Reduction Behaviour of Wüstite Doped with MgO

Compacts made from pure wüstite and compacts doped with 2% MgO were annealed at 1000°C for 3 hrs in 50%CO‐CO₂ gas mixtures. The annealed samples were isothermally reduced at 800‐1100°C in H₂ gas. Selected samples were isothermally reduced at 1000°C with pure CO and 50%H₂‐CO gas mixture to investigate the effect of gas composition on the reduction processes. The oxygen weight loss resulting from the reduction of the samples was recorded as a function of time. X‐ray diffraction (XRD), scanning electron microscopy (SEM), optical microscopy and porosity measurements were used to characterize the annealed and reduced samples. Magnesio‐wüstite (MgO·FeO) phase was formed during the annealing of MgO doped wüstite. The MgO·FeO in turn decreased the porosity of the annealed doped samples compared to pure wüstite compacts. The influence of temperature, gas composition and MgO content on the reduction behaviour and the morphology of the annealed samples was investigated. The values of the apparent activation energy were calculated from Arrhenius plots and correlated with the reduction mechanism. The reduction rate increased with reaction temperature. In doped compacts, the MgO·FeO phase was not completely reduced both at lower reduction temperature (800°C) and during reduction with pure CO. From the activation energy values, the initial reaction stage was controlled by the combined effect of chemical reaction and gas diffusion while solid state diffusion controlled the final stage of reduction. Morphologically, metallic iron was formed in different shape structures under the effect of MgO addition and reduction conditions.     

Effect of Ca(II)‐ion in Wüstite on its Reduction to Iron

Fired compacts of hematite doped with different contents of CaO (mass fractions were 3.5, 4.0, 4.5, 5.0 %) were isothermally reduced to wüstite in a CO‐CO₂ gas mixture at 1173K. It was found from X‐ray diffraction investigation that the lattice parameters of wüstite increased with CaO content dissolved in wüstite crystal. The pure wüstite as well as the wüstite doped with CaO were reduced at different temperature. The results showed that the reductions were promoted with increasing CaO content. The higher the content of CaO dissolved in wüstite crystal was, the larger the lattice parameter and the interplanar distances of wüstite became. This expansion was helpful to the migration of Fe ions and enhanced the reduction. At the early stage, the reduction of pure and doped wüstite compacts was controlled by the interfacial chemical reaction step. At the latter stage, the gaseous diffusion was the rate‐determining step for both sample types.     

Effect of basicity on wüstite sinter reducibility under simulated blast furnace conditions

Abstract

In this study, synthetic sinters with different basicity (CaO/SiO₂?=?0, 0·5 and 2·0) were prepared at 1300°C and prereduced at 900°C using low potential reducing gas (LPRG; 20%CO, 20%CO₂, 5%H₂ and 55%N₂). The prereduced sinters were subsequently reduced to metallic iron at 950–1100°C using relatively high potential reducing gas (HPRG; 30%CO, 5%CO₂, 10%H₂ and 55%N₂). Both LPRG and HPRG were selected to simulate the gas composition in the blast furnace upper and lower shaft respectively. High pressure mercury porosimeter, X-ray phase analysis, optical and scanning electron microscope were used for the analysis of the prepared and reduced sinters. In the original basic sinter, calcium ferrite (CaFe₂O₄) and dicalcium silicate (Ca₂SiO₄) phases were identified as well as the main Fe₂O₃ phase, whereas wollastonite [Ca_2·87Fe_0·13(SiO₃)₃] and silica (SiO₂) were formed in the acidic sinter. The prereduction in sinters with LPRG at 900°C resulted in the formation of wüstite (Fe_0·902O) phase. The subsequent reduction in wüstite sinters to metallic iron using HPRG at 950–1100°C was found to be the highest for basic sinter and the least for acidic sinter. The higher reduction rate of basic sinter was attributed to the enhancement of wüstite reducibility through the formation of calcium ferrites. The lower reduction rate of wüstite in acidic sinter was attributed to the formation of hard reducible fayalite (Fe₂SiO₄) and ferrobustamite [(Ca_0·5Fe_0·5)SiO₃] phases. The rate controlling mechanism during the reduction process was estimated by the correlation between apparent activation energy calculation and microstructure investigations.     

Formation and decomposition of C4F7 type calcium ferrites in superfluxed magnetite based pellets during oxidation and reduction

Abstract

In the current work the reactions of magnetite based pellets with large additions of calcite (3%CaO) during reduction have been investigated. This made it possible to use both X-ray diffraction (XRD) and scanning electron microscopy (SEM) to detect reaction phases that normally occur in very small amounts. The main binding phase in the pellets after oxidation was (CaO,MgO,FeO)₄(Fe₂O₃)₇, whereas the one commonly reported in the literature is (CaO)(Fe₂O₃)₂. During reduction at 500–700°C severe cracking occurred in these pellets, especially in the calcium ferrite phase. However, the decomposition of this phase began at 600°C, and therefore it is believed that the reason for the cracks is low strength of the phase itself, rather than weakness induced by reduction of the phase. Upon reduction of magnetite into wüstite at 800°C, the calcium began dissolving in the wüstite, and at 900°C porous calciowüstite had formed in the entire sample, except for some remaining magnetite left in the pellet cores.     

Multistep Reduction Kinetics of Fine Iron Ore with Carbon Monoxide in a Micro Fluidized Bed Reaction Analyzer

The reduction kinetics of Brazilian hematite by CO is investigated in a Micro Fluidized Bed Reaction Analyzer (MFBRA) using an analyzing method based on Johnson-Mehl-Avrami (JMA) model at temperatures of 973 K (700 °C), 1023 K (750 °C), 1073 K (800 °C), and 1123 K (850 °C). The solid products at different reduction stages are evaluated by SEM/EDS and XRD technologies. Results indicate that the reduction process is better to be discussed in terms of a parallel reaction model that consists of the reactions of hematite to wüstite and wüstite to iron, rather than a stepwise route. Meanwhile, the controlling mechanism of the reduction process is found to vary with temperature and the degree of conversion. The overall process is controlled by the gas–solid reaction occurring at the iron/wüstite interface in the initial stages, and then is limited by the nucleation of wüstite, and finally shifts to diffusion control. Moreover, the reactions of hematite to wüstite and wüstite to iron take place simultaneously but with different time dependences, and the apparent activation energies of hematite to wüstite and wüstite to iron are determined as 83.61 and 80.40 KJ/mol, respectively.     

The diffusivity of sulfur in wüstite and its solubility in wüstite and γ iron in equilibrium with each other and a liquid oxysulfide

The solubility of sulfur in wüstite in equilibrium with γ iron and liquid oxysulfide was found to be 0.011 wt pct at 1050°C. The sulfur solubility in γ iron in equilibrium with wüstite and liquid oxysulfide was also determined at 1050°, 1150°, and 1250°C and found to be 135, 165, and 160 ppm respectively. These values are considerably lower than the sulfur-solubility in y iron in the binary Fe-S system saturated with pyrrhotite. The diffusivity of sulfur in wüstite was determined by oxidizing Fe-S alloys in mixtures of CO and CO₂, and analyzing the entire sample for sulfur afterwards. From the amount of sulfur diffused through the growing wüstite layer into the gas phase, the diffusivity of sulfur in wüstite was evaluated, and found to be 4.1 × 10⁻⁸ and 9.6 × 10⁻⁷ cm²/s at 1050° and 1250°C respectively. These values are of the same order as the self-diffusivity of iron in wüstite in equilibrium with iron at the same temperatures.     

Formation of a Network Structure in the Gaseous Reduction of Magnetite Doped with Alumina

Reduction of un-doped magnetite is developed topochemically with the formation of a dense iron shell. However, the reduction of alumina-doped magnetite to wüstite proceeds with the formation of a network-like structure which consists of criss-crossed horizontal and vertical plates of wüstite. Reduction of magnetite includes the conversion of Fe³⁺ to Fe²⁺ and the movement of iron cations from the tetrahedral sites on the {400} and {220} planes of magnetite to the octahedral sites on the {200} planes of wüstite. Alumina has a negligibly small solubility in wüstite. In the reduction of magnetite doped with Al₂O₃, rejected Al³⁺ cations from wüstite diffuse to the magnetite–hercynite solid solution. Enrichment of the Fe₃O₄–FeAl₂O₄ solution with alumina in the vicinity of the reduction interface restricts the growth of {220} planes of wüstite and nucleation of {220} planes adjusted to the existing planes, preventing the merging of wüstite plates during the reduction process. Reduction of magnetite from the magnetite–hercynite solid solution practically stops when the Al³⁺ content at the interface approaches the solubility limit. Wüstite in the separated plates is reduced further to iron.     

Blast Furnace Operating Conditions Manipulation for Reducing Coke Consumption and CO2 Emission

A comparative reduction behavior of wüstite samples prepared from iron ore sinter was investigated to find the optimum way for reducing coke consumption and CO₂ emission in blast furnace technology. A series of wüstite reduction experiments was carried out using different gas mixtures. A correlation between the experimental results and real data of blast furnaces at Egyptian Iron and Steel Company (EISCO) was demonstrated in order to optimize the coke consumption inside blast furnaces. Different theoretical models were applied on real data of blast furnaces to calculate the effect of operation parameters on the coke consumption. It was found that the wüstite reducibility can be controlled and enhanced using certain ratio of H₂ and CO gases. Such kind of enhancement decreases the remaining quantity of unreduced wüstite which descends to the high temperature region of blast furnace. The theoretical analysis of real data using certain values of H₂ and CO shows that increasing the amount of natural gas injection in blast furnace of EISCO will decrease the degree of direct reduction of iron oxide and consequently will decrease the amount of coke consumption.     

Mineralogical evaluationof ladle slags at voestalpine Stahl GmbH

Abstract

A large number of steel grades are produced at voestalpine Stahl GmbH. The ladle slag must meet different requirements according to the specific steel grade. The mineralogical structure of ladle slag is highly dependent on its basicity. At a basicity of <0·8, spinels occur, and, at higher values, periclase or (Mg,Fe)-wüstite occurs, depending on the degree of slag deoxidation. At basicities between 0·6 and 1·6, calcium aluminates C₁₂A₁₇ and C₃A (C=CaO; A=Al₂O₃) are predominant. Calcium silicates C₂S and C₃S (S=SiO₂) occur throughout the basicity range, and CaO _free occurs at basicities >1·2, both depending on the SiO content. This enables the metallurgist to specifically adjust the ladle slag according to the steel grade requirements. Knowledge of the slag condition is essential in order to achieve certain metallurgical results, such as desulphurisation or degree of oxide purity.     

The mechanism of whisker growth in the reduction of wüstite

The growth of iron whiskers on wüstite is explained on the basis of C. Wagner’s mechanism for the reduction of nonstoichiometric oxides. The first metal nucleus develops into a whisker if the iron accumulation in the supersaturated wustite particle is nearly uniform, a condition which is favored by chemical control. A mathematical model is proposed in which the “filling ratio” of wüstite by iron prior to nucleation is expressed in terms of the reaction constant, the chemical diffusion coefficient of iron, the particle radius, the Fe/o ratio at equilibrium with the gas, and the critical Fe/o ratio for nucleation at the most favorable site. The morphology of iron in the early stages of reduction, which goes from a single cylindrical whisker, through sponge iron to a smooth layer, can be predicted by comparing at all times the map of the actual Fe/o ratio at the surface with the map of the local Fe/O ratio for nucleation. A satisfactory test of the theory is obtained through a survey of the experimental evidence available about the effect of several physico-chemical factors of reduction on the iron morphology,e.g. surface characteristics, particle size, composition of the reducing gas, solute cations in wüstite, temperature, and gas transport. Formerly with the Ecole Centrale des Arts et Manufactures, Chătenay-Malabry     

Reduction of iron oxides from liquid slags using a graphite rotating disc

Research work has been carried out on the reduction of FeO from liquid slags of the CaO‐FeO‐SiO₂ ternary system using a graphite rotating disc technique. The investigations were conducted on slags with a basicity of CaO/SiO₂ = 1.27 and FeO contents of 20 and 60%, at temperatures of 1350 and 1420°C. The calculated viscosity range for these slags is within 2.53 – 0.43 dPa·s. It has been found that the factor controlling the reduction process is diffusion of FeO towards the disc surface, both in the case of the reduction from the slag with 20% FeO and in the case of the reduction from the slag with 60% FeO fraction. The diffusion coefficient of FeO at the reduction temperature of 1350°C is of the order of magnitudes of 10^?7 cm²/s, while at 1420°C it reaches the order of 10^?6 cm²/s. The calculated thickness values for the limiting diffusion layers range from 8.54·10^?3 to 0.70·10^?3 cm. It has been found that with increasing reduction rate also Boudouard's reaction starts to be important to the overall reduction rate. The limiting reduction rate at which Boudouard's reaction starts to be important to the entire process is dependent on temperature, being approximately 10.0·10^?6 mol FeO/cm² s at 1350°C, and approximately 15.0·10^?6 mol FeO/cm² s at 1420°C.     

Untersuchung zur Gleichgewichtseinstellung der festen Phasen im System CaO–2CaO · SiO2–3CaO · P2O5

Bedeutung der Kenntnis der Phasenbeziehungen im Bereich CaO–2CaO · SiO_2–3CaO · P₂O₅ des Systems CaO–P₂O₅–SiO₂ für die Weiterverarbeitung von Eisenhüttenschlacken. Untersuchung des Einflusses des 3 CaO · P₂O₅ auf die Beständigkeit von 3 CaO · SiO₂ mit Hochtemperatur-Röntgenaufnahmen. Angabe der Phasengleichgewichte im System CaO–2 CaO · SiO_2–3 CaO · P₂O₅ für den festen Zustand unterhalb 1450 °C. Erklärung der Primärausscheidung von Silikophosphaten aus technischen Schlackenschmelzen.     

Reduction swelling behaviour of haematite/magnetite agglomerates with addition of MgO and CaO

Abstract

The influence of three kinds of CaO and MgO additives (dolomite, burnt lime and serpentine) on the reduction swelling behaviour of haematite–magnetite (H–M) concentrates pellets was studied. Burnt lime and dolomite increased the reduction swelling index of H–M oxidised pellets, while the reduction swelling index was able to be reduced when serpentine was added. CaO accelerated the formation and growth of metallic iron whiskers and led to abnormal swelling of the magnetite briquettes, while MgO was able to be dissolved in wüstite and reduced the migration rate of Fe²⁺; therefore, there was no catastrophic swelling in either the haematite or magnetite briquettes. As far as H–M concentrate pellets were concerned, because the solubility of CaO in magnetite was greater than that in the primary haematite and the secondary haematite generated from magnetite during the oxidation was easy to be reduced to wüstite, there was abnormal swelling in the reduced H–M pellets with CaO addition.     

Study on refining slags targeting high cleanliness and lower melting temperature inclusions in Al killed steel

Abstract

Three high basicity slags (A, B and C) were used in laboratory to refine Al killed steel to target high oxide cleanliness and low melting temperature inclusions. Inclusions were of CaO–MgO–Al₂O₃–SiO₂ system after 90 min reaction, parts of which were MgO based. Total oxygen were in the range of 0·0007–0·0010 and 0·0005–0·0010% respectively when slag A (CaO/SiO₂, 6–8; Al₂O₃, ～40%) and slag B (CaO/SiO₂, 6–8; Al₂O₃, ～30%) were applied, with inclusions all in spherical shape and mainly <5 μm. Inclusion composition concentrated in or around the lower melting point region (<1500°C) under slag A, while it became more scattered under slag B. Total oxygen varied between 0·0008 and 0·0011% under slag C (CaO/SiO₂, 3–4; Al₂O₃, about 20–25%). Many of the inclusions were in larger size, irregular morphology and located far away from the lower melting point region. Formation of MgO based inclusions closely related to solubility behaviour of MgO in the slag.     

Characterisation studies on swelling behaviour of iron ore pellets

At JSW Steel Limited (JSWSL), pellets form the major part of the iron-bearing feed to corex and blast furnace. JSWSL produces low-basicity pellets ((CaO/SiO₂) – 0.40 to 0.50). The quality of the pellet is affected by the raw material chemistry (gangue content), flux proportion and their subsequent heat treatment to produce the fired pellets. The raw material silica, limestone addition, i.e. basicity – CaO/SiO₂ of pellet decides the mode, temperature and the amount of melt formed. The properties of the pellets are, therefore, largely governed by the form and degree of bonding achieved between ore particles and also by the stability of these bonding phases during the reduction of iron oxides. In the present study, laboratory pelletisation experiments have been carried out to know the effects of silica and basicity on the microstructure and swelling behaviour of pellets during reduction. Phase analysis was carried out using image analyser, and chemical analysis of oxide and slag phases was carried out using SEM–EDS. From the laboratory studies, it was observed that the swelling index of the pellets decreased with an increase in silica content due to the decrease in porosity. The presence of higher silica in pellet hinders the reduction step of haematite to magnetite at lower temperatures. Pellets with basicity range 0 to 0.1 exhibited lower swelling index due to the formation of high melting point fayalite phase and also at this basicity range the structure is held together by the seam-like compounds between Fe₂O₃ and SiO₂ primarily at high silica content. Higher swelling index was observed at the basicity range 0.3 to 0.7 due to the presence of low melting point calcium olivines (1115°C) between fayalite (FeSiO₄) and dicalcium silicate (Ca₂SiO₄). Low melting point slag phase enhances the swelling index of the pellets. Swelling index of the pellets considerably dropped between the basicity range 0.9 to 1.1 due to the formation of calcium ferrite phases with a close pore structure.     

Evaluation of the reduction of iron oxide from liquid slags using a graphite rotating disk

The rotating disk methodology has been used for examination of the reduction of FeO from CaO-FeO-SiO₂ liquid slags (20 and 60 pct FeO) with a CaO/SiO₂ ratio equal to 0.66 and 1.27, in the temperature range 1350 °C to 1420 °C. It has been found that the reduction proceeds under diffusion control. The calculated diffusion coefficients fall in the range 0.76·10⁻⁷ to 1.6·10⁻⁶ cm²/s. Comparison of these values with those given in the literature suggests that the calculated coefficients are related to the diffusion of oxygen ions in the slag. The calculated thickness of the limiting diffusion layer, δ, ranges from 0.65·10⁻³ to 5.25·10⁻³ cm, depending on the reduction conditions. The largest decrease in the limiting diffusion layer thickness takes place at low rotational speeds, i.e., 100 and 400 rev/min. The maximum value of the mass transfer coefficient is 1.71·10⁻³ cm/s for reduction from slag with a CaO/SiO₂ ratio of 1.27, 60 pct FeO, at 1420 °C and 2000 rev/min, and the minimum value is 0.27·10⁻⁴ cm/s for reduction from slag with a CaO/SiO₂ ratio of 0.66, 20 pct FeO, at 1350 °C and 100 rev/min. Good agreement has been found between experimental and calculated reduction rates at low disk rotations (100 and 400 rev/min).     

Verhalten des Sauerstoffs beim Elektro-Schlacke-Umschmelzen

Einflußvon Stahl- und Schlackenzusammensetzung im System Calciumfluorid-Kalk-Tonerde-Kieselsäure[-Eisen(II)-oxid] [CaF₂–CaO–Al₂O₃–SiO₂(–FeO)] auf die Sauerstoffgehalte in elektro-schlacke-umgeschmolzenen Stählen. Aussagen über die Entmischungstendenz des Eisen(II)-oxids in calciumfluoridhaltigen Schlacken unter den Bedingungen des Elektro-Schlacke-Umschmelzverfahrens. Folgerungen über die geschwindigkeitsbestimmenden Reaktionsteilschritte bei der Sauerstoffaufnahme durch Stahl und Schlacke während des Umschmelzens.     

Effect of Compressive stresses on Microstructure of Scales Formed on Pure Iron Under Continuous Air Cooling

 The microstructure development of oxide scale on pure iron under the mutual effect of compressive stress and cooling conditions was investigated. Oxide-scale structure was examined by optical microscopy (OM) and scanning electron microscopy (SEM). It was found that oxide scale formed under normal cooling conditions had a structure mainly consisted of an outer magnetite and an inner wüstite layer. When a compressive stress was applied, numerous magnetite precipitates formed within wüstite layer homogeneously at starting cooling temperature of 900℃, and the wüstite layer in the scale transformed into a mixture of mostly magnetite/iron eutectoid and magnetite layer at starting cooling temperature of 700℃. The wüstite decomposition and precipitation of magnetite in wüstite under compressive stress was discussed.     

Kinetics of the Reaction of CO<Subscript>2</Subscript>/CO Gas Mixtures with Iron Oxide

The kinetics of the oxygen exchange reaction between carbon dioxide and carbon monoxide were measured on iron, wüstite, and magnetite surfaces. This was done through the use of an isotope exchange technique. The measured rate constants are dependent on the oxygen activity. This dependence is expressed by k_a = k_oa_O^−m. The parameter m was found to have values between 0 and 1. It was found that, in the iron region, the apparent rate constant was independent of the oxygen partial pressure (i.e., m = 0) at 1123 K (850 °C) and that it was inversely dependent on the oxygen partial pressure (i.e., m = 1) for the magnetite region at 1123 K (850 °C) and 1268 K (995 °C). In the wüstite region, m was found to be equal to 0.51, 0.66, and 1.0 for the w₁, w₂, and w₃ pseudo phases, respectively, at 1268 K (995 °C). At 1123 K (850 °C), in wüstite, m was found to be equal to 0.59 and to 1.0 for the w₁′ and w₃′ pseudo phases, respectively.