氧化钨氢还原动力学的研究进展

超细钨粉是目前制备多种硬质合金和结构材料的重要原料,其主要制备方法为氢还原法.在对大量文献资料进行查阅及分析的基础上,综述了氧化钨氢还原动力学的研究现状,包括氧化钨氢还原的基本原理、不同动力学条件下的还原历程以及物相转换.介绍了国内外学者在不同钨氧化物的氢还原过程中,通过改变动力学参数对产物钨粉的粒度、形貌、结构和性能的影响,以及实际工业生产的现状和存在的问题.重点介绍了目前氢还原动力学的几种模型及其方程,并对可能适用于氧化钨氢还原动力学研究的动力学模型进行了展望.      

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 nickel oxide reduction by hydrogen; Measurements in a fluidized bed and in a gravimetric apparatus

The suitability of a batch fluidized bed laboratory reactor for measuring the rates of gas-solid reactions was investigated. Experiments were carried out on the reduction of Falconbridge nickel oxide by hydrogen in a batch fluidized bed reactor within the tem-perature range 550 K to 650 K using particles in the range of 60 to 100 mesh. The reactor was operated at approximately atmospheric pressure and gas flow rates were in the range of two to four times the minimum fluidization velocity at temperature. The results showed internal consistency and rough agreement with the results of previous workers. The re-sults were interpreted and correlated by means of a structural model for gas-solid reac-tions. As a check on the fluidized bed measurements, experiments were also carried out using the conventional gravimetric technique to measure the rate of reduction of compac-ted pellets of nickel oxide by hydrogen. When due allowance was made for the change of surface area of the oxide during compaction, the results were in close agreement with the fluidized bed results. Rate measurements using hydrogen-nitrogen mixtures revealed that the reaction is not first order with respect to hydrogen, as usually assumed, but is ap-proximately of order two-thirds at one atmosphere hydrogen partial pressure. Formerly Graduate Student at Berkeley     

Investigation of the kinetics of reduction of nickel tungstate by hydrogen

In the present work, the kinetics of reduction of nickel tungstate, NiWO₄, by hydrogen was investigated by a thermogravimetric method in the temperature range 891 to 1141 K. The experiments were conducted under both isothermal and nonisothermal conditions. The products were examined by X-ray diffraction analysis. The results indicate that the reduction reaction proceeds in two steps; first, reduction of NiWO₄ to nickel as well as WO₂ and then WO₂ to tungsten. From the isothermal experiments, the activation energies of the two reaction steps were calculated to be 95.3 ± 4.9 and 80.8 ± 6.4 kJ · mol⁻¹, respectively. The activation energy value obtained from nonisothermal experiments for the first step is in agreement with the isothermal experiments. The values are compared with the activation energies reported in other literature for the individual oxides. Formerly with Royal Institute of Technology, Stockholm, Sweden     

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.     

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 kinetics of reduction of lead oxide by CO + CO2 gas mixtures

Abstract

The kinetics of reduction of lead monoxide by CO + CO₂ gas mixtures have been investigated in the temperature range 538° to 630°C. The effect of temperature on the over-all rate seems to be slight; an activation energy of 4.8 k cal/g mole was computed from the average rates. The results were interpreted using a diffusion model that considers the reduction to take place primarily at the boundary separating the partially reduced and the unreacted parts of the oxide compact. The over-all resistance to the reduction process appears to consist of two components: gas phase film diffusion and pore diffusion through partially reduced compact. For the most part, the reduction process is dominated by the pore diffusion. In the initial stage (t < 4 min.), the resistance offered by the film diffusion may be significant.

Résumé

La cinétique de réduction du monoxyde de plomb par le mélange gazeux CO + CO₂ a été étudiée pour des températures allant de 538°C à 630°C. L'effet de la température sur le taux global semble faible; une énergie d'activation de 5.8 Kcal/g. mole a été calculée à partir des taux moyens. Les résultats ont été interprétés à l'aide d'un modéle de diffusion qui considére que la réduction se passe d'abord à la frontière entre l'oxyde partiellement rèduit et l'oxyde non réduit de la pastille. La résistance globale au processus de réduction semble formée de deux composantes: la diffusion à travers la couche limite gazeuse et la diffusion dans les pores de l'oxyde partiellement réduit. Pour sa plus grande partie, le processus de réduction est contrô1é par la diffusion dans les pores. Dans le stade initial (t < 4 min), la résistance offerte par la couche limite peut être importante.     

Intrinsic kinetics of the hydrogen reduction of Cu2S

Intrinsic kinetics of the hydrogen reduction of cuprous sulfide (Cu₂S) have been measured. Experiments were carried out in the temperature range 823 to 1023 K using a thermogravimetric analysis method. The reaction was studied in detail using both thin pellets and powder samples. The reaction followed first-order kinetics with respect to the solid reactant concentration as well as the hydrogen concentration. An activation energy of 92.0 kJ/mol (22.0 kcal/g-mole) was obtained for the reaction. Copper produced from the reaction formed filaments which sintered above about 1000 K. Is now Senior Metallurgist with Cyprus Metallurgical Process Corporation     

温度对氢气还原氧化铁影响及电化学性能研究

采用纯的氧化铁进行直接还原的实验研究,利用H2作为还原气体在高温管式炉中进行焙烧还原.结果表明,随着反应时间的增加,氧化铁块还原率相应的增大.反应温度的不同,氧化铁块还原速率及电导率也相应的不同.600~900℃下经过一定时间的氢还原,还原产物的还原速率随温度的增加而增加,均可达到95%以上.通过交流阻抗法对还原产物的电导率的测试,得到了还原产物的电导率随时间和还原度的变化曲线.随着温度及时间的不同,电导率也相应变化,电导率的不同,可间接反映还原过程中产生了导电性不同的物质.     

Intrinsic kinetics of the hydrogen reduction of copper sulfate: Determination by a nonisothermal technique

The direct hydrogen reduction of metal sulfates may provide an attractive method for producing high-purity metals. In this work the intrinsic kinetics, unaffected by diffusional and mass transfer effects, of the hydrogen reduction of copper sulfate particles have been determined using a nonisothermal technique. The reduction rate shows a first-order dependence on the hydrogen partial pressure and the amount of unreacted copper sulfate. The activation energy of the reaction was determined to be 63.7 kJ/mol. SUN K. KIM, formerly Graduate Student in the Department of Metallurgy and Metallurgical Engineering, University of Utah.     

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.     

氢含量对氧化亚铁粉体还原动力学的影响

采用热重法获得了873~1 173 K氧化亚铁在不同氢含量H2-Ar还原气中的等温还原动力学曲线,发现在973~1 023 K温度范围,随着氢气含量(体积分数)的增加,反应达到的还原程度降低.结合产物的显微结构分析,在该温度范围,随着氢气含量(体积分数)增加,氧化亚铁还原的化学反应速率加快,新生成的铁相黏结加剧,阻碍反应气体的扩散,最终影响整个反应进程.同时发现,反应速率与氢含量不符合线性规律.     

Influence of pressure on the kinetics of synthetic llmenite reduction in hydrogen

In-situ thermogravimetric measurements were used in the hydrogen reduction of poly-granular synthetic ilmenite discs at temperatures in the range 823 to 1173 K and at pressures in the range 1.2 to 13 atm. A symmetrical beam microbalance was used, coupled with twin reactors and twin furnaces, to minimize buoyancy and drag effects. Stable operation was achieved at high gas flow rates where gas film transport effects were negligible. Polishing the ilmenite discs prior to reduction eliminated the formation of dense surface metallic iron films that can impede gas diffusion into the discs. Macroscopically, the reduction reaction proceeded topochemically and a shrinking core reaction model was found to be appropriate to predict conversion-time relationships. It was necessary to allow for water vapor adsorption onto the reacting interface in order to model the effect of pressure on the reduction kinetics. The observed reduction rate increased sharply with pressure up to approximately 3 atm and then approached a plateau with further pressure increase. The porosity in the reduced ilmenite samples was very fine, with pore diameters of typically 0.05 to 0.3 μm. Intragrain gas pressure buildup in the fine pores due to the influence of Knudsen diffusion was incorporated into the modeling of the kinetic data.     

氢气还原褐铁矿实验研究与动力学分析

本文旨在研究不同温度下氢气对褐铁矿还原度和还原速率的影响,并以此为基础分析动力学相关问题.通过热重分析深入了解褐铁矿失水状况.使用粒度为8~12 mm褐铁矿焙烧后,分别在750~950℃五个不同温度下使用4 L·min^-1 H₂进行还原,并分析还原率和还原速率随时间的变化关系.研究发现：随着反应进行,试样还原率逐渐增大,五条还原率曲线在t>28 min后与还原温度排序一致.通过动力学研究计算得反应表观活化能E=15.323 kJ·mol^-1,从而确定扩散为还原反应的限制环节.     

Interfacial kinetics of hydrogen with liquid slag containing iron oxide

Interfacial kinetics on the hydrogen reduction of liquid Fe _t O in Fe _t O-M _x O _y slag (M _x O _y = CaO, SiO₂, Al₂O₃, and TiO₂) has been studied at 1673 K. Because the rate of hydrogen reduction was very fast, the rate was controlled by gas-phase mass transfer under most of the experimental conditions. The effect of CaO or SiO₂ addition on the interfacial chemical reaction rate of hydrogen reduction was empirically evaluated as a function of the ferrous-ferric ratio in the slag. The observed interfacial chemical reaction rates in Fe _t O-CaO and Fe _t O-SiO₂ slags showed reasonable agreement with the estimated values. Most of the available literature data on the reduction rate of liquid iron oxide by solid carbon, hot metal, and reducing gases were also reviewed and compared with the results of the present work. It was found that the rate of hydrogen reduction of liquid iron oxide slag is much faster than that with other reducing agents such as solid carbon, carbon dissolved in the liquid iron, and CO gas. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.     

Hydrogen reduction kinetics of nickel sulfide in the presence of calcium oxide

Recovery of pure nickel from nickel sulfide (Ni₃S₂) was studied by following to completion the hydrogen reduction reaction in the presence of calcium oxide. The effects of reaction temperature, molar ratio of calcium oxide to nickel sulfide, bed depth, and particle size of the nickel sulfide powder on the reaction were experimentally investigated. A simple empirical integrated rate equation describing the relationship among these variables over the temperature range 773 to 973 K was derived. The activation energy for the scavenged reaction was found to be 101.9 kJ from the experimental data. Over the range of experimental conditions, both globular and fibrous forms of metallic nickel were observed.     

生物质碳包覆球状纳米氧化铁非等温还原动力学

采用水热法,以九水硝酸铁和尿素为原料成功制备出球状纳米氧化铁。以生物质葡萄糖为碳源,采用水热炭化法制备了碳包覆球状纳米氧化铁。在N₂气氛下使用高温综合热分析仪测量了不同升温速率下的TG-DTA曲线,通过Flynn-Wall-Ozawa法和Coats-Redfern法相互验证确定碳包覆纳米氧化铁还原的动力学参数。结果表明：包覆生物质碳与纳米氧化铁在180℃就可以发生反应,降低了还原温度,反应活化能约为79.48 kJ/mol,反应模型为三维扩散模型。