The kinetics of reduction of hematite by carbon

Reduction kinetics of mixtures of hematite and carbon powders were investigated in the temperature range of 850° to 1087°C. Experiments were carried out under argon atmosphere and the isothermal weight loss of the samples was determined as a function of time. The effects of carbon particle size, hematite/carbon ratio of the mixture, and addition of promotive or inhibitive reagents were also investigated. The results were summarized in the form of fractional reaction vs time plots. A kinetic model developed on the basis of carbon solution-loss reaction as rate-controlling represented the results fairly well. An enthalpy of activation of 72 kcal/per mole was calculated, within the range of 957° to 1087°C. The observed effects of Li₂O and FeS on the reduction kinetics are consistent with the influence these reagents are known to exercise on the solution-loss reaction.     

Microstructural changes on the reduction of hematite to maanetite

The microstructures resulting from the reduction of hematite to magnetite have been examined for a wide range of temperatures and gas conditions. A transition in product morphology from plate or lath magnetite to porous magnetite was found to occur at a constant free energy difference between the reducing gas mixture and the hematite over a range of reaction temperatures. Direct observations of lath nucleation and growth are reported and show the self-accelerating effect of the Fe₂O₃ → Fe₃O₄ transformation. The limits of porous growth are discussed in terms of established theories of discontinuous precipitation and a mechanism for the formation of lath magnetite is proposed.     

Morphology of hematite to magnetite reduction zone

The reduction process of hematite to magnetite results in distinct changes in morphology of magnetite. These changes depend on structural properties of parent hematite and reduction conditions. The reduction experiments were performed in 3% CO and 97% CO₂ gas mixture at 450 and 850°C on selected crystals of natural hematite. The phase composition and morphological characteristics of the reduced layer were determined on the basis of microscopic analysis. Singular blasts or blastoidal colonies of magnetite were formed in 450°C on the boundaries of the hematite grains. They began to grow and joined the layer. Magnetite formed at 450°C is distinctly microporous. Cracks and desintegration of hematite grains appear together with reduction of hematite. At 850°C nucleation of the magnetite is quite different then at 450°C. The formation of singular magnetite lamellae or a palisade of crystallographically oriented magnetite lamellae were observed. Their growth results in the formation of the magnetite layer. Tunnel-shaped pores in magnetite layer show the same direction as lamellar front of reduction.     

High voltage microscopy of the reduction of hematite to magnetite

The microstructural changes associated with the formation of magnetite in hematite have been studied in specimens which have been partially reduced outside the microscope, thinned until electron transparent, and then examined in the normal way. Three types of structure have been observed in varying proportions which depend on the reduction temperature. At low temperatures, magnetite grows by the propagation of a cellular interface, the gas phase being transported to the cell boundary by a network of tunnels. At intermediate temperatures, magnetite plates are formed, whereas at high temperatures, both plate magnetite and blocky magnetite appear. It is proposed that the factor controlling the morphology which develops is the ratio of the cell boundary diffusivity to volume diffusivity of ferrous ions. It is noted that the decomposition morphologies of hematite and austenite have many similarities. The basic reason for this similarity is that both transformations involve substantial redistribution of elements in the solid state and the microstructures which develop are those that perform this redistribution the most efficiently at the temperatures involved. This paper is based in part on a thesis submitted by N. J. Tighe in partial fulfillment for the Degree of Doctor of Philosophy at the University of London.     

Structural changes and kinetics in the gaseous reduction of hematite

The mechanism of the gaseous reduction of hematite grains to magnetite was studied. Grav-imetric measurements were carried out for the reduction of Carol Lake hematite pellets and grains in CO-CO₂ atmospheres over the temperature range 500 to 1100°C. The pore size distribution in the reduced magnetite was measured by mercury porosimetry. Partially reduced grains were examined by optical microscopy. At temperatures below 800°C, the reduction of a hematite grain to magnetite occurred at a well-defined shrinking-core inter-face. The average pore size in magnetite formed at 600°C was found to be 0.03 μm. An es-timate of the rate of CO diffusion through pores of this size indicated that the reaction rate at 600°C was controlled by a step near the hematite-magnetite interface. At temperatures above 800°C, the reaction mechanism became altered due to the preferential growth of magnetite along a single direction in each hematite grain. The reduction rate decreased with an increase in temperature, and no microporosity was present in magnetite formed at 1000°C and above. It was postulated that the reaction rate was controlled by the rate of formation of fresh nuclei and by their rate of subsequent growth. Formerly Professor of Applied Metallurgy, Imperial College     

The importance of convective mass transfer in the reduction of hematite

The relative importance of transport phenomena, as opposed to chemical phenomena, in controlling the kinetics of hematite reduction has been debated for a long time. Recent measurements of gaseous diffusion coefficients in the porous iron and intermediate oxide layers produced by the reduction have shown that gaseous diffusion plays an increasingly important role, especially towards the end of the reaction. Convective mass transfer, however, is still assumed to play a negligible role, principally because of the way in which the reduction rate of hematite particles varies with the gas flow rate, and with the particle diameter. The established theories of convective mass transfer are used in this paper to show that the observed variations would occur whatever contribution convective mass transfer was making to controlling the reduction rate. The observed variations, therefore, give no indication as to the relative importance of convective mass transfer. This paper then, makes a quantitative comparison between the mass transfer rates necessary to sustain the reaction rates observed in recent hematite reduction experiments and the rates predicted by the established theories of mass transfer. This comparison shows that convective mass transfer can play a major role in controlling the reduction rate, although the relative magnitude of its contribution varies with particle size, and with reduction temperature.     

The importance of convective mass transfer in the reduction of hematite

The relative importance of transport phenomena, as opposed to chemical phenomena, in controlling the kinetics of hematite reduction has been debated for a long time. Recent measurements of gaseous diffusion coefficients in the porous iron and intermediate oxide layers produced by the reduction have shown that gaseous diffusion plays an increasingly important role, especially towards the end of the reaction. Convective mass transfer, however, is still assumed to play a negligible role, principally because of the way in which the reduction rate of hematite particles varies with the gas flow rate, and with the parti-cle diameter. The established theories of convective mass transfer are used in this paper to show that the observed variations would occur whatever contribution convective mass transfer was making to controlling the reduction rate. The observed variations, therefore, give no indication as to the relative importance of convective mass transfer. This paper then, makes a quantitative comparison between the mass transfer rates necessary to sus-tain the reaction rates observed in recent hematite reduction experiments and the rates predicted by the established theories of mass transfer. This comparison shows that con-vective mass transfer can play a major role in controlling the reduction rate, although the relative magnitude of its contribution varies with particle size, and with reduction temperature.     

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.     

鲕状高磷赤铁矿熔融还原过程脱磷技术研究

针对"宁乡式"鲕状高磷赤铁矿的矿物学特点,研究用熔融还原法处理"宁乡式"鲕状赤铁矿脱磷的技术。热力学及动力学分析和熔融还原脱磷实验表明,碱度、萤石比例等因素影响脱磷率。鲕状高磷赤铁矿熔融还原过程中配加CaO、CaF2脱磷效果显著,脱磷率最高可达97.3%。合适的二元碱度在1.8~2.2之间,CaF2的合理加入量在6%~9%。     

On the mechanism of iron oxide reduction by carbon

A theoretical analysis has been made to determine the conditions under which the reduction of iron oxide by carbon takes place according to the 2 step mechanism involving the Boudouard reaction. This is based on the concept of minimum temperature of reduction (T _min) below which the Boudouard reaction does not affect the reduction process. The effect of variables such as carbon reactivity, total pressure and so forth onT _min has been studied. TheT _min can be used to determine if metallization is possible under a given set of conditions.     

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.     

Dephosphorisation of high phosphorus oolitic hematite by carbon composite pre-reduction and fast melting separation

High phosphorus oolitic hematite deposit is a kind of refractory iron ore resource of huge amount. At present, it is difficult to be utilised by traditional physical and chemical technology efficiently and economically. A novel process for utilisation of the high phosphorus oolitic hematite based on carbon composite pre-reduction and fast melting separation has been put forward in the paper. High grade pig iron nugget of low phosphorus could be obtained in the present research. The influence of experimental conditions, such as pre-reduction temperature, C/O (molar ratio) and basicity, on the dephosphorisation behaviours was studied in detail. The thermodynamic basis and reduction and melting separation process were also analysed. The phosphorus content in the iron nugget decreased with the increasing of basicity and increased with the increasing of C/O. The optimum parameters were pre-reduction temperature of 1200°C for 30?min, C/O of 0.95 and basicity of 1.7. After melting separation of molten iron and slag at 1400°C for 10?min, the iron nugget containing 0.02?wt-% [P] would be obtained. The dephosphorisation degree and iron yield in the form of iron nugget were 97.5% and 96.9% respectively. The iron nugget may be directly used as the raw materials of steelmaking from the view point of its high grade.     

Recovery of metallic iron from high phosphorus oolitic hematite by carbothermic reduction and magnetic separation

Abstract

This study aims to provide theoretical and technical basis for economical and rational use of high phosphorus oolitic hematite. Following physical, chemical and microscopic characterisation of high phosphorus oolitic hematite ore the feasibility of separation of phosphorus and metallic iron by reduction roasting and magnetic separation process were investigated. The results indicate that such a process is a feasible and efficient method for iron and phosphorus separation of high phosphorus oolitic hematite. The recovery of metallic iron and dephosphorisation rate is relatively low without additives but is significantly improved by appropriate CaO and Na₂CO₃ addition. With 8%CaO and 3%Na₂CO₃ the recovery of metallic iron and dephosphorisation rate reach 95.1 and 94.0% respectively.     

Kinetics of reduction of hematite with hydrogen gas at modest temperatures

The kinetics of hydrogen reduction of thin, dense strips of hematite were investigated in the range 245 °C to 482 °C. Pure hydrogen gas at 1 atm was used as the reducing agent. Because of the relative thinness (only 136 /μm thick) of the specimens used, the pore-diffusion of gases offered no significant resistance to the reduction process. The interfacial-reaction-rate constantk _s ^* , which has been corrected for film-mass-transfer effects, is found to be given by logk _s ^* = −1.032 (±0.138) -[7860 (±200)]/2.303r where k _s ^* is in g · atom O · cm⁻² · s⁻¹ · atm⁻¹. The activation energy for the reduction process is found to be 65,325 (±1650) J · mol⁻¹; the rate-controlling step appears to be the Fe₃O₄ → Fe conversion step.     

赤铁粉矿在H2气氛中的闪速还原行为

通过高温下的动力学实验对铁粉矿在H_2气氛中的闪速还原行为进行了研究.采用XRD、SEM和金相显微镜对反应后铁粉矿的物相组成和单个粉矿颗粒表面及内部微观形貌的演变规律进行了分析,采用化学分析法获得了反应后赤铁粉矿的还原度.结果表明:铁粉矿高温下发生的闪速还原仍然遵循Fe_2O_3→Fe_3O_4→FeO→Fe的逐级还原规律.粉矿颗粒的剖面由未反应核和产物层构成,符合未反应核模型的描述.采用模型函数配合法得出铁粉矿与H_2在高温下发生闪速还原反应的限制性环节是界面化学反应,进一步基于动力学模型计算得到,闪速还原的表观活化能为311 kJ/mol.