy. In 1963 he was elected a Fellow of the Metallurgical Society of the AIME, and in 1964 he gave the AIME Howe Memorial Lecture. Professor Richardson delivered the Hatfield Memorial Lecture in 1964, the May Lecture of the Institute of Metals in 1965, and the Wernher Memorial Lecture of The Institution of Mining and Metallurgy in 1967. He was elected a Member of Council of the Iron and Steel Institute in 1967, having been an Honorary Member since 1962. In 1968 he became a Vice-President of the Institution of Mining and Metallurgy. In that year he was also elected a Fellow of the Royal Society and awarded the Bessemer Gold Medal of the Iron and Steel Institute, both honors for his contribution to the understanding of the thermodynamics and kinetics of metallurgical processes. In 1970 the honorary degree of Doktor-Ingenieur was conferred on him by the Technische Hochschale, Aachen. The 1971 Extractive Metallurgy Division Lecture, “Drops and Bubbles in Extractive Metallurgy.” was delivered on Wedresday, March 3, 1971.     

Influence of surface tension-driven flow on the kinetics of oxygen absorption in molten copper

Absorption measurements have revealed that surface tension-driven flow, the Marangoni effect, can enhance the liquid phase mass transfer coefficient by at least an order of magnitude during the top jetting of molten copper with oxygen. This factor has been established by comparing the mass transfer coefficient for copper to the value measured for molten silver which does not display the surface phenomenon under similar jetting conditions. The spontaneous motion on the copper surface was found to arise under conditions of starvation oxygen transfer in the gas phase in which copper oxide forms in a localized region beneath the lance tip and spreads radially outward at about 30 cm s^-1. The mass transfer coefficient was correlated satisfactorily to unsteady state diffusion theory using measured spreading velocities. Sulfur initially present in the copper bath at concentrations of 0.01 to 0.5 pct and silicon at 20 ppm had no effect on the mass transfer coefficient under the spreading conditions. By virtue of the size of the enhancement factor and the lack of influence of dissolved sulfur and silicon on mass transfer, it is suggested that the Marangoni effect may play an important role in the transfer of oxygen into process baths. R. G. BARTON formerly Graduate Student.     

Physical and mathematical models of gas‐liquid fluid dynamics in LD converters

Fluid dynamics of gas‐liquid interactions in a LD converter to refine steel was physically and mathematically simulated. Using a water model three cases of gas supply were considered, top blowing, bottom injection and combined process top blowing‐bottom injection. Mixing time in top blowing increases with bath height and the distance between the lance of the gaseous jet and the bath surface. The jet penetration was found to be dependent on the modified Froude number. The unstable and unsteady behaviour of the bath topography, as affected by the gaseous jet, was well simulated through a multiphase momentum transfer model. In top blowing, three zones of liquid splashing were found, penetration with low splash, heavy splash and dimpling with low splash intensity. These zones depend on the gas flow rate and the distance from the lance to the bath surface. During bottom injection mixing times decrease with the number of tuyères, increases of bath height and gas flow rate. In a combined process mixing time decreases considerably due to the recirculating flow formed by the action of the top jet and the submerged jets. When a submerged jet is located just below the top jet the mixing time does not decrease as compared with the separated processes either top blowing or bottom stirring.     

Phase transformations during heating of llmenite concentrates

llmenite concentrates were heated in argon and oxygen in the temperature range 700 °C to 1000 °C to study the behavior of the pseudorutile phase and other changes which occur. Pseudorutile does not persist in argon or oxygen in the temperature range studied. In argon at 700 °C, pseudorutile decomposes into hematite and rutile, while at 1000 °C, it combines with ilmenite to form ferrous-ferritic pseudobrookite solid solution. A new phase “Fe₂O₃-2TiO₂” was identified as an intermediate product during the heating of ilmenite or pseudorutile in oxygen. This compound decomposes into hematite and rutile below 800 °C and to pseudobrookite and rutile above 800 °C. The sequence of reactions during the heating of ilmenite and pseudorutile in oxygen is proposed. Formerly with the Department of Metallurgy, Imperial College of Science, Technology and Medicine, London. Formerly with the Department of Metallurgy, Imperial College of Science, Technology and Medicine, London.     

Reduction behavior of llmenite with carbon at 1240 °C

A considerable body of literature“[1-17] has been published on various aspects of the solid-state reduction of ilmenite (FeTiO₃) using various reductants. All these investigations were limited up to 1200 °C. The present investigation describes the reduction behavior of two ilmenite concentrates produced by Westralian Sands Limited (WSL) and Western Mineral Sands (WMS) with Collie coal at 1240 °C. These materials were from Australia and were supplied by Tioxide International Limited, United Kingdom. The chemical analyses of these materials, given in Table I, were provided with the materials. The unmilled concentrates were sieved, and the size fraction of 10₄ to 152 μm, which accounted for ≈70 pct of the original material, was used for the experiments. The Collie coal was crushed and sieved to produce particles of the same size fraction. The purpose of this investigation is to study the effect of temperature higher than 1200 °C on the kinetics of the reduction and the formation of various phases after reduction. Formerly with the Department of Materials, Imperial College of Science, Technology and Medicine, London P. Grieveson, formerly Professor of Applied Metallurgy, Department of Materials, Imperial College of Science, Technology and Medicine, London     

Rate processes and structural changes in gaseous reduction of hematite particles to magnetite

Carol Lake hematite particles were reduced to magnetite at 600 and 1000 °C using CO + CO₂ mixtures. The rate of reduction was measured in gravimetric tests and structural changes followed by optical microscopy, BET surface area, and mercury porosimetry. At 600 °C, the reduction of each grain approximately followed the shrinking core model, and fine pores were created in the magnetite produced. The volume of pores was constant at a porosity level of 8.8 pct, but the average pore size depended on the rate of oxygen removal, and finer pores were obtained under conditions of fast reduction. The reaction followed a different path at 1000 °C and proceeded by sideway thickening of a finite number of magnetite lamellae formed parallel to each other in each grain. Reduced particles showed reentrant surface depressions and cracks, but no porosity. Reaction mechanisms were postulated to relate these structural features to the progress of the reaction. Examination of the reaction steps indicated that the separation of oxygen from solid surfaces was likely to be the rate determining step at both temperatures. This was reflected in rate measurements by a strong dependence on CO pressure while the influence of oxygen activity (as represented by CO₂/CO ratio) was of secondary importance at 1000 °C and negligible at 600 °C. A detailed analysis of reaction rates could not be made, however, because the particles were of a wide range of sizes and their structure changed during reduction. formerly Research Student at Imperial College, London, formerly Professor of Metallurgy, Imperial College, London,     

Electrode mass transfer under conditions of natural and forced convection

In order to investigate the mass transfer to a cathode under conditions of natural and forced convection, silver was plated under conditions where the deposition rate was controlled by mass transfer. Rather than analyzing chemically for the deposited silver, it was determined locally by electrochemical stripping. This enabled the mass transfer coefficients to be determined accurately over the whole surface of the electrode under conditions of natural convection, electrolyte jetting, and gas sparging. In general, the mass transfer coefficient was enhanced significantly only in the very close vicinity of the agitation source. An improved sparger design with closely spaced multiple bubble sources was found to enhance overall mass transfer. formerly Graduate Student, Department of Mate-rials Science and Metallurgy, University of Cambridge     

转炉超音速氧枪喷头磨损后的吹炼特性变化

 转炉氧枪喷头会随枪龄的增加发生不同程度的侵蚀,为了探究氧枪喷头侵蚀程度对超音速气体射流吹炼特性的影响,建立了120 t转炉及超音速氧枪的三维全尺寸几何模型,研究了氧枪喷头不同磨损角度对气体射流特性、熔池速度及壁面侵蚀的影响。发现随着磨损角度增加,射流速度衰减加快,射流核心区长度缩短,同一等速线长度缩短,射流中心最大速度和最大速度点距中心距离增大。射流动压衰减速度随磨损角度增加而加快,磨损角度由0增至20°,距喷头端面1.5 m处最大动压减小了14.84%,14 000 Pa等压线包围面积由0.038 m2减小至0.002 m2。钢液面处高速区面积随着磨损角度增加而减小,死区面积随着磨损角度增加而增大。熔池纵截面高速区域主要分布在冲击凹坑和底吹元件附近,低速区域主要分布在熔池底部,死区主要分布在熔池底部中心和炉壁下部区域。当熔池深度小于0.6 m时,顶吹气流对熔池的搅拌起主要作用,磨损角度增加,熔池搅拌能力变弱,熔池横截面高速区面积减小,低速区和死区面积增大;当熔池深度大于0.6 m时,底吹气流对熔池搅拌起主要作用,高速区面积基本不变。渣-金作用区域和底吹流股附近流体湍动能较大、壁面剪切应力较为集中,该部位耐火材料侵蚀严重。熔池壁面附近流体湍动能和壁面剪切力随磨损角度增加而降低,转炉炉衬侵蚀速度减小。     

The solidification of pure metals (and eutectics) under uni-directional heat flow conditions: II. Solidification in the presence of superheat

The generalized integral-profile method previously developed² to predict the solidification rates of pure metals and eutectic alloys under a range of cooling conditions has been extended to treat solidification rates in the presence of super heat. Heat transfer within the liquid metal has been characterized in terms of a single parameter for which a heat balance on the liquid yields an ordinary nonlinear equation. This equation is additional to the two equations derived previously for the growth of the solid metal, and the three equations have been solved simultaneously using a Runge-Kutta technique. Solutions have been obtained for heat transfer conditions that can be reproduced accurately in laboratory experiments. The results of a series of such experiments are also presented and shown to agree very well with the theoretical predictions. In carrying out the analysis, it has been shown that a number of different cooling and solidification modes can occur during the solidification of superheated liquid metal, the solidification process following one of two possible routes through these modes. This presents a useful approach to the analysis of this solidification problem and, indeed, of more complicated problems since the logic of the computer program used in the analysis is closely related to the logic of the solidification process. The work described in this paper was carried out while the authors were with the John Percy Research Group in Process Metallurgy at Imperial College, London.     

The Penetration Behavior of an Annular Gas–Solid Jet Impinging on a Liquid Bath: Comparison with a Conventional Circular Jet

The top-blow injection technique of a gas–solid mixture through a circular lance is used in the Mitsubishi Continuous Smelting Process. One of the inherent problems associated with this injection is the severe erosion of the hearth refractory below the lances. A new configuration of the lance to form an annular gas–solid jet rather than a circular jet was designed in the laboratory scale. With this new configuration, solid particles leave the lance at a much lower velocity than the gas, and the penetration behavior of the jet is significantly different than with the circular lance in which the solid particles leave the lance at the same high velocity as the gas. The results of cold model tests using an air-sand jet issuing from a circular lance and an annular lance into a water bath showed that the penetration of the annular jet is much less sensitive to the variations in particle feed rate as well as gas velocity than that of the circular jet. Correlation equations for the penetration depth for both circular and annular jets show agreement among the experimentally obtained values.     

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.     

Transferring Characteristics of Momentum/Energy during Oxygen Jetting into the Molten Bath in BOFs: A Computational Exploration

This paper presents a numerical study of transferring characteristics of momentum/energy during oxygen jetting into the slag–metal molten bath in basic oxygen furnaces by a multi‐fluid volume of fluid model. The evolution of the momentum/energy and turbulence of jets along their axial travel were studied. The results were quantitatively compared within a wide range of oxygen supply pressures, at which the jets may be at under‐ or over‐expanding state. The efficiency of the momentum/energy transfer from the jets to the molten bath was also assessed with respect to lance height and operation pressure. The numerical results show that the momentum and kinetic energy for the jets suffering from shock waves likely cause more intensive damping compared to the jets with expansion waves. The turbulence kinetic energy and turbulence dissipation rate are observed to firstly increase and then decrease. This effect is more pronounced at a lower operation pressure. Based on these results, the optimum lance height was identified for a rapid slagging operation. It is also shown that the efficiency of the energy transfer from the jets to the molten bath is very low. Decreasing lance height or increasing operation pressure promotes the efficiency of the momentum transfer from the jets to the molten bath but lowers the efficiency of the kinetic energy transfer.     

转炉顶吹过程渣-金-气三相作用特性

基于VOF界面追踪技术,建立了氧气转炉顶吹过程渣-金-气多相传输行为的数学模型,研究了渣-金-气三相界面行为,给出了渣/金界面特征的演化过程和熔池内流场变化特征,对冲击坑形态进行了具体的描述,并对穿透深度和冲击坑直径进行了定量化的表征。结果表明:由于冲击坑表面波的传播,冲击坑以及渣/金/气界面具有明显的瞬态特征且熔池是振荡的;钢液熔池内涡旋中心位置随着吹炼时间以及渣的运动而变化;低枪位加强了钢液表面的波动,增加了钢液表面的粗糙度,增大了射流与熔池的接触面积,从而有利于射流向熔池的动量传递,促进熔池的搅拌。     

转炉局域传质和混匀效果

采用冷态转炉对转炉熔池局域流动和传质效果进行了研究.选用不同氧枪喷头、枪位和熔池形状进行实验,通过测量熔池各区域的电导率值来研究熔池局域传质和混匀效果.根据实验结果,分析了各因素对熔池传质、死区分布、混匀时间及熔池速度均匀性等的影响.研究结果发现:标准熔池(径深比为3.1)中,熔池死区主要位于熔池底部侧壁和环流中心处;浅型熔池(径深比为5.2)中,熔池死区主要位于熔池侧壁.适当增加氧枪喷孔倾角和熔池径深比,有利于增大熔池环流半径,改善熔池内部流动,减小熔池内部死区.      

Diffusivity of hydrogen in liquid nickel and copper

The diffusivity of hydrogen in liquid nickel was determined from 1468° to 1550°C by a capillary gas-reservoir technique. The diffusion cell was semiinfinite in length and consisted of Specpure nickel in an alumina capillary. An argon and hydrogen gas flow maintained a constant hydrogen potential at the metal/gas interface. A controlled furnace hot zone of length 23 cm was obtained using eight separate windings of Pt-40 pct Rh wire. The temperature profile in this zone was adjusted so that the top of the cell was hotter than the bottom, to eliminate convection. The experiments were terminated by a rapid nonaqueous quench. The diffusion columns were then sectioned and analyzed by vacuum extraction. Diffusivities were calculated using a solution to Fick’s Second Law. At 1468°C,D = 3.17 x 10⁻³ sq cm per sec with σ = ±0.76 × 10⁻³; at 1550°C,D = 3.48 × 10⁻³ sq cm per sec with σ = ±0.54 × 10⁻³. The diffusivity of hydrogen in liquid copper was determined using a shallow melt and analyzing for the total diffusate content; at 1101°C,D = 0.99 x 10⁻³ sq cm per sec with a = ±0.25 × 10⁻³; at 1201°C,D = 1.26 × 10⁻³ sq cm per sec with σ = ±0.16 × 10⁻³. Formerly with the Nuffield Research Group in Extraction Metallurgy at Imperial College, London, England This paper is based upon a thesis submitted by J. H. WRIGHT in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the University of London.     

Simulation of Flow Fluid in the BOF Steelmaking Process

The basic oxygen furnace (BOF) smelting process consists of different chemical reactions among oxygen, slag, and molten steel, which engenders a vigorous stirring process to promote slagging, dephosphorization, decarbonization, heating of molten steel, and homogenization of steel composition and temperature. Therefore, the oxygen flow rate, lance height, and slag thickness vary during the smelting process. This simulation demonstrated a three-dimensional mathematical model for a 100 t converter applying four-hole supersonic oxygen lance and simulated the effect of oxygen flow rate, lance height, and slag thickness on the flow of molten bath. It is found that as the oxygen flow rate increases, the impact area and depth increases, which increases the flow speed in the molten bath and decreases the area of dead zone. Low oxygen lance height benefits the increase of impact depth and accelerates the flow speed of liquid steel on the surface of the bath, while high oxygen lance height benefits the increase of impact area, thereafter enhances the uniform distribution of radial velocity in the molten steel and increases the flow velocity of molten steel at the bottom of furnace hearth. As the slag thickness increases, the diameter of impinging cavity on the slag and steel surface decreases. The radial velocity of liquid steel in the molten bath is well distributed when the jet flow impact on the slag layer increases.     

120 t转炉旋流氧枪的射流搅拌特性

 熔池流动状态及反应速度是实现转炉高效冶炼的关键,主要取决于氧气射流与熔池的相互作用及底吹搅拌强度。建立了120 t转炉旋流氧枪的三维全尺寸几何模型,利用数值模拟研究了不同旋流角旋流氧枪的射流特性,并对比分析了其对转炉熔池的冲击搅拌效果。结果表明,随着旋流角的增大,氧气流股的射流核心区长度不断减小,射流中心距氧枪轴线距离增大,氧枪射流交汇点距喷孔出口距离不断增大,射流聚合现象被抑制;当旋流角由0°增加至15°时,氧气射流的冲击深度减小了40%,冲击半径增加了13%;熔池纵截面上的高速区域分布在冲击凹坑附近,横截面上的高速区域分布在冲击凹坑及相邻凹坑连接处延长线外部区域。     

Modeling of an Impinging Oxygen Jet on Molten Bath Surface in 150 t EAF

 A transient three-dimensional mathematical model has been developed to analyze the three-phase flow in a 150 t EAF (electric arc furnace) using oxygen. VOF (multiphase volume of fluid) method is used to simulate the behaviors of molten steel and slag. Numerical simulation was conducted to clarify the transient phenomena of oxygen impingement on molten bath. When oxygen jet impinges on the surface of molten bath, the slag layer is broken and the penetrated cavity in molten steel is created. Simultaneously, the wave is formed at the surface of uncovered steel on which the slag layer is pushed away by jet. The result of numerical simulations shows that the area and velocity of uncovered steel created by impingement, jet penetration depth change from 0.10 m2, 0.0125 m/s, 3.58 cm to 0.72 m2, 0.1445 m/s, 11.21 cm, when the flow rate of an oxygen lance varies from 500 to 2000 m3/h. The results have been validated against water model experiments. More specially, the relation between the penetration depth and oxygen flow rate predicted by numerical simulation has been found to agree well with that concluded by water model.     

Fluid dynamics and mass transfer in submerged gas-particle jets

A room temperature model of a submerged gas-particle jet was used to investigate the hydrodynamics and gas-liquid mass transfer in such systems. Air or CO₂ was used to inject particles of silica sand into water. In some cases, the sand was coated with resin to produce a hydrophobic surface. The flow regimes of behavior were observed: In the bubbling flow regime large bubbles of gas are formed and penetrated by a stream of particles which did not entrain gas, and in the steady jet flow regime the gas and particles travel together in a narrow cone. The second flow regime is favored by a high gas velocity, a small particle size, and a high ratio of particles to gas in the jet. The surface characteristics of the injected particles do not appear to affect this transition. A CO₂-NaOH solution model was used to determine the effects of inert particle injection of the rate of mass transfer from gas to liquid. The rate of mass transfer was higher in steady cone jets, because under these conditions, the gas is dispersed into finer bubbles and carried deeper in the bath. Formerly Graduate Student in the Department of Civil Engineering, Mechanics, and Metallurgy, University of Illinois at Chicago