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INTRODUCTION

ALUMINUM has been produced by the HallHeroult process for more than 100 years. Carbon anode and liquid aluminum cathode are used in this process to decompose alumina dissolved in molten NaF-AlF3based cryolite, in which the cryolite ratio (the mole ratio of NaF to AlF3) ranges from 2.0 to 2.7. However, the electrolytic process is operated at temperature of 1183 K to 1233 K (910 C to 960 C), which is a high-energyconsumption process. Lowering the operating temperature of the aluminum reduction cell can reduce the energy consumption efficiently; therefore, producing aluminum at lower temperature [below 1123 K (850 C)] is a popular research field studied by many laboratories.[1–5] One key problem is to find a solvent for alumina with a lower liquidus temperature and suitable physicochemical properties. The KF-AlF3-based electrolyte has a much lower liquidus temperature[6] and a much wider range of lowtemperature liquid composition than that of the NaFAlF3 system.[7,8] Furthermore, the alumina solubility in KF-AlF3-based electrolyte is higher than that in NaFAlF3-based electrolyte. Thus, the KF-AlF3-based electrolyte is considered a promising electrolyte used in aluminum electrolysis. However, the KF-AlF3 system will be changed into KF-NaF-AlF3 system inevitably because of the accumulation of sodium (which is brought in by alumina in the electrolysis process) in the electrolyte. As a whole, the KF-NaF-AlF3 system is the most possible low-temperature electrolyte for HENGWEI YAN, Research Scholar, is with the School of Metallurgical Science and Engineering, Central South University, Changsha 410083, P.R. China, and with the Zhengzhou Research Institute of CHALCO, Shangjie, Zhengzhou 450041, P.R. China. Contact e-mail: [email protected] JIANHONG YANG, Vice President and WANGXING LI, President, are with the Zhengzhou Research Institute of CHALCO. SHAZI CHEN, Senior Engineer, is with the School of Metallurgical Science and Engineering, Central South University. Manuscript submitted February 13, 2011. Article published online June 17, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B

aluminum electrolysis. It can promote the development of inert anode and help to realize the industrialization of low-temperature aluminum electrolysis as well. The published physical-chemical data of KF-NaFAlF3 system electrolyte is deficient because it is a new electrolyte system. Belyaev et al.,[9] Barton et al.,[10] and Danielik and Gabcova[11] reported a partial rough-phase diagram of KF-NaF-AlF3 system, but the detailed data cannot be obtained in the phase diagram. Apisarov et al.[12] measured the liquidus temperature and conductivity at [KF]/([KF] + [NaF]) molar ratio ranging from 0 to 1 and at the fixed values of ([KF] + [NaF])/[AlF3] molar ratio (CRt) equal to 1.3, 1.5, and 1.7. In our previous work,[13] the liquidus of the KF-NaF-AlF3based electrolyte has been measured at the CRt is 1.3, 1.41, and 1.5 with the content of NaF ranging from 0 mol pct to 50 mol pct. Little information is available on alumina solubility i