SnO 2 -Based Gas (Methane) Anodes for Electrowinning of Aluminum
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I.
INTRODUCTION
ALUMINUM
and aluminum alloys have good properties and are used in wide areas such as transportation, construction, and packaging. Primary aluminum is produced via the Hall–He´roult process invented in 1886.[1] Carbon anodes are used with the emission of CO2 in this process, and they require to be adjusted and replaced periodically resulting in disturbing the cell performance and increasing the corresponding energy consumption.[1] Owing to the disadvantages of using the consumable carbon anode, nonconsumable (inert) anodes for oxygen evolution have attracted significant attention. When using nonconsumable anodes, the anode is dimensionally stable and the need to adjust and change the anode is eliminated. Thus the whole cell has a more stable performance and decreased energy consumption is required. In addition, oxygen is produced at the anode, rather than the greenhouse gas CO2. However, for aluminum electrolysis, the theoretical cell voltage for using a nonconsumable anode is 2.2 V, which is about 1 V higher than that of a consumable carbon anode, likely to result in a higher requirement of electrical energy. As a result, a porous gas anode is applied as an alternative for substituting the consumable carbon anode. A reducing gas (e.g., CH4) is introduced to the anode/electrolyte interface, which participates in the anode reaction, with the overall reaction being shown below: Al2 O3 þ 3=4CH4 ¼ 2Al þ 3=4CO2 þ 3=2H2 O:
½1
S. XIAO, Lecturer, is with the Anhui University of Technology, Maanshan, P.R. China. T. MOKKELBOST and O. PAULSEN, Research Scientists, are with the SINTEF Materials and Chemistry, Trondheim, Norway. A.P. RATVIK, Department Head, and GEIR MARTIN HAARBERG, Professor, are with the Norwegian University of Science and Technology, Trondheim, Norway. Contact e-mail: [email protected] Manuscript submitted October 3, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
Compared with a carbon anode, the emissions of CO2 are reduced by 50 pct when producing the same amount of aluminum metal. In addition, the gas anode maintains most of the above mentioned advantages of using a nonconsumable anode. The theoretical cell voltage for reaction [1] is ~1.2 V at 1123 K (850 C), similar to that of using a consumable carbon anode and ~1 V lower than that of using a nonconsumable anode, shown in Table I which gives the standard potentials for different electrodes in molten cryolite at 1123 K (850 C). The standard Gibbs energies for calculating the potentials in Table I were obtained by the thermodynamic software program HSC.[2] Investigations on introducing reducing gases to the anodes for electrowinning in molten salts have been reported before. Porous carbon was mainly used as the anode and a small depolarization effect was observed in molten cryolite for aluminum electrolysis.[3–5] In addition, magnetite has also been studied as the anode, but it was unstable in molten cryolite during the electrolysis.[4] Platinum was also applied as the gas anode with the introduction of either methane or
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