Molten Salt Electrorefining of Indium Metal from Indium Tin Alloy

Wednesday, 31 May 2017: 14:10
Norwich (Hilton New Orleans Riverside)
S. H. Choi (KIRAM, KITECH, South Korea, In-Ha University, South Korea), B. Ali (KIRAM, KITECH, South Korea), S. K. Hyun (In-Ha University, South Korea), J. J. Sim (KIRAM, KITECH, South Korea, In-Ha University, South Korea), W. Ju (KIRAM, KITECH, South Korea, Hanyang University, South Korea), J. H. Lim (KIRAM, KITECH, South Korea, In-Ha University, South Korea), T. S. Kim, and K. T. Park (KIRAM, KITECH, South Korea)
Indium is an essential element for the fabrication of transparent conductive oxide (TCO) materials. However, it is very rare element and is obtained as a byproduct of zinc metal refining. Moreover, its recycling rate is less than 1%. This provokes the need to recycle indium from its end products. Indium and tin (Sn) are especially well suited for molten salt electrolysis because of their low melting points. In this paper, reduction processes on molybdenum electrode at 500oC in molten LiCl–KCl eutectic (55.7-44.3 mol %) containing varying amount of InCl3 was studied by Cyclic Voltammetry (CV) and Chronopotentiometry (CP). In-Sn alloy was used as raw material. Thermodynamic calculations showed poor stability of InCl3 with metal based crucibles. So, Al2O3 crucible was utilized for the experiments. Concentration of InCl3 was changed from 1 wt% to 3 wt% and it was found that the reduction of indium ions take place by a two-step electrochemical reaction: In3+/In+ and In+/Ino at the potentials of 0.78 and 0.24 V versus Ag/AgCl. Number of transferred electrons and diffusion coefficient of In+3 ion species in molten salt with varying InCl3 content was estimated with the help of CV measurements. However, at 500oC, InCl3 shows chemical reactivity with Sn metal according to the following reaction: InCl3 + 1.5Sn → 1.5SnCl2 + In. This problem was overcome by shifting the molten chloride system to molten fluoride system (LiF-KF-InF3) as fluoride based salts shows high chemical stability with metals. As a result of electrorefining, In-Sn alloy was recycled into pure indium with low levels of impurities.