Studies on the corrosion and the behavior of inert anodes in aluminum electrolysis
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I.
INTRODUCTION
THE primary extraction of aluminum is carried out by the so-called Hall-Heroult electrolytic process with the cell reaction 112AI~O, + 3/4C = AI + 3/4CO,
[1]
However, the consumable carbon anode used in this process has some inherent disadvantages. Many attempts have been made to develop inert anodes to replace the carbon anodes for aluminum electrowinning. "'21 Inert anodes offer possible savings in operating costs by eliminating the consumable carbon anode. The anode product will be oxygen instead of CO~, the cell reaction being 1/2A1=O3 = AI 4- 3/402
[2]
and hence the use o f inert anodes would eliminate the emissions of CO,_, carbon tetrafluoride (CF4) and polycyelic aromatic hydrocarbons (PAH) related to aluminum production today. Despite the fact that the reversible potential for an oxygen-evolving anode is approximately 1 V higher than that of a carbon anode, i.e., ~2.2 V for Eq. [2] vs ~1.2 V for Eq. [1], this can be offset by lower overvoltage and lower ohmic losses by reducing the anode-cathode distance (ACD). Further savings can be expected by a more optimal cell design by the use of inert anodes, wettable cathodes, and ledge-free sidewalls. In a discussion of future trends in aluminum production, Grjotheimm proposed electrolysis with inert anodes and wet-table cathodes
H. XIAO, formerly with the Department of Electrochemistry, No~vegian Institute of Technology, is Postdoctoral Research Associate, Chemistry Department, UniversiW of Tennessee, Knoxville, TN 379961600. R. HOVLAND, Research Scientist, and S. ROLSETH, Senior Research Scientist, are with SINTEF Materials Technology, N-7034 Trondheim, Norway. J. THONSTAD. Professor, is with the Department of Electrochemistry, Norwegian Institute of Technology., N-7034 Trondheim. Norway. Manuscript submitted February 1, 1995. METALLURG{CALAND MATERIALSTtL4NSACTIONS ~
at lower temperatures (below 800 ~ vs - 9 6 0 ~ for the present Hall-Heroult process). Early work on inert anode materials was concentrated on metalst2~ and ceramic oxides. Metals have high electrical conductivities, but with the exception of some of the precious metals, they tend to be subject to massive oxidation under oxygen evolution in cryolitic melts. Ceramic oxides have in most cases fairly low solubilities in cryolite-alumina melts (0.02 to 1 wt pct), and some oxides are good semiconductors. Tracing back to the 1930s when Belyaev and Studentsovt~J and Belyaev tsl first tested some pure oxides and ferrites as inert anodes, problems with corrosion, disintegration of the anodes, and contamination of the metal were encountered. Since then, extensive studies have been conducted and some progress has been made, particularly during the last few decades.tr-tgl The most significant developments are briefly mentioned subsequently. Tin oxide has received much attention as an inert anode material. AlderV-'l and Klein~*Jj developed various doped SnO_,-based anodes, typically with 2 pct Sb~O3 + 1 pct CuO as doping and sintering agents. The doped anodes possessed good physical and
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