METALLURGICAL AND MATERIALS TRANSACTIONS B
ABSTRACTS
Volume 28B, No. 4, August 1997

This Month Featuring: The 1997 Extraction and Processing Distinguished Lecture; Hydrometallurgy; Pyrometallurgy; Transport Phenomena; Physical Chemistry; Solidification; Solid State Reactions; Welding & Joining; Mathematical Modeling. View August 1997 Contents.

THE 1997 EXTRACTION AND PROCESSING DISTINGUISHED LECTURE

Metal Extraction from Ores by Heap Leaching
ROBERT W. BARTLETT
Heap leaching low-grade ores has become a major contributor to the extraction of economically important metals, notably copper and gold. The state-of-the-art in heap leaching is reviewed with emphasis on process engineering. Rock leaching, including rock pore diffusion and mineral kinetics, solution flow, and retention in ore heaps during percolation leaching, and bio-oxidation of sulfidic ores are covered. Oxygen transport into heaps by gaseous diffusion, natural convection, and forced air ventilation is discussed. Strategies for optimizing heap leaching include ore crushing, ore agglomeration, low-cost air ventilation of sulfide ore heaps undergoing bio-oxidation using fans, and the use of aggregate metal extraction rate constants in making metallurgical business decisions about heap leaching.

HYDROMETALLURGY

Communication: Influence of Untrasound in Ammoniacal Leaching of a Copper Oxide Ore
K. SARVESWARA RAO, K.L. NARAYANA, K.M. SWAMY, and J.S. MURTY

PYROMETALLURGY

Thermodynamic Properties of Solid Pt-Mn, Pt-Cr, and Pt-Mn- Cr Alloys at 1500°C
A.M. GARBERS-CRAIG and R.J. DIPPENAAR
Equations for the activity coefficients of manganese and chromium in platinum at 1500°C were determined from activity-composition relations in the Pt-Mn, Pt-Cr, and Pt-Mn-Cr systems. This was done by equilibrating platinum wire with, respectively, MnO, Cr2O3, and an equimolar mixture of Cr2O3 and (Mn, Cr)3O4, under oxygen partial pressures that ranged from 10-6.5 to 10-10.0 atm. It was found that equations for the activity coefficients can be represented by Darken's quadratic formalism. The experimental data confirmed that the Pt-Mn, Pt-Cr, and Pt-Mn-Cr alloys studied show strong negative deviations from ideality, as is expected from the respective phase diagrams.

Evaluation of Unified Interaction Parameter Model Parameters for Calculating Activities of Ferromanganese Alloys: Mn-Fe-C, Mn-Fe-Si, Mn-C-Si, and Mn-Fe-C-Si Systems
HONGJIE LI and ARTHUR MORRIS
Interaction parameters for Mn-based alloys were evaluated using both carbon solubility and activity data for species in binary and ternary manganese alloys. The parameters at 1400°C are the following

CC = -4.5; CCC = 58.8; FeC = 1.5; FeCC = 2.5; ln 0C = -1.15

SiSi = 15.5; SiSiSi = 2.69; FeSi = 2.5; CSi = 12.4; CCSi = 27.5; ln 0Si = -7.46

The unified interaction parameter model (UIPM) was used to calculate the activity coefficients of species and the solubility of carbon in ferromanganese alloys (up to quaternary Mn-Fe-C-Si). The results were in good agreement with experimental data. In particular, this model provides an approach for controlling the silicon content of standard ferromanganese.

An Assessment of the Fe-O-Si System
MALIN SELLEBY
The thermodynamic properties of the Fe-O-Si system have been studied with particular emphasis on the slag system FeO-Fe2O3-SiO2. The liquid phase is described using an ionic two-sublattice model, while fayalite is treated as a stoichiometric phase. The available experimental data were critically examined and a self-consistent set of data was chosen to be used in the assessment, which was carried out taking into account higher order information from the Ca-Fe-O-Si system.

An Assessment of the Ca-Fe-O-Si System
MALIN SELLEBY
The thermodynamic properties of the Ca-Fe-O-Si system have been studied with particular emphasis on the slag system CaO-FeO-Fe2O3-SiO2. One of the subsystems, Fe-O-Si, was evaluated simultaneously to be able to take into account the higher order information from the present system. The assessment was succeeded by a study of different modifications of the ionic two-sublattice model that was used to describe the liquid phase. This model allows description of the slag and the liquid metal with one set of parameters. A model has been chosen which is optimal with regard to the fit of experimental data, compatibility with related systems, and number of parameters needed. The assessment is based on recent evaluations of the subsystems. All the solid-solution phases have been described using the compound-energy model.

TRANSPORT PHENOMENA

Nonwetting Flow of a Liquid through a Packed Bed with Gas Cross-Flow
G.S. GUPTA, J.D. LITSTER, E.T. WHITE, and V.R. RUDOLPH
In this study, the effect of a cross-flow gas field on the percolating flow of a non-wetting liquid through a packed bed was investigated. Experiments were conducted to measure the liquid shift, due to the cross-flow of air, for the flow of aqueous barium chloride solutions and mercury percolating through beds of polyethylene and expanded polystyrene particles. An X-ray technique was used to visualize the liquid flow pattern through the packed bed. The liquid percolates through a packed bed as a series of rivulets and droplets which are continuously breaking up and coalescing. A mathematical model to predict the direction of the liquid rivulet/droplet flow under the influence of a gas flow field was developed. The model treats the liquid as a discrete phase and includes the effects of gravity, gas drag, and inertial and viscous bed resistance. The effective droplet/rivulet size is an important model parameter, and the model postulates that the droplet/rivulet size is a function of both the effective capillary size of the bed and the liquid flow rate. A simplified population balance analysis for droplet coalescence is used to predict the effect of liquid flow rate on droplet/rivulet size. The model predictions are consistent with the experiments.

Establishment Time of Liquid Flow in a Bath Agitated by Bottom Gas Injection
MANABU IGUCHI, TSUNEO KONDOH, and KEIJI NAKAJIMA
The establishment time of gas-liquid two-phase flows in a cylindrical bath agitated by bottom gas injection through a central single-hole bottom nozzle was investigated. Because the turbulence intensity in the bath was comparable to or larger than the unity, the conventional definition of the flow establishment time based on the history of mean velocity was not suitable for the present case. In fact, it was difficult to determine the flow establishment time based on the well-known 90 or 99 pct criterion for the mean velocity. Accordingly, two methods of determining the flow establishment time by focusing on the turbulence components instead of the mean velocity components were proposed. Velocity measurements were made with a two-channel laser Doppler velocimeter. The flow establishment time was correlated as a function of gas flow rate. Close agreement was obtained by the two methods.

PHYSICAL CHEMISTRY

Kinetic Studies of the Reduction of FeO and FeWO4 by Hydrogen
J.A. BUSTNES
The kinetics of the reduction of FeO and FeWO4 have been investigated by thermogravimetric method in the temperature ranges of 1023 to 1173 K and 823 to 1173 K, respectively. The experiments were conducted under isothermal conditions in flowing hydrogen gas. The activation energies for the reduction of the pure iron oxide and iron tungstate were found to be 42.7 kJ/mol and 85.1 kJ/mol, respectively. The results obtained show that the reduction of iron tungstate proceeds in one single step from FeWO4 to Fe3W2 and W. The experimental data are compared with the information available in the literature, and the correlations between the results obtained in pure oxide systems and complex systems are discussed.

Activity-Composition Relations of MnO in MnO-CrOx-CaO-SiO2 Containing Melts
A.M. GARBERS-CRAIG and R.J. DIPPENAAR
The MnO activities in (MnO-CrOx-CaO-SiO2)-containing melts, which were saturated with the (Mn, Cr)3O4 spinel phase, were determined at 1500°C under an oxygen partial pressure of 10-8.99 atm. This was done by equilibrating the samples with platinum. The activity of MnO in the melt was then calculated from the activity coefficient of manganese in the resultant Pt-Cr-Mn alloy. Darken's quadratic formalism for ternary metallic solutions was used to calculate the activity coefficient of manganese in the Pt-Cr-Mn system, in which platinum was considered to be the solvent. It was found that an increase in the concentration of MnO in the melt increases both the MnO activity and the activity coefficient of MnO. For a constant MnO concentration in the (MnO-CrOx-CaO-SiO2)- containing melts, the activity of MnO can be increased by increasing the basicity of the melt. In order to obtain high-manganese recoveries from (MnO-CrOx-CaO-SiO2)-containing melts into an alloy phase, basic slags in which the activity coefficient of MnO is high should therefore be used.

Evaluation of Interaction Coefficient in Al-Cu-H Alloy
HUA LIU and MICHEL BOUCHARD
The theory for interaction coefficient of alloying additions on hydrogen activity in Al alloys has been generally discussed. In particular, the interaction coefficient of Cu addition on the hydrogen activity for the Al-Cu-H system has been evaluated based on hydrogen solubility values in pure aluminum and in Al-Cu alloy measured at different Cu concentrations by means of Sieverts' apparatus. The Cu addition was found to reduce the hydrogen solubility, and its effect on hydrogen solubility was evaluated by the interaction coefficient CuH, which is a function of Cu-addition concentration and melt temperature. The evaluated interaction coefficient was converted into an alloy correction factor CF(A) and a temperature correction factor CF(T), which are employed in industrial hydrogen measurement techniques such as TELEGAS and ALSCAN.

Kinetic Studies on the Dissolution of Nitrogen in CaO-Al2O3, CaO-SiO2, and CaO-CaF2 Melts
HIDEKI ONO, KAZUKI MORITA, and NOBUO SANO
The rate of nitrogen dissolution in CaO-Al2O3, CaO-SiO2, and CaO-CaF2 melts was measured by 14N-15N isotope exchange reaction. The rate constant of nitrogen dissolution in CaO-based oxide melts, which is defined as first order with respect to nitrogen partial pressure, was found to be much smaller than that in molten iron-based alloys investigated in our previous work. The activation energies for nitrogen dissolution in 40 mass pct CaO-60 Al2O3 and 50 CaO-50 SiO2 melts are 224 and 581 kJ/mol, respectively. For CaO-Al2O3 and CaO-SiO2, dependence of the rate constant on composition is very similar to that of nitride capacities. Moreover, it was confirmed that the rate constant was not affected by oxygen or nitrogen partial pressure.

Influence of Chromium and Nickel on the Dissociation of CO2 on Carbon-Saturated Liquid Iron
C.P. PETIT and R.J. FRUEHAN
At 1600°C, under conditions where the rate was not significantly affected by liquid-phase or gas- phase mass transfer, the rate of dissociation of CO2 was determined from the rate of decarburization of iron-based carbon-saturated melts containing varying amounts of chromium and nickel. The rate was determined by monitoring the change in reacted gas composition with an in-line spectrometer. The results indicate that neither chromium nor nickel had a strong effect on the kinetics of dissociation of CO2 on the surface of the melt. Sulfur was found to significantly decrease the rate, as is the case for alloys without chromium or nickel, and the rate constant is given by

where k0 denotes the chemical rate on pure iron, KS is the adsorption coefficient of sulfur, s is the activity of sulfur corrected for Cr, and kr represents the residual rate at a high sulfur level. The rate constants and adsorption coefficient were determined to be:

k0 = 1.8 x 10-3 mol/cm2 s atm
kr = 6.1 x 10-5 mol/cm2 s atm
KS = 330 ± 20

Experiments run at lower carbon contents showed that only a very small quantity of chromium was oxidized, immediately forming a protective layer. However, this oxidation occurred at a higher carbon content (2 pct) than what was expected from the thermodynamics.

The Liquidus Surface and Tie-Lines in the Iron-Cobalt-Sulfur System between 1473 and 1623 K
M. SOLTANIEH, J.M. TOGURI, and R. SRIDHAR
The liquidus surface and tie-lines in the iron-cobalt-sulfur ternary system have been determined between 1473 and 1623 K. The experiments were conducted by equilibrating the liquid sulfide phase with the metallic alloy phase. The liquid sulfide phase was sampled and chemically analyzed. The alloy phase was analyzed by electron microprobe. Combining the present results with the available literature data, the thermodynamic properties of this system were calculated.

Communication: Discussion of "Use of Solid Electrolyte Galvanic Cells to Determine the Activity of CaO in the CaO-ZrO2 System and Standard Gibbs Free Energies of Formation of CaZrO3 from CaO and ZrO2"
K.T. JACOB

Communication: Discussion on Thermodynamic Properties of Ternary Molten Salts from Wilson Equation
DONG-PING TAO and XIAN-WAN YANG

SOLIDIFICATION

Prediction of Dendrite Arm Spacings in Unsteady- and Steady-State Heat Flow of Unidirectionally Solidified Binary Alloys
DOMINIQUE BOUCHARD and JOHN S. KIRKALDY
Various theoretical dendrite and cell spacing formulas have been tested against experimental data obtained in unsteady- and steady-state heat flow conditions. An iterative assessment strategy satisfactorily overcomes the circumstances that certain constitutive parameters are inadequately established and/or highly variable and that many of the data sets, in terms of gradients, velocities, and/or cooling rates, are unreliable. The accessed unsteady- and steady-state observations on near-terminal binary alloys for primary and secondary spacings were first examined within conventional power law representations, the deduced exponents and confidence limits for each alloy being tabularly recorded. Through this analysis, it became clear that to achieve predictive generality the many constitutive parameters must be included in a rational way, this being achievable only through extant or new theoretical formulations. However, in the case of primary spacings, all formulas, including our own, failed within the unsteady heat flow algorithm while performing adequately within their steady-state context. An earlier untested, heuristically derived steady-state formula after modification,

ultimately proved its utility in the unsteady regime, and so it is recommended for purposes of predictions for general terminal alloys. For secondary spacings, a Mullins and Sekerka type formula proved from the start to be adequate in both unsteady- and steady-state heat flows, and so it recommends itself in calibrated form,

for future predictions.

The Effect of Macroscopic Solute Diffusion in the Liquid upon Surface Macrosegregation
HÅVARD J. THEVIK and ASBJØRN MO
An existing one-dimensional mathematical model that predicts the macrosegregation formation due to solidification shrinkage has been modified to also account for the macroscopic diffusion of solute in the liquid. It is shown both numerically and analytically that such solute diffusion has an almost negligible influence on the predicted solute profile, except in a very thin layer near the chill surface, where severe solute depletion develops. This layer is related to a discontinuity in the diffusive solute flux at the surface, and for a moderately cooled Al-4.5 pct Cu alloy, the layer thickness is of the order 100 µm. When the lever rule is imposed, the solute concentration at the surface becomes equal to the partition coefficient multiplied by the nominal alloy concentration, and the boundary layer solidifies completely once the temperature drops below the liquidus temperature of the (initial) melt. This indicates that assuming local thermodynamical equilibrium at a domain boundary when simultaneously accounting for the macroscale solute diffusion should be reconsidered in macrosegregation modeling.

SOLID STATE REACTIONS

Kinetics of Zinc Ferrite Formation in the Rate Deceleration Period
DAN K. XIA and CHRISTOPHER A. PICKLES
The initial surface chemical reaction involved in zinc ferrite formation is very rapid at any given temperature, and the reactants quickly become covered with zinc ferrite. Then, the reaction rate becomes decelerated. In this work, the kinetics of zinc ferrite formation during this rate deceleration period was studied in the temperature range of 873 to 1073 K. The experimental data were found to be best described by Jander's model and the activation energy was found to be about 168 kJ mol-1. This activation energy indicates that diffusion is rate controlling. The effect of briquetting pressure was more significant at low compaction pressures, and sample size had no effect in the range studied. These results contribute to an improved understanding of the formation mechanism of some soft ferrites, such as MnZn-ferrite and NiZn-ferrite, which are employed in the electronics industry, and also the formation of zinc ferrite during roasting and dust formation in pyrometallurgical processes.

WELDING & JOINING

A Mathematical Model of Gas Tungsten Arc Welding Considering the Cathode and the Free Surface of the Weld Pool
W.-H. KIM, H.G. FAN, and S.-J. NA
A two-dimensional axisymmetric numerical model, including the influence of the cathode and the free surface of the weld pool, is developed to describe the heat transfer and fluid flow in gas tungsten arc (GTA) welding. In the model, a boundary-fitted coordinate system is adopted to precisely describe the cathode shape and deformed weld-pool surface. The current continuity equation has been solved with the combined arc plasma-cathode system, independent of the assumption of current density distribution on the cathode surface, which was essential in the previous studies of arc plasma. It has been shown that the temperature profile, the current, and the heat flux to the anode show good agreement with the experimental data. Moreover, the current and the heat-flux distributions may be affected by the shape of the cathode and the free surface of the weld pool.

The Effects of Process Variables on Pulsed Nd:YAG Laser Spot Welds: Part II. AA 1100 Aluminum and Comparison to AISI 409 Stainless Steel
D.C. WECKMAN, H.W. KERR, and J.T. LIU
In this two-part article, the weldabilities of AA 1100 aluminum and AISI 409 stainless steel by the pulsed Nd:YAG laser welding process have been examined experimentally and compared. The effects of laser pulse time and power density on laser spot weld characteristics, such as weld diameter, penetration, melt area, melting ratio, porosity, and surface cratering, have been studied and explained qualitatively in relation to material-dependent variables such as absorptivity and thermophysical properties. The weldability of AISI 409 stainless steel was reported in Part I of this article. In the present article, the weldability of AA 1100 aluminum is reported and compared to that of AISI 409 stainless steel. Weld pool shapes in aluminum were found to be influenced by the mean power density of the laser beam and the laser pulse time. Both conduction-mode and keyhole-mode welding were observed in aluminum. Unlike stainless steel, however, drilling was not observed. Conduction-mode welds were produced in aluminum at power densities ranging from 3.2 to 10 GW/m2. The power density required for melting aluminum was approximately 4.5 times greater than stainless steel. The initial transient in weld pool development in aluminum occurred within 2 ms, and the aspect ratios (depth/width) of the steady-state conduction-mode weld pools were approximately 0.2. These values are about half those observed in stainless steel. The transition from conduction- to keyhole-mode welding occurred in aluminum at a power density of about 10 GW/m2, compared to about 4 GW/m2 for stainless steel. Weld defects such as porosity and cratering were observed in both aluminum and stainless steel spot welds. In both materials, there was an increased propensity for large occluded vapor pores near the root of keyhole-mode welds with increasing power density. In aluminum, pores were observed close to the fusion boundary. These could be eliminated by surface milling and vacuum annealing the specimens, suggesting that such pores were due to hydrogen. Finally, excellent agreement was obtained between experimental data from both alloys and an existing analytical model for conduction-mode laser spot welding. Two nondimensional parameters, the Fourier number and a nondimensional incident heat flux parameter, were derived and shown to completely characterize weld pool development in conduction-mode welds made in both materials.

MATHEMATICAL MODELING

Steady-State Modeling of Zinc-Ferrite Hot-Acid Leaching
DIMITRIOS FILIPPOU and GEORGE P. DEMOPOULOS
Mathematical modeling of leaching processes involving porous solid particles can be of crucial importance for the design, performance analysis, and further improvement of several hydrometallurgical plants. In this work, a mathematical model is developed for the description and analysis of the steady-state response of the hot-acid leaching (HAL) process, whereby zinc-ferrite porous particles are leached inside a cascade of continuous stirring-tank reactors by relatively concentrated sulfuric acid solutions at temperatures close to 373 K. The model incorporates the intrinsic reaction kinetics and the particle dissolution pattern, as the last can be described by the grain model for surface-reaction control. In particular, the mathematical basis of the model is a population-balance equation for dissolving ferrite grains, which is coupled together with appropriate mass-balance equations for all dissolved species, and conversion values are estimated for each reactor. The model shows that it can predict very successfully the performance of an industrial HAL circuit, and for this reason, it has been used for exploring operation alternatives for that particular circuit.

Wall-to-Bed Heat Transfer in a Circulating Fluidized Bed for the Reduction of Iron Ore Particles
Y.B. HAHN and Y.H. IM
A model is presented that describes the wall-to-bed heat transfer in a circulating fluidized bed (CFB) used for the prereduction of iron ore in the smelting-reduction iron-making process. The model incorporates the core-annulus type flow structure and the wall emulsion layer growing downward along the surface. Model predictions showed good agreements with measured data taken from the literature. The hydrodynamic behavior near the wall surface was able to be properly described by the core-annulus flow structure. A higher heat-transfer coefficient with higher solid circulation flux was obtained in the upper part of the bed because of the heat input caused by the lateral diffusion of particles from the core. The predicted and measured data also showed the minima in the heat- transfer coefficients in the lower part of the bed. Model predictions indicated that in the CFB for the reduction of iron ore particles, it is important to properly control the inlet temperature of the reducing gas, rather than that of the solid particles. The implications of the behavior of heat transfer in the CFB are discussed for the reduction of iron oxides.

Communication: The Modeling of Flow Phenomena in Air-Agitated Pachuka Tanks
HIMANSHU SINGH and DIPAK MAZUMDAR

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