PFC Evolution Characteristics During Aluminium and Rare Earth Electrolysis

Ole S Kjos,Thor Aarhaug,Anne Støre,Henrik Gudbrandsen,Camilla Sommerseth,Karen Sende Osen,Heiko Gaertner,Asbjørn Solheim,Ana Maria Martinez

doi:10.1007/978-3-319-72284-9_189

Abstract

In addition to aluminium electrolysis, the electrolysis of rare earth (RE) metals from fluoride melts is a significant source of perfluorocarbon (PFC) emissions to the atmosphere. These processes have many similarities, they are both based on molten fluoride salt electrolysis at temperatures around 1000 °C, and are utilizing carbon materials as the anode. Although PFC emissions from aluminium industry and rare earth electrolysis have similar overall reactions, they are often reported to have different characteristics. In order to get a better understanding of these differences and similarities, different laboratory experiments focusing on anode reactions and gas compositions in Al2O3 saturated cryolite and REF3-LiF melts during aluminium and rare earth metal electrolysis were studied. The results obtained, combined with thermodynamic data analysis allowed to better understand onset, evolution and termination behaviour of PFC evolution in molten fluoride systems of different chemistries.

Highlights

Both aluminium and rare earth (RE) electrolysis are conducted in molten fluoride salts, PFC emissions from aluminium production have received substantial focus over the last 10 years, whereas the focus on PFC emissions from rare earth emissions have been less
This can be explained by an insignificant yearly production of rare earth compared to aluminium, but high PFC emissions from the process does matter in an environmental setting; in addition the fluoride loss can give process control challenges
The anode reactions for aluminium and rare earth electrolysis will in principle be identical, evolution of CO, CO2 or C-Fx gases from a molten fluoride salt containing dissolved oxide

Summary

Introduction

Both aluminium and rare earth (RE) electrolysis are conducted in molten fluoride salts, PFC emissions from aluminium production have received substantial focus over the last 10 years, whereas the focus on PFC emissions from rare earth emissions have been less. The electrodes are usually vertically positioned, compared to the horizontal electrode arrangement in a Hall-Heroult aluminium cell. This will result in different convection patterns, and different holding time for gas bubbles at the anode surface that in theory could affect how an anode gets blocked. The simple fact that the small dimensions in a laboratory cell requires the use of insulating refractory materials to regulate current distribution on the electrodes will be a priority. This necessitates a greater care when evaluating experimental results. The experiments were conducted in various REF3-LiF (50:50 molar ratio) -REO (1-2 wt%) systems and Cryolite respectively

Theory

Experimental

Results and Discussion

Conclusions