A model test for emulsion in gas–stirred ladles

At high gas-stirring rates of liquid metals in ladles covered with slag emulsification of slag droplets into the metal melt occurs. Emulsification intensifies the rate of slag-metal extracting reactions. A theoretical model of the emulsification process was developed. The equilibrium between buoyancy, surface, and inertial force at the location where the slag droplets are formed determines the critical liquid metal velocity for drop formation. The amount of droplets formed per time depends on the flow of kinetic energy in the thin slag layer accelerated by momentum transfer from metal to slag. Experiments on a water model were carried out to determine the drop sizes, the number of droplets formed per time and the drop flow paths. Consequences for the amount of emulsification in steel melts are drawn.     

Mixing models for gas stirred metallurgical reactors

The mixing of liquids in ladles, (0.5 ≦L/D ≦ 2.0), agitated by a centrally rising bubble plume, has been analyzed both theoretically and experimentally. An exhaustive review of previous metallurgical literature on mixing in ladles and furnaces demonstrates that the majority of previous investigators in the field consider mixing to be brought about primarily by turbulent diffusion phenomena. The present study clearly shows that mixing is a combination of both convection and eddy diffusion processes, neither of which can be disregarded for gas stirred systems. For predicting mixing times during such gas injection procedures, a simple empirical equation is proposed for axisymmetric systems:τ _mαε_m ^−1/3L⁻¹R^5/3. Hereτ _m is the 95 pct mixing time,ε _m is the specific energy input rate,R is the vessel radius, andL is the depth of liquid. On the basis of physical and mathematical modeling, the rate of liquid mixing in conventional gas injection ladle metallurgy operations is compared with those observed in C.A.S. (composition adjustment by sealed argon bubbling) systems. It was found that mixing in C.A.S. operations is relatively slow and highly insensitive to gas flowrates(i.e., specific energy input rates).     

Modelling criteria for flow simulation in gas stirred ladles: experimental study

Abstract

Criteria for modelling isothermal flows encountered in typical gas stirred ladles have been investigated both theoretically and experimentally. To this end, the phenomena of fluid mixing in ladles have been investigated in order to deduce the relationship between model and full scale gas flowrates, needed for maintaining dynamic similarity between the two. Starting with the governing equation for material transport, mixing times in geometrically and dynamically similar gas stirred systems were first correlated theoretically. On the basis of this, it is shown that, in the Froude dominated flow regime (typical of industrial ladle refining operations), the ratio of mixing times in geometrically and dynamically similar gas stirred systems can be represented in terms of the geometrical scale factor λ(=L _mod /L _fs ) according to τ_m,mod /τ_{m,fs = λ}^1/2. To assess the adequacy and appropriateness of various scaling equations reported in the literature (namely Q_{mod /Q fs} =λⁿ, proposed values of nbeing 1·5, 2·5, and 2·75, respectively), extensive experimental measurements of mixing times were carried out in three differently sized water model ladles. To measure mixing times, the conventional conductivity measurement technique was adopted. Comparisons of experimental ratios of mixing times with the corresponding theoretical ratio (=λ^1/2 ) confirm that, in the Froude dominated flow regime, the most appropriate criterion for dynamic similarity between model and full scale ladles is Q _mod /Q _fs = λ^2·5. Such findings were also corroborated through consideration of empirical mixing time correlations reported for Froude dominated ladle flows.     

Special porous plugs for the production of super clean steels

Fine bubblers to be used as special porous plugs (SPP), have been devised for fine bubbling within steel melts for improving the cleanliness efficiently by floating the micro‐inclusions and removing the dissolved gases. The development principles of the magnesia‐spinel porous plugs are presented. Laboratory water simulator tests have shown that small (< 1 mm), non‐coagulating bubbles, with uniform distribution are formed. The characterisation and testing of the SPP has shown that under steelmaking conditions clogging from metal infiltration is limited. Erosion, cracking or changes in microstructure were not detected in high temperature long‐term endurance tests. With combined treatment in a vacuum induction furnace under a 3 kPa pressure and Ar purging of 18 min through the new SPP, the total oxygen content of the steel melt was decreased from 65 to 7.7 ppm. An application has been filed for a patent for the method for the production of fine bubblers to be used as porous plugs     

Numerical modelling studies of flow and mixing phenomena in gas stirred steel ladles

Abstract

A three-dimensional computational fluid dynamics (CFD) model was developed to simulate the fluid flow and mixing phenomena in a gas stirred ladle. Particular attention was paid to incorporate the effect of slippage between rising gas bubbles and surrounding fluid in the numerical model, to capture the relevant flow physics in a more effective manner. Various parametric studies were undertaken to examine the effects of gas flow rate, bottom nozzle configurations and tracer addition locations on mixing time. It was observed that the arrangement of bottom nozzles has a great effect on the mixing behaviour in a gas stirred ladle, with off centric gas injection producing shorter mixing time. Mixing time was found to be sensitive to the tracer addition position, particularly for the axisymmetric bottom gas injection system. The predicted results were compared with reported experimental observations and a very good agreement was observed in this regard, thereby establishing the authenticity of the proposed formulation.     

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.     

Slag eye formation in single and dual bottom purged industrial steelmaking ladles

ABSTRACT

Argon gas is often injected from the bottom of the ladle during steel refining operations. The injected gas interacts with the liquid (metal and slag) bath and enhances the momentum, heat, and mass transfer rate in the melt. However, during these gas–liquid interactions, an opening of the slag layer called slag eye is formed, which exposes the molten metal surface to the atmosphere, which is generally undesirable. In the current work, a transient, three-dimensional mathematical model is used to study the turbulent gas–liquid interactions in single as well as dual bottom blown industrial steelmaking ladles. A Coupled Level Set Volume of Fluid (CLSVOF) model is used for tracking the steel-argon, steel-slag, and argon-slag interfaces, from which the slag-eye area has been predicted. It is found that the inlet gas purging rate, melt height, slag layer thickness, angular and radial positions of the gas inlets affect the slag opening area. Non-dimensional empirical correlations are proposed to predict the slag opening area in both single as well as dual purged ladles, using non-linear regression analysis.     

两段强制搅拌调浆的混合特性及对煤泥浮选的影响

基于强化煤泥浮选前的矿浆预处理过程,设计了具有折叶开启式涡轮的两段强制搅拌装置.通过分析两段强制搅拌体系的流量准数、功率准数、剪切特性、循环特性以及混合效率来评价搅拌体系的混合特性,并对永城矿区的无烟煤煤泥进行了两段强制模式的调浆浮选试验.随着转速的增加,输入两段强制搅拌体系的能量以及混合效率增加,单位能耗的矿浆循环性能和剪切性能均减小,剪切性能减小的趋势小于循环性能减小的趋势.在相同转速下,大直径叶片的单位能耗具有较强的剪切特性及混合效率、较小的循环/剪切比.在两段强制搅拌调浆模式下,旋流静态微泡浮选床可获得较好的浮选指标和较大的处理能力.大直径和高转速条件下的能量输入可促进矿浆中难浮颗粒的回收,在合适的处理能力条件下能够加强粗颗粒的回收,而在处理能力较强的条件下可以提高浮选尾煤中的细颗粒灰分.     

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.     

Residence time calculation for chemotactic bacteria within porous media

Local chemical gradients can have a significant impact on bacterial population distributions within subsurface environments by evoking chemotactic responses. These local gradients may be created by consumption of a slowly diffusing nutrient, generation of a local food source from cell lysis, or dissolution of nonaqueous phase liquids trapped within the interstices of a soil matrix. We used a random walk simulation algorithm to study the effect of a local microscopic gradient on the swimming behavior of bacteria in a porous medium. The model porous medium was constructed using molecular dynamics simulations applied to a fluid of equal-sized spheres. The chemoattractant gradient was approximated with spherical symmetry, and the parameters for the swimming behavior of soil bacterium Pseudomonas putida were based on literature values. Two different mechanisms for bacterial chemotaxis, one in which the bacteria responded to both positive and negative gradients, and the other in which they responded only to positive gradients, were compared. The results of the computer simulations showed that chemotaxis can increase migration through a porous medium in response to microscopic-scale gradients. The simulation results also suggested that a more significant role of chemotaxis may be to increase the residence time of the bacteria in the vicinity of an attractant source.     

Spinel and carbide formation in oxide-carbon refractories for steel-casting ladles

In the converter shop at OAO Nizhnetagil’skii Metallurgicheskii Kombinat (NTMK), the steel-casting ladle is lined with oxide-carbon refractories. The aggressiveness of slags toward the refractory lining is explained. The composition of the linings is studied, along with the processes within the refractories at high temperatures. The basic requirements on refractory components supplied to OAO NTMK are formulated. The life of casting-ladle linings in the converter shop is increased.     

Effect of holding time and surface cover in ladles on liquid steel flow in continuous casting tundishes

Mathematical modeling of fluid flow and heat transfer of melt in a typical two-strand slab caster tundish has been done for a complete casting sequence. The complete casting sequence consists of 1 minute of tundish emptying period during the ladle transfer operation followed by 1 minute of tundish filling period by the new ladle and pouring at the normal operating level of the tundish for 46 minutes. The effect of varying ladle stream temperature conditions on the melt flow and heat transfer in the continuous casting tundish has been studied. When the ladle stream temperature decreases appreciably over the casting period, corresponding to heat loss of the melt in the ladle from the top free surface, the incoming melt temperature becomes lower than that of the melt in the bulk of the tundish after about 30 minutes from the start of teeming. This results in melt flow along the bottom of the tundish instead of the normal free surface directed flow. The ladle melt stream temperature shows little variability when the ladle has an insulated top. Corresponding to this situation, the temperature of the incoming melt remains higher than that of the melt in the bulk of the tundish and the normal free surface directed flow is maintained throughout the casting period. The product cast under such condition is expected to have a uniformly low inclusion content. The heat loss condition from the top of the ladle has been shown to be the dominant factor in determining fluid flow and heat-transfer characteristics of the melt in the tundish rather than the holding time of the melt in the ladle. Formerly Graduate Student, Department of Materials Science and Engineering, Ohio State University     

连铸用钢包的耐火内衬优化及疲劳寿命预测

摘要：连铸技术对钢包的性能要求越来越高,而传统钢包炉衬构件普遍存在使用寿命短、消耗高等问题。通过研究耐火材料特性,优化内衬结构布置,设计出一种长寿命、超保温的新型钢包,并基于数值模拟技术,对新型钢包与传统钢包在典型工况下的温度与应力进行对比分析。温度场模拟结果表明,新型钢包在保温性能上有较大的提升,钢包壳最高温度较传统钢包降低54℃。同时,应力场结果表明,新型钢包壳的最大应力减小了66.7MPa且整体应力分布更加均匀。最后将温度场和应力场的分析结果反馈到钢包的生产、制造、维护上,并进行实验验证。实验结果表明,新型钢包在保温与长寿等性能方面表现更好,内衬寿命提高了119炉次,达到了钢包设计、制造、维护一体化的效果。