Influence of Al 2 O 3 on the reoxidation of aluminum in molten cryolite
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[1]
4Al(/) + 3CO2(g) ~ 2AlzOj(/) + 3C(s)
[2]
where the subscripts l, s and g represent electrolyte, solid and gaseous phases. 3 Thonstad has shown that for electrolytes with the cryolite composition less than one percent of the CO2 is reduced to c a r b o n ? Accordingly, Reaction [2] can be ignored for this experimental investigation. Moreover, formation of CO by the Boudouard reaction COz(g) + C(s) = 2CO(g)
[3]
although highly favored at temperatures above 1273 K does not occur except at very low current densities? 9 The solubility of CO z in molten cryolite is extremely small (10 -6 tool CO2 per cm 3 of cryolite for one atm COz). 1~ Accordingly, reaction processes involving dissolved CO2 have been dropped in favor of processes involving oxidation of the metal at the gas-electrolyte interface. The process involves three steps: 1) Reaction of the metal with the electrolyte at the metal-electrolyte interface. 2) Transport of the products through the D. C. LYNCH is Assistant Professor of Metallurgy in the Department of Mining, Metallurgical, and Ceramic Engineering, University of Washington, Seattle, WA 98195. M. K. HAN, formerly a Graduate Fellow with the University of Washington, is now a Graduate Student in the Department of Metallurgy and Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. Manuscript submitted August 25, 1980. METALLURGICAL TRANSACTIONS B
bulk of the electrolyte. 3) Oxidation of the cathodic products by CO2 at the gas-electrolyte interface. Steps 1 and 3 are chemical processes involving high activation energies, responding to changes in temperature and activity. Step 2 is a physical process subject primarily to agitation and stirring. Numerous investigators have examined Reactions [1] and [2] with some reporting step 1 as the rate-determining process while others have found step 2 to limit the reaction rate. zl The sheer numbers of previous works on the reoxidation reaction precludes separate review of each study. However, those works directly examining Reactions [1] and [2] and the significant conclusions found in each are outlined in Table I. A m o n g the sixteen investigations, only three identified the rate of metal dissolution to be the rate determining step. One of these studies, conducted by Thonstad and Solbu, 4 found the dissolution of metal to be the rate limiting step only when the electrolyte was highly agitated and when the oxidation rate reached 21 9 10-6 mol CO per cm 2 per min. Revazyan, 5 and Gerlach and Weber 6 also reported metal dissolution as the rate limiting step with reaction rates at 25 ~ 10-6 and 50 9 10-6 mol CO per cm 2 per min, respectively. In this latter study it is likely that transport processes would have influenced the reaction rates had convection in the electrolyte been decreased. F r o m all the data it appears that mass transport of dissolved aluminum is the rate determining step in the reoxidation process when reaction rates are less than 25 ~ 10-6 tool CO per cm 2 per min. Several authors have noted a decrease in current effici
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