Reduction of lead minerals by CO/CO2 gas mixtures: Application of the grain model

The grain model is shown to describe satisfactorily the reduction of lead oxide and lead calcium silicate pellets by CO/CO₂ gas mixtures. The reduction of lead calcium silicate, the principal lead bearing phase in the feed material to a commercial blast furnace, is initiated at about 750 °C. The activation energy is 161 kJ mol⁻¹. These values are much higher than values for the reduction of lead oxide. The lack of chemical reaction in the upper reaches of the commercial lead blast furnace is attributed to the low reactivity of the feed material. This is a consequence of the low porosity, large particle size of the feed material, and the presence of lead in the form of complex silicates.     

The kinetics of S35 exchange between SO2/CO/CO2 gas mixtures and copper sulfide melts at 1523 K

The kinetics and mechanisms of oxidation of copper sulfide melts have been investigated using a radioisotope exchange technique. Copper sulfide melts were doped with S³⁵. The transfer of the radioisotope between the melt and SO₂/CO/CO₂ gas mixtures in chemical equilibrium with the melt was monitored by analyzing the changes in radioactivity of the gas. Analysis of the results indicates that the rate-limiting chemical reaction involves the formation of an activated complex SO, and the rate of exchange of the sulfur isotope at 1523 K is described by the relationshipR = 6.4(±2) (P _CO /P _CO2 )P _SO2 g atom S m⁻² s⁻¹. formerly Research Assistant, University of Queensland formerly Postdoctoral Fellow, University of Queensland     

氧化铁气基还原过程的气体氧化动力学

 为了研究氧化铁气基还原过程的气体氧化过程,给出了气基还原单个/单层氧化铁颗粒（球团）的气体利用率计算公式,并建立了氧化铁还原及还原气体氧化的耦合动力学模型。结合氢气还原单颗粒氧化铁以及氧化铁固定床、流化床还原试验得出,在还原分数较低时,气体利用率较高,但是随着还原分数的提高,气体利用率不断下降、还原时间明显延长。缩小颗粒粒度、提高反应速率常数（温度、优质还原剂、催化剂等）等措施有利于提高还原分数和气体利用率;单纯提高气体速度（增加气矿比）,有利于提高还原分数,但是使气体利用率降低。     

The kinetics of the reduction of chromium oxide by hydrogen

The reduction kinetics has been studied as function of hydrogen partial pressures,pH₂O/pH₂ ratio, gas flow rate, and temperature. The reduction follows a linear time law and is dependent on the gas flow rate below a value of approximately 10 cm · s^-1, since the rate is determined by the removal of the water vapor being formed. In this range the reduction rate may be calculated from gas dynamical data. At sufficiently high flow rates the phase boundary reaction is rate determining. The activation energy is 123 kJ · mol^-1. The reduction rate is proportional to the square root of hydrogen pressure and decreases with increasing water vapor content. Formerly Research Associate with Dechema-Institut     

Electronically driven transport of oxygen from liquid iron to CO + CO2 gas mixtures through stabilized zirconia

Transport of oxygen in the following electrochemical system was investigated;O (liquid iron) Oⁱⁿ²⁻ (in ZrO₂2−CaO) O₂ (CO + CO₂) An alumina crucible was charged with liquid iron containing 580 ± 10 ppm oxygen. A calcia-stabilized zirconia tube (closed at one end) was immersed in the liquid iron. The inside of the zirconia tube was flushed with a stream of CO + CO2 gas mixture. Oxygen was removed from liquid iron to the CO + COO₂ gas mixture without application of an external current. Kinetics of oxygen transport in this system are discussed in terms of mixed ionic and electronic conduction of the zirconia, and also diffusion of oxygen in liquid iron. The rate controlling step for this oxygen removal process was found to be transport of oxygen across a boundary layer in the melt at the melt/electrolyte interface. M. IWASE, on leave from the Department of Metallurgy, Kyoto University, Kyoto, Japan M. TANIDA, Formerly Graduate Student at Kyoto University     

Isothermal reduction kinetics of self-reducing mixtures

Isothermal reduction of haematite carbon mixtures was investigated at temperatures 750–1100°C under inert atmosphere. Mass loss curves proved the stepwise reduction of haematite to metallic iron. The non-linear feature of haematite to magnetite reduction kinetics was observed and an activation energy of 209?kJ?mol^?1 was calculated. Irrespective of carbon-bearing material type, reduction rate of magnetite was linear. Activation energy values were calculated to be 293–418?kJ?mol^?1. Significant increase in the reduction kinetics in the last step (Wustite reduction) was observed and explained by the catalytic effect of freshly formed metallic iron. During the initial stages of wustite reduction, the activation energy values were calculated to be in the range of 251–335?kJ?mol^?1 for all carbon-bearing materials.     

The effects of impurity elements on the reduction of wustite and magnetite to iron in CO/CO2 and H2/H2O gas mixtures

The reduction of dense wustite and magnetite samples in CO/CO₂ and H₂/H₂O gas mixtures has shown that impurity elements in solid solution in the oxides can significantly affect the reaction mechanisms operative during reduction and the conditions for porous iron growth. In general, the presence of P, Mg, Ti, Si, Ca, K, and Na in wustite favors, in order of increasing strength, the formation of the porous iron product morphology. Aluminum, on the other hand, significantly reduces the range of gas conditions over which the porous iron microstructure may be obtained. S. GEVA, formerly Research Assistant, Department of Mining and Metallurgical Engineering, University of Queensland. D.H. St. JOHN, formerly Senior Lecturer, Department of Mining and Metallurgical Engineering, University of Queensland.     

Kinetics of dense magnetite formation during oxidation of wüstite and reduction of hematite in CO/CO2 gas mixtures

Thermogravimetric study of the wüstite oxidation. Influence of the cation diffusion in magnetite and of the oxygen transfer at the phase boundary magnetite/gas on the wüstite oxidation and on the hematite reduction. Linear and parabolic growth of dense magnetite.     

Solid-state reduction of chromium oxide by methane-containing gas

Reduction of chromium oxide, Cr₂O₃, was investigated in a fixed bed laboratory reactor in the temperature range 900 °C to 1200 °C using CH₄-H₂-Ar gas mixture. The extent and rate of reduction as functions of gas composition and temperature were determined by on-line off-gas analysis using a mass spectrometer. Samples at different stages of reduction were examined by scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis. The chromium oxide was reduced to chromium carbide Cr₃C₂ with a degree of reduction close to 100 pct. The rate of reduction increased with temperature and methane content in the reducing gas. Carbon monoxide, added to the reducing gas, strongly retarded the rate of Cr₂O₃ reduction. The hydrogen content had a slight effect on the reduction rate. High extent and rate of reduction by methane-containing gas in comparison with carbothermal reduction were attributed to high carbon activity in the reducing gas—15 to 50 (relative to graphite).     

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.     

Reduction of solid wustite in H2/H2O/CO/CO2 gas mixtures

The reduction of dense wustite in H₂/H₂O/CO/CO₂ gas mixtures has been carried out at temperatures between 1073 and 1373 K. The critical conditions for the formation of porous iron product morphologies have been identified and the results discussed in relation to the breakdown of dense iron layers on wustite surfaces. Formerly with the University of Queensland.     

The kinetics of reduction of iron oxide from molten slag

This investigation primarily consists of measurement of the rate of reduction of FeO from sulfur-free slag. Preliminary measurements of the effect of sulfur on the rate of reduction and the sulfide capacity of the iron-rich slags have also been made. The results show that with increasing SiO₂ contents the rate of reduction is decreased. The influence of sulfur could not definitely be clarified.     

Microstructural features produced by the reduction of wustite in H2/H2O gas mixtures

The systematic study of the reduction of pure wustites (FeO) between 600 and 1100°C in H₂/H₂O gas mixtures has revealed a number of important morphological changes. It has been shown that dense wustite can decompose to form a highly porous wustite before iron nucleation takes place. The product morphologies of iron formed on the wustite on reduction have been classified into three types, (a) porous iron, (b) porous wustite covered by dense iron, and (c) dense wustite covered by dense iron.     

CO2 conversion and decarburization kinetics of CO2 gas and liquid Fe-C alloy at 1873 K

The reactions between CO2 gas and liquid Fe-C alloy with different initial carbon concentrations at 1873 K were investigated using experimental results,thermody...     

The determination of oxygen in gas mixtures by electromotive force measurements using solid oxide electrolytes

The behavior of impervious ZrO₂+ 10 mole pct CaO electrolyte tubes was studied at 500∮ to 1100° in oxygen concentration cells of the type (−) Ar-O₂or CO-CO₂/ZrO₂-CaO/O₂(+) The equilibrium oxygen pressures imposed by Ar-O₂(1 to 10⁻⁶atm) and CO-CO₂ (10⁻⁶to 10⁻¹⁸ atm at 1000°) mixtures of known compositions were determined from electromotive force measurements. The measured and theoretical electromotive forces were compared. For Ar-O₂ mixtures, the oxygen pressures can be measured with accuracies of ±0.4 pct at 1 to 10⁻² atm and ±4 pet at 10⁻⁴ to 10⁻⁵ atm. An accuracy of ±11 pct can be achieved with CO-CO₂ mixtures. The electromotive force of the above cell increases as the gas flow rate at the anode increases at low flow rates and becomes independent of flow rate at high flow rates. The minimum flow rates required to eliminate any significant flow rate dependence are presented as a function of oxygen pressure. Formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto, Toronto, Ontario, Canada     

Hematite ore reduction to magnetite with CO/CO2 – kinetics and microstructure

Uncertainties and discrepancies are common in the first step of the reduction of hematite ore to magnetite, especially as regards the experimental conditions governing the transition from porous to lamellar magnetite. Hence the problem has been reinvestigated with an ore from Itabira (Brazil). This ore has been carefully characterized with X-ray diffraction, X-ray photoelectron spectroscopy, optical and scanning electron microscopy and electron microprobe analysis. It has been established that α-quartz and goethite are present, and iron combined on the surface with S or Si; most particles, in the range 50–200 μm, are hematite single crystals with 0.1% Al as the only substitution; there are also smaller crystals agglomerated by the gangue. Numerous experiments have shown that the transition from porous to dense lamellar magnetite is enhanced by an increase of temperature and a decrease of CO %. This is explained by a competition between the chemical reaction and solid state diffusion of ions along the hematite-magnetite interface. The shrinking-core model has been applied to the domain of porous magnetite, yielding a chemical rate constant ? = 15.5 exp (?69500/RT), roughly twice lower than in the reduction of hematite synthetic single crystals. The nucleation frequency increases sharply with temperature to ～700°C where it reaches ～10⁹ m^?2·s^?1, as with pure hematite.     

应用Fick定律计算CO/H2+N2混合气体还原铁矿石动力学参数的偏差

为了讨论采用Fick定律未反应核模型(Fick模型)计算CO/H2+N2混合气体还原铁矿石动力学参数的偏差,建立基于Maxwell-Stefan关系式的未反应核模型(M-S模型),对Fick模型计算得到的动力学参数和M-S模型中设定的动力学参数进行比较分析.结果表明:N2对CO/H2还原FeO的阻碍作用与其摩尔分数成正比;N2对正反应速率常数的计算没有影响;CO-N2体系中Fick模型所用的互扩散系数小于实际值,而H2-N2体系中其所用的互扩散系数大于实际值.     

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.     

The gaseous reduction of solid calciowustites in Co/Co2 and H2/H2O gas mixtures

The reduction of calciowustites has been carried out in CO/CO₂ and H₂/H₂O gas mixtures at temperatures between 1073 and 1373 K. The effect of lime additions to the wustite is to extend considerably the range of gas conditions over which porous iron morphologies are observed compared to those found on reduction of pure wustite. The products obtained at low oxygen potentials consisted of porous iron containing a dispersion of CaO particles, at intermediate oxygen potentials a two phase structure consisting of porous iron and dicalcium ferrite was formed, and at high oxygen potentials a dense iron layer over the oxide surface is observed throughout reduction. F. NAKIBOGLU, formerly Graduate Student, Department of Mining and Metallurgical Engineering, University of Queensland, Brisbane, Australia D.H. St. JOHN, formerly Postdoctoral Fellow, University of Queensland     

Oxidation kinetics of zinc vapor in CO∶CO2 mixtures: Part I. Comparison with past literature

The methods employed and the results obtained during a recent investigation of the oxidation of gaseous zinc by CO∶CO₂ mixtures are described. The kinetic data are tested against the predictive models derived and employed by previous investigators of this reaction. Although the experimentally determined reaction rates are highly reproducible, no consistent agreement can be found with the models presented in the prior literature. Efforts to account for the kinetics on the basis of the oxygen potential of the gas mixture are also unsuccessful. It is concluded that previous attempts to understand the kinetics in terms of the reaction Zn _(g) +CO _2(g) ⇒ZnO _(s) +CO _(g) are oversimplistic. A novel interpretation which appears to resolve the discrepancies of the literature is detailed in the second of two articles on the subject.