Kinetics and mechanism of reactions between iron oxides and iron‐carbon melts

The reaction between iron oxides and iron‐carbon melts was studied in the temperature range 1523–1973 K. Pellets made from three different sources of iron oxides were added onto the melt surface, and the reaction time was measured through the constant volume pressure increase (CVPI) method. The effects of reaction temperature, oxide and melt surface areas, oxide type and weight on reaction time were evaluated. Analysis of partially reduced pellets through SEM was also performed. It has been determined that the reaction time increases as the pellet weight increases, and decreases as the temperature, the contact area between the oxide and the melt and the surface area of the melt increase. Examination of partially reduced pellets had shown that the reduction occurs topochemically. Based on the results, it is proposed that the overall reaction rate is determined by the consecutive reactions of dissociation of FeO and formation of CO.     

Reaction mechanism of FeO reduction by solid and dissolved carbon

The reduction reactions of FeO by carbon have been studied in order to be able to understand the fundamental phenomena occurring in smelting reduction process. The reduction of pure FeO by solid carbon proceeds mostly according to the same reaction mechanism as that by dissolved carbon in iron, the rate of which was experimentally determined to be controlled by the interfacial chemical reaction between Fe-C melt and intermediate CO₂ gas. Hence, the reduction rate of pure FeO by solid carbon is also chemically controlled by the Boudouard reaction between the dissolved carbon and CO₂ at the interface of by-product Fe droplet/gas phase, the activation energy of which was found to be about 193.2 kJ/mol. In addition, the reduction reaction of FeO in CaO-SiO₂-Al₂O₃-FeO slags by the dissolved carbon in Fe melt was also investigated over the FeO mass content less than 20 %. The reduction rate shows first order dependence with respect to FeO concentration. The surface active sulphur content in iron does not affect the reduction rate, and the temperature dependence of reduction rate gives the activation energy of 24.78 kJ/mol. Therefore, the reduction rate of FeO in slags by the dissolved carbon can be safely mentioned to be controlled by the liquid phase mass transfer of FeO through the slag phase diffusion-resistant boundary layer over the limited FeO concentration range. The empirical expression for the mass transfer controlled reactioe, deren Aktivierungsenergie ca. 193.2 kJ/mol beträgt. Außerdem wurde die Reduktion von FeO in CaO-SiO₂-Al₂O₃-FeO-Schlacken mit dem in der Eisenschmelze gelöstem Kohlenstoff fär FeO-Massengehalte von weniger als 20% untersucht. Die Reduktionsgeschwindigkeit weist hinsichtlich der FeO-Konzentration eine Abhängigkeit 1. Ordnung auf. Der Anteil an oberflächenaktivemn rate was determined as r = 5.94(±0.07).10^?6.exp(-24780/RT).(%FeOP)^0.96 over the reaction conditions employed.     

Reduction of FeO in smelting slags by solid carbon: Experimental results

The reduction of CaO-SiO₂-Al₂O₃-FeO slags containing less than 10 wt pct FeO by solid carbonaceous materials such as graphite, coke, and coal char was investigated at reaction temperatures of 1400 °C to 1450 °C. The carbon monoxide evolution rate from the system was measured using stationary and rotating carbon rods, stationary horizontal carbon surfaces, and pinned stationary spheres as the reductants. The measured reaction rate ranged from 3.25 × 10^?7 mol cm^?2 s^?1 at 2.1 pct FeO under static conditions to 3.6 × 10^?6 mol cm^?2 s^?1 at 9.5 pct FeO for a rotating rod experiment. Visualization of the experiment using X-ray fluoroscopy showed that gas evolution from the reduction reaction caused the slag to foam during the experiment and that a gas film formed between the carbon surface and the slag at all times during experimentation. The reaction rate increased with increased slag FeO contents under all experimental conditions; however, this variation was not linear with FeO content. The reaction rate also increased with the rotation speed of the carbon rod at a given FeO content. A small increase in the reaction rate, at a given FeO content, was found when horizontal coke surfaces and coke spheres were used as the reductant as compared to graphite and coal char. The results of these experiments do not fit the traditional mass transfer correlations due to the evolution of gas during the experiment. The experimental results are consistent, however, with the hypothesis that liquid phase mass transfer of iron oxide is a major factor in the rate of reduction of iron oxide from slags by carbonaceous materials. In a second article, the individual rates of the possible limiting steps will be compared and a mixed control model will be used to explain the measured reaction rates.     

Kinetic model for reduction of iron oxide in molten slags by iron-carbon melt

Rate of reduction of iron oxide in iron and steelmaking slags by mass contents of dissolved carbon (>3%) in molten iron depends upon activity of FeO, temperature, mixing of bulk slag and other experimental conditions. A general kinetic model is developed by considering mass transfer of FeO in slag, chemical reaction at gas-metal interface and chemical reaction at gas-slag interface, respectively, as the three rate controlling steps. A critical analysis of the experimental data reported in literature has been done. It is shown that in the case of slags containing mass contents of less than 5% FeO, the reduction of FeO is controlled by mass transfer of FeO in slag plus chemical reaction at gas-metal interface; when slags contain more than 40% FeO, the reduction of FeO is controlled by chemical reaction at gas-metal interface plus chemical reaction at gas-slag interface; at intermediate FeO mass contents (between ～ 5 and 40% FeO), the reduction of FeO is controlled by all three steps, namely, mass transfer of FeO in slag, chemical reaction at gas-metal interface and chemical reaction at gas-slag interface. The temperature dependence of rate constant for the gas-slag reaction is obtained as: In k₂ = –32345.4(&6128)/ T + 19.0(&3.42); σ_lnk2,1/T = &0.3. where k₂ is expressed in mol m^-2 s^-1 bar^-1. The mass transfer coefficient of iron oxide in bulk slag is found to vary in the range 1.5 × 10^-5 to 5.0 × 10^-5 m/s, depending upon the slag composition as well as experimental conditions.     

Kinetics of Reduction of MnO in Molten Slag with Carbon Undersaturated Liquid Iron

The reduction of MnO in molten slag with carbon undersaturated iron was studied. It was found that the process is affected by the carbon content of molten metal and the temperature. The higher the carbon content and the temperature, the faster both the reduction and the emerging of the hump on curve of ωFeO, the larger the difference betwe en ωFeO, max and ωFeO, e. The phenomena were explained with three-step reaction model.     

Kinetics of the solid-state carbothermic reduction of wessel manganese ores

Reduction of manganese ores from the Wessel mine of South Africa has been investigated in the temperature range 1100 °C to 1350 °C with pure graphite as the reductant under argon atmosphere. The rate and degree of reduction were found to increase with increasing temperature and decreasing particle sizes of both the ore and the graphite. The reduction was found to occur in two stages: (1) The first stage includes the rapid reduction of higher oxides of manganese and iron to MnO and FeO. The rate control appears to be mixed, both inward diffusion of CO and outward diffusion of CO₂ across the porous product layer, and the reaction of carbon monoxide on the pore walls of the oxide phase play important roles. The values of effective CO-CO₂ diffusivities generated by the mathematical model are in the range from 2.15 x 10⁻⁵ to 6.17 X 10⁻⁵ cm².s⁻¹ for different ores at 1300 °C. Apparent activation energies range from 81. 3 to 94.6 kJ/kg/mol. (2) The second stage is slower during which MnO and FeO are reduced to mixed carbide of iron and manganese. The chemical reaction between the manganous oxide and carbon dissolved in the metal phase or metal carbide seems to be the rate-controlling process The rate constant of chemical reaction between MnO and carbide on the surface of the impervious core was found to lie in the range from 1.53 x 10⁻⁸ to 1.32 x 10⁻⁷ mol . s⁻¹ . cm⁻². Apparent activation energies calculated are in the range from 102.1 to 141.7 kJ/kg/mol. Formerly Doctoral Student, Department of Metallurgy and Materials Engineering, University of the Witwatersrand, Johannesburg,     

The rate of reduction of iron oxides by carbon

The rate of reduction of Fe₂O₃ and FeO by coconut charcoal, coal char and coke, in an inert atmosphere within the temperature range 900 to 1200°C was investigated. The effects of pressure, particle size, and the amount of carbon were determined. The results indicate that the reaction takes place by means of the gaseous intermediates CO and CO₂, and that the overall rate is controlled by the oxidation of the carbon by CO₂. The rates of reduction of FeO and Fe₂O₃ by CO are relatively fast, and the CO₂/CO ratio for the oxidation of carbon is determined by their equilibria. The reduction of Fe₂O₃ by carbon is accomplished in two stages, with FeO forming first. The reduction of Fe₂O₃ to FeO is faster than that of FeO to Fe because its CO₂/CO equilibrium ratio is higher and hence the rate of oxidation of carbon is faster. A direct comparison was made between the rate constants for the reduction of FeO by carbon and those for the oxidation of carbon in the appropriate CO₂-CO gas mixtures, and they are in good agreement. Apparently the iron formed by the reduction does not significantly catalyze the oxidation of carbon; whereas for the reduction of NiO by carbon, the Ni formed does catalyze the oxidation of carbon.     

The Reaction Behavior of Direct Reduced Iron (DRI) in Steelmaking Slags: Effect of DRI Carbon and Preheating Temperature

An experimental study was conducted to quantify the rate of direct reduced iron (DRI) decarburization in a steelmaking slag using the constant volume pressure increase technique. Experiments were conducted by dropping DRI pellets into molten slag at temperatures from 1773 K to 1873 K (1500 °C to 1600 °C). Subsequent experiments were carried out in which the DRI pellets were preheated while the slag temperature remained constant. The effect of the initial carbon content and the preheating temperature of the DRI on the reaction rate was investigated. The decarburization of DRI seems to comprise two stages, a reaction between the FeO and DRI followed by decarburization through the iron oxide of slag. Carbon has a significant effect on the kinetics of both stages, whereas the preheating temperature mainly influences the rate of decarburization between FeO and carbon inside the pellet.     

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.     

30MVA直流电弧炉冶炼钛渣配碳比研究

工业生产中,为生产出合格的钛渣必须加入适量的碳作为还原剂,将高价氧化物还原为低价氧化物。云南某公司30 MVA大型密闭直流电弧炉(DC炉)生产运行过程中,通过控制无烟煤用量与钛精矿用量之比——配碳比(ratio of anthracite to ilmenite,简称AIR),使生产在输入能量一定、钛精矿成分稳定的条件下力求获得良好的产品品质。生产通过中空石墨电极将钛精矿和无烟煤加入DC炉内,熔炼温度控制为1973～2023 K;熔炼输入功率为15 MVA;入炉钛精矿粒度为0.1～0.33 mm;入炉无烟煤粒径为5～25 mm的比例大于85%。理论上熔炼还原1 t钛精矿,将会产出526 kg渣和368 kg金属铁,O/I比率约为89.4%,理论配碳比约为7.895%。通过生产物料衡算得出,一定熔炼周期内的AIR平均值为12.228%,O/I比率平均值为81.317%。在配碳量不足的情况下,钛精矿中的FeO易于离解出氧并与碳结合,使FeO还原反应优先于TiO2等氧化物,碳最大可能的消耗在FeO的还原上;配碳量越高,则碳将用于还原难还原的氧化物(如MgO,CaO,MnO等)上,使FeO的还原受到抑制。配碳比还会影响DC电炉熔渣流动性和挂渣层。试生产熔炼周期内,通过调整AIR,实现了钛渣中TiO2品质的提高,其含量可从82%提高到89%以上。     

Reduction Behaviour of BOF Slags by Carbon in Iron

Experiments were carried out in a system with BOF slags from industrial operations in order to optimize the conditions of recycling BOF slags produced in the steelmaking process. Reduction reactions of FeO and P₂O₅ proceeded steadily and the FeO reduction rate was almost identical to that of P₂O₅. The reduction reaction of FeO and P₂O₅ in BOF slag at the slag/gas interface is the rate‐controlling step. The reaction rates of FeO and P₂O₅ by dissolved carbon in molten iron are of first order with respect to their respective concentrations. The reduction reactions of FeO and P₂O₅ by dissolved carbon in iron are much closer to the equilibrium state compared with the reduction by solid carbon. It is necessary to control the portion of phosphorus vaporization during reduction treatment in order to obtain efficient operational conditions for BOF slag reduction.     

高炉粉尘造泡沫渣过程中FeO还原动力学的研究

为了循环利用高炉粉尘,研究了用宝山钢铁股份有限公司高炉粉尘与沥青焦粉混合后加入电弧炉造泡沫渣过程中FeO的还原动力学。结果表明,随粉尘加入量的增加和温度的升高,FeO的还原速率加快;用固体碳还原渣中FeO的反应为表观二级反应,其表观活化能为276kJ／mol;用固体碳还原渣中FeO的反应总速率由CO还原FeO的界面化学反应和炉渣的流动传质共同控制。     

Effect of CaO Addition on Iron Recovery from Copper Smelting Slags by Solid Carbon

We investigated the effect of flux (lime) addition on the reduction behavior of iron oxide in copper slag by solid carbon at 1773 K (1500 °C). In particular, we quantified the recovery of iron by performing typical kinetic analysis and considering slag foaming, which is strongly affected by the thermophysical properties of slags. The iron oxide in the copper slag was consistently reduced by solid carbon over time. In the kinetic analysis, we determined mass transfer coefficients with and without considering slag foaming using a gas holdup factor. The mass transfer of FeO was not significantly changed by CaO addition when slag foaming was ignored, whereas the mass transfer of FeO when slag foaming was considered was at a minimum in the 20 mass pct CaO system. Iron recovery, defined as the ratio of the amount of iron clearly transferred to the base metal ingot to the initial amount of iron in the slag phase before reduction, was maximal (about 90 pct) in the 20 mass pct CaO system. Various types of solid compounds, including Mg₂SiO₄ and Ca₂SiO₄, were precipitated in slags during the FeO reduction process, and these compounds strongly affected the reduction kinetics of FeO as well as iron recovery. Iron recovery was the greatest in the 20 mass pct CaO system because no solid compounds formed in this system, resulting in a highly fluid slag. This fluid slag allowed iron droplets to fall rapidly with high terminal velocity to the bottom of the crucible. A linear relationship between the mass transfer coefficient of FeO considering slag foaming and foam stability was obtained, from which we concluded that the mass transfer of FeO in slag was effectively promoted not only by gas evolution due to reduction reactions but also by foamy slag containing solid compounds. However, the reduced iron droplets were finely dispersed in foamy and viscous slags, making actual iron recovery a challenge.     

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.     

Role of Heat Transfer in Early Stage Decarburization of DRI in Slag

The gas generation from reactions between direct reduced iron (DRI) pellets and steelmaking slags is known to take place in two stages; (1) the reaction of FeO and carbon within DRI, i.e., pellet internal reaction, followed by (2) the reduction of slag FeO with DRI carbon at the pellet?Cslag interface, if any carbon remains from the first step. To understand the controlling mechanism of the reaction between FeO and C inside DRI, the rate of the gas release and the temperature of pellets suspended in a slag-free atmosphere were quantified. The results were used to determine the apparent thermal conductivity of DRI that showed values of approximately 0.5 to 2 W.m^?1.K^?1 for a temperature range of 573?K to 1273?K (300?°C to 1000?°C). Furthermore, it was found that the experimental gas evolution rates are consistent with the values predicted by a heat?Ctransfer based model, confirming that the FeO-C reaction within pellet is controlled by the rate of heat transfer from the slag to the DRI pellet.     

LEACHING RATES OF ICEL-YAVCA DOLOMITE IN HYDROCHLORIC ACID SOLUTION

Additives can give rise to obvious, step-wise changes both in the oxidation process and in the sintering process. Therefore, the oxidation and sintering characteristics measured in dried pellets prepared from pure magnetite concentrates can not be representative for those characteristics in dried pellets containing additives. The oxidation and sintering characteristics of magnetite iron ore pellets balled with a novel complex binder (namely MHA) were mainly investigated by batch isothermal oxidation measurements in this research. Combined results reveal that the thermal decomposition of MHA binder influences the oxidation and sintering processes of dried pellets. Oxidation rate of pellets increases obviously with increasing the oxidation temperature in the range from 800°C to 1000°C. And the remaining FeO content declines gradually when separately heated for 10 min at low temperature (<1000°C). However, the oxidation rate of pellets decreases distinctly when oxidation temperature is higher than 1000°C. In addition, when oxidation temperature increases from 1000°C to 1250°C, the FeO content of pellets goes up obviously, particularly at 1250°C. The FeO content in the core of sintered pellets heated at 1250°C can even reach 29.68%. SEM spectrum analysis demonstrate that some iron appears in forms of wustite in sintered pellets, which indicates that the reduction reaction of iron oxide occurs during the high temperature sintering process. This is explained by the occurrence of reducing atmospheres because of the pyrogenic decomposition of MHA binder.     

Processing of Agglomerated Red Filter Dust in the Converter Operation from Metallurgical Point of View

Red filter dust (RFD) from steel works contains up to 50 mass% iron, which therefore can serve as raw material for steel production. It should be possible to recycle a fraction of the RFD in the converter process of a steel works wherein also scrap for recycling is used. The aim was the investigation of the reduction behavior of the iron oxide in the RFD. This was accomplished by contact of the dust with pig iron containing up to 3.9 mass% carbon and also by addition of bio‐char to the dust, creating self‐reducing briquettes. The experimental results were compared to the theoretical achievable iron oxide reduction. The reaction time of selected briquettes was calculated by a kinetic approach. Additional the behavior of lead and zinc in the dust was investigated. The mass balance of the converter process indicated the influence of the dust recycling especially regarding the zinc mass flow.     

基于热轧全流程氧化铁皮控制的耐蚀性工艺

 针对热轧生产流程实际工况,系统研究了热轧、卷取阶段的三次氧化铁皮演变规律,旨在不增加生产成本的前提下,通过调整热轧生产工艺控制氧化铁皮结构,利用热轧后生成的氧化铁皮作为防护屏障,提高钢材耐蚀性能。结果表明,在不同轧制温度下,三次氧化铁皮结构从外到里分别为Fe₂O₃、Fe₃O₄和FeO,由于FeO中的阳离子空位密度大,导致其比例最大,并且随着轧制温度增加,氧化铁皮中的FeO层厚度逐渐增厚,并且其比例也逐渐增加。通过模拟连续冷却试验发现氧化铁皮结构转变关系呈现出“C”曲线的形式。在450～550 ℃温度范围内卷取时,FeO发生共析反应程度达到峰值,同时可以看出在高温下卷取可以有效抑制共析转变的发生。通过大量的试验研究表明,获得以先共析Fe₃O₄为主的完整氧化铁皮的结构类型,是有效提高热轧钢材耐蚀性的主要控制方向。因此在国内某钢厂热连轧生产线进行了基于氧化铁皮控制的耐蚀性工艺试轧试验和盐雾试验,结果表明,氧化铁皮完整致密,并且其结构类型主要为先共析Fe₃O₄,因此利用轧制工艺调整改变钢板表面氧化皮结构,钢材耐蚀性能得到显著提高。     

铁粉矿在HIsarna工艺中预还原行为的研究

通过高温实验与理论分析研究了铁粉矿颗粒在高温下的热分解和熔化行为,以及熔化后气体与熔融粉矿液滴之间的还原动力学.当温度高于FeO熔点且产物层中有FeO生成时,铁粉矿颗粒会出现熔化现象.还原反应前210 ms伴随着剧烈的热分解反应,主要是Fe_2O_3分解成Fe_3O_4.熔化后的铁粉矿颗粒产物层是液态的FeO,颗粒中心是未反应的固态Fe_3O_4,还原反应发生在颗粒表面.Fe~(3+)在产物层中的扩散是还原反应的限制性环节,通过计算得到气体与熔融铁粉矿颗粒还原反应的表观活化能约为141 kJ/mol.     

氧化亚铁颗粒氢还原过程的结构演变

采用热重法实验研究了773~1273K氧化亚铁的等温氢还原动力学,发现873K温度以上,反应动力学曲线有明显转折,说明反应机理发生了变化．在973~1073K的温度范围,出现了反常的温度效应．即反应速率随温度升高而减小．为讨论产物结构对反应动力学的影响,分别对不同温度的反应产物,以及一定温度不同还原状态（不同反应时间）的产物进行形貌观察．结果显示．随着反应温度升高,还原产物表面的孔洞增多,枝状特征显著增加,而973K和1023K时表面的烧结现象明显．一定温度下,随着反应进行,表面的孔洞增多,并逐渐出现烧结．973K和1023K温度条件下反应产物大体保留原来的大颗粒外形,而1173K时还原2min开始,就大量出现枝状产物,并逐渐烧结．结合产物形貌变化和反应动力学曲线,反应前期为界面化学反应控速,随着反应进行．还原的金属铁发生烧结现象,致密的结构阻碍了产物气体向外扩散,反应控速环节转变为产物气体的外扩散,还原速率也随之降低．