The Effect of the Transition Metal Oxide Content of a Mold Flux on the Radiation Heat Transfer Rates

Heat transfer in the mold of a steel continuous casting machine strongly influences cast surface quality. Transition metal oxides have been widely used in mold slags in continuous casting to aid in liquid pool formation and enhance melting rates. Although a few research studies have been carried out to investigate the thermal properties of mold slags containing transition metal oxides, very few studies have directly correlated their effect on radiation heat transfer rate in continuous casting. This study investigates the radiation heat transfer rates across a mold flux film and determines the influence of the transition metal oxides, MnO and Fe₂O₃, on these rates. It is found that additions of 10% MnO or 5% Fe₂O₃ reduce the radiation heat transfer rate by approximately 25% and increase the adsorption coefficient from 400 to 1800 m^?1.     

Oxidation of molten copper matte

An amount of 80 mg of molten copper matte of a pseudo-ternary Cu₂S-FeS-Fe system contained in a slender alumina sample tube was oxidized at 1503 and 1533 K in a mixed O₂-Ar gas stream and the oxidation path was followed, comparing the overall rate of oxidation with the gaseous diffusion in the sample tube. The following successive reactions were found to be controlled by gas diffusion. Initially, Fe was oxidized to form FeO. After the melt composition reached a pseudo-ternary Cu₂S-FeS-FeO system, FeS was oxidized to form FeO. As the amount of FeO increased, Fe₃O₄ was also formed and subsequently Cu was produced by the oxidation of Cu₂S. In the latter stage, the Cu was oxidized, and the final product under the condition of gas diffusion control was composed of Cu₂O, Fe₃O₄, and CuFeO₂. On the other hand, the rate of formation of Fe₂O₃, CuO, and CuFe₂O₄ was much slower and they were not formed during the oxidation duration where the overall rate of oxidation was controlled by gas diffusion.     

Electronically driven transport of oxygen from liquid iron to CO + CO2 gas mixtures through stabilized zirconia

Transport of oxygen in the following electrochemical system was investigated;O (liquid iron) Oⁱⁿ²⁻ (in ZrO₂2−CaO) O₂ (CO + CO₂) An alumina crucible was charged with liquid iron containing 580 ± 10 ppm oxygen. A calcia-stabilized zirconia tube (closed at one end) was immersed in the liquid iron. The inside of the zirconia tube was flushed with a stream of CO + CO2 gas mixture. Oxygen was removed from liquid iron to the CO + COO₂ gas mixture without application of an external current. Kinetics of oxygen transport in this system are discussed in terms of mixed ionic and electronic conduction of the zirconia, and also diffusion of oxygen in liquid iron. The rate controlling step for this oxygen removal process was found to be transport of oxygen across a boundary layer in the melt at the melt/electrolyte interface. M. IWASE, on leave from the Department of Metallurgy, Kyoto University, Kyoto, Japan M. TANIDA, Formerly Graduate Student at Kyoto University     

A structural study of oxidation in a zirconia-toughened alumina fiber-reinforced Fe3Al composite

A zirconia-toughened alumina fiber-reinforced Fe₃Al-based intermetallic composite was fabricated by pressure casting. The chemical stability of the composite at 1100 °C in vacuum and air was studied by optical, scanning, and transmission electron microscopy. Fiber/molten metal interaction during pressure casting resulted in the rejection of ZrO₂ from the fiber into the molten metal. The fiber/matrix interface in the cast composite was in some areas covered with thin ZrC and Fe₂AlZr layers. Vacuum annealing resulted in the dissolution of Fe₂AlZr and precipitation of ZrC and (Ti, Nb)C particles within the matrix. The density of carbides was very low. Air annealing led to the oxidation of ZrC to ZrO₂, Fe₂AlZr to a mixture of A1₂O₃ + ZrO₂, and preferential growth of α-Al₂O₃ over the ZrO₂. Depletion of Al from the matrix as a result of oxidation gave way to the precipitation of (a) coarse (Fe, Al)₂(Nb, Al) particles and (b) fine cuboidal-shaped particles within the matrix during slow cooling from the oxidizing temperature. Oxidation of the matrix ended with the conversion of Fe(Al, Cr) into (Fe, Al, Cr)₂O₃. The Fe₂O₃ was observed to wet the grain boundaries of the A1₂O₃ fiber, which resulted in the disintegration of the fiber. Zr-containing plate-like precipitates with a {10-14} habit plane were occasionally observed in Fe₂O₃. Diffusion of oxygen through the fiber and/or the fiber/matrix interface is believed to be responsible for the rapid oxidation of the composite.     

Reaction mechanism for the acid ferric sulfate leaching of chalcopyrite

The acid ferric sulfate leaching of chalcopyrite, CuFeS₂ + 4Fe⁺³ = Cu⁺² + 5Fe⁺² + 2S⁰ was studied using monosize particles in a well stirred reactor at ambient pressure and dilute solid phase concentration in order to obtain fundamental details of the reaction kinetics. The principal rate limiting step for this electrochemical reaction appears to be a transport process through the elemental sulfur reaction product. This conclusion has been reached in other investigations and is supported by data from this investigation in which the reaction rate was found to have an inverse second order dependence on the initial particle diameter. Furthermore, the reaction kinetics were found to be independent of Fe⁺³, Fe⁺², Cu⁺² and H₂SO₄ in the range of additions studied. The unique aspect of this particular research effort is that data analysis, using the Wagner theory of oxidation, suggests that the rate limiting process may be the transport of electrons through the elemental sulfur layer. Predicted reaction rates calculated from first principles using the physicochemical properties of the system (conductivity of elemental sulfur and the free energy change for the reaction) agree satisfactorily with experimentally determined rates. Further evidence which supports this analysis includes an experimental activation energy of 20 kcal/mol (83.7 kJ/mol) which is approximately the same as the apparent activation energy for the transfer of electrons through elemental sulfur, 23 kcal/ mol (96.3 kJ/mol) calculated from both conductivity and electron mobility measurements reported in the literature. formerly Metallurgy Graduate Student, University of Utah.     

Effects of CaO, Al2O3, and MgO additions on the copper solubility, ferric/ferrous ratio, and minor-element behavior of iron-silicate slags

The effects of CaO, Al₂O₃, and MgO additions, singly or in combination, on the copper solubility, the Fe³⁺/Fe²⁺ ratio in slag, and on the minor-element behavior of silica-saturated iron silicate slags were examined at 1250 °C and a p _O2 of 10⁻¹² to 10⁻⁶ atm. The results indicated that copper solubility in slag was lowered with the addition of CaO, MgO, and Al₂O₃, in decreasing order. The Fe³⁺/Fe²⁺ ratio in the slag decreased with the additions, but this effect was smaller at lower oxygen potentials. The presence of small amounts (about 4 pct) of CaO, Al₂O₃, and MgO in the slag resulted in increased absorption of Bi and Sb into molten copper, but had a smaller effect at large additions (about 8 to 11 pct). The distribution behavior of Pb was a function of oxygen partial pressure, which indicates the oxidic dissolution of Pb in the slag as PbO, while the behavior of Bi, Sb, and As was found to be independent of oxygen potential, supporting the atomic (neutral) dissolution hypothesis of these elements in the slag. The distribution behavior of Pb and As was not significantly affected by the additions. The activity coefficients of Bi and Sb in the slags were determined to be as follows: (1) for no addition, γ _Bi=40 and γ _Sb=0.4; (2) for small additions (about 4.4 pct), γ _Bi=70 to 85 and γ _Sb=0.8; and (3) for large additions (about 8 to 11 pct), γ _Bi=60 to 75 and γ _Sb=0.5 to 0.7.     

Study on electrical conductivity of FexO–CaO–SiO2–Al2O3 slags

The composition dependences of electrical conductivity of Fe_xO–CaO–SiO₂–Al₂O₃ slags at different oxygen potentials and temperatures have been studied experimentally in the present work. From the experimental results, the total electrical conductivity and electronic conductivity for all the slags monotonously decrease as increasing CO/CO₂ ratio from about 0 to 1. With the increase of Fe_xO content, the total electrical conductivity and electronic conductivity increase at a fixed CO/CO₂ ratio. It is also found that the ionic conductivity of all the studied slags increases as increasing the CO/CO₂ ratio, which is resulted from the increase of Fe²⁺ ion concentration. In addition, the temperature dependences of ionic, electronic and total conductivity for different compositions obey the Arrhenius law. The electronic transference number exhibits a strong relationship with oxygen potential, but is independent of temperature.     

Electrochemical Transport in Molten Lead Silicates

Abstract

Ionic transport in the molten system PbO-SiO₂ is discussed based upon available data on lead and silicon tracer diffusivities, inter diffusion coefficients, electrical conductivity and on measured values of lead and silicon transference numbers. The interrelationship of these parameters is illustrated through a theoretical treatment of the phenomenological transport equations. The Onsager transport coefficients for the independent set of equations are calculated for melt compositions between 0.3 < x _SiO2 < 0.6 at 850°C. The coincidence of large changes in the transport parameters between the orthosilicate and metasilicate compositions and the large changes in the silicate structure is discussed.

Résumé

Les phénomènes de transport ionique dans le systeme PbO-SiO₂ à l'état fondu sont analysés et discutés a partir des valeurs déjà publiées concernant la diffusion des isotopes traceurs de Pb et Si, les coefficients d'interdiffusion de PbO et SiO₂, les conductivityés ioniques et les mesures des nombres de transport pour ces éléments. Un traitement théorique des équations phénoménologiques de transport illustre les différentes relations entre ces paramètres. Les coefficients de transport d'Onsager sont calculés, à l'aide d'un système indépendant d'équations, pour les compositions comprises entre 30 et 60 % de SiO₂ en mole, a 850°C. La possibilité de coincidence des fortes variations des propriétés de transport avec les changements de structure ionique des bains silicates est discutée, pour des compositions correspondant à des structures d'ortho- ou de méta-silicates.     

The reaction behavior of Fe-C-S droplets in CaO-SiO2-MgO-FeO slags

The rate of reduction of FeO in the slag by carbon in iron droplets (2.9 wt pct C, 0.01 wt pct S) was studied for CaO-SiO₂-MgO slags containing between 3 and 35 wt pct FeO and temperatures ranging from 1643 to 1763 K. The effects of Fe₂O₃ additions to the slag and sulfur variations in the metal on the reaction rate were also studied. It was found that the behavior of the metal droplets in the slag, as observed by X-ray fluoroscopy, changed significantly with FeO content in the slag. Below 10 wt pct FeO, the droplet remained intact while reacting with the slag; however, above this FeO concentration, the droplet became emulsified within the slag. The large increase in surface area of the metal droplet due to emulsification caused the rate of reaction to be one to two orders of magnitude faster than for droplets that did not become emulsified. It was suggested that when the droplet is emulsified, the surface area and reaction kinetics are greatly increased, and the rate becomes controlled by mass transfer of FeO as Fe²⁺ and O²⁻ ions in the slag to the emulsified droplet. At low FeO contents for which the droplet does not emulsify, the rate is controlled by dissociation of CO₂ on the metal. It was also found that a critical temperature exists for a given FeO content at which point the rate of CO evolution increases dramatically. Additions of Fe₂O₃ to the slag and sulfur to the metal caused significant changes to the rate of reaction possibly by affecting the emulsification behavior of the droplet.     

Electronic conductivities of commercial ZrO2 + 3 to 4 wt pct CaO electrolytes

Potentiostatic electrochemical experiments with liquid metal electrodes have been analyzed to provide values for the mixed electrical conduction parameters (PM_ion, oø_o, O_o , P_o,P _o, and E O,C.) associated with commercial-grade ZrO₂ + 3 to 4 wt pct CaO electrolytes. Good agreement between experimental and calculated values for the open-circuit cell voltage,E o. c., for relatively high oxygen concentrations in the melt demonstrates that the currently accepted interpretation of electronic conduction in solid electrolytes is valid. In addition, these results provide an experimental verification that n-type electronic conductivity is proportional toPo ₂ ^-1/4.     

Effect of the oxidation level of iron on the viscosity of FeO x -CaO melts

Viscometry methods are used to estimate the effect of iron oxidation level at the Fe³⁺/Fe_tot ratio from 0.1 to 0.8 and additions of slag-forming oxides (MgO, SiO₂, Al₂O₃, and ZnO) up to 5% on the viscosity of FeO _x -CaO melts. The iron(III) oxide content is shown to correlate with primary solidification temperature T _p.s and viscous flow activation energy E _η. The dependences of T _p.s and E _η on the melt composition are found as a result of processing the experimental data by mathematical statistics methods.     

Effects of Mn ore on the Dephosphorization and Desulphurization in Hot Metal Pretreatment

In order to examine the possibility of utilizing Mn ore in the pretreatment process of liquid iron, experiments were carried out in a system with liquid iron and CaO‐SiO₂‐MgO‐Al₂O₃‐FeO slag with additives (Mn ore and Fe₂O₃+MnO₂) in a magnesia crucible at 1673K. When Mn ore was added to the pretreatment slag, decarburization, desiliconization, dephosphorization and desulphurization proceeded simultaneously with the reduction of Mn ore. The reduction rate of MnO and the Mn concentration in the melt increased with increasing initial Si content. The maximum dephosphorization was obtained when the additive consisted of 66.6 mass% Fe₂O₃ and 33.4 mass% MnO₂. The desulphurization ratio increased with increasing the relative amount of MnO₂ in the additives. The amount of additives comprising Fe₂O₃ and MnO₂ required for a targeted manganese content could be predicted using the mass balance. Effects of the additives on dephosphorization were also estimated.     

Measurement of electronic conduction in cryolite alumina melts and estimation of its effect on current efficiency

The electronic conductivity of molten Na₃AlF₆ saturated with A1₂O₃ was determined as a function of the activity of Na at 1000 °C by using the Wagner polarization technique. Typical results showed electronic conductivities of 0.298 ohm^-1 cm^-1 at a_Na = 0.04 and 0.084 ohm^-1 cm^-1 at a_Na = 0.01. A transient technique employing the same experimental cell was used to determine the diffusion coefficient and the mobility of electrons in this melt. The mobility is 0.33 cm² V^-1 s^-1. A model calculation for electrolysis of a melt of cryolite saturated with alumina to produce aluminum metal is presented to show how electronic conductivity effects current efficiency (CE). Under various assumed conditions, which are tabulated, CEs ranged from around 85 to 98 pct at 1000 °C. Formerly Chemist Formerly Scientist     

Oxidation of Aluminum Melts with Rare-Earth Metals

Thermogravimetry studies show that the oxidation of aluminum–rare-earth metal (R) melts obeys a parabolic law. The true oxidation rate of the melts is on the order of 10^–4–10^–3 kg/(m² s). In the Al–R systems, the minimum oxidation rate corresponds to the compositions of intermetallic compounds. The oxidation rate of a melt is shown to increase with temperature. It is found by IRS (infrared spectroscopy) and XRD (X-ray diffraction) that the melt oxidation products consist of γ-Al₂O₃, R₂O₃ (R = La, Ce, Pr, Nd, Y, Sc), CeO₂, and rare-earth metal monoaluminates RAlO₃ (CeAlO₃, LaAlO₃, NdAlO₃).     

Thermodynamic properties of PbO-SiO2 slags by emf measurements

Abstract

The thermodynamic properties of PbO-SiO₂ liquid slags have been investigated by measuring the emfs of the following cell:

Pt, O₂(1 atm)/0.90 ZrO₂ + 0.10 CaO/(PbO-SiO₂)_liq.sol'n Pb_(l)

From the results, the activities and the other partial molar properties of the system have been calculated for compositions between 40 and 85 mole % PbO in the temperature range 720 to 110°C.

Résumé

Les propriétés thermodynamiques des scories liquids PbO-SiO₂ ont été recherchees en mesurant la f.e.m. de la pile suivante:

Pt,O₂(1 atm.)/0.90 ZrO₂ + 0.10 CaO/(PbO-SiO₂)_liq.sol'nPb_(l)

A partir des résultats, les activités et les autres propriétés molales partielles de ce système ont été calculées pour des compositions molales variant de 40 à 85% en PbO, sur un intelvalle de température s'etendant de 720 a 1100°C.     

Oxidation Studies of SiAlON/MgAlON Ceramics with Fe2O3 and CaO Impurities, Part II: Phase Evolution

The oxidation behavior of composite SiAlON/MgAlON phases, synthesized from the leaching residue after the aqueous treatment of salt cake from aluminum remelting, is compared with the oxidation of corresponding synthetic samples. The samples were subjected to oxidation under air as the oxidant atmosphere in the temperature range of 1373 K to 1773 K (1100 °C to 1500 °C). The phases present were analyzed by scanning electron microscopy (SEM)-electron-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) to arrive at the evolution of the various phases formed during oxidation. From the experimental results, especially by the characterization of the oxidation products, the mechanism of the oxidation reaction was deduced as follows: With the progress of oxidation, the composition of the material being oxidized moved toward the Al₂O₃-rich corner of MgO-Al₂O₃-SiO₂ and CaO-MgO-Al₂O₃-SiO₂ phase diagrams relevant to the SiAlON/MgAlON composite. At lower temperatures, the addition of Fe₂O₃ and CaO facilitated the formation of cordierite and anorthite, respectively. With increasing temperature, islands of silicate melt were formed dissolving these oxides, with the liquidus temperature getting lowered as a consequence. The liquid phase formed engulfed the adjacent solid phases providing strong mobility for the cations and enabling the crystal growth. As a result, intermediate products, i.e., cordierite, anorthite, and spinel, which were formed earlier during oxidation, are found to get dissolved in the liquid phase.     

Oxidation of Fe-7Cr-12Ni-(0-6)Al-(0-7)Si alloys

A series of Fe-7Cr-12Ni-(0-6)Al-(0-7)Si alloys was oxidized at test temperatures between 800° and 1000 °C. The master alloy, Fe-7Cr-12Ni, had no oxidation resistance at temperatures above 800 °C. Aluminum additions to the master alloy produced a protective A1₂O₃ layer over the substrate metal which prevented rapid oxidation and allowed time for a Cr₂O₃ layer to develop underneath. Silicon addition to the master alloy produced an SiO₂ oxide sublayer at the metal-oxide interface which, when formed, provided an effective barrier to oxidation. A combination of 1.5-pct-Al and 2.0-pct-Si additions provided the best oxidation protection.     

High-temperature isopiestic studies on the ternary slag PbO-SiO2-B2O3 at 1273 K

An improved high-temperature isopiestic technique has been applied to the ternary slag PbO-SiO₂-B₂O₃ and its binary subsystems PbO-SiO₂ and PbO-B₂O₃ at 1273 K. The isopiestic measurements agree well with the literature data for the two binaries. Each entire isoactivity curve of PbO in the ternary slag was determined by only one experiment. The activities of SiO₂ and B₂O₃ were then calculated analytically. The measured and calculated results for the ternary slag approximately fit a partial ideal solution model proposed previously.     

Kinetics of As,Sb, Bi and Pb volatilization from industrial copper matte during Ar+O<Subscript>2</Subscript> bubbling

A kinetic study on minor elements removal during copper matte oxidation was designed under the presumptions of low FeO activity and of no fayalite slag formation. Copper matte with a mass fraction of Cu of 59 pct was mixed by Ar gas blowing during preheating. The matte was oxidized at 1523 and 1673 K by bubbling Ar+O₂ gas through a submerged nozzle. The effects of melt temperature and input oxygen content on the oxidation rate of matte and the volatilization rate of minor elements in copper matte are discussed. The competition reaction composed of the oxygen dissolution into matte and SO₂ gas evolution rate results in the preferential oxidation of FeS in copper matte. The desulfurization rate of matte and the volatilization of minor elements in copper matte were primarily controlled by the mass-transfer rate through the gas film boundary layer around rising gas bubbles. The As, Pb, and Bi were significantly removed during Ar gas blowing with the volatilization rates of Bi and Pb markedly increasing with the melt temperature. However, the dependences of the volatilization rates on the input oxygen partial pressure were affected in a complicated way by the effects of the melt temperature and the reduced amount of exhaust gas.     

Kinetics of Reduction of SiO2 in SiO2-Al2O3-CaO Slags by Al in Fe-Al(-Si) Melts

Kinetic models considering mass transport in, (i) metal phase only and (ii) both metal and slag phases (mixed control or two-phase mass transfer) were developed for the reduction of SiO₂ in a SiO₂-Al₂O₃-CaO slag by Al in an Al-Fe melt. The models were validated with experiments of the reaction with Fe-Al melt and SiO₂-Al₂O₃-CaO-MgO_sat slags at 1873 K (1600 °C). The models predict that the rate of reaction is slower in the mixed control model because of the added resistance of slag phase mass transport. The mixed control becomes applicable when the slag contains low amounts of SiO₂. In this case, when the initial Al content in the metal increases, the normalized rate of reaction decreases. The increased Al content in the metal retards the reaction due to the limited SiO₂ provided to the reaction interface in the mixed control model. Sensitivity analyses were done using the models for the ratios of mass transfer coefficients of Si to Al, and Al₂O₃ to Si, along with slag density, which did not impose a significant effect.