Abdel-Hady A. El-Geassy

Behaviour of manganese oxides during magnetising reduction of Baharia iron ore by CO–CO2 gas mixture

Abstract High manganese containing iron ore samples were isothermally reduced with a CO–CO2 gas mixture at 600–1000°C. The course of reduction was followed by a weight loss technique. The influence of reducing gas composition and temperature on the reduction kinetics was investigated. The different phases formed during reduction were identified by X-ray phase analysis, while their structures were microscopically examined. The reduced samples were magnetically tested by means of a Davis tube tester. The effect of grain size, drum speed, and cleaning conditions on the efficiency of magnetic separation was studied using a Box-Mag wet low intensity magnetic separator. The separation efficiency was determined by analysing total iron, manganese, and acid insoluble contents in both magnetic and non-magnetic fractions. Best testing results were obtained on separation of the sample reduced with 80CO–20CO2 (vol.-%) at 800°C. The optimum grain size for magnetic separation is below 0·15 mm while that of the drum speed is 100 rev min-1 . The cleaning of the magnetic fraction increases the iron content and decreases the manganese and acid insoluble contents.

Isothermal reduction behaviour of MnO2 doped Fe2O3 compacts with H2 at 1073–1373 K

Abstract Pure Fe2O3 and Fe2O3 doped with 2, 4, and 6 mass% of MnO2 (>99%) compacts annealed at 1473 K for 6 h were isothermally reduced with H2 at 1073–1373 K. The O2 weight loss resulted from the reduction of compacts was continuously recorded as a function of time using thermogravimetric analysis (TGA). High pressure mercury porosimeter, optical and scanning electron microscopes, X-ray phase analysis and vibrating sample magnetometer were used to characterise both the annealed and reduced samples. In MnO2 containing samples, manganese ferrite (MnFe2O4) was identified. The rate of reduction of pure and doped compacts increased with temperature and decreased with the increase in MnO2 content. Unlike in pure compacts, the reduction of MnO2 containing samples was not completed and stopped at different extents depending on MnO2 (mass%). At initial reduction stages, the decrease in the rate was due to the presence of poorly reducible manganese ferrite (MnFe2O4) phase which was partially reduced to iron manganese oxide (FeO0.899, MnO0.101) at the final stages. The reduction mechanism was predicted from the correlation between the reduction kinetics and the structure of partially reduced samples at different temperatures. The reduction of pure and doped samples was controlled by a combined effect of interfacial chemical reaction and gaseous diffusion mechanism at their initial stages. At final stages, the interfacial chemical reaction was the rate controlling mechanism.

Behaviour of iron ore-fuel oil composite pellets in isothermal and non-isothermal reduction conditions

Abstract Self-fluxing iron ore pellets as an alternative to the agglomeration process led to the use of low price fuel oil as a binder and reducing material. Composite pellets containing 5–15% fuel oil were isothermally and non-isothermally reduced at 750–1000°C in a flow of H2 or N2 gases. The total weight loss resulting from O2 removal from the reduction of Fe2 O3 and from the thermal decomposition of fuel oil was continuously recorded as a function of time at different reduction conditions. The actual reduction extent at a given time was calculated from the chemical analysis of partially reduced samples at a given time and temperature. Microscopic examination and X-ray phase analysis were applied to characterise the reduction products. The isothermal reduction of composite pellets indicated that the reduction rate increased with the increase in fuel oil content at the early stages. At the later stages, the reduction rate increased in the order 12>10>5> 15% fuel oil containing pellets. The non-isothermal reduction of composite pellets in N2 atmosphere showed the presence of an incubation period at initial reduction stages. The low intensity magnetic separation technique was applied with the aim of increasing the iron content at the expense of associated impurities. The magnetic and non-magnetic fractions were analysed and the overall recovery was determined.

Simultaneous reduction nitridation for the synthesis of tungsten nitrides from Ni–W–O precursors

Abstract Tungsten nitrides were synthesised from NiO–WO3 and NiWO4 precursors at 973–1273 K in a flow of H2–N2 gas mixture. The reduction–nitridation reactions were carried out isothermally in fluidised bed reactor, and the off-gas from the reactions was continuously analysed by gas chromatography. The effect of reaction temperature and precursor composition on the rate of formation of Ni–W nitrides was studied. The different phases developed during the reduction–nitridation reactions were identified by X-ray diffraction analysis technique. The morphology and the grain structure of the precursors were examined by SEM, and the elemental composition in the structure was analysed by electron dispersive spectrometry. The results showed that the reduction of Ni–W–O precursors proceeded in a stepwise manner (NiWO4→Ni–WO3→Ni–WO2→Ni–W). Tungsten nitrides (WN and WN2) were formed from the reaction of the freshly reduced W metal with N2 gas and WN was the predominant phase detected at higher temperatures. The reaction mechanisms were elucidated from the apparent activation energy values and the application of different formulations derived from the gas–solid reaction model at early and later stages of reactions. It was concluded that the interfacial chemical reaction is the rate determining step at initial stages, while a combined effect of gaseous diffusion and interfacial chemical reaction controlled the reaction at later stages. At final stages, the nitridation reactions contributed to the reaction mechanism leading to produce tungsten nitrides.

Reduction of iron oxide compacts with simulated blast furnace top and shaft gases to mitigate CO2 emissions

Abstract The mitigation of CO2 emissions has become a global priority in the iron and steel industry. One promising solution to decrease CO2 emissions is recycling of the blast furnace top gas to be reused in the reduction of iron oxide. In this study, iron oxide compacts were reduced isothermally with simulated conventional blast furnace top gas (BFTG: 20%CO, 20%CO2, 5%H2, 55%N2) at 700–900°C using a thermogravimetric technique. The reduction reached 30–34% depending on the applied temperature. The compacts were reduced completely to wüstite (Fe0·925O or Fe0·971O) with a few grains of metallic iron, which appeared only at 900°C. To investigate the reduction behaviour at the later stages, the compacts reduced with BFTG at 900°C were followed by isothermal reduction with simulated blast furnace shaft gas (BFSG: 30%CO, 5%CO2, 10%H2, 55%N2) at 950–1100°C. The total reduction extents were increased to 66–90% at 950–1100°C respectively. The remaining unreduced wüstite (Fe0·974O) has a higher Fe/O ratio compared with that formed by reduction with BFTG. At the initial stages of reduction, the rate controlling mechanism was interfacial chemical reaction, while at the later stages, solid state diffusion was the rate controlling mechanism. Reflected light microscope, scanning electron microscope, X-ray diffraction and Poresizer techniques are used to estimate the reduction kinetic and mechanisms.

Developed model for reduction mechanism of iron ore pellets under load

Abstract A laboratory scale shaft furnace for testing the reduction of commercial iron ore pellets under load was constructed. A series of reduction experiments of iron ore pellets with a synthetic reformed natural gas was carried out under various operational conditions of varying temperature, type of ore pellets and applied load. The reduction tests were carried out at 750–1000°C under different loads, i.e. 0·2, 0·4 and 0·57 kg cm−2. The sticking phenomenon accompanying reduction of iron ore pellets under load was investigated and was found to be dependent on temperature, chemical composition and applied load. The experimental results were analysed to investigate the reduction kinetics and mechanism of iron ore pellets under controlled conditions of temperature and applied load. A model was developed depending on the shrinking core modulus and mixed control ratio to give a definite and quantified measure of reduction reaction. This model allows good analysis of the actual effect of reduction temperature and applied load upon controlling mechanism and furthermore, it indicates the relative effect of the different mechanisms in the mixed control regime.

Reduction-Carburization of the Oxides of Ni and W Towards the Synthesis of Ni-W-C Carbides

Ternary Ni-W-C cemented carbides were synthesized directly from mixture powder of NiO-WO3 by simultaneous reduction-carburization in mixed H2-CH4 gas environment in a thin bed reactor in the temperature range 973-1273K. The kinetics of the reaction was closely followed by monitoring the mass change using thermogravimetric method (TGA). The nascent particles of the metals formed by reduction could react with the gas mixture with well-defined carbon potential to form a uniform product of Ni-W-C. The gas mixture ratio was adjusted in such a way that the Ni-W-C formed was close to the two phase tie line. In view of the fact that each particle was in direct contact with the gas mixture, the reaction rate could be conceived as being controlled by the combined reduction-carburization reaction. From the reaction rate, the Arrhenius activation energies were evaluated. Characterization of the carbides produced was carried out by using X-ray diffraction, SEM-EDS as well as high resolution electron microscope (HREM). The grain sizes were also determined. Correlations were found between the carbide composition as well as grain size and the process parameters such as temperature of the reduction-carburization reaction as well as the composition of the gas mixture. The results are discussed in the light of the kinetics of the reduction of oxides and the thermodynamic constraints.

Alternative Reducing Agents in Metallurgical Processes: Gasification of Shredder Residue Material

Shredder residue material (SRM) contains plastic material, which has a potential to replace metallurgical coal for reduction during bath-smelting processes. Among the important parameters affecting its implementation are the gasification and the reactivity of char. Therefore, prior to considering its application in metallurgical processes, the gasification characteristics of the produced char need to be studied. Although the char produced from SRM contains lower fixed carbon compared with coal char, it has a porous structure and high surface area, which makes it highly reactive during gasification experiments. In addition to physiochemical properties, the catalytic effect of ash content of SRM char is attributed to its higher reactivity and lower activation energy compared with coal char. Furthermore, the effect of devolatilization heating rate on the gasification characteristics of produced char is investigated. It was found that the devolatilization heating rate during char production has a considerable effect on morphological properties of the char product. Moreover, the gasification reactivity of char produced at a fast devolatilization heating rate was the highest, due to the less crystalline structure of the produced char.

Alternative Reducing Agents in Metallurgical Processes: Devolatilization of Shredder Residue Materials

Plastic-containing shredder residue material has the potential to be used as an alternative reducing agent in nonferrous bath smelting processes. This would lead to not only decreased dependency on primary sources such as coal or coke but also to an increase in the efficiency of utilization of secondary sources. This calls for systematic scientific investigations, wherein these secondary sources are compared with primary sources with respect to devolatilization characteristics, combustion characteristics, reactivity, etc. As a first step, in this paper, devolatilization characteristics of plastic-containing shredder residue material (SRM) are compared to those of coal using thermogravimetric analysis. Proximate analysis has shown that SRM mainly decomposes by release of volatiles, while coal shows high fixed carbon content, which is reported to contribute to reduction reactions. To study the reduction potential of the evolved materials, composition of evolved off-gas was continuously monitored using quadrupole mass spectroscopy. The composition of volatiles shows H2, CO, and hydrocarbons which are known to have reduction potential. Therefore, it is essential that SRM would be used in a process that could utilize the evolved volatiles for reduction. Furthermore, to understand the potentials of different plastic materials as reducing agents, the devolatilization mechanisms and volatile composition of three common plastics, namely, polyethylene, polyurethane, and polyvinylchloride and their mixtures have been studied. The results show the interaction between the plastics within the binary and ternary mixtures. Similar phenomena may occur during devolatilization of SRM, which contains different type of plastics.