Reduction of chromites to sponge ferrochromium in methane-hydrogen mixtures

Abstract

Reduction of synthetic and natural chromites in methane-hydrogen mixtures obtained by the hydrogenation of carbon and carbides has been studied by reducing chromite-carbon composite pellets in hydrogen.

Kinetic data from temperature-programmed and isothermal reduction experiments indicate favourable conditions for the production of sponge ferrochromium. Reduction starts above 800°C, with high reduction rates obtainable at temperatures as low as 1000°C.

Reduction of the iron and chromium components of synthetic chromites proceeds simultaneously; other oxides present are not reduced at these temperatures. The product gases have a high reducing potential and consist of a mixture of CO, H₂ and CH₄, with no CO₂ or H₂O. Ferrochromium with controlled carbon content can be produced by adjusting reduction parameters.

The reduction occurs in a single step if sufficient carbon is available for complete reduction of FeO.Cr₂O₃ to Fe₃C,Cr₃C₂. When the stoichiometric amount of carbon is added to the composite pellet for reduction to Fe,Cr, the reduction proceeds in steps. In the first step, Fe₃C and CraC₂ are formed; methane is produced by the hydrogenation of the added carbon, with some of the chromite remaining unreduced. In the subsequent steps, reduction of the remaining chromite proceeds, with methane formed by the hydrogenation of Fe₃C, Cr₃C₂ and lower carbides, ultimately giving ferrochromium with almost no carbon.

The reduction proceeds topochemically and a shrinking-core kinetic model is applicable. From isothermal experiments, activation energies for the reduction of FeO.Cr₂O₃ were found to be 47 and 67 kcal/mole for the first and second steps, respectively.

Résumé

La réduction des chromites synthétiques et naturelles par des mélanges méthane-hydrogéne, obtenue par l'hydrogénation de carbone et de carbures, a été étudiée en réduisant les boulettes composites chromite-carbone dans l'hydrogène.

Les données cinétiques des expériences, à température constante et à. température programmée, indiquent les conditions favorables pour la production du ferro chrome spongieux. La réduction deb ute au-dessus de 800°C, on attaint des taux de reduction eleves3. partir de 1000°C.

La réduction des constituants de fer et de chrome, des chromites synthétiques, se produit simultanément; les autres oxydes présents ne sont pas réduits à. ces températures. Les gaz produits ont un haut potentiel réducteur et consistent en un mélange de CO, H₂ et CH₂, sans CO₂ ou H₂O. Le ferrochrome, avec des quantités de carbone contrôlées, peut être produit en ajustant les paramètres de réduction.

La réduction se déroule en une seule étape s'il y a suffisamment de carbone disponible pour la réduction complète du FeO.Cr₂O₃ au Fe₃C.Cr₃C₂. Lorsque la quantité stoëchiométrique de carbone est ajoutée aux boulettes composites pour la réduction au Fe,Cr, cette réduction procède en plusieurs étapes. Au moment de la première étape, Fe₃C et Cr₃C₂ sont formés avec Ie méthane produit par l'hydrogénation du carbone ajouté, et avec quelques chromites restantes non-réduites. Aux étapes subséquentes, la réduction des chromites sa produit avec le méthane formé par l'hydrogénation de Fe₃C₁Cr₃C₂ et de carbures non saturés, finalement donnant du ferro chrome sans carbone en excès.

La réduction est topochimique et un modéle de réaction purement interfaciale est applicable. A partir d'expériences isothermes, les énergies d'activation pour la réduction de FeO.Cr₂O₃ sont 47 et 67 Kcal/mole pour la prèmiere et seconde étape respectivement.     

Carbothermic reduction of domestic chromites

Maximizing minerals recovery from domestic resources and minimizing the energy requirements of mineral processing constitute two goals of the Bureau of Mines, U.S. Department of the Interior. Accordingly, the Bureau has determined that coal char is generally the preferred reductant among commercially available carbonaceous materials in laboratory scale reduction experiments in argon between 1100 and 1500°C on a domestic metallurgical grade chromite with respect to the degree of reduction and metallization. For a domestic high iron chromite, coal char is preferred between 1100 and 1300°C, whereas metallurgical coke is the reductant of choice at 1400 and 1500°C. Both domestic chromites display generally similar reduction characteristics. The degrees of reduction and metallization are proportional to the time and/or temperature used and generally the rate of reduction is greatest during the first 15 min. The high iron chromite is more easily reduced, especially at the higher temperatures. Relatively simple kinetic equations cannot adequately describe the reduction mechanism for both chromites. The reduction may be nucleation controlled, especially under conditions of interest to commercial operations. The results could be used as a guideline for prereducing domestic chromites for subsequent smelting operations.     

High-temperature synthesis of tungsten carbide in a methane-hydrogen gas medium

Tungsten carbide synthesis in a methane-hydrogen medium have been studied and the conditions for obtaining WC power with no free-carbon impurity determined. Hard alloys based on a “gas” carbide have been shown to exhibit high ductility. The results of the study have been used to develop a commercial process to produce high-ductile hard alloys for high-pressure equipment, stamping dies, and drilling tools.     

Effect of methane-hydrogen mixtures on flow and combustion of coherent jets

Coherent jets are widely used in electric arc furnace(EAF)steelmaking to increase the oxygen utilization and chemical reaction rates.However,the influence of fuel gas combustion on jet behavior is not fully understood yet.The flow and combustion characteristics of a coherent jet were thus investigated at steelmaking temperature using Fluent software,and a detailed chemical kinetic reaction mechanism was used in the combustion reaction model.The axial velocity and total temperature of the supersonic jet were measured via hot state experiments.The simulation results were compared with the experimental data and the empirical jet model proposed by Ito and Muchi and good consistency was obtained.The research results indicated that the potential core length of the coherent jet can be prolonged by optimizing the combustion effect of the fuel gas.Besides,the behavior of the supersonic jet in the subsonic section was also investigated,as it is an important factor for controlling the position of the oxygen lance.The investigation indicated that the attenuation of the coherent jet is more notable than that of the conventional jet in the subsonic section.     

Magnetic properties of rare earth HoCrO_3 chromites

The temperature dependence of the magnetic properties was systemically studied by dc/ac magnetization and specific heat measurement for heavy rare earth HoCrO3 chromites.The results revealed the existence of complex phase coexistence and competitive magnetic behavior in HoCrO3 chromites.It was found that,in the region of higher temperature above 141.0 K,HoCrO3 behaved as a typical Curie-Weiss paramagnetic(PM).And in the region of low temperature,a novel magnetization behavior was observed with negative magnetization(diamagnetism-like) characteristics under an external field of 100 Oe and M-T curves exhibited two symmetrical branches for field cooling(FC) and zero field cooling(ZFC) modes.This behavior indicated the coexistence of canted antiferromagnetic(CAFM) and weak ferromagnetic(FM) phase.These also exhibited the existence of competition mechanism below characteristic temperature TN1=141.0 K and the magnetic order of Ho ion below 7.5 K.The current complex magnetization might be attributed to the interaction between paramagnetic Ho3+ moments and canted Cr3+ moments.     

Mechanism of low-temperature synthesis of tungsten carbide and WC-Co mixtures in a methane-hydrogen atmosphere

Translated from Poroshkovaya Metallurgiya, No. 10, pp. 31–35, October, 1992.     

Kinetics of the carburization of iron alloys in methane-hydrogen mixtures and of the decarburization in hydrogen

The surface reaction kinetics have been investigated for the reaction CH₄ (gas) = C (dissolved) + 2H₂ (gas) on Fe and Fe-M foils (M = Mn, Cr, Co, Ni, Sn) at temperatures between 800 and 1000°C. The resistance relaxation method was applied in a flow apparatus. The following rate equation was observed for the rate of carbon uptake where is the carbon concentration and k and k′ are the rate constants for carburization and decarburization respectively. For Fe, Fe-10 Ni and Fe-10 Co the value of v = 3, for the alloys with > 20% Ni as well as for all Fe-Mn and Fe-Cr alloys the value v = 4 results from the measurements, which indicates differences in the rate determining step. With increasing Cr, Mn, Ni, Co content the rate constants k and k′ increase. The decrease of the equilibrium carbon concentration with increasing Ni content in Fe-Ni alloys is correlated to a fast increase of k′ and a slow increase of k with increasing Ni content. Beyond the minimum of at about 70 wt.% Ni both rate constants show a strong increase. By holding Ni at high carbon activities (a_C > 0.7 at 1000°C) segregation of carbon induces a surface reconstruction which is characterized by formation of (111) planes and which reduces the reaction rates. The increase of the equilibrium carbon concentration in the systems Fe–Mn and Fe–Cr with increasing content of alloying element is correlated to a slow increase of k′ and a fast increase of k with increasing Mn or Cr content. Sn segregates to the surface and retards the reaction rate by blocking the reaction sites for the surface reaction, similar to the effects of P and S.     

电位滴定法测定铬矿石中三氧化二铬的微波溶样法研究

详细探讨、优化了超高压条件下微波消解铬矿石的条件,并对来自各国的大部分铬矿石进行消解试验。结果表明,在称样量为0.20 g,加入7 mL硫酸、5 mL磷酸和1 mL氟硼酸及在1 200 W的超高压输出功率下消解40 min,能很好的溶解铬矿石样品。在此消解条件下消解样品,并利用电位滴定法对2个不同三氧化二铬含量的铬矿石国家一级标准物质测定10次,测定结果的平均值与认定值的相对误差为0.018 %~0.024 %,相对标准偏差为0.11 %~0.14 %;用相同的方法对来自不同国家的铬矿石进行消解和测定,三氧化二铬测定结果在实验室允许误差范围内。     

Reduction Rate of Ferrous Oxide in Smelting Reduction

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Geochemistry and mineralogy of platinum-group elements (PGE) in chromites from Centralnoye I, Polar Urals, Russia

The Polar Urals region of northern Russia is well known for large chromium(Cr)-bearing massifs with major chromite orebodies,including the Centralnoye I deposit in the Ray-lz ultramafic massif of the Ural ophiolite belt.New data on platinum(Pt)-group elements(PGE),geochemistry and mineralogy of the host dunite shows that the deposit has anomalous iridium(It)values.These values indicate the predom-inance of ruthenium-osmium-iridium(Ru-Os-lr)-bearing phases among the platinum-group mineral(PGM)assemblage that is typical of mantle-hosted chromite ores.Low Pt values in chromites and increased Pt values in host dunites might reflect the presence of cumulus PGM grains.The most abundant PGM tbund in the chromite is erlichmanite(up to 15 μm).Less common are cuproiridsite(up to 5 μm),irarsite(up to4-5 μm),and laurite(up to 4 μm).The predominant sulfide is heazlewoodite,in intergrowth with Ni--Fe alloys,sporadically with pentlandite,and rarely with pure nickel.Based on the average PGE values and esti-mated Cr-ore resources,the Centralnoye I deposit can be considered as an important resource of PGE.     

降低攀钢高炉渣碳化电耗的途径

分析了攀钢高炉渣还原碳化的理论电耗构成，并与实际情况进行了对比，找到了造成电耗高的原因，指出“合理利用”从高炉排出的熔融高炉渣的显热、改善冶炼动力学条件、提高电炉的热效率是降低电耗的3个主要途径，同时还应从理论上深入研究电炉冶炼条件下的相关机理和理论，才能解决经济可行性问题。并对电耗可降低的限度进行了预测。     

高还原势气氛下球团矿还原行为的研究

炉顶煤气循环氧气高炉采用纯氧鼓风以及炉顶煤气循环利用工艺使得炉内煤气成分与传统高炉相比发生了改变,炉内的煤气成分主要以H2和CO为主.为了研究还原性气体H2和CO对球团矿还原行为的影响,分别用H2-N2、CO-N2、H2-CO混合气体在1173K下通过热重的方法进行试验.研究发现还原度随着混合气体中H2或CO比例的增加而增加,但是H2的还原能力明显比CO要强.在H2-CO混合气体中H2的加入有利于还原进行.用H2-CO混合气体还原得到的还原速率不能用H2-N2和CO-N2混合气体下得到的还原速率相加得到.微观结构观察发现,用H2进行还原时得到的铁结构较致密,而用CO还原时铁会破裂为许多小碎片.在用含一氧化碳的混合气体进行还原时,还原度曲线在还原后期由于碳沉积导致出现下降的趋势.还原气体中氢气的存在会加剧碳沉积现象,而氮气的存在会抑制这一现象.对还原后试样进行X射线衍射以及化学分析表明试样中的碳以碳化铁(Fe3C)和石墨形式存在.     

中国发展非高炉炼铁的现状及展望

非高炉炼铁是钢铁工业发展的前沿技术之一。目前,直接还原已成为世界钢铁工业不可缺少的组成部分。熔融还原虽实现工业化生产的环境优越性得以公认,但其低能耗、低成本等优点还有待于实践证实。发展直接还原铁生产弥补废钢的短缺是中国钢铁工业、装备制造业发展的急需。以国内技术为基础,利用国内铁矿资源,以煤制气 竖炉为主导工艺是中国发展直接还原铁生产的主要方向。加强对国外熔融还原技术发展的跟踪,强化国内熔融还原技术的开发力度,尤其是消化COREX技术以及实现其装备的国产化是中国熔融还原发展的重要方向。     

轻压下技术在高碳钢方坯连铸应用中的参数选取

轻压下技术作为改善铸坯偏析的有效措施之一,被广泛应用于高碳钢方坯连铸中,其压下参数的选取对于轻压下技术至关重要.通过理论分析并结合其他厂的生产经验,认为压下位置在中心固相率为0.2～0.8、总压下量为4～8 mm时,轻压下效果最佳.合理地应用轻压下技术,可以有效地减轻铸坯的V型偏析、降低中心碳偏析并改善铸坯的宏观组织.     

含碳球团还原反应及其技术

对含碳球团的自还原反应和工艺中涉及的燃烧和氧化反应进行了论述,分析了煤的性质、氧化性气氛对含碳球团自还原性的影响;介绍并且评价了现有和正在研发的含碳球团直接还原法和熔融还原法。     

熔融还原过程硅还原氧化行为

试验研究熔融还原过程条件下SiO₂还原氧化行为及对过程的影响.研究表明:喷吹煤粉燃烧温度、煤的灰份组成影响SiO₂还原过程.通过适当选择煤种和控制氧煤喷枪的喷吹位置,可以减轻SiO₂还原挥发对过程的不利影响.     

镁热法生产海绵钛还原设备换热过程分析

以热工测试数据为基础,对还原过程中冷却空气的散热进行计算和分析,建立TiCl4加料速度和空气散热强度之间的代数关系式,并以此关系式为基础对TiCl4加料速度进行评估。结果表明,加料速度与风速和一定范围内冷却空气的出口温度存在一定的关系,可以根据冷却空气风速和出口温度得到加料速度的评价值,一定程度上也可以预测加料速度范围。