Mathematical Modeling of Fluid Flow in Bath during Combined Side and Top Blowing AOD Refining Process of Stainless Steel: Application of the Model and Results

The mathematical model developed for the molten steel flow in the combined side and top blowing AOD refining process of stainless steel has been used to compute and analyze the flow fields of the liquid phases in the baths of the 120 t AOD converter and its water model unit with a 1/4 linear scale. The influence of the side tuyere number and the angle between each tuyere on the flows has been examined. The results demonstrate that the mathematical model can quite reliably and well model and predict the fluid flow in an AOD bath with the combined blowing. The liquid flow in an AOD converter bath with the combined blowing is resulted from the gas side blowing streams under the influence of a gas top blowing jet. The streams play a governing role on it; and the liquid in the whole bath is in active agitation and circulatory motion during the gas blowing process. The gas jet from the top lance does not change the essential features of the gas stirring and liquid flow in the bath, but can make the local flow pattern of the bath liquid obviously vary and its turbulent kinetic energy enhance. The changes in the tuyere position and number have similarly not altered the basic characteristics and patterns of the gas agitation and liquid flow and turbulent kinetic energy distribution in the bath. At a given tuyere number and gas side blowing rate or a given angular separation between each tuyere and gas side blowing rate, however, the variation of the angle between each tuyere or the tuyere number can locally change them. Using 6 tuyeres with 27° can reach the more uniform flow field and turbulent energy distribution of the liquid in the bath than taking 7 tuyeres with 18° or 22.5° and 6 tuyeres with 22.5°.     

Preliminary Investigation of Fluid Mixing Characteristics during Side and Top Combined Blowing AOD Refining Process of Stainless Steel

The fluid mixing characteristics in the bath during the side and top combined blowing AOD (argon‐oxygen decarburization) refining process of stainless steel were preliminarily investigated on a water model unit of a 120 t AOD converter. The geometric similarity ratio between the model and its prototype (including the side tuyeres and the top lances) was 1:4. On the basis of the theoretical calculations for the parameters of the gas streams in the side tuyeres and the top lances, the gas blowing rates used for the model were more reasonably determined. The influence of the tuyere number and position arrangement, and the gas flow rates for side and top blowing on the characteristics was examined. The results demonstrated that the liquid in the bath underwent vigorous circulatory motion during gas blowing, without obvious dead zone in the bath, resulting in a high mixing effectiveness. The gas flow rate of the main tuyere had a governing role on the characteristics, a suitable increase in the gas flow rate of the subtuyere could improve mixing efficiency, and the gas jet from the top lance made the mixing time prolong. Corresponding to the oxygen top blowing rate specified by the technology, a roughly equivalent and good mixing effectiveness could be reached by using six side tuyeres with an angle of 27 degrees between each tuyere, and five side tuyeres with an angular separation of 22.5 or 27 degrees between each tuyere. The relationships of the mixing time with the gas blowing rates of main‐tuyeres and sub‐tuyeres and top lance, the angle between each tuyere, and the tuyere number were evaluated.     

Numerical Simulation of Fluid Flow in Bath during Combined Top and Bottom Blowing VOD Refining Process of Stainless Steel

The fluid flow in a bath in combined top and bottom blowing vacuum‐oxygen decarburization (VOD) refining process of stainless steel has numerically been simulated. The three‐dimensional mathematical model used is essentially based on that proposed in our previous work for the flow in combined side and top blowing argon‐oxygen decarburization (AOD) process, but considering the influence of reduced ambient pressure. Applying it to the flow in the bath of a 120 t VOD vessel under the refining conditions, the results present that the model can fairly well simulate and estimate the flow phenomena. The flow pattern of molten steel in the bath with the combined blowing is a composite result under the common action of the jets from a three‐hole Laval top lance and gas bottom blowing streams. The jets have a leading role on it; the molten steel in the whole bath is in vigorous stirring and circulatory motion during the blowing process. The streams do not alter the basic features of the gas agitation and liquid flow, but can evidently change the local flow pattern of the liquid and increase its turbulent kinetic energy to a certain extent. The flow field and turbulent kinetic energy distribution in the combined blowing with three tuyeres are more uniform than those in the blowing with double tuyeres. Increasing properly the tuyere eccentricities is of advantage for improving the velocity and turbulent kinetic energy distributions, the stirring and mixing result in the practical VOD refining process.     

Model study of mixing and mass transfer rates of slag-metal in top and bottom blown converters

Water model experiments have been conducted to clarify mixing rates of molten steel and mass transfer rates between slag and metal in LD and Q-BOP furnaces using six different circular tuyere arrangements. Splashing and ‘spitting’ were also examined with a view to finding a quiet bath with minimum mixing time and maximum mass transfer rate. Froude’s similarity criterion was fulfilled to determine gas flow rate and bath depth. Complete mixing time of water determined by tracer technique had been 0.9 second to 1.8 seconds for Q-BOP as compared to 6 seconds to 13 seconds for LD. This shows that the stirring intensity in Q-BOP is remarkably larger than that of LD. A simple relationship τ = 5.9(Q/N) ^−0.49 was obtained with gas flow rateQ and number of tuyereN. This indicates that flow rate of gas per tuyere should be intensified to realize better mixing. Mass transfer coefficient K_Ba for bottom blowing was found to be almost double that for top blowing. Of all the tuyere configurations studied for Q-BOP’s, a half circular tuyere arrangement was found to be the best considering all aspects of mixing, mass transfer, and bath agitation.     

Effects of process conditions on mixing between molten iron and slag in smelting reduction vessel via water model study

Abstract

The present study has been conducted to investigate the effects of operating conditions, which include gas flowrate, tuyere size, tuyere number, and height of iron phase, on the extent of mixing between molten iron and molten slag in the direct iron ore smelting reduction process. A transparent acrylic water model, 30% of the size of the actual smelter, was constructed to study the mass transfer phenomena. In the water model, water and spindle oil were used to simulate molten iron and molten slag, respectively, while air was used to replace the bottom blown nitrogen gas. In addition, thymol (C₁₀H₁₄O₆) was used as a tracer material in the water model, added to the water at the beginning of the experiment. As mixing between water and spindle oil proceeded owing to stirring by the bottom blown gas, the concentration of thymol in the water decreased and that in the spindle oil increased. Water samples were taken from the bottom and 12 cm above the bottom of the water model at various operating times. Concentrations of thymol were then measured using a diode array ultraviolet visible spectrophotometer. By analysing the concentration data, the mass transfer rate k_wA, which is a direct index for evaluating the mixing efficiency, could be derived. The process conditions under investigation included 40-500 L min^-1 gas flowrate, 0·3^-1 cm tuyere size, four or five tuyeres, and 20-30 cm height of the water phase. The test results indicate that when the gas flowrate increases, the value of k_wA increases, which indicates better mixing between oil and water phases. However, as the gas flowrate approaches 40 L min^-1, the improvement becomes less obvious. The smaller tuyere gives better mixing, and the design of five tuyeres results in better mixing compared with four tuyeres when they are blown with the same total gas flowrate. However, mixing efficiency decreases with increased height of the water phase. Also, as the gas flowrate of bottom blowing approaches 40 L min^-1, gas blowing from the top has little effect on the mixing behaviour in the liquid bath. For a four tuyere system, the process conditions of height of oil phase 5 cm, height of water phase 25 cm, diameter of tuyere 0·75 cm, and gas flowrate for each tuyere 40 L min^-1, appear to be the optimal design.     

Flow regimes in submerged gas injection

The behavior of gas discharging into a liquid has been investigated in the labora-tory and in plant. The laboratory work has involved the injection of different gases from a submerged, horizontal tuyere into water, zinc-chloride solution, and a mercury bath. High speed cinematography and pressure measurements in the tuyere have been carried out to characterize the flow regimes. In the case of the mercury bath, a novel “half-tuyere” has been developed to permit visual observation of the gas. In this way, two regimes of flow, bubbling and steady jetting, have been delineated as a function of the modified Froude number and the ratio of gas to liquid densities. Pressure measurements at the tuyere tip have been correlated to the different stages of bubble growth in the bubbling regime, and can be used to distinguish one flow regime from the other. The measured bubble frequency and volume correspond reasonably well to predictions of a simple model of bubble growth under conditions of constant flow. The forward penetration of the jet centerline from the tuyere tip has been measured and found to depend both onN _Fr′ andρg/ρl. In the industrial tests, pressure taps have been installed in the tuyeres of a nickel converter to monitor the pressure wave of the jets under normal, low pressure blowing operations. The measurements show that the converter jets operate in the bubbling mode with a bubble frequency of 10 to 12 s⁻¹, similar to a gas jet in mercury. Tests involving higher pressure injection indicate that the steady jetting, or underexpanded, regime obtains at pressures of about 340 kPa (50 psi). Based on equivalent experiments in the laboratory, it is clear that low pressure blowing has the disadvantage of poor penetration of air into the bath so that the jets rise close to the back wall and locally accelerate refractory wear. Moreover between the formation of successive bubbles, the bath washes against the tuyere mouth and contributes to accretion formation. This necessitates periodic punching of the tuyeres which also contributes to refractory wear at the tuyere line. The use of high pressure injection to achieve steady jetting conditions, as currently practiced in the new bottom blown steelmaking processes, should be considered to solve these prob-lems, and possibly usher in a new generation of nonferrous converters.     

复吹转炉钢-渣间容量传质系数的水-油模型

在熔池直径880mm、深258mm的冷态模型中，利用水模拟钢水、机油模拟渣、苯甲酸模拟钢-渣间传输物质来研究熔池直径5285mm、深1545mm的复吹转炉吹炼工艺参数对钢-渣间容量传质系数的影响。结果表明，枪位350mm，顶吹流量117m^3/h，底吹流量1．68m^3/h至2．36m^3/h时，传输物质苯甲酸的容量传质系数变化最显著，在采用枪位350mm，顶吹流量140m^3/h，底吹流量1．68m^3/h，底枪布置方式为8孔对称布置在0．66直径的圆上时，传输物质的容量传质系数最大。     

Bath mixing behaviour in top–bottom–side blown converter

Abstract

The influence of blowing process parameters on bath stirring was investigated in a model of a top–bottom–side blown converter using physical modelling experiments. It was shown that the side blowing gas flowrate has an important influence on bath mixing time which decreases as side tuyere gas flowrate increases up to a critical flowrate and then plateaus. Bottom gas injection is favourable for bath mixing for top–bottom–side blown converters; however, top lance height, top gas flowrate and bath level have little influence.     

Preliminary Investigation of Mathematical Modeling of Stainless Steelmaking in an AOD Converter: Mathematical Model of the Process

Mathematical modeling of stainless steelmaking in an AOD (argon‐oxygen decarburisation) converter with side and top combined blowing has been preliminarily investigated. The actual situations of the side and top combined blowing AOD process were analysed. A mathematical model for the whole refining process of stainless steel has been proposed and developed. The model is based on the assumption that one part of the oxygen blown through a top lance reacts with CO escaping from the bath, another part of the oxygen oxidizes the elements in the molten steel droplets splashed by the oxygen jet, and the remaining oxygen penetrates and dissolves into the molten steel through the pit stroked by the jet. All the oxygen entering into the bath oxidizes C, Cr, Si, and Mn dissolved in the steel and also the Fe of the steel melt, but the FeO generated is also an oxidant of C, Cr, Si, and Mn in the steel. During the process, all possible oxidation‐reduction reactions occur simultaneously and reach their equilibria, respectively their combined equilibrium, in competition at the liquid/bubble and liquid/slag interfaces. In the simple side blowing after the top blowing operation is finished, the possible reactions take place simultaneously and reach a combined equilibrium in competition at the liquid/bubble interfaces. The overall decarburization rate in the refining process is the sum of the contributions of both the top and side blowing processes. It is also assumed that at high carbon concentrations, the oxidation rates of elements are mainly dependent upon the supplied oxygen rate, and at low carbon contents, the rate of decarburisation is primarily related to the mass transfer of carbon from the molten steel bulk to the interface. It is further assumed that the non‐reacting oxygen blown into the bath does not accumulate in the steel and will escape from the bath and react with CO in the atmosphere above the bath. The study presents calculations of the refining rate and the mass and heat balances of the system for the whole process. Additionally, the influences of the operating factors, including addition of slag materials, scrap, and alloy agents, the non‐isothermal conditions, the changes in the amounts of metal and slag during the whole refining process, and others have all been considered.     

Cold Simulation of Oxygen Transfer Rate in BOF

 By measuring the mass transfer coefficient of between water and oil, the oxygen transfer rate of bath in the BOF smelting process is researched, and the influence of top and bottom blowing gas flow rate on the mechanism of mass transfer between metal and slag is discussed. The results show that when the bottom blowing gas flow rate increases on the conditions of top blowing, the mass transfer rate evidently increases, and the influence ratio of top blowing on the mass transfer is 10 percent of bottom blowing; The relation among top gas flow rate, bottom gas flow rate and lance height are established by the stirring power density. The equation between the mass transfer coefficient between metal and slag is formed, which furnishes reference for optimizing process parameters of BOF. The relation between the emulsification ratio between water and oil and the bottom blowing gas flow rate on the condition of top and bottom blowing is obtained. The result shows that with the increase of the bottom blowing gas flow rate the emulsification ratio increases in linearity, which increases the mass transfer rate of benzoic acid between water and oil.     

Physical simulation of converter steelmaking with powder injection

A water model of top and bottom blown converter with top lance powder injection and bottom tuyere powder injection was established to investigate the powder injection. The results show that the powder penetration ratio under the condition of top lance injection is greater than that under the condition of bottom tuyere powder injection. In both cases, the powder penetration ratio increases with the increase of solid/gas ratio and powder particle size. Powder uniform dispersion time with top lance powder injection is longer than that with bottom tuyere powder injection. Top lance powder injection, lance height of 258?mm, bottom blowing rate of 1·96?Nm³?h^?1 and powder particle size of 0·212–0·380?mm are suggested as the optimum powder injection operation under the experimental condition. The corresponding optimum operation for prototype is top lance powder injection with lance height of 1550?mm, powder size of 1–5?mm and bottom blowing rate of 450?Nm³?h^?1.     

Mathematical Modeling of Fluid Flow in Bath during Combined Side and Top Blowing AOD Refining Process of Stainless Steel: Mathematical Model of the Fluid Flow

Considering that the liquid flow field under the conditions of the combined side and top blowing would be a combined result from the common action of the side blowing gas streams and a gas top blowing jet, as the first attempt, the three‐dimensional mathematical models for the flows of molten steel in an AOD converter bath during the simple side and top blowing processes have been proposed and developed, respectively. And the mathematical model of the flow in the bath during the combined blowing AOD refining process of stainless steel has been given by the composition and superposition of the two models. In the composed model, the gas‐liquid two‐phase flow is described and treated in terms of the two‐fluid (Eulerian‐Eulerian) model. The especially modified two‐equation k?ε model for the turbulence in the liquid phase is employed. And, the surface of the sunken pit formed by impact of the gas jet blown from a top lance at the central location of the bath liquid surface is regarded as a revolution paraboloid. The related details of the composed model are shown.     

Design and calculation of gas property parameters for constant area lance under conditions of friction flow with heating

Abstract

Formulae for calculating the outlet property parameters of gas heating and friction streams in tubular and annular type lances with constant area (tuyeres) are given, and have been applied to the case of an annular type used for an AOD (argon–oxygen decarburisation) vessel of 18 t capacity. The distributions of both the inner wall temperatures of the tuyere and the gas stagnation temperatures along its length have been more reasonably fixed. The friction factors for the gas flows through the main and subtuyeres during blowing refining have been determined by comparison of the practically measured P–Q relationships with the results from trial calculations. The outlet parameters of the gas streams for the central tube (main tuyere) and annular slit pipe (subtuyere) of the tuyere have been calculated. The influences of the gas supply pressure, the length and diameter of the tuyere, and the type and composition of the gases, as well as the heating effect, on the gas outlet parameters have been considered. The results obtained may be expected to offer useful information and a reliable basis for tuyere design and determination, control, and optimisation of the gas blowing parameters and technology, as well as for the investigation of hydraulic modelling of the blowing processes.     

顶底侧吹转炉炼钢新技术开发研究

在顶底复吹转炉熔池侧壁上安装侧吹枪,形成顶底侧吹转炉。通过实验室物理模拟研究了顶底侧吹条件下转炉熔池的混匀行为;在工业试验中,对比了顶底侧吹转炉和顶底复吹转炉炼钢的冶金效果。实验室研究结果表明,顶底侧吹技术可以显著提高转炉熔池的搅拌能力,大幅度降低转炉熔池的混匀时间,存在一个临界侧吹气量,当侧吹气量大于该临界值后,熔池混匀时间变化不大。工业试验结果表明,转炉采用顶底侧吹技术,可以降低钢铁料消耗,吨钢石灰消耗可降低将近3kg,提高了转炉的脱磷能力,降低炉渣和钢水的氧化性,平均出钢碳氧积为0.0025×10-4,钢水氧化性的降低提高了合金收得率。     

Injection phenomena in nonferrous processes

Injection phenomena in a Peirce-Smith copper converter and a slag fuming furnace have been investigated during blowing. A tuyerescope attached to the back of a given tuyere has been employed to observe the build-up, and to obtain samples, of accretions while a piezoelectric transducer has been used to measure pressure fluctuations in the tuyere. It has been found that in both processes the accretions grow upward by a freezing mechanism from the bottom of the horizontal tuyere. Probing the shape of an accretion in the fuming furnace, without coal injection, showed that the accretion grew into the bath 30 to 40 mm from the tuyere tip. A study of the shape and frequency of the pressure pulses as well as the duration of intervals of low pressure in the tuyere has revealed that the closely spaced tuyeres may operate independently or may interact. Tuyeres in a full slag fuming furnace act independently of one another, and the air discharges in the classical bubbling regime at 5 to 6 bubbles per second. In the copper converter there is more tuyere interaction as bubbles growing at adjacent tuyeres coalesce to form a horizontal unstable gas envelope. In both processes if the tuyeres are shallowly submerged the gas may channel directly to the surface without forming bubbles. The influence of bath viscosity, the extent of wall erosion at the tuyere line, and tuyere submergence on these injection phenomena have been elucidated qualitatively. G.G. RICHARDS, formerly Graduate Student     

双流体模型在AOD熔池流场数学模拟中的应用

基于气液双流体模型和湍流的修正k-ε模型,考虑了多股气流侧吹操作对熔池流场的影响,以及AOD熔池内气液两相流的行为和两相间的动量传输,建立了AOD多股气流侧吹精炼过程中熔池内流体流动的数学模型,并对宝钢股份不锈钢事业部120 tAOD原型和线尺寸为其1/4的水模型熔池内流体的流动作了模拟,结果表明,确实可以采用双流体模型来模拟AOD精炼过程中熔池内流体的流动;用该模型计算的结果表明,整个熔池流体处于活泼的搅拌和循环运动状态。     

地转偏向力对侧顶复吹AOD炉流场影响的理论分析

基于地转偏向力的原理和120 t侧顶复吹AOD炉的构造,对AOD炉和地转偏向力的关系进行了理论分析和计算,认为地转偏向力通过对AOD炉内侧枪和顶枪气流的作用影响其流场.针对本研究的AOD炉,计算表明地转偏向力使顶枪气流从出口到熔池液面发生7.92×10^-8(°)的偏转,使侧枪气流发生1.88×10^-4(°)的偏转,说明地转偏向力对侧枪气流的影响更大.     

Study of solid mushroom formation in direct iron ore smelting reduction process using low temperature water model

Abstract

In the direct iron ore smelting reduction process, molten iron near the bottom blowing gas tuyere is cooled by low temperature/endothermic gas and forms a mushroom shaped solid on top of the tuyere. The formation of an appropriate solid mushroom, which covers the tuyere, can protect the tuyere and the surrounding refractory. In the present study, a water model with a low temperature gas system was established to investigate formation of the solid mushroom and the effects of operating conditions on its shape and dimensions. Transparent acrylic was used to construct the water model, which was 40% of the size of the actual furnace. Water was used to simulate the molten iron. Low temperature air, obtained by passing air through a heat exchanger cooled by liquid nitrogen, was blown into the water bath through a bottom tuyere. The air temperature was able to reach-188±1°C. In the water model experiments, water near the tuyere was cooled, and formed an ice mushroom surrounding the tuyere. The effects of operating conditions, mainly gas flowrate and mould material surrounding the tuyere, on the parameters of the solid mushroom were investigated. The parameters of the solid mushroom included whether it could be formed and duration of the solid mushroom, as well as the shape, dimensions, and weight of the solid mushroom. Attempts were also made to relate the temperature-time and pressure-time relationships of the blown gas to the parameters of the solid mushroom. With copper used as mould material surrounding the tuyere, the water model experiments were conducted with flowrate of the bottom blown gas set in the range 30-90 NL min^-1. The results show that as the gas flowrate was increased, the highest water temperature which allowed the solid mushroom to form in the water model was increased. Three different types of pressure-time curve were obtained under different gas flowrates in the present study. They also corresponded to different forms of solid mushroom. As peaks appeared in the pressure-time curve, they revealed ice capsulation and subsequent bursting to release the pressure. A gas flowrate of 80 NL min^-1 and water temperature of 19·2°C with copper plate as the bottom material are considered to be optimal conditions of the water model for growth of the appropriate ice mushroom. These data are rather consistent with the gas flowrate and superheat for the actual direct iron ore smelting reduction unit, which are 2700 NL min^-1 and 120°C (equivalent to 70 NL min^-1 and 22·7°C in the water model).     

Cold model studies of mixing and mass transfer in steelmaking vessels

Abstract

Room temperature model studies using water to simulate 'metal' and paraffin oil (when required) as 'slag' were conducted to study the extent of mixing and the rate of mass transfer between metal and slag in the 130 t basic oxygen furnaces (BOFs) in operation in Tata Steel. Several systems of gas injection including top blowing, combined blowing and exclusive bottom purging were investigated. Similar work was undertaken in a room temperature model of an 80 t energy optimising furnace (EOF), in operation for a brief period earlier in Tata Steel. Details of the optimum blowing conditions, including the number/distribution of bottom tuyeres for the BOFs, are elaborated in the present paper. How mixing/mass transfer in an EOF compares with the BOF case(s) is also highlighted.     

Splashing mechanism in combined blowing

Abstract

In combined BOF blowing, lance parameters and the combination of bottom or side wall tuyeres have an influence on splashing behaviour. The aim of this study was to clarify the interaction of the lance jet cavity with the bottom blowing plume and the side wall blowing jet and to determine its effects on splashing. According to the water model tests, three basic axioms existed in the combined blowing. First, when the bottom tuyere (or side wall tuyere) was located exactly beneath the lance jet, the lowered cavity turned the direction of splashes to lower trajectories. Second, the total amount of splashing was constant and the splashing peak was generated on the wall above the bottom plume. Third, both the plume and the side wall jet formed a so called protected zone beyond it. The model experiments showed clearly that the combination of bottom tuyeres and interaction of cavities and plumes play a very important role in splash generation in real converters.