Inclusion Flotation Research Articles

Bubble plume effects on the flotation kinetics of nonmetallic inclusions based on experimental observations

AbstractSteelmaking has shown an increasing concern toward nonmetallic inclusions, leading to new technologies in the secondary metallurgy of steel. Although the typical inclusion removal procedure is by injecting inert gas into the ladle, this vessel does not fulfill all the requirements to accept a porous structure tailored to produce “clean steels.” Consequently, the spotlight has moved to the tundish, the last vessel before solidification, in which gas injection can continuously operate. Therefore, this work focuses on understanding the influence of typical gas flow rates (10–60 NL/min) on the kinetics of inclusion flotation, considering two bubble diameters (0.6 and 1.1 mm). For this purpose, experimental measurements were conducted in a water model, where glass hollow spheres played the role of inclusions, and their concentration was fitted by an exponential decay. In general, injecting bubbles into the system contributed positively to a faster and greater flotation of particles. The smaller bubbles led to a higher maximum efficiency, whereas the larger ones allowed a shorter time scale (i.e., a faster removal), defining a trade‐off to tune the bubble size. Regarding the gas flow rate, the results indicate an optimum range to decrease the time scale, and suggestions for bubble curtains in tundishes are drawn.

Physical simulation study on flow field characteristics of molten steel during argon blowing in shroud of tundish

In the current study, water models are used to study the effect of the flow control devices, the casting velocity, gas flow rate and gas inlet position in the shroud on the flotation of inclusions and molten steel transfer behaviour at the tundish. Microbubbles were generated by using a ladle shroud. The results show that the inclusion flotation efficiency decreases and the dead volume fraction increases as the casting velocity becomes faster. Meanwhile, with the installation of the flow control devices, the residence time of molten steel becomes longer and the inclusion flotation efficiency is getting higher. Bubble adhesion plays an important role in removing inclusions. Smaller bubbles are favourable for inclusion flotation. As the flow rate of gas becomes larger and the gas inlet position installs a topper in the shroud, the bubbles become small. In this experiment, the flotation rate of inclusions in the tundish can be improved to 95.28% by changing the blowing parameters.

Stabilizing Flow of Molten Steel in Mold during Slab Continuous Casting through Electromagnetic Swirling Flow in Nozzle

Bias flow of molten metal in slab mold caused by unstable flow from submerged entry nozzle can deteriorate the slab quality during continuous casting. In this study, a novel electromagnetic swirling flow in nozzle (EMSFN) technique, which utilizes electromagnetic force to stabilize the flow in the nozzle and subsequently control the flow in the mold, is proposed. The mechanism for controlling unstable flow through EMSFN is explored through numerical simulation and water model experiments. In the results, it is shown that the molten steel is rotated under the effect of magnetic field and flows into the mold along the upper edge of the nozzle outlet, which weakens the impact of molten steel on the bottom of nozzle and stabilizes the outflow from nozzle. The flow field is consistently symmetrical on both sides of the nozzle. The impact depth of molten steel flowing down along the narrow sides of the mold on both sides is greatly reduced. The surface velocity on both sides of the nozzle changes uniformly. The range of surface velocity variation with EMSFN is only 10% of that with a conventional nozzle. The EMSFN technique can help prevent slag entrapment and improve the flotation of inclusions and bubbles.

Water Model Experiment on Coagulation, Entrapment by Liquid/Liquid Interface and Bubble Flotation of Inclusions in Molten Copper

Water Model Experiment on Coagulation, Entrapment by Liquid/Liquid Interface and Bubble Flotation of Inclusions in Molten Copper

Effect of Impeller and Gas Stirring on Agglomeration Behavior of Polydisperse Fine Particles in Liquid

Agglomeration, coalescence and flotation of non-metallic inclusions in steel melt are effective for obtaining “clean steel.” In this study, the agglomeration and breakup behaviors of particles with a primary particle size distribution (hereinafter, polydisperse particles) in a liquid under impeller and gas stirring were compared by numerical calculations and model experiments. The particle-size-grouping (PSG) method in the numerical agglomeration model of particles was combined with a breakup term of agglomeration due to bubble bursting at the free surface. Polydisperse and monodisperse polymethylmethacrylate (PMMA) particles were used in the agglomeration experiments. The agglomeration rate of the polydisperse particles under impeller stirring was increased by an increasing energy input rate, whereas the agglomeration rate under gas stirring decreased under this condition due to the larger contribution of the breakup of agglomerated particles during bubble bursting in gas stirring. At the same energy input rate, agglomeration of polydisperse particles was larger under impeller stirring than under gas stirring. The agglomeration rate of polydisperse particles was larger than that of monodisperse particles under both impeller and gas stirring at the same energy input rate. The computational temporal changes in the total number of particles were in good agreement with the experimental results. This means that the difference in the agglomeration behaviors observed in impeller and gas stirring can be explained by the turbulent coagulation and subsequent agglomerated particle breakup in gas stirring. The computational temporal change in the number of each group approximately agreed with the experimental change in both impeller and gas stirring.

Water Model Experiment on Bubble Flotation of Inclusions in Molten Copper

Water Model Experiment on Bubble Flotation of Inclusions in Molten Copper

Investigation of Inclusion Agglomeration and Flotation During Levitation Melting of Ni-Based Superalloy in a Cold Crucible

Inclusion agglomeration and flotation during levitation melting in a cold crucible were investigated. The morphology, number and chemical composition of inclusions were characterized by scanning electron microscopy and transmission electron microscopy with energy-dispersive x-ray spectroscopy analysis. In the billet, inclusions were mainly single mixed inclusions (oxide as the core, nitride as the shell), and their size was in the range of 1–7 μm. After levitation melting, approximately 81% of inclusions existed in the form of clusters. The number of clusters was higher at the top, and the size was larger at the top (150–250 μm). The mechanism of levitation melting removing inclusions was discussed by collision theory. First, nitrides dissolved, and the core oxides were exposed. Then, oxides collided, agglomerated, and floated. Turbulent collision played the leading role in inclusion agglomeration, while Stokes collision was predominant for inclusion flotation. Finally, nitrides precipitated again during solidification.

Development of a Reliable Kinetic Model for Ladle Refining

Kinetic modeling of ladle refining can be used by steel plants to improve steelmaking and to produce cleaner steels. It can also help researchers to understand the process better. There have been several recent attempts to develop a kinetic model that can predict changes in steel, slag, and inclusion compositions with time. Often the models require parameters that can be difficult to measure under plant conditions, and model limitations have not been discussed in detail. In this study, a two-parameter kinetic model has been developed to predict changes in steel, slag, and inclusion compositions during ladle refining; the two parameters are the mass-transfer coefficient in steel and the inclusion flotation rate constant. The model was based on FactSage macro-processing. Examples of results show that the model can be used to diagnose effects of steel and slag sampling practices and to estimate alloy dissolution time. The model results demonstrate that the presence of solids in slag can significantly reduce the rate of steel–slag reaction while maintaining a high inclusion flotation rate. Some limitations of the model are discussed: the overprediction of calcium transfer to alumina inclusions and the absence of information on inclusions originating from entrained slag.

Flow Interaction in Continuous Casting Tundish Due to Bubble Curtain Operation

Tundish argon bubbling has been studied in literature regarding homogenization and non‐metallic inclusion removal. Argon bubbling not only alters the flow within the tundish, which might aid the natural flotation of inclusions, argon bubbles also can directly capture inclusions, when bubbles and inclusions collide. The ascending bubbles from the line sparger induce a recirculating flow within the tundish. This recirculating flow interacts with the incoming liquid jet, emanating from the ladle shroud. For a sufficiently strong recirculating flow, the incoming liquid jet is deflected in a similar manner as a jet in cross‐flow. For this paper, computational fluid dynamics (CFD) simulations have been carried out with OpenFOAM to reproduce experiments, studying the jet deflection in a 1:3 water model due to gas bubbling. In the course of the CFD simulations, several modeling approaches are tested. A significantly large jet deflection might adversely affect the internal flow patterns of the tundish, as the liquid jet might hit a present turbulence inhibitor off‐center or miss it entirely. Tundish jet deflection constitutes an adverse effect of gas bubbling in a tundish leading to an off‐design, internal flow, which has not been studied so far, to the best of the authors’ knowledge.

Oxygen-driven impurities scavenging before solidification of Fe-based metallic glasses

Oxygen and impurities are basically unavoidable during the manufacturing of Fe-based metallic glasses, frequently impeding the products' performance and greatly increasing the processing cost. Until now, how oxygen affects the glass-forming ability remains controversial, and eliminating the adverse effects of oxygen and impurities is still a huge challenge. Here we discover a new force of oxide inclusion flotation that drives the occurrence of purification, by blowing oxygen into the Fe-based melts, which help to mix the melts well to accelerate the oxidization and elimination of impurities from commercial raw materials. The synergistic reaction of oxygen and impurities not only improves the glass forming ability, magnetic and mechanical properties of the products, but also makes the stringent processing requirements, e.g. high-purity starting materials and/or high-purity argon, unnecessary. This study uncovers the mystery of oxygen in the glass formation, extends the innovation of fabrication process and highlights a significant technological breakthrough to reduce industrial production cost of Fe-based metallic glasses.

Decarburization and Inclusion Removal Process in Single Snorkel Vacuum Degasser

Abstract In the current work, the coupled mathematical models for decarburization, fluid flow model and inclusion collision-aggregation model were solved to investigate the spatial distribution of carbon, inclusion’s collision-aggregation and removal in a single snorkel vacuum refining furnace (SSF). The numerical results show that the turbulence kinetic energy of ladle in SSF is much greater than that in RH, which can shrink the dead zone and prompt the mixing in the ladle. The overall decarburization reaction rate can be described as a first-order reaction. On the condition of the same gas flow rate, the volumetric mass transfer coefficient for decarburization in SSF is almost twenty times bigger than that in RH, which leads to a much greater decarburization rate in SSF. The spatial distribution of carbon mass fraction in SSF is quite different from that in RH. There is the greater mass fraction of carbon at the recirculation zone under up-snorkel in RH, but this phenomenon disappears in SSF. The inclusion removal can be simplified as the mass transfer between liquid steel to slag, refractory wall and bubble surface. And the overall inclusion removal rate can be regarded as a first-order reaction. The volumetric mass transfer coefficient for inclusion removal in SSF is about three times as that in RH, the inclusion removal rate in SSF is greater than that in RH. The inclusions with different size have different removal rates in SSF. For inclusion flotation after deoxidization, the treatment time in SSF is less than that in RH.

Numerical model of liquid metal flow in steel making tundish with flow modifiers

The optimum condition for clean steel production in the tundish of a continuous casting process reactor can be obtained using numerical modelling. Five different arrangements of flow modifier in the form of impact pad systems deployed in an eight ton, delta shaped, and two strand bloom caster tundish are analysed and optimum design of the impact pad to improve the inclusion removal efficiency is evolved. Reynolds Averaged Navier-Strokes (RANS) equations with standard k-ε model of turbulence and energy equation are used to study fluid flow and inclusion flotation in the tundish. The inclusion separation efficiency is evaluated by solving the inclusion transport equation. Height variations along with additional notch amongst different impact pads yield best micro inclusion separation efficiency.

Tundish Open Eye Formation: A Trivial Event with Dire Consequences

Inert gas shrouding is a traditional practice in tundish metallurgy and has several benefits such as, protecting the melt stream from air aspirations, aiding inclusion flotation with argon bubbles, and also possible thermal and chemical homogenization. On the down side, it displaces the protective slag layer on the top of the melt, and exposes the steel to the ambient atmosphere. This region is often referred to as the slag eye or open eye. This exposed area leads to higher radiative heat losses, reoxidation of the liquid steel, nitrogen pickup, and subsequent inclusion formation. Although perceived as a trivial event by many, Tundish Open Eye (TOE) has dire consequences. In the present work, TOE formation and its consequences have been investigated. The mathematical modeling of this turbulent multiphase system is performed using the Volume of Fluid (VOF) method, and discrete phase method (DPM), coupled with the standard k‐ϵ turbulence model. The mathematical model is compared with the water model results and plant trials. The main objective is to ensure that the steelmaking tundish acts as a refiner and not as a contaminator.

Development of Slag-Forming Mixture for Steel-Pouring Ladle and its Application Technology During Production of Steels with High Aluminum and Sulfur Content

Flotation and assimilation of nonmetallic inclusions from metal slag in steel-pouring ladles and tundishes during pouring steels with increased aluminum and sulfur content are considered. The authors study the formation of nonmetallic inclusions in a steel-pouring ladle during steel extra-furnace treatment, the process of submerged nozzle clogging during continuous casting, the change in slag composition in a tundish, and development of an assimilation mixture composition for a steel-pouring ladle. Laboratory studies are made of the physicochemical properties of a developed SFM. It is established that use of the new SFM makes it possible to improve nonmetallic inclusion assimilation in a steel-pouring ladle and does not lead to a marked change in slag chemical composition and physical properties in a tundish. As a result of this, there is less steel contamination with nonmetallic inclusions and a reduction in submerged nozzle clogging.

Formation mechanism of channel segregation in carbon steels by inclusion flotation: X-ray microtomography characterization and multi-phase flow modeling

Recent experimental dissections of steel ingots and multi-scale simulations have led to the discovery of a potential driving force for channel segregation: the flotation of oxide-based inclusion (D. Li et al., Nat. Commun. 5:5572 (2014)). Further experimental analysis and numerical modeling are necessary to clarify this mechanism in detail. In this work, the inclusions in a carbon steel ingot that exhibits severe channel segregations were characterized by the 3D X-ray microtomography, which revealed a significant enrichment and growth of inclusions in the channels. Based on above microtomography characterization, a 2D macrosegregation model encompassing the inclusion flotation was established. In the model, the motions of solid inclusions and liquid were described using the multi-phase flow scheme within the Euler-Lagrange framework. The benchmark simulations showed that sufficient inclusion populations with appropriate sizes are capable of altering the local flow patterns and destabilize the mushy zone, initiating the subsequent channel segregation. The continuous interplay between melt convection, inclusion flotation and solidification eventually causes the formation of macroscale channel. The predicted sizes and volume fraction of inclusions that are able to trigger the channel segregation effectively are consistent with the data obtained via microtomography characterization. The macrosegregation model was then applied to predict the channel segregations in an industrial carbon steel ingot. A rather good agreement of A-segregates was achieved between the simulation and the dissected ingot.

A Comparative Study of Fluid Flow and Mass Transfer in a Trumpet-Shaped Ladle Shroud Using Large Eddy Simulation

The advantages of trumpet-shaped ladle shrouds (TLS) have been frequently demonstrated over conventional straight-bore ladle shrouds (CLS) with respect to production efficiency and molten steel quality in continuous casting practices. The present study is to shed some lights on why the TLS are better than the CLS design by examining the fluid dynamics and mass transfer using large eddy simulation. The obtained numerical results were validated with particle imaging velocimetry experiments. Flow velocity, deformation, turbulent energy dissipation, and mixing kinetics of tracer were discussed. The results showed that the entering jet of the CLS flowed straight down into the tundish with a relatively high speed (average at 0.710 to 0.815 m/s) and turbulent kinetic energy. However, the trumpet section of a TLS intensified velocity differences, strain rates, and vortices, and promoted an increase on turbulence dissipation rate in the interior of the ladle shroud. The average speed of the entering jet to the tundish was decreased to 0.270 to 0.410 m/s from the 0.708 m/s of the inlet speed. The entering jet from the TLS swung, twisted and well mixed with surrounding fluid in the tundish, and dissipated its kinetic energy. Consequently, the turbulence of the whole flow field as well as the mean skin friction coefficient of tundish wall and the velocity of free liquid surface were reduced. A tracer experiment was carried out to study mass transfer and flow mixing behavior, and the results demonstrated that the use of the TLS increased the plug volume and decreased the dead zone, thereby enhancing inclusion flotation.

Role of Tundish Argon Diffuser in Steelmaking Tundish to Improve Inclusion Flotation with CFD and Water Modelling Studies

Role of Tundish Argon Diffuser in Steelmaking Tundish to Improve Inclusion Flotation with CFD and Water Modelling Studies

Numerical Investigation of Fluid Flow and Heat Transfer Phenomenon Inside a Single Strand Tundish of Slab Caster

The tundish design is an important factor which governs the steel quality. An effective tundish design can enhance inclusion flotation through development of required liquid steel flow pattern. The well-shaped bottom is one of the developments in the area of tundish design. This tundish is in practice and has significantly contributed towards improving the productivity of the steel plants. The current work investigates the liquid steel flow phenomenon in this type of tundish. The 3D CFD model was used for present investigation. The results obtained shows that well-shaped bottom tundish produces better performance as compared to tundishes having flat bottom. The effect of bath height, throughput and depth of well on flow behavior inside the tundish having well-shaped bottom was studied in the current work. The results of RTD analysis show the need to optimize these parameters. CFD simulations were also performed to capture the non isothermal phenomenon in a well-shaped bottom tundish.

Inclusion flotation-driven channel segregation in solidifying steels.

Channel segregation, which is featured by the strip-like shape with compositional variation in cast materials due to density contrast-induced flow during solidification, frequently causes the severe destruction of homogeneity and some fatal damage. An investigation of its mechanism sheds light on the understanding and control of the channel segregation formation in solidifying metals, such as steels. Until now, it still remains controversial what composes the density contrasts and, to what extent, how it affects channel segregation. Here we discover a new force of inclusion flotation that drives the occurrence of channel segregation. It originates from oxide-based inclusions (Al2O3/MnS) and their sufficient volume fraction-driven flotation becomes stronger than the traditionally recognized inter-dendritic thermosolutal buoyancy, inducing the destabilization of the mushy zone and dominating the formation of channels. This study uncovers the mystery of oxygen in steels, extends the classical macro-segregation theory and highlights a significant technological breakthrough to control macrosegregation.

Flotation of nonmetallic inclusions during argon injection into the tundish of a continuous-casting machine. Part 3

Flotation of nonmetallic inclusions during argon injection into the tundish of a continuous-casting machine. Part 3