Trace Elements in the Si Furnace-Part II: Analysis of Condensate in Carbothermal Reduction of Quartz
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INTRODUCTION
REDUCING agents in carbothermal reduction of silica include carbon (coke and/or char), SiO(g), CO(g), and SiC. In the process of silica reduction, impurity oxides present in the charge material are also reduced. Less stable oxides than silica, such as oxides of Ni, Co, Fe, Pb, Cu, Cr, Mn, Zn, Na, and K are easily reduced and partitioned between silicon, oxide, and gas phases, while stable oxides of Ca, Al, Ti, and Mg are not reduced. The reduced species can be removed from quartz in the form of volatile gaseous phases. Gaseous species comprise pure metals, as well as suboxides and ELENA DAL MARTELLO, Ph.D., and GABRIELLA TRANELL, Professor, are with the Department of Materials Science and Engineering, NTNU, Trondheim, Norway. Contact e-mails: [email protected]; [email protected]. OLEG OSTROVSKI, Professor, and PRAMOD KOSHY, Research Associate, are with UNSW Materials Science and Engineering, Sydney, NSW, Australia. GUANGQING ZHANG, Lecturer, is with UOW, Wollongong, NSW, Australia. OLA RAANESS, Senior Advisor, and KAI TANG, Research Scientist, are with SINTEF Materials and Chemistry, Trondheim, Norway. RUNE BERG LARSEN, Professor, is with the Department of Geology and Mineral Resources Engineering, NTNU, Trondheim, Norway. Manuscript submitted June 15, 2012. Article published online September 27, 2012. 244—VOLUME 44B, APRIL 2013
combination of metal oxides and hydroxides. The partial pressure of the gaseous species depends on the activities of elements in raw materials and reaction temperature. Myrhaug and Tveit studied the distribution of trace elements present in the metal, ‘‘microsilica’’ (condensed silica fume) and off gases for a Fe-Si furnace[1] and Si furnace.[2,3] The trace elements found in the microsilica were mainly K, Zn, Na, Mg, and Pb, whereas the elements found in the Fe-Si alloy were primarily Cu, Cr, P, Mn, Al, Cr, Ni, Fe, B, and Ti. The behavior of P is dependent on the Fe content in the charge: In the Si furnace, 85 pct of P input reports to microsilica, while in the Fe-Si furnace, only 25 pct. Myrhaug[4] described the behavior of trace elements in the silicon furnace with a model based on the boiling temperature of pure elements. The model assumes that an element evaporates when its boiling point is lower than the process temperature. The boiling point model refers to the case of partial pressure of the elements over the pure element P0EL equals 1 atm, when activity of the elements aEL equals to 1, which is not representative of the real case. Moreover, the model does not take into account the fact that gaseous compounds other than the pure elements may form. Impurities found in the microsilica and in the off gases do not necessarily represent the total amount METALLURGICAL AND MATERIALS TRANSACTIONS B
of gaseous compounds formed in the furnace: Gaseous compounds may condense and circulate in the furnace, which is a process known in the manganese production (Zn(g) and K(g) react with CO(g) and CO2(g) forming ZnO(s) and K2CO3(l,s), respectively[5–7]). The formation of gaseous
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