Electrochemical Removal of AlCl 3 from LiCl-KCl Melts
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TRODUCTION
CURRENTLY, the only way to produce primary lithium in industry is by molten salt electrolysis from KCl-LiCl melts at 693 to 703 K (420 to 430 C).[1–6] Usually, the KCl and LiCl used in industry have a purity of 98 to 99 wt pct containing an amount of impurities, such as NaCl, AlCl3, MgCl2, and CaCl2. These impurities can be deposited as the corresponding metals during the Li electrolytic process due to a lower theoretical decomposition voltage than that of LiCl; therefore, the primary Li contains a purity of 98 to 99 wt pct with 0.1 to 0.8 wt pct Na, 1 wt pct K, 0.03 wt pct Al, and 0.01 to 0.05 wt pct Ca.[7] These impurities have to be further removed by vacuum distillation in a stainless steel reactor at temperatures from 873 to 1073 K (600 to 800 C), and sometimes followed by zone melting,[8–10] to satisfy the purity requirement for use in batteries, alloys, and nuclear power generation. About 52 kWh/kg Li has to be used to purify the primary lithium from 98.5 to 99.9 wt pct accompanied with a serious corrosion of the reactor. Hence, it is necessary to explore an economically feasible method to remove these impurities instead of the traditional one. Lou et al.[11] and Gulens et al.[12] reported that a kind of lithium ion conductive material Li1.3Ti1.7Al0.3(PO4)3 was used to remove Na+ ions from LiCl solution to purify LiCl salt. Jin et al.[13] suggested an alcohol extraction method to prepare a highly purified LiCl salt with lower Na and K content. In our laboratory, electrochemical methods are proposed to separate those M. SHEN, Postdoctoral Student, B. LI and J.G. YU, Professors, and S.Z. LI, M.A.Sc Student, are with the National Engineering Research Center for Integrated Utilization of Salt Lake Resource, East China University of Science of Technology, Shanghai 200237, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted September 5, 2010. Article published online December 21, 2011 1662—VOLUME 43A, MAY 2012
impurities that have lower theoretical decomposition voltages than LiCl from the LiCl-KCl melts prior to Li electrolysis. As one of the impurities in LiCl-KCl melts, AlCl3 presents a much lower theoretical decomposition voltage than LiCl; it is supposed to be deposited and separated from the AlCl3-KCl-LiCl melts by potentiostatic electrolysis prior to Li electrochemical reduction. Usually, the Al3+ ion concentration in the KCl-LiCl melts is low and gradually gets lower as the electrolysis proceeds. Since the kinetics of aluminum deposition in chloride melts by electrochemical techniques is controlled by ion diffusion according to Yan et al.,[14] Bouteillon and Marguier,[15] Gabcˇo et al.,[16] and Zhang et al.,[17] the Al3+ electrochemical reaction rate will become lower and lower. The main challenges during the removal of Al3+ ions from the KCl-LiCl melt are if Al3+ ions can be completely removed, how long the process will take, and how to improve the electrochemical reaction kinetics of Al deposition in the melts. In this article, cyclic voltammetry (CV) is employ
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