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RODUCTION

MULTICOMPONENT fluoride mixtures are the only available media for the development of new environmentally friendly and energy-saving technologies for aluminum production. Electrolytes with high electrical conductivity, low viscosity, and low vapor pressure should be used. The possibility to carry out the process at a much lower temperature than is possible in the frame of the traditional Hall–Heroult technology is also important. In the last 20 years, some developments have been proposed to reduce operating temperature through a cardinal change of bath composition.[1–3] To obtain an essential decrease of the electrolyte liquidus temperature, the mixture of potassium and sodium cryolites with a low cryolite ratio (CR) can be used. For example, Beck and Brooks[1] used a low melting electrolyte based on the mixture of NaF-AlF3 and KF-AlF3 eutectics with an addition of LiF. Because of the low alumina solubility, a slurry electrolyte is formed in this case that leads to the low efficiency of electrolysis. The electrolytes on the base of potassium cryolite possess much higher alumina solubility.[4–6] Therefore, the potassium cryolite with a low CR was suggested as a basic component of a low-temperature electrolyte for aluminum electrolysis.[7,8] The supposed operating temperature interval is 1023 K to 1173 K (750 °C to 850 °C), which corresponds to the KF-NaFAlF3 electrolyte compositions at CRs of 1.3 to 1.7. Such additives as CaF2 and NaF, which can be introduced into an electrolyte (as oxides) with alumina, significantly change the properties of a bath. The first systematic data on a phase diagram of the NaF-KF-AlF3 ternary system were presented by Belyaev et al.[9] and Barton.[10] Unfortunately, this information is given mainly in graphical form and is not detailed enough. Part of the NaF-KF-AlF3 ternary ALEXEI APISAROV and ALEXANDER DEDYUKHIN, Research Scientists, ELENA NIKOLAEVA, OLGA TKACHEVA, and ALEXANDER REDKIN, Senior Researchers, PAVEL TINGHAEV, Ph.D. Student, and YURII ZAIKOV, Director, are with the Institute of High Temperature Electrochemistry, Yekaterinburg 620219, Russia. Contact e-mail: [email protected] Manuscript submitted May 17, 2010. Article published online December 21, 2010. 236—VOLUME 42B, FEBRUARY 2011

system in the range of up to 50 mol pct AlF3 was measured using the thermal analysis method by Danielik and Gabcova.[11] The liquidus temperatures of the ternary system NaF-KF-AlF3 at a low CR were measured by Dedyukhin et al.[12] The liquidus temperature in the quasibinary K3AlF6-Na3AlF6 system was obtained by Chin,[13] Grjothiem,[14] and Fellner.[15] The most recent results on the temperature of primary crystallization in the Na3AlF6-K3AlF6-AlF3 for some compositions were published by Wang et al.[16] The liquidus temperature in the LiF-KF-AlF3 system was obtained only for CR = 3 by Mal’tsev and Bukhalova.[17] The influence of calcium fluoride on liquidus temperatures of the sodium cryolite was investigated by Craig and Brown,[18] Rolin,[19] and Fenerty.[20] The ternary system NaF-AlF3-CaF2