Fluid dynamics of vertical submerged gas jets in liquid metal processing systems

High speed cinematography and a pressure trace technique have been used to investigate the fluid dynamics of inert gas jets injected vertically upward into water, molten tin, lead-tin alloy, and iron. Two flow regimes of jet behavior were observed: one in which unstable bubbles were produced at the jet nozzle, and one in which a steady cone of gas emerged from the nozzle and broke up continuously into small bubbles. The transition between bubbling and continuous jet flow was controlled by the mass flow of gas per unit area of the jet and occurred at a flow rate of approximately 40 g/cm² s in all of the systems studied.     

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.     

Oxidation of zinc sulfide in a fluidized bed

Kinetics of oxidation of ZnS particles in a batch-type fluidized bed were studied at temperatures between 800 and 910°C. A two-phase model was employed for the fluidized bed, and the partial pressure of oxygen and the gas-film mass transfer coefficient on the particle surface were separately evaluated in gas bubbles and in the emulsion phase. The calculated fractional reaction coincided well with the experimental results. The difference in O₂ partial pressure between gas bubbles and emulsion phase was found to be fairly large especially under the vigorous fluidizing condition. Furthermore, it was shown from the mathematical model that the reaction of ZnS particles in the gas bubbles is negligible because of the extremely low solid concentration and that the overall rate of reaction in the emulsion phase is virtually controlled by the rate of gas-film mass transfer at higher temperature. The resistance of interfacial reaction within the particle also becomes significant when the temperature is lowered. Y. Fukunaka and T. Monta are both former Graduate Students, Kyoto University, Kyoto, Japan.     

The injection of solids using a reactive carrier gas

The injection of nonwettable powders into melts in the bubbling regime was studied experimentally using a cold-model system. Polyethylene powder was injected into a cylindrical vessel containing water, through a vertical top-submerged lance, with insoluble (air) and soluble (ammonia) carrier gases. The concentration of particles in the liquid and the penetration length of the particle-liquid jet into the bath were measured, as the carrier gas composition, the gas and solids flow rates, and the particle size were varied. It was found that the concentration of particles retained in the liquid was up to 10 times higher, and the penetration length of the jet was up to three times higher when the soluble carrier gas was used instead of the insoluble carrier gas. For both carrier gases, the dispersed particle concentration increased with increasing gas flow rate and increasing particle size, whereas the penetration length of the jet increased with increasing gas and solids flow rates.     

Numerical Simulation of CO2 Used for Slag Splashing Process in Converter

Utilizing CO₂ for the slag splashing process presents a novel approach that enhances CO₂ utilization in the steel industry and promotes efficient slag splashing. Herein, numerical simulation is employed to investigate the kinetic feasibility of this technique on a 100 t converter. The volume of fluid (VOF) model is utilized to trace the gas–slag interface, while the standard k–ε model is selected to describe the turbulent flow of each phase. Initially, the model is validated via isentropic theory and experimental data. Subsequently, the effects of gas and oxygen lance replacement are evaluated. The results indicate that employing CO₂ solely instead of N₂ leads to a reduction in jet velocity and slag mass flow rates at different positions. However, the utilization of the innovative oxygen lance slows down the jet decay and increases the slag mass flow rate at various heights and sidewalls, except for the lower cone, resulting in a satisfactory slag splashing performance. The present study verifies the feasibility of this new technology and its potential to contribute to CO₂ reduction in the industry positively.     

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.     

Experimental investigation and three-dimensional computational fluid-dynamics modeling of the flash-converting furnace shaft: Part II. Formulation of three-dimensional computational fluid-dynamics model incorporating the particle-cloud description

A fluid-dynamics computer model of the flash-converting furnace shaft, which is based on basic principles, is presented. The model is fully three-dimensional and incorporates the transport of momentum, heat, and mass and the reaction kinetics between the gas and particles in a particle-laden turbulent gas jet. The k-ɛ model was used to describe gas-phase turbulence in an Eulerian framework. The particle-cloud model was used to track the particle phase in a Lagrangian framework. The coupling of gas and particle equations was performed through the source terms in the Eulerian gas-phase governing equations. Copper matte particles were represented as Cu₂S · yFeS _x . Based on experimental observation, the oxidation products were assumed to be Cu₂O, CuO, Fe₃O₄, and SO₂. A reaction mechanism involving the external mass transfer of oxygen from the gas to the particle surface and diffusion of the oxygen through the successive layers of Cu₂O-Fe₃O₄ and CuO-Fe₃O₄ was proposed. The predictions of the computer model were compared with the experimental data collected in a large laboratory furnace. Reasonable agreement between the model predictions and the measurements was obtained in terms of the fractional completion of the oxidation reactions and the sulfur remaining in the reacted particles. The relevance of the computational model for further analysis and optimization of an industrial flash-converting operation is discussed.     

Mass transfer between a horizontal,submerged gas jet and a liquid

The rate of reaction between a horizontal, submerged gas jet and a liquid has been measured in a model system under conditions where mass transfer in the gas phase is rate limiting. The gas was 1 pct SO₂ in air, and the liquid was a 0.3 pct solution of hydrogen peroxide in water. SO₂ absorption rates were measured as a function of jet Reynolds number (10,000 < N_Re < 40,000) and jet orifice diameter (0.238 < d₀ < 0.476 cm). The product of the gas phase mass transfer coefficient and the interfacial area per unit length of jet trajectory, k_SO2 α was found to increase linearly with increasing Reynolds number and to be a strong function of the orifice diameter. The ratio of k_so2 α to volumetric gas flow rate was shown to be independent of Reynolds number for a given orifice diameter. Extrapolated values of k_so2 α are lower than the coefficients measured for vertical CO jets blown upward through liquid copper. Extrapolation of the measured mass transfer data to the jet conditions in copper matte converting and in the gaseous deoxidation of copper has indicated that the gas utilization efficiencies in these processes should approach 100 pct if gas phase mass transport is rate controlling.     

A unified approach to bubbling-jetting phenomena in powder injection into iron and steel

The injection of powder into liquids has been investigated by physical modeling and by multi-phase fluid dynamic modeling. The transition from gas-particle jets which penetrate deeply into the liquid and a gas bubbling regime was found to depend on the coupling between gas and particle phases in the conveying line; fine particles at high loading couple well and form jets, whereas coarse particles separate from the gas during bubble formation. The measured penetration depths of submerged jets in water and lead and top jets in water were very well described by equations balancing the momentum of the jet and its buoyancy. A regime of particle-liquid jets that forms in conjunction with bubbling also appears to depend on coupling, between the particle and liquid phases. The effect of surface tension on the particle penetration through a bubble interface was modeled for the single particle and multi-phase cases and compared with the work of others. On the basis of this modeling, the expected regime of flow for many powder injection conditions can be predicted. The flow regimes of existing processes are discussed, and guidelines for the design of processes employing various types of reactions are presented. Formerly with McMaster University during the course of this work,     

The breakup of bubbles into jets during submerged gas injection

There has never been any fundamental explanation presented for the transition from the bubbling regime to the jetting regime when gas is injected into liquid at high velocity through submerged tuyeres. This is an important issue in metallurgical processes, since the flow regime is known to influence refining rates, refractory erosion, and the penetration of the liquid into the tuyere. Based on the observation that many small droplets of liquid and gas bubbles are formed to create the jets, a combined Kelvin-Helmholtz and Rayleigh-Taylor instability analysis has been applied to bubbles forming at submerged tuyeres. For particular wavelengths of disturbances, the interface will be unstable and create bubbles and droplets. The critical injection velocity for instability depends on surface tension, tuyere diameter, and the gas-to-liquid density ratio, which can be summarized by We = 10.5(ρ*)^−1/2, where We is the Weber number based on the gas velocity and density and tuyere diameter, and ρ* is the gas-to-liquid density ratio. The importance of surface tension had not been appreciated previously for this regime of gas injection. There is considerable controversy in the literature concerning the measurement of the transition from bubbling to jetting. The 70 pct “linking” point, proposed by Ozawa and Mori, describes the situation where 70 pct of the bubbles link with the preceding bubbles and produce a reasonably steady jet. The theoretical correlation developed above predicts the velocity to reach this point ±20 pct (95 pct confidence level) in a variety of systems from six different groups of workers. The theoretical analysis demonstrates that the instabilities are primarily capillary in nature, not gravity waves, which explains the observation that orientation has little effect on the jetting transition.     

Study on Mass Transfer Characteristics in an AOD Converter Bath under Conditions of Combined Side and Top Blowing

The mass transfer characteristics in a steel bath during the AOD refining process with the conditions of combined side and top blowing were investigated. The experiments were conducted on a water model unit of 1/4 linear scale for a 120‐t combined side and top blowing AOD converter. Sodium chloride powder of analytical purity was employed as the flux for blowing, and the mass transfer coefficient of solute (NaCI) in the bath was determined under the conditions of the AOD process. The effects of the gas flow rates of side and top blowing processes, the position arrangement and number of side tuyeres, the powdered flux particle (bubble) size and others on the characteristics were examined. The results indicated that, under the conditions of the present work, the mass transfer coefficient of solute in the bath liquid is in the range of (7.31×10^?5‐3.84×10^?4) m/s. The coefficient increases non‐linearly with increasing angle between each tuyere, for the simple side blowing process at a given side tuyere number and gas side blowing rate. The gas flow rate of the main tuyere has a governing influence on the characteristics, and the gas jet from the top lance decreases the mass transfer rate, the relevant coefficient being smaller than that for a simple side blowing. Also, in the range of particle (bubble) size used in the present work and with all other factors being constant, raising particle (bubble) size increases the coefficient. Excessively fine powder particle (bubble) sizes are not advantageous to strengthening the mass transfer. With the oxygen top blowing rate practiced in the industrial technology, the side tuyere arrangements of 7 and 6 tuyeres with an angular separation of 22.5° and 27° between each tuyere, as well as 5 tuyeres with an angle of 22.5° between each tuyere can provide a larger mass transfer rate in the bath. Considering the relative velocity of the particles to the liquid, the energy dissipation caused by the fluctuation in the velocity of the liquid in turbulent flow and regarding the mass transfer as that between a rigid bubble and molten steel, the related dimensionless relationships for the coefficient were obtained.     

Convective Mass Transfer onto Multiple Gas Bubbles

An analysis has been performed on the fluid flow and convective mass transfer in a system consisting of many gas bubbles. Both interference effects of neighboring bubbles and the influence of the circulation induced inside the bubbles on the flow field and the mass transfer rate were taken into account in the analysis. Analytical solutions were obtained for the mass transfer rate. The results indicate that both the packing density and the viscosity ratio of fluid spheres and surrounding fluid have significant effects on the flow field and the mass transfer rate. It was found that the mass transfer rate in a system of bubbles becomes much higher than that for a system of solid spheres. Based on the analytical results, discussions are provided concerning the mass transfer onto gas bubbles from liquid flow.     

The trajectories and distribution of particles in a turbulent axisymmetric gas jet injected into a flash furnace shaft

Numerical computations have been performed for the behavior of a vertical turbulent particle-laden gas jet exemplified by the shaft region of a flash-smelting furnace. The two-equation(k-ε) model was used to describe turbulence. Model predictions for the gas and solid flow fields give a satisfactory representation of experimental data taken from the literature. The predictions of flow properties of the two phases under flash-smelting conditions have been obtained for various inlet conditions, particle sizes, particle loading, and oxygen enrichment. Model predictions show that the axial velocity of the particle phase is substantially higher than that of the gas phase. The presence of solid particles causes the axial velocity of the gas phase to be greater near the centerline and lower in the outer region than in a single-phase gas jet. A more uniform distribution of particles was obtained by introducing a strong radial velocity of the distribution air at the inlet. The implications of the behavior of a particle-laden gas jet on flash-smelting processes arc discussed.     

Rate of reactions between carbon dioxide and graphite

Solid graphite rods have been oxidized at temperatures between 1020 and 1510 °C using CO₂ containing gases. The activation energy was found to be 270 kJ/mol in the temperature range from 1020 to 1170 °C where the reaction is chemically controlled. At higher temperatures the reaction is controlled by external mass transfer of CO₂ with an activation energy of 86 kJ/mol. The shift from chemical to mass transfer control depends on the CO₂ pressure and the gas flow behaviour. Since per mol of carbon consumed one net mol of gas is produced, there is a net gas flow away from the graphite surface. This makes the transport of CO₂ to the surface more difficult, retarding the rate at high temperatures.     

Particulate flow analysis in inclined pipes and transfer chutes using tomography imaging,discrete element simulations and continuum modeling approaches

In particulate material transfer systems,traditional shear test based steady state analysis can provide some insight into the strength of the bulk material and subsequent resistive frictional forces during flow.For fast flowing transfer points,dynamic flow conditions dominate and additional modelling techniques are required to improve design guidance.The research presented shows the evolution of a design solution which utilises two distinct processes;a continuum method and a discrete element method(DEM). Initially,the internal structure of dense granular flow,down vertical and inclined pipes was investigated using a twin sensor,12 electrode electrical capacitance tomography device.Subsequently,DEM simulations were conducted using the commercial software,PFC3D.Initially,two particle types and their flow behaviours were analysed:plastic pellets and sand.The pipe angle was varied between 0°and 45°to the vertical.For both the plastic pellets and the sand,good qualitative agreement was found with the spatial particle concentration analysis.Generally,the flow had a dense particle region at its core with the particle concentration reducing away from this core.As expected,at 0°, the core was centrally located within the pipe for both the plastic pellets and sand.At pipe angles 5°or greater,the dense core of particles was located on or near the pipe wall.Average flow velocity analysis was also conducted using the results of wall friction test analysis.The velocity comparisons also showed good agreement between the ECT image analysis and the DEM simulations. Subsequently,the DEM method was used to analyse a complex transfer system(or chute) with the continuum method providing comparative flow analysis with the DEM flow analysis.     

The separation of the solids from the carrier gas during submerged powder injection

A fundamental aspect of submerged powder injection into melts which is not well understood is the extent to which the particles separate from the carrier gas upon injection, particularly under high solids loading conditions. In this study, the injection of nonwettable powders was investigated using a cold-model system at solids loadings from 1 to 25. Polyethylene powder was injected through a top-submerged lance into a cylindrical water bath under bubbling conditions. Air was used as the carrier gas. The apparatus was designed so that the particles remaining with the gas phase could be collected separately from those which escaped from the bubbles. The gas velocity (5.15 to 10.3 m/s), surface tension (0.03 to 0.072 N/m), lance diameter (4.7 to 7.4 mm), and particle size (< 500 μm) were independently varied. The separation of the powder from the primary gas bubbles was found to increase with increasing solids loading when the gas velocity, surface tension, and lance diameter were held constant. At constant solids loading, the separation increased with increasing gas velocity, increased with increasing lance diameter, and decreased with increasing surface tension. The separation was found to be independent of the particle size of the powder in the range of solids loadings tested. A theoretical relationship between the penetration efficiency and the particle jet Weber number successfully correlated with the experimental data.     

The Penetration Behavior of an Annular Gas–Solid Jet Impinging on a Liquid Bath: The Effects of the Density and Size of Solid Particles

Top-blow injection of a gas?Csolid jet through a circular lance is used in the Mitsubishi Continuous Smelting Process. One problem associated with this injection is the severe erosion of the hearth refractory below the lances. A new configuration of the lance to form an annular gas?Csolid jet rather than the circular jet was designed in this laboratory. With this new configuration, the solid particles fed through the center tube leave the lance at a much lower velocity than the gas, and the penetration behavior of the jet is significantly different from that with a circular lance where the solid particles leave the lance at the same high velocity as the gas. In previous cold-model investigations in this laboratory, the effects of the gas velocity, particle feed rate, lance height of the annular lance, and the cross-sectional area of the gas jet were studied and compared with the circular lance. This study examined the effect of the density and size of the solid particles on the penetration behavior of the annular gas?Csolid jet, which yielded some unexpected results. The variation in the penetration depth with the density of the solid particles at the same mass feed rate was opposite for the circular lance and the annular lance. In the case of the circular lance, the penetration depth became shallower as the density of the solid particles increased; on the contrary, for the annular lance, the penetration depth became deeper with the increasing density of particles. However, at the same volumetric feed rate of the particles, the density effect was small for the circular lance, but for the annular lance, the jets with higher density particles penetrated more deeply. The variation in the penetration depth with the particle diameter was also different for the circular and the annular lances. With the circular lance, the penetration depth became deeper as the particle size decreased for all the feed rates, but with the annular lance, the effect of the particle size was small. The overall results including the previous work indicated that the penetration behavior of an annular jet is much less sensitive to the variations in operating variables than that of a circular jet. Correlation equations for the penetration depth that show good agreements with the measured values have been developed.     

Wear and Dissolution of MgO–C Refractory Lining in Belly and Top Cone Regions of BOF Vessel

The MgO–carbon bricks of varying carbon concentration are used as refractory lining material in BOF. Laser profiling of the lining is done at regular intervals to keep track of refractory wear in different parts of vessels. The main factors which affect the kinetics of dissolution of graphite flakes lying between the MgO grains in the belly region are attack by CO₂ in gas and FeO in slag, and temperature. FeO can easily penetrate the MgO grains along grain boundaries and reach those places where graphite flakes are present. Kinetic models of refractory wear are analyzed on the basis of data obtained from actual laser profile measurements. The bricks salvaged from the top cone region at the end of the campaign have been subjected to scanning electron microscope (SEM) and electron probe micro analyzer (EPMA) investigations. The possible cause of wear in top cone region is also the oxidation of carbon in the brick by CO₂ gas and direct attack by FeO thrown from the jet impact region.     

On the oxidation of cyanides in the stack region of the blast furnace

Computed results are reported on the oxidation of KCN particles carried by blast furnace gas through the stack region of the furnace. The calculations were performed on the assumption that the oxidation process is mass transfer controlled, but proper allowance was made for the variable CO/CO₂ ratios and temperatures to which the particles are exposed during their passage through the stack. The results of these calculations show that for cyanide particles in the range 0.02 to 0.1 cm in diameter the system may be extremely sensitive both to the temperature profiles and to the CO/CO₂ ratios within the stack; thus the control of these parameters could provide a means of controlling cyanide emissions.