The Solubility of Aluminum in Cryolite-Based Electrolyte-Containing KF
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RODUCTION
ALUMINUM is produced by the electrolysis of alumina dissolved in cyrolite-based melts. Na3AlF6AlF3-Al2O3 containing 2 to 5 wt pct CaF2 is usually used as the basic system. Other additions like LiF or MgF2 may be present as well. High current efficiency and low energy consumption are always the main targets in the production of aluminum. However, aluminum dissolves into the electrolyte by the means of physical dissolution or chemical reaction and is oxidized by anode gas, which is one of the most significant reasons for the loss of current efficiency.[1] Therefore, the dissolution of aluminum has captured much attention for a long time. The solubility of aluminum is generally used to describe the capacity of aluminum dissolving in the electrolyte. The studies on solubility of aluminum in cryolite-based electrolyte have been intensely reported. In early 1965, Thonstad[2] measured the solubility of aluminum in NaF-AlF3-A12O3 melts with cryolite ratio (termed CR which denotes the molar ratio of NaF/ AlF3) increasing from 1.7 to 6.7 at 1273 K (1000 °C). He pointed out the importance of sealing crucible, and he holds the view that the solubility difference between slow cooling and rapid quenching of melts is probably due to metal loss by evaporation during sampling. Odegard[3] obtained aluminum solubility for cryolite ratio ranging YU ZHANG, formerly Master Degree Student with the School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China, is now Ph.D. Student with Tianjin University, Tianjin 300072, China. JIANGYU YU, Research Assistant, BINGLIANG GAO, ZHONGNING SHI, and ZHAOWEN WANG, Professors, YIBAI LIU, Undergraduate Student, and XIANWEI HU, Associate Professor, are with the School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China. Contact e-mail: blgao@mail. neu.edu.cn Manuscript submitted August 16, 2015. Article published online January 19, 2016. 1296—VOLUME 47B, APRIL 2016
from 0.9 to 4.0 between 1233 K and 1333 K (960 °C and 1060 °C). Danielik[4] reported some latest data on aluminum solubility for cryolite ratio ranging from 1.4 to 2.2 at temperature range of 1073 K to 1233 K (800 °C to 960 °C). A thermodynamics model by assuming the existence of Na, AlF, and AlF2- ions was suggested according to their data. Qiu et al.[5–7] observed the phenomenon of aluminum dissolution through a see-through cell, and he indicated that the phenomenon of metal fog could be explained by electrochemical dissolution, which was restrained under cathodic polarization . However, research concerning the influence of additives on aluminum solubility was relatively few. Wang et al.[8] analyzed the effect of 1 to 10 wt pct LiF on the total metal solubility in cryolite-alumina melts at 1268 K (995 °C) by tin extraction-stripping technique. The results showed that metal solubility decreased with increase of LiF concentration. In some Chinese aluminum smelters, the aluminum bath contains high content of KF, which comes from alumina as the type of potassium oxide. The cells always behave
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