METALLURGICAL AND MATERIALS TRANSACTIONS B
ABSTRACTS
Volume 27B, No. 5, October 1996

This Month Featuring: Pyrometallurgy; Electrometallurgy; Transport Phenonema; Process Control; Physical Chemistry; Solidification; Mathematical Modeling. View October 1996 Table of Contents.

PYROMETALLURGY

Reduction of FeO in Smelting Slags by Solid Carbon: Experimental Results
B. SARMA, A.W. CRAMB, and R.J. FRUEHAN
The reduction of CaO-SiO2-Al2O3-FeO slags containing less than 10 wt pct FeO by solid carbonaceous materials such as graphite, coke, and coal char was investigated at reaction temperatures of 1400°C to 1450°C. The carbon monoxide evolution rate from the system was measured using stationary and rotating carbon rods, stationary horizontal carbon surfaces, and pinned stationary spheres as the reductants. The measured reaction rate ranged from 3.25 x 10-7 mol cm-2 s-1 at 2.1 pct FeO under static conditions to 3.6x10-6 mol Cm2-2 s-1 at 9.5 pct FeO for a rotating rod experiment. Visualization of the experiment using X-ray fluoroscopy showed that gas evolution from the reduction reaction caused the slag to foam during the experiment and that a gas film formed between the carbon surface and the slag at all times during experimentation. The reaction rate increased with increased slag FeO contents under all experimental conditions; however, this variation was not linear with FeO content. The reaction rate also increased with the rotation speed of the carbon rod at a given FeO content. A small increase in the reaction rate, at a given FeO content, was found when horizontal coke surfaces and coke spheres were used as the reductant as compared to graphite and coal char. The results of these experiments do not fit the traditional mass transfer correlations due to the evolution of gas during the experiment. The experimental results are consistent, however, with the hypothesis that liquid phase mass transfer of iron oxide is a major factor in the rate of reduction of iron oxide from slags by carbonaceous materials. In a second article, the individual rates of the possible limiting steps will be compared and a mixed control model will be used to explain the measured reaction rates.

Preoxidation and Hydrogen Reduction of Ilmenite in a Fluidized Bed Reactor
P.L. VIJAY, RAMANI VENUGOPALAN, and D. SATHIYAMOORTHY
Studies on preoxidation and hydrogen reduction of Quilon-grade ilmenite have been carried out in a fluidized bed reactor. During preoxidation, the effect of various parameters such as temperature,gas flow rate, and period of reaction has been studied. Hydrogen reduction studies have been carried out both for raw and preoxidized ilmenite. Results obtained on the conversion rate of iron oxide to metallic iron for the preoxidation as well as the reduction period showed three distinct stages: (1) initial slow induction stage; (2) intermediate acceleratory stage; and (3) final slowing down process. Kinetic data plot on -ln (1-x) vs time for metallization of iron oxide showed a linear trend for preoxidation and hydrogen reduction.

ELECTROMETALLURGY

Liquidus Temperatures for Primary Crystallization of Cryolite in Molten Salt Systems of Interest for Aluminum Electrolysis
ASBJORN SOLHEIM, SVERRE ROLSETH, EGIL SKYBAKMOEN, LISBET STOEN, ASMUND STERTEN, and TROND STORE
Temperatures for primary crystallization of Na3AlF6 in multicomponent electrolyte systems of interest for the aluminum electrolysis process were determined by thermal analysis. The results are presented as binary and quasibinary diagrams and discussed in view of the literature data. An empirical equation describing liquidus temperatures for primary crystallization of Na3AlF6 was derived:

where t is the temperature in degree Celsius and the square brackets denote the weight percent of components in the system Na3AlF6-AlF3-CaF2-AlO3-LiF-MgF2-KF. The composition limitations are , and [Al2O3] up to saturation.

TRANSPORT PHENOMENA

Mathematical Modeling of Tundish Operation and Flow Control to Reduce Transition Slabs
HUIQING SARAH CHEN and ROBERT D. PEHLKE
Minimization of transition slabs in continuous casting operations is critical for optimum quality and productivity. The number of these slabs is closely related to the tundish geometry and operating practice. Therefore, tundish flow and mixing with various tundish configurations and different tundish operating levels are investigated through computer simulation. Residence time distributions and transitional concentration profiles are predicted. Finally, practical operations for a grade change to reduce transition slabs are assessed.

A Water Model Study of the Flow Asymmetry Inside a Continuous Slab Casting Mold
D. GUPTA and A.K. LAHIRI
The work studies the extent of asymmetric flow in water models of continuous casting molds of two different configurations. In the molds where fluid is discharged through multiple holes at the bottom, the flow pattern in the lower portion depends on the size of the lower two recirculating domains. If they reach the mold bottom, the flow pattern in the lower portion is symmetrical about the central plane; otherwise, it is asymmetrical. On the other hand, in the molds where the fluid is discharged through the entire mold cross section, the flow pattern is always asymmetrical if the aspect ratio is 1: 6.25 or more. The fluid jet swirls while emerging through the nozzle. The interaction of the swirling jets with the wide sidewalls of the mold gives rise to asymmetrical flow inside the mold. In the molds with lower aspect ratios, where the jets do not touch the wide side walls, the flow pattern is symmetrical about the central plane.

Model Study of Bubble and Liquid-Flow Characteristics in a Bottom Blown Bath under Reduced Pressure
MANABU IGUCHI, HIROSHI UEDA, TOMOYUKI CHIHARA, and ZEN-ICHIRO MORITA
Gas injection techniques are widely used in metals refining processes. Pressure on the bath surface of reactors is sometimes highly reduced to enhance the efficiency of refining. Many fundamental and practical investigations have been made to clarify the effects of reduced surface pressure on the mixing time and reaction rates of decarburization or desulfurization in the bath. However, details of these effects are not fully understood yet. Since the mixing time and chemical reaction rates are closely associated with fluid flow phenomena in the bath, information on, for example, the total surface area of bubbles rising in the bath and liquid flow induced by the buoyancy force of the bubbles should be accumulated as much as possible. In this study, the so-called water-model experiments were carried out to reveal the effects of reduced surface pressure on the bubble and liquid-flow characteristics using a two-needle electroresistivity probe and a two-dimensional laser Doppler velocimeter. At an axial position near the nozzle, each bubble expanded to a volume corresponding to the hydrostatic pressure. The bubble and liquid-flow characteristics in the axial region located farther than this axial position were found to be approximately the same as those obtained under an atmospheric surface pressure.

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

The Production of Nickel-Zinc Alloys by Powder Injection
D.E. LANGBERG and M. NILMANI
Alloys of nickel and zinc are used for the hot dip galvanizing of reactive steels containing around 0.1 pct Si. The production of these alloys is hindered by the low solubility of nickel in molten zinc at normal zinc alloying temperatures (450°C to 550°C). The kinetics of the dissolution of nickel in molten zinc were investigated in the temperature range 450°C to 550°C to show that the dissolution rate is controlled by mass transfer in the liquid boundary layer. The experiments involved dipping nickel plates into a crucible of zinc, which could be rotated to provide controlled convection around the stationary nickel plates. The dissolution rate of nickel plates immersed vertically in a static zinc melt was measured to determine the diffusion coefficient of nickel in zinc as a function of temperature and the activation energy of diffusion. A mathematical model was formulated to predict the dissolution time of nickel particles suspended in molten zinc. The dissolution times observed during plant-scale alloying tests to produce 0.15 wt pct Ni alloys using powder injection to introduce the nickel into the zinc melt agreed with the predictions of the mathematical model. The plant tests demonstrated that alloying times of less than 10 minutes can be achieved at normal alloying temperatures if the nickel is added as a powder.

The Transported Entropy of Na+ in Solid State Cryolite
V.S. SHARIVKER and S. KJELSTRUP RATKJE
The transported entropy of Na+ in mixtures of NaF (s) and Na3AlF6 (s) is determined from thermocell experiments. The experiments were favorably described by the electric work method. The variation observed in the thermocell electromotive force (emf) with composition can be explained from the probable path of charge transfer in the electrolyte. The transported entropies are for cryolite and for sodium fluoride between 380°C and 500°C. The value obtained for sodium in the solid cryolite makes us predict that the transported entropy for Na in the molten electrolyte mixture for aluminum production is substantial and that the reversible heat effects in the aluminum electrolysis cell are the same.

PROCESS CONTROL

Reference Electrode of Simple Galvanic Cells for Developing Sodium Sensors for Use in Molten Aluminum
L. ZHANG, D.J. FRAY, J.C. DEKEYSER, and F. DE SCHUTTER
A simple galvanic cell, Na (ref) Na (in molten Al), was used in developing electrochemical sensors determining the sodium content in molten aluminium. A review of possible sodium reference electrodes has shown that (Al2O3+air) and (Fe2O3+'Na10Fe16O29+'air) are particularly good candidates. The reference electrode using ferrite was believed to be more rapid in response and was used in sensors tested in 50 kg molten aluminium. At 998 K and with sodium content of 5 to 158 ppm, the performance of three sensors is reported.

PHYSICAL CHEMISTRY

Preparation of Glass-Forming Materials from Granulated Blast Furnace Slag
M. ALONSO, E. SAINZ, and F.A. LOPEZ
Glass precursor materials, to be used for the vitrification of hazardous wastes, have been prepared from blast furnace slag powder through a sol-gel route. The slag is initially reacted with a mixture of alcohol (ethanol or methanol) and mineral acid (HNO3 or H2SO4) to give a sol principally consisting of Si, Ca, Al, and Mg alkoxides. Gelation is carried out with variable amounts of either ammonia or water. The gelation rate can be made as fast as desired by adding excess hydrolizing agent or else by distilling the excess alcohol out of the alkoxide solution. The resulting gel is first dried at low temperature and ground. The powder thus obtained is then heat treated at several temperatures. The intermediate and final materials are characterized by thermal analysis, infrared (IR) spectroscopy, X-ray diffraction, scanning electron microscopy (SEM), and chemical analysis. From the results, the operating conditions yielding a variety of glass precursors differing in their composition are established. The method, in comparison with direct vitrification of slag, presents a number of advantages: (1) the glass precursor obtained devitrifies at higher temperatures; (2) it enables the adjustment, to a certain extent, of the chemical composition of the glass precursor; and (3) it permits recovering marketable materials at different stages of the process.

Critical Evaluation and Optimization of the Thermodynamic Properties of Liquid Tin Solutions
MARIE-CLAUDE HEUZEY and ARTHUR D. PELTON
Thermodynamic and phase equilibrium data for the following 18 elements in molten Sn were collected and critically evaluated: Al, Ca, Ce, Co, Cr, Cu, Fe, H, Mg, Mo, Na, Ni, O, P, S, Se, Si, and Ti. Binary and ternary data were optimized to give polynomial expressions for the excess Gibbs energies as functions of temperature and composition. For some solutes, the optimized expressions are valid over the entire composition range . In other cases, the expressions apply to Sn-rich solutions. Solute-solute interaction terms were estimated where data were not available. The optimized Gibbs energy expressions are also presented in the form of interaction parameters, and the equivalence between the polynomial and interaction parameter formalisms is discussed. Through the Kohler equation, or the modified interaction parameter formalism, the thermodynamic properties of the multicomponent solution of 18 elements in Sn can be calculated. The database is suitable for computer storage and manipulation.

Phase Equilibria in the Metal-Sulfur-Oxygen System and Selective Reduction of Metal Oxides and Sulfides: Part I. The Carbothermic Reduction and Calcination of Complex Mineral Sulfides
ANIMESH JHA, SANCHUAN TANG, and ANDREAS CHRYSANTHOU
The difference in the standard Gibbs free energy for the formation of any two oxides or sulfides is the chemical potential for selective reduction of metals from complex minerals. The magnitude of the Gibbs free energy difference is shown by plotting the univariant relationships for relevant sulfides and oxides. In this investigation, three examples of mineral sulfides are considered, and the experimental results are compared with the predicted thermodynamic calculations. These examples include the reduction conditions for nickel and iron sulfides and pentlandite (Fe,Ni)9S8 and chalcopyrite (CuFeS2) minerals. The reduction behavior of mineral sulfides, such as those of nickel, cobalt, iron, and copper, is illustrated by referring to both the sulfide and alloy phase equilibria. In particular, the solution thermodynamic properties of the metallic phase equilibria are featured for determining the physical chemistry of preferential or selective reduction of the metal oxides and sulfides. The mechanism for the reduction of the aforementioned sulfide minerals is explained with the aid of the governing phase equilibria for the calcination process. The results from the carbothermic reduction of sulfide minerals are also compared. The important roles of lime and calcium sulfate in controlling the emission of sulfurous gases during the reduction reaction are explained. A qualitative analysis of reduction reactions of nickel and iron sulfides is reviewed to provide a comparison of the mechanism for complex nickel-bearing minerals. The importance of these results in producing alloy and pure metallic phases is also examined.

Thermodynamic Properties of Oxygen in Yttrium-Oxygen Solid Solutions
T.H. OKABE, T.N. DEURA, T. OISHI, K. ONO, and D.R. SADOWAY
The oxygen potential in yttrium-oxygen (Y-O) solid solutions was measured by equilibration with titanium-oxygen (Ti-O) solid solutions. Yttrium and titanium samples were immersed in calcium-saturated CaCl2 melts at temperatures between 1108 and 1438 K, and oxygen levels in the two metals were measured. With the Ti-O system acting as a reference, oxygen potentials in Y-O solid solutions were determined. By this technique, it was possible to make reliable measurements of extremely low oxygen potentials (as low as 10-44 atm at 1273 K), far beyond the range of solid oxide electrolyte sensors.

Effects of O, Se, and Te on the Rate of Nitrogen Dissolution in Molten Iron
HIDEKI ONO, HIROKAZU FUKAGAWA, KAZUKI MORITA, and NOBUO SANO
The effects of oxygen, selenium, and tellurium on the rate of nitrogen dissolution into molten iron have been investigated at 1973 K using an isotope-exchange reaction and the results are summarized as follows. The rate constant of nitrogen dissolution measured at lower oxygen concentration ([mass pct O] < 0.015) is larger than previously reported ones under an atmospheric pressure and agrees well with the value from desorption rate under reduced pressures. Selenium and tellurium retard the nitrogen dissolution into liquid iron more significantly than oxygen, and the degree of the retarding effect is in the order of tellurium, selenium, and oxygen. The adsorption coefficients are calculated to be KO=144, KSe=1120, and KTe=1640 with respect to mass pct solute from present results. A model that surface active elements and nitrogen are adsorbed on the same site at the interface and the dissociation reaction of nitrogen molecule on the site represented by the equation is the rate-determining step reasonably expresses the retarding effect of the surface active elements on the reaction rate on the assumption that all sites at the metal surface have a uniform adsorption energy for each solute.

Modeling and Experimental Study of Gaseous Oxidation of Liquid Iron Alloys
HAIPING SUN and ROBERT D. PEHLKE
On the basis of the experimental results and thermodynamic and kinetic theories, a system reaction model was developed for liquid iron reacting with O2/CO2/CO/N2 gases. For verification of the model, laboratory-scale experiments using a levitation melting technique were carried out on the kinetics of simultaneous oxidation of carbon, silicon, and manganese in a liquid metal droplet by oxygen and/or carbon dioxide in nitrogen gas. Both reaction model predictions and experiments show that for medium or high-carbon (1.64 and 3.38 pct carbon, respectively) liquid iron, oxidation of silicon and manganese occurs at 1873 K only after cessation of the vigorous decarburization reaction, but that they proceed from the beginning in a low-carbon (0.4 pct carbon) run. The oxidation of silicon was accompanied by oxidation of manganese because of the reduction of the activity of manganese oxide once an oxide formed on the surface of the metal droplet. In a low-temperature run (< 1633 K), the metal surface was covered by a solid or very viscous oxide layer and the reaction proceeds far more slowly. The reaction model does not interpret reaction behavior at lower temperature because diffusion in the oxide layer may be the rate-controlling step, and this mechanism has not been included in the model.

SOLIDIFICATION

High-Speed Imaging and Analysis of the Solidification of Undercooled Nickel Melts
JOHN W. LUM, DOUGLAS M. MATSON, and MERTON C. FLEMINGS
Rapid solidification of undercooled pure nickel has been imaged at sufficiently high spatial resolution (64 X 64 pixels) and temporal resolution (40,500 frames/s) to observe interface shape and motion at solidification velocities exceeding 45 m/s. Imaging was of 8 g, quartz-enclosed melts at undercoolings of 70 to 300 K. Dendrite velocities within the melt were calculated from the surface velocities observed employing a simple geometric model of growth. Solidification was found to proceed invariably from a single nucleation point; growth velocity then followed an approximate power-law relationship with respect to undercooling up to some critical value , where 150K < < 180K. At higher undercoolings, velocity increased less rapidly than predicted by the power-law relationship and the interface morphology changed in appearance from angular to macroscopically smooth.

MATHEMATICAL MODELING

Generalized Enthalpy Method for Multicomponent Phase Change
A. MACKENBROCK and K.-H. TACKE
Phase change problems with coupled transport of heat and species occur in solidification and other fields of materials processing. This article describes a new formulation for numerical modeling of coupled phase transformation. The technique is an extension of the conventional heat diffusion enthalpy method and uses the control volume approach. Numerical time-steps are separated into a transport section which updates extensive molar quantities from flux expressions and balance equations, and a thermodynamic section which computes intensive quantities from state functions through local entropy maximization. Heat and species transport are handled in a symmetrical manner with flux expressions from irreversible thermodynamics. The scheme is general enough to cover an unrestricted number of species and phases in various systems and is appropriate for three-dimensional modeling. Applications of the algorithm are illustrated by computations on isomorphous, eutectic, and peritectic binary systems; a simplified dendritic growth problem with coupled heat and species transport is also presented.
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