Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 2008"

Kinetics of Aqueous Leaching and Carbonization of Steelmaking Slag

Abstract: Sequestration of carbon dioxide by steelmaking slag was studied in an atmospheric three-phase system containing industrial slag particles, water, and CO2 gas. Batch-type reactors were used to measure the rate of aqueous alkaline leaching and slag particle carbonization independently. Four sizes of slag particles were tested for the Ca leaching rate in deionized water at a constant 7.5 pH in an argon atmosphere and for carbonate conversion with CO2 bubbled through an aqueous suspension. Conversion data (fraction of Ca leached or converted to carbonate) were evaluated to determine the rate-limiting step based on the shrinking core model. For Ca leaching, the chemical reaction is the controlling mechanism during the initial period of time, which then switches to diffusion through the developed porous layer as the rate-limiting step. Carbonate conversion proceeded much slower than leaching conversion and was found to be limited by diffusion through the product calcium carbonate layer. The calculated value of diffusivity was found to be 5 × 10−9 cm2/s, which decreased by an order of magnitude with increasing carbonization conversion as a result of changing density of the product layer. The experimental data fit the shrinking core model well after correction for the particle specific surface area.

Flow Control with Local Electromagnetic Braking in Continuous Casting of Steel Slabs

Abstract: A computational fluid flow model is applied to investigate the effects of varying submerged entry nozzle (SEN) submergence depth and electromagnetic brake (EMBr) field strength on flow in the mold cavity. The three-dimensional, steady K-e model of the nozzle and liquid cavity in the mold used the magnetic induction method in FLUENT to incorporate the localized-type static EMBr field measured at a steel plant. The model was validated by comparing results with an analytical solution and with nail board and oscillation mark measurements collected at the plant. Increasing EMBr strength at a constant SEN depth is found to cause a deeper jet impingement, weaker upper recirculation zone and meniscus velocity, and a smaller meniscus wave. Increasing SEN depth without EMBr caused the same trends. Increasing SEN depth at a constant EMBr strength brought about the opposite: higher meniscus velocity, larger meniscus wave, and deeper penetration depth. Using the knowledge gained from this model, electromagnetic forces can be controlled to stabilize the fluid flow in the mold cavity and thereby minimize casting defects.

Efficient Melt Stirring Using Pulse Sequences of a Rotating Magnetic Field: Part II. Application to Solidification of Al-Si Alloys

Abstract: The present study considers the solidification of an Al-7 wt pct Si alloy under the influence of electromagnetic melt stirring using a rotating magnetic field (RMF). The effect of a continuously applied RMF is compared with an RMF pulse sequence of alternating direction (RMF-PSAD). The resulting flow structure in a cylindrical liquid metal column has been measured by isothermal experiments using the ternary alloy GaInSn. The solidification experiments performed with the Al-7 wt pct Si alloy confirm our numerical predictions concerning the temperature field during solidification and the distribution of primary crystals and eutectic phase in the solidified samples. The application of the RMF-PSAD regime at suitable frequencies of the reversals of the magnetic field direction fP delivers an equiaxed microstructure without macrosegregation.

Flow Transport and Inclusion Motion in Steel Continuous-Casting Mold under Submerged Entry Nozzle Clogging Condition

Abstract: Clogging of the submerged entry nozzle (SEN) is a serious problem during the continuous casting of steel, due to its influence on the casting operations and product quality. Fluid-flow-related phenomena in the continuous casting mold region with the SEN clogging are investigated in the current article, including the quantitative evaluation of inclusion removal, slag entrainment, heat transfer, and the prediction of breakouts. The calculations indicate that, in order to accurately simulate the fluid flow in the mold region, the SEN should be connected with the mold region and the two should be calculated together. In addition, the whole mold region has to be calculated. Clogging at the SEN at one side induces asymmetrical jets from the two outports; thus, the fluid flow in the mold is asymmetrical. In addition, more inclusions are carried by the flow to the top surface of the nonclogged side, and the slab at the nonclogged side has a lower quality. With SEN one-sided clogging, inclusions travel a much larger distance, on average, before they escape from the top or move to the bottom. The overall inclusion entrainment fraction from the entire top surface for inclusions of any size is less than 10 pct. A higher turbulence energy and a larger surface velocity induce more inclusion entrainment from the top surface. Smaller inclusions are more easily entrained into the steel than are larger ones. More >200-μm inclusions can be entrained into the molten steel from the top slag with SEN clogging than without clogging. The SEN one-sided clogging generates an asymmetrical temperature distribution in the mold; it also generates temperatures higher than the liquidus temperature at some locations of the solidified shell, which increases the risk of breakouts. The SEN clogging should be minimized in order to achieve a uniform steel cleanliness, a cleaner steel, and a safe continuous casting operation.

A Study of the Crystallization Behavior of a New Mold Flux Used in the Casting of Transformation-Induced-Plasticity Steels

Abstract: Transformation-induced-plasticity (TRIP) steels are one of a new generation of steel grades that are under development for use in automotive products. Because of the addition of significant quantities of aluminum to the chemistry of some TRIP steels, one of the challenges in continuous casting is to design a mold flux that is compatible with this steel chemistry and that allows sequence casting. This article documents the solidification behavior of a mold flux that was developed to be more compatible with high-aluminum-containing steels and compares its solidification behavior to a commercial mold flux used in the casting of low-carbon (LC) aluminum-killed steel. This new mold flux precipitates calcium fluoride at high temperatures and does not form a glass at the cooling rates that are normally found in a continuous caster.

Experimental Investigation of Mechanical Properties of Metallic Hollow Sphere Structures

Abstract: Metallic foam was fabricated from 316L stainless steel spheres, where the bonding of the spheres was achieved by a sintering process. The mechanical behavior of a low-density material (0.3 g/cm3) with 2- and 4-mm sphere diameter and a high-density material (0.6 g/cm3) with 4-mm sphere diameter was investigated in compression and tension. The cell wall material of this hollow sphere structure (HSS) had different morphologies: dense and porous sintered walls were investigated. The cell wall morphology affects the Young’s modulus (stiffness) and the ductility of the HSS material. Defects, such as the cell wall porosity, lower the ductility of the material. Besides the quasi-static measurements, the HSS material was tested with a resonance frequency method (dynamic measurement), to obtain detailed information on the stiffness at different temperatures up to 700 °C. In-situ compression and tension tests were carried out to understand the deformation mechanisms on the scale of the single hollow spheres. The failure mechanisms in the vicinity of the sintering neck of the spheres was investigated. A doubling of the density leads to an increase of the plateau stress and the ultimate tensile stress of the material, whereas the ductility (strain to fracture) depended mainly on the cell wall morphology. Due to the mainly tensile loading of the cell walls in the vicinity of the sinter neck, the ultimate tensile strength doubled for the high-density HSS, in good agreement with theoretical considerations. In compression, the gain in the plateau stress was not as distinctive compared with the theoretical considerations assuming a bending dominated deformation. The influence of structural parameters, such as cell wall morphology, cell wall thickness, and sphere diameter, on the mechanical behavior is discussed.

Thermodynamics of the Formation of MgO-Al2O3-TiOx Inclusions in Ti-Stabilized 11Cr Ferritic Stainless Steel

Abstract: The equilibration between CaO-SiO2-MgO-Al2O3-CaF2 (-TiO2) slag and Fe-11 mass pct Cr ferritic stainless steel melts was investigated at 1873 K in order to clarify the effect of Al and Ti addition as well as that of slag composition on the formation of complex oxide inclusions. The activity of oxygen calculated from the classical Wagner formalism changes from about a O = 0.0002 to 0.001 and the values of a O from [Al]/(Al2O3) and that from [Si]/(SiO2) equilibria are in relatively good agreement with each other with some scatters. The phase stability diagram of the inclusions and the equilibrium iso-[O] lines in the Fe-11 mass pct Cr-0.5 mass pct Si-0.3 mass pct Mn-0.0005 mass pct Mg steel melts was constructed by using FACTSAGE 5.5 program as a function of Al and Ti contents. The computed iso-[O] lines were slightly larger than the values estimated from the slag-metal equilibria. The composition of the inclusions could be plotted on the computed MgO-Al2O3-TiO x phase diagram. The inclusions in the steel melts equilibrated with the basic slags are located in the “spinel + liquid” region, while those in equilibrium with the less basic slags are mostly in the “liquid” single phase. This is in good accordance to the observed morphology of the inclusions. However, in cases of high concentration of Ti and Al, the inclusions were found to be spinel + liquid, even though the less basic slags are equilibrated. When plotted on logarithmic scales, the mole ratio $$ {\left( {{X_{{{\text{MgO}}}} \times X_{{{\text{Al}}_{{\text{2}}} {\text{O}}_{{\text{3}}} }} } \mathord{\left/ {\vphantom {{X_{{{\text{MgO}}}} \times X_{{{\text{Al}}_{{\text{2}}} {\text{O}}_{{\text{3}}} }} } {X_{{{\text{Ti}}_{{\text{2}}} {\text{O}}_{{\text{3}}} }} }}} \right. \kern- ulldelimiterspace} {X_{{{\text{Ti}}_{{\text{2}}} {\text{O}}_{{\text{3}}} }} }} \right)} $$ of the inclusions (spinel potential) was expressed as a linear function of $$ {\left\lfloor {{a_{{{\text{Mg}}}} \times a^{2}_{{{\text{Al}}}} \times a_{{\text{O}}} } \mathord{\left/ {\vphantom {{a_{{{\text{Mg}}}} \times a^{2}_{{{\text{Al}}}} \times a_{{\text{O}}} } {a^{2}_{{{\text{Ti}}}} }}} \right. \kern- ulldelimiterspace} {a^{2}_{{{\text{Ti}}}} }} \right\rfloor } $$ of the steel melts with a slope of unity theoretically expected. Also, the spinel potential is very low and nearly constant when the activity of Al2O3 is less than that of TiO2 in the slag saturated by MgO, whereas it linearly increases by increasing the $$ \log \;{\left( {{a_{{{\text{Al}}_{{\text{2}}} {\text{O}}{}_{{\text{3}}}}} } \mathord{\left/ {\vphantom {{a_{{{\text{Al}}_{{\text{2}}} {\text{O}}{}_{{\text{3}}}}} } {a_{{{\text{TiO}}_{{\text{2}}} }} }}} \right. \kern- ulldelimiterspace} {a_{{{\text{TiO}}_{{\text{2}}} }} }} \right)} $$ at $$ {\left( {{X_{{{\text{Al}}_{{\text{2}}} {\text{O}}{}_{{\text{3}}}}} } \mathord{\left/ {\vphantom {{X_{{{\text{Al}}_{{\text{2}}} {\text{O}}{}_{{\text{3}}}}} } {X_{{{\text{TiO}}_{{\text{2}}} }} }}} \right. \kern- ulldelimiterspace} {X_{{{\text{TiO}}_{{\text{2}}} }} }} \right)} > 1 $$ .

Novel Approach to Link between Viscosity and Structure of Silicate Melts via Darken’s Excess Stability Function: Focus on the Amphoteric Behavior of Alumina

Abstract: The effect of alumina on the relationship between viscosity and structure of the CaO-SiO2-Al2O3-MgO system is investigated by employing viscometer using the rotating cylinder method and Fourier transform–infrared (FT-IR) spectra, respectively. In addition, the original Darken’s excess stability function was introduced in order to understand the thermophysical phenomena and the role of alumina based on thermodynamics. Alumina behaves as an amphoteric oxide in the CaO-SiO2-Al2O3-MgO melts, and this is not only experimentally confirmed but also thermodynamically proved by taking the Darken’s excess stability function into account.

Subsolidus phase equilibria of the Fe-Ni-O system

Abstract: The phase equilibria of the Fe-Ni-O system in the temperature range between 800 °C and 1600 °C, at various oxygen partial pressures, in the subsolidus region, have been experimentally studied using equilibration and quenching techniques followed by electron-probe X-ray microanalysis (EPMA). The pseudo-binary ‘Fe2O3’-NiO phase diagram in air and isothermal Fe-Ni-O phase diagrams at low oxygen partial pressures, at temperatures between 1000 °C and 1200 °C, have been constructed. In the conditions studied, it was found that the solubilities of iron in bunsenite-(Ni, Fe)O and solubilities of nickel in wustite-(Fe, Ni)O phases are higher than reported in the previous studies. It is suggested that the improved experimental techniques used in the present study ensure the achievement of the true equilibrium state in the system. The isothermal ternary phase diagrams of FeO-NiO-Fe2O3 at 1000 °C, 1100 °C, and 1200 °C, modeled using FactSage, have also been presented. The present work has provided new phase equilibria data in the Fe-Ni-O system, as well as provided the basis for further experimental studies and for development of thermodynamic models of higher order complex such as Fe-Ni-X-O (X = Mg, Al, Cr), important to improve extraction of Ni from nickel laterite ores.

A Semi-empirical Mathematical Model to Estimate the Duration of the Atmosphere within a Double Oxide Film Defect in Pure Aluminum Alloy

Abstract: It has been shown that the oxygen and nitrogen within the atmosphere of a double oxide film defect can be consumed by the surrounding Al melt. Experimentally determined reaction rates were used to construct a semi-empirical model to predict the change in volume with time of a bubble of air trapped in an Al melt, with the model including the diffusion of H from the metal into the bubble. Comparison with experimental results showed that the model predicted the change in volume well. The model was then used to estimate the duration of the internal atmosphere within double oxide film defects, which suggested that these would be consumed in a time of up to 3 minutes, depending upon assumptions made about the initial defect size.

Transient Mold Fluid Flow with Well- and Mountain-Bottom Nozzles in Continuous Casting of Steel

Abstract: Nozzle shape plays a key role in determining the flow pattern in the mold of the continuous- casting process under both steady-state and transient conditions. This work applies computational models and experiments with a one-third scale water model to characterize flow in the nozzle and mold to evaluate well-bottom and mountain-bottom nozzle performance. Velocities predicted with the three-dimensional k-e turbulence model agree with both particle- image velocimetry and impeller measurements in the water model. The steady-state jet velocity and angle leaving the ports is similar for the two nozzle-bottom designs. However, the results show that nozzles with a mountain-shaped bottom are more susceptible to problems from asymmetric flow, low-frequency surface-flow variations, and excessive surface velocities. The same benefits of the well-bottom nozzle are predicted for flow in the steel caster.

A Model for Predicting the Austenite Grain Size at the Surface of Continuously-Cast Slabs

Abstract: During the continuous-casting process, retarded cooling of the strand surface below oscillation marks and surface depressions results in the formation of coarse austenite grains. These coarse grains have proven to dramatically reduce the ductility of steel within the second ductility trough, and thus increase the risk of surface crack formation. In addition to the thermal history the composition of the steel, in particular the content of carbon and precipitation-forming elements, plays a decisive role in the development of the austenite grain size. The present work addresses the development and validation of an experimental and numerical model for predicting the austenite grain size in the continuous-casting process. In a first step, the previous austenite grain size on the surface of slabs was determined by metallographic examinations for several slabs with various carbon content. Next, a solidification experiment was adjusted in order to simulate the cooling conditions in the mold of a slab caster, but also to suppress the precipitation of nitrides and carbo-nitrides by subsequent accelerated cooling. Thus, it was possible to study the influence of steel composition on austenite grain growth at temperatures close to the solidus temperature, unaffected by precipitates. The results of both the plant and laboratory experiments point to a maximum austenite grain size with a carbon content of approximately 0.17 mass pct. The parameters of a grain size prediction model were fitted to the results of the experiment. The resultant model was coupled with a precipitation model and then applied to the slab casting process. The measured and calculated grain size values at the surface and immediately below the surface of the slabs agree very closely. The model was finally applied to calculate the grain growth in the center of a virtual oscillation mark under simplified assumptions. Although only the surface temperature in the mold diverges significantly from the original solution, the difference of the initial cooling conditions results in an increase of the final grain size by up to 40 pct.

Reduction of Iron-Oxide-Carbon Composites: Part II. Rates of Reduction of Composite Pellets in a Rotary Hearth Furnace Simulator

Abstract: A new ironmaking concept is being proposed that involves the combination of a rotary hearth furnace (RHF) with an iron-bath smelter. The RHF makes use of iron-oxide-carbon composite pellets as the charge material and the final product is direct-reduced iron (DRI) in the solid or molten state. This part of the research includes the development of a reactor that simulated the heat transfer in an RHF. The external heat-transport and high heating rates were simulated by means of infrared (IR) emitting lamps. The reaction rates were measured by analyzing the off-gas and computing both the amount of CO and CO2 generated and the degree of reduction. The reduction times were found to be comparable to the residence times observed in industrial RHFs. Both artificial ferric oxide (PAH) and naturally occurring hematite and taconite ores were used as the sources of iron oxide. Coal char and devolatilized wood charcoal were the reductants. Wood charcoal appeared to be a faster reductant than coal char. However, in the PAH-containing pellets, the reverse was found to be true because of heat-transfer limitations. For the same type of reductant, hematite-containing pellets were observed to reduce faster than taconite-containing pellets because of the development of internal porosity due to cracking and fissure formation during the Fe2O3-to-Fe3O4 transition. This is, however, absent during the reduction of taconite, which is primarily Fe3O4. The PAH-wood-charcoal pellets were found to undergo a significant amount of swelling at low-temperature conditions, which impeded the external heat transport to the lower layers. If the average degree of reduction targeted in an RHF is reduced from 95 to approximately 70 pct by coupling the RHF with a bath smelter, the productivity of the RHF can be enhanced 1.5 to 2 times. The use of a two- or three-layer bed was found to be superior to that of a single layer, for higher productivities.

Formation of a Mineral Layer during Coke Dissolution into Liquid Iron and Its Influence on the Kinetics of Coke Dissolution Rate

Abstract: The formation and development of the mineral layer that forms between coke and liquid iron during carbon dissolution has been characterized. Coke particles (−2 mm, +0.5 mm) were added to the top surface of an iron 2 mass pct C melt at representative iron-making temperatures, for periods of time between 2 and 120 minutes, before being quenched. The quenched samples were then sectioned, and the solidified coke-melt interfacial region analyzed in the scanning electron microscope (SEM). Analysis showed that a mineral layer was present at the interface at all experimental temperatures (1450 °C to 1550 °C) from 2 minutes and persisted beyond 120 minutes. The mineral layer was found to be composed of calcium aluminate phases, with the proportions of these phases dictating its morphology. Further, changes observed in the rate of carbon dissolution from the coke were related to the composition and morphology of the mineral layer. The effect of this mineral layer on the rate of carbon dissolution has been interpreted as a change in the reaction control mechanism.

Carbothermal Reduction of Manganese Oxide in Different Gas Atmospheres

Abstract: Carbothermal reduction of manganese oxides was studied in hydrogen, helium, and argon at different temperatures and carbon-to-manganese oxide ratios. Isothermal and temperature programmed carbothermal reduction experiments were conducted in a fixed bed reactor in a vertical tube furnace, with on-line monitoring of gas composition by the CO-CO2 infrared sensor. The extent of reduction was calculated using the off-gas composition and LECO oxygen contents in the reduced samples. In all gas atmospheres, the reaction rate increased with temperature. The reduction rate of manganese oxide in hydrogen was higher than in helium, and in helium higher than in argon. This was attributed to the involvement of hydrogen in the reduction process and the difference in CO and CO2 diffusion coefficients in helium and argon.

Factors Affecting the Precipitation of Potassium Jarosite in Sulfate and Chloride Media

Abstract: The factors affecting the precipitation of potassium jarosite in both sulfate and chloride media were systematically investigated for the range of conditions likely to be encountered in hydrometallurgical practice. In sulfate solutions at 97 °C, the amount of precipitate increases with increasing retention times up to 10 hours, with increasing temperatures to approximately 100 °C, and with increasing ferric-ion concentrations. Despite the variations in the amount of precipitate, the composition remains nearly constant and is characteristic of that of potassium jarosite. The presence of potassium jarosite seed accelerates the rate of precipitation and results in near-maximum amounts of precipitate after just a few hours of reaction. The presence of ferrous sulfate has a negligible effect on both the product yield and composition. The amount of precipitate decreases with increasing acid concentrations, but the composition of the precipitates remains nearly constant, even for acid concentrations as high as 1.0 M H2SO4. Both the amount and composition of the precipitates vary as the K2SO4 concentration increases; in the presence of excess ferric ions, nearly complete K precipitation occurs. Small amounts of Cu2+, Zn2+, and Pb2+ are incorporated in the structure of potassium jarosite, and the amount increases with increasing concentrations of the divalent metal ions. Potassium jarosite is readily formed in chloride media at 140 °C, provided that an independent source of sulfate ions is available. A minimum K concentration is required in order to avoid the formation of hematite; at higher K concentrations, however, the product composition becomes nearly constant and reflects that of potassium jarosite. Both the product yield and composition are independent of the chloride concentration, added as LiCl, and the ferrous ion concentration, added as FeCl2. Although increasing concentrations of HCl result in a near-linear decrease in the amount of precipitate, the composition is independent of the HCl concentration and is characteristic of that of potassium jarosite. Increasing temperatures to 200 °C result in an increase in the product yield, but have no significant effect on the composition of the potassium jarosite precipitates.

Effect of Process Parameters on the Melting Ratio in Overlap Pulsed Laser Welding

Abstract: A study of melting ratio in overlap pulsed laser welding has been done on St14 carbon steel sheet to investigate the effect of process parameters. Pulse duration, pulse energy, pulse frequency, and travel speed were varied in the experimental procedure. The results of the melting ratio have been presented by reforming the related formulas. Formulas have been modified based on overlapping and preheat effect factors. A new parameter is defined to show the actual energy entrance to the spot region in overlapping pulsed laser welding. It is shown that keyhole formation in pulsed laser welding has an essential role in increasing the melting ratio. Moreover, it is shown that the role of pulse energy is more effective on the melting ratio than pulse duration and overlapping process variables. The effect of overlapping has been studied by varying the travel speed and pulse frequency separately, and an optimum range of overlapping for maximizing the melting ratio in the full penetration keyhole mode was established.

Kinetic Studies on Hydrogen Reduction of MoO3 and Morphological Analysis of Reduced Mo Powder

Abstract: The studies on hydrogen reduction of MoO3 indicated two main stages, namely, MoO3 to MoO2 and MoO2 to Mo. In both stages, temperature and time of reduction progressively increased on diluting hydrogen from 100 to 10 pct. Detailed kinetic analysis of the MoO2 to Mo stage was conducted by carrying out isothermal reduction between 625 °C and 900 °C by pure hydrogen. The kinetic equation was found to be $$ g{\left( \alpha \right)} = {\left[ { - \ln {\left( {1 - \alpha } \right)}} \right]}^{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 n}}\right.\kern- ulldelimiterspace} \!\lower0.7ex\hbox{$n$}}} = {\text{k}}t $$ , with n ranging from 1.49 to 2.13. The rate constant k obeyed the Arrhenius temperature dependence with the associated activation energy of 136 kJ mol−1. The X-ray diffraction (XRD) analysis of the product confirmed the phases predicted by thermal analysis. The scanning electron microscope (SEM) analysis of molybdenum powder revealed the presence of a greater number of pores and cracks in the individual particles produced at lower reduction temperatures and the tendency of acquiring spherical morphology with increasing reduction temperature. Based on the studies conducted, the optimum conditions for MoO3 to Mo reduction were predicted.

Reduction of Iron-Oxide-Carbon Composites: Part I. Estimation of the Rate Constants

Abstract: A new ironmaking concept using iron-oxide-carbon composite pellets has been proposed, which involves the combination of a rotary hearth furnace (RHF) and an iron bath smelter. This part of the research focuses on studying the two primary chemical kinetic steps. Efforts have been made to experimentally measure the kinetics of the carbon gasification by CO2 and wustite reduction by CO by isolating them from the influence of heat- and mass-transport steps. A combined reaction model was used to interpret the experimental data and determine the rate constants. Results showed that the reduction is likely to be influenced by the chemical kinetics of both carbon oxidation and wustite reduction at the temperatures of interest. Devolatilized wood-charcoal was observed to be a far more reactive form of carbon in comparison to coal-char. Sintering of the iron-oxide at the high temperatures of interest was found to exert a considerable influence on the reactivity of wustite by virtue of altering the internal pore surface area available for the reaction. Sintering was found to be predominant for highly porous oxides and less of an influence on the denser ores. It was found using an indirect measurement technique that the rate constants for wustite reduction were higher for the porous iron-oxide than dense hematite ore at higher temperatures (>1423 K). Such an indirect mode of measurement was used to minimize the influence of sintering of the porous oxide at these temperatures.

Three-Dimensional Numerical Study of Impinging Water Jets in Runout Table Cooling Processes

Abstract: Cooling from impinging water jets in runout table (ROT) processing depends on the fluid flow and depth of water accumulated in the water pool that forms on the surface of the moving steel strip. This effect is investigated with a three-dimensional (3-D) computational model of fluid flow, pressure, and free surface motion for realistic banks of nozzles within the flow rate region of the ROT process (2400 to 9200 L/min m2). The volume of fluid (VOF) method with the high-resolution interface capturing (HRIC) scheme was implemented to handle the free surface flow of the water jet, and the k-e model was used for turbulence. The governing equations are discretized by a second-order accurate scheme and solved with the computational fluid dynamics (CFD) code Fluent. The model was validated with experimental measurements of free-surface shape and hydraulic jump position for a single water jet impinging onto a moving surface that included turbulent flow and multiphase regions of mixed bubbles and water. For banks of water jets impinging onto the surface of the moving strip in a realistic ROT, the free surface shape, velocity, and pressure distributions have been calculated for various flow rates and surface widths. A deeper water pool is expected on the moving surface with increasing water flow rate and with increasing width. In addition, as the water pool height increases, the pressure variations on the moving surface below the water jets decrease. A simple relation to predict the water pool height from the water flow rate per unit area and strip width has been derived. The predictions agree well with both the 3-D calculations and measurements from water model experiments.

Simulation of Fluid and Inclusions Dynamics during Filtration Operations of Ductile Iron Melts Using Foam Filters

Abstract: The use of ceramic foam filters in ductile iron foundries to reduce the number of inclusions that reach the casting has been widely accepted. However, the exact mechanisms contributing to foam filter effectiveness are not yet known; this limits the ability to maximize filter performance and inclusion reduction. The objective of this work is to qualify and quantify the effects of the foam filter structure on inclusion retention. This has been accomplished through the development of a three-dimensional (3-D) mathematical model, based on physical water modeling and mathematical simulations. It was found that the casting rate and inclusion density play minor roles in the capture ratio, while inclusion size is the most influent variable. One mechanism for capturing inclusions involves the direct impact of an inclusion on the web wall and its adhesion after crossing over the liquid film. Two additional mechanisms involve the entrainment of inclusions by buoyancy-lift forces into low-velocity fields and the ulterior adhesion through buoyancy effects. The second mechanism is the entrainment of inclusions into microrecirculating flows; the inclusions remain in these flows for times that exceed the mold filling time. The latter mechanism has limited intensity for inclusions approximately 30 to 100 μm in size. In order to enhance the effects of this mechanism in this range of sizes, the vorticity magnitude in the microfree shear flows in the filter’s pores must be increased, through changes in the structure geometry of this device.

Phase Equilibria in Ferrous Calcium Silicate Slags: Part I. Intermediate Oxygen Partial Pressures in the Temperature Range 1200 °C to 1350 °C

Abstract: Ferrous calcium silicate slags are used in primary and secondary metallurgical processes (described by the FeO-Fe2O3-CaO-SiO2 system) Despite the industrial and scientific importance of this system, the phase equilibria have not been fully investigated Characterization of this slag system is necessary to improve the design and optimization of new and existing metallurgical processes, particularly in relation to fluxing practice and operating temperatures Experimental methods have been developed to investigate the phase equilibria of these slags involving the equilibration of samples at fixed oxygen partial pressures, rapid quenching, and the analysis of the compositions of solid and liquid phases using electron probe X-ray microanalysis (EPMA) with wavelength dispersive detectors Liquidus and solidus data are reported for the primary phase fields of spinel, pseudo-wollastonite, and tridymite in the temperature range of 1200 °C to 1350 °C at an oxygen partial pressure of 10−6 atm, and at 1250 °C at an oxygen partial pressure of 10−5 atm The resulting data have been used to construct liquidus and solidus isotherms in the “FeO”-CaO-SiO2 system directly relevant to industrial processes

Activity of Iron Oxide in Steelmaking Slag

Abstract: Most refining reactions in steelmaking involve oxidation of impurity element(s). The product(s) of oxidation either dissolve in the slag or escape as gaseous phase. The activities of oxygen in the metal (h O), and that of “FeO” in slag (a FeO), are major factors controlling these chemical reactions. The activities of oxygen and “FeO” are thermodynamically related, provided equilibrium distribution of oxygen between the slag and the metal is attained. This enables direct estimation of one parameter from the other. A thorough knowledge of the variation in activity of FeO, and factors affecting the same, is therefore of great importance in the process metallurgy of steelmaking. The present work experimentally measures the activity of FeO in steelmaking slags and attempts to develop a correlation for estimation of γ(FeO) as a function of temperature and chemical composition of the slag.

Kinetics of Scrap Melting in Liquid Steel: Multipiece Scrap Melting

Abstract: This article extends the study of single-steel bar melting discussed in a previous article[1] to the investigation of two-bar and multibar melting kinetics. Experiments involving multiple bars reveal that the interbar spacing and the initial solid and liquid steel temperatures influence the final melting time by altering the degree of “steel iceberg” formation. Simulations of scrap melting using a recently developed phase-field model of steel scrap melting[1] are shown to follow the trends of the two-bar melting experiments. The phase-field methodology is also extended to examine melting of randomly distributed scrap in the liquid steel bath, a poorly understood situation that is difficult to access experimentally. Two types of simulations were performed. The first type assumed a constant heat-transfer coefficient and liquid steel temperature, corresponding to the limiting case of melting with perfect stirring in the liquid steel bath. Results for this case reveal that the final melting time was controlled by the largest of a group of isolated steel icebergs, which formed in regions of low scrap porosity. The second type examined the case of melting dominated by heat conduction, using an effective thermal conductivity to model low-level natural convection in the liquid steel. In this case, phase-field simulations show that, under certain conditions, melting could be well approximated by a simple one-dimensional (1-D) analytical melting model with effective parameters related to the scrap distribution, scrap preheating, and liquid bath temperatures.

Surface Segregation during Directional Solidification of Ni-Base Superalloys

Abstract: Some aspects pertaining to the increased microsegregation at the external casting surface during directional solidification of a typical Ni-base superalloy, CMSX 10N, are presented. Increased eutectic coverage was observed at the external surface along the solidification length. This eutectic appears as a thin segregated layer proud of the secondary dendrite arms preventing them from impinging onto the mold wall. The extent of surface eutectic coverage was represented as a fractional measure of the ingot perimeter. Possible mechanisms focusing on the following: (1) interaction between mold and metal, (2) inclination of primary dendrite, and (3) contraction of the dendrite network have been investigated in relation to the observed phenomenon. We deduce that the most likely explanation is associated with the contraction of the dendritic network, which qualitatively accounts both for the observed morphology and the increased eutectic fraction at the external surface of the casting.

Basic Nickel Carbonate: Part I. Microstructure and Phase Changes during Oxidation and Reduction Processes

Abstract: A significant industrial problem associated with the production of nickel from basic nickel carbonate has been identified. Fundamental studies of the change of phase, product surface, and internal microstructures taking place during oxidation and reduction processes at temperatures between 110 °C and 900 °C have been carried out. The various elemental reactions and fundamental phenomena that contribute to the change of the physical and chemical characteristics of the samples during the processes taking place in Ni metal production through gas/solid-reduction processes have been identified and thoroughly investigated. The following phenomena affecting the final-product microstructure were identified as follows: (1) chemical changes, i.e., decomposition, reduction reactions, and oxidation reactions; (2) NiO and Ni recrystallization and grain growth; (3) NiO and Ni sintering and densification; and (4) agglomeration of the NiO and Ni particles.

Thermodynamics for Leaching of Scheelite—Pseudo-Ternary-System Phase Diagram and Its Application

Abstract: For thermodynamic reasons, it has been considered that NaOH cannot be used for the digestion of scheelite. Though the caustic thermal mill process has been applied in industry for a long time, people still do not understand it well. In this article, a method called the “pseudo-ternary-system diagram method” is proposed and is used as a tool for carrying out thermodynamic analysis of the NaOH digestion of scheelite. A theoretical explanation is also provided for the concentration effect of Na2CO3 on scheelite digestion.

Reduction of Iron-Oxide-Carbon Composites: Part III. Shrinkage of Composite Pellets during Reduction

Abstract: This article involves the evaluation of the volume change of iron-oxide-carbon composite pellets and its implications on reduction kinetics under conditions prevalent in a rotary hearth furnace (RHF) that were simulated in the laboratory. The pellets, in general, were found to shrink considerably during the reduction due to the loss of carbon and oxygen from the system, sintering of the iron-oxide, and formation of a molten slag phase at localized regions inside the pellets due to the presence of binder and coal/wood-charcoal ash at the reduction temperatures. One of the shortcomings of the RHF ironmaking process has been the inability to use multiple layers of composite pellets because of the impediment in heat transport to the lower layers of a multilayer bed. However, pellet shrinkage was found to have a strong effect on the reduction kinetics by virtue of enhancing the external heat transport to the lower layers. The volume change of the different kinds of composite pellets was studied as a function of reduction temperature and time. The estimation of the change in the amount of external heat transport with varying pellet sizes for a particular layer of a multilayer bed was obtained by conducting heat-transfer tests using inert low-carbon steel spheres. It was found that if the pellets of the top layer of the bed shrink by 30 pct, the external heat transfer to the second layer increases by nearly 6 times.

Kinetics of Pressure Dissolution of Enargite in Sulfate-Oxygen Media

Abstract: Enargite (Cu3AsS4) is an increasingly common impurity in Chilean copper concentrates and its presence complicates the conventional treatment of the concentrates by smelting-converting because of the environmental risk of arsenic emissions to the atmosphere. Therefore, the recovery of copper from concentrates with high arsenic content must be carried out by nonconventional technologies. Sulfuric acid leaching processes are viable alternatives to treat copper concentrates with high content of enargite. Hence, in this article, the pressure leaching of enargite in the sulfuric acid-oxygen system is discussed. The leaching was studied at 160 °C to 220 °C and partial pressures of oxygen of 303 to 1013 kPa. The enargite dissolution was determined to occur as predicted by thermodynamics according to \( {\text{Cu}}_{3} {\text{AsS}}_{4}+{\text{8.75O}}_{2}+2{\text{.5H}}_{2} {\text{O}}+{\text{2H}}^{+} =3{\text{Cu}}^{2+}+{\text{H}}_{3} {\text{AsO}}_{4} +{\text{4HSO}}^{-}_{4}.\) The leaching rate increased substantially with increasing temperature. Complete dissolution of enargite with particle size 64 μm was obtained at 220 °C and 689 kPa of oxygen partial pressure in 120 minutes. The dissolution kinetics was analyzed by using the shrinking core model for spherical particles with surface chemical control. The rate of reaction was found to be 1/3 order with respect to the oxygen partial pressure and zero order with respect to sulfuric acid concentration. Activation energy of 69 kJ/mol was estimated for the dissolution reaction, which is a typical value for a chemically controlled process.

Kinetics of spinel formation and growth during dissolution of MgO in CaO-Al2O3-SiO2 slag

Abstract: The formation and growth of MgAl2O4 spinel crystals on a single-crystal MgO substrate submerged in a 40 pct CaO, 40 pct SiO2, and 20 pct Al2O3 slag were directly observed using high-temperature microscopy. This showed that the crystals initially form on the MgO surface, but may break off and be carried out into the liquid slag. Still pictures extracted from digitally recorded images were used to measure the size of these crystals at 1420 °C, 1440 °C, and 1460 °C as a function of time. Growth of the crystals was found to follow the parabolic rate law, with rates increasing with temperature. An activation energy of 564 kJ mol−1 was estimated from the experimental data. This was found to be comparable with previously published results from different types of experiments on spinel formation.