Hematite Pellets Research Articles

Kinetic Analysis of the Iron Oxide Reduction Using Hydrogen-Carbon Monoxide Mixtures as Reducing Agent

The kinetics of the reduction of hematite pellets using hydrogen-carbon monoxide mixtures as reducing agent was described by using the grain model. This model involves the particle size and the porosity of the pellet asmain structural parameters which affect directly the kinetics of the hematite pellets during the reduction process. The predictions of the model were compared with the experimental results. Fired hematite pellets were reduced at 850°C using hydrogen, carbon monoxide and Midrex gas. The weight loss technique was used to follow the reduction process. The reduction of iron oxide pellets using hydrogen or carbon monoxide is a mixed controlled system, where chemical reaction and internal gas diffusion are competing processes during the first stage of the reduction, while internal gas diffusion becomes controlling step at the last stage of the process. The reduction of iron oxide pellets using Midrex gas is a mixed controlled system throughout the whole reduction process.

A FIRST PRINCIPLE SIMULATOR OF AN IRON OXIDE PELLET INDURATION FURNACE – AN APPLICATION TO OPTIMAL TUNING

First principle models are useful tools to increase the comprehension of complex processes such as the hematite pellet induration furnace that is simulated in this study. The first principle model of the furnace integrates heat transfer phenomena between the solids and the gas, mass transfer phenomena due to the pellet drying, coke combustion, carbonate decomposition, magnetite oxidation, gas flow phenomena in the pellet bed and piping system as well as pellet sintering mechanisms. A steady state simulator was constructed that generates gas and pellet temperatures in the pellet bed and gas flow rates and pressures in the different zones of the furnace. The simulator also calculates the statistical distribution of the sintered pellet mechanical properties. The simulator capabilities are illustrated by several optimization tests using different performance criteria and constraints. These simulation tests help to generate furnace operating guidelines to select air flow and fuel distributions inside the furnace as well as the grate speed and the bed height.Les modèles phénoménologiques sont des outils puissants pour la compréhension des procédés à comportements complexes comme le four de frittage de boulettes d’hématite qui fait l’objet de cette étude de simulation. Le modèle décrit les phénomènes de transfert de chaleur entre le gaz et les solides, les phénomènes de transfert de masse se produisant dans les réactions chimiques comme le séchage, la combustion du coke, la calcination des fondants, l’oxydation de la magnétite, les phénomènes d’écoulement des gaz dans les tuyauteries et à travers le lit de boulettes, et enfin les phénomènes de frittage. Un simulateur est construit permettant de simuler les régimes stationnaires de fonctionnement du four et donc de générer les températures et pression du gaz ainsi que les températures des boulettes en tout point du lit de boulettes. Le simulateur génère aussi la distribution de la résistance en compression des boulettes frittées. L’utilisation du simulateur est illustrée succinctement pour aider à la conduite de l’opération, déterminer les goulots d’étranglement limitant son fonctionnement et rechercher un réglage optimal du four pour différents critères de performance contraints.

Wetting and Penetration Behavior of Calcium Ferrite Melts to Sintered Hematite

Effects of adding SiO 2 or Al 2 O 3 on the wetting and penetration characteristics of calcium ferrite melts, CaO.Fe 2 O 3 (CF) and CaO.2Fe 2 O 3 (CF 2 ), to sintered hematite substrate have been examined. And the wetting and penetration characteristics were discussed from the viewpoints of the relation to the viscosity and surface tension of calcium ferrite melts. The effects of the addition of calcium ferrite slag with SiO 2 or Al 2 O 3 on the strength of sintered hematite pellets were also examined. The slag tablets of CF 2 -based slag neither melt completely nor spread uniformly on the hematite substrate at the experimental temperature at 1 300°C in this study. The wettability of CF-based slag melts on sintered hematite substrate was improved by the addition of SiO 2 or Al 2 O 3 , and the spreading rate was increased with increasing the value of η/γ (viscosity/surface tension of slag melts). The penetration of CF-based slag melts into sintered hematite substrate was depressed by the addition of SiO 2 or Al 2 O 3 , and the penetration depth was increased with increasing the value of η/γ surface tension/ viscosity of slag melts). The sintered hematite pellet with CF-5 mass%Al 2 O 3 slag sintered at 1 200°C had the highest compressive strength in this study. Because the liquidus temperature of CF slag is decreased to about 1 200 °C by adding 5 mass% Al 2 O 3 , and the sufficient amount of liquid phase, which has the appropriate wetting and penetrating characteristics controlled by the surface tension and the viscosity, as a binder was formed.

Influence of Direct Reduction Condition of Hematite Pellets with H2/CO on the Oxidation Behaviour of DRI in Air

Direct reduced iron (DRI) is the product of some commercial direct reduction (DR) of iron ore on base of natural gas. DRI tends to oxidize in air generally above 300 °C and then follows spontaneous combustion. To control the oxidation mechanism, several investigators have used different iron samples and methods. This paper gives the results of experimental work carried out for determination of DRI oxidation. The behaviour of DRI oxidation in air after isothermal reduction of hematite pellets with different size, temperature and H2 / CO mixture is investigated.

H<SUB>2</SUB>-CH<SUB>4</SUB>ガスによるヘマタイトペレットからの炭化鉄の生成

H<SUB>2</SUB>-CH<SUB>4</SUB>ガスによるヘマタイトペレットからの炭化鉄の生成

Aplicación de técnicas cinéticas no isotérmicas en la reducción de tres minerales de hierro de grado comercial

The mechanism of reduction of iron ore agglomerates by both isothermal and nonisothermal TG studies was investigated, and the work was complemented with the structural characterization of the total and partially reacted samples. Three different commercial hematite pellets were studied. The mechanisms of reduction were obtained under isothermal conditions, resulting in a fitting to chemical reaction models. Nonisothermal reduction was carried out using a TGA system (CAHN TG-171) from 600 to 1,000 °C maintaining a lineal heating rate of 4, 7 and 10 °C/min, and the reducing atmospheres used were H2 (100 %) and H2-CO (95:5). The kinetic parameters were evaluated by Coats & Redfern, Dixit & Ray and Prakash & Ray techniques. It was found that the lower the heating rate, the higher the reduction degree and the higher activation energy. The activation energy for reduction with the mixture H2-CO was always higher than that obtained with pure H2.

Autocatalytic Reduction of Hematite with Hydrogen under Conditions of Surface Control: A Vacancy-Based Mechanism

An autocatalytic mechanism is shown to explain the kinetics of the surface reaction occurring during the low-temperature reduction of sintered hematite pellets with hydrogen between 300 and 500°C. The results satisfactorily fit the predictions of a mechanism based on nondissociative chemisorption of hydrogen on the oxygen vacancies at the surface of the hematite pellet. This chemisorption process activates the hydrogen molecule by substantially decreasing its bond energy and increasing its bond length. The calculated activation energy of the surface reaction over this temperature range was found to be much smaller than those reported earlier. The larger values are believed to have been affected by some diffusion control within the solid phase, as a result of the limitations of the weight loss technique which was used in those measurements.

Kinetics and morphological aspects of the dissolution and reduction of hematite pellets in oxygen steelmaking converters

The kinetics and mechanism of hematite pellet dissolution in liquid iron containing carbon and in liquid low carbon steel have been studied both on laboratory and plant scale. Microscopy shows that hematite reduces to magnetite due to heating the pellet in the bath. Carbon from the liquid iron reduces magnetite to wüstite and FeO to Fe ; magnetite melts before complete reduction to wüstite.

Dissolution of Fe2O3 and FeO Pellets in Bath Smelting Slags.

The rate of the dissolution of hematite and wustite pellets in bath smelting type slags was investigated at 1723 K. The rates were measured by the change in their diameter and the rate of gas evolution from the pellet. The diameter of the pellets, as a function of time, was obtained from recorded X-ray radiograph pictures. For unstirred slags it was found that the rate of the dissolution of hematite pellets was faster than that of wustite pellets due to gas evolution from the hematite pellets which increases heat and mass transfer. The rate of the dissolution of wustite and hematite pellets were much faster in slags stirred with argon and, in this case, wustite dissolved faster. The rate of gas evolution during the dissolution of hematite with an inert gas over the slag was also measured by using a mass flowmeter. The rate of gas evolution was also determined under different CO-CO2 gas mixtures by analyzing the off gas with a mass spectrometer. It was found that the amount of oxygen evolved increases with decreasing the CO2/CO ratio. From the experimental results the heat transfer coefficient for the present conditions was estimated. These values were extrapolated to the actual operating conditions of a bath smelter and the melting time and the amount of undissolved pellets at the steady state in the slag were calculated. It was estimated that the pellets melt in less than 10 sec and the fraction of unmelted pellets, in the slag in a typical practice, is less than 1% of the volume of the slag. The rate of production in a bath smelter depends on the amounts of Fe2+ and Fe3+ resulting from the dissolution of the pellets. The present results indicate that for Fe2O3 pellets the rate of iron production will be about 25% less than for FeO.

In Situ measurement of effective gas diffusivity through hematite pellets during stepwise reductions

The Wicke-Kallenbach (W-K) method for effective gas diffusivity measurements could only be used in the vicinity of ambient temperatures. However, in the present study, this technique has been extended to temperatures of about 1273 K through use of a high-temperature cement. This newly developed high-temperature W-K method was applied toin situ measurements of the diffusive and viscous fluxes through hematite pellets during stepwise reductions. When the sam-ple (acid and basic pellets) is reduced, it swells significantly. However, a gas-tight seal between the holder and specimen was successfully maintained by use of a high-temperature cement. This cement, composed mainly of Na2O (20 mass pct) and SiO2, separates into a solid and liquid phase at elevated temperatures. Thus, it can move in the same direction as the expansion of sample and thereby maintain the gas-tight seal. With this new technique, the structural param-eters of gases based on the “dusty gas model” for D’Arcy’s flow, Knudsen diffusion, and mo-lecular diffusion were obtained. These parameters were compared with those estimated through pore structure models.

Effect of coexistence of potassium and sulphur on abnormal swelling during reduction of hematite pellets.

In order to elucidate the effect of coexistence of potassium and sulphur on abnormal swelling during reduction of iron oxides, hematite pellets containing potassium carbonate have been reduced with sulphur bearing hydrogen base gas under the condition of gas starvation in the temperatures of 800 to 1 000°C.The addition of 0.12.0 mol% K2CO3 shows that normal swelling amounts to below 40% with macro- and microscopic cracks and that abnormal swelling reaches 300% at 800°C with coarse iron whiskers. This abnormal swelling due to only potassium takes place considerably regardless of any reduction history.The abnormal swelling due to gaseous sulphur is observed in range of 10-21 as sulphur activity of gas (aS) relative to Fe/FeS equilibrium. The maximum swelling is 500% with fine iron whiskers in aS of 0.1 at 900°C. It is found that sulphur has stronger abilities for the swelling than potassium.The largest swelling is 600% and it is observed under coexistence of potassium and sulphur without the formation of liquid phase at 800°C.Actions of both additives during metallization is discussed in the light of authors' previous researches dealing with the effect of sulphur on reduction of wustite.

Comparison of isothermal and nonisothermal reduction

Experimental investigation of the isothermal and nonisothermal reduction of low silica hematite pellets in H2/CO mixture between 600 and 1234°C shows that the reduction rate in H2 rich gas mixture for nonisothermal condition is lower than for isothermal case at comparable temperatures. On the basis of the two period reduction model, some of the physical and chemical characteristics of pellets and reducing gas is calculated. Because of the advantage shown for isothermal reduction, a new technique for direct reduction of pellets is suggested.

The influence of carbon deposition on the reduction kinetics of commercial grade hematite pellets with CO, H2, and N2

Experimental measurements are reported on the rate at which commercial grade, low silica hematite pellets react with a gas mixture consisting of CO, H2, and N2 over the temperature range 500 °C to 1200 °C. Systems of this type are of considerable practical interest, both regarding the operation of direct reduction processes and ironmaking in the blast furnace. The results of the work may be summarized as follows: No carbon deposition was found when operating the system above 900 °C and in the absence of CO gas. When operating the system below 900 °C carbon deposition occurred, which in effect prevented the normal conversion from reaching completion. The maximum rate of carbon deposition was found to occur between 500 °C and 600 °C. In general hydrogen (in the presence of CO) tended to promote carbon deposition, while the presence of nitrogen appeared to retard the deposition process. When the reaction process was being carried out below 900 °C with CO + H2 gas mixtures, the reduction process occurred simultaneously with carbon deposition. At lower temperatures, say around 500° to 600 °C, the deposition process dominated, while at the higher temperatures, and particularly at a high hydrogen content of the reactant gas, the reduction process was dominant. The structural examination of the partially reacted specimens has shown that the carbon deposited was found primarily in the form of elemental carbon rather than cementite. Furthermore, X-ray analysis of the free pellet surface has indicated that iron was present in the carbon deposit phase. The practical industrial implications of these findings are discussed in the paper.

各種ヘマタイトペレット水素還元速度の多段反応帯モデルによる解析

各種ヘマタイトペレット水素還元速度の多段反応帯モデルによる解析

Experimental Study of the Resistance due to the Rate of Gas Flow on the Hydrogen Reduction of an Iron Oxide Pellet

Single basic hematite pellets (porosity 24%) have been reduced in two reactors of 54 and 156 mm I.D, over a temperature range 600 to 1000°C and a flow-rate range 0.1 to 20Nl/min. In the initial stage of reduction, hydrogen is diluted by nitrogen which flows through a reactor until it reaches a set temperature, and the reaction retards especially at low flow rates. In the course of reduction, hydrogen is also diluted by the product gas (H2O). For these dilution processes, the dispersion model has been introduced into the unreacted-core shrinking model for one interface in consideration of the resistance due to the rate of gas flow proposed by Clair (1965).Contribution of this resistance, denoted by k(0≤k≤1, k=0 and 1 imply the analyses without and with the resistance, respectively), has been examined by comparing calculated reduction curves with experimental ones: The value of k increases as the flow rate decreases. With appropriate selection of kinetic constants, the modified model produces a greatly improved fit to experimental data even at low flow rates, where sigmoidal reduction curves occur; such curves cannot be described by the analysis in which only time lag is taken into account on the assumption of plug flow.

多孔質ヘマタイトペレットの水素還元に対する固相内拡散を考慮した多段反応帯モデル

多孔質ヘマタイトペレットの水素還元に対する固相内拡散を考慮した多段反応帯モデル

多孔質ヘマタイトペレットのガス還元に対する多段反応帯モデル

A mathematical model is presented for describing the multi-stage reaction of a porous hematite pellet with a reducing gas on the basis of the ISHIDA and WEN'S intermediate model. This model represents non-topochemical behavior; cross section of a pellet is devided into four layers-iron layer and the three reaction zones of hematite-magnetite, magnetite-wüstite, and wüstite-iron layers, in which chemical reaction and gaseous diffusion proceed simultaneously. Semi-analytical solutions to reducing gas and oxygen concentrations in a pellet are derived and fractional reduction and boundary radii are calculated numerically. Acid and basic hematite pellets (porosity_??_0.23) were reduced with hydrogen of 5 Nl/min over a temperature range 700° to 1000°C. Calculated reduction curves and boundary radii agreed well with experimental data except in the final stage of reduction of the basic pellet at 1000°C.

Improved theory on the rate of reduction of single particles and fixed beds of iron oxide pellets with hydrogen.

Single hematite pellets having not so large porosity are reduced over a temperature range 600-1000°C and a flow-rate range 0.1-20Nl/min. In the initial stage of reduction, hydrogen is diluted by nitrogen which flows through a reactor until it reaches a set temperature, and the reaction retards especially at low flow rates. In the course of reduction, hydrogen is also diluted by the product gas. For these dilution processes, the dispersion model is introduced into the unreacted-core shrinking (UCS) models for one and three interfaces in consideration of the resistance due to the rate of gas flow proposed by Clair. With appropriate selection of kinetic constants, the modified models produce greatly improved fit to experimental data.Porous wustite pellets are reduced and their reduction rates are analyzed by using modified grain and intermediate models; for this case the latter analysis gives rather satisfactory results. Then, multi-stage diffusional (MSD) model is derived on the basis of the intermediate model, and the calculated results are compared with hematite-reduction data. Both reduction curves based on this model and the three interface UCS model reproduce experimental ones very well, while interface radii based on the MSD model simulate cross-sectional view much better than those based on the three interface model. When basic hematite pellets are reduced at 1000°C, the reduction rate decreases extremely in the final stage. An equation of solid-state diffusion is introduced into the MSD model; fairly good agreement is seen up to the final stage.Packed beds of Hamersley pellets are reduced over a temperature range 600-1000°C and a flow-rate range 1-45Nl/min. Not only reduction curves and exit hydrogen flow-rate but core radii and local fractional-reduction in beds reduced partially are measured. The equations of mass balance for a fixed bed are combined with the UCS models for one and three interfaces and numerical calculations are done by the method of characteristics or by an approximate approach; the results calculated by both methods are much the same. Calculated results based on the three interface model reproduce experimental data much better than those based on the Rist model or the one interface model.Porous pure-hematite pellets and basic pellets (fluxed pellets) are reduced at 800 and 1000°C, respectively, and their rates are analyzed by the approximate approach based on the MSD models with and without the solid-state diffusion; each analysis reproduces respective experimental data very well.

ヘマタイトペレットの水素還元速度に及ぼす各種因子の影響

ヘマタイトペレットの水素還元速度に及ぼす各種因子の影響

The Behavior of Softening and Melting of Hematite Pellet and Sinter during Heating in a Reducing Atmosphere

The softening and melting characteristics off some pellets and sinter during the reduction and the heating were examined by using an X-ray TV apparatus. The structure of the samples after the experiment were also examined by a microscope and an electron microprobe analyzer.For the runs without load, the melt-down temperatures varied from 1250°C to the melting point of iron according to the reducing conditions and rose with the increasing reduction degree.An application of load lowered the melt-down temperature of each sample, which strongly depended upon the properties of the formed slag.The EPMA analysis shows that the slag containing a small amount of iron oxide is formed in Hirohata P. of high melt-down temperature, but much iron oxide, about 50%, is contained in the slag formed in Robe River P., which melts at a low temperature.