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Electrochemical Deposition of Terbium from Molten Salts
Chemical and Materials Engineering
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AbstractTerbium is a heavy rare earth element with an atomic number of 65, and an atomic mass of 158.925 amu. Terbium is used in several applications, of which the most important are in defense and clean energy technology. Defense applications involve using terbium in its alloyed form, Terfenol-D - alloyed with iron and dysprosium, as a magnetostrictive material in naval sonar system. Terbium is also a phosphor that is used in high efficiency lighting systems. Environmental concerns and exhaustion of fossil fuels have increased the need to use clean energy technologies in everyday life. This has resulted in a change in the materials supply-demand scenario. Rare earth supply is estimated to fall short of the demand in the next 5-15 years owing to recent rare earth mining problems within the U.S and China's monopoly over the rare earth supply. This short supply is of critical concern for the Department of Defense and thus these are considered strategic materials. This critical scenario calls for improved extraction and recycling techniques.This thesis explores a new method for recycling of terbium, namely, via a molten salt electrochemical route. All the experiments were conducted inside a specially built anaerobic system. TbCl3 was chosen as the precursor for the electrochemical deposition of terbium and LiCl-KCl was the salt medium. The circuit consisted of a two electrode system immersed in the molten salt mixture containing the electroactive species Tb(III) ions. A graphite anode was used in all the experiments and either tungsten or stainless steel 316 (SS316) served as the cathodes. Terbium was successfully deposited in all the scenarios, but the current efficiencies varied depending on the process variables. Spectroscopic techniques were used to qualitatively confirm terbium electrodeposition, and gravimetric measurements were used for quantitative analysis. The temperature range investigated was 400-8000C. Process variables studied include cathode material, temperature, concentration, deposition potential, current density, and period of deposition. Relationship between these various parameters and their effects on the electrodeposition of terbium are discussed.