Study of Cu-Ni-Fe Alloys as Inert Anodes for Al Production in Low-Temperature KF-AlF 3 Electrolyte
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INTRODUCTION
ALUMINUM is the most commonly used metal in the world after steel. Global Al production was about 64 million tons (Mt) in 2018[1] and is projected to grow substantially in the forthcoming years because of higher consumption across a wide range of sectors, especially transport, buildings, and engineering.[2] Al production is a major green-house gas (GHG) emitter, releasing about 861 Mt of CO2-equivalent in 2012, which includes process emissions, and indirect emissions from electricity production. This corresponds to an average of 18.4 tons of CO2eq. per ton of produced Al.[3] The current Al production is based on the Hall–Heroult process in which alumina, dissolved in molten cryolite (Na3AlF6) at about 950 C, is electrochemically reduced to aluminum according to the overall reaction: Al2O3 + 3/2 C fi 2 Al + 3/2 CO2. Today, the best-performing Al smelters using GHG-free energy sources such as hydroelectricity emit ~ 2 tons of
SYLVAIN JUCKEN, DANIEL GUAY, and LIONEL ROUE´ are with the INRS-E´nergie Mate´riaux Te´le´communications; 1650 Boulevard Lionel Boulet; Varennes, QC J3X 1S2, Canada. Contact e-mail: [email protected] BERNARD TOUGAS is with the Centre de Me´tallurgie du Que´bec (CMQ); 3095 Rue Westinghouse Parc Industriel des Hautes-Forges, Trois-Rivie`res, QC G9A 5E1, Canada. BOYD DAVIS is with the Kingston Process Metallurgy Inc.; 759 Progress Avenue, Kingston, ON K7M 6N6, Canada. Manuscript submitted March 27 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
CO2eq. per ton of aluminum produced. Significantly, further GHG emission reductions will require technology breakthroughs. A promising approach is to replace the consumable carbon anodes with inert anodes which emit O2 instead of CO2 during Al electrolysis according to the overall electrochemical reaction: Al2O3 fi 2 Al + 3/2 O2. The use of inert anodes would also lead to a significant reduction in the operating and infrastructure costs of Al smelters by preventing frequent anode lowering and replacement operations, by eliminating the carbon anode production plant, and by enabling more compact and energy-efficient vertical electrode cell design when combined with wettable cathodes.[4] The development of inert anodes is, however, very challenging as they need to meet hardly compatible requirements such as a good electrical conductivity, a good resistance to thermal shocks, and an excellent resistance to corrosion in the cryolite electrolyte. Materials that have been considered for inert anodes include metals, ceramics, and cermets, each with their specific advantages and drawbacks.[5,6] To date, the viability of inert anodes in industrial Al smelters has not yet been proven mainly due to their insufficient long-term stability, the difficulty of manufacturing them in large and defect-free shape with robust electrical connections and/ or their inability to tolerate cell operation disturbances. Decreasing the operating temperature is a practical approach to reducing the anode corrosion rate in addition to allowing more anode material options, especially fo
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