The reduction of iron oxides by volatiles in a rotary hearth furnace process: Part II. The reduction of iron oxide/carbon composites

The reduction of iron oxide/carbon composite pellets with hydrogen at 900 °C to 1000 °C was studied. Compared to hydrogen, the reduction by carbon was negligible at 900 °C and below. However, significant carbon oxidation of the iron oxide/graphite pellets by H₂O generated from the reduction of Fe₂O₃ by H₂ was observed. At higher temperatures, reduction by carbon complicates the overall reduction mechanism, with the iron oxide/graphite composite pellet found to be more reactive than the iron oxide/char composite pellet. From the scanning electron micrographs, partially reduced composite pellets showed a typical topochemical interface with an intermediate region between an oxygen-rich unreacted core and an iron-rich outer shell. To determine the possibility of reduction by volatiles, a layer of iron oxide powders was spread on top of a high volatile containing bituminous coal and heated inside a reactor using infra-red radiation. By separating the individual reactions involved for an iron oxide/coal mixture where a complex set of reactions occur simultaneously, it was possible to determine the sole effect of volatile reduction. It was found that the light reducing gases evolve initially and react with the iron oxide, with complex hydrocarbons evolving at the later stages. The volatiles caused about 20 to 50 pct reduction of the iron oxide.     

Non-Topochemical reduction of iron oxides

Non-topochemical behavior was studied during reduction of porous spheres of hematite by stages through the intermediate oxides and also continuously to iron by CO/CO₂ mixtures at temperatures of 600 to 900°C (873 to 1173 K). The behavior became more nearly topochemical as temperature increased. Shrinking occurred during the reduction of hematite to magnetite and of magnetite to wüstite, whereas swelling was observed during the reduction of wiistite to iron. Shrinking was greater, and swelling less, at higher temperatures. The total surface area of the solid decreased with increasing extent of reduction during each of the three stages. A non-topochemical model was developed which satisfies, better than previously proposed models, the reduction data for the single reactions and the three reactions occurring simultaneously. The model provides for variation in particle size and local changes in porosity and effective diffusivity. An empirical “sintering exponent” was introduced to describe changes in reacting surface area.     

Reduction of iron and chromium from oxides by carbon

The reaction of iron and chromium oxides with carbon is considered. The decomposition of carbon monoxide is an important source of carbon at the surface of the iron oxides. In the reaction of Cr₂O₃ with carbon, the first stage includes dissociation of the oxides, with the liberation of atomic and molecular oxygen, and the formation of carbon oxides. Rapid reduction of chromium is ensured by the reaction of Cr₂O₃ with C₃O₂ and elementary carbon, which is produced from oxides.     

The kinetics of reduction of iron oxides at moderate temperatures

The rates of reduction of wustite with hydrogen were measured in the temperature range 511 to 690 K. The wustite specimens were prepared by oxidizing thin iron foils, 50μm thick. These were reduced in a Cahn electrobalance under isothermal conditions and the weight loss sustained was continuously monitored. The effect of the phase transformation of wustite on the reduction kinetics was investigated. By using thin oxide specimens and a rapid flow of reducing gas, the respective influences of pore diffusion and “film” mass transfer were rendered insignificant. The reduction of the oxides, in the temperature range investigated, appears to be chemically controlled. The fractional reduction (α)time(t) plots are sigmoid shaped and exhibit three distinct features: incubation, acceleratory and decaying periods. The rate constantk _s was evaluated from the middle region of the α-t plots. Its temperature dependence may be expressed by: $$\bar k_s = 0.263( \pm 0.047)\exp \left[ {\frac{{ - 71,550 \pm 900}}{{RT}}} \right]g atom O/cm^2 s atm$$ g· atom o/cm² · s · atm. The experimental data was interpreted by means of a nucleation and growth model.     

System analysis of the reduction of iron oxides

The parameters of the equilibria that take place during the reduction of iron oxides in the water gas medium (CO-CO₂-H₂-H₂O) in the presence and absence of carbon are calculated. The possibilities of the diagram technique of representing results are discussed. The following two spatial diagrams are plotted: one of them reflects a set of three-phase equilibrium surfaces, and the curved surface in the other diagram consists of a set of six three-phase equilibrium regions separated by four-phase equilibrium lines. The last diagram has a single point that determines the parameters of invariant five-phase equilibrium. A method is proposed to graphically determine possible water gas compositions in equilibrium with the following mixtures: Fe₃O₄-C, Fe₃O₄-FeO-C, FeO-C, FeO-Fe(C)-C, Fe(C)-C, and Fe₃O₄-Fe(C)-C.     

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.     

等离子体氢还原铁氧化物的研究进展

等离子体氢因其化学活性的优势,迅速成为氢冶金领域的研究热点。目前等离子体氢还原铁氧化物的基础理论研究仍不完善,主要缺少对其微观机理、杂质元素迁移规律等方面的研究。通过对等离子体氢还原铁氧化物的相关研究进行回顾,从热等离子体氢的还原入手,再到冷等离子体氢的还原,对国内外等离子体氢还原铁氧化物的研究方法进行概括,并对等离子体的还原机理和应用进展进行详细分析。结果表明,与氢气还原铁氧化物相比,热等离子体氢和冷等离子体氢在还原过程中更具热力学和动力学优势,并且由于体系中有较高浓度的振动激发分子氢,冷等离子体氢被认为拥有更大的发展潜力。研究结果为后续学者的试验设计提供参考,并为其研究方向提供建议。     

On the mechanism of iron oxide reduction by carbon

A theoretical analysis has been made to determine the conditions under which the reduction of iron oxide by carbon takes place according to the 2 step mechanism involving the Boudouard reaction. This is based on the concept of minimum temperature of reduction (T _min) below which the Boudouard reaction does not affect the reduction process. The effect of variables such as carbon reactivity, total pressure and so forth onT _min has been studied. TheT _min can be used to determine if metallization is possible under a given set of conditions.     

Solid structural changes during the reduction of iron oxides

Abstract

The changes in the solid structure, i.e., grain size, shape and pore structure, during the reduction. of reagent-grade hematite to iron by CO?CO₂ mixtures at temperatures between 725 and 925°C were investigated. The reduction of hematite to wustite was found to produce little structural change. In contrast, three mechanisms of structural change occurred during the reduction of wustite to iron. These included the thermal sintering of the wustite, the growth of fibrous iron filaments from the partially reduced wustite grains, and the sintering or “coarsening” of the reduced iron. A quantitative model for the sintering of the wustite is described.

The effects of the structural changes on the reduction kinetics are reviewed. The sintering of the wustite reduced the over-all rate of reduction; the growth of the fibrous iron increased the rate of reduction. The sintering of the iron had little effect on the kinetics. The interaction between the sintering of the wustite and the growth of the fibrous irons, which change the character of the reduction from nontopochemical to topochemical, is also discussed.

Résumé

Les variations de la structure solide, i.e., la grosseur du grain, la forme et la porosité,ont été étudiées au moment de la réduction d'hématite en fer, dans des mélanges de CO?CO₂ aux températures comprises entre 725 et 925°C. La réduction de l'hématite à la wustite s'accompagne d'un léger changement de structure. Au contraire, trois mécanismes de changements structuraux se sont présentés au cours de la réduction de la wustite en fer. Ce sont: le frittage thermique de la wustite, la croissance des filaments fibreux de fer provenant des grains de wustite partiellement réduits et aussi le frittage du fer réduit. Un modéle quantitatif décrit Ie frittage de la wustite.

Les effets des variations structurales sur la cinétique de réduction sont revisés. Le frittage de la wustite a diminué le taux de réduction, alors que la croissancé du fer fibreux a augmente ce taux. Le frittage du fer a un effet minime sur la cinétique. L'interaction entre le frittage de la wustite et la croissance des fibres de fer fait varier Ie caractère de la réduction, de non-topochimique à topochimique.     

Reduction of iron oxides by wet gas in the presence of carbon

The equilibrium parameters in the reduction of iron oxides by wet gas (CO + CO₂ + H₂ + H₂O) in the presence of carbon are calculated. Graphical representation of the results is discussed, and a diagram is plotted consisting of a set of three-phase equilibrium surfaces and four-phase equilibrium curves, whose point of intersection determines the parameters of invariant five-phase equilibrium. The composition of the wet gas in equilibrium with the following mixtures is established: Fe₃O₄-C, Fe₃O₄-FeO-C, FeO-C, FeO-Fe(C)-C, Fe(C)-C, and Fe₃O₄-Fe(C)-C. A method of graphical determination of the possible wet gas compositions in equilibrium with these mixtures is proposed.     

Diffusion of H2-H2O through porous iron formed by the reduction of iron oxides

Measurements were made of the rate of equimolar counterdiffusion of hydrogen and water vapor through porous iron formed by the reduction of dense hematite, magnetite, and commercial iron ore pellets with hydrogen. The experiments were conducted at temperatures between 400° and 1000°C and at pressures between 0.1 and 40 atm. It is demonstrated that the structure of the porous iron is primarily a function of reduction temperature and that the diffusion process at the higher reduction temperatures is normal. The effect of gaseous diffusion on the rate of reduction of dense hematite with hydrogen is discussed. It is shown that gaseous diffusion limits the rate at the higher temperatures and pressures.     

The effect of carbon content on the rate of reduction of FeO in slag relevant to iron smelting

In the iron smelting, or bath smelting, process the tapped metal contains high amounts of sulfur and the slag contains high amounts of FeO, relative to blast furnace slag. After tapping, the FeO can be further reduced by carbon in the metal, which will also lead to better desulfurization. Although there have been many studies of the reaction of carbon in iron with FeO in slag, discrepancies exist with regards to the effect of carbon in iron on the rate of FeO reduction in slag, which is the subject of this study. Experiments were conducted at 1723 K, using a slag with basicity close to one with an FeO mass content of 5 %. The rate of reduction was measured using a pressure increase technique. For moderate and high sulfur contents, as in the case of iron smelting, the rate is primarily controlled by the dissociation of CO₂ on the surface of the molten iron. Furthermore, if the effect of carbon on sulfur is taken into account, for the range of carbon mass contents of 2 to 4.5 %, there is no effect of the carbon level on the rate of FeO reduction. At low sulfur contents it was found that there is considerable slag foaming, which inhibits mass transfer of FeO in the slag, and significantly reduces the rate. Even when there is no slag foaming at low sulfur contents, mass transfer of FeO in the slag can influence the rate of FeO reduction.     

The reduction of iron oxides by volatiles in a rotary hearth furnace process: Part I. The role and kinetics of volatile reduction

With iron ore reduction processes using coal-ore pellets or mixtures, it is possible that volatiles can contribute to reduction. By simulating the constituents of the individual reducing species in the volatiles, the rates for H₂ and CO were investigated in the temperature and reduction range of interest; hydrogen is the major reductant and was studied in detail. The kinetics of the reduction by H₂ has been found to be a complex mechanism with, initially, nucleation and growth controlling the rate. There is a catalytic effect by the existing iron nuclei, followed by a mixed control of chemical kinetics and pore diffusion. This results in a topochemical reduction of these iron oxide particles. Up to 1173 K, reduction by H₂ is considerably faster than by carbon in the pellet/mixture or by CO. It was also found that H₂S, which is involved with the volatiles, does not affect the rate at the reduction range of interest.     

Reaction mechanism on the smelting reduction of iron ore by solid carbon

The kinetics of the smelting reduction of iron ore by a graphite crucible and carbon-saturated molten iron was investigated between 1400 °C and 1550 °C, and its reaction phenomena were continuously observed in situ by X-ray fluoroscopy. In the smelting reduction by graphite, it was shown from the observation results that the smelting reduction reaction proceeded by the following two stages: an initial quiet reduction without foaming (stage I) and a following highly active reduction with severe foaming (stage II). At 1500 °C, by the graphite crucible, the reduction rate of iron ore was found to be 8.88×10⁻⁵ mol/cm² · s, and by the molten iron, 8.25×10⁻⁵ mol/cm²·s. The activation energies for the reduction by the graphite crucible and the molten iron were 24.1 and 22.9 kcal/mol, respectively. Based on the results of kinetic research and X-ray fluoroscopic observations, it can be concluded that these two types of smelting reduction reactions of iron ore by the graphite crucible and by the molten iron are essentially the same.     

A study on the kinetics of iron oxide reduction by solid carbon

Rate of reduction of ferric oxide in the presence of solid carbon was measured in the laboratory using a thermogravimetry setup. Iron oxide in the form of powder and micropellets were used. Coconut char of high reactivity was employed as carbonaceous material. Product gas analysis was carried out to calculate the rate of carbon loss during reduction. Ferric oxide reduction was found to take place in a stage-wise manner. For the powder system, the overall reaction was found to be exclusively controlled by the gasification process. Gasification rates of coconut char in carbon dioxide were utilized to predict the rates of carbon loss during reduction. The predicted and experimental rates of carbon loss during reduction of ferric oxide by carbon were compared and possible explanations were given for the observed trends.