Model calculations for mass transfer with mixing in ladle metallurgy

It is well known that in secondary metallurgy during gas stirring, zones with differing degrees of homogeneity can occur in the liquid steel. By means of model calculations it will be demonstrated under which conditions mixing has to be considered for mass-transfer phenomena, as for example desulphurization. Under unfavourable conditions, i.e. large regions with low flow velocities – so-called dead zones – and low values for the exchange volume flow between these regions and the bulk volume, the equalization of concentration will become the rate-determining step for the total mass transfer. In order to avoid or minimize dead zones, the general flow pattern in the ladle has to be optimized.     

Emulsification and mass transfer in ladle metallurgy

Measurements of slag emulsification in gas-stirred ladles were carried out in cold-model systems of different geometric sizes. Detaching of slag droplets – necessary for efficient emulsification – only takes place if the flow velocity at the slag/metal interface exceeds a certain level. The use of a centric nozzle leads at high gas flow rates to considerably larger degrees of emulsification than eccentric stirring. The reason of this phenomenon is that the recirculation flow during centric gas injection transports larger amounts of emulsified droplets into deeper regions of the melt while during eccentric stirring there is more time for reseparation of slag droplets into the top slag. Comparing emulsification results with mass-transfer measurements, the dependence between rate constants as well as degrees of emulsification and Froude number shows similar behaviour.     

Reaction technique in ladle metallurgy

Process models are very useful to control high efficient industrial metallurgical processes.However their accuracy depends strongly on the choice of boundary conditions and thermodynamic as well as kinetic data used. Whereas the commercial data base FactSage or Thermocalc is used as the source of thermodynamic data the kinetic parameter are characteristic for each process and process design.Therefore it is essential to estimate the kinetic parameter in well designed experiments supported by using of numerical methods. In this paper the steel melt flow parameter,gas-melt interfacial area and mass transfer coefficient obtained in 30t industrial gas stirred ladles are described.On the example of nitrogen absorption and desorption the predictive process model for nitrogen control while ladle treatment and decarburisation process is presented for different steel alloys.The modelling results are compared with results from industrial processes.     

Interfacial phenomena and mass transfer in extractive metallurgy

Interfacial phenomena play an important role in pyrometallurgical processes and knowledge of them and of physical properties involved is necessary for understanding the mechanisms and the kinetics of such reactions. A large number of measurements, performed at Irsid under equilibrium conditions, are presented in this review. Several experimental techniques were used and more particularly: – Sessile drop method for measurement of liquid metals surface tension and contact angle liquid metal/solid oxide; – measurement of the contact angle between a liquid slag drop and its liquid metal substrate from which the interfacial tension can be derived; – direct determination of the interfacial tension from X-ray pictures of metal drops immersed in the slag. The systems studied consisted, for the metal phase, of binary and ternary Fe alloys containing C, Mn, Si, O, S and, for the slag phase, binary and ternary mixtures made from CaO, SiO₂, Al₂O₃, MnO, iron oxides, CaF₂ and Na₂O. A strong effect of O and S potentials was observed. For non-equilibrium conditions, however, the dynamic interfacial tension between liquid metal and slag decreases sharply when an intense mass transfer occurs through the interface. The potential consideration of interfacial turbulence phenomena (Marangoni effect) in metallurgical reactions is also discussed.     

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.     

Kinetic model for the influence of carry-over slag in ladle metallurgy

A kinetic model for the refining of steel by powder injection and by top slag reaction is presented. The influence of carry-over slag on the indepentent and on the combined action of both processes is evaluated. The calculated concentration-time curves clearly show the detrimental influence of carry-over slag and the benefits of different processing ways for different thermodynamic, kinetic and process parameters. An estimation of the numerical values of the transport coefficients in the metal phase and in the injected particles is included.     

Hydrodynamic modeling of some gas injection procedures in ladle metallurgy operations

Experimental studies of flows generated in a 0.30 scale water model of a 150 ton steelmaking ladle are reported. These were used to test the adequacy of a generalized two-dimensional computational scheme for predicting flows generated by fully submerged and partially submerged gas injection lances. The roles of turbulence models and grid configuration were assessed. Predictions for flow fields generated in a 150 ton steelworks ladle with and without tapered side walls, and with and without surface baffles around the rising plume were considered.     

A correlation for estimation of mass transfer rates of solids in gas stirred ladle systems

Mass transfer between solid and liquid in gas stirred cylindrical vessels has been analysed both theoretically and experimentally. To this end, starting from an appropriate “single particle-fluid correlation”, a relationship applicable to a system of suspended solids has been deduced and represented according to: . On the basis of this, dissolution of solids in two different aqueous models of gas stirred ladles has been investigated and it has been demonstrated that the correlation produces estimates which are in reasonable agreement with the experimental observations. Furthermore, it is shown that the present correlation has a form analogous to the one, derived much earlier by Kolmogoroff from the theory of isotropic turbulence.     

A novel dual plugs gas blowing mode for efficient ladle metallurgy

A novel gas blowing mode with different flowrates for two plugs of metallurgical ladle is explored and studied through a sophisticated water model. The results show that this mode can efficiently decrease the mixing time and the total area of the slag eye for most cases, as compared with the conventional mode with same flowrates for two plugs. Generally, a relatively close angle between the porous plugs and a small radial position are beneficial to a decrease in the mixing time of bath, while a relatively far plug radial position leads to a smaller slag eye. In addition, tracers fed from the middle of the dual plugs are proven to be very beneficial to the mixing of the ladle. The slag layer will prolong the mixing time due to its consumption on the stirring energy compared with the situation without slag.     

Modeling of gas-liquid reactions in ladle metallurgy: Part I. Physical modeling

A physical model of a ladle degassing operation was developed to simulate the reactions at rising bubbles and at the free surface. Carbon dioxide desorption from a sodium hydroxide solution was used to simulate the liquid-phase diffusion-controlled decarburization of liquid steel. It was found that under reduced pressure, the reactions were faster than attributable to solely the increase in volumetric flow rate. It was possible to separate the reactions with the bubbles from the free surface reactions; 20 to 40 pct of the reactions occurred at the free surface, depending on injection conditions. The free surface desorption rate depended on the gas flow rate and the number of injectors. The mass transfer coefficients to the bubbles were in reasonable agreement with previous work. Plume bending was observed when small bubbles were influenced by the bulk liquid flow patterns.     

Modeling of gas-liquid reactions in ladle metallurgy: Part II. Numerical simulation

A combined three-dimensional, Lagrangian-Eulerian model for gas-liquid flow in a ladle was developed. The model compared very well with available experimental results in Wood’s metal in terms of void fraction, liquid velocity, and plume bending. From the model, it was clear that the lateral lift force is responsible for plume spreading, whereas lateral drag forces bend the plume. The model was extended to include mass transfer to rising bubbles and at the free surface. The rate of reaction compared very well with the results of part I on the desorption of carbon dioxide from sodium hydroxide solutions.     

Properties of through hardening bearing steels produced by BOF blowing metallurgy and by electric arc furnace with ladle metallurgy

Melting in EAF and BOF-TBM and different methods of aluminium deoxidation in the subsequent ladle treatment result in a similar degree of cleanness. With respect to micro-inclusions the air melted ingot and continuous cast steels achieve nearly the same level as remelted steels. Considering macro-inclusions there still remains a decisive difference: remelted steel is sure to be free from macro-inclusions, whereas some occasionally occur in air melted steel. The random test and metallographic evaluation systems do not allow uniform classification of the various steel groups. None of the applied methods can determine definite inclusion contents. The number and sizes of the evaluated types of inclusions only can define the statistical probability of their occurrence in a larger lot of heats. Up to now they cannot be related to the fatigue behaviour of the steels. Therefore, further investigation is necessary to reveal the effect of critical sizes and chemistry of the various inclusions on the fatigue behaviour. First tests reveal that the machinability seems to be governed by the carbide grain sizes more than by the sulphur contents in the investigated range of 0.003-0.015%. The comparison of test results in terms of cleanness and segregation shows that continuous casting should allow the same application as ingot casting.     

Model study on mixing and mass transfer in ferroalloy refining processes

An experimental study has been performed to investigate the bath mixing intensity induced by a high-strength submerged gas injection in a bottom blown air-stirred one-seventh water model of Creusot-Loire Uddeholm (CLU) reactor using three different tuyere configurations. Experimental results have been discussed in terms of the mass transfer rate and mixing time. The air flow rates varied from 0.00599 to 0.01465 m³/s. The mixing time was determined at various gas flow rates, bath heights, and nozzle orientations, both in the presence and absence of a second phase. The mixing time was found to decrease with increasing gas flow rate and decreasing bath height. The influence of bath mixing intensity on mass transfer between metal (water) and slag (paraffin) was studied by measuring the transfer of benzoic acid from the gas-stirred water bath to paraffin as a function of the gas injection parameters. The bath mixing intensity was characterized by the value of the mass transfer rate constant. The rate constant of mass transfer between the metal and slag was found to increase with increasing gas injection rate and decreasing bath height.     

Modern trends in the development of ladle metallurgy and the problem of nonmetallic inclusions in steel

The main trends in the development of the ladle treatment of steel have been analyzed. The most complex problem is shown to be the formation of nonmetallic inclusions having a certain type and a low concentration. The main parameters of the ladle treatment of steel that control the type and amount of nonmentallic inclusions in ready products are analyzed using the data of industrial heats, laboratory experiments, and physicochemical simulation and computation. The principles of control of steel purity in nonmetallic inclusions are formulated, and the main trends in the development of the concepts of nonmetallic inclusions are determined.     

钢包底换热系数的计算

根据钢包出钢完毕后的传热特点,推导出包底耐火材料的自然对流换热系数和辐射换热系数计算公式,并由此计算出包底的对流辐射换热系数.通过举例计算发现,在包底温度介于1400～1600℃时,对流换热系数远小于辐射换热系数,且其大小随温度升高略有降低;对流辐射换热系数随温度的升高而增加,且其大小与温度基本呈线性关系.     

Optimizing the technology for making automobile sheet 08Yu based on the physicochemical principles of ladle metallurgy

The main method of eliminating the possibility of having the steel contaminated by nonmetallic inclusions is optimizing the parameters of the ladle treatment on the basis of reliable physicochemical representations of the processes of refining, alloy, and finishing of the steel, the interactions of the metal and slag phases, and other aspects of ladle metallurgy. This article examines the main steps being taken in this direction at the Cherepovets Metallurgical Combine Severstal’ in the production of high-quality automobile sheet 08Yu. __________ Translated from Metallurg, No. 8, pp. 58–65, August, 2007.