Kinetics of Lime-Enhanced Hydrogen Reduction of Solid Nickel Sulphide

Investigations have been carried out on the hydrogen reduction of solid nickel sulphide (β-NI₃S₂) in the presence of lime. The effects of the charge composition, temperature (500-700°C). hydrogen flow rate, time of reduction and particle size of the sulphide have been studied. Lime was found to tremendously enhance the reduction process and drastically stifle H₂S emission into the off-gas. Temperature as well as hydrogen flow rate were found to affect the reduction process and best results were achieved (in static bed experiments) with 200% CaO addition at 630°C in 2 hr employing a hydrogen flow rate of 0.2 l/min. Thermodynamic considerations and several experimental findings indicate that the progress of the Ni₃S₂-CaO-H₂ reaction is governed by the intrinsic kinetics of the Ni₃S₂-H₂ reaction. Kinetic analysis reveals the observance of Jander's linear rate equation indicative of phase boundary control at the sulphide/gas interface. Scanning electron microscopic studies on the reduced nickel sulphide pellets show that like in solid state transformations, discontinuous precipitation (cellular morphology) is exhibited.     

Reduction of Nickel Oxide Powder and Pellet by Hydrogen

Reduction of nickel oxide (NiO) powder and pellet by hydrogen was studied in a thermogravimetric apparatus. The variables studied were temperature (573, 673, 773, 873, 973?K) and hydrogen flow rate (100, 150, 200?cc?min^?1). With NiO powder 70?C80?% reduction and with NiO pellets about 95?% reduction were achieved. With both NiO powder and pellets, the rate of reduction increased with increasing temperature and hydrogen flow rate and the linear nature of the fractional reduction versus time (F vs. t) plot, for the most part of the reduction at all temperatures, supported a rate control by gas film diffusion. The activation energies for the reduction of NiO powder and NiO pellet were found to be 20.14 and 19.21?kJ?mol^?1, respectively. The SEM images showed that the grain size of nickel (Ni) produced was 1?C2???, and the XRD analysis established the presence of Ni (and the absence of NiO) in the reduced sample.     

Reduction of Oxide Scale with Hydrogen

During hot rolling process metals will inevitably oxidize because of high temperature and air condition.In order to guarantee the surface quality,acid pickling is applied to remove the oxide scale while waste acid will do harm to the environment.Faced with the problem,by means of reduction process of hot-rolled plates,the oxide scale will be reduced to iron,so that acid pickling is unnecessary.One pass cold rolling procedure was applied.The compression ratios of hot-rolled plates with oxide scale were 10%,18%,26%and 31%,respectively.After that,samples mentioned above including a sample without deformation were separately reduced under hydrogen atmosphere condition(5% H2 +95% Ar in volume percent)at 600-1 000℃.The thermal gravimetric apparatus(TGA)was used to establish accurate experimental condition and obtain complete mass loss data.Field emission electron probe microanalysis(EPMA)was applied to analyze scale morphology change and composition distribution through the oxide scale.It was found that the sample with 26%compression ratio could be reduced completely at 900℃ which was favorable to galvanization.     

Reduction of Solid Iron Oxide by Carbon

The reduction of pre‐reduced iron oxide by carbon was investigated from a kinetic viewpoint. The experimental procedure included thermogravimetric analysis to measure the weight change of the iron oxide pellet. The activation energy of the overall reaction was calculated to be 210 kJ/mol in the temperature range from 1273 to 1573 K. The overall reduction rate of FeO by carbon is believed to be mixed‐controlled by the Boudouard reaction and by the FeO reduction by CO gas in the temperature range investigated. The critical carbon concentration for the FeO reduction was estimated under the assumption that the activities of FeO and Fe are unity.     

Reduction Kinetics of Metal Oxides by Hydrogen

In this paper, a new kinetic model has been developed for reduction of metal oxides with hydrogen under both isothermal and non‐isothermal conditions. This model describes the kinetics of single reductive reaction and double reductive reactions by considering the diffusion and chemical reaction controlling mechanisms. In particular, the model is in the analytic form of expressing the reduction extent as an explicit function of time, temperature, radius of the particle, and hydrogen partial pressure, which is convenient for using and theoretical analysis. The reduction kinetics of nickel oxide, natural ilmenite, and Fe₂MoO₄ agree well with the theoretical results by the present model.     

Influence of Phase Transitions on the Macrokinetics of the Gaseous Reduction of Iron Oxide

The reduction of magnetite pellets is studied by thermogravimetry in a hydrogen flow upon linear heating. A multiple decrease in the reduction rate is observed at the degrees of metallization higher than 70% in a temperature range of 920–950°C. The observed effect of a decrease in the reduction rate by four to five times on heating is related to the α → γ phase transition of iron. Temperature cycling (heating–holding–heating–holding…) in the phase transition range shows that this effect is reversible; i.e., the reduction rate increases on cooling, unlike that on heating. The average temperature of the beginning of the effect in heating–cooling cycles (913°C) turns out to be close to the thermodynamic temperature (911°C) of the α → γ phase transition of iron. Isothermal studies of the reduction rate of pellets in a temperature range of 900–1000°C also confirm this phenomenon. The effect of a decrease in the reduction rate is assumed to appear due to a decrease in the effective coefficient of gas diffusion in γ-iron as compared to α-iron.     

Kinetics of the Carbothermic Reduction of Manganese Oxide from Slag

Metallurgical and Materials Transactions B - Experiments were performed using a range of test conditions to elucidate the rate controlling step during the reaction of liquid iron-carbon droplets...     

Mechanisms and Kinetics of Reduction of Solid NiO in CO/CO2 and CO/Ar Gas Mixtures

Metallurgical and Materials Transactions B - Experimental studies have been carried out on the reduction of dense nickel oxide in CO/CO2 and CO/Ar gas mixtures at temperatures between...     

Analysis of Product Morphologies and Reaction Mechanisms on Gaseous Reduction of Iron Oxides

It is well established that in gaseous reduction processes solid iron oxides exhibit a wide range of reducibilities. Using the interface stability criteria developed to describe the decomposition of metal compounds in reactive gas atmospheres, together with microstructural evidence obtained under well characterised reaction conditions, the relationships between product structures, and the mechanisms and the kinetics of reduction of iron oxides have been examined. It is shown that the structures and reaction rates can be explained through the occurrence of four principal mechanisms; continuous gas pore formation, dense metal layer growth, discontinuous metal layer breakdown and continuous coupled growth of metal and pores. Through identification of the critical conditions for these different mechanisms, the maximum rates of reduction of a given starting material as a function of temperature, thermal history and gas conditions can be more clearly understood.     

Effect of CaCO3 on the Gaseous Reduction of Tin Oxide Under CO-CO2 Atmosphere

Calcite (CaCO₃), one of the main impurities, has an uncertain effect on the volatilization behavior of tin oxide in the tin-bearing iron ore concentrates during the selective reduction volatilization process. In this study, natural calcite powders were added into the tin-bearing iron ore concentrates to investigate tin volatilization behavior affected by CaCO₃ under different roasting temperature, CO content, and roasting time. In addition, the effect of CaCO₃ on the phase transformation of SnO₂ was determined using XRD, SEM-EDS combined with thermodynamics calculation. The results indicated that CaCO₃ had a significantly adverse effect on the Sn volatilization ratio, because Ca₂SnO₄ was easily formed under CO-CO₂ atmosphere. It was also found from the study that the generation of SnO_(s) was the most critical intermediate for promoting the formation of Ca₂SnO₄.     

Kinetics of Reduction of Low-Grade Nickel Laterite Ore Using Carbon Monoxide

Reduction of a low-grade nickel laterite ore with carbon monoxide to produce Fe-Ni alloy was investigated using a thermo-gravimetric analysis (TGA) method. Non-isothermal reduction tests with a fixed heating rate of 10 °C/min from room temperature to 1200 °C were carried out to determine the different reduction stages and reaction products in each state. Combining measured mass losses with theoretically calculated values together with X-ray diffraction analysis, the products of different reduction stages were identified and a reaction path was established. Isothermal reduction tests with temperatures ranging from 500 °C to 1100 °C were performed to evaluate the temperature dependence of the reduction kinetics. Various kinetic models were fitted to the experimental data to further determine the rate-controlling step in the isothermal tests. Then, two groups of TG experiments were carried out to study the effect of CO flow rate and sample mass on the rate of reaction. The results indicated that the reduction rate increases with the increase of the reduction temperature from 500 °C to 1100 °C. More alloy products are formed and the apparent activation energies increase from 8.6 to 14.7 kJ/mol with the increase of the reduction temperature from 700 °C to 1100 °C. Accordingly, it was proposed that diffusion of CO in the gas bulk and through the pores of the laterite ore sample bed are the rate limiting steps.     

Stability Criteria for Product Microstructures Formed on Gaseous Reduction of Solid Metal Oxides

A range of different solid oxide and metal product morphologies can be formed on gaseous reduction of metal oxides. These product morphologies have been shown in previous studies to be critical in determining the rate limiting reaction mechanisms and the overall rates of reduction. By considering (1) established criteria for the stability of moving interfaces in a thermodynamic potential gradient, (2) the relative rates of chemical reactions on the oxide and metal surfaces, and (3) key process phenomena and physico-chemical properties of the solid phases, the conditions for the formation of various product morphologies are identified. The analysis also demonstrates the theoretical basis for the development of morphology maps that define the product morphologies as a function of thermodynamic driving force for reaction and reaction temperature. The methodology is shown to be general and can be applied to the analysis of any system involving the decomposition of metal compounds in reactive gas atmospheres.     

蓝色氧化钨氢还原制取超细钨粉的工艺研究

以蓝钨为原料，通过氢还原制取超细钨粉末，研究了还原温度、装舟量和氢气流量对制取钨粉粒度的影响。结果表明：还原温度是制取超细钨粉的关键因素，在适当的工艺条件下，可以使用蓝钨制得超细钨粉。     

Kinetics of Reduction of Iron Oxide in Mixtures of Oxide and Carbon: A Critical Appraisal

In the area of alternative ironmaking, processes using mixtures of fines of iron ore with fines of carbonaceous material (coal/char/coke) have become popular and are growing. For about three decades the present author and his co-workers carried out several investigations on kinetics of various aspects of reduction of iron oxide and gasification of carbon, including reduction in mixtures and in composite pellets of fines of iron oxide/ore and graphite/char/coal. The present paper is a brief and critical review, primarily on the basis of the above studies. It is contended that the issues focused here are relevant even now.     

低温下氢气还原氧化铁的动力学研究

 用热重分析法研究了低温下不同粒度氧化铁的氢还原动力学,得出在同一温度下,铁矿粉粒度从107.5 μm降到2.0 μm后,由于粉体的表面积大幅度增加,提高了粉气接触面积,从而使得化学反应的速度提高了8倍左右,还原反应的表观活化能从78.3 kJ/mol降低到36.9 kJ/mol;当反应速度相同时, 粒度6.5 μm的粉体的反应温度比107.5 μm的降低了80 ℃左右。同时,通过理论推导和实验结果表明,当反应扩散层厚度相同时,铁矿粉粒度越小,反应扩散层厚度越薄,其还原率越高。