Date of Award
Many applications utilize rare earths as a part of their essential composition, such as: Ni-MH batteries, glass screen, magnets, fluorescent lightning, etc. These reactive elements are normally found as an oxide compound due to their affinity with oxygen. Their oxide formation exhibits a high standard free energy, which make them really stable. Consequently, extracting rare earths cannot be easily done with a simple acid-base reaction.
It has been known that there are two reduction ways to extract rare earth, metallothermic reduction and molten salt electrolysis process. Metallothermic method is determined by standard free energy formation, melting point, boiling point, vapor pressure, viscosity, and density. As for molten salt electrolysis, it is not only determined with those factors, but also a decomposition potential. Molten salt or fused salt is used as an electrolyte, because it has an excellent electric conductivity, heat capacity, and can also act as a solvent.
This work presents a comprehensive study on thermodynamic considerations of molten salt electrolysis for rare earth metals. Related publications' results are analyzed and summarized. It reveals that a mixture of molten salts can help reduce the melting point (at its eutectic point) and increase their electric conductivity. Furthermore, rare earth fluoride as a solvent can help increase the rare earth oxide solubility as the feed, while alkali metals exhibit a contrast result. A higher temperature also result in higher solubility. In chloride molten salt electrolysis, a stable divalent ion will be produced and result in a low current efficiency. Fluoride molten salt electrolysis has higher current efficiency than the chloride system, yet it requires more energy consumption due to a higher decomposition potential in fluoride system.
E-pO2- diagram was constructed, modeled and implemented for providing essential information in the reduction process of rare earths by using STABCAL program. This work selects neodymium in a 75mole%LiF-25mole%NdF3 system to produce neodymium by molten electrolysis at 750oC. E-pO2- diagram showed the value of the decomposition potential of Nd, which is -5.08 volts calculated from Kubaschewski databases and -4.98 volts calculated from HSC databases. This is confirmed with the similar decomposition potential determined by V.A. Grebnev and V.P. Dmitrienko (2007) experiment that is 4-8 volt using similar conditions. At 5.5volts applied potential, the E-pO2- diagram also showed a tendency of Nd metal to decompose rather than Li metal (higher driving force) with an overvoltage () 0.425volts for Nd and 0.153volts for Li.
Gunawan, Arwin, "Thermodynamic Considerations in Molten Salt Electrolysis for Rare Earth Metals" (2015). Graduate Theses & Non-Theses. 22.