Kinetics and Mechanism of Low-Grade Manganese Ore Reduction by Natural Gas
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INTRODUCTION
THE carbothermic reduction of manganese ore by solid carbon source is the conventional route in the production of manganese ferroalloys. Because of involvement of gases in the smelting-reduction process of submerged arc furnace (SAF), the gaseous reduction was investigated to improve the production process. Moreover, developing a gaseous reduction process dedicated to manganese ores is desired especially for sustainable development. While the reduction of manganese ores by either CO or H2 (in the absence of a solid carbon source) proceeds to MnO, further gaseous reduction of MnO can only be achieved by using methane.[1,2] In comparison with carbothermic reduction,
ALIREZA CHERAGHI and HOSSEIN YOOZBASHIZADEH are with the Sharif University Technology (SUT), Azadi Avenue, 1458889694 Tehran, Iran. Contact e-mails: [email protected]; [email protected] ELI RINGDALEN is with the SINTEF, Materials and Chemistry, 7465 Trondheim, Norway. JAFAR SAFARIAN is with the Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway. Manuscript submitted October 5, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
gaseous reduction of manganese ores by methane has several advantages such as higher reduction kinetics at lower temperatures.[1,3–7] Methane is thermodynamically unstable at 823 K.[8,9] However, higher temperatures are required for carbide formation from manganese oxides. High activity of methane at elevated temperatures (proper for carbide formation) is reported to be the reason for the high reduction kinetics.[1,6,10] It was suggested that the methane reduction mechanism was by the adsorbed active carbon on the ore/oxide surface, supplied from methane stepwise cracking.[1] High methane partial pressure, which resulted in methane decomposition and carbon deposition on the surface, is reported to impede further reduction.[1,6,11] Hence, adding H2 to methane-containing gas was suggested to decrease the methane partial pressure and to achieve complete manganese carbide formation.[1,4–6] Experimental studies indicated that manganese carbide formation by methane containing gases started at temperatures higher than 1033 K.[1,5] Although increasing the temperature to 1473 K enhanced the reduction rate,[1,5] SiO2 presence in the manganese ore has lowered the extent and the rates of the reduction.[5,6] Mn7C3 is the most reported manganese carbide that was formed in reduction by methane.[1,5] During reduction by methane, formation of other carbides such as Mn5C2 at 1473 K has also been reported.[3,4]
Because of the higher reducibility of iron oxides compared to manganese oxides (for given conditions) and iron carbides’ role as heterogeneous nucleation sites for manganese carbide formation, the presence of iron oxide in the ore increased the rate of carbothermic reduction of manganese ores.[12] While the co-existing iron oxide of the ore was carburized by methane,[3,4,13] it has been reported that the carbide species was changed through the reaction progress from Fe7C3 to Fe3C.[3
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