Determination of the phase boundaries of the wustite solid solution within the context of reduction tests

Oxygen reduction measurements performed isothermally on Fe₂O₃ with CO/CO₂ gas mixtures make it possible, in combination with a system of equations describing the homogeneous wustite phase FeO_n, to describe also the boundaries of this solid solution mathematically. Compared with corresponding data determined by Giddings and Gordon on the basis of a literature evaluation, the boundaries of the wustite field are found to shift to a much lower reduction potential. The developed system of equations makes it possible to plot the curves of the same oxygen concentration in the homogeneous wustite phase field of the Bauer-Glaessner diagram. When comparing the reduction equilibria of the left- and right-hand wustite solid solution boundary with the appropriate literature data there is a good agreement particularly with the measurements of Darken and Gurry. Significantly lower oxygen concentrations are found in the FeO phase diagram, however, especially at high temperatures. New concentration data derived from the systems of equations are also given for the melt equilibria of the wustite phase, which lead to the assertion that the liquidus curves of magnetite, wustite and iron are also shifted to a lower oxygen concentration. The decomposition point of the wustite is found at 570°C, 49.5 %, CO'₂ and 23.2 % oxygen.     

The correlation of the thermodynamic properties of wustite by a gaussian based formalism

A Gaussian-based formalism is used to correlate precisely the data of Darken and Gurry¹ for the wustite phase of the industrially important system Fe-0 within and at the boundaries of this phase. Equations are also given for the free energy and heats of formation of metastable FeO, wustite in equilibrium with γ-Fe, and Fe₃O₄ from y-Fe and oxygen at 1 atm.     

Studies on the reduction of hematite by carbon

Single pellet experiments have been carried out in a nitrogen atmosphere to study the reduction of hematite by graphite in the temperature range 925 to 1060°C. The effect of variables such as c/Fe₂O₃ molar ratio, pellet size, and so forth, has been investigated. Gas analysis data show a continuous decrease in CO₂/CO ratio during reduction, the values being far away from Fe/FeO equilibrium for wustite reduction by CO. The activation energies associated with different degrees of reduction appear to be widely different suggesting a possible changeover in reaction mechanism during the progress of reduction. X-ray diffraction studies confirm the stepwise nature of hematite reduction. Formerly Research Scholar in the Department of Metallurgy, Indian Institute of Science, Bangalore-560012, India,     

Morphological changes during reduction of magnetite compacts

Magnetite superconcentrate compacts were reduced to wustite and iron at various temperatures (1173-1423 K) and the morphological changes occurring at various stages of reduction were observed under the scanning electron microscope. It has been demonstrated that considerable pitting and fragmentation occur during solid state reduction of Fe₃O₄ to FeO by solid iron. During the reduction of FeO the iron nuclei formed have a tetrahedral shape. At low reduction temperatures (1173 K) the large number of iron nuclei formed on wustite sinter together forming a dense iron layer. At high temperatures sustained growth of nuclei in the vertical direction leads to a sinuous porosity. Genesis of porosity creation has also been explained in terms of formation of holes, slits, cracks and fragmentation in magnetite and wustite and growth and sintering of iron nuclei.     

Phase Composition of Oxide Scales during Reheating in Hot Rolling of Low Carbon Steel

Low carbon steel was oxidized over the temperature range 1000‐1250°C in O₂‐CO₂‐H₂O‐N₂, O₂‐H₂O‐N₂, and O₂‐CO₂‐N₂ gas mixtures. Oxidation times were 12‐120 min. and the scales were 50‐2000 μm thick. The variations of these parameters were chosen to elucidate the phase composition of oxide scales under conditions similar to those of reheating furnaces in hot strip mills, using either thin slab casting or conventional casting and rolling technology. Two types of scales have been observed which are influenced by the furnace atmosphere, oxidation time, and temperature. The first type is a crystalline scale with an irregular outer surface, composed mostly of wustite (FeO), and a negligible amount of magnetite (Fe₃O₄). The second type is the classical three‐layer scale, composed of wustite (FeO), magnetite (Fe₃O₄), and hematite (Fe₂O₃). In general, the experiments showed that an increase in oxidation time decreased the percentage of wustite while the percentages of magnetite and hematite increased. A rise in oxygen concentration in the gas mixture increased the percentages of magnetite and hematite, confirming earlier experimental findings. In water vapour‐free atmospheres O₂‐CO₂‐N₂, the oxide scales had a low percentage of wustite, and high percentage of magnetite and hematite. Carbon dioxide showed a small influence at 1100°C, and a negligible one at 1250°C.     

Pressure increase and “dynamic effective diffusivity” within a powder bed during reaction-reduction of iron oxide powders by hydrogen at 900°C

α-Hematite powder having an average diam of 8.1 μm was packed in a vertical quartz reactor (I.D. 14.6 mm) and was reduced by feeding hydrogen toward the top of the packed bed at 900°C under atmospheric pressure. A large increase in pressure as high as 118 mmHg was observed within the powder bed during the course of reaction. The stoichiometric condition that the diffusion rate of the reactant gas, H₂, should be equal to that of the product gas, H₂O, gave rise to this pressure increase within the bed. When the pressure within the bed were uniform, the ratio of the diffusional fluxes of the reactant gas, H₂, and of the product gas, H₂O, would be equal to the square root of the ratio of the molecular weight of H₂O and of H₂. An effective diffusivity which includes not only the diffusional term but also the hydrodynamic term was introduced into the equations for the flow of gases with chemical reactions. The pressure variations within the powder bed calculated from the proposed flow equations agreed quite well with those observed during the course of the three-step reduction of hemattte (Fe₂O₃) to magnetite (Fe₃O₄), then to wustite (Fe_1?y O), and finally to iron. It was also found that, during the reduction, the presence of pressure gradient within the bed reduced the effective diffusivity of hydrogen to about one third of that without the pressure gradient.     

The gaseous reduction of solid calciowustites in Co/Co2 and H2/H2O gas mixtures

The reduction of calciowustites has been carried out in CO/CO₂ and H₂/H₂O gas mixtures at temperatures between 1073 and 1373 K. The effect of lime additions to the wustite is to extend considerably the range of gas conditions over which porous iron morphologies are observed compared to those found on reduction of pure wustite. The products obtained at low oxygen potentials consisted of porous iron containing a dispersion of CaO particles, at intermediate oxygen potentials a two phase structure consisting of porous iron and dicalcium ferrite was formed, and at high oxygen potentials a dense iron layer over the oxide surface is observed throughout reduction. F. NAKIBOGLU, formerly Graduate Student, Department of Mining and Metallurgical Engineering, University of Queensland, Brisbane, Australia D.H. St. JOHN, formerly Postdoctoral Fellow, University of Queensland     

The activities of sulfide and oxide components and the solubility of oxygen in copper-iron-sulfur-oxygen mattes at 1300 °C

The activities of iron and copper and the solubilities of oxygen in copper-iron-sulfur-oxygen mattes have been determined by equilibrating mattes with CO−CO₂−SO₂ gas mixtures of fixed partial pressures of oxygen and sulfur and equilibrating a small mass of platinum with the melt. Iron and copper transferred from the matte to form a platinum-iron-copper alloy in which the activities of iron and copper are the same as in the matte. The activities of iron and copper in the matte were then determined from knowledge of the activities of iron and copper in the system platinum-iron-copper. Sulfides ofW _Fe=0.1, 0.3, and 0.5 were studied, whereW _Fe=wt pct Fe/(wt pct Fe+wt pct Cu), and sulfur pressures of 0.005, 0.0158, and 0.025 atm and oxygen pressures of 3.16×10⁻¹⁰, 7.94×10⁻¹⁰, 2.00×10⁻⁹, and 3.16×10⁻⁹ were used. The activity of copper, which varied in the range 0.06 to 0.165, decreases with increasingp _{O 2} at constantW _Fe andp _{S 2} and decreases with increasingp _{S 2} at constantW _Fe and constantp _{O 2}. The activity of iron, which varied in the range 0.002 to 0.06, increases with increasingp _{O 2} at constantW _Fe andp _{S 2} and decreases with increasingp _{S 2} at constantW _Fe andp _{O 2}. The activities of the components Cu₂S, FeS, Cu₂O, FeO, and Fe₃O₄ were calculated from the activities of iron and copper, the partial pressures of oxygen and sulfur, and the approapriate equilibrium constants. The variations of the activities of these components with matte grade, oxygen pressure, and sulfur pressure are presented and discussed. Within the range of experimental conditions studied, the solubility of oxygen in the melts is given by wt pct O=2.59pO₂/0.225pS₂/−0.18 (1+9.0W _Fe)^1.86     

Reduction behaviour of powder mixtures of iron oxide and carbon in reactive atmospheres

Mixtures of fines of iron ore and carbon were kept in hot zone of furnace for various durations under flowing argon or mixture of hydrogen and carbon dioxide. Experiments were conducted at temperatures of 1150, 1250 and 1300 K. Other variables were inlet H₂/CO₂ ratio both oxidising and reducing to wustite, Fe₂O₃/C ratio in sample and sample size. 2 types of carbon were selected, viz. graphite and activated char. The degree of reduction of oxide (F) was determined by subsequent hydrogen reduction of reaction products. F vs. time data did not follow any set pattern due to complex mechanism and kinetics. Equilibrium calculations predict that, inlet H₂+CO₂ mixture which undergoes water gas shift reaction inside the furnace, reduces to wustite at a H₂/CO₂ ratio of 3. As expected, generally it yielded highest value of F. However at 1300 K, even the gas with H₂/CO₂ ratio of 1.5/1, although oxidising to wustite, gave comparably high value of F, presumably due to enhanced gasification rate at higher temperature. Although reactivity of graphite was much lower than that of activated char, they exhibited comparable extents of reduction.     

Application of thermodynamic and kinetic principles in the reduction of metal oxides by carbon in a plasma environment

The application of plasma technology to metal oxide reduction is discussed with reference to established thermodynamic and kinetic principles. ΔG°-T diagrams for the corresponding metal oxide, metal carbide, and C-CO reactions are presented and the important role played by thep _CO/P _{CO 2} ratio examined. On the basis of these theoretical considerations, supported by some earlier experimental results conducted on the reduction of iron and chromium oxide concentrates in the form of taconite and chromite by carbon within a plasma reactor, the tendency to form either elemental metals or carbides is discussed. It is also suggested that the reduction of taconite by carbon takes place in two stages within the plasma medium. In the first stage, ferric oxide is reduced to wustite by carbon, and in the second stage wustite is reduced to metal. It is also postulated that in the first stage of reduction, ferric oxide may also be reduced to wustite through an exchange reaction between ferric oxide and iron, without CO evolution. The rate controlling step for the first stage of taconite reduction is thought to lie at the gas/slag interface generated within the plasma environment, while the second stage of reduction is controlled by carbon gasification by CO₂. Formerly Postdoctoral Fellow with Mineral Research Center, University of Minnesota     

Microstructural features produced by the reduction of wustite in H2/H2O gas mixtures

The systematic study of the reduction of pure wustites (FeO) between 600 and 1100°C in H₂/H₂O gas mixtures has revealed a number of important morphological changes. It has been shown that dense wustite can decompose to form a highly porous wustite before iron nucleation takes place. The product morphologies of iron formed on the wustite on reduction have been classified into three types, (a) porous iron, (b) porous wustite covered by dense iron, and (c) dense wustite covered by dense iron.     

Phase equilibria in the system Fe-Zn-O at intermediate conditions between metallic-iron saturation and air

The phase equilibria in the FeO-Fe₂O₃-ZnO system have been experimentally investigated at oxygen partial pressures between metallic iron saturation and air using a specially developed quenching technique, followed by electron probe X-ray microanalysis (EPMA) and then wet chemistry for determination of ferrous and ferric iron concentrations. Gas mixtures of H₂, N₂, and CO₂ or CO and CO₂ controlled the atmosphere in the furnace. The determined metal cation ratios in phases at equilibrium were used for the construction of the 1200 °C isothermal section of the Fe-Zn-O system. The univariant equilibria between the gas phase, spinel, wustite, and zincite was found to be close to pO₂=1 · 10⁻⁸ atm at 1200 °C. The ferric and ferrous iron concentrations in zincite and spinel at equilibrium were also determined at temperatures from 1200 °C to 1400 °C at pO₂ = 1·10⁻⁶ atm and at 1200 °C at pO₂ values ranging from 1 · 10⁻⁴ to 1 · 10⁻⁸ atm. Implications of the phase equilibria in the Fe-Zn-O system for the formation of the platelike zincite, especially important for the Imperial Smelting Process (ISP), are discussed.     

Influence of metal-wustite separation on the reduction of wustite with hydrogen

The reduction rate of wustite with hydrogen at 1133 to 1233 K and for hydrogen pressures between 1.6 and 5 mbar has been measured gravimetrically upon varying the separation between tablets made of wustite and iron or nickel. The reduction rate of wustite is increased by a factor 2.4 prior to precipitation of iron on wustite when the wustite-metal distance is changed from 2 mm to 0.35 at H₂ pressures of 1.6 mbar and 2.3 mbar respectively. It has thus been demonstrated for the first time, that the catalytic effect of the metal phase on wustite reduction also exists when metal and wustite are separated. To describe this catalytic effect mathematical relations have been derived on the basis of a reaction model. These relations are in good accord with the results of the measurements. From recently published experimental findings, it may be concluded that, as a result of heterogeneous reactions, translationally ‘hot’ H₂ molecules are desorbed from the metal surface. These induce a rapid reaction at wustite unless they have been deactivated by molecular collisions within the bulk gas between the tablets.     

Thermodynamics of manganese distribution between liquid iron and the CaO–Al2O3–SiO2–FeO–MnO system

The thermodynamics of distribution of constituents between liquid iron and the CaO–Al₂O₃–SiO₂–FeO–MnO system at 1600°C was studied using electrochemical indication of the equilibrium partial pressure of oxygen in both phases. The results show that oxidation potential of the Fe(l)–CaO–Al₂O₃–SiO₂–FeO–MnO system, expressed in terms of log p(O₂), is directly proportional to log (x(MnO) · x(FeO)/w| Mn |). Manganese distribution coefficient, L'_mn, in intersection CaO/Al₂O₃ = 1 decreases with increasing slag basicity expressed in terms of activity a(CaO) or 1/γ(MnO). Experimentally determined equilibrium constant K_Mn/Fe is equal to 2.7 for 1600°C. The number of exchanged electrons between Fe-O-Mn-Si electrode and the slag approaches the theoretical value.     

The reduction of hematite to wustite in a laboratory fluidized bed

Particles (approximately 180 to 250 Μm across) were reduced by CO-CO₂, by H₂-H₂O, and by CO-CO₂-H₂-H₂O in a small fluidized bed, with facilities for automatic sampling of off-gas. Structural changes in the preheating period and during reduction were followed by surface area measurements and by microscopy. During reduction, surface areas increased initially and then decreased, as porosity created by chemical reaction was reduced by sintering; ‘uniform internal reduction’ was observed from magnetite to wustite. Although the bed was operated under vigorously bubbling conditions, with flow rates 7 to 15 times the minimum fluidization velocity at temperature, gas utilization was high for abstraction of about two-thirds of the maximum amount of oxygen removable by the inlet gas. The utilization fell sufficiently in the remaining stages of the reaction for approximate rate constants to be estimated. The rate increased (a) with increasing temperature, (b) with increasing gas flow, (c) with increasing reducing potential of the inlet gas, and (d) with increasing hydrogen content of inlet gas. The off-gas analyses showed the importance of the water gas shift reaction within the pores of the fluidized particles.     

Solubility of oxygen and sulfur in copper-iron mattes

The concentrations of oxygen and sulfur in unsaturated and magnetite-saturated Cu-Fe mattes were measured as a function of oxygen and sulfur pressures and iron metal weight fraction of the matte (W _Fe = wt Fe/(wt Fe + wt Cu)). The liquid matte samples were equilibrated with streams of gas of known pressures of S₂ and O₂ at 1468 K. Empirical correlation equations were developed to describe the experimental results. The correlation for oxygen in unsaturated matte is wt pct O = 2.50P _{O 2} ^0.200 P _{S 2} ^-0.142 (1 + 9.0W _Fe ^2.19, and in magnetite-saturated matte it is wt pct O = 0.14 + 2.39W _Fe + 12.0W _Fe ² for 0.001 <P _{S 2} ≤ 0.01 atm and it is wt pct O = −3.06 + 2.39W _Fe + 12.0W _Fe ² − 1.60 logP _{S 2} for 0.01 <P _{S 2} < 0.023 atm. A single complex equation ofP _{O 2},P _{S 2}, andW _Fe describes the sulfur concentrations in both unsaturated and magnetite-saturated mattes. An erratum to this article is available at .     

Economic analysis of reduction processes in blast furnaces

On the basis of a mathematical model, blast-furnace performance is analyzed as a function of the utilization of the furnace gas??s reducing capacity ??_ac in the indirect reduction of ferrous oxide. Economic analysis of the reducing processes is based on the consumption V _s of reducing gas per unit atomic oxygen in wustite. The influence of the gas??s attainment of equilibrium composition on the furnace performance is assessed by means of V _s -??_ac plots.     

Sulfides and oxides in Fe−Mn alloys: Part I. Phase relations in Fe−Mn−S−O system

This is a critical review of available equilibrium data between phases involving Fe, Mn, S, and O. Using the Morey-Williamson theorem, and that modified by Darken, the sulfur and oxygen potential diagrams are constructed for the Fe?S?O system involving nine univariant and three invariant equilibria. The solubility of sulfur in wustite-saturated iron is evaluated; the sulfur content of iron in equilibrium with wustite and liquid oxysulfide reaches a maximum of 143 ppm at 1200°C; this is about one half of that corresponding to the solidus of the Fe?S system. An estimate is made of the phase equilibria in the Fe?Mn?S?O quaternary system involving gamma iron and Mn(Fe)O phases. There is a eutectic invariant at ～900°C, and the liquid miscibility gap invariant is estimated to be at ～1225°C. From the expected phase relations and equilibria, it is deduced that if sufficient oxygen and sulfur,i.e. Mn(Fe)O and Mn(Fe)S phases, are present in Fe?Mn alloys, there may be a liquid oxysulfide phase present at temperatures above 900°C, depending on the concentration of manganese in solution. The higher the manganese content in solution, the higher is the temperature above which a liquid phase is present,e.g. for 10 ppm Mn, 900°C and for ～90 pct Mn, ～1225°C. A mechanism is suggested for the precipitation of sulfides and oxysulfides near the surface of steel during heating in an oxidizing atmosphere. In the Appendix by Darken and Gurry, results are given of the melting temperatures of mixtures of wustite and pyrrhotite in equilibrium with iron, from an investigation carried out about thirty years ago.     

Diffusion-limited sulfidation of wustite

The sulfidation of wustite in H₂S−H₂O−H₂−Ar atmospheres has been studied at temperatures of 700, 800, and 900°C with thermogravimetric techniques. Polycrystalline wustite wafers were equilibrated in a flowing H₂O−H₂−Ar gas stream and then sulfidizedin situ. During an initial transient stage a protective layer of FeS formed on the sample, and an intermediate layer of Fe₃O₄ formed between the FeO and FeS layers. Subsequently, the reaction followed a parabolic rate law. The parabolic rate constant varied from 0.22×10⁻² mg² cm⁻⁴ min⁻¹ at 700°C to 6.5×10⁻² mg² cm⁻⁴ min⁻¹ at 900°C. The reaction rate was limited by the diffusion of iron through the intermediate Fe₃O₄ layer which grew concurrently with the FeS layer and at the expense of the FeO core. After the FeO core was completely converted to Fe₃O₄, the process entered a passive stage during which no further mass changes could be detected. SCOTT McCORMICK, formerly Graduate Student, Purdue University is currently Assistant Professor, Department of Metallurgical and Materials Engineering, Illinois Institute of Technology, Chicago, Illinois 60616.     

Thermodynamics of Calcium Ferrite Slags at 1200 and 1300°

Abstract

Oxygen isobars and liquidus isotherms of the system CaO-FeO-Fe₂O₃ at 1200 and 1300°C were determined by quenching samples equilibrated with CO₂–CO mixtures, The iron liquidus and the melt coexisting with two solids were carefully examined in terms of their composition as well as the equilibrium oxygen partial pressures, p _o2 . At 1200°C, p _o2 was 10^?7.70 atm when the slag coexisted with magnetite and dicalcium ferrite, At 1300°C, the melt region extends to the CaO–Fe₂O₃ join, where p _o2 was 10^?0.68 atm (air) or higher. Within the range of p _o2 from one order above that at iron saturation to 10^?4 atm, the slag composition, p _o2 , and the temperature T are related by the equation:

log (Fe^{+ + +} /Fe^{+ +} ) ? 0.170logp _O2 + 0.018 (wt% CaO) + 5500/T ? 2.52.

Activities of CaO(s), FeO(l), and Fe₃O₄ (S) in the slag were calculated from the p _o2 data by combining the available thermal data and/or by Gibbs-Duhem equation.

Résumé

Les isobares d'oxygene et les isothermes du liquidus du systeme CaO–FeO–Fe₂O₃ à 1200 et 1300°C ont été déterminées en trempant des échantillons équilibrés avec des mélanges CO₂–CO. Le liquidus du fer et le mélange coexistant avec deux solides ont été soigneusement étudiés en terme de composition et de pression partielle d'oxygène à l'équilibre, p _o2 . A 1200°C, p _o2 était de 10^?7.7 atm quand la scorie était en équilibre avec la magnétite et la ferrite de dicalcium. A 1300°C, le domaine du liquide s'étend jusqu'au joint CaO–Fe₂O₃ où p _o2 valait 10^?0.68 atm (air) ou plus. A l'intérieur du domaine de p _o2 qui s'étend d'un ordre au dessus de celle à la saturation en fer jusqu'à 10^?4 atm, la composition de la scorie, p _o2 et la température T sont reliées par l'équation:

log(Fe³⁺/Fe²⁺) ? 0.170logp _o2 + 0.018(pd %CaO) + 5500/T ? 2.52.

Les activités de CaO(s), FeO(l) et Fe₃O₄(S) dans la scorie ont été calculées a partir des valeurs de p _o2 en combinant les données thermodynamiques disponibles et/ou l'équation de Gibbs-Duhem.