The Influence of Sulfur Content on the Carbothermal Reduction of SiMn Slag
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INTRODUCTION
THE industrial production of silicomanganese (SiMn) alloys involves the simultaneous carbothermal reduction of MnO and SiO2 dissolved in a liquid slag at high temperatures of 1773 K to 1923 K (1500 C to 1650 C).[1,2] The dominating components of the SiMn slag are MnO, SiO2, Al2O3, CaO, and MgO. MnO and SiO2 are reduced into a molten alloy solution either by solid carbon in coke or by carbon dissolved in metal according to the following reactions[3,4]: MnO ðlÞ þ C ðs; lÞ ¼ Mn ðlÞ þ CO ðgÞ
½1
SiO2 ðlÞ þ 2 C ðs; lÞ ¼ Si ðlÞ þ 2 CO ðgÞ:
½2
In recent decades, the kinetics of MnO reduction in the field of ferromanganese (FeMn) production processes, where solid MnO still exists, have been studied extensively.[5–8] In this process, the activity of MnO is hence much higher than in SiMn slag.[5–8] It has been observed that stirring has no effect on the kinetics of
XIANG LI is with the School of Energy and Power Engineering, Jiangsu University, Jiangsu 212013, China and also with the Department of Materials Science and Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway. Contact e-mail: [email protected] MERETE TANGSTAD is with the Department of Materials Science and Engineering, Norwegian University of Science and Technology. Manuscript submitted February 13, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
MnO reduction, while the reduction rate is highly sensitive to temperature. Therefore, it has been concluded that MnO reaction kinetics are not controlled by any transport mechanism but are controlled by chemical reaction.[5,6] The rate and extent of MnO reduction are affected by the slag chemistry, including concentrations of different oxides and trace elements. In a homogenous liquid slag, higher basicity was found to produce a higher reduction rate of MnO.[6–8] This finding could be due to the higher driving force given by the effect of CaO and MgO on the activity of MnO or by the effect these oxides have on the viscosity of the slag. SiO2 reduction is more difficult than MnO reduction and requires higher temperatures. While MnO is reduced by both solid carbon and dissolved carbon in metal, the formation of Si requires a metal phase with low Si activity.[1] A linear relationship between the activity of SiO2 in the slag and the reduction rate was reported by several investigations, indicating that its kinetics are also controlled by the interfacial chemical reaction.[9–11] Some trace elements in carbon and ore are known to have important effects on reduction rates as catalysts. The sulfur content is of particular interest. Sulfur is known as a surface-active element and significantly decreases the surface tension of the liquid iron on refractories.[12–15] Xu et al.[6,16] observed that the rate of MnO reduction by carbon-saturated iron is decreased with the increase in sulfur content from 0.027 to 0.079 wt pct. It was explained that the effective reaction area decreases due to the adsorption of sulfur in the slag-metal boundary layer. In contrast, other researchers reported a p
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