Identifying an efficient endpoint for oxygen extraction from lunar regolith simulant pellets using molten salt electrolysis

Extracting oxygen and metals from lunar regolith would support exploration activities on the Moon and beyond. This study examines various factors that influence the efficiency and long-term viability of lunar resource extraction processes, specifically focussing on molten salt electrolysis, or the FFC-Cambridge process. Pellets of lunar mare simulant were reduced for a range of different times, to explore the behaviour throughout the extraction process. Experiment sets were carried out with both carbon anodes and oxygen-evolving tin oxide anodes showing that, while the reduction rate was faster with carbon anodes, similar trends in the rate and efficiency of the process could be observed for mare regolith simulant regardless of anode material. Comparative experiments with lunar highland simulant material demonstrated that the process can be used to extract oxygen for both materials, but that an overall higher current efficiency can be achieved for highland regolith material. Analysis of the solid product at different stages throughout the reduction of mare simulant demonstrated the important role of Ca in the molten salt reduction process and highlighted the non-simultaneous nature of the reduction process in a bulk sample. Overall, the results described indicate that targeting 60 % oxygen extraction may afford the most oxygen in the least time, while targeting 70–80 % extraction would maximise power efficiency. However, to maintain a balanced electrolyte and ensure the process is self-sustaining, it may be necessary to extract 90 % of the total oxygen available in the regolith and accept a lower power efficiency and processing rate. This work shows that multiple factors must be considered in conjunction when selecting process parameters for lunar oxygen extraction. • Extraction of oxygen from mare and highland lunar regolith simulant with molten salt electrolysis. • Examination of the regolith reduction behaviour throughout the molten salt extraction process. • Identification of the most efficient percentage of oxygen extract for lunar surface operations.