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.     

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.     

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.     

Bubble formation during horizontal gas injection into downward-flowing liquid

Bubble formation during gas injection into turbulent downward-flowing water is studied using high-speed videos and mathematical models. The bubble size is determined during the initial stages of injection and is very important to turbulent multiphase flow in molten-metal processes. The effects of liquid velocity, gas-injection flow rate, injection hole diameter, and gas composition on the initial bubble-formation behavior have been investigated. Specifically, the bubble-shape evolution, contact angles, size, size range, and formation mode are measured. The bubble size is found to increase with increasing gas-injection flow rate and decreasing liquid velocity and is relatively independent of the gas injection hole size and gas composition. Bubble formation occurs in one of four different modes, depending on the liquid velocity and gas flow rate. Uniform-sized spherical bubbles form and detach from the gas injection hole in mode I for a low liquid speed and small gas flow rate. Modes III and IV occur for high-velocity liquid flows, where the injected gas elongates down along the wall and breaks up into uneven-sized bubbles. An analytical two-stage model is developed to predict the average bubble size, based on realistic force balances, and shows good agreement with measurements. Preliminary results of numerical simulations of bubble formation using a volume-of-fluid (VOF) model qualitatively match experimental observations, but more work is needed to reach a quantitative match. The analytical model is then used to estimate the size of the argon bubbles expected in liquid steel in tundish nozzles for conditions typical of continuous casting with a slide gate. The average argon bubble sizes generated in liquid steel are predicted to be larger than air bubbles in water for the same flow conditions. However, the differences lessen with increasing liquid velocity.     

Investigation of the kinetics of breakup of gas bubbles in liquid metals with digital simulation method

The method of digital system simulation can be effectively used to quantify the complex multiphase interactions within a gas injection process. Process simulation results yield a better understanding and a better aimed engineering of gas dispersion techniques in metallurgical processes. In this paper the breakup phenomenon of gas bubbles in stagnant liquids is simulated and the dependencies between breakup of bubbles and various parameters of a gas dispersion process such as operative parameters, system parameters and mass transfer rates are investigated. The bubble diameter after breakup is almost independent of the nozzle diameter and gas flow rate. The frequency of bubble breakup and critical bubble size depend on the rate of mass transfer into the bubble. An almost constant rising velocity is achieved only in those cases investigated where mass transfer and bubble breakup are considered. In all other cases no stationary rising velocity is obtained. The interplay between bubble size, rising velocity and the inertia of the surrounding liquid and the influence of mass transfer and breakup are investigated. Simulation results reveal that the behaviour of an ascending bubble is strongly influenced by the mass transfer rate, i. e. by the composition of the melt. Verification of the simulation results with empirical equations from literature shows a very good agreement in all dispersion systems investigated.     

Jet penetration and liquid splash in submerged gas injection

Jet penetration, bubble dispersion, and liquid splash were studied in the nitrogen-water system. Among the effects evaluated were those due to lance design, nozzle dimensions, gas driving pressure, and liquid density. In side-nozzle injection, penetration is found to increase with jet force number,N, given by the product of the gas driving pressure and the nozzle diameter. In top-submerged injection, horizontal and vertical penetrations increase with the horizontal and vertical components, respectively, of the jet force number. Liquid splash is greater in the side-nozzle injection than in top-submerged multiple-orifice injection, and appears to decrease as the number of orifices increases.     

Fluid dynamics in channel reactors stirred by submerged gas injection

This work describes the fluid flow and associated local and longitudinal mixing phenomena which influence the behavior and characteristics of continuous flow reactors, such as the Noranda reactor and the Q-S process. In the present work, mixing in channel reactors agitated by submerged gas injection along the length has been studied using a water model. The effects of gas injector separation, gas flow rate, depth of water, lateral configuration of injectors, submersion depth of gas injectors, and width of the channel have been investigated. It has been found that the longitudinal mixing depended significantly on the locations of the gas injectors. For constant values of other variables, there existed an optimum injector separation at which maximum longitudinal mixing was found. Industrial applications of this study are described.     

Fluid dynamics in channel reactors stirred by submerged gas injection

This work describes the fluid flow and associated local and longitudinal mixing phenomena which influence the behavior and characteristics of continuous flow reactors, such as the Noranda reactor and the Q-S process. In the present work, mixing in channel reactors agitated by submerged gas injection along the length has been studied using a water model. The effects of gas injector separation, gas flow rate, depth of water, lateral configuration of injectors, submersion depth of gas injectors, and width of the channel have been investigated. It has been found that the longitudinal mixing depended significantly on the locations of the gas injectors. For constant values of other variables, there existed an optimum injector separation at which maximum longitudinal mixing was found. Industrial applications of this study are described.     

Mixing of concentric gas jets issuing vertically into a liquid

Mixing of two concentric jets of dissimilar gases blown vertically upward through wafer was investigated experimentally. Air was blown through the inner pipe and carbon dioxide through the annular space between the inner and the outer pipes. The two jets mixed rapidly upon emergence from the nozzle. This made physical shielding of the air jet by CO₂ ineffective. It was caused by the fact that the air bubbles during and upon emergence from the nozzle were large. Moreover, they exhibited oscillations which made them spread over the annulus. Pursuing this argument it has been inferred that physical shielding would be ineffective in OBM/Q-BOP converter process of steelmaking as well. Therefore, protection of tuyere lining by shrouding gas seems to be due to thermal and chemical shielding, primarily. N.B. BALLAL, formerly a Graduate Student at Indian Institute of Technology, Kanpur, is presently     

Heat transfer and lance clogging during submerged powder injection

Nitrogen and silica particles of 30, 130, and 450 μm average diameters were injected at solid-to-gas loadings up to 280 kg/m³ into liquid lead at 400°C through a steel lance equipped with four thermo-couples. The lance was positioned adjacent to a transparent wall in the lead retort so that the flow patterns could be photographed. It was found that 130 and 450 μm particle injection produced bubling in the lead and clogging at high loadings, while the 30 μm particles produced jetting with no clogging. Analysis of the thermocouple responses permitted the determination of the heat transfer coefficients at the inner and outer lance surfaces. The inner surface heat transfer coefficient increased with loading, whereas the one at the outer surface was independent of loading. A two-phase, unsteady-state, one-dimensional model was developed for momentum and heat transfer in the lance permitting the calculation of gas and particle velocities, volume fractions, and temperatures as well as the lance temperatures. Using the experimentally determined heat transfer coefficients, it is shown that the gas and particles are heated only 20 to 40 K in the lance. Nevertheless, this is a large heat demand which chills the lance so that clogging will occur in the bubbling regime.     

Physical characteristics of gas jets injected vertically upward into liquid metal

The time-averaged structure of plumes has been measured with a two-element electroresistivity probe during upward injection of nitrogen or helium into mercury in a ladle-shaped vessel. From these measurements and data obtained earlier for air jets in water, general correlations linking the spatial distribution of gas fraction with the Froude number and gas/liquid density ratio have been developed. Early evidence suggests that these correlations should be applicable to gas-stirred metallurgical baths. Measurements of the profiles of bubble velocity and bubble pierced length reveal that the kinetic energy of the gas is dissipated close to the nozzle, and buoyancy dominates flow over most of the plume. Castillejos E., formerly with the Centre for Metallur-gical Process Engineering, The University of British Columbia,     

Characterization of gas bubbles injected into molten metals under laminar flow conditions

Velocity and volume measurements of gas bubbles injected into liquid metals under laminar flow conditions (at the orifice) have been achieved. A novel experimental approach utilizing noises generated by bubbles was used to collect the necessary data. Argon gas was bubbled through tin, lead, and copper melts, and gas bubble formation frequencies (and hence bubble sizes) were determined. It was found that the bubble size generated for a particular orifice diameter was dependent upon the magnitudes of the orifice Froude and Weber numbers. Maximum formation frequencies increased slightly with decreasing orifice diameter, and the transition point from varying to constant frequency occurred at an orifice Weber number of approximately 0.44. Velocities of gas bubbles rising through the metals were greater than those previously reported for studies in which only one bubble was in the melt at any time. Effective drag coefficients of the rising bubbles were found to agree with data previously generated in aqueous systems. Formerly Graduate Student, Michigan Technological University     

Characterization of gas bubbles injected into molten metals under laminar flow conditions

Velocity and volume measurements of gas bubbles injected into liquid metals under laminar flow conditions (at the orifice) have been achieved. A novel experimental approach utilizing noises generated by bubbles was used to collect the necessary data. Argon gas was bubbled through tin, lead, and copper melts, and gas bubble formation frequencies (and hence bubble sizes) were determined. It was found that the bubble size generated for a particular orifice diameter was dependent upon the magnitudes of the orifice Froude and Weber numbers. Maximum formation frequencies increased slightly with decreasing orifice diameter, and the transition point from varying to constant frequency occurred at an orifice Weber number of approximately 0.44. Velocities of gas bubbles rising through the metals were greater than those previously reported for studies in which only one bubble was in the melt at any time. Effective drag coefficients of the rising bubbles were found to agree with data previously generated in aqueous systems. R. J. ANDREINI, Formerly Graduate Student, Michigan Technological University,     

Spot turbulence,breakup, and coalescence of bubbles released from a porous plug injector into a gas-stirred ladle

Many metallurgical processes are connected with gas injection into liquid metals for refining purposes. For this reason, considerable effort has been made during the past 2 decades to investigate gas-injection operations in steelmaking ladles. Numerous physical and mathematical models are available in the literature as well as experiments (most of them performed in the air-water model). In theoretical works, usually, the bubble size is assumed constant, but this approximation is good just at low gas flow rates. When the gas flow rate increases, three different types of bubble dispersion patterns are observed in experiments. This situation cannot be predicted by means of the turbulence multiphase models normally implemented in commercial CFD codes. Their results predict a smooth (and wrong) bubble-size increase and not a sudden transition from a pattern to another, as in experiments. In this articles a new idea for approaching the bubble turbulence in the ladle, called “spot turbulence,” is presented and comparison with experimental data shown.     

Model investigations on liquid velocity induced by submerged gas injection in steel bath

The technique of dissolution rate measurement is used to determine the liquid velocity induced by different locations of submerged-lance gas injection. As a result of the study the equations are proposed to calculate the liquid velocity for any location of submerged lance. It has been shown that off-centre lance location increases considerably the potential of submerged gas injection in terms of attaining the uniform liquid velocity in the bath and decreasing the total dissolution time of the additive.     

Bubble and liquid flow characteristics during horizontal cold gas injection into a water bath

In refining processes such as the AOD process cold gas is blown horizontally into the molten metal bath of the processes. The spatial distribution of bubbles in the bath is one of the important factors influencing the efficiency of the processes. In this study, a water model study was carried out to understand the characteristics of bubbles and liquid flow generated by horizontal gas injection. The bubble and liquid flow characteristics were measured using an electro‐resistivity probe and a laser Doppler velocimeter, respectively. In the flow field near the nozzle the bubble characteristics for the horizontal cold gas injection can be predicted by empirical equations derived for isothermal gas injection systems. The liquid flow characteristics could not be measured in this region. On the other hand, in the region far from the nozzle the two characteristics for the cold gas injection became different from those for the isothermal gas injection because of enhanced buoyancy force acting on expanding cold bubbles due to heat transfer.     

Kinetics of deoxidation of liquid copper by graphite particles during submerged injection

Liquid copper can be deoxidized by submerged injection of inert gas in the presence of graphite particles. This paper describes the results of experiments in which approximately 20 kg copper melts were deoxidized by injection of N₂, CO₂, and air in the presence of low sulfur graphite particles. The apparent kinetics of the deoxidation process are first order with respect to the concentration of dissolved oxygen, and the concentration of oxygen in the melt could be reduced to less than 10 ppm by weight after less than 30 minutes of injection. The kinetics are consistent with mixed rate control with both mass transfer and chemical reaction rate affecting the rate of deoxidation. In these experiments, the rate of deoxidation under a graphite covering was slower when particles were injected with the gas stream than when gas was injected alone, although this result may have been influenced by the small size of the melt. Y. W. Chang, formerly Graduate Student with the Department of Civil Engineering, Mechanics, and Metallurgy, University of Illinois at Chicago This paper is based on a presentation made in the T.B. King Memorial Symposium on “Physical Chemistry in Metals Processing” presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS.