Improvement of Impeller Blade Structure for Gas Injection Refining under Mechanical Stirring简

Abstract： The impeller blade structure for gas injection refining under mechanical stirring has been explored by water model experiments. A sloped swept-back blade impeller is＇proposed for the purpose. The central part of the impeller is disk- or plate-shaped, and the blades are fitted to the side of the disk or plate. In addition, a disk is put on the top side of the impeller blades. The impeller can strengthen the radial and downward flow between the blades and weaken the swirl flow in the zone above the impeller. These effects on flow phenomena are favorable for disintegration and wide dispersion of bubbles which are injected from a nozzle attached to the center of the underside of the impeller. In addition, the sloped swept-back impeller requires less power consumption. The impeller shaft should be placed away from the vessel center so as to disperse the injected bubbles widely in the bath under mechanical stirring even with unidi- rectional impeller rotation and without installing baffles. The number of gas holes in the nozzle and the direction of gas injection have a little effect on the bubble disintegration and dispersion in the bath. Highly efficient gas injection refining can be established under the conditions of proper impeller size, larger nozzle immersion depth, larger eccen- tricity and rotation speed of the impeller. The sloped swept back blade impeller can decrease the power consumption and vet improve the bubble disintegration and wide dist~ersion in the bath.     

Experimental Research of External Desulfurization in Situ Mechanical Stirring

This paper presents a new idea about desulfurization with in-situ mechanical stirring method on the basis of desulfurization by single blow grain magnesium and KR method, that is, the inner gases carry the magnesium vapor formed in-site in molten iron by magnesium-based desulfurization, and bubble dispersed and disintegrated under the condition of mechanical stirring, thence to improve the efficiency of desulfurization by single blow grain magnesium .It has been proved by research of cold water model experiment that the bubble dispersion and disintegration can not only improve the desulphurization efficiency but also increase the utilization rate of magnesium. Obviously, the bubble dispersion and disintegration of magnesium vapor is the key problem in improving the desulphurization efficiency and increasing the utilization rate of magnesium. Thus the research focus on exploring the performance of bubble dispersion and disintegration on the base of refining process and gas-liquid mass transfer. According to the literature and cold water model experimental result basing on principle of similitude, the influencing factors and interaction of bubble dispersion and disintegration have been studied from the perspectives of physical and numerical simulation. The study would provide the theoretical and experimental data for the new method of desulfurization with in-situ mechanical stirring.     

Improvement of Impeller Blade Structure for Gas Injection Refining under Mechanical Sirring

The impeller blade structure for gas injection refining under mechanical stirring has been explored by water model experiments.A sloped swept-back blade impeller is proposed for the purpose.The central part of the impeller is disk-or plate-shaped,and the blades are fitted to the side of the disk or plate.In addition,a disk is put on the top side of the impeller blades.The impeller can strengthen the radial and downward flow between the blades and weaken the swirl flow in the zone above the impeller.These effects on flow phenomena are favorable for disintegration and wide dispersion of bubbles which are injected from a nozzle attached to the center of the underside of the impeller.In addition,the sloped swept-back impeller requires less power consumption.The impeller shaft should be placed away from the vessel center so as to disperse the injected bubbles widely in the bath under mechanical stirring even with unidirectional impeller rotation and without installing baffles.The number of gas holes in the nozzle and the direction of gas injection have a little effect on the bubble disintegration and dispersion in the bath.Highly efficient gas injection refining can be established under the conditions of proper impeller size,larger nozzle immersion depth,larger eccentricity and rotation speed of the impeller.The sloped swept-back blade impeller can decrease the power consumption and yet improve the bubble disintegration and wide dispersion in the bath.     

CO2吸收模型中容积传质系数的确定

气泡微细化是“原位机械搅拌法铁水炉外脱硫技术”的关键.气液传质系数是研究气液吸收过程的基本参数.本文根据相似性原理建立水模型实验装置,并通过测定NaOH吸收CO2的速率来研究气泡微细化过程,同时根据吸收原理定量计算出容积传质系数Ak及CO2气体利用率η.当溶液pH值从12降低到9的过程中,容积传质系数为2.938×10 4m3/s,本实验所用CO2的利用率的公式可简化为:η=18.98/Qt.本论文的研究结果可为进一步研究吸收速率提供理论依据.     

Study on Absorption Rate by Eccentric Mechanical Stirring in Gas Injection Refining for Iron and Steel Making

In gas injection refining processes, a great amount of gas is injected into molten metal in short time, so that very large bubbles are inevitably formed. Wide dispersion of small bubbles in the bath is indispensable for high refining efficiency. Eccentric mechanical stirring with unidirectional impeller rotation was tested using a water model for pursuing better bubble disintegration and dispersion. Absorption rate are used to research on the influence law of the bubble dispersion and disintegration and gas-liquid absorption by the influence of, rotation mode, rotation speed and gas flow rate. Compared to the experimental results of absorption rate under eccentric stirring and centric stirring ,provide the scientific experimental and theoretical guidance for high-temperature experiment of hot metal desulfurization .According to experimental and theoretical analysis, this paper has studied various factors effecting on gas absorption process and volumetric mass transfer coefficient using the system of CO2-NaOH-H2O.The results show that:the volumetric mass transfer coefficient and absorption efficiency of CO2 can be increased under eccentric stirring mode, Because bubble disperse quickly with eccentric mechanical stirring, which results in promoting complete reaction between CO2 and NaOH, and improving the mass transfer coefficient and absorption. Volumetric mass transfer coefficient and efficiency of CO2 increase with the increasing rotation speed under the condition of eccentric stirring .But volumetric mass transfer coefficient and efficiency of CO2 decrease with the increasing rotation speed under the condition of centric stirring.     

Behaviour of Ar‐1%Mg Bubbles in Desulfurization of Hot Metal by Magnesium Injection

A mathematic model for an argon bubble containing magnesium vapour in hot metal was developed for interpreting the desulfurization process by injection of granulated magnesium. The change of rising velocity, pressure, size and magnesium vapour content of the bubble and the rates of desulfurization, deoxidation, magnesium dissolution by the bubble were investigated with the aid of the model. Most of the magnesium vapour in an argon bubble is consumed by the reaction with hot metal while the bubble is rising from 3.5 m depth in hot metal to the bath surface. The magnesium vapour content in the bubble decreases to a minimum halfway on the bubble's rising and slightly increases thereafter as the bubble approaches the bath surface. During the initial or middle stages of the injection process, magnesium vapour in a gas bubble is mainly consumed by the desulfurization and deoxidation of hot metal. During the final stage or within the active zone of the process, magnesium dissolution into hot metal and unreacted magnesium vapour in the bubble arriving at the bath surface become significant.     

金属镁粒铁水脱硫过程动力学

通过对金属镁粒铁水脱硫过程动力学的研究发现,金属镁粒的粒径越大,镁粒气化后的气泡在铁液中的停留时间和平均上浮速率就越大,铁液中镁脱硫反应的传质系数减小;上浮速率及停留时间与铁液温度几乎没有关系,但传质系数随温度的增加而增加;在一定的铁液深度和铁液硫含量时,金属镁粒铁水脱硫时镁的利用率随镁粒的粒径增加而减小;温度对镁的利用率的影响在镁粒粒径小时比粒径大时要大,在铁液硫含量小时比硫含量大时影响要大;就脱硫过程动力学而言,温度越高越有利于铁液中镁的脱硫反应,这与热力学结论相反;铁液温度为1573 K,铁液中的w([S])为0.06%,铁水包的深度为3 m,在镁粒半径范围为(3～16)×10-4 m时,理论计算镁脱硫时镁的平均利用率为83.1%,与宣化钢铁公司生产得到的数据进行了对比,得到很好的吻合.     

喷嘴结构对铁水脱硫气泡微细化的影响

针对铁水预处理镁蒸气脱硫的气泡微细化问题,本文通过水模型实验,研究了喷嘴结构对气泡在熔池中的分散、CO_2吸收速率和利用率以及均混时间的影响规律.结果表明:使用SSB-D桨,搅拌转速200 r/min,通气流量为1.0 m~3/h,偏心度0.4,浸入深度250 mm时,透气砖结构的喷嘴可以使熔池内的气泡分布"死区"减小,同时缩短了均混时间,提高了镁蒸气的容积传质系数和利用率.     

底吹钢包内离散气泡运动及界面波动对脱硫行为影响

为了分析钢包内离散气泡和界面波动对脱硫效率的影响,本文采用数值模拟法,选择大涡模拟和离散相模型,结合界面追踪法建立三维瞬态渣-金-气三相流模型,模拟气泡行为及渣层变化规律,同时利用相间传质求解发生于钢包内渣-金界面处的脱硫过程.本模型设置压力为101.325 kPa,反应温度为1873 K,考虑了气泡运动、变形、破碎、...     

Physical modeling of gas/liquid mass transfer in a gas stirred ladle

The absorption of gas through the plume eye and of an injected gas in a steelmaking ladle process was investigated, using a physical model of CO₂ absorption into a NaOH solution. The results show that the inert gas escaping through the plume eye is ineffective in protecting the bath from the atmosphere, and placing an oil layer (simulated slag) decreases the absorption rate significantly. Increasing the flow rate of the inert gas not only exposes more of the liquid surface to the CO₂ atmosphere, but also increases the mass transfer coefficient at the surface. The overall mass transfer between an injected CO₂ gas and NaOH solution includes the mass transfer through the surface of the bath as well as the mass transfer in the bubble dispersion zone. The difference between the mass transfer in the bubble dispersion zone and the overall mass transfer was found to be significant for relatively low gas flow rates. The mass transfer coefficient of CO₂ in the bubble dispersion zone was estimated using available information regarding the bubble size and velocity. Mass transfer coefficient estimated for the constant bubble frequency regime shows a dependence on gas flow rate. However, if a constant characteristic size of bubbles is assumed as an alternative approach, the mass transfer coefficient is independent of the gas flow rate.     

The kinetics of molten iron desulfurization using magnesium vapor

Magnesium vapor was injected into 60 kg heats of carbon-saturated iron at 1523 K. The magnesium dissolution and desulfurization rates as well as magnesium bubble size were monitored over sulfur contents ranging from 0.0002 to 0.2 pct S. The magnesium dissolution mass transfer coefficient was found to be 0.046 ± 0.034 mm · s⁻¹ (for 30 mm diam bubbles). Further analysis indicated that most of the desulfurization took place at magnesium sulfide inclusions present in the bath; there was good agreement between the experimental precipitation rate constant and that predicted for diffusion to the number of inclusions observed by chemical analysis and inclusion counts. The seed magnesium sulfide inclusions were probably stripped from the ascending magnesium bubbles. These particles were quickly eliminated from the melt by bubble and induction stirring, resulting in a steady-state number of inclusions. This allowed a pseudosteady-state model to be developed.     

Improvement of desulfurization efficiency of molten iron with magnesium vapor produced <Emphasis Type="Italic">In situ</Emphasis> by aluminothermic reduction of magnesium oxide

Based on the desulfurization process of molten iron with magnesium vapor produced in situ by the aluminothermic reduction of magnesium oxide, methods for improving the desulfurization efficiency of magnesium (η _S,Mg) have been studied. By use of the mixed-control model of gas- and liquid-phase mass transfer developed in a previous article, it is shown that η _S,Mg can be improved by reducing the bubble size, increasing the lance immersion depth, dividing MgO-Al pellet charging into several portions, and lowering the operating temperature. A new method for improving η _S,Mg, by adjusting the initial magnesium molar ratio in the bubble (N _Mg,0) through a change in the Ar carrier gas flow rate, is proposed. The optimum initial magnesium molar ratio in the bubble (N _Mg,opt) for obtaining the maximum η _S,Mg value exists, and its value changes with the sulfur concentration in the molten iron. Under the condition of N _Mg,0=N _Mg,opt, reducing the bubble diameter improves η _S,Mg and decreases the Ar carrier gas flow rate that is needed to adjust N _Mg,0 to N _Mg,opt. The aluminum consumption is decreased by the use of the 4MgO-2Al pellet instead of the 3MgO-2Al pellet and is further decreased by lowering the temperature from 1673 to 1623 K. Experimental examinations of the influences of lance immersion depth, pellet charging method, and carrier gas flow-rate pattern have been conducted. Reasonable agreement between the calculated and the experimental results indicates the validity of the present optimization method.     

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.     

A Detailed Single Bubble Reaction Sub‐Model for AOD Process

A new detailed model that describes the chemical reactions, mass transfer and heat transfer taking place on the surface of a single gas bubble in liquid steel is presented in this paper. By using this model, locally occurring small scale physical and chemical mechanisms can be effectively studied. This information is required later in developing a simplified reaction sub‐model to be used in CFD simulation of an operating AOD vessel. To demonstrate the capabilities of the new model, the behaviour of a single bubble under two example conditions was simulated. In the case of high carbon content of the steel, here 1%, a contribution analysis showed that the major fraction of the oxygen goes to oxidize dissolved C. When 50% of the carbon in the bath is burned and if the same gas composition (90% O₂, N₂) is still used, the main product is initially Cr₂O₃, indicating that the gas composition should have been changed if this had been a real process in question. To verify, a series of O₂/N₂ ratios 0.1…0.95 were simulated at 50% C conversion to see how more optimal product yield can be obtained. In addition, time dependent profiles of temperature and all species in and around the bubble are presented. The results presented here are applicable only to a local position in the AOD vessel. To be applicable to a whole AOD vessel, the model should be implemented as a source term into CFD software or a corresponding process simulation tool. This will be our future work.     

A mathematical approach for the mass transfer between liquid steel and an ascending bubble

A mathematical model which is adaptable to practical conditions has been put forward to deseribe adequately the purging of liquid steel with an inert gas. The mass transfer between liquid steel with varying C, O, H, N, and S contents and an ascending argon bubble has been investigated. The simultaneous mass transfer of all possible gaseous compounds is considered as a function of the initial bubble mass, height of the steel bath, the bulk concentrations in the melt, and the external pressure. The model takes into account the change in size and form of the bubble resulting from its rise and the mass transfer between the bubble and the melt, and also the influence of surface active agents such as sulfur and oxygen. The following general conclusions can be drawn: 1) the gases flushed (out of the melt) by the bubble can be related to the amount of argon injected into the bath. 2) Increase in the initial bubble size and the content of the gas in the bath to be purged, and decrease in the external pressure result in more pronounced deviations from equilibrium saturation within the bubble. 3) Lower external pressure, increasing supersaturation of CO, and greater amounts of purging gas at higher dispersion are the chief factors responsible for increasing, purging efficiency for H, N₂, CO in steel melts. 4) Surface-active agents decrease the purging ratio for nitrogen in a carbon-free melt, but, in carbon-containing melts, increasing amounts of oxygen alone lead to a considerable increase in the purging ratios for nitrogen due to the high mass transfer of CO to the bubble. In the latter case, the effect of increasing sulfur content on the purging ratios is of no significance. KAZUO OKOHIRA, formerly Graduate Student at the Institut für Eisenhüttenkunde at Aachen HERMANN SCHENCK, Dr.-Ing., Dr.-Ing. E.h., Dr.h.e., formerly Director of the Institut für Eisenhüttenwesen, President of the German Iron and Steel Institute (VDEh) from 1950 to 1968 This paper is based in part on a thesis submitted by KAZUO OKOHIRA in partial fulfillment of the requirements for degree of Doktor-Ingenieur at the Technische Hochschale, Aachen.     

进气方向对文丘里微气泡发生器气泡直径的影响

文丘里微气泡发生装置常采用自吸式进气的方式,在工程应用中可能存在微气泡通量不足的问题。采用压缩空气进气,并通过照相法重点考察了错流、逆流和并流3种进气方向对文丘里管微气泡发生器生成气泡直径的影响。结果表明：喉管处液速超过4.72 m/s时所产生的湍流剪切场才能将进入的气泡破碎成~200 μm级别的微气泡;添加3-戊醇能够稳定生成的微气泡,抑制生成的气泡在从文丘里管到测试槽表面逸出过程中的聚并和破碎过程,从而使测试槽中不同位置处气泡直径能保持生成时的微气泡的直径;3种进气方向中,错流进气因气泡进入后更贴近壁面流动,所生成气泡直径最大;而并流进气气泡脱离时间更短,使得生成气泡尺寸最小。在相同条件下,并流进气生成的微气泡比表面积最大,约是错流进气的3倍,最有利于气液传质。     

金属镁冶炼还原渣脱硫性能

为充分利用固废资源，降低脱硫成本，采用传统皮江法冶镁工艺中产生的金属镁渣为基质，利用实验室湿式脱硫系统，对不同镁渣粒径、烟气气体流速、二氧化硫浓度、固液质量比（简称固液比）以及氧气含量条件下的脱硫性能进行了研究。结果表明，样品粒径越小，流速越慢，二氧化硫流速越低，固液比越低，脱硫量越高；反之，则脱硫量越小。在粒径0.106 mm、烟气流速340 mL/min、二氧化硫流速10 mL/min、固液比0.1 g/g、饱和脱硫时间1 575 min的条件下，二氧化硫吸附量为1 354.62 mg/g。     

Computer simulation on the hydrodynamics of gas injection process in liquid steel

It is shown that with the aid of digital simulation methods complex multiphase interrelated systems, such as gas-injection process can be analysed. Interdependencies can be revealed and quantitative evaluation of characteristic system quantities are provided. The method of digital system simulation is a very convenient tool for process analysis or system engineering. Results of the computer-aided process simulations (Caps) yield a better understanding of complex phenomena and better aimed engineering of gas dispersion techniques in metallurgical processes. A particular interest of this investigation is to reveal the effect of mass-transfer rate on the hydrodynamic behaviour of a gas-injection process. The combined effects of total flow rate of injected gas and mass-transfer rate on the system quantities such as mixing power, induced liquid flow rate, holdup, interfacial area and volumetric mass transfer coefficient are evaluated under steady state conditions of the investigated systems and illustrated in simulation plots. The liquid velocity has a minor effect on bubble size at some distance from the orifice but controls the location of bubble breakup. The frequency of bubble breakup and final bubble size depends on the intensity of mass transfer. Mixing power due to gas bubbles and circulation velocity of the steel bath increase appreaciably if there is a chance of bath reactions producing more gas. The integral mean values of mixing power, induced velocity of liquid and holdup in plume, specific interfacial area and volumetric mass-transfer coefficient increase with increasing total flow rate of injected gas and intensity of mass transfer.     

Palladium stripping rates in PGM refining

This study investigates the stripping (back-extraction) of palladium from the perspective of mixer–settler hydrodynamics. A hydroxyoxime extractant and hydrochloric acid medium are used. Firstly, the palladium stripping rate (chemistry) is determined using a vigorously stirred laboratory unit cell. This increases with operating temperature and stripping acid concentration. At optimal chemical conditions (25 °C, 6 N HCl), the stripping rate is sufficiently fast for equilibrium to be reached within 5–10 minutes. Secondly, the palladium stripping rate (hydrodynamics) is determined using a pilot-scale mixer–settler. The mixer–settler is operated with a 4-bladed radial disc and a 6-bladed Rushton turbine impeller, over a range of impeller speeds corresponding to power intensities of 2 to 5 kW/m³. The palladium stripping rate increases with increasing impeller speed for both the radial disc and Rushton turbine impellers. The overall mass transfer coefficient also increases with increasing power intensity, up to power intensities 100% higher than those used in typical plant stripping mixer vessels. When benchmarked against the power intensity, not much difference between the two impellers is noted. The study concludes that the stripping of palladium by solvent extraction as used in the PGM industry can be improved by operating at higher power intensities.     

Prediction of Bubble Size Distribution in Aluminium Foam as a Function of %Titanium Hydride Addition

Foaming of liquid aluminium by addition of foaming agent (TiH₂ particles) is numerically simulated using population balance equations. Phenomena such as hydrogen release by the TiH₂ particles, heterogeneous nucleation of bubbles in oxide surface cavities, and diffusion based bubble growth are modelled. A simple mass transfer coefficient, which varies inversely with the bubble size is used to estimate the bubble growth rate. Simulation is performed to study the effect of TiH₂ content on the final bubble size distribution, total number of bubbles and average bubble size. In general, the average properties of the predicted distributions are close to the experimental values, whereas the spread in the bubble size is observed to be considerably narrower for the predicted values. The deviation in the spread of the distributions is attributed to the inverse bubble size dependent growth rate and non-inclusion of bubble coalescence in the model.