Hematite Compacts Research Articles

Influence of Manganese Oxide and Silica on the Morphological Structure of Hematite Compacts

AbstractDry compacts of pure Fe2O3 and Fe2O3 doped with either (2–6 mass%) MnO2, (2.5–7.5 mass%) SiO2 or with both (2–6% MnO2 + 7.5% SiO2) were indurated at 1373 K for 6 hours and physically and chemically characterized. The fired compacts were isothermally reduced with pure CO gas at 1073–1373 K. The O2‐weight loss was continuously recorded as a function of time using TGA technique. The external volume of pure and doped compacts was measured at different reduction conditions and the volume change was calculated. The structural changes accompanying the reduction process were visually and microscopically examined and the different phases were identified by X‐ray diffraction analysis. After firing, manganese ferrite (MnFe2O4) phase was identified in MnO2‐doped compacts. In pure Fe2O3 compacts, the external volume of compacts was increased with reduction temperature, showing a maximum swelling value at 1198 K. Catastrophic swelling was observed in MnO2‐doped Fe2O3 compacts, the volume change increased with MnO2 content showing catastrophic swelling in compacts containing 6%MnO2 at 1248 K. The catastrophic swelling was attributed to the formation of dense metallic iron whiskers and plates in a highly porous structure. Unlike in MnO2‐doped samples, no considerable volume changes were detected in SiO2‐doped Fe2O3 and (MnO2 + SiO2)‐doped Fe2O3 compacts where the presence of silica greatly hindered the swelling phenomenon at all reduction temperatures.

Influence of Artificially Induced Porosity on Strength and Reduction Behavior of Hematite Compacts

With the objective to gain more fundamental understanding of the influence of pore structure on the low-temperature reduction behavior of iron ore agglomerates, laboratory-scale induration experiments were performed using pure hematite compacts doped with naphthalene as a porosity-generating agent. The pore structure was thus controlled up to 60 % porosity and 1.4 mm pore diameter, and no mineralogical changes were observed in these ranges after induration. The cold strength of indurated compacts was higher with pores smaller than 0.125 mm in diameter, as well as with pores larger than 1 mm. Changes in the cold crushing behavior of indurated compacts are discussed based on matrix length and distribution theory. Reduction tests in CO-CO2-N2 gas at temperatures in the range of 350-800°C revealed high strength after reduction of compacts with fine pores less than 0.125 mm in diameter. Reasons for this high strength were the isotropic stress distribution within the compacts, as well as lower expansion due to development of a complex reduction front.

Influence of sintering conditions on the reduction behaviour of pure hematite compacts

This paper is centered upon possible effects of the peak temperature attained during sintering upon subsequent reduction of hematite. To this end, pure hematite compacts were prepared and sintered at various temperatures in the 1100–1400 °C range, and analysed microscopically before and after reduction. The microstructure undergoes marked changes during high temperature sintering (1370–1400 °C). This change was found to be associated with magnetite formation and subsequent re-oxidation during cooling. Fine cracks (discontinuities) developing along grain boundaries and the coarse pores and irregular shape of re-oxidised hematite, associated with the earlier formation of magnetite, influenced reduction behaviour particularly at 550 °C, resulting in expansion and severe loss of strength. In isothermal reduction tests of pure hematite tablets conducted between 400 and 1200 °C, four types of reduction behaviour were observed. Expansion and strength after reduction were both affected by such changes of the reduction mechanism. Reduction behaviour was influenced by both the reduction temperature and nature of the microstructure prior to reduction.

Influence of reduction conditions on the expansion and microtexture of sintered hematite compacts during the transition to magnetite

Permeability in the blast furnace shaft is adversely affected by low-temperature reduction degradation of sinters. The fundamental cause of this is the expansion of the iron oxide phase resulting from the reduction of hematite to magnetite. The present study shows that expansion behavior during the hematite/magnetite reduction step and the resulting micro structure are strongly influenced by the reduction temperature (partial oxygen pressure), the reducing agent and the nature of the hematite. The reduction rate is of minor importance. A reduction mechanism is proposed that can explain the dilation behavior and the magnetite microst nature. The results provide better insight into the reduction degradation behavior.

Influence of doping with CaO and/or MgO on stepwise reduction of pure hematite compacts

Fired compacts made from pure Fe2O3 and Fe2O3 doped with 1·0%CaO and/or 1·0%MgO were isothermally reduced in a stepwise manner at 1173–1473 K. At each temperature, hematite was first reduced to magnetite in 10CO–90CO2, then to wustite (Fe0·934O) in the appropriate mixture of CO–CO2, and finally to iron in pure CO. The influence of doping with CaO and/or MgO on structural changes and the reduction kinetics and mechanism was intensively studied. The reduction rate of Fe2O3 to Fe3O4 was increased in the order CaO–MgO–Fe2O3 > MgO–Fe2O3 > CaO–Fe2O3 > Fe2O3, and this transformation step was controlled by gaseous diffusion in the early stages and by interfacial chemical reaction at later stages. The reduction of magnetite to wustite was controlled by a combined gaseous diffusion and interfacial chemical reaction mechanism. The reduction of wustite was also promoted, in the early stages, in the presence of CaO and/or MgO. At a level of ≥40%, the presence of MgO retarded reduction, resulting in a slowing down of the rate at lower temperatures. The wustite–iron step was controlled by a mixed reaction in the early stages. At later stages, the reduction of CaO–FeO specimens was controlled by an interfacial chemical reaction, while solid state diffusion had a pronounced effect on the reduction of pure and MgO containing compacts.

ICaO and/or MgO on stepwise reduction of pure hematite compacts

ICaO and/or MgO on stepwise reduction of pure hematite compacts

Reduction of hematite compacts by H2‐CO gas mixtures

The reduction behaviour of hematite compacts by H2‐CO gas mixtures was investigated at 1073‐1223 K. The total porosity, pore size distribution and surface area of the compact was measured using mercury pressure porosimeter. The reduction tests were carried out using Cahn balance. The reduction behaviour could not be described in terms of a single rate‐determining step; the reduction process was initially controlled by the chemical reaction at the oxide/iron interface, controlled by the intraparticle diffusion through the reduced layer towards the end of reduction, and the mixed control, in between. Over the whole range, the reduction rate decreased with CO content in the gas mixture. The chemical reaction rate constants were two to three times higher for H2 reduction than those of CO reduction, and the effective diffusivities of H2 reduction were three to four times higher than those of CO reduction. Values of activation energy for chemical reaction were found to be 19.8‐42.1 kJ/mol depending on the gas compositions; 100% CO showing the lowest.

Kinetics and Mechanisms of Re-oxiation of Freshly Reduced Iron Compacts.

Hematite compacts were isothermally reduced with hydrogen at 973-1273 K. The reduction was carried out up to 90-95% reduction degree using thermogravimetric technique. The reduced products were subjected to re-oxidation in dry air. The rates of re-oxidation at 473-1073 K as a function of time was studied. Microscopic examination, and porosimetric measurements were used to elucidate the kinetics and mechanisms of reduction and re-oxidation. During reduction of iron oxide, the rate was increased with temperature and the structure of sponge iron produced was temperature dependent. Three transition temperatures were identified during re-oxidation. Reoxidation at the initial stages was controlled by interfacial chemical reaction whereas at both the intermediate and later stages, solid-state diffusion was the rate controlling step.

Effect of Sintering on the Structure of Hematite and Its Behaviour During Reduction

Hematite compacts were sintered at 973–1273 K for up to 2 h in air. The physical characteristics of green and sintered compacts (total porosity, pore surface area, pore size distribution and particle sizes) were measured and correlated with the sintering conditions. The surface structure of the compacts was examined with scanning electron microscopy. A mathematical model was proposed to correlate the variation of porosity with sintering parameters.Sintered compacts were isothermally reduced with H2 at 773–1273 K using a weight-loss technique. The mechanisms and kinetics of reduction were studied and correlated with the structure of the Fe2O3 compacts. Another model was derived to predict the apparent activation energy at the initial stages of the reduction of Fe2O3 compacts having a certain pore surface area.

Influence of original structure on the kinetics and mechanisms of carbon monoxide reduction of hematite compacts.

Green, porous and dense Fe2O3 compacts having 54, 35 and 8% porosity respectively were obtained at different temperatures. The structure of these compacts was examined and correlated to their thermal history. These compacts were isothermally reduced with carbon monoxide at 500-1100°C using weight-loss technique. The structural changes accompanying the reduction process were examined with mercury pressure porosimeter and with optical and scanning electron microscopes while the different phases formed during reduction was identified by X-ray diffraction technique. At the initial stages of reduction, the rate was increased with temperature and with increase in original porosity of compacts. With progress of reduction, a reduction rate minimum was observed at 950-975°C and was attributed to sintering and densification of γ-iron. The retardation in the rate was not detected during the reduction of dense compacts due to the disintegration of iron grains resulting from carbon deposition and subsequent reactions. The values of apparent activation energy were calculated and correlated with both the original structure of compacts and the reduction mechanisms.

Influence of the original structure on the kinetics of hydrogen reduction of hematite compacts.

Green compacts of hematite (54% porosity) were sintered in air at 1000°C for 30min and at 1300°C for 180min to produce porous (35% porosity) and dense (8% porosity) compacts, respectively. Hematite compacts were isothermally reduced with hydrogen at 500-1100°C using weight-loss technique as a function of time. The structural changes accompanying sintering and reduction processes were examined with mercury pressure porosimeter and optical and scanning electron microscopes. The reduction of porous and dense samples reveals the presence of reduction rate minimum at 650°C which was attributed to sintering and densification of the freshly reduced iron around the oxide grains. The retardation in the rate at 650°C was increased with the increase in the reduction extents and with decrease in the porosity of the compacts. The computed values of activation energy were correlated with the corresponding reduction mechanisms.

Expansion and Contraction during Hydrogen Reduction of Green and Pre-heated Hematite Compacts Containing Foreign Oxides

Effects of the addition of SiO2 and Al2O3 to green and pre-heated hematite compacts on the expansion/contraction of the two compacts when reduced in a hydrogen atmosphere were studied by measuring changes in the electrical resistance and thermal expansion/contraction.The results obtained are as follows:(1) Green compacts are reduced faster than pre-heated compacts. The reduction of green compacts proceeds simultaneously at the inner parts as well as at the surfaces, while pre-heated compacts are reduced slowly in accordance with the core model where the reduction proceeds gradually from the outer to the inner parts.(2) In case of green compacts, the smaller the oxide particle size is, the slower the reduction rate is and also the less the shrinkage due to reduction is.(3) Cracking of green and pre-heated compacts containing foreign oxides is considered to be due mainly to the local stresses in compact matrix caused by the difference in shrinkage between the contained oxides of different sizes and, in some cases, to the so-called phase-separated structure.(4) The electrical resistance is clearly related to expansion/contraction over 95% reduction in case of green compacts except the compact containing γ-alumina; in case of pre-heated compacts, over 90% reduction when silica is contained and over about 60% reduction when alumina is contained.

異種酸化物含有ヘマタイト圧粉体とその焼成体の水素還元過程における膨張と収縮

Effects of the addition of SiO2 and Al2O3 to green and pre-heated hematite compacts on the expansion/contraction of the two compacts when reduced in a hydrogen atmosphere were studied by measuring changes in the electrical resistance and thermal expansion/contraction.The results obtained are as follows:(1) Green compacts are reduced faster than pre-heated compacts. The reduction of green compacts proceeds simultaneously at the inner parts as well as at the surfaces, while pre-heated compacts are reduced slowly in accordance with the core model where the reduction proceeds gradually from the outer to the inner parts.(2) In case of green compacts, the smaller the oxide particle size is, the slower the reduction rate is and also the less the shrinkage due to reduction is.(3) Cracking of green and pre-heated compacts containing foreign oxides is considered to be due mainly to the local stresses in compact matrix caused by the difference in shrinkage between the contained oxides of different sizes and, in some cases, to the so-called phase-separated structure.(4) The electrical resistance is clearly related to expansion/contraction over 95% reduction in case of green compacts except the compact containing γ-alumina; in case of pre-heated compacts, over 90% reduction when silica is contained and over about 60% reduction when alumina is contained.

The effect of firing conditions on the Strength of Hematite compacts

The strengths of fired hematite compacts have been measured and the results correlated with changes in the other physical properties of the compacts brought about by the sintering treatments. The decrease in porosity during sintering was the main factor in strength development. Empirical equations were developed which predict the porosity and strength of the compacts as a function of the time/temperature treatments. Structural observations indicated that at high porosity values the fracture mode was intergranular, whereas at low porosities the mode was mainly transgranular. Regardless of the time/temperature treatments, the compact strengths tended towards a maximum limiting value.