METALLURGICAL AND MATERIALS TRANSACTIONS B
ABSTRACTS
Volume 27B, No. 2, April 1996

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

HYDROMETALLURGY

Behavior of Antimony(III) during Copper Electrowinning in Chloride Solutions
H.K. LIN and X. WU
Contamination of cathodic copper by Sb during electrowinning in chloride solutions is a surface phenomenon. A digitized scanning electron microscopy (SEM) micrograph indicates that the Sb is concentrated on the surface of the cathode. Energy-dispersive X-ray (EDX) analysis reveals that the Sb-containing layer is a complex salt of Cu, Sb, Cl, and O. Electrochemical measurements show that the adsorption of Sb or Cu species decreases with the increase of acidity of the solution when the solution contains antimony chloride or cuprous chloride. The adsorption increases with the increase of the acidity when the solution contains both Sb and Cu. The discharge of cuprous ions in the adsorbed complex salt releases antimonious ions and then forms a new layer of the complex salt with cuprous ions from the solution. This newly formed complex salt is readsorbed on the surface of the cathode. Thus, Sb concentrates on the surface of the cathode instead of being evenly distributed throughout the copper product. This suggested mechanism also explains the fact that the presence of Sb in the electrolyte enhances the electrodeposition of Cu.

PYROMETALLURGY

Communication: Zinc Reduction of MoO3 in a Self-Propagating High-Temperature Synthesis Process
S.K. KO, C.W. WON, S.S. CHO, and B.S. CHUN

Kinetics of the Flash Converting of MK (Chalcocite) Concentrate
G.J. MORGAN and J.K. BRIMACOMBE
The chemical kinetics and mechanism of the flash converting reaction of MK concentrate (approximately 93 pct chalcocite and 7 pct pentlandite) in oxygen-containing atmospheres were determined. The reaction path of individual particles undergoing flash oxidation in a laminar flow furnace was followed by observing the particle temperature and changes in apparent particle diameter with a fast response, two-wavelength pyrometer; and these observations were compared to the predictions of a mathematical model of a single reacting particle. Flash reaction products were examined under a scanning electron microscope to determine chemical composition and particle morphology. A dust generation mechanism was elucidated, whereby preferential evaporation of copper from the surface of the particle leaves it encased in a nickel-rich shell. Continued oxidation of sulfides and metallic copper leads to ongoing particle heating and the generation of gas (sulfur dioxide and copper vapor) which either vents through a rupture or causes catastrophic fragmentation of the nickel-rich shell.

ELECTROMETALLURGY

Effect of Baking Temperature and Anode Current Density on Anode Carbon Consumption
Z. KUANG, J. THONSTAD, S. ROLSETH, and M. SØRLIE
The electrolytic anode carbon consumption (CC) was studied as a function of anode baking temperature (BT) and anode current density in a closed laboratory cell, where the anode gas was absorbed and also monitored on a CO/CO2 analyzer. The CC referred to Faraday's law was determined from the weight loss of the anode, the carbon gasification (CG) from the amount of absorbed anode gas, and the formation of carbon dust (CD) by the difference. The CC decreased by about 9 pct per 100°C increase in BT, and it increased with decreasing anode current density, particularly at low current densities. The CG and CD showed similar trends. The excess CG, which was attributed to the Boudouard reaction (CO2 + C = 2CO), showed a relative decrease with increasing current density, although the overall rate of the reaction increased.

Studies on the Corrosion and the Behavior of Inert Anodes in Aluminum Electrolysis
H. XIAO, R. HOVLAND, S. ROLSETH, and J. THONSTAD
The corrosion rates of inert anodes based on tin oxide and nickel ferrite cermet materials were studied as a function of some operating parameters. To reach a better understanding of the corrosion mechanism, the behavior of the anodes was observed under some specific conditions, such as in pure cryolite, at high current densities, at different potentials, and at varying cathode surface areas. It was confirmed that low alumina concentrations led to catastrophic corrosion of the anodes and that high current densities and high as well as low NaF/AlF3 molar ratios were also detrimental. The corrosion rate of tin oxide based anodes showed a minimum (so-called ``normal corrosion'') at anodic potentials of 2.2 to 2.4 V with respect to aluminum. The normal corrosion is due to chemical dissolution of the anode material and reduction of the corrosion products into the cathode metal. The corrosion rate increased with increasing cathode surface area. At potentials higher than ~2.5 V, the anodes showed catastrophic corrosion. Catastrophic corrosion can be ascribed to decomposition of the anode material by depletion of alumina at the anode surface provoked by low bulk concentration of alumina and/or high current density.

TRANSPORT PHENONEMA

A Multiphase Fluid Mechanics Approach to Gas Holdup in Bath Smelting Processes
H. GOU, G.A. IRONS, and W.-K. LU
In slag-based, smelting-reduction processes, the overflow of slag from the vessel is considered a major limitation to productivity; this phenomenon is commonly called slag foaming. While much has been learned from laboratory-scale studies of foaming, the superficial gas velocities are well below those encountered in production (centimeters per second compared to meters per second). A multiphase fluid dynamic model was developed to determine the relationship between gas velocity and void fraction (holdup) at industrial production rates. In the model, the drag between the gas and slag is balanced against the weight of the slag. Within the framework of the model, the only unknown quantity is a drag factor which can be extracted from experimental data. Values of this factor from water models, smelting-reduction converters, and other slag systems fall in a narrow range. The model can be used to estimate slag height in smelting-reduction vessels. The behavior of slags at high rates of gas injection is markedly different from foaming observed at low flow rates.

Effects of Nickel on the Sintering Behavior of Fe-Ni Compacts Made from Composite and Elemental Powders
K.S. HWANG and M.Y. SHIAU
Injection-molded Fe-Ni parts made from composite and elemental powders were prepared, and the effect of nickel on the sintering of iron compacts was investigated. Dilatometry analyses showed that the alpha-gamma phase transformation temperature of the Fe-Ni compact changed from a fixed 912°C for pure iron to a temperature range between 700°C and 912°C where two phases coexisted. The microstructure indicated that nickel impeded surface diffusion and slowed down the neck growth rate of iron powder in the early sintering stage. The dual phase and the small neck size at low temperatures suppressed the exaggerated grain growth, which usually occurs on carbonyl iron powders at 912°C. It was also observed that nickel impeded the grain growth of iron at high temperatures. Thus, by reducing the exaggerated grain growth during phase transformation, impeding the grain growth at high temperatures, and with high diffusion rates of iron in Ni-rich areas, enhanced densification was obtained for Fe-Ni systems, particularly for those systems made from composite powders. However, when coarse nickel powder was added, expansion was observed due to the presence of large pores around nickel powders. These pores were formed because of the particle rearrangement which was caused by the Kirkendall effect.

Dispersed-Phase Holdup in Liquid-Liquid Emulsions Generated by High-Strength Bottom Gas Injection
M.S. LEE and H.Y. SOHN
A cold model study on the effects of various operating conditions on the dispersed-phase holdup in a liquid-liquid emulsion generated by bottom gas injection has been performed using water and kerosene. Dimensional analysis was used to correlate the holdup with the operating conditions and the sampling locations. The holdup increased with gas velocity and decreased with axial and radial distances. The radial distribution of holdup follows a Gaussian function. The holdup increased with increasing ratio of the heavy phase to light phase heights and decreased with the height of the heavy phase. The variation of the holdup within the plume was correlated by a single equation involving a set of dimensionless numbers.

Heat-Transfer and Pressure-Drop Considerations in the Design of Sirosmelt Lances
C.B. SOLNORDAL and N.B. GRAY
The Sirosmelt submerged combustion smelting process utilizes top injection lances to deliver fuel and air into a metallurgical melt. Helical vanes are used within the annular lance to impart swirl to the flowing air and enhance heat transfer from the lance wall to the air. To improve the understanding of transport phenomena within the lance, a detailed study of the fluid-flow and heat-transfer characteristics of decaying swirling flow in a heated annulus has been performed. Swirl strength and type were varied, the outer wall of the test section was heated uniformly, and Reynolds numbers ranged from 85,000 to 175,000. The swirl decay characteristics, heat-transfer coefficient, pressure losses, and heat transfer per unit pumping power were determined, and heat-transfer mechanisms were identified. It was found that the entrance pressure losses associated with the helical vane swirlers contributed up to 80 pct of the total pressure loss, indicating that improved swirler entrance design could significantly reduce operation costs. Recommendations are made for optimizing the shape of the swirler by using variable pitch/variable span swirlers and optimizing swirler position by aligning the entrance of the swirlers with the local flow angle. These changes will significantly improve the heat transfer per unit pumping power within a Sirosmelt lance.

PROCESS CONTROL

Characteristics of Eccentric Bubble Plumes in Liquids
MARCO A.S.C. CASTELLO-BRANCO and KLAUS SCHWERDTFEGER
The hydrodynamics of air/water plumes in a large scale model of a metallurgical ladle was investigated. The dimensions of the cylindrical vessel were 1600 mm i.d. and 2250 mm total height. The air was injected through an eccentric nozzle positioned halfway between the center and the wall of the vessel. The gas concentration, bubble frequency, and liquid and gas velocities were measured using electrical resistivity probes and a propeller flowmeter. Particular attention was paid to the liquid velocity. Its field is complex and nonstationary. Light scattering experiments on a small scale model were carried out to supplement the velocity data measured in the large vessel. From the analysis of the data, it was found that the maximum values of the gas fraction (time average), the upward liquid velocity, and the gas velocity can be described with the same dimensionless correlations derived previously.

PHYSICAL CHEMISTRY

Applicability of Butler's Equation in Interpreting the Thermodynamic Behavior of Surfaces and Adsorption in Fe-S-O Melts
J.P. HAJRA and M. DIVAKAR
Expressions for various second-order derivatives of surface tension with respect to composition at infinite dilution in terms of the interaction parameters of the surface and those of the bulk phases of dilute ternary melts have been presented. A method of deducing the parameters, which consists of repeated differentiation of Butler's equations with subsequent application of the appropriate boundary conditions, has been developed. The present investigation calculates the surface tension and adsorption functions of the Fe-S-O melts at 1873 and 1923 K using the modified form of Butler's equations and the derived values for the surface interaction parameters of the system. The calculated values are found to be in good agreement with those of the experimental data of the system. The present analysis indicates that the energetics of the surface phase are considerably different from those of the bulk phase. The present research investigates a critical compositional range beyond which the surface tension increases with temperature. The observed increase in adsorption of sulfur with consequent desorption of oxygen as a function of temperature above the critical compositional range has been ascribed to the increase of activity ratios of oxygen to sulfur in the surface relative to those in the bulk phase of the system.

Electrical Conductivity of Molten Cryolite-Based Mixtures Obtained with a Tube-Type Cell Made of Pyrolytic Boron Nitride
J. HÍVES, J. THONSTAD, Å. STERTEN, and P. FELLNER
A pyrolytic boron nitride tube-type cell was used to measure the electrical conductivity for molten cryolite, for binary mixtures of cryolite with Al2O3, AlF3, CaF2, KF, Li3AlF6, and MgF2, and for ternary mixtures Na3AlF6-Al2O3-CaF2 (MgF2) and Na3AlF6-AlF3-KF (Li3AlF6). The cell constant was about 40 cm-1. The temperature and concentration dependence of the conductivity in the investigated concentration range was described by the equation

where T represents the temperature in Kelvin and the brackets represent the mole fractions of the additions. The standard deviation was found to be 0.026 S cm-1 (~1 pct). For practical reasons, it is often desired to express composition in weight percent. In that case, it holds that

where T represents the temperature in Kelvin and the brackets denote the concentration of the additives in weight percent. However, in this case, the maximum relative error of the conductivity equation can reach up to 2.5 pct.

Activities in MnO-SiO2-Al2O3 Slags and Deoxidation Equilibria of Mn and Si
HIROKI OHTA and HIDEAKI SUITO
The activities of MnO and SiO2 along the liquidus line in the MnO-SiO2-Al2O3-FeO (1.2 to 6.7 mass pct) system were determined at 1823 and 1873 K by using a slag-metal equilibration technique. On the basis of the re-evaluated MnO iso-activity curves, the SiO2 and Al2O3 iso-activity curves were determined by using the ternary Gibbs-Duhem relation. The control of inclusions composition in Si-Mn killed steels is discussed based on the equilibria between inclusion and steel with respect to Si, Mn, Al, and O.

Phase Relations of a Silicide/Silica Reaction Couple at 2273 K
MARIA PEÑA, CARLOS RAMOS, and ARTURO BRONSON
The phase relations of a zirconium silicide/silica reaction couple have been investigated at 2273 K in air. After annealing times from 4 to 49 hours, the reaction couples, created by encapsulating zirconium disilicide in a quartz ampule, developed an interdiffusion zone and an inner core consisting of solidified zirconium silicide. The interdiffusion zone consisted of a silica layer, zirconia precipitates, and prior liquid silicide globules dispersed in prior liquid silica. Zirconia precipitates formed from the oxidation of the silicide melt between the protective silica layer and the two liquid regions of silicide and silica, as a result of oxygen diffusion. At 2273 K, the inner core consisted of liquid zirconium silicide (ZrSi)L, although the microstructure analysis indicated formation of proeutectic ZrSi2 and a eutectic microstructure of ZrSi2 and Si upon solidification. A constant oxygen potential developed within the interdiffusion zone and protected the silicide from oxidation even after 49 hours at temperature.

Communication: Interdissolution Kinetics on Oxide Powder Mixture Using High-Temperature X-Ray Diffraction Technique
A. JAKOBSSON, D. SICHEN, and S. SEETHARAMAN

Communication: Thermal Decomposition of Silicon Carbides: Discussion of "The Effect of an Electric Field on Self-Sustaining Combustion Synthesis, Parts I and II"
M. NAGAMORI, J.-A. BOIVIN, and A. CLAVEAU

Communication: Author's Reply
A. FENG and Z.A. MUNIR

SOLIDIFICATION

Transient Thermal Analysis of Solidification in a Centrifugal Casting for Composite Materials Containing Particle Segregation
C.G. KANG and P.K. ROHATGI
One-dimensional heat-transfer analysis during centrifugal casting of aluminum alloy and copper base metal matrix composites containing Al2O3, SiCp, and graphite particles has been studied. The model of the particle segregation is calculated by varying the volume fraction during centrifugal casting, and a finite difference technique has been adopted. The results indicate that the thickness of the region in which dispersed particles are segregated due to the centrifugal force is strongly influenced by the speed of rotation of the mold, the solidification time, and the density difference between the base alloy and the reinforcement. In the case where the base alloy density is larger than that of the particles, the thickness of the particle-rich region near the inner periphery decreases with an increase in speed, thereby increasing the volume fraction of dispersion. The solidification time of the casting is also dependent upon the speed of rotation of the mold, and it decreases with an increase in speed. This study also indicates that the presence of particles increases the solidification time of the casting.

Effect of Superheat on the Solidification Structures of AISI 310S Austenitic Stainless Steel
S. OZBAYRAKTAR and A. KOURSARIS
An experimental study was carried out to investigate the evolution of macrostructure and microstructure in AISI 310S stainless steel during solidification. Experimental findings suggested that the macrostructure and the microstructure of the cast material responded differently to variations in casting temperature. As the casting temperature decreased, the macrostructure was refined, as expected, but the microstructure coarsened. A relationship was established between the proportion of equiaxed zone and superheat as follows:

pct equiaxed zone = a+b ln (1/T)

where a and b are constants. The relationship between grain width and superheat could be expressed by the equation

gw = e (c+d/T)

where c and d are constants determined by the distance from the edge of the ingot. The relationship between primary arm spacing and superheat could be expressed by the equation

1 = p+q ln (1/T)

where p and q are constants determined by the distance from the edge of the ingot. The parameter ``grain width ratio'' has been introduced to describe the relationship between the shape and the nucleation and growth kinetics of the columnar grains.

MATHEMATICAL MODELING

An Extended Two-Dimensional Mathematical Model of Vertical Ring Furnaces
S. PETER, A. CHARETTE, R.T. BUI, A. TOMSETT, and V. POTOCNIK
An extended two-dimensional (2-D) mathematical model of vertical anode baking furnaces has been developed. The work was motivated by the fact that a previous 2-D model was unable to predict the nonuniform baking in the transverse direction, i.e., perpendicular to the longitudinal axis of the furnace. The modeling strategy based on dividing each section in four zones (underlid, pit, underpit, head wall and fire shaft zones) and introducing two symmetry planes in the exterior pits is explained. The basic heat-transfer relations used are also detailed. Selected results shown include draught and oxygen concentration profiles in the flue, gas and anode temperature distributions and fuel consumption in the back fire ramp. Simulation and experimental results are compared.

Dynamic Behavior of a Liquid/Liquid Interface at an Oscillating Wall
HONG SHA, RALF DIEDRICHS, and KLAUS SCHWERDTFEGER
The dynamic behavior of the meniscus between two liquids has been investigated experimentally using mercury and silicon oil and theoretically by solution of the equations of motion applying the marker and cell (MAC) method. The liquids were contained in a plexiglass tube which could be moved relative to the liquids with constant and superposed sinusoidal velocities. Special attention was focused on the movement of the three-phase contact line (TPL) at the wall. Depending on the parameters of the motion, the TPL may be drawn above the flat part of the oil/mercury meniscus so that the normally convex contour of the meniscus disappears and a concave contour is formed. Good agreement was found between the measured and computed shapes of the meniscus.

Communication: Discussion of "Derivation and Consistency of the Partial Functions of the Ternary System Involving Interaction Coefficients"
W.W. MEYER

Communication: Author's Reply
J.P. HAJRA

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