Structural and morphological changes during reduction of hematite to magnetite and wustite in hydrogen rich reduction gases under fluidised bed conditions

Abstract

During the reduction of iron ore fines structural changes in particles have a significant influence on the rate of reduction. Investigations regarding porosity, specific surface area and mean pore diameters in the reduction of hematite with hydrogen rich reducing gases were performed by mercury porosimetry. Morphological changes were examined by metallographic analyses of polished sections in reflected light. In the magnetite equilibrium phase, significant influence of temperature on structural parameters and sintering effects were found.

For wustite phases, the influence of temperature was less pronounced. For the reduction of hematite to magnetite and magnetite to wustite topochemical phase growth and microporous product layers were observed. In the single step, reduction of hematite to wustite progressive conversion and significant increase in the mean pore diameter were found. The results presented in this work are of high importance for understanding the reaction kinetics of iron ore fines and essential for modelling heterogeneous reactions.     

Investigation of iron oxide reduction by TEM

An “environmental cell” located in a high voltage transmission electron microscope has been used to study the reduction of single crystal iron oxides by hydrogen and hydrogen-argon mixtures. The cell enables a direct observation of the solid during reaction, thus permitting the nucleation and growth of solid reaction products to be observed. Hematite was reduced at temperatures in the range 387 to 610°C with gas pressures up to 5.3 kP. Reduction with pure hydrogen was considerably faster than when argon was present. Lath magnetite which rapidly transforms to porous magnetite and thence (more slowly) to porous iron was observed. The reduction of magnetite and of wustite single crystals was observed in the temperature range 300 to 514°C using both hydrogen and hydrogen-argon mixtures at gas pressures up to 6.6 kP. Incubation periods were found for magnetite reduction; during these periods faceted pits formed in the oxide. Iron formed in the early stages was epitaxial with the host magnetite; at later stages the epitaxy was lost and fissures frequently formed in the metal. The morphology of the iron differed between the gas mixtures. Disproportionation accompanied the reduction of wustite, producing intermediate polycrystalline magnetite despite reducing conditions. The disproportionation appeared to be promoted by the reduction reaction. For both oxides, reduction in the hydrogen-argon mixture was slower than in pure hydrogen.     

Gaseous reduction of iron oxides: Part IV. mathematical analysis of partial internal reduction-diffusion control

In this mathematical analysis of gaseous reduction of iron oxides, the partial internal reduction of the porous oxide and gas diffusion in the porous iron layer are considered simultaneously in deriving the rate equation. The rate equation, derived by partly analytical and partly numerical solutions, is well substantiated by the experimental results obtained previously. The following parameters, determined previously, are used in the application of the rate equation: i) specific rate constant for the gas reaction on the pore walls of wustite, ii) pore surface area of wustite, iii) effective gas diffusivity in the porous wustite formed during reduction of hematite, and iv) effective gas diffusivity in the porous iron layer. The effective depth of the internal reduction zone at the wustite-iron diffuse interface increases steeply with the progress of reduction beyond about 50 pct O removal. For reduction of 1 to 2 cm diam hematite spheroids in 100 pct H₂, the gas composition at the diffuse iron-wustite interface is within 10 to 20 pct of that for the iron-wustite equilibrium; beyond about 50 pct O removal, the rate of reduction is controlled primarily by gas diffusion in the porous iron layer. From the mathematical analysis it is found that the relative depth of internal reduction increases with decreasing particle size and increasing temperature.     

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.     

Reduction of Molten Copper Slags with Mixed CO-CH4-Ar Gas

The reduction of magnetite from slags plays an important role in many metallurgical operations. It has been found that the losses of copper metals generally increase with the increasing magnetite content in the slag. The main objective of the current article was to study the reduction process of iron oxides by mixed CO-CH₄-Ar gas from commercial copper slags. The magnetite content decreased gradually, while fayalite phase increased during the reduction process. The reduction rates of the oxides were found to be of the first-order kinetics. The rate-limiting step was the interfacial mass transfer between the gas and liquid phase which fitted the penetration model. The effects of reduction temperature and injection gas flow on the deoxidation rate are also mentioned in the study.     

Gaseous reduction of iron oxides: Part III. Reduction-oxidation of porous and dense iron oxides and iron

The internal reduction of high-grade granular hematite ore in hydrogen and carbon monoxide, and also the internal oxidation of porous iron granules in CO₂-CO mixtures have been investigated. To assist the interpretation of the rate data for porous iron and iron oxides, rate measurements have been made also with dense wustite, previously grown on iron by oxidation. The iron formed by reduction of dense wustite is porous, similar to that observed when porous hematite is reduced. It is found that the rate of dissociation or formation of water vapor or carbon dioxide on the iron surface is about an order of magnitude greater than that on the surface of wustite. The results of the previous investigations using dense iron and wustite are in general accord with the present findings. The rate of reduction of hematite increases with increasing pore surface area of the reduced oxide. The results indicate that the rate of reduction of granules is controlled primarily by the formation of H₂O or CO₂ on the pore walls of wustite. The specific rate constants evaluated from internal reduction, using the total pore surface area, are about 1/50 to 1/100 of those for dense wustite. These findings indicate that with porous wustite or iron, the effective pore surface area utilized is about 1 to 2 pct of the total pore surface area. The rate of reduction in H₂-CO mixtures is in accord with that derived from the rate constants for reduction in H₂ and CO.     

Behavior of direct reduced iron and hot briquetted iron in the upper blast furnace shaft: Part I. Fundamentals of kinetics and mechanism of oxidation

It is considered that the use of prereduced ferrous materials and sources of metallic iron such as direct reduced iron (DRI) or hot briquetted iron (HBI) improves the productivity of the blast furnace (BF). However, oxidation of DRI/HBI can occur in the upper zone of the BF, which may increase the content of the reducing gases but may not decrease the coke rate substantially. The behavior of DRI and HBI was investigated by measuring the rate of oxidation of the materials in CO₂ gas in a temperature range of 400 °C to 900 °C. In addition, the microstructure of “as-received” and oxidized materials was examined. The iron oxide phases formed due to oxidation were determined using X-ray diffraction (XRD) and a vibrating sample magnetometer. The results of isothermal experiments indicated that the kinetics of oxidation of metallic iron is slow at 400 °C. In DRI samples, the initial rate is controlled by the limited mixed control of chemical kinetics at the iron/iron oxide interface and pore mass transfer, whereas gas diffusion in pores is the rate governing step during the final stages of oxidation. The oxidation of wustite from iron is found to be faster than the oxidation of the former to magnetite. The structure of DRI after oxidation resembled a “reverse topochemical-oxide on the surface metal in the center” structure at 600 °C to 700 °C. The final iron oxide phase formed in DRI after oxidation was magnetite and not hematite. The oxidation of HBI was limited to the surface of the samples at lower temperatures; at 900 °C, moderate oxidation was observed and a topochemical iron oxide layer was formed.     

Rate of the reduction of the iron oxides in red mud by hydrogen and converted gas

The drying and gas reduction of the iron oxides in the red mud of bauxite processing are studied. It is shown that at most 25% of aluminum oxide are fixed by iron oxides in this red mud, and the other 75% are fixed by sodium aluminosilicates. A software package is developed to calculate the gas reduction of iron oxides, including those in mud. Small hematite samples fully transform into magnetite in hydrogen at a temperature below 300°C and a heating rate of 500 K/h, and complete reduction of magnetite to metallic iron takes place below 420°C. The densification of a thin red mud layer weakly affects the character and temperature range of magnetizing calcination, and the rate of reduction to iron decreases approximately twofold and reduction covers a high-temperature range (above 900°C). The substitution of a converted natural gas for hydrogen results in a certain delay in magnetite formation and an increase in the temperature of the end of reaction to 375°C. In the temperature range 450–550°C, the transformation of hematite into magnetite in red mud pellets 1 cm in diameter in a converted natural gas is 30–90 faster than the reduction of hematite to iron in hydrogen. The hematite-magnetite transformation rate in pellets is almost constant in the temperature range under study, and reduction occurs in a diffusion mode. At a temperature of ～500°C, the reaction layer thickness of pellets in a shaft process is calculated to be ～1 m at a converted-gas flow rate of 0.1 m³/(m² s) and ～2.5 m at a flow rate of 0.25 m³/(m² s). The specific capacity of 1 m² of the shaft cross section under these conditions is 240 and 600 t/day, respectively. The use of low-temperature gas reduction processes is promising for the development of an in situ optimum red mud utilization technology.     

Kinetics of Reduction of CaO-FeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>-MgO-PbO-SiO<Subscript>2</Subscript> Slags by CO-CO<Subscript>2</Subscript> Gas Mixtures

Kinetics of the reaction of lead slags (PbO-CaO-SiO₂-FeO _x -MgO) with CO-CO₂ gas mixtures was studied by monitoring the changes in the slag composition when a stream of CO-CO₂ gas mixture was blown on the surface of thin layers of slags (3 to 10 mm) at temperatures in the range of 1453 K to 1593 K (1180 °C to 1320 °C). These measurements were carried out under conditions where mass transfer in the gas phase was not the rate-limiting step and the reduction rates were insensitive to factors affecting mass transfer in the slag phase. The results show simultaneous reduction of PbO and Fe₂O₃ in the slag. The measured specific rate of oxygen removal from the melts varied from about 1 × 10^?6 to 4 × 10^?5 mol O cm^?2 s^?1 and was strongly dependent on the slag chemistry and its oxidation state, partial pressure of CO in the reaction gas mixture, and temperature. The deduced apparent first-order rate constant increased with increasing iron oxide content, oxidation state of the slag, and temperature. The results indicate that under the employed experimental conditions, the rate of formation of CO₂ at the gas-slag interface is likely to be the rate-limiting step.     

Reduction-Carburization of NiO-WO<Subscript>3</Subscript> Under Isothermal Conditions Using H<Subscript>2</Subscript>-CH<Subscript>4</Subscript> Gas Mixture

Ni-W-C ternary carbides were synthesized by simultaneous reduction–carburization of NiO-WO₃ oxide precursors using H₂-CH₄ gas mixtures in the temperature range of 973 to 1273 K. The kinetics of the gas–solid reaction were followed closely by monitoring the mass changes using the thermogravimetric method (TGA). As a thin bed of the precursors were used, each particle was in direct contact with the gas mixture. The results showed that the hydrogen reduction of the oxide mixture was complete before the carburization took place. The nascent particles of the metals formed by reduction could react with the gas mixture with well-defined carbon potential to form a uniform product of Ni-W-C. Consequently, the reaction rate could be conceived as being controlled by the chemical reaction. From the reaction rate, Arrhenius activation energies for reduction and carburization were evaluated. Characterization of the carbides produced was carried out using X-ray diffraction and a scanning electron microscope (SEM) combined with electron dispersion spectroscopy (SEM-EDS) analyses. The grain sizes also were determined. The process parameters, such as the temperature of the reduction–carburization reaction and the composition of the gas mixture, had a strong impact on the carbide composition as well as on the grain size. The results are discussed in light of the reduction kinetics of the oxides and the thermodynamic constraints.     

The effects of impurity elements on the reduction of wustite and magnetite to iron in CO/CO2 and H2/H2O gas mixtures

The reduction of dense wustite and magnetite samples in CO/CO₂ and H₂/H₂O gas mixtures has shown that impurity elements in solid solution in the oxides can significantly affect the reaction mechanisms operative during reduction and the conditions for porous iron growth. In general, the presence of P, Mg, Ti, Si, Ca, K, and Na in wustite favors, in order of increasing strength, the formation of the porous iron product morphology. Aluminum, on the other hand, significantly reduces the range of gas conditions over which the porous iron microstructure may be obtained. S. GEVA, formerly Research Assistant, Department of Mining and Metallurgical Engineering, University of Queensland. D.H. St. JOHN, formerly Senior Lecturer, Department of Mining and Metallurgical Engineering, University of Queensland.     

Estimation of rate parameters for reduction of iron ore–graphite composite pellets in packed bed reactor using genetic algorithm

Abstract

A semiempirical kinetic model has been developed to determine the course of reduction of iron ore–graphite composite pellets over time in a laboratory scale side heated packed bed reactor attached with a tailor made bottom hanging thermogravimetric set-up. The rate parameters in the model, especially the three sets of apparent activation energy values and frequency factors associated with the reduction of iron oxides in three elementary steps, namely hematite to magnetite, magnetite to wustite and wustite to iron, have been estimated based on experimental data by employing an optimisation tool, the genetic algorithm (GA). The difference between the predicted and experimental degree of reduction is minimised to obtain the rate parameters. The experimental degree of reduction is calculated based on mass loss data during reduction and the exit gas analysis. Estimated values of apparent rate parameters were found to be of the same order of magnitude to their intrinsic counterparts reported in literature. Finally, by using the predicted rate parameters the temporal evolution of various oxide phases as well as pure iron has been evaluated.     

Kinetics of iron oxidation with water vapour at temperatures between 1300 and 1450°C

In this paper measurements of the kinetics of iron oxidation in H₂O-containing gas mixtures of various compositions in the temperature range of 1300 to 1450°C, and metallographic examinations are described. The reaction product is solid wustite below and liquid iron saturated iron oxide above 1377°C, this temperature being the incongruent melting temperature of iron saturated wustite. The experiments were carried out by measuring the weight increase during the oxidation of an iron specimen connected to a thermo-balance with a platinum wire. The water vapour was generated by means of a water vapour saturator. The rate law is linear in the beginning of the single experiment and later becomes parabolic. The linear law was interpreted as being caused by two resistances connected in series: the transport of the oxidising gas through the adjacent gas boundary layer, and the phase boundary reaction at the oxide interface. The parabolic law was interpreted as being determined by the transport of iron ions and vacancies through the growing oxide layer. The resistance of gas transport becomes negligible above a certain critical gas velocity which is, for example, 23 cm/s at 1342°C. The temperature-dependent values of the phase boundary reaction rate constant were calculated with the help of known theories from the results of those experiments in which the gas velocities were above the critical value for gas transport at the respective temperatures. The parabolic law did not apply, when the oxidation product was liquid as under these circumstances the formed oxide dropped off the specimen during the experiment and hence, became no thicker than 22 μm. For all the experiments the oxide layer was composed of wustite, even when the oxygen potential of the reaction gas was far higher than that for equilibrium of the gas with wustite and magnetite. The surface structure of the oxide layer and the grain sizes varied with temperature. At lower temperatures the grains were relatively small while at higher temperatures they became extremely large up to a diameter of 6 mm.     

非化学计量对铁氧化物还原热力学平衡的影响

铁氧化物是具有非化学计量比的化合物,非化学计量对铁氧化物的还原过程带来一系列影响.本文采用Dieckmann缺陷模型和Weiss的浮氏体理想固溶体模型分别对非化学计量比的磁铁矿和浮氏体进行了热力学计算.同时根据电荷守恒和物质守恒,对铁氧化物固溶体的综合缺陷度δ与还原失重率和亚铁含量的关系进行了分析,以期对实验终产物的判定提供依据.通过理论分析与计算,最终明确了不同化学计量比的磁铁矿和浮氏体在不同温度下的平衡还原势P_CO(H2),即相应的优势区图.在给定还原势的纯赤铁矿等温还原过程(未有金属Fe生成时),当失重率小于6%时,还原产物属于Fe³⁺占优势的磁铁矿区域;当失重率高于6%时,反应进入Fe²⁺占优势的浮氏体区域.     

Structural changes and kinetics in the gaseous reduction of hematite

The mechanism of the gaseous reduction of hematite grains to magnetite was studied. Grav-imetric measurements were carried out for the reduction of Carol Lake hematite pellets and grains in CO-CO₂ atmospheres over the temperature range 500 to 1100°C. The pore size distribution in the reduced magnetite was measured by mercury porosimetry. Partially reduced grains were examined by optical microscopy. At temperatures below 800°C, the reduction of a hematite grain to magnetite occurred at a well-defined shrinking-core inter-face. The average pore size in magnetite formed at 600°C was found to be 0.03 μm. An es-timate of the rate of CO diffusion through pores of this size indicated that the reaction rate at 600°C was controlled by a step near the hematite-magnetite interface. At temperatures above 800°C, the reaction mechanism became altered due to the preferential growth of magnetite along a single direction in each hematite grain. The reduction rate decreased with an increase in temperature, and no microporosity was present in magnetite formed at 1000°C and above. It was postulated that the reaction rate was controlled by the rate of formation of fresh nuclei and by their rate of subsequent growth. Formerly Professor of Applied Metallurgy, Imperial College     

Effect of Firing Temperature and Reducing Gas Composition during Low-Temperature Reduction of Nanocrystalline Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>

Pure nanocrystalline hematite (40 to 100 nm) compacts were prepared and sintered at various temperatures (300 °C to 600 °C) and then reduced with 100 pct H₂ at 500 °C. On the other hand, fired compacts at 500 °C were reduced with a H₂-Ar gas mixture containing different concentration of hydrogen (100, 75, 50, and 25 pct) at 500 °C using thermogravimetric techniques. Nanocrystalline Fe₂O₃ compacts were characterized before and after reduction with X-ray diffraction, scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and reflected light microscope. It was found that the fired compacts at 400 °C to 600 °C have relatively faster reaction behaviors compared to that at lower firing temperature 300 °C. By decreasing the firing temperature to 300 °C, partial sintering with grain growth was observed clearly during reduction. Also, it was found that the reduction rate increased with increasing hydrogen content in the reducing gas. Comparatively, grain growth and partial coalescence took place during reduction with 25 pct H₂ due to long reaction time.

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Oxidation of Low-Carbon Steel in 17H<Subscript>2</Subscript>O-N<Subscript>2</Subscript> at 900 °C

The oxidation kinetics of two low-carbon steels in a flowing 17H₂O-N₂ gas mixture at 900 °C and the scale structures developed are examined. Similar linear and parabolic oxidation kinetics are observed for the two steels, although some differences are observed within the first 5 minutes of oxidation and in the linear-to-parabolic transition period. The oxidation behaviors observed in the linear kinetics stage are more consistent with published results, exhibiting typical surface-reaction-controlled patterns. However, the observed parabolic oxidation rates are two orders of magnitude smaller than those of iron and steel oxidation in air and oxygen as well as that predicted using Wagner’s parabolic oxidation theory. Similar oxide scale structures are observed on the two steels for the samples oxidized for more than 15 minutes. The surfaces of the scales exhibit pyramidal, faceted grain structures with growth ledges developed on some crystal faces and growth pits at the peaks of the pyramidal grains. In their cross sections, the scales have a columnar structure and appear two layered, with a thin, outer magnetite layer and an inner, growing wustite layer. The wustite grains coarsen with increased oxidation time and develop a growth texture with preferred (111) and (110) orientations in parallel to the sample surface after oxidation for longer than 60 minutes. Conventional oxidation theories cannot provide a satisfactory explanation of the apparently conflicting results observed during the parabolic oxidation stage.     

Gaseous reduction of iron oxides: Part I. Reduction of hematite in hydrogen

The reduction of high-grade hematite ore in hydrogen has been investigated. There is an unusual temperature effect for small granules with a dip in the rate at about 700°C, similar to those reported by previous investigators for different types of iron oxides. The particlesize effect on the time of reduction suggests that there are three major limiting rate-controlling processes: i) uniform internal reduction, ii) limiting mixed control and iii) gas diffusion in porous iron layer. Processes (ii) and (iii) are special cases of a so-called topochemical mode of reduction associated with the formation of product layers. Unidirectional reduction experiments revealed the significant role played by gas diffusion in porous iron layer as a rate-controlling process. The effective H₂-H₂O diffusivity in porous iron derived from, the reduction data is found to decrease markedly with decreasing reduction temperature. This is consistent with the fracture surfaces of porous iron as viewed by scanning electron microscopy. The present interpretation of the rate of reduction of hematite ore is found to apply equally well to previously published data on the hydrogen-reduction of natural and synthetic hematite pellets.     

Kinetics of iron oxidation with CO2 between 1300 and 1450 °C

The paper describes measurements of the kinetics of iron oxidation in CO₂-CO mixtures of various compositions in the temperature range of 1300 to 1450 °C. The reaction product is solid wustite below and liquid iron saturated iron-oxygen melt above 1377 °C, the incongruent melting temperature of iron saturated wustite. The experiments were carried out by measuring the weight increase during the oxidation of an iron specimen connected to a thermo-balance with a platinum wire. The specimen had the shape of a flat plate, and the oxidising gas flowed past this plate with a defined velocity. Among the experiments the flow velocities were varied. The rate law is linear in the beginning of the single experiment and later becomes parabolic. The linear law was interpreted as caused by two resistances connected in series: the transport of the oxidising gas through the adjacent gas boundary layer, and the phase boundary reaction at the oxide interface. The parabolic law was interpreted as determined by the transport of iron ions and vacancies through the growing oxide layer. The resistance of gas transport becomes negligible above a certain critical gas velocity which is 24 cm/s at 1400 °C. The critical gas velocity increases with rising temperature. The gas transport resistance was described by known theories of mass transfer in front of a solid wall. The temperature-dependent values of the phase boundary reaction rate constant were calculated with the help of known theories from the results of those experiments where the gas velocities were above the critical value for gas transport. From the parabolic law the diffusion coefficient of vacancies in wustite was calculated. The parabolic law does not appear, when the oxidation product is liquid as under these circumstances the formed oxide drops off the specimen during the experiment and hence, becomes not thicker than 22 μm. All the results join well with older results for lower temperatures known from literature.     

Effect of magnetic field on reduction of magnetite

The reduction behaviour of magnetite oxide by hydrogen below Curie temperature was investigated in the presence of external magnetic field by thermogravimetric analysis. The reduction rate of magnetite powder increased with increasing external magnetic field strength below Curie temperature of magnetite. In order to figure out the effect of external magnetic field on reduction of magnetite, two types of magnetite, powder and pellet, were studied. It was possible to enhance the reduction rate of magnetite powder, because the particles of magnetite in the presence of an external magnetic field exposed more surface to the reducing gas. The effects of reduction temperature, reducing agent, iron oxide type, particle size and specimen shape on the metallisation behaviour of magnetite were visually clarified below the Curie temperature under the influence of external magnetic field. Despite of the increase in reduction degree by applied magnetic field, the rate controlling step was not changed due to the formation of porous metallic iron layer that keeps the path for reducing agent to approach the unreduced iron oxide in the core of magnetite particle.