Estimation of rate parameters for reduction of iron ore–graphite composite pellets in packed bed reactor using genetic algorithm

Abstract

A semiempirical kinetic model has been developed to determine the course of reduction of iron ore–graphite composite pellets over time in a laboratory scale side heated packed bed reactor attached with a tailor made bottom hanging thermogravimetric set-up. The rate parameters in the model, especially the three sets of apparent activation energy values and frequency factors associated with the reduction of iron oxides in three elementary steps, namely hematite to magnetite, magnetite to wustite and wustite to iron, have been estimated based on experimental data by employing an optimisation tool, the genetic algorithm (GA). The difference between the predicted and experimental degree of reduction is minimised to obtain the rate parameters. The experimental degree of reduction is calculated based on mass loss data during reduction and the exit gas analysis. Estimated values of apparent rate parameters were found to be of the same order of magnitude to their intrinsic counterparts reported in literature. Finally, by using the predicted rate parameters the temporal evolution of various oxide phases as well as pure iron has been evaluated.     

High Temperature Thermodynamics and Applications of Rare Earth Oxides and Sulphides in Ferrous Metallurgy

The thermodynamic behavior of the rare earth metals lanthanum and cerium in liquid and steel as predicted by the high temperature (1400-1600°C) thermochemistry of their oxides, oxysulphides and sulphides, is compared to experimentally-determined thermodynamic behavior reported in the literature. The comparison of experimental results includes data from high temperature equilibrium studies and electrochemical cells.

Phase stability diagrams, constructed in terms of the Henrian activities of oxygen and sulphur, are used to compare thermodynamic data bases and to determine some of the applications of lanthanum and cerium in ferrous metallurgy.

M-S-O (M-Ce, La. Ca and Mg) phase stability diagrams arc used in the comparison of hot-metal desulphurization processes, and the control of graphite morphology in the production of nodular and compacted graphite iron.     

Absorption of gaseous oxygen by liquid cobalt,copper, iron and nickel

An experimental investigation of the rates of oxygen solution in molten cobalt, copper, iron and nickel was carried out using pure oxygen and a constant-volume Sieverts’ method. It was found that the volume of gaseous oxygen which initially reacted with the inductively stirred metals was strongly dependent on the physical nature of the oxide film which formed during the first stage of reaction. The initial temperature of the molten iron, cobalt, and nickel was 1600°C, and for copper was 1250°C. For initial oxygen pressures above the melt of about one atmosphere both molten iron and copper, which formed liquid surface oxides, initially absorbed nearly 20 cm³ (STP) O₂/cm² of melt surface area, while molten cobalt and nickel, which formed solid oxides, absorbed about 6 cm³ (STP) O₂/cm² under the same experimental conditions. For approximately 30 s after the initial reaction between these liquid metals and gaseous oxygen, the oxygen absorption rate was proportional to the square root of the oxygen pressure above the melt, and proportional to the melt surface area, but independent of melt volume. The rate-limiting step for oxygen absorption by liquid iron, cobalt and copper can be described by dissociative adsorption of oxygen molecules at the gas/oxide interface. After 30 s of reaction, the rate of oxygen absorption became less dependent on the oxygen pressure above the melt. This indicated that the rate-controlling step was changing from a surface reaction to growth of the oxide layer by cationic diffusion in the bulk oxide. The oxidation rate of liquid nickel appears to be too complex to be described by models for dissociative adsorption of oxygen molecules at the gas/oxide interface and parabolic growth of the oxide layer. The formation of a thin layer of nickel oxide which allows oxygen to migrate through cracks or grain boundaries may be responsible for the relatively high oxygen absorption rate compared to that of liquid cobalt. R. H. RADZILOWSKI, formerly a Graduate Studient at The University of Michigan     

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。     

Absorption of gaseous oxygen by liquid cobalt, copper, iron and nickel

An experimental investigation of the rates of oxygen solution in molten cobalt, copper, iron and nickel was carried out using pure oxygen and a constant-volume Sieverts' method. It was found that the volume of gaseous oxygen which initially reacted with the inductively stirred metals was strongly dependent on the physical nature of the oxide film which formed during the first stage of reaction. The initial temperature of the molten iron, cobalt, and nickel was 1600‡C, and for copper was 1250‡C. For initial oxygen pres-sures above the melt of about one atmosphere both molten iron and copper, which formed liquid surface oxides, initially absorbed nearly 20 cm³ (STP) O₂/cm² of melt surface area, while molten cobalt and nickel, which formed solid oxides, absorbed about 6 cm³ (STP) 0₂/cm² under the same experimental conditions. For approximately 30 s after the initial reaction between these liquid metals and gaseous oxygen, the oxygen absorption rate was proportional to the square root of the oxygen pressure above the melt, and pro-portional to the melt surface area, but independent of melt volume. The rate-limiting step for oxygen absorption by liquid iron, cobalt and copper can be described by dissocia-tive adsorption of oxygen molecules at the gasJoxide interface. After 30 s of reaction, the rate of oxygen absorption became less dependent on the oxygen pressure above the melt. This indicated that the rate-controlling step was changing from a surface reaction to growth of the oxide layer by cationic diffusion in the bulk oxide. The oxidation rate of liquid nickel appears to be too complex to be described by models for dissociative ad-sorption of oxygen molecules at the gasJoxide interface and parabolic growth of the oxide layer. The formation of a thin layer of nickel oxide which allows oxygen to migrate through cracks or grain boundaries may be responsible for the relatively high oxygen ab-sorption rate compared to that of liquid cobalt. Formerly a Graduate Student at The University of Michigan     

Hydrogen Plasma Processing of Iron Ore

Iron is currently produced by carbothermic reduction of oxide ores. This is a multiple-stage process that requires large-scale equipment and high capital investment, and produces large amounts of CO₂. An alternative to carbothermic reduction is reduction using a hydrogen plasma, which comprises vibrationally excited molecular, atomic, and ionic states of hydrogen, all of which can reduce iron oxides, even at low temperatures. Besides the thermodynamic and kinetic advantages of a hydrogen plasma, the byproduct of the reaction is water, which does not pose any environmental problems. A review of the theory and practice of iron ore reduction using a hydrogen plasma is presented. The thermodynamic and kinetic aspects are considered, with molecular, atomic and ionic hydrogen considered separately. The importance of vibrationally excited hydrogen molecules in overcoming the activation energy barriers, and in transferring energy to the iron oxide, is emphasized. Both thermal and nonthermal plasmas are considered. The thermophysical properties of hydrogen and argon–hydrogen plasmas are discussed, and their influence on the constriction and flow in the of arc plasmas is considered. The published R&D on hydrogen plasma reduction of iron oxide is reviewed, with both the reduction of molten iron ore and in-flight reduction of iron ore particles being considered. Finally, the technical and economic feasibility of the process are discussed. It is shown that hydrogen plasma processing requires less energy than carbothermic reduction, mainly because pelletization, sintering, and cokemaking are not required. Moreover, the formation of the greenhouse gas CO₂ as a byproduct is avoided. In-flight reduction has the potential for a throughput at least equivalent to the blast furnace process. It is concluded that hydrogen plasma reduction of iron ore is a potentially attractive alternative to standard methods.     

Activity of Chromium Oxide in CaO‐SiO2 based Slags at 1873 K

Thermodynamic properties of chromium oxides in molten slags are very important for optimization of stainless steel refining processes as well as reduction processes of chromium ores. The solubility of chromite into molten slags has been found to vary drastically with oxygen partial pressure and slag composition in the former studies by the authors. In the present study, activity data and redox equilibria of chromium oxides measured under moderately reducing conditions, P_O2= 6.95×10^?11 atm, at 1873 K are summarized. For the CaO‐SiO₂‐CrO_x system, the activity coefficient of chromium oxide increased with increasing basicity and the optimized slag composition for stainless steel refining is assessed as that saturated with CaCr₂O₄ and Cr₂O₃ using the phase relations determined. On the other hand, the presence of MgO and Al₂O₃ brings about different behaviour of chromium oxide activity and redox equilibria and the 44 mass per cent CaO ‐ 39 mass per cent SiO₂ ‐11 mass per cent Al₂O₃ ‐ 6 mass per cent MgO slag is recommended to reduce the chromium oxidation loss in the practical stainless steel refining process at 1873 K.     

Reduction and foaming of FeO containing slag

Abstract

Smelting reduction processes being developed for producing liquid iron using coal and oxygen are attractive because they allow the use of ore fines directly and do not depend on coke. This paper presents a brief review of some aspects of smelting reduction and some results of an experimental investigation carried out on the reduction of 5–20 wt-% FeO in a synthetically prepared slag by various reductants in a plasma reactor. Some results of a simulation of the smelting reduction process by carrying out post-combustion with oxygen lancing over the slag surface are also presented. It has been possible to achieve a steady state condition, namely, 1–2 wt-% FeO in the slag with a slag height of 4–5 cm during periodic addition of a charge consisting of iron ore, coal, and flux. A kinetic analysis of FeO reduction with various reductants is presented in detail.     

Thermodynamics of oxygen solutions in Fe-Ni-V melts

The oxygen solutions in Fe-Ni melts with up to 5% V are analyzed thermodynamically. The results of the works in which the fields of the vanadium-deoxidized oxide phases in iron and nickel were determined are generalized. The thermodynamic model developed for the calculation of the deoxidation of iron-nickel alloys with vanadium is shown to be adequate. The deoxidizing capacity of vanadium decreases insignificantly as the nickel content in the melt increases to 20% and increases substantially as the nickel content increases further. The oxygen solubility curves pass through a minimum, whose position changes from 2.3192% V for pure iron to 0.7669% V for pure nickel. We determined the equilibrium point [V]* between the (Fe, Ni)V₂O₄ and V₂O₃ oxide phases for alloys of six compositions at 1873 K. In nickel, [V]* is almost 200 times lower than in iron. The deoxidation of the Fe-40% Ni melt with vanadium is studied experimentally, and the experimental results agree satisfactorily with the calculated data.     

The kinetics of formation of h2o and co2 during iron oxide reduction

The kinetics of the chemical reaction-controlled reduction of iron oxides by H₂/H₂O and CO/CO₂ gas mixtures are discussed. From an analysis of the systems it is concluded that the decomposition of the oxides takes place by the two dimensional nucleation and lateral growth of oxygen vacancy clusters at the gas/oxide interface. The rates of decomposition of the oxides under conditions of chemical reaction control are dependent not only on the partial pressures of the reacting gases at the reaction temperature but also on the oxygen activity of the prevailing atmosphere. Application of this model to the kinetic data leads to the determination of the maximum chemical reaction rate constants for the decomposition of the iron oxide surfaces. Assuming the reactions H₂ (g) + O(ads) → H₂O(g) andCO(g) + O(ads) → CO₂ (g) to be rate controlling the maximum chemical reaction rate constants for the reduction of iron oxides are given by $$\Phi _{{\text{H}}_{\text{2}} } = 10^{.00} exp \left( {\frac{{ - 69,300}}{{RT}}} \right)mol m^{ - 2} s^{ - 1} atm^{ - 1} $$ and $$\Phi _{CO} = 10^{4.40} \exp \left( {\frac{{103,900}}{{RT}}} \right)mol m^{ - 2} s^{ - 1} atm^{ - 1} $$ The maximum chemical reaction rate constants do not necessarily indicate the maximum rates which can be achieved in practice since these will depend on the limitations imposed by mass transport in the systems. The rate constants are important however since they indicate for the first time the upper limit of any reduction rate in these systems. The fractions of reaction sites which appear to be active on wüstite surfaces in equilibrium with iron are calculated. A direct relationship between chemical reaction rates on liquid iron surfaces and rates on atomically rough iron oxide surfaces is postulated.     

Oxidation state and activities of chromium oxides in CaO-SiO2-CrOx slag system

The activities of chromium oxides in a CaO-SiO₂-CrO _x slag system were determined with the electromotive force (EMF) method by equilibrating with metallic chromium at 1873 K. The effect of slag basicity on the activity coefficients of CrO and CrO_1.5 was analyzed. The results showed that increasing the slag basicity increased the activity coefficient of CrO; however, the effect on that of CrO_1.5 was not significant. The oxidation state of chromium in CaO-SiO₂-CrO _x slags was systematically investigated at both 1873 and 1863 K. It was found that divalent and trivalent chromium coexists in the slags. Divalent chromium oxide is favored, instead of trivalent chromium oxide, because of low slag basicity and low oxygen potential. It was concluded that the oxidation state of chromium in the slag system varied greatly from almost pure “CrO” to a composition corresponding to Cr₃O₄. In addition, the thermodynamic data in the slag system were assessed based on the regular solution model to mathematically describe the activities of chromium oxides in the slags. A group of model parameters were obtained. The calculated activities of chromium oxides were comparable to the measured data.     

Phase equilibria in the metal-sulfur-oxygen system and selective reduction of metal oxides and sulfides: Part I. The carbothermic reduction and calcination of complex mineral sulfides

The difference in the standard Gibbs free energy for the formation of any two oxides or sulfides is the chemical potential for selective reduction of metals from complex minerals. The magnitude of the Gibbs free energy difference is shown by plotting the univariant relationships for relevant sulfides and oxides. In this investigation, three examples of mineral sulfides are considered, and the experimental results are compared with the predicted thermodynamic calculations. These examples include the reduction conditions for nickel and iron sulfides and pentlandite (Fe,Ni)₉S₈ and chalcopyrite (CuFeS₂) minerals. The reduction behavior of mineral sulfides, such as those of nickel, cobalt, iron, and copper, is illustrated by referring to both the sulfide and alloy phase equilibria. In particular, the solution thermodynamic properties of the metallic phase equilibria are featured for determining the physical chemistry of preferential or selective reduction of the metal oxides and sulfides. The mechanism for the reduction of the aforementioned sulfide minerals is explained with the aid of the governing phase equilibria for the calcination process. The results from the carbothermic reduction of sulfide minerals are also compared. The important roles of lime and calcium sulfate in controlling the emission of sulfurous gases during the reduction reaction are explained. A qualitative analysis of reduction reactions of nickel and iron sulfides is reviewed to provide a comparison of the mechanism for complex nickel-bearing minerals. The importance of these results in producing alloy and pure metallic phases is also examined.     

Application of thermodynamic and kinetic principles in the reduction of metal oxides by carbon in a plasma environment

The application of plasma technology to metal oxide reduction is discussed with reference to established thermodynamic and kinetic principles. ΔG°-T diagrams for the corresponding metal oxide, metal carbide, and C-CO reactions are presented and the important role played by thep _CO/P _{CO 2} ratio examined. On the basis of these theoretical considerations, supported by some earlier experimental results conducted on the reduction of iron and chromium oxide concentrates in the form of taconite and chromite by carbon within a plasma reactor, the tendency to form either elemental metals or carbides is discussed. It is also suggested that the reduction of taconite by carbon takes place in two stages within the plasma medium. In the first stage, ferric oxide is reduced to wustite by carbon, and in the second stage wustite is reduced to metal. It is also postulated that in the first stage of reduction, ferric oxide may also be reduced to wustite through an exchange reaction between ferric oxide and iron, without CO evolution. The rate controlling step for the first stage of taconite reduction is thought to lie at the gas/slag interface generated within the plasma environment, while the second stage of reduction is controlled by carbon gasification by CO₂. Formerly Postdoctoral Fellow with Mineral Research Center, University of Minnesota     

Thermodynamic Activities of “FeO” in some Binary “FeO”‐Containing Slags

In the present investigation, experimental measurements of the thermodynamic activities of iron oxide in the Al₂O₃‐“FeO”, CaO‐“FeO” and “FeO”‐SiO₂ systems were performed in the temperature range 1823‐1873 K by using gas equilibration technique. The molten slag, kept in a Pt‐crucible was brought to equilibrium with a gas mixture of known oxygen partial pressure. A part of the Fe from the “FeO” was reduced during the equilibration and got dissolved in the Pt phase. The samples were quenched after the required equilibration time and the slag phase as well as the platinum crucible was subjected to chemical analysis. The activities of “FeO” in the slag were calculated from the experimental data using thermodynamic information on the Fe‐Pt binary metallic system generated and assessed earlier. The experimental results are compared with earlier thermodynamic studies of the slag systems. Reassessment with the KTH slag model is performed and the results are compared with other thermodynamic models, viz. F*A*C*T? and Thermo‐Calc? respectively. The experimental activities predicted by the KTH slag model are in good agreement with the experimental data available in the literature. A general agreement between the various models is also observed.     

含碳球团直接还原熔分机理

为了探究含碳球团还原熔分机理,将分析纯的Fe₂O₃、氧化物和不同还原剂固结成球并进行等温还原实验,研究了温度、还原时间、配碳量、还原剂种类等条件对球团还原熔分行为的影响.进一步采用X射线衍射、扫描电子显微镜等手段表征了含碳球团在不同还原时间的微观结构及物相变化.实验结果表明:焙烧温度过低或过高含碳球团都不能良好熔分,配碳量增加可以提高球团还原和熔分速率,适宜的温度、碳氧摩尔比、还原剂分别是1400℃、1.2和煤粉.含碳球团还原熔分包括直接还原反应、间接还原反应、碳的气化反应、渗碳反应和铁的熔化反应,最后实现渣铁分离.      

Calculation of the standard heat capacities of crystalline oxides in the Fe-O-Ti system

Our earlier experimental studies of the solid-phase reduction of disseminated lump ores demonstrate that an oxide lattice transforms into a metal lattice via the saturation of the oxide crystal lattice by charged oxygen vacancies. Low-charge metal cations appear in the oxide crystal lattice, and they are related to oxygen vacancies by the condition of local electrical neutrality. As oxygen vacancies are accumulated (i.e., during reduction), the number of oxygen vacancy-low charge cation complexes in the initial oxide increases. The total composition of the oxide phase in the range of a crystal lattice of a certain type changes continuously from the initial oxide to the end product of reduction, i.e., to the lower oxide or a metal. Therefore, it is necessary to determine the thermal characteristics of not only all possible stoichiometric compounds in the M-O system but also MO_x oxides of variable compositions. Equations for calculating the standard heat capacities of complex stoichiometric oxides and oxides of variable compositions in the Fe-O-Ti system are derived using a mathematical model developed earlier, and these characteristics are calculated.     

Possible failure of emf oxygen sensor in liquid iron containing dissolved calcium or magnesium

The solubility products of CaO and MgO in liquid iron, measured using the emf oxygen sensor, are several orders of magnitude greater than those calculated from the thermochemical data. This would imply that the values of Δ and Δ derived from the solubility products are, by a large amount, less negative than those given in the compiled thermochemical tables. As is shown in this paper, the values of Δ derived indirectly from numerous experimental data on various gas-solid (-liquid) reactions involving pure CaO, are in general accord with the compiled thermochemical data. It is surmised from these observations that the oxygen activities in liquid iron saturated with CaO or MgO, measured by the emf cell, giving high solubility products of CaO and MgO, may be attributed to the failure of the emf oxygen sensor in liquid iron containing dissolved calcium or magnesium. Two reaction mechanisms are discussed in the paper to account for the malfunction of the oxygen sensor under highly reducing conditions, which will prevail in liquid iron containing even a small amount of dissolved calcium or magnesium. These two mechanisms are (i) redox reaction at the (Y₂O₃)ThO₂ electrolyte/melt interface and (ii) oxygen flux through the electrolyte from the reference electrode Cr–Cr₂O₃ to the melt/electrolyte interface. Suggestions are made for some experimental work to confirm or refute the argument presented in this communication.     

The effect of alkali salt catalyst on the carbothermic reduction of nickel oxide

The purpose of this communication is to report the experimental measurements on the rate of carbothermic reduction of nickel oxide with and without catalyst added. While kinetic studies on the molten salt enhanced carbothermic reduction of iron oxides^[1,2] and zinc oxide^[3] have been reported, no report has been found on the kinetics of carbothermic reduction of nickel oxide with catalyst added. Formerly with the Department of Metallurgical Engineering, The Ohio State University, Columbus, OH