Physicochemical Properties of Industrial Aluminum Electrolytes Enriching Li and K: The Liquidus Temperature
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UCTION
WITH the rapid development of the aluminum electrolysis industry, the demand of bauxite is growing. Therefore, low-grade bauxite has to be used due to the reduction of high-grade bauxite. The low-grade bauxite contains significant amounts of lithium salts and potassium salts. Then, alumina contains plenty of Li2O and K2O as a result of being extracted from these low-grade bauxites. Therefore, LiF and KF will accumulate in the aluminum electrolytes that are fed with this alumina. Thus, the composition and physicochemical properties of electrolytes are changed. According to investigation, the weight percentage of LiF and KF is in the range 4.0 to 8.0 wt pct. The electrolysis temperature has reduction to 1183 K to 1193 K (910 °C to 920 °C), and some are even lower than 1173 K (900 °C) due to the high content of LiF and KF in electrolytes. But the current efficiency decreases when the operation temperature decreases, some even down to 89 to 91 pct. The Al2O3 concentration is difficult to control, resulting in the increase of the anode effect coefficient. Therefore, investigating the characteristics and properties of the electrolytes enriching Li and K not only can provide instruction on the aluminum electrolysis process, it can also enrich the basic database of the molten salts. It is meaningful to study the properties of the industrial electrolytes enriching Li and K. The XIAO-JUN LV, YA-JING SHUANG, JIE LI, SHI-YUE CHEN, YAN-QING LAI, HONG-LIANG ZHANG, and YE-XIANG LIU are with the School of Metallurgy and Environment, Central South University, Changsha, 410083, P.R. China. Contact e-mail: [email protected] Manuscript submitted November 24, 2014. Article published online January 18, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
objective of this article is to investigate the liquidus temperature, and the research on alumina solubility of electrolytes will be presented in another article. Several groups have developed model equations describing the liquidus temperature for compositions corresponding to conventional electrolytes.[1–9] Solheim[9] has studied the liquidus temperature based on the system Na3AlF6-AlF3-LiF-MgF2-CaF2-Al2O3-KF and has produced the empirical equation. Although the scope of this empirical equation is broad, the test data based on the Na3AlF6-AlF3-Al2O3-CaF2-LiF system was limited. For example, the composition limitation of AlF3 was 20 wt pct, but it only takes four tests in the range of 5.36 to 5.8 wt pct, four tests in 11.92 to 12.42 wt pct, and eight tests in 21 to 25 wt pct, which is deviated from the industrial aluminum electrolytes. The test data are too few to ensure the accuracy of the equation due to the complexity of melts. The research on potassium salts impacting the liquidus temperature mainly focused on the electrolyte system Na3AlF6-K3AlF6-AlF3.[10–14] Wang[10] has studied the liquidus temperature for the electrolyte system Na3AlF6-K3AlF6-AlF3. The composition limitations were KR (K3AlF6/(K3AlF6 + Na3AlF6)) 0 to 50 wt pct, w(AlF3): 0 to 30 wt pct. His study results showed the
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