ISIJ International
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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559

ISIJ International Vol. 33 (1993), No. 11

  • Effect of CO Gas Formation on Reduction Rate of Iron Oxide in Molten Slag by Graphite

    pp. 1125-1130

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    DOI:10.2355/isijinternational.33.1125

    An idea of vacuum suction through a porous graphite tube has been applied to maintain direct contact between FeO bearing molten slag and graphite by continuous removal of CO gas generated by the reduction. In this way examination of relative importance of the direct and indirect reductions has become possible. The primary slag had the composition of 20.5%Li2O-38.4%CaO-41.4%SiO2 (molar ratio 1:1:1). The initial concentration of FeO in the molten slag was varied between 1.5 and 12.5%. Experiments were made at 1300°C under argon gas stream. The rotation speed of the porous graphite tube was kept constant at 100 rpm.
    The net effect of vacuum suction is the resultant of two opposing effects of enhancement by interfacial agitation by CO bubble evolution and interference by gas bubbles around the graphite tube. These two effects balance each other when the initial FeO concentration is below about 2%. For concentrations high than 2%, the rate of reduction is larger when the gas is removed by vacuum suction. The positive net effect indicates that the indirect reduction by CO gas is not so important as the direct reduction by graphite.
    A possible mechanism has been proposed to explain how the direct reduction can be responsible for the reduction of iron oxide in molten slag by graphite.
  • Kinetic Studies on the Dissolution of Nitrogen into Molten Iron by 14N-15N Isotope Exchange Reaction

    pp. 1131-1135

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    DOI:10.2355/isijinternational.33.1131

    The dissolution of nitrogen into molten iron in the temperature range from 1823 to 1923 K has been investigated using an isotope exchange reaction. The feature of this technique is that the chemical reaction rate can be determined without the effect of mass transport in liquid iron. It was experimentally shown that the dissolution of nitrogen is a first order reaction with respect to the partial pressure of nitrogen. The rate constant (k) for an extremely purified molten iron is expressed as follows with an activation energy of 95.0 kJ/mol.
    log k (mol/cm2·s·atm)=–4960(±1350)/T–1.83(±0.03)
    Even a very small amount of oxygen contained in the gas has been observed to retard the reaction considerably.
  • Feasibility Study on Blast Furnace Ironmaking System Integrated with Methanol Synthesis for Reduction of Carbon Dioxide Emission and Effective Use of Exergy

    pp. 1136-1143

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    DOI:10.2355/isijinternational.33.1136

    Partial replacement of coke by natural gas and production of methanol from the off gas will result in the integrated blast furnace ironmaking system with methanol synthesis. According to the requirement of reduction of CO2 emission and energy savings, the feasibility study of the integrated system was conducted. Operating data of the new system was calculated by a mathematical model based on the heat and mass balances. The effect of operating conditions, such as replacement amount of coke and conversion factor to methanol, on the total emisson of greenhouse effect gas (GEG) and on the exergy loss was examined. The exergy loss and the GEG emission decreased with increase in the replacement amount of coke by natural gas and the conversion factor. In the operation case that has 300 kg-coke/thm and 219 Nm3-natural gas/thm in fuel ratio and 2.3% in conversion factor, the total amount of GEG decreased from 542 to 454 kg/thm and the sum of exergy loss also decreased from 9.0 to 8.2 GJ/thm.
    x

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    1. Mechanisms of Hanging Caused by Dust in a Shaft Furnace ISIJ International Vol.31(1991), No.9
    2. Prediction of a Blast Furnace Burden Distribution Variable ISIJ International Vol.36(1996), No.9
    3. Suppression of the Formation of Large Pores in the Assimilated Parts of Sinter Produced Using Pisolitic Ores ISIJ International Vol.36(1996), No.11
  • Methanol Synthesis from Blast Furnace Off Gas

    pp. 1144-1149

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    DOI:10.2355/isijinternational.33.1144

    Carbon dioxide emission from the blast furnace can be suppressed by the establishment of the blast furnace operation with natural gas injection and methanol synthesis process from the off gas. Natural gas injection leads to the decrease in concentrations of CO and CO2 and to the increase in H2, which are advantageous to the methanol synthesis reaction. Catalysts of Cu-ZnO/Al2O3 system were found active in the methanol synthesis from the off gas, although the composition of a Cu-ZnO/Al2O3 catalyst used in the conventional methanol synthesis industry showed the formation of only a small amount of methanol. The catalysts of Cu-ZnO/Al2O3 system with the higher concentrations of Cu and ZnO were highly active and selective to methanol where the selectivity reached 90%. The reduction temperture of the catalysts exerted remarkable influence on the methanol yield. The Cu-ZnO/Al2O3 catalyst with the optimal composition of Cu, ZnO, and Al2O3 demonstrated excellent stability in the methanol synthesis during a long time run.
    x

    Readers Who Read This Article Also Read

    1. Mechanisms of Hanging Caused by Dust in a Shaft Furnace ISIJ International Vol.31(1991), No.9
    2. Prediction of a Blast Furnace Burden Distribution Variable ISIJ International Vol.36(1996), No.9
    3. Suppression of the Formation of Large Pores in the Assimilated Parts of Sinter Produced Using Pisolitic Ores ISIJ International Vol.36(1996), No.11
  • In-flight Reduction of Fe2O3, Cr2O3, TiO2 and Al2O3 by Ar-H2 and Ar-CH4 Plasma

    pp. 1150-1158

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    DOI:10.2355/isijinternational.33.1150

    The in-flight reductions of Fe2O3, Cr2O3, TiO2 and Al2O3 were carried out in the Ar-H2 and Ar-CH4 plasma in order to elucidate the reduction ability of hydrogen and carbon in thermal plasma.
    The Ar-H2 plasma reduces Fe2O3 and Cr2O3 to their metals. TiO2 is reduced to Ti2O3 and Ti3O5. Al2O3 is not reduced. The products contain non-spherical and spherical particles, which are quenched and cooled from gas and liquid respectively. The reaction mechanisms are explained as follows, from the experimental observations, the phase diagram calculation and the heat conduction calculation. The oxide particles in the plasma are heated quickly, and then they melt and vaporize. Fe2O3 and Cr2O3 are vaporized as a metallic state in Ar-H2 plasma. They are quenched and precipitated as non-spherical particles of their metals. TiO2 and Al2O3 are vaporized as their suboxides such as TiO, AlO and Al2O. They are precipitated as non-spherical particles and oxidized to stable oxides such as Ti2O3, Ti3O5 and Al2O3 during the cooling. The particles which are not vaporized completely are cooled from liquid phase as spherical particles. The phase diagram calculation is used to predict the quenched phase and the quenched temperature. The results of the heat conduction calculation of the particles in the plasma support this vaporizing and quenching mechanism.
    The Ar-CH4 plasma reduces Fe2O3 and Cr2O3 to their metals. Chromium carbide and titanium carbide are obtained from Cr2O3 and TiO2 by this treatment. The mechanisms are induced as follows. Fe, Cr, Ti and Al are vaporized as metallic sates or the suboxides. When they are cooled, Cr and ti make their carbides and Al precipitates as Al2O3. The production of carbides makes it difficult to obtain metals by reduction of oxide with carbon.
  • Mathematical Modelling of Reversed Flow and Heat Transfer in the Combustion Chamber of the LB Furnace

    pp. 1159-1167

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    DOI:10.2355/isijinternational.33.1159

    A mathematical model has been developed to describe the turbulent and reversed flow with combustion reaction and radiation heat transfer in a cylindrical combustion chamber. Measured gas velocity and tracer concentration in the cold flow compared well with the mathematical modelling results, and by using the same model for the high temperature reacting flow enables the correct trends in overall heat transfer rates to be predicted. The model was applied to the design of the combustion chamber of the LB furnace. The influences of the location of the combustion air inlet and the preheating temperature of air on the overall heat transfer rate from the flame to the reduction tube, where heat is required, were investigated for process optimization. Through modelling work it is found that 1) radiation dominates the overall heat transfer process in the chamber, 2) the heat transfer rate to the reduction tube in the chamber may be greatly enhanced by adjusting the gas flow and combustion pattern through selecting the air inlet location and preheating the combustion air, and 3) the model is useful as a design tool.
  • Kinetics of Gaseous Reduction of Iron Ore Fines

    pp. 1168-1173

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    DOI:10.2355/isijinternational.33.1168

    Unsintered beds of blue dust of varying thicknesses were subjected to reduction in flowing reducing gases. The temperature range was 898 to 1123 K for hydrogen reduction and 1073 to 1373 K for reduction by carbon monoxide. Reduction by CO was carried out in two-stages, and only the data of second stage (wustite→iron) were analysed further. Several approaches to data analysis were attempted. The most satisfactory one was to take the slope of the initial linear region of fractional reduction vs. time curve as a measure of rate constant (k). Extrapolation of k vs. bed depth data to hypothetical zero bed depth for H2-reduction and to 0.4 mm depth for CO-reduction allowed determination of chemical rate constants (kc). In kc vs. 1/T plots were not straight lines. This and other anomalous behaviours were attributed to structural changes in the bed during reduction.
  • Effects of Sponge Iron on the Electric Arc Furnace Operation

    pp. 1174-1181

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    DOI:10.2355/isijinternational.33.1174

    Data obtained on a 70-ton UHP electric arc furnace have been used for studying the effects of the sponge iron proportion in the metallic charge on important technological parameters of the steelmaking operation. The results obtained show that an increase in the sponge iron proportion leads to an increase in the consumptions of electric power, graphite electrodes, furnace refractories, lime and deoxidizers. The metallic yield decreases, whereas the slag weight per ton of liquid steel, the total ferrous oxide in the slag and the oxygen content of the bath increase with increasing sponge iron in the charge. The effects of metallization of the direct reduced pellets on power consumption and metallic yield are the reverse of the effects of the sponge iron proportion. The levels of the residual metals Cu, Ni and Cr, and the concentrations of the impurities P and S decrease with increasing sponge iron in the furnace feed. The use of sponge iron also improves the segregation of C, S and N in the steel. The yield point and tensile strength of hot-rolled plain steel bars decrease slightly, whereas elongation increases with increasing sponge iron in the charge. The actual steelmaking time decreases when the sponge iron proportion grows.
  • Effect of Chemical Composition on Apparent Viscosity of Semi-solid Alloys

    pp. 1182-1189

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    DOI:10.2355/isijinternational.33.1182

    Stirring experiments of Al, Cu and Fe-based semi-solid alloys on continuous cooling were carried out to investigate the effect of chemical composition on the apparent viscosity of semi-solid alloy slurries. The apparent viscosity for these alloys increased with increasing solidification rate and decreasing shear rate, and the critical fraction of solid for fluidity increased with decreasing solidification rate and increasing shear rate. The equation for apparent viscosity ηa of semi-solid alloy was proposed as follows:
    ηaLa{1+(αρmC1/3γ−4/3)/[2(1/fs−1/(0.72−βC1/3γ−1/3))]}    (Pa · s)
    α=2.03×102(X/100)1/3
    β=19.0(X/100)1/3
    where ηLa is the apparent viscosity of liquid, ρm is the density of alloy, C is the solidification rate, γ is the shear rate and fs the fraction of solid. The α and β values depended on chemical composition of alloys and became larger with increasing solute content X. These results were explained by the shape parameter value of suspended particles and the volume fraction of liquid trapped between suspended particles in semi-solid alloys.
  • Effects of Strengthening Mechanisms on Sulfide Stress Cracking Resistance of Low Strength Steels

    pp. 1190-1195

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    DOI:10.2355/isijinternational.33.1190

    Sulfide stress cracking (SSC) of low strength steels are often thought as being related to non-metallic inclusions. However, effects of intrinsic metallurgical factors such as chemical composition and micro structure have not yet been clarified. In this paper, effects of strengthening due to grain refining, precipitation, solid solution and dislocation were studied using tensile type SSC tests. Threshold stress (σth) is in proportion to intragrain hardness irrespective of strengthening mechanism and grain size. Further analysis shows that σth is related to σo defined in the Hall-Petch relationship. The ratio of σth to yield strength therefore increases with an increase in grain size, which is contrary to the case of high strength steel. The difference originates from the fact that SSC of low strength steels is transgranular whereas SSC of high strength steel contains integranular fractures.
  • Crystallographic Analysis of Electrodeposited Zinc Crystals on Fe Substrate

    pp. 1196-1201

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    DOI:10.2355/isijinternational.33.1196

    The morphology and the crystallography of η-zinc crystals electrodeposited on a steel substrate have been examined mainly by scanning electron microscopy and X-ray Laue back reflection technique. The morphology of η-crystals varies with increasing the current density from thin layered plates to dendrite structures. The layered crystals form directly on the steel substrate with a specific variant of the Burgers' orientation relationship described as {110}α//(0001)η and <111>α//<1210>η. The average size of these layered crystals increases with decreasing the angle between the <111>α//<1210>η close-packed direction and the substrate surface. These crystallographic features can be explained in terms of the η-crystal nucleation at the steps on the substrate surface.

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