Phase relations associated with the aluminum blast furnace: Aluminum oxycarbide melts and Al-C-X (X=Fe, Si) liquid alloy
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INTRODUCTION
I N recent years there has been growing interest in methods of producing reactive metals such as aluminum in blast furnaces. 1-6In such processes, the heat required for the large endothermic reaction is provided by coke combustion, and there is need for insight into the chemistry of reducing alumina, in addition to existing knowledge of electrothermal processes. Early investigations7-11relating to electrothermal processes have shown that the reduction of alumina by carbon exhibits complicated features, summarized as follows:10.1~ (1) severe volatilization of aluminum components; (2) carbide formation and high carbon solubilities in liquid metals; (3) complicated reaction path to metals. These chemical features are of primary concern also in establishing a blast furnace process. Frey et al. ,~ who first proposed a method of producing an aluminum-silicon alloy in a blast furnace, pointed out that the liquid alloys produced can be carbidized in the presence of excess carbon. Bruno and coworkers2'3'4 placed an emphasis on volatilization in an aluminum-silicon blast furnace; their experimental results and thermodynamic considerations showed that such a furnace may not be feasible unless the volatilization can be reduced considerably. They suggested (1) pressurizing the furnace; (2) adding iron to decrease the activity of liquid aluminum and silicon; (3) giving up the complete coke-combustion heating. All are related to the gas-condensed phase equilibria. We started this investigation in 1977 and showed experimentally that it is really difficult to reduce alumina-silica ores in a blast furnace because of the formation of a rigid bridge. 5 Our current objective 5'6 is therefore to make HARUMI YOKOKAWA, MASAO FUJISHIGE, and SEIICHI UJIIE, Research Chemists, and MASAYUKI DOKIYA, Section Chief, are with Energy Chemistry Division of National Chemical Laboratory for Industry, Tsukuba Research Center, Yatabe, Ibaraki-305, Japan. Manuscript submitted July 29, 1986. METALLURGICALTRANSACTIONS B
aluminum-iron-silicon alloys. The present assessment of the thermodynamic properties has been made to clarify the effects of adding iron on the chemical stability of the aluminum compounds. In particular, the aluminum oxycarbide melt and the liquid aluminum alloys become the key compounds because (1) The aluminum oxycarbide melt is the intermediate in the reduction in the absence of other slag components. This melt does not tend to react with carbon, but easily vaporizes; this tendency should be affected by a change in temperature, pressure, or other thermodynamic conditions. (2) To reduce the volatilization of aluminum, alloying with other metals may be adequate; knowing the aluminum activity in various alloy systems becomes important. (3) The stability of the liquid aluminum alloys against the formation of carbide phases is also controlled by the activity. To obtain such thermodynamic properties, we need the activities in the aluminum oxycarbide melt and the liquid aluminum alloys in the region of 2000 to 2500 K. Unfort
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