Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 2010"

A Review on Die Attach Materials for SiC-Based High-Temperature Power Devices

Abstract: Recently, high-temperature power devices have become a popular discussion topic because of their various potential applications in the automotive, down-hole oil and gas industries for well logging, aircraft, space exploration, nuclear environments, and radars. Devices for these applications are fabricated on silicon carbide-based semiconductor material. For these devices to perform effectively, an appropriate die attach material with specific requirements must be selected and employed correctly. This article presents a review of this topic, with a focus on the die attach materials operating at temperatures higher than 623 K (350 °C). Future challenges and prospects related to high-temperature die attach materials also are proposed at the end of this article.

Development of an Inert Anode for Electrowinning in Calcium Chloride–Calcium Oxide Melts

Abstract: Studies were performed investigating the anodic testing of calcium ruthenate for electrowinning in calcium chloride–calcium oxide melts. The results showed that calcium ruthenate may be suitable as an inert anode in calcium chloride containing melts as it exhibited a low rate of corrosion in melts containing a small amount of calcium oxide, capable of producing oxygen on its surface, and did not contaminate the melt. To reduce the amount of ruthenium in the anode, solid solutions of calcium ruthenate in calcium titanate were investigated. At low concentrations, the solid solution is a semiconductor with a relatively low conductivity at room temperature, but at the temperature of operation, 1173 K, the material is an excellent electronic conductor. The other way of reducing the amount of ruthenium is to coat the solid solution onto a substrate. In this way, the substrate would give the mechanical strength while the coating would give the electrical conductivity and corrosion protection. Calcium ruthenate-based anodes can endure long-term use in the laboratory under an applied electrical field with oxygen being liberated on the anode indicating that these materials are candidates for the electrowining in calcium chloride–calcium oxide melts.

Modeling of Blast Furnace with Layered Cohesive Zone

Abstract: An ironmaking blast furnace (BF) is a moving bed reactor involving counter-, co-, and cross-current flows of gas, powder, liquids, and solids, coupled with heat exchange and chemical reactions. The behavior of multiple phases directly affects the stability and productivity of the furnace. In the present study, a mathematical model is proposed to describe the behavior of fluid flow, heat and mass transfer, as well as chemical reactions in a BF, in which gas, solid, and liquid phases affect each other through interaction forces, and their flows are competing for the space available. Process variables that characterize the internal furnace state, such as reduction degree, reducing gas and burden concentrations, as well as gas and condensed phase temperatures, have been described quantitatively. In particular, different treatments of the cohesive zone (CZ), i.e., layered, isotropic, and anisotropic nonlayered, are discussed, and their influence on simulation results is compared. The results show that predicted fluid flow and thermochemical phenomena within and around the CZ and in the lower part of the BF are different for different treatments. The layered CZ treatment corresponds to the layered charging of burden and naturally can predict the CZ as a gas distributor and liquid generator.

Computational Fluid Dynamics Simulation of Supersonic Oxygen Jet Behavior at Steelmaking Temperature

Abstract: Supersonic oxygen jets are used in steelmaking and other different metal refining processes, and therefore, the behavior of supersonic jets inside a high temperature field is important for understanding these processes. In this study, a computational fluid dynamics (CFD) model was developed to investigate the effect of a high ambient temperature field on supersonic oxygen jet behavior. The results were compared with available experimental data by Sumi et al. and with a jet model proposed by Ito and Muchi. At high ambient temperatures, the density of the ambient fluid is low. Therefore, the mass addition to the jet from the surrounding medium is low, which reduces the growth rate of the turbulent mixing region. As a result, the velocity decreases more slowly, and the potential core length of the jet increases at high ambient temperatures. But CFD simulation of the supersonic jet using the k−e turbulence model, including compressibility terms, was found to underpredict the potential flow core length at higher ambient temperatures. A modified k-e turbulence model is presented that modifies the turbulent viscosity in order to reduce the growth rate of turbulent mixing at high ambient temperatures. The results obtained by using the modified turbulence model were found to be in good agreement with the experimental data. The CFD simulation showed that the potential flow core length at steelmaking temperatures (1800 K) is 2.5 times as long as that at room temperature. The simulation results then were used to investigate the effect of ambient temperature on the droplet generation rate using a dimensionless blowing number.

Transformation of Alumina Inclusions by Calcium Treatment

Abstract: The objectives of this study were to investigate reactions of calcium with Al2O3 by different model experiments both on the laboratory and on the industrial scale. Experiments with solid Al2O3 and CaO were performed between 1350 °C and 1600 °C. Reaction rate constants were determined based on scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) observations of reaction products and weight measurements of the Al2O3 reacted via dissolution of the CaO bearing phases from the specimens after the annealing period. The results showed that the formation of calcium aluminate phases proceeded rapidly at temperatures greater than 1405 °C when a liquid calcium aluminate was formed. In the lowest temperature range (1350 °C–1405 °C), when the formation of liquid phase ceased, the reaction rate was several orders of magnitude lower. Industrial trials including Ca-alloy injection into steel, sampling and SEM/EDS analyses, as well as an inclusion rating in the samples show the concept of rapid transformation of the alumina inclusions with Ca treatment.

Influence of forced/natural convection on segregation during the directional solidification of Al-based binary alloys.

Abstract: We analyzed the columnar solidification of a binary alloy under the influence of an electromagnetic forced convection of various types and investigated the influence of a rotating magnetic field on segregation during directional solidification of Al-Si alloy as well as the influence of a travelling magnetic field on segregation during solidification of Al-Ni alloy through directional solidification experiments and numerical modeling of macrosegregation. The numerical model is capable of predicting fluid flow, heat transfer, solute concentration field, and columnar solidification and takes into account the existence of a mushy zone. Fluid flows are created by both natural convection as well as electromagnetic body forces. Both the experiments and the numerical modeling, which were achieved in axisymmetric geometry, show that the forced-flow configuration changes the segregation pattern. The change is a result of the coupling between the liquid flow and the top of the mushy zone via the pressure distribution along the solidification front. In a forced flow, the pressure difference along the front drives a mush flow that transports the solute within the mushy region. The channel forms at the junction of two meridional vortices in the liquid zone where the fluid leaves the front. The latter phenomenon is observed for both the rotating magnetic field (RMF) and traveling magnetic field (TMF) cases. The liquid enrichment in the segregated channel is strong enough that the local solute concentration may reach the eutectic composition.

Removal of Inclusions from Aluminum Through Filtration

Abstract: Filtration experiments were carried out using both an AlF3 slurry-coated and an uncoated Al2O3 ceramic foam filter to study the removal of nonmetallic inclusions and impurity elements. The results showed that the 30-ppi ceramic foam filter removed up to 85 pct inclusions from aluminum. Several pictures of two- and three-dimensional morphologies of both nonmetallic and intermetallics inclusions also have been presented. The following contributing mechanisms for the removal of nonmetallic inclusions in the deep-bed filtration mode are proposed: (1) collision with walls and interception effect and (2) the formation of both intermetallic and nonmetallic inclusion bridges during filtration. Fluid dynamics modeling of inclusion attachment to the filter walls showed that most inclusions, especially those with larger sizes, are entrapped at the upper part of the filter, whereas smaller inclusions are dispersed well throughout the filter. The calculated inclusions removal fractions for the 30-ppi filter showed that almost all inclusions >125 μm are removed, and inclusions ~5 μm in size are removed up to 85 pct. The interfacial energy between two collided same-size inclusions was calculated, indicating that a strong clustering of inclusions may result within the filter window. Magnesium impurities were removed up to 86 pct by the AlF3 slurry-coated filter. The filter acted in active filtration mode in addition to the contribution of the air oxidation of dissolved [Mg], which was calculated to be 13 pct. The total mass transfer coefficient of dissolved [Mg] to the reaction interface was calculated to be 1.15 × 10−6 m/s.

Mass Transfer of Phosphorus in Silicon Melts Under Vacuum Induction Refining

Abstract: An experimental investigation into the mass transfer of phosphorus in molten silicon under vacuum induction refining has been carried out. In a pilot-scale experiment, in the temperature range 1773 K (1500 °C) to 1873 K (1600 °C) and a vacuum of 0.1 to 0.035 Pa smelting for 7200 seconds (2 hours), phosphorus is decreased from 15 ppmw to 0.08 ppmw, which achieved the target for solar-grade silicon of less than 0.1 ppmw. Lab-scale experiments are used to determine the effects of vacuum, refining time, and temperature on the rate of mass transfer of phosphorus during vacuum refining. Hardly any phosphorus was removed when the vacuum pressure is greater than 100 Pa. Mass-transfer coefficients are nearly independent of pressure at 1783 K (1510 °C) when pressures are below 0.1 Pa and are highly correlated with vacuum pressures above 0.1 Pa. A model of vacuum refining of inductively stirred silicon melts is discussed to explain the transfer path of phosphorus out of the melt.

Computational Fluid Dynamics Modeling of Supersonic Coherent Jets for Electric Arc Furnace Steelmaking Process

Abstract: Supersonic coherent gas jets are now used widely in electric arc furnace steelmaking and many other industrial applications to increase the gas–liquid mixing, reaction rates, and energy efficiency of the process. However, there has been limited research on the basic physics of supersonic coherent jets. In the present study, computational fluid dynamics (CFD) simulation of the supersonic jet with and without a shrouding flame at room ambient temperature was carried out and validated against experimental data. The numerical results show that the potential core length of the supersonic oxygen and nitrogen jet with shrouding flame is more than four times and three times longer, respectively, than that without flame shrouding, which is in good agreement with the experimental data. The spreading rate of the supersonic jet decreased dramatically with the use of the shrouding flame compared with a conventional supersonic jet. The present CFD model was used to investigate the characteristics of the supersonic coherent oxygen jet at steelmaking conditions of around 1700 K (1427 °C). The potential core length of the supersonic coherent oxygen jet at steelmaking conditions was 1.4 times longer than that at room ambient temperature.

Removal of Low-Content Impurities from Al By Super-Gravity

Abstract: Super-gravity segregation was investigated to enrich and remove the low-content impurities from Al by solidifying Al under a super-gravity field. The macrosegregations of Fe (0.19 wt pct) and Si (0.09 wt pct) were remarkable within 1.5 cm along the direction of super gravity, and the concentration ratios between two sides under super gravity of 1000 g reached 4.05 and 2.80, respectively. The microstructures demonstrated that Fe- and Si-rich phases formed and gathered at the bottom along the direction of super-gravity field.

Modification and Minimization of Spinel(Al2O3·xMgO) Inclusions Formed in Ti-Added Steel Melts

Abstract: High-melting-point inclusions such as spinel(Al2O3·xMgO) are known to promote clogging of the submerged entry nozzle (SEN) in a continuous caster mold. In particular, Ti-alloyed steels can have severe nozzle clogging problems, which are detrimental to the slab surface quality. In this work, the thermodynamic role of Ti in steels and the effect of Ca and Ti addition to the molten austenitic stainless steel deoxidized with Al on the formation of Al2O3·xMgO spinel inclusions were investigated. The sequence of Ca and Ti additions after Al deoxidation was also investigated. The inclusion chemistry and morphology according to the order of Ca and Ti are discussed from the standpoint of spinel formation. The thermodynamic interaction parameter of Mg with respect to the Ti alloying element was determined. The element of Ti in steels could contribute to enhancing the spinel formation, because Ti accelerates Mg dissolution from the MgO containing refractory walls or slags because of its high thermodynamic affinity for Mg \( ( {e_{\text{Mg}}^{\text{Ti}} = - 0. 9 3 3}). \) Even though Ti also induces Ca dissolution from the CaO-containing refractory walls or slags because of its thermodynamic affinity for Ca \( \left( {e_{\text{Ca}}^{\text{Ti}} = - 0.119} \right), \) dissolved Ca plays a role in favoring the formation of calcium aluminate inclusions, which are more stable thermodynamically in an Al-deoxidized steel. The inclusion content of steel samples was analyzed to improve the understanding of fundamentals of Al2O3·xMgO spinel inclusion formation. The optimum processing conditions for Ca treatment and Ti addition in austenitic stainless steel melts to achieve the minimized spinel formation and the maximized Ti-alloying yield is discussed.

Numerical Simulation of the Multiphase Flow in the Rheinsahl–Heraeus (RH) System

Abstract: Knowledge of gas–liquid multiphase flow behavior in the Rheinsahl–Heraeus (RH) system is of great significance to clarify the circulation flow rate, decarburization, and inclusion removal with a reliable description. Thus, based on the separate model of injecting gas behavior, a novel mathematical model of multiphase flow has been developed to give the distribution of gas holdup in the RH system. The numerical results show that the predicted circulation flow rates, the predicted flow velocities, and the predicted mixing times agree with the measured results in a water model and that the predicted tracer concentration curve agrees with the results obtained in an actual RH system. With a lower lifting gas flow rate, the rising gas bubbles are concentrated near the wall; with a higher lifting gas flow rate, gas bubbles can reach the center of the up-snorkel. A critical lifting gas flow rate is used to obtain the maximum circulation flow rate.

Carbothermal reduction of a primary ilmenite concentrate in different gas atmospheres

Abstract: The carbothermal reduction of a primary ilmenite concentrate was studied in hydrogen, argon, and helium. Ilmenite and graphite were uniformly mixed and pressed into pellets. Reduction was studied in isothermal and temperature-programmed reduction experiments in a tube reactor with continuously flowing gas. CO, CO2, and CH4 contents in the off-gas were measured online using infrared sensors. The phase composition of reduced samples was characterized by X-ray diffraction (XRD). Oxygen and carbon contents in reduced samples were determined by LECO analyzers (LECO Corporation, St. Joseph, MI). The main phases in the ilmenite concentrate were ilmenite and pseudorutile. The reaction started with the reduction of pseudorutile to ilmenite and titania, followed by the reduction of ilmenite to metallic iron and titania. Titania was reduced to Ti3O5 and even more to Ti2O3, which was converted to titanium oxycarbide. Reduction was faster in hydrogen than in helium and argon, which was attributed to involvement of hydrogen in the reduction reactions. The formation of titanium oxycarbide in hydrogen started at 1000 °C and was completed in 300 minutes at 1200 °C, and 30 minutes at 1500 °C. The formation of titanium oxycarbide in argon and helium started at 1200 °C and was not completed after 300 minutes at 1300 °C.

Numerical Investigation of Residual Stress in Thick Titanium Alloy Plate Joined with Electron Beam Welding

Abstract: A finite-element (FE) simulation process integrating three dimensional (3D) with two-dimensional (2D) models is introduced to investigate the residual stress of a thick plate with 50-mm thickness welded by an electron beam. A combined heat source is developed by superimposing a conical volume heat source and a uniform surface heat source to simulate the temperature field of the 2D model with a fine mesh, and then the optimal heat source parameters are employed by the elongated heat source for the 3D simulation without trial simulations. The welding residual stress also is investigated with emphasis on the through-thickness stress for the thick plate. Results show that the agreement between simulation and experiment is good with a reasonable degree of accuracy in respect to the residual stress on the top surface and the weld profile. The through-thickness residual stress of the thick plate induced by electron beam welding is distinctly different from that of the arc welding presented in the references.

Transient Behavior of Inclusion Chemistry, Shape, and Structure in Fe-Al-Ti-O Melts: Effect of Gradual Increase in Ti

Abstract: During the processing of liquid steels, nonmetallic inclusions precipitate and evolve under conditions that often involve transient changes in chemistry or temperature, which could be reflected in the final products unless sufficient time is provided for equilibration to be established. The current study is focused on documenting the changes that inclusions undergo in terms of chemistry, shape, and structure when Ti is added in smaller batches, to avoid reactions caused by locally high Ti concentrations and result in a final melt chemistry with a Ti/Al ratio of 1 in the melt corresponding to the chemistry of interstitial free (IF) steel melts in the ladle furnace. When Ti was added in two increments, the inclusion composition was altered from spherical and irregular Al2O3 to mostly irregular inclusions that included both Al and Ti after the first addition. The second addition did not cause any change, but with time, the inclusion chemistry reverted back to Al2O3 with the morphology change remaining. For the case when Ti was added in four increments, however, the inclusion chemistry was modified largely after the first Ti addition, but the inclusion morphology did not change to the irregular-dominant case until the second Ti addition was made. Part of the Ti-containing inclusions was the result of the dissolution of TiOx into Al2O3. It seems that a critical Ti/Al ratio exists in between 1/4 and 1/2, which determines the morphological change. This finding might be coincident with the required increase in Ti and the decrease of local oxygen, which causes a precipitation of a new TiOx phase as opposed to dissolution of TiOx in Al2O3. Prolonging the interval between each Ti addition would allow the inclusion change in composition, reverting from the Ti-containing dominant stage to primarily Al2O3, but not in morphology.

Modeling of Heat Transfer and Fluid Flow in the Laser Multilayered Cladding Process

Abstract: The current work examines the heat-and-mass transfer process in the laser multilayered cladding of H13 tool steel powder by numerical modeling and experimental validation. A multiphase transient model is developed to investigate the evolution of the temperature field and flow velocity of the liquid phase in the molten pool. The solid region of the substrate and solidified clad, the liquid region of the melted clad material, and the gas region of the surrounding air are included. In this model, a level-set method is used to track the free surface motion of the molten pool with the powder material feeding and scanning of the laser beam. An enthalpy–porosity approach is applied to deal with the solidification and melting that occurs in the cladding process. Moreover, the laser heat input and heat losses from the forced convection and heat radiation that occurs on the top surface of the deposited layer are incorporated into the source term of the governing equations. The effects of the laser power, scanning speed, and powder-feed rate on the dilution and height of the multilayered clad are investigated based on the numerical model and experimental measurements. The results show that an increase of the laser power and powder feed rate, or a reduction of the scanning speed, can increase the clad height and directly influence the remelted depth of each layer of deposition. The numerical results have a qualitative agreement with the experimental measurements.

Addition of Dispersoid Titanium Oxide Inclusions in Steel and Their Influence on Grain Refinement

Abstract: In this article, the addition of dispersoid titanium oxide inclusions into liquid steel, the effect of additions on the inclusions found in the steel and on grain refinement, and acicular ferrite formation were studied. Different TiO2-containing materials and addition procedures into liquid steel were tested in experimental heats to obtain inclusions that promote grain refinement and acicular ferrite formation in C-Mn-Cr steel. Different additives with metallic Ti and TiO2 were added into the steel melt just before casting or into the mold during casting to create Ti-containing inclusions. The aluminum content in steel was lowered by an addition of iron oxide. The samples taken from steel melts and ingots were studied with a scanning electron microscope to find inclusions and to analyze them. Thermodynamic calculations showed that the Al content should be low (<50 ppm) to obtain Ti oxide dominating inclusions, whereas Al2O3 were formed at higher Al contents. When TiO2 was added late before casting, the oxide inclusions were Ti oxides and were mixed with Ti, Al, and Mn oxides. Small inclusions around 1 μm were detected in the samples with TiO x or TiN as the main component. It could be concluded that the additions resulted in a clearly higher number and in a smaller size of TiO x inclusions than just by adding metallic Ti. Selected samples were brought for subsequent hot rolling and heat-treatment experiments to find out the grain-refining effect and the eventual formation of acicular ferrite. Grain refinement was observed clearly, but the presence of acicular ferrite could not be confirmed definitely.

Electrochemical Reduction of Tungsten Compounds to Produce Tungsten Powder

Abstract: The production of tungsten by direct current reduction has been investigated. Experimental studies involved the electrochemical reduction of the solid tungsten compounds tungsten trioxide (WO3) and calcium tungstate (CaWO4) in the form of an assembled cathode of porous pellets attached to a current collector. Molten calcium chloride and a molten solution of calcium chloride and sodium chloride at eutectic composition, 48 pct mol NaCl, were used as the electrolytes. Reduced samples were characterized by means of X-ray diffraction analyses and scanning electron microscopy. The results of X-ray analyses, supported with thermodynamic computations, showed that WO3 cannot be used without loss in processes that involve the use of CaCl2 at high temperatures because it reacts with CaCl2 by releasing volatile tungsten oxychloride. In the electrochemical reduction of CaWO4, X-ray diffraction results indicated the presence of tungsten with significant concentrations of calcium compounds. Metallic tungsten was obtained after treating the reduced samples with dilute hydrochloric acid solutions.

Confocal Microscopic Studies on Evolution of Crystals During Oxidation of the FeO-CaO-SiO2-MnO Slags

Abstract: The current work investigates dynamic phenomena at the microstructural level during iron and manganese recovery from the liquid FeO-CaO-SiO2-MnO slags using an oxidation method. A hot-stage-equipped confocal scanning laser microscope (CSLM) was used to analyze the kinetic behavior of crystallization in synthetic slags. Based on observed precipitations on cooling in the 1273 K (1000 °C) to 1873 K (1600 °C) temperature range, a time–temperature–transformation (TTT) diagram has been created. The crystallization studies were conducted in air.

Simple Method for Estimating the Electrical Conductivity of Oxide Melts with Optical Basicity

Abstract: The electrical conductivity of oxide melts is an important physicochemical property for designing the electric smelting furnaces. Although the data of many slag systems have been measured, the quantitative relationships of electrical conductivity to slag composition and temperature are still limited. In this article, a model is proposed based on the optical basicity corrected for the cations required for the charge balance of $$ {\text{AlO}}_{ 4}^{ 5- } $$ , in which Arrhenius Law is used to describe the relationship between electrical conductivity and temperature. In this model, the activation energy is expressed as a linear function of the corrected optical basicity. Successful applications to CaO-MgO-Al2O3-SiO2 and CaO-Al2O3-SiO2 systems indicate that this model can work well in the electrical conductivity estimation.

Modeling Energy Dissipation in Slag-Covered Steel Baths in Steelmaking Ladles

Abstract: Physical and mathematical modeling of energy dissipation phenomena in a gas-stirred ladle with, and without, an overlying second-phase liquid have been carried out at relatively low gas flow rate and specific energy input rate. Data from the literature are applied to infer the extent of energy dissipation caused by various mechanisms. An analysis reveals that bubble slippage and friction at the vessel walls dominate energy dissipation in such systems, each contributing roughly one third of the input energy. The remainder is dissipated because of turbulence in the bulk of the liquid, the formation of a spout, and interactions between the upper phase and the bulk liquid when an overlying liquid is present. Remarkably, the overlying liquid despite its small volume (~3 pct to 13 pct of the bulk), is found to dissipate about 10 pct of input energy. To understand the way the total input energy is dissipated via the overlying liquid, flow and mixing studies were carried out with different types of upper phase liquids. Tracer dispersion studies conducted with Petroleum ether as the overlying liquid show reasonably intense flow within the upper phase with no noticeable entrainment around the spout. In contrast, a thick layer of highly viscous upper phase liquid such as mustard oil shows extensive deformation of the upper phase around the spout, but no discernable motion within. However, remarkably, the thickness of the upper phase rather than its physical properties was found to influence bath hydrodynamics and mixing most significantly. A mechanism based on the rerouting of the surfacing plume and the attendant reversal of flow in the vicinity of the spout is advocated to explain energy dissipation caused by the overlying liquid. This finding is rationalized with our experimental results on composition adjustment with sealed argon bubbling (CAS) alloy addition procedures reported more than two decades ago, wherein flow reversal caused by the baffle in the immediate vicinity of the surfacing plume was shown to cause significant energy dissipation, leading to much sluggish flow and slower mixing in the bulk of the liquid, in comparison with an equivalent unbaffled situation.

High-Temperature Volatilization Mechanism of Stibnite in Nitrogen-Oxygen Atmospheres

Abstract: The volatilization of stibnite (Sb2S3) in nitrogen and mixtures of nitrogen-oxygen was investigated in the temperature range 973 K to 1423 K (700 °C to 1150 °C). The overall volatilization reaction study was carried out using a thermogravimetric analysis technique under various gas flow rates. The results indicated that in an inert atmosphere, stibnite can be volatilized most efficiently as Sb2S3(g) with a linear rate up to about 1173 K (900 °C). At temperatures above 1223 K (950 °C), stibnite decomposes to antimony and sulfur gas, impairing the antimony volatilization. For linear behavior in nitrogen gas, kinetic constants were determined, and an activation energy of 134 kJ/mol was calculated for the volatilization reaction. However, in the presence of oxygen, antimony can be volatilized efficiently as valentinite (Sb2O3) at low oxygen concentrations (approximately 1 to 5 pct) at approximately 1173 K to 1223 K (900 °C to 950 °C); otherwise, at higher partial pressures of oxygen, the volatilization of antimony is limited by the formation of nonvolatile cervantite (SbO2). In highly oxidizing atmospheres, a high vaporization of antimony could be achieved only at temperatures higher than 1423 K (1150 °C) where cervantite becomes unstable and decomposes into SbO(g) and 0.5O2(g).

Experimental Studies on the Oxidation States of Chromium Oxides in Slag Systems

Abstract: In view of the importance of the thermodynamic behavior of chromium in the slag phase as well as the serious discrepancies in the earlier reports on the valence state of chromium in slag melts, the oxidation state of chromium oxides in CaO-SiO2-CrOx and CaO-MgO-(FeO-) Al2O3-SiO2-CrOx were investigated experimentally in the present study using two different experimental techniques. The gas–slag equilibrium technique was adopted to study the CaO-SiO2-CrOx system between 1823 K (1550 °C) and 1923 K (1650 °C) and equilibrated with mixtures of CO-CO2-Ar gases corresponding to three different oxygen partial pressures (between 10−4 and 10−5 Pa). After equilibrating, the samples were quenched and subjected to analysis using the X-ray absorption near edge structure method to determine the distribution ratio of Cr2+/Cr3+ in the slags. The second technique examined the applicability of the high-temperature mass spectrometric method combined with the Knuden effusion cell for quantifying the valence states of Cr in the multicomponent system CaO-MgO-(FeO-) Al2O3-SiO2-CrOx up to a maximum temperature of 2000 K (1727 °C). The results showed that the Knudsen cell-mass spectrometric method could be used successfully to estimate the valence ratio for Cr in silicate melts. According to the present study, the Cr2+/Cr3+ ratio increased with increasing temperature and a decreasing slag basicity as well as the oxygen partial pressure prevailing in the system. A mathematical correlation of X CrO/X CrO1.5 as a function of temperature, oxygen partial pressure, and basicity was developed in the present work based on the present results as well as on those assessed from earlier data.

Thermodynamic Modeling of Arsenic in Copper Smelting Processes

Abstract: Published data on the activity coefficients of arsenic in liquid copper, matte and, slag have been reviewed, assessed, and used in the development of thermodynamic databases for solution models of melts. The databases were validated against the literature data on the equilibrium distribution of arsenic between the matte and the slag. The models and databases were used in investigating the effects of matte grade, slag chemistry, SO2 partial pressure, arsenic loading, and temperature on the equilibrium distribution of arsenic between the melts and gas phase during copper smelting and converting. The results obtained show that the continuous smelting processes operates close to equilibrium between condensed phases with most arsenic reporting to the gas phase. A comparison of the batch and continuous converting processes showed a considerable difference with respect to the elimination of the arsenic from condensed phases. These results indicate batch processes to be more efficient in the removal of arsenic through the gas stream.

Mechanism and Kinetics of Thermal Decomposition of MgCl2 × 6H2O

Abstract: The reaction mechanism and kinetic behavior of thermal decomposition of MgCl2 × 6H2O were studied by thermal gravimetric analysis. The results showed that the thermal decomposition process of MgCl2 × 6H2O could be divided into six stages. In the first two stages, four crystalline waters were lost. The dehydration and hydrolysis coexisted during the third and fourth stages. The fifth stage corresponded to the evaporation of 0.3 crystalline waters, and one molecular hydrogen chloride was eliminated in the last stage. The kinetic analysis of the thermal decomposition process was performed using the Doyle, Coats–Redfern, and Malek methods. The results suggested that the mechanisms of six stages were two-dimensional phase boundary mechanism, three-dimensional phase boundary mechanism, nucleation and nuclei growth mechanism (Avrami–Erofeev equation n = 3), two-dimensional phase boundary mechanism, three-dimensional diffusion mechanism (cylinder and G-B equation), and nucleation and nuclei growth mechanism (Avrami–Erofeev equation n = 1), respectively. The apparent active energies of six stages were 66.8 kJ × mol−1, 138.0 kJ × mol−1, 77.2 kJ × mol−1, 135.6 kJ × mol−1, 77.4 kJ × mol−1, and 92.2 kJ × mol−1, respectively. The frequency factors were 3.6 × 109 s−1, 8.8 × 1017 s−1, 4.6 × 109 s−1, 3.0 × 1014 s−1, 78.6 s−1, and 1.2 × 103 s−1, respectively.

Physical and Mathematical Modeling of Inert Gas-Shrouded Ladle Nozzles and Their Role on Slag Behavior and Fluid Flow Patterns in a Delta-Shaped, Four-Strand Tundish

Abstract: Inert gas shrouding practices were simulated using a full-scale, four-strand water model of a 12-tone, delta-shaped tundish. Compressed air was aspirated into the ladle shroud to model volumetric flow rates that range between 2 and 10 pct of steel entry flows. Bubble trajectories, slag layer movements, and flow fields, were visualized. Flow fields were visualized using particle image velocimetry (PIV). A numerical model also was developed using discrete phase modeling (DPM) along with the standard k-e turbulence model with two-way turbulence coupling. Predicted flow fields and bubble trajectories corresponded with the water model experiments.

Thermodynamics of Arsenic in FeOx-CaO-SiO 2 Slags

Abstract: The distribution of arsenic between calcium ferrite slag and liquid silver (wt pct As in slag/ wt pct As in liquid silver) with 22 wt pct CaO and between iron silicate slag with 24 wt pct SiO2 and calcium iron silicate slags was measured at 1573 K (1300 °C) under a controlled CO-CO2-Ar atmosphere For the calcium ferrite slags, a broad range of oxygen partial pressure (10–11 to 021 atm) was covered, whereas for the silicate slags, the oxygen partial pressure was varied from 10–9 to 31 × 10–7 atm The measured relations between the distribution ratio of As and the oxygen partial pressure indicates that the oxidation state of arsenic in these slags is predominantly As3+ or AsO15 The measured distribution ratio of arsenic between the calcium ferrite slag and the liquid silver was about an order of magnitude higher than that of the iron silicate slag In addition, an increasing concentration of SiO2 in the calcium-ferrite-based melts resulted in decreases in the distribution of arsenic into the slag Through the use of measured equilibrium data on the arsenic content of the metal and slag in conjunction with the composition dependent on the activity of arsenic in the metal, the activity of AsO15 in the slags was deduced These activity data on AsO15 show a negative deviation from the ideal behavior in these slags

Alumina Solubility and Diffusion Coefficient of the Dissolved Alumina Species in Low-Temperature Fluoride Electrolytes

Abstract: The solubility of alumina was measured by rotating an alumina cylinder (~500 rpm) in a high-purity melt for ~3 to 6 hours, crushing and sampling the frozen melt, and determining the oxygen content in a Leco analyzer. The alumina solubilities determined were as follows: (1) 3.2 ± 0.3 wt pct in NaF-AlF3 eutectic at 1023 K (750 °C); (2) 3.0 ± 0.3 wt pct in NaF-AlF3-CaF2 (5 wt pct) at 1023 K (750 °C); and (3) 5.2 ± 0.5 wt pct in a KF-AlF3 eutectic at 1003 K (730 °C). The alumina solubility in the KF-AlF3 eutectic was 2 wt pct more than in the sodium analogue, offering the possibility of operating a low-temperature aluminum smelting cell without the need for an alumina slurry. The diffusion coefficient of the dissolved alumina species was determined in the NaF-AlF3 eutectic at 1023 K (750 °C) using the rotating disc method and applying the Levich equation. Through a limited range of rotation rates, the system seemed to be mass-transfer controlled, and the diffusion coefficient was estimated to be in the range 1.8 to 2.2 × 10−6 cm2 s−1. This value is about five times lower than the values encountered at traditional aluminum smelting temperatures (~1233 K (960 °C)) and would result in relatively low mass transfer coefficients.

Simulation of Fluid Flow and Oscillation of the Argon Oxygen Decarburization (AOD) Process

Abstract: The oscillation of argon oxygen decarburization (AOD) converters is flow related and depends on the process parameters (eg, vessel geometry, melt fill height, process gas type and blowing rate, vessel tilting angle, as well as geometry, number, and arrangement of the side-wall nozzles) For a 120-ton AOD converter with seven submerged side-wall nozzles, plant tests, physical simulations on a 1:4 scale water model, and computational fluid dynamics simulations have been done The investigations show that the penetration depth of an inert gas jet into the melt does not exceed approximately 04 m The plumes are located close to the nozzle-side converter wall and induce a large-scale primary vortex as well as intensive surface movements; both are responsible for the oscillation Several process mechanisms were investigated The oscillation is highest in the last stage of the dynamic blow and is still high during the reduction stage As the amount of inert gas increases, the vibration level also increases Inert gas has a greater influence on the oscillation than oxygen Tilting the converter around 8 deg clearly leads to more intensive oscillations Increasing the blowing rate increases the forces and torques acting on the vessel, whereas the oscillation frequency remains nearly constant A varying fill level does not influence the vibration level the same way as the blowing rate The operational test shows, for example, that the maximum torque does not depend on the heat size when the latter varies between –8 pct and +21 pct of the nominal heat size The water model test shows decreasing forces and torques with a rising fill level