Non-Topochemical reduction of iron oxides

Non-topochemical behavior was studied during reduction of porous spheres of hematite by stages through the intermediate oxides and also continuously to iron by CO/CO₂ mixtures at temperatures of 600 to 900°C (873 to 1173 K). The behavior became more nearly topochemical as temperature increased. Shrinking occurred during the reduction of hematite to magnetite and of magnetite to wüstite, whereas swelling was observed during the reduction of wiistite to iron. Shrinking was greater, and swelling less, at higher temperatures. The total surface area of the solid decreased with increasing extent of reduction during each of the three stages. A non-topochemical model was developed which satisfies, better than previously proposed models, the reduction data for the single reactions and the three reactions occurring simultaneously. The model provides for variation in particle size and local changes in porosity and effective diffusivity. An empirical “sintering exponent” was introduced to describe changes in reacting surface area.     

Some aspects on porous properties of iron oxides containing foreign oxides reduced by hydrogen

The objective of this study is to prepare suitable iron ores for industrial reduction processes. Therefore, the properties of porous, green and indurated hematite compacts as well as iron ore were investigated by quantitatively measuring pore volume, BET surface area and pore size distribution. Furthermore, the influence of foreign oxides on porous properties was investigated. Based on these data, the mechanism of sintering process of the compacts was discussed. The results obtained here suggest that the porosity of the compacts after almost 100 pct reduction does not depend upon whether the compacts were indurated or not. Therefore, it would be concluded that indurating the compact is not necessary for pulverized iron ores containing foreign oxides when reduced in the process such as a fluidized bed system.     

Whisker growth in reduction of oxides

Cobalt ferrite (CoFe₂O₄) was reduced in CO-CO₂ gas mixtures at 1173 K at total pressure between 6.6 × 10³ and 3.3 × 10⁴ Pa, and at CO/CO₂ ratios between 2.9 and 11.8. The reduction led to the formation of metal whiskers. The experiments and analysis emphasized the behavior of the whisker diameter during reduction. Impurities such as calcium and potassium stimulate metal nucleation but appear to inhibit catalysis of gas reactions at the metal/gas/oxide triple junction. The steady state whisker diameter was found to be inversely proportional to the total gas pressure at constant CO/CO₂ ratio. A new model is proposed to explain whisker development. It considers metal/oxide interface diffusion coupled with a metal/oxide/gas triple junction reaction at the whisker base as the process determining the whisker diameter.     

Reactions of metal oxides with carbon

The reaction of metal oxides with carbon is investigated. Baikov’s hypothesis regarding the role of atomic oxygen in the reduction of metals and the oxidation of carbon is confirmed theoretically and experimentally. The initial stages in the process include conjugate chemical reactions: dissociation of the oxide with the liberation of atomic and molecular oxygen; and the oxidation of carbon. The reaction of thermodynamically stable metal oxides with carbon is slowed by the accumulation of carbon monoxide in the system and the blocking of the reducing agent’s surface by metal carbides and oxycarbides. The production of silicon ferroalloys from briquetted batch is shown to be promising.     

Phase transformation in the oxides

This paper reviews phase transformations in the oxides, and briefly examines the similarities and dissimilarities with the phase transitions in metals. A few topics selected for the discussion, include, displacive phase transitions, order-disorder transitions, and clustering and precipitation. The displacive transition involves the oxygen polyhedra, and is usually found in ferroelectric and antiferroelectric oxides, such as, BaTiO₃, SrTiO₃, PbZrO₃, and other oxides of perovskite structure. In lattice dynamics studies, this transition has been associated with an instability of a long-wave optical phonon. The condensation of the ferroelectric soft mode causes the static distortion of the lattice at the phase transition. The order-disorder transformation in the oxides may occur in different forms. It may be the hydrogen ordering in KDP, the catio distributions in spinel MgFe₂O₄,the Li-Fe ordering of the octahedral sites in LiFe₅O₈, or the formations of layer structures in the double perovskites, such as Ba₂CoWO₆, Ba₂CoReO₆, and others. Clustering and precipitations are commonly observed phenomena in oxide systems. They result in phase separations in both the crystalline, and the glassy systems. Heat treatments to control the crystallization processes in glasses result in glass ceramics, which have been proven to be superior to both the original glass, and to the precipitated crystalline phase. This paper is based on a presentation made at a symposium on “Phase Transformations in Less Common Metals: A Dialogue,” held at the Fall Meeting in Cleveland on October 16, 1972, under the sponsorship of the Phase Transformations Activity, Materials Science Division, American Society for Metals.     

Hot pressing of oxides of refractory metals

Conclusions A study was made of the sintering process in the hot pressing of several powdered oxides of transition metals, namely, TiO₂, ZrO₂, V₂O₅, Nb₂O₅, and Cr₂O₃. It was established that, at any given treatment duration, raising the temperature steadily increases the rate of densification and density of specimens. At each temperature, the densification process is characterized by a specific final density.Translated from Poroshkovaya Metallurgiya, No. 11 (107), pp. 22–25, November, 1971.     

氧化钨物理特征的分析

对铵钨青铜((NH_4)_(0.5)WO_3,ATB)、氢钨青铜(H_(0.33)WO_3,HTB)、紫色氧化钨(W_(18)O_(49),TVO)和蓝色氧化钨(WO_3+WO_(2.9),TBO)4种氧化钨粉的氮气吸附/脱附等温线数据的分析,获得了其表面积、微孔体积、微孔分布、中孔体积、平均孔径和分数维维数等物理参数。结果表明:TVO粉末具有最大的中孔体积、最小的微孔体积、最窄的孔径分布、最小的分数维维数和最大的平均孔径,有利于氢还原制取超细钨粉,而不利于掺杂工艺。HTB粉末具有最大的微孔体积,最宽的孔径分布,最高的分数维维数和最小的平均孔径,对于掺杂工艺来说是有利的。ATB和TBO的上述参数介于TVO和HTB的参数之间。     

Gaseous reduction of iron oxides: Part III. Reduction-oxidation of porous and dense iron oxides and iron

The internal reduction of high-grade granular hematite ore in hydrogen and carbon monoxide, and also the internal oxidation of porous iron granules in CO₂-CO mixtures have been investigated. To assist the interpretation of the rate data for porous iron and iron oxides, rate measurements have been made also with dense wustite, previously grown on iron by oxidation. The iron formed by reduction of dense wustite is porous, similar to that observed when porous hematite is reduced. It is found that the rate of dissociation or formation of water vapor or carbon dioxide on the iron surface is about an order of magnitude greater than that on the surface of wustite. The results of the previous investigations using dense iron and wustite are in general accord with the present findings. The rate of reduction of hematite increases with increasing pore surface area of the reduced oxide. The results indicate that the rate of reduction of granules is controlled primarily by the formation of H₂O or CO₂ on the pore walls of wustite. The specific rate constants evaluated from internal reduction, using the total pore surface area, are about 1/50 to 1/100 of those for dense wustite. These findings indicate that with porous wustite or iron, the effective pore surface area utilized is about 1 to 2 pct of the total pore surface area. The rate of reduction in H₂-CO mixtures is in accord with that derived from the rate constants for reduction in H₂ and CO.     

Solutions of iron oxides in molten cryolite

All the iron oxides (FeO, Fe₃O₄, Fe₂O₃, and FeAl₂O₄) dissolve in cryolite-alumina melts to give solutions containing both Fe(II) and Fe(III). The factor controlling the Fe(II)/Fe(III) ratio is the oxygen pressure, and experimental results are interpreted on that basis. Predictions are made of the variation of solubility with oxygen pressure, and the standard potential of the Fe²⁺/Fe³⁺ redox couple is calculated. The anode and anode gas of an industrial Hall-Heroult cell appear to be insufficiently oxidizing to cause significant conversion of Fe(II) to Fe(III). An anomaly in the liquidus diagrams for FeF₂ – Na₃AlF₆ and FeO – Na₃AlF₆ is accounted for in terms of solid solution of FeF₂ in cryolite. This article is based on a presentation made at “The Milton Blander Symposium on Thermodynamic Predictions and Applications” at the TMS Annual Meeting in San Diego, California, on March 1–2, 1999, under the auspices of the TMS Extraction and Processing Division and the ASM Thermodynamics and Phase Equilibrium Committee.