METALLURGICAL AND MATERIALS TRANSACTIONS B
ABSTRACTS
Volume 28B, No. 1, February 1997

This Month Featuring: Hydrometallurgy, Pyrometallurgy, Electrometallurgy, Transport Phenomena, Physical Chemistry, Solidification, Solid State Reactions, and Mathematical Modeling. View February 1997 Table of Contents.

HYDROMETALLURGY

The Electrochemical Behavior of Gold in Ammoniacal Solutions at 75°C
RAJIB DASGUPTA, Y. CHARLES GUAN, and KENNETH N. HAN
The dissolution behavior of gold in ammoniacal solutions at 75°C has been investigated. The variables investigated include concentration of lixiviants, pH of the solution, and various oxidants. The anodic dissolution of gold was studied in the absence of oxidants using the potentiodynamic polarization technique. The cathodic reaction of oxidants were studied on a platinum electrode. Tafel polarization tests were also performed to determine the dissolution rate. The overall dissolution of gold in ammoniacal solutions was examined using a mixed potential theory. A comparative study on the role of cupric ammine, oxygen, hydrogen peroxide, and sodium hypochlorite as oxidants for the dissolution of gold in ammoniacal solutions has been carried out.

Recovery of Copper through Decontamination of Synthetic Solutions Using Modified Barks
I. GABALLAH, D. GOY, E. ALLAIN, G. KILBERTUS, and J. THAURONT
Decontamination of synthetic acetate, chloride, nitrate, and sulfate solutions containing 10, 100, 1,000, 10,000, and 50,000 ppm of copper using chemically treated barks has been studied. Metal percentage removal from solutions depends on the pH, the initial concentration, and, to some extent, the anion. It varies from 40 through 99 pct of the initial metallic ion's content in the solution. The average retention capacity of the treated bark is about 43 mg of Cu/g of dry modified bark (0.68 mmole/g of dry bark). Extraction of copper cations from the saturated modified bark was made possible with dilute acid. Regeneration of bark for reuse as an ion exchanger was possible. Bark loaded with copper was analyzed by scanning electron microscopy (SEM) and infrared (IR) spectroscopy. Copper was uniformly distributed in the bulk of the bark. No copper segregation was observed. It seems that copper was bound to the acidic (phenolic) sites of the bark. Anions were not detected on the copper-loaded bark with either SEM electron probe microanalysis or IR spectroscopy. Incineration of the bark loaded with copper resulted in ashes containing about 77 pct of copper oxides, while pyrolysis of the same sample led to ashes containing 10 pct of metallic copper and about 85 pct carbon.

Leaching of Marine Manganese Nodules by Acidophilic Bacteria Growing on Elemental Sulfur
YASUHIRO KONISHI, SATORU ASAI, and YUICHI SAWADA
This article describes the bioleaching of manganese nodules by thermophilic and mesophilic sulfur-oxidizing bacteria, in which oxidized sulfur compounds are biologically produced from elemental sulfur added to liquid medium and are simultaneously used to leach nodules. The thermophile Acidianus brierleyi solubilized the manganese nodules faster at 65°C than did the mesophiles Thiobacillus ferrooxidans and Thiobacillus thiooxidans at 30°C. Leaching experiments with A. brierleyi growing on elemental sulfur were used to optimize various process parameters, such as medium pH, initial sulfur-liquid loading ratio, and initial cell concentration. The observed dependencies of the leaching rates at a pH optimum on the initial amounts of elemental sulfur and A. brierleyi cells were qualitatively consistent with model simulations for microbial sulfur oxidation. Under the conditions determined as optimum, the leaching of nodule particles (-330 +500 mesh) by A. brierleyi yielded 100 pct extraction of both copper and zinc within 4 days and high extractions of nickel (85 pct), cobalt (70 pct), and manganese (55 pct) for 10 days. However, the iron leaching was practically negligible.

Kinetics of Hydrothermal Oxidation of Granular Pb Metal to PbO Powder in Sodium Hydroxide Solutions
N. YAMASAKI, XIANG LAN, and Q.I. FENG
The oxidation kinetics of granular Pb metal in sodium hydroxide solutions were investigated in an autoclave, under temperatures ranging from 110°C to 175°C, oxygen pressures of up to 3.0MPa, particle size fractions from 0.52 to 5 mm, and NaOH concentrations of up to 1.6 mol/L. In most instances, the 1-2/3 - (1-)2/3 vs time relationship, indicative of a diffusion-controlled reaction, was closely obeyed. The oxidation rate increased significantly with increasing temperature, and the apparent activation energy was found to be 29.6 kJ/mol. The oxidation process of lead metal can be improved evidently by using a stronger stirring method to decrease the product layer. The distribution of the various Pb complexes calculated from available thermodynamic data showed that the prominent component in alkaline solution was HPbO2- ion which determined the amount of Pb2+ available for the formation of PbO.

PYROMETALLURGY

Kinetics of Chlorination of Niobium Pentoxide by Carbon Tetrachloride
P.K. JENA, E.A. BROCCHI, and R.I. GARCIA
Kinetics of the chlorination of Nb2O5 powder by CCl4 vapor in mixture with N2 in a static bed in the temperature range of 698 to 853K were carried out at different partial pressures of CCl4 (pCCI4), varying from 0.10 to 0.75 atm. The fraction of Nb2O5 chlorinated R at pCCl4 of 0.6 atm in the temperature range of 698 to 773K was found to be proportional to time t, and the activation energy E is calculated to be 112 kJ/mole. Results on the effect of pCCI4 (0.4, 0.6, and 0.75 atm) at 723K suggest that the rate v (R/min) is proportional to p1.5CCI4. However, at pCCI4 of 0.2 atm, R is not linear with t, rather, R1/2 is linear with t. Based on these results, two mechanisms, one at low pCCI4 (0.2 atm) and another at higher pCCI4 values, in the temperature range of 698 to 773K have been suggested. Similar studies in the higher temperature range (793 to 853K), where pCCI4 used to decompose to elemental chlorine and carbon, were also carried out. At all temperatures and pCCI4 values, R is found to be directly proportional to t. At two higher pCCI4 (0.4 and 0.6 atm), v is proportional to pCCI4, whereas at two lower pCCI4 (0.1 and 0.2 atm) it is proportional to p0.5CCI4. The E values obtained in the temperature range of 793 to 853K at pCCI4 of 0.6 and 0.2 atm are found to be 57 and 115 kJ/mole, respectively. In this higher temperature range, two different reaction mechanisms have been proposed.

ELECTROMETALLURGY

Effect of FeO in the Slag and Silicon in the Metal on the Desulfurization of Hot Metal
P.K. IWAMASA and R.J. FRUEHAN
Work has been conducted to investigate the effects of FeO in the slag and silicon in the metal on hot metal desulfurization. Laboratory experimental results show that FeO decreases and silicon increases the rate of desulfurization. Silicon in the metal is consumed by the reduction of FeO and also by the desulfurization reaction. A mathematical kinetic model was developed to describe both the effects of silicon and FeO on desulfurization for the laboratory scale. The model predicts the sulfur and silicon content in the metal and the FeO and sulfur content in the slag as a function of time. It is based on four-component simultaneous mass transfer: sulfur and silicon in the metal and FeO and sulfur in the slag. Experimental results, the development of the kinetic model, and a comparison of the model and experimental results are presented.

Spouted Bed Electrowinning of Zinc: Part I. Laboratory-Scale Electrowinning Experiments
JUAN CARLOS SALAS-MORALES, J.W. EVANS, O.M.G. NEWMAN, and P.A. ADCOCK
Two types of laboratory cells have been constructed to electrowin zinc from sulfate electrolytes: one cell was cylindrical while the other had a rectangular (flat) geometry. Cells were operated on industrial or synthetic electrolytes to electrodeposit zinc onto a spouted bed of zinc particles in the range of 0.75 to 1.45 mm. Current efficiencies and cell voltages have been measured during the course of batch experiments, enabling the calculation of the energy consumption per kilogram of zinc deposited. Electrolyte samples have been analyzed. Current densities (current per unit of cell cross-sectional area) were in the range of 1380 to 6200 A/m2. Most catholytes were initially neutral and contained on the order of 150 g/L of zinc. Final acid contents were in the range of 39 to 114 g/L of sulfuric acid. The performance of the cells (particularly with respect to current efficiency) was superior to prior work on fluidized bed electrowinning from similar electrolytes. The flat cell was superior to the cylindrical cell and showed energy consumptions of less than 3 kWh/kg zinc at current densities up to 3500 A/m2 when used to take the zinc content from 150 to 100 g/L zinc. Current efficiencies in this application ranged from 91 to 92 pct.

Spouted Bed Electrowinning of Zinc: Part II. Investigations of the Dynamics of Particles in Large Thin Spouted Beds
A. VERMA, JUAN CARLOS SALAS-MORALES, and J.W. EVANS
The behavior of particles in thin spouted beds, mostly equipped with draft tubes, has been investigated. Three apparatuses have been used: a laboratory-scale cylindrical bed, a 2-m-tall "flat" (rectangular cross section) bed and a 2-m-wide flat bed, the last equipped with multiple draft tubes. Most of the results were obtained on the tall bed. Minimum spouting flow rate, pressure distribution, particle velocities, and solid circulation rates were determined as a function of bed geometry (including draft tube dimensions and position). Observations were made of the direction of liquid flow in the bed outside the draft tube and of the occurrence of zones in the bed where the particles appeared stationary. The wide bed was used to determine that there is a maximum separation between draft tubes beyond which particles cannot be kept in motion across the whole width of the bed.

Alumina Solubility in Molten Salt Systems of Interest for Aluminum Electrolysis and Related Phase Diagram Data
EGIL SKYBAKMOEN, ASBJØRN SOLHEIM, and ÅMUND STERTEN
The solubility of alumina in molten Na3AlF6 containing various amounts of AlF3, CaF2, and LiF was determined by measuring the weight loss of a rotating sintered corundum disc. The results were fitted to the following empirical expression:

[Al2O3]sat= A(t/1000)R

where
A=11.9-0.062[AlF3]-0.0031[AlF3]2-0.50[LiF]

-0.20[CaF2]-0.30[MgF2]+

B=4.8-0.048[AlF3]+

where the square brackets denote weight percent of components in the system Na3AlF6-Al2O3 (sat)-AlF3-CaF2-MgF2-LiF and t is the temperature in degree Celsius. The standard deviation between the equation and the experimental points in the temperature range from 1050°C to about 850°C was found to be 0.29 wt pct Al2O3. A series of revised phase diagram data of interest for aluminum electrolysis was derived based on the present work and recently published data for primary crystallization of Na3AlF6 in the same systems.

TRANSPORT PHENOMENA

Model Study of Turbulence Structure in a Bottom Blown Bath with Top Slag Using Conditional Sampling
MANABU IGUCHI, TADATOSHI NAKATANI, and HIROSHI UEDA
Water model experiments were carried out to study the effect of top slag on the turbulence structure in a molten steel bath agitated by bottom gas injection. Water and silicone oil were used as models of molten steel and slag, respectively. Air was injected through a single-hole bottom nozzle so that the reverse emulsification of the silicone oil occurred at the silicone oil-water interface. Silicone oil droplets thus generated were carried deeply into the lower water layer. Turbulence measurements were made using a two-channel laser Doppler velocimeter (LDV) in the presence and absence of the top oil layer. A conditional sampling method called the four-quadrant classification method was applied to detect large scale coherent motions in the vertical bubbling jet as well as in the recirculation region. The structure and intensity of turbulence inside the bubbling jet were strongly affected by bubbles. In the absence of the top oil layer, higher momentum fluid motions directed from the centerline of the bubbling jet to the sidewall of the vessel were mainly responsible for the turbulence production in the bubbling jet, while lower momentum fluid motions directed from the sidewall toward the centerline governed the turbulence production in the recirculation region. On the other hand, in the presence of the top oil layer, the coherent motion in the bubbling jet was also affected slightly by the top layer except for the center of the bubbling jet, whereas any distinguished coherent [iw-1]motion was not observed in the recirculation region, and hence, turbulence production was weak there.

Mass Transfer between Solid and Liquid in a Gas-Stirred Vessel
AMARENDRA K. SINGH and DIPAK MAZUMDAR
Mass transfer between solid and bulk liquid in an axisymmetric gas-stirred water model of a metallurgical reactor has been investigated both experimentally and theoretically. To this end, mass transfer rates from benzoic acid compacts submerged in an aqueous gas bubble driven system were measured via a weight loss technique. In conjunction with the weight loss measurements, liquid velocity and turbulence kinetic energy distributions in the bath were also mapped via laser doppler velocimetry (LDV). From the detailed LDV measurements, relevant dimensionless groups such as: Reloc,r and Ret were estimated. Experimental measurements indicated that flow parameters varied from one location to another within the system. The corresponding variation in dissolution rates was, however, less pronounced. Such a trend was observed for all three gas flow rates studied. It was found that experimentally measured dissolution rates can be correlated with the measured flow and turbulence parameters (viz., and u) in terms of a previously reported dimensionless correlation, viz., Sh = 0.73 (Reloc,r)0.25 (Ret)0.32 (Sc)0.33. Parallel to flow measurements, a two-phase turbulent flow model was also applied to numerically compute the distributions of mean and fluctuating velocity components in the vessel. Embodying the predicted velocity components in the aforementioned correlation, mass transfer rates were recalculated. A comparison between the two sets of Sherwood numbers (estimated on the basis of the experimentally measured and theoretically predicted flow fields) suggests that solid-liquid mass transfer rates in a gas-stirred vessel can be predicted reasonably well via an axisymmetric, steady-state, two-dimensional turbulent flow model.

PHYSICAL CHEMISTRY

Solubility of Platinum in Molten Fluxes as a Measure of Basicity
SHIGEKO NAKAMURA and NOBUO SANO
The solubility of platinum in the molten BaO-Al2O3, BaO-SiO2, CaO-Al2O3, CaO-SiO2, Na2O-SiO2, and CaO-Al2O3-SiO2 systems has been measured in order to seek a new measure of basicity. The solubility of platinum increases with increasing content of basic oxide. The correlations among the solubility of platinum, carbonate, sulfide, and phosphate capacities, basic oxide activities, and theoretical optical basicity are discussed.

A Model for Calculating Interaction Coefficients between Elements in Liquid and Iron-Base Alloy
F.M. WANG, X.P. LI, Q.Y. HAN, and N.X. ZHANG
A new model for predicting ln 0 and an activity interaction coefficient j has been proposed by use of free volume theory, Miedema's semiempirical formation enthalpy model for binary alloys, and the Toop model. The results calculated are in good agreement with experimental values except for the systems which contain nitrogen and hydrogen gas elements.

SOLIDIFICATION

Numerical Simulation and Fourier Thermal Analysis of Solidification Kinetics in High-Carbon Fe-C Alloys
E. FRAS, W. KAPTURCKIEWICZ, A. BURBIELKO, and H.F. LÓPEZ
The aim of this work was to carry out both experimental and numerical simulations of cast iron solidification under various conditions. The experimental work was based on a novel technique of thermal analysis known as the Fourier method, whereas solidification modeling was possible by solving the Fourier equation with a heat source. Moreover, a comparison between the Fourier and the Newtonian method indicated that their predictions are appreciably different. The Newtonian method is rather insensitive to the actual thermal gradients and predicts a clear maximum in heat generation at the onset of solidification. In contrast, the Fourier method incorporates the effect of actual thermal gradients and predicts two successive heat generation peaks of increasing magnitude as solidification proceeds. In particular, it was found that the experimental outcome of solidified volume fractions agrees closely with the predictions of the Fourier method. In this case, both experimental and computer simulations on 30- and 40-mm-diameter cylindrical specimens indicated that the solidified fraction followed a sinusoidal trend. Moreover, it was found that under normal solidification conditions, secondary nucleation of fine grains can occur near the center of a cylindrical cast iron specimen. Secondary grain nucleation is attributed to the development of a second undercooling maximum which easily exceeds the initial one. Finally, the effects of inoculation were investigated in plain cast iron and as a function of the inoculation time. Accordingly, in all cases, the computer simulations were in close agreement with the experimental outcome.

Communication: Primary Particle Melting Rates and Equiaxed Grain Nucleation
Q. HAN and A. HELLAWELL

SOLID STATE REACTIONS

Isothermal Precipitation of Commercial 3003 Al Alloys Studied by Thermoelectric Power
NEY JOSÉ LUIGGI
We have used thermoelectric power (TEP) techniques to study the precipitation process in samples of cold-rolled commercial 3003 Al alloys, in which we guarantee the initial microstructure by diverse thermal treatments. Kinetics are obtained, isothermally, at several aging temperatures, proving that the manganese impurity controls the precipitation process. The Nordheim-Gorter rule is used to determine the effect of impurities on the diffusional TEP, permitting us to know the amount of Mn in solid solution for the diverse work conditions. The time-temperature-transformation (TTT) curves, obtained from the isothermal kinetics plots, indicate the existence of different precipitate phases which depend upon the thermal treatment applied to the sample. For homogenized samples, the observed behavior was associated to an equilibrium phase with a nose toward 540°C and with an activation energy of (41 ± 2) Kcal/mol. For strained samples, we observe a metastable phase with an activation energy of (19 ± 2) Kcal/mol and also a stable phase with an activation energy similar to the one obtained for the homogenized samples. For as-cast and as-cast-heated samples, we predict the existence of two precipitated phases, the activation energy of the precipitated phase at low temperature being (31 ± 2) Kcal/mol. The energy of the second phase seems to be the same for all microstructural conditions. The results obtained for the stable phase agree with the results reported in the literature.

MATHEMATICAL MODELING

Mathematical Simulation and Experimental Verification of Melting Resulting from the Coupled Effect of Natural Convection and Exothermic Heat of Mixing
HONGFA HU and STAVROS A. ARGYROPOULOS
When melting processes are associated with an exothermic heat of mixing, unique coupled transport phenomena take place. In this article, a mathematical model has been developed to simulate these unique coupled heat and mass transfer events. The model was based on the control-volume finite difference approach and on an enthalpy method. In order to verify the mathematical model, a low-temperature physical model was established consisting of ice and sulfuric acid solutions. In this physical model, both temperature and velocity measurements were carried out. The model predictions were compared with experimental measurements, and they were found to be in good agreement. The model was also applied to a high-temperature system, namely, the melting of silicon metal in liquid high carbon iron. The predictions distinguished two periods present in the entire melting process. In the first period, the silicon was heated up. The second period, i.e., free melting period, occurred in tandem with the exothermic reaction, and consequently, the melting process was greatly accelerated. As was the case with the low-temperature physical model, as with the high-temperature system, good agreement was obtained between the predicted results and the experimental measurements.

Analysis of Thermoelectric Power Measurements in the Study of Precipitation Kinetics in 3003 Al Alloy
NEY JOSÉ LUIGGI
Multiphase precipitation kinetics for a 3003 aluminum alloy, pointed out in our previous work and obtained under different microstructural conditions by thermoelectric power measurements, is now modeled under the light of the theory of reaction rates. Our multiphase model encompasses the precipitation kernels of Johnson-Mehl-Avrami (JMA) and of Fujita-Damask (FD), which adequately reproduce the experimental kinetics. The JMA kernel generates almost constant values for the exponent n. These values are less than one in temperature ranges where the log of the rate constant k grows linearly with the inverse of the temperature. The FD kernel provides an idea of the average critical size of the phase which is formed. Our multiphase results are compared with those obtained from the traditional monophase considerations rendering evident important differences between both models. The total kinetics is deconvoluted, allowing us to characterize the different metastable and stable phases formed according to the different microstructural conditions. The activation energy for kinetics in "as-cast" samples behaves differently from that obtained from the microstructures in homogenized and homogenized-strained samples. The activation energy for each of these cases is reported.

Kinetics of Simultaneous Two-Phase Precipitation in the Fe-C System
NEY LUIGGI and ANGEL BETANCOURT
A theoretical approach for the interpretation of the kinetics of simultaneous stable and metastable phase precipitation in a binary system is proposed. The model, based on the nucleation and growth theory, defines a critical size different for each phase. The size of the clusters evolves by adding or subtracting a single atom one at a time. A set of coupled differential equations is obtained for the chemical rate whose solution reproduces the kinetics of thermoelectric power measurements in the Fe-C multiphase system. Suppositions about the growing and dissolution rate constants reduce the size of the equation system with a gain in computation time.

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