Study on Liquidus Temperature of NaF-KF-LiF-AlF 3 System with Low Cryolite Ratio

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Hall–He´roult molten salt aluminum electrolysis process is used for the industrial production of aluminum. During the aluminum electrolysis process, excessive electrolysis temperature results in high energy consumption, high carbon consumption and large CO2 emissions.[1] One possible solution to alleviate these problems is to reduce the electrolysis temperature. However, the current industrial electrolyte is the Na3AlF6-AlF3 system, with a NaF/AlF3 cryolite ratio (CR, defined as the molar ratio of NaF-AlF3) of 2.6, and a reduction of the electrolysis temperature will lead to problems such as reduced conductivity, reduced current efficiency, crust formation at the bottom of the cathode, and increased cathode voltage drop (CVD).[1,2] Therefore, it will be necessary to develop a new low-temperature aluminum electrolyte system for the aluminum electrolysis process.

BINGXU CHEN, JIANPING PENG, YAOWU WANG, and YUEZHONG DI are with the School of Metallurgy, Northeastern University, Shenyang, 110819, P.R. China. Contact e-mail: [email protected] Manuscript submitted October 31, 2019.


Addition of AlF3 can significantly reduce the liquidus temperature of the electrolyte system. However, the main disadvantages of the existing electrolyte system are the low solubility and dissolution rate of alumina and the systems’ low electrical conductivity.[3,4] To overcome the disadvantages produced by reduction of the liquidus temperature, the physicochemical properties of the electrolyte can be changed by additives. Addition of potassium fluoride can beneficially influence the liquidus temperature of the low cryolite-ratio NaF-AlF3 system and enhance the alumina solubility of the system.[5] The solubility of alumina in the NaF-AlF3 system containing 24.6 pct (molar fraction) potassium fluoride reached 4.38, which is higher than the alumina solubility in the NaF-AlF3 system at CR = 1.3 when heating temperature reached 1023 K.[6,7] Lithium fluoride can also reduce the liquidus temperature of low cryolite-ratio NaF-AlF3 system and contribute to overcome the disadvantage of low conductivity. For example, Na3AlF6-Li3AlF6-AlF3 (CR = 2) can reach 2.0 S cm1 at 1123 K.[3,8] However, there are some disadvantages of potassium fluoride and lithium fluoride when added to the electrolyte system. Potassium fluoride can reduce the conductivity, and lithium fluoride can result to lower the alumina solubility. While taking the advantages and disadvantages of adding these two additives separately

into account, researchers also considered a combined simultaneous addition of lithium fluoride and potassium fluoride to the NaF-KF-LiF-AlF3 system. Research on the liquidus temperature for a new aluminum electrolyte system contributes to choose the selection of the appropriate temperature range for electrolysis and forms the basis for understanding of the aluminum electrolysis process. The liquidus temperature of the NaF-AlF3 system when enriched lithium and potassium has been investigated.[9,10] In systems with cryolite rat