Anodic Corrosion Behavior of NiFe 2 O 4 -Based Cermet in Na 3 AlF 6 -K 3 AlF 6 -AlF 3 for Aluminum Electrolysis

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INTRODUCTION

THE Hall–He´roult process which is currently being used in the primary aluminum production requires large amounts of energy and has several disadvantages, including high carbon consumption (>400 kg/t Al), emission of CO2, CF4, and polycyclic aromatic hydrocarbons. CO2 is produced throughout the oxidation of carbon anode. The perfluorocarbons are the result of the so-called anode effect caused by the low alumina concentration in the bath during electrolysis, and are among the longest-lived and most polluting greenhouse gases. One kg of CF4 in the atmosphere has the same effect as about 6.5 metric tons of CO2 in a 100-year time period.[1] The implementation of non-consumable (inert) anode could solve these problems and has become a research focus for several decades. After many years’ of study, the materials of inert anode for aluminum electrolysis were focused on cermet and alloys.[2–4] Among them, NiFe2O4-based cermet, which possesses not only good electrical conductivity of metal but also good corrosion resistance of ceramic to molten cryolite, was regarded as one of the promising materials for inert anode of aluminum electrolysis.[2,5] The ceramic phase was generally ferrite, such as NixFe3xO4, NiyFe1yO and NiFe2O4 + NiO. The metallic phase was metals or alloy, such as Cu, Cu-Ni, Ni–Fe, Cu-Cr, and Cu-Ag. Aluminum Company of America (Alcoa) conducted, with the support by the US Department of Energy, a considerable work about 17 pct(mass ZHONGLIANG TIAN, Associate Professor, YANQING LAI, Professor, SHU YANG, Student, JIE LI, Professor, and YEXIANG LIU, Professor, are with the School of Metallurgy and Environment, Central South University, Changsha 410083, P.R. China. Contact e-mail: [email protected] JIANN-YANG HWANG, Professor, is with the Department of Materials Science and Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted June 16, 2014. Article published online March 31, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B

percent, the same below)Cu + 83 pct(51.7 pctNiO, 48.3 pctFe2O3) material and a 6kA pilot cell scale was also carried out.[6,7] In China, 17 pctNi + 83 pct (10NiO-NiFe2O4) cermet inert anode was conducted for the electrolysis testing.[3,8] However, the results from these researches show that none of the inert anode materials has demonstrated long-term viability under current aluminum electrolytic conditions, and the major challenge is still on the anti-corrosion performance of NiFe2O4-based cermets. In order to decrease corrosion rate of inert anode under electrolytic conditions, the research works were focused on two aspects. On one hand, the material composite and the preparation technique were optimized to improve the corrosion resistance performance of the material itself.[8,9] On the other hand, the use of a low-temperature electrolyte for aluminum electrolysis could greatly decrease its corrosion rate and increases the utility of inert anode. Thus, the new electrolyte system was developed to meet the material for ‘‘low temperature, high a