Perfluorocarbon Emissions During Dysprosium Electrolysis from Fluoride- Based Melts

Current environmental policies demand that the entire electrolysis process of greening and cleaning be implemented, from metal extraction to material synthesis. Therefore, there is an urgent need to develop an advanced new green metal extraction process to meet the stringent environmental protection requirements [1]. In general, RE electrolysis generates unnecessarily high emissions of perfluorocarbon gases (PFCs), such as tetrafluoromethane (CF4) and hexafluoroethane (C2F6), which are potent greenhouse gases and are not filtered or destroyed in the off-gas [2]. To address this issue, understanding of the micro-generation mechanism of the greenhouse gases (GHGs) in the Dy electrolysis is crucial in an effort to largely reduce the PFC emission and keep the process in the green process window. To apprehend the mechanism of the greenhouse gas formation CO, CO2 and perfluorocarbon gases (CF4 and C2F6), in-situ measurements of the chemical identity and amount of the anode gases during electrodeposition of dysprosium metal from fluoride-based melts DyF3 – LiF = 50 – 50% with 1% Dy2O3 were performed. In the electrolysis process at constant potentiostatic mode which leads to the electrodeposition of Dy metal from fluoride-based melts with a carbon anode and a molybdenum cathode PFC emission is involved as well. However, in the potential range where Dy3+ is reduced to Dy metal, the experimentally measured FTIR results from the on-line anode off-gas analysis indicate that the anode gas products mainly consist of CO and CO2, while negligible amounts of CF4 were recorded to have been evolved, and C2F6 has been detected just above or below the detection limit. The results showed that the CF4 concentration was not higher than 0.1 ppm in the Dy electrolysis process. Fluorides and oxyfluoride complexes such as [REFx]y- and [REOFx]y- are probably formed during the dissolution of Dy2O3 in fluoride-containing melts. The complexes are involved in the reaction with the GC anode, and CO2/CO and CF4/C2F6 are formed depending on the working potential of the anode [3]. A comparison of the experimental results obtained from Dy2O3+DyF3+LiF electrolyte has shown that the relatively low Dy deposition overpotential prevents fluorocarbon evolution at the anode. The approach presented here to the reduction of the PFC gases emission during RE electrolysis can be considered as an innovative method due to its positive impact on the environment.