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.     

温度对高磷鲕状赤铁矿含碳球团直接还原影响

对高磷鲕状赤铁矿含碳球团直接还原进行了研究。结果表明,1100℃以下时,提高温度可以显著提高球团金属化率;1100℃以上时,继续提高温度对球团金属化率影响不大。球团金属化率越高,磁选精矿铁品位越高。还原温度不仅显著影响球团的金属化率,还影响金属铁相的长大及磁选效果。因此,控制适宜的温度对高磷鲕状赤铁矿含碳球团直接还原至关重要。     

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.     

微细粒贫赤铁矿碳热还原的动力学

通过对微细粒贫赤铁矿进行非等温动力学研究,考察其反应控速环节,并探究反应配碳量及升温速率对还原过程的影响,研究结果表明:还原过程由界面反应控制;配碳量增加及升温速率提高均对还原反应起促进作用;碱金属盐对还原具有催化作用,降低了反应活化能。     

还原剂对高磷鲕状赤铁矿直接还原过程铁还原的影响

采用添加脱磷剂直接还原焙烧-磁选的工艺制备直接还原铁,研究了不同还原剂对高磷鲕状赤铁矿直接还原过程铁还原的影响.实验结果和扫描电镜分析表明,还原剂中固定碳和挥发分含量对于焙烧产物中金属铁晶粒的聚集、增多和长大以及所得还原铁指标影响较大.焦炭和无烟煤所得焙烧产物中金属铁晶粒与脉石矿物结合较紧密,难以在磨矿过程中实现单体解离.褐煤所得焙烧产物中金属铁晶粒出现明显的连接和长大,且与脉石矿物界限分明,嵌布粒度较粗,有利于铁颗粒与脉石矿物的解离,从而其铁回收率较其他还原剂高.     

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.     

鲕状赤铁矿微波加热还原提铁脱磷机制

采用微波加热还原鲕状赤铁矿内配碳球团,考察了还原温度、碱度及添加剂用量对球团含磷组元迁移的影响,对微波碳热还原提铁脱磷机制进行了分析。结果表明,随着还原温度的升高含磷组元逐渐被还原,当还原温度达到1 150℃以上时含磷矿物被大量还原,并且富集到还原铁中造成还原铁粉磷含量过高。在较低还原温度下,通过选择合适的碱度和脱磷剂用量,能有效地抑制含磷组元的还原,促进铁氧化物的还原和聚集。实验采用原矿粒度0.8 mm、碱度0.8、碳氧摩尔比1.0、钠盐添加剂用量20%(质量分数)、还原温度为950℃保温10 min的条件对物料进行还原,将还原物料研磨到0.074 mm在65 mT的场强下进行磁选可得到全铁质量分数82.79%、回收率86.49%、P质量分数0.34%的指标,所得到的还原铁粉杂质较少,而含磷物质主要以磷酸盐的形式存在于磁选渣中。     

铁酸钙与赤铁矿非等温还原动力学

采用非等温热重的方法,在30% CO+70% N₂(体积分数)气氛下,以10 K·min^-1升温至1123 K的过程中,比较了铁酸钙与赤铁矿的逐级还原过程及其还原动力学.结果表明:铁酸钙和赤铁矿开始还原温度分别为873 K和623 K;由反应速率与反应度的关系及分阶段X射线衍射物相分析发现,铁酸钙还原过程为两段式反应(CaO·Fe₂O₃→2CaO·Fe₂O₃→Fe),而赤铁矿还原过程为传统的三段式反应(Fe₂O₃→Fe₃O₄→FeO→Fe).通过Freeman-Carroll法计算得知铁酸钙和赤铁矿的还原平均活化能分别为49.88和43.74 kJ·mol^-1;铁酸钙还原过程符合随机成核随后生长模型,动力学模式函数为Avrami-Erofeev方程,其积分形式为[-ln (1-α)]ⁿ;而赤铁矿还原过程动力学机理分为两部分,在还原度α为0.1~0.5时,为三级化学反应模型,模式函数积分形式为1-(1-α)³;在α为0.5~0.9时,符合二维圆柱形扩散模型,动力学模式函数为Valensi方程,其积分形式为α+(1-α)ln (1-α).     

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     

难选鲕状赤铁矿深度还原-磁选实验研究

针对国内某种难选鲕状赤铁矿的特点,进行了深度还原-磁选实验研究,探讨了还原温度、还原时间、二元碱度、磨矿细度和磁场强度等不同实验条件对渣铁分离效果和产品指标的影响.通过光学显微分析、X射线衍射分析、SEM和化学分析等手段确定了原矿与产品的物相组成与特点.在还原温度为1200℃、还原时间为2h及二元碱度为0.2的工艺条件下,获得了品位为91.94%、回收率为95.85%的铁精矿粉.分析表明,所得铁精粉的品位高,有害杂质少.     

低品位赤铁矿球团成核剂强化还原机理研究

对某低品位赤铁矿球团直接还原过程中成核剂作用机理进行了研究.结果表明:在直接还原过程中,成核剂能够使金属铁晶粒形核位垒降低50%以上,有效促进了金属铁晶粒快速形核.同时,添加成核剂后,球团的还原表观活化能由18.10kJ·mol^-1降低到10.15 kJ·mol^-1,降低了43.92%.表观活化能越低,还原反应越容易进行,即成核剂能够有效促进铁矿物还原,从而提高还原球团的金属化率.     

Effect of fluxing agents on reduction degradation behaviour of hematite pellets

During induration at a high temperature, a considerable amount of slag/melt phase forms inside the iron ore pellets, comprising SiO₂, Al₂O₃, CaO, MgO and FeO. After cooling, the slag phase solidifies and acts as an important bonding phase in the finished pellets and influences their room temperature as well as high temperature properties, especially reduction degradation. Fluxing agents play an important role in forming these bonding phases depending on the type and amount of flux. In the present study, the effect of different fluxing agents, namely, limestone, dolomite, magnesite and pyroxenite, on melt formation and microstructure during induration and on reduction degradation behaviour during reduction was examined. From the results, it was understood that to reduce the disintegration during reduction it is essential to increase the amount and distribution of bonding phases like silicates, which are more stable as compared to oxide phases like hematite. Acid pellets exhibited highest reduction degradation due to the presence of more hematite bonds and less silicate bonds. In limestone fluxed pellets, reduction degradation index dropped considerably with increasing CaO content due to the formation of more amount of bonding phase. Dolomite–pyroxenite pellets, on the other hand, showed lower reduction degradation index up to 0.4 basicity, and beyond that, higher degradation was observed due to the increased pore size, which resulted in poor strength of the reduced pellet matrix and hence more degradation. Low reduction degradation observed in pyroxenite and magnesite fluxed pellets could be due to the formation of magnesioferrite and silicate melt, which are more stable phases compared to hematite.     

超细赤铁矿粉非熔态还原实验研究

研究了平均粒度为2μm的超细赤铁矿粉和粒度分布为0.18～0.154mm的常规细矿粉在非熔态下的还原情况.以矿粉的还原度为目标,用改造后的L₁₆(2¹⁵)正交表安排4×2³次试验,以排除还原气氛、还原温度和还原时间等因素的干扰.结果表明,在纯H₂、100%CO两种气氛下,温度为650～850℃时,均能实现不同程度Fe₂O₃→Fe的还原,与常规粒度矿粉相比,超细矿粉的还原度较高,获得相同还原度时所需的还原温度约低365℃.试验中,各因子之间均无相关性,可分别估计其定量的影响,建立线性回归式.超细赤铁矿粉非熔态还原后的粒度分布观察表明,颗粒间并未发生烧结,还原产物仍为粉状.     

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.     

微细粒赤铁矿配加SiO2直接还原试验

采用赤铁矿配加 SiO2制成的团块进行直接还原,研究了微细粒嵌布赤铁矿直接还原焙烧及其分选机理,应用 X射线衍射扫描电镜及能谱等对还原产物进行分析.结果表明,SiO2含量较低时,SiO2对微细粒嵌布赤铁矿直接还原影响较小,SiO2含量增加时,还原团块金属化率、分选指标显著降低.还原温度较高时,SiO2对微细粒嵌布赤铁矿的直接还原-分选的影响加大.采用反应活性较好的烟煤作还原剂,能提高还原过程中铁氧化物还原速率,加快铁氧化物的还原,降低 SiO2对还原-分选指标的影响.SiO2含量升高,还原焙烧产物中硅酸铁含量增加,金属铁含量减少,同时金属铁颗粒粒径也随之减小,SiO2的存在对微细粒嵌布赤铁矿直接还原有显著的不利影响,适宜的还原温度和良好的还原煤有利于改善微细粒嵌布赤铁矿直接还原-磁选指标.     

MgO对赤铁矿粉矿烧结产物低温还原过程的影响

铁矿粉烧结中添加MgO有助于稳定磁铁矿和抑制二次赤铁矿的形成,可以改善烧结矿低温还原粉化性能,但其对烧结矿低温还原过程的影响尚不是十分明确。通过热重法测定不同MgO含量的烧结产物在550℃还原过程中质量的变化,定量地了解MgO对烧结产物低温还原性的影响。同时结合XRD和光学显微镜跟踪试样在还原前后矿物组成、结构及裂纹的变化,揭示了MgO改善烧结矿低温还原粉化的机理。结果表明,MgO对烧结产物低温还原过程的影响与Al_2O_3含量有关,不含Al_2O_3时,MgO抑制烧结产物的低温还原,添加Al_2O_3后,MgO对烧结产物低温还原的抑制作用减弱。另外,烧结产物还原过程中裂纹的形成与烧结产物的矿物组成及结构有关,适当的孔隙有利于减少裂纹的形成,这很好地解释了实际铁矿石烧结生产中添加少量的MgO可以改善烧结矿的低温还原粉化性。