Electrochemical Reduction of Indium and Tin Oxides from Molten Salts
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Indium is considered to be a rare element. The largest end use for indium is ITO powder. ITO is indium oxide combined with 10 percent tin oxide and this represent approximately 72 percent of the indium market. ITO can be spread as thin film on glass or plastic to act as a transparent electrical conductor and infrared reflector. Apart from scarcity, indium has recycling rates less than 1%, all in all making availability of In a challenging issue. Various methods for recycling of indium from secondary sources containing indium have been investigated, mainly based on hydrometallurgical methods. The recovery rate of indium is high through hydrometallurgy. However, the use of various solvents, including corrosive acids and hazardous extraction substances, increases the potential environmental risks. Comparing with pyrometallurgy, separation by chlorination can be operated at lower temperatures. However, it needs a lot of additives and a subsequent purification procedure to obtain metallic indium. These two methods are still in laboratory stage. All these inspired the study of new indium electrorefining processes in fused salt media. In this work, the electrochemical reduction processes in molten LiCl-KCl eutectic containing InCl3, InCl2 and InCl were studied on glassy carbon electrode at 450 °C. The cathodic discharge of indium ions to form metallic indium was shown to take place by a consecutive two-step electrochemical process. This process is identified as: In3++ 2e-= In+ and In+ + e- =In. The reversibility of the system In3+/In+ was confirmed by the superimposition of the direct and the reverse convoluted voltammograms. Mass transport towards the electrode is a simple diffusion process and chronoamperometry was applied to determine the diffusion coefficient of the In3+ ion. Metallic indium deposition on the glassy carbon electrode occurs at a potential of around -0.85 V vs. the Ag+/ Ag reference electrode. InCl2 disproportionates to form InCl and InCl3. The CV curves for InCl2 and InCl were essentially identical to the one obtained from the reduction of InCl3 after a certain time. Metallic indium was recovered from InCl3 by constant voltage electrolysis. The electrochemical deposition of tin was studied with cyclic voltammetry at glassy carbon electrodes in the fused LiCl-KCl at 450 °C. Metallic tin deposition on the glassy carbon electrode occurs at a potential of around -0.56V vs. Ag+/Ag reference electrode. Metallic tin of high purity was obtained. At higher anodic voltage, Sn2+ is oxidised to Sn4+. Mass transport towards the electrode is a simple diffusion process and the diffusion coefficient of Sn2+ was calculated. The current efficiency as the ratio between the charges needed for cathodic deposition and subsequent anodic dissolution of tin was investigated. The electrochemical behaviour of indium and tin oxides was studied in molten LiCl- KCl. The direct de-oxidation of indium and tin oxides were investigated by using different electrodes such as Mo wire, liquid zinc, liquid indium and liquid tin. A new design of direct electrolytic reduction was proposed taking into account the low melting points of indium and tin. An ITO covered glass electrode has been investigated by electrochemical means in molten LiCl-KCl at 450 °C using Mo wire current collector. The findings, together with SEM and EDX analyses, confirmed ITO layer has been reduced.