A study of the reduction rate of FeO in slag by solid carbon

The reduction reaction of FeO in slag by carbon plays an important role in bath smelting reduction processes. In this study, the rate of this reaction was measured to understand the kinetic behavior of FeO reduction in slag by using the mass spectrometer technique. The present experimental results implied that the rate-determining step would change from the mass transfer of FeO at a low FeO content (<5 wt pct) to the chemical reaction at the gas/carbon interface at a high FeO content (>30 wt pct), while the total reduction rate would increase with an increasing FeO content in the slag. Based on the results of this study and comparisons with thermodynamical data for FeO in slag, the reduction rate of FeO can be expressed by the following equation:

Application of sessile drop technique (SDT) for study of reduction of FeO in slags: reduction by solid carbon as well as solute carbon

Abstract

With growing importance of smelting reduction processes, experimental research to understand reduction of FeO in molten slag and the foaming behaviour associated therein have assumed greater relevance. In a previous paper, experimental data on reduction of FeO in molten slag in a 30 kW capacity induction furnace have been reported. The present study used around 250 g slag in each experiment was concerned with influence of FeO concentration in molten slag and temperature on foaming and kinetics. The present paper describes a study on the same system carried out using the sessile drop technique (SDT) which, although it uses very small (～25 mg) sample size, is extremely versatile. Moreover, SDT allows one to actually see the reaction occur. An attempt has been made to correlate kinetic data obtained using the present set-up with the data where much larger sample sizes were used. Both solid carbon and solute carbon have been employed in reduction studies.     

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.     

高铅渣还原过程渣性能及其还原特性的热力学分析

液态高铅渣的还原过程中炉渣物理化学性能变化对其还原特性至关重要。本文在分析高铅渣的主要物化性能指标及其研究方法及分析检测手段基础上,对目前铅渣物理化学性能及还原特性的研究现状进行了概述。并运用热力学软件分析计算了高铅渣还原过程的热力学特性,结果表明,在高铅渣还原反应中,从热力学角度,渣系的所有物相中,无论是和C反应还是和C0反应,Pb_8ZnSi_6O_(21)都是最容易被还原的。在温度700℃条件下,Pb_8ZnSi_6O_(21)和Pb_4SiO_6与C直接还原反应比与C0还原反应更容易进行,但是对Pb_2SiO_4和Pb0没有明显的趋势。     

两段碳还原处理铬渣的工艺实践

利用C和CO的还原性,能有效地将Cr^6 还原成Cr^3 或者金属Cr．韶钢利用烧结、炼铁工艺,有效地回收和利用铬渣中的有效成分,同时利用烧结炼铁工艺中的C和CO在高温下的还原反应,达到解毒目的,实现铬渣的资源化和无害化．     

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.     

Kinetics of manganese oxide reduction from basic slags by carbon dissolved in liquid iron

The reduction of manganese oxide from a basic slag by carbon dissolved in liquid iron is a slow reaction, failing to approach equilibrium closely in 20 hr. Furthermore, the rate of stirring has no apparent effect on the reaction rate. This identifies the rate-controlling step as a chemical reaction at the interface. Only the model for the reactionO ²⁻ =O + 2e ⁻ gave a consistent interpretation as the melt geometry, and concentration of manganese oxide and carbon were varied. The rate constant for this reaction was found to be 1.28 × 10⁻⁵ mole per sq cm per min at 155O°C. The effect of temperature is substantial with a calculated energy of activation for the system of 25 kcal per mole. Formerly Graduate Student, The University of Michigan This paper is based on a portion of a thesis submitted by W. L. DAINES in partial fulfillment of the requirements for the degree Doctor of Philosophy at The University of Michigan.     

固体颗粒对转炉炉渣黏度的影响

通过高温试验考察了固体颗粒对转炉炉渣黏度的影响.高温试验采用Ca2SiO4析晶-熔渣和MgO析晶-熔渣两种体系.试验结果表明:体系的黏度随固相体积分数的升高而显著增加.模型检验结果显示:Roscoe、Ruiz和Zholkov模型预测与试验结果相近(相对误差小于5.4％).该研究对含固相转炉炉渣黏度的预测有一定的指导意义.     

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.     

Kinetics of reduction of lead oxide in liquid slag by carbon in iron

The reduction of lead oxide in dilute solution in CaO-Al₂O₂-SiO₃ slag by carbon dissolved in iron was investigated using a composite crucible as a container so as to exclude graphite from the system. The variables studied to elucidate the reaction mechanism were pressure inside the crucible, carbon content of the metal, lead oxide concentration in slag, and slag composition. The experimental results are best explained by postulating the existence of a gas film at the slag metal interface. It is suggested that the rate controlling step for the lead oxide reduction by carbon is a chemical reaction at the gas/slag interface. The rate constant for up to 3 wt pct PbO in the slag and 2.0 to 4.3 pct C in iron at 1400 °C as calculated from the present study is 4.6 x 10^-4 mol/cm²/min/atm.     

铜渣熔融还原过程中硫的行为特征

对铜渣熔融还原过程中硫的行为特征进行了研究.结果表明,熔渣碱度从0.8增至1.4,渣硫容量增大脱硫作用增强,铁水硫含量由0.6%降至0.13%;铁水脱硫为吸热反应,熔渣温度由1 773 K升至1 823 K渣脱硫能力提升,铁水含硫由0.13%降至0.089%.熔渣-铁水硫理论分配比远大于实验时间条件下硫分配比,保温时间延长铁水脱硫率提高.熔渣碱度1.4、保温温度1 823 K和保温时间40 min时,处理后铁水含硫为0.11%,含量仍较高,需进一步对铁水进行脱硫预处理才可用于炼钢.     

Influence of carbon and nitrogen on solid solution decay

“475 °C embrittlement” of high-chromium ferritic steels with Cr content from 15 to 35 wt pct and different commercial impurities (C, N) has been investigated. The influence of preliminary treatment (600 °C to 1250 °C) and chemical composition of the alloy (Cr, C, N, Mo, Ti, Nb) on kinetics has been established. Internal friction (IF) was used to determine the contribution of interstitial atoms to the formation of Cr-modulated structure during different stages of embrit- tlement of high-chromium steels. By use of IF, static and dynamic (impact) mechanical tests with different states of stress, transmission and scanning electron microscopy of structures and fracture surfaces, and the application of diffraction methods, thermodynamic diagrams of the aging of high-chromium steels have been obtained. In the temperature range of 475 °C em- brittlement, the following sequence of processes has been established: (1) decay of solid solution supersaturated with interstitial atoms by dislocation pinning, (2) formation of substitutional- interstitial (s-i) complexes of interstitial (C, N) and substitutional (Cr) atoms, and (3) formation of zones enriched in Cr.     

Post-combustion and heat transfer at blowing of oxygen into a carbon monoxide containing slag foam

A combined post-combustion model (CPM) for smelting reduction processes was developed in a multi-national research project supported by the European Coal and Steel Commission. The project partners were CSM, Rome, Hoogovens, Ijmuiden, MPI, Düsseldorf, and TUB, Berlin. This paper reports about a heat transfer model developed by TU Berlin within this project. The batch-type smelting reduction reactor has a two-layered slag: an upper foamy and a lower less foamy slag. A bubble stream of (CO+H₂) gas originating from the iron oxide reduction reaction with coal in the lower slag flows upwards. The rising (CO+H₂) gas is post-combusted by three oxygen jets blown horizontally into the upper part of the slag. A flame zone, and above the flame a mixing and a bubble zone form, in which post-combustion reaction and transfer of the post-combustion heat to the slag take place. The modelling of the flame zone was the subject of a previous paper. The present report describes models of the mixing and the bubble zone and of the occurrences in the gas space above the slag. The macro-kinetics of the overall heat transfer process including slag recirculation and heat transfer from the upper foamy to the lower dense slag are presented further. The model calculations provide information about the distribution of the post-combustion and the heat transfer processes over the single zones as functions of the important internal process parameters. Further, the oxygen utilisation, the heat efficiency and the temperatures at various locations of the process are described as functions of the ratio of post-combustion oxygen flow rate to (CO+H₂) evolution rate. In all the calculations a specific gas through-put of carbon monoxide of 3 mol/t?s is assumed. This value corresponds to 510 mol/s for the assumed melt of 170 t. The model shows that heat transfer efficiencies of more than 90 % and slag temperatures of less than 1700°C are possible, if the slag circulation rate is 300 kg/s. Lower circulation rates lead to higher slag temperatures and worse heat transfer efficiencies. Controlled slag circulation is thus an important process tool.     

Flame formation and post-combustion at blowing of oxygen into a carbon monoxide containing slag foam

A combined post-combustion model (CPM) for smelting reduction was developed in a multi-national research project supported by the European Coal and Steel Community. The project partners were CSM, Rome, Hoogovens, Ijmuiden, MPI, Düsseldorf, and TUB, Berlin. This paper is a report about a flame model developed by TUB, Berlin. An oxygen jet is blown into a carbon monoxide containing slag foam. The jet entrains carbon monoxide and slag droplets. Carbon monoxide is combusted by oxygen to carbon dioxide and the developed heat is transferred by radiation from the gas to the surrounding slag and by radiation and convection to the entrained droplets. The droplets are mixed with the slag at the flame end so that also their heat content is finally transferred to the bulk slag. The model consists of a detailed treatment of the entrainment processes, the combustion reaction taking into account the carbon dioxide dissociation equilibrium, the enthalpy changes, and the heat transfer processes. One obtains two ordinary differential equations describing the temperature and composition of the flame gas as functions of the flame pass-way. They are solved numerically by the Runge-Kutta method. As the results, the main flame properties, namely the flame velocity, the diameter and the upwards angle of the flame, the amount of gas and slag entrained from the surroundings to the flame, the oxygen utilisation, from which the post-combustion degree is calculated, the flame temperature along the flame pass-way, and the total heat transfer from the flame gas to the slag, are described as functions of various internal and external parameters. The presented flame model is part of a general post-combustion and heat transfer model of a smelting reduction process with post-combustion in the slag.     

The reduction of zinc from slags by an iron-carbon melt

An experimental investigation was undertaken to study the mechanism of reduction of zinc from slags in the presence of a carbon-saturated iron melt. Batch tests were performed at 1400 °C, and the variation of the zinc and iron concentration in the slag during reduction was determined by sampling the slag at intervals during the test. In graphite crucibles, zinc in slags containing iron was reduced faster than zinc in iron-free slags, both when an iron bath was present and when it was absent. Zinc was reduced faster from slags containing iron when an iron bath was present than when an iron bath was absent. The dominant mechanism of reduction of zinc from slags containing iron appears to be the reaction of Zn²⁺ ions with Fe²⁺ ions to form zinc vapor and Fe³⁺ ions. When an iron bath is present, the Fe³⁺ ions are reduced back to Fe²⁺ predominantly by reaction with iron from the bath. Mass transfer of Fe³⁺ ions in the slag appears to be the rate-controlling step. Reduction of iron from slag by carbon occurred in parallel with the reduction of zinc, and whether there was a net increase or decrease of iron in the slag depended on the relative rates of production and consumption of iron. Lead and copper in the slag were reduced to low levels. The lead volatilized and the copper dissolved in the alloy.     

Rates and mechanisms of FeO reduction from slags

The rate of FeO reduction from CaO?SiO₂ slags has been determined in a stationary magnesia crucible with a graphite rod as the reductant. At 1650°±35°C, with a slag basicity CaO/SiO₂=1.2±0.3, and in the total iron concentration,C, range of 10 pct<C<40 pct, the specific rate,Q _E , of reduction in g-moles per min per sq cm was found to beQ _E =8.25×10^?6 C ^1.77 According to this and previous literature results, the reduction rates span three possible stages. The order of reaction in regard to concentration varies from first order in the low concentrations (0.1 pct<C<1.5 pct) to second order in the intermediate concentrations (1.5 pct<C<15 pct) and finally to 1.77th order in the high concentrations (15 pct<C<40 pct). The reduction reaction can best be interpreted by convective mechanisms because the observed rates fall between the rates of the chemically limited graphite gasification and the rates predicted from molecular diffusion.     

Ladle desulfurization of steel by solid slag mixtures

When using solid slag mixtures immediately after discharge from the converter, the desulfurization of the steel is greatly improved. The refined slag formed in the casting ladle permits the removal of more than 40% of the sulfur from the steel. Reducing the initial sulfur content in the metal shortens the subsequent ladle treatment by 10 min and lowers the power consumption.