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.     

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.     

On the Gibbs energy of formation of wustite

Gibbs energy change for the reactionxFe_(s) + 1/2O_2(g) = Fe _x O_(s) has been redetermined using the galvanic cell (−) Fe_(s), Fe _x O_(s)∥ZrO₂ − CaO∥NiO_(s), Ni_(s)(+) in the temperature range 866 to 1340 K. The results are at variance with earlier works in that they reflect the transformations occurring in the iron phase. The Gibbs energy function is represented by two nonlinear equations,viz., ΔG° (866 to 1184 K) = −251480 − 18.100T + 10.187T lnT ± 210 J/mol and ΔG° (1184 to 1340 K) = −286248 + 181.419T - 13.858T lnT ± 210 J/mol. Formerly Research Assistant at the Department of Theoretical Metallurgy, The Royal Institute of Technology, Stockholm     

The nucleation of iron on dense wustite: A morphological study

Dense wustite was reduced at temperatures from 430 to 1100 °C in CO-A, CO-CO₂, H₂-A, and H₂-H₂O mixtures. Most of the experiments were conducted on a hot stage microscope, providing a clear record of the sequence of events. The morphology of phases was further studied on the optical and the scanning electron microscopes. Two types of nuclei were identified: (1) dense nuclei, ranging from regular whiskers to simple protrusions, around which flat bases develop to form a protective film, and (2) porous nuclei with lenticular shapes, which remain level with the sample surface as they grow (both radially and into the wustite) to form a porous layer. Mild gases and moderate temperatures favor dense iron, whereas strong gases and extreme temperatures (either low or high) favor porous iron. The domains of the two types are represented in morphology maps. They overlap in transition zones where mixed nuclei are found; in this case the porous nuclei appear later than the dense ones but they grow much faster. The morphology maps support the view that sponge iron in iron ore reduction does not originate from porous iron nuclei but develops as a maze of dense iron growths. Experiments with cold-worked and annealed samples indicate that dislocations are essential to the growth of whiskers. Formerly at the Ecole Centrale des Arts et Manufactures, Chatenay-Malabry, France The present paper is abstracted from a thesis prepared by S. El Moujahid at the Ecole Centrale des Arts et Manufactures and presented for the degree of Docteur d’Etat at the University of Nancy. For this work, S. El Moujahid received the 1984 Carl Wagner Award from the Max Planck Institut für Biophysikalishe Chemie of the University of G?ttingen, Germany.     

The nucleation of iron on dense wustite: A morphological study

Dense wustite was reduced at temperatures from 430 to 1100 °C in CO-A, CO-CO₂, H₂-A, and H₂-H₂O mixtures. Most of the experiments were conducted on a hot stage microscope, providing a clear record of the sequence of events. The morphology of phases was further studied on the optical and the scanning electron microscopes. Two types of nuclei were identified: (1) dense nuclei, ranging from regular whiskers to simple protrusions, around which flat bases develop to form a protective film, and (2) porous nuclei with lenticular shapes, which remain level with the sample surface as they grow (both radially and into the wustite) to form a porous layer. Mild gases and moderate temperatures favor dense iron, whereas strong gases and extreme temperatures (either low or high) favor porous iron. The domains of the two types are represented in morphology maps. They overlap in transition zones where mixed nuclei are found; in this case the porous nuclei appear later than the dense ones but they grow much faster. The morphology maps support the view that sponge iron in iron ore reduction does not originate from porous iron nuclei but develops as a maze of dense iron growths. Experiments with cold-worked and annealed samples indicate that dislocations are essential to the growth of whiskers. Formerly at the Ecole Centrale des Arts et Manufactures, Chatenay-Malabry, France     

Microreaction mechanism in reduction of magnetite to wustite

In order to understand the microreaction mechanism of the reduction of magnetite to wustite, hydrogen ions were implanted into magnetite at room temperature by an ion accelerator. The crystalloid transformation during the reduction process was investigated by using selected-area electron diffraction patterns. The experimental results showed that {220} planes on the surface of magnetite were changed first because the concentration of oxygen ions on the {220} planes is higher than other planes to follow the reaction of oxygen ions with hydrogen ions, leaving the {220} planes and resulting in rearrangement of ions. On the other hand, oxygen ions migrate more difficulty than iron ions in magnetite; therefore, {220} planes in the bulk are more stable than other planes. Based on the experimental facts, two kinds of microreaction mechanisms in reduction of magnetite to wustite are suggested. It was found that (1) wustite with [001] direction was formed on the magnetite with [001] direction, (2) (220) and (200) planes of wustite were parallel with (220) and (400) planes of magnetite, respectively, in crystal structure between parent phase and new phase, (3) some {220} planes were formed earlier than other ones in wustite during the reduction process. These results can be considered as due to the similar geometric distribution of oxygen ions between magnetite and wustite.     

Surface segregation of calcium oxide in wustite and its effects on the reduction

The segregation of calcium oxide to the surface of wustite was investigatedin situ by Auger electron spectroscopy at temperatures in the range of 973 to 1223 K. Calcium oxide indicated a strong tendency to segregate, with the molar fractions at the surface being 10 to 1000 times the molar fractions in the bulk (X _CaO ^bulk ). The segregation isotherms showed maxima at intermediate molar fractions in the bulk (X _CaO ^bulk = 0.003 or 0.005). The enthalpy change for the segregation increased from −43.3 to −5.20 kJ/mol with increasing X _CaO ^bulk . The main driving force of the segregation may be the relaxation of the strain energy generated around calcium ions. Calcium oxide also segregates at the surface of iron in contact with the wustite. By the segregation of calcium oxide, the surface energies of wustite and iron may decrease whereas the wustite/iron interfacial energy may increase. The enhancement of the reduction of wustite by calcium oxide is concluded to essentially occur by the good and continuous direct contact of wustite surface with reducing gases owing to the suppression of new iron nucleus and owing to the contraction of the wustite/iron-nuclei interfacial area compared with the free-surface area of wustite in order to decrease the total energy.     

Heat transfer and melting characteristics of metallized wustite pellets

The melting procedure of the metallized wustite pellet under Ar atmosphere was examined in this work. The temperature at the center and surface of the pellet was measured during pellet heating from 200 to 1550 °C. The variation of the pellet shape and the movement of melting interface were continuously monitored by means of an X-ray image system during pellet melting. The adoption of digital image processing made a significant accuracy improvement of the X-ray image analysis compared with the previous works. On the basis of experimental results, a modified unit cell model was built for describing the internal structure and the effective thermal conductivity of the metallized porous pellet. The calculated results based on the model agreed well with the experimental ones.     

Mechanisms of porous iron growth on wustite and magnetite during gaseous reduction

The iron/iron oxide interface morphologies formed during the reduction of dense wustite and magnetite samples have been examined using scanning electron microscopy. Under conditions where porous iron products are obtained, a range of structures, which depend critically on the gas composition, reaction temperature, and bulk oxide composition, may be formed at the inter-face during reduction. Three principal mechanisms of porous iron growth have been identified: (1) a continuous coupled reaction, involving cooperative pore and iron growth, (2) a continuous dendritic growth mechanism, in which pores advance ahead of the iron formation, and (3) a discontinuous mechanism, involving the successive formation and breakdown of dense iron lay-ers on the oxide surface. The reaction mechanisms are explained in terms of the relative mag-nitudes of the various chemical reactions and mass transport processes which occur during the decomposition of the solids. S.P. MATTHEW, formerly Graduate Student, Department of Min-ing and Metallurgical Engineering, University of Queensland. T.R. CHO, formerly Graduate Student, Department of Mining and Metallurgical Engineering, Uni-versity of Queensland.     

水煤气反应对浮氏体还原影响的实验研究

在900℃和1 000℃时,气体成分为(CO H2)/(CO CO2 H2)=0.85,H2/(CO H2)=0.3和(CO H2)/(CO CO2 H2)=0.75,H2/(CO H2)=0.3的条件下做了浮氏体还原实验.得出在温度高于810℃时,水煤气反应影响浮氏体还原,混合气体中CO2≥25%时表现明显,混合气体中CO2≤15%时影响较小.     

An investigation of the critical influence of potassium on the reduction of wustite

An investigation of the influence of potassium on the reduction of wustite single crystals has been carried out at temperatures ranging from 600 to 1000 °C in 30 pct CO-10 pct CO₂-3 pct H₂-57 pct N₂, a gas composition close to that of the reserve zone of the blast furnace. Three doping techniques have been used in order to perform either an insertion of potassium in the wustite lattice, by annealing, by surface doping at room temperature, or a doping by a potassium loaded reducing gas during the reduction. Regardless of which doping technique is used, no potassium has been detected in the wustite lattice. All dopings affect only the surface of the particles before the reduction. Potassium markedly increases the rate of reduction of wustite. This is explained first by the change in the morphology of the iron produced. In addition, potassium also acts on the chemical stage of the reduction by changing the nucleation and the growth of the iron. Another important result is the following: when potassium is added to partly reduced wustite, it changes the iron properties in the course of a subsequent reduction (this iron becomes much more permeable to the gas, increasing the transfer of the reactants).     

Heating and melting of wustite pellets in liquid slag

The melting behaviour of the metallized, porous wustite pellets immersed in liquid slag, as well as the influences of the metallization ratio, pre-heating temperature of the pellet and slag temperature were examined in this work by means of an X-ray imaging system. The internal structure of the pellets after having been immersed in slag was checked by optical microscope and EPMA. The adoption of digital image processing improved the image analysis dramatically and, as a result, some important phenomena, such as solid slag shell forming and melting, slag penetration, wustite component dissolution as well as the influence of experimental conditions, were quantified.     

Establishment of product morphology during the initial stages of wustite reduction

The product morphologies obtained on the reduction of wustite in CO/CO₂ gas mixtures between 1073 and 1373 K are reported. Three types of product morphology are identified, namely, type A (porous iron), type B (porous wustite covered with dense iron), and type C (dense wustite covered with dense iron). The reactions which occur during the reduction of wustite in CO/CO₂ and H₂/H₂O systems both before and after iron nucleation are examined. The product morphologies obtained on reduction are explained qualitatively in terms of the relative rates of the chemical reaction with the gas and the mass transport processes both in and on the solid. Formerly Postdoctoral Fellow at the Department of Mining and Metallurgical Engineering, University of Queensland, St. Lucia, Brisbane, Australia An erratum to this article is available at .     

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.     

The decomposition of wustite under mixed chemical reaction/mass transport limitations

Numerical solutions have been obtained for the mixed chemical reaction/diffusion limited planar decomposition of a wustite slab prior to iron metal nucleation. The results of the analysis are presented in dimensionless parameters and predict the iron concentration profile within the slab during decomposition, the total loss of oxygen from the sample, and the concentration of iron at the gas/wustite interface, as a function of time. The formulation of the problem allows solutions to be obtained for reduction in H₂/H₂O or CO/CO₂ gas mixtures at reaction temperatures between 873 K and 1573 K.     

The Influence of calcia and magnesia in wustite on the kinetics of metallization and iron whisker formation

Calcium oxide andJor magnesium oxide are introduced in different ways to high purity wustite powders, followed by metallization in mixtures of CO, CO₂, and N₂ at 750, 900, and 1100 °C. A new technique is developed to study the nucleation and growth of iron where wustite plates are coated with thin films of CaO of MgO prior to metallization. Calcia, in solid solution with wustite, influences the kinetics of metallization and the morphology of iron. It is reasonably confirmed that nucleation and outward growth of iron, to form iron whiskers, occur at sites higher in CaO concentration on the wustite surface. MgO, on the other hand, has almost no noticeable effect on the iron whisker formation.     

Mathematical modelling of wustite pellet reduction: grain model in comparison with USCM

A mathematical time-dependent and non-isothermal model has been carried out on the basis of the grain model to study the behaviour of a wustite pellet undergoing chemical reactions with reducing gases. The behaviour of wustite pellet reduction analysed by the grain model has been compared with unreacted shrinking core model (USCM). The results show, unlike the grain model, USCM cannot properly predict the impact of gas mixture parameters and pellet characteristics on the local reduction degree. The results also display that when the grain diameter, temperature and tortuosity increase, the diffusivity resistance in the pellet increases which causes more heterogeneous reduction. However, an increment in porosity causes gases to easily diffuse in the pellet and as a result a less heterogeneous reduction will occur.