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INTRODUCTION

ALUMINUM is produced electrochemically by Hall-Heroult process.[1] This technology uses carbon anode and liquid aluminum cathode to decompose alumina dissolved in molten NaF-AlF3-based electrolyte, in which cryolite ratio (CR) ranges from 2.0 to 2.7. Cryolite Ratio (CR) is an important parameter for electrolyte used in aluminum reduction process. Traditional cryolite ratio is defined as the mole ratio of NaF to AlF3 in electrolyte: CR = ½NaF=½AlF3 

½1

Production of aluminum with low temperature (below 900 °C) has been an attractive topic for many years[2–5] because it has the possibility to reduce the energy consumption. One of the problems that needs to be solved is to find an electrolyte with lower liquidus temperature and suitable physicochemical properties. The NaF-KF-AlF3 electrolyte with low cryolite ratio (1 < CR < 2) is a kind of promising electrolyte for low-temperature aluminum production[6–12] because

HENGWEI YAN, ZHANWEI LIU, and WENHUI MA are with State Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province/The National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan, 650093, China, and also with the Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan, 650093, China. Contact e-mails: [email protected] and [email protected] JIANHONG YANG is with the School of Material Science and Engineering, Jiangsu University, Zhenjiang, Jiangsu, 212013, China. CHENGZHI WANG is with the Zhengzhou Research Institute of CHALCO, Zhengzhou, Henan, 450041, China. Manuscript submitted September 4, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS B

solubility of alumina in this electrolyte is higher than that in NaF-AlF3 electrolyte with the same CR. However, for the NaF-KF-AlF3 system electrolyte, the definition of the cryolite ratio is different due to the presence of two kinds of alkali fluorides, potassium fluoride (KF) and sodium fluoride (NaF). The CR should be defined as the mole ratio of NaF and KF to AlF3: CR ¼ ð½NaF þ ½ KFÞ=½AlF3 

½2

For the NaF-KF-AlF3 system electrolyte, some researchers already used this CR definition in their published reports.[6–8] The CR to a large extent determines the physicochemical properties of the electrolyte, especially for the NaF-KF-AlF3 system electrolyte with low CR, in which a change of 0.1 in the CR could lead to 20 °C to 40 °C fluctuation in the liquidus temperature.[6,7,9,10] The CR of the NaF-KF-AlF3 system electrolyte also has significant influence on alumina solubility,[7,11] as well as density, surface tension,[12] and conductivity[7,8,13,14] of the bath. However, the CR is always changing due to the evaporation of fluoride, so the accurate measurement on the CR is vitally important for the chemical composition control of the electrolyte, and the electrolyte composition can be adjusted according to the measurement result. CR analysis is a routine work in many traditional aluminum smelters. The analysis of