Characterization of metal products from the molten salt electrolysis of lunar highland regolith simulants

Abstract

The FFC (Fray-Farthing-Chen) molten salt electrolysis process is one of the proposed technologies for oxygen and metal extraction from lunar regolith. Previous work demonstrated its capacity to extract up to 96 % of the oxygen present in mare type regolith simulant without requiring regolith melting. The FFC process also allows direct recovery of the reduced regolith, which has been shown to comprise particles and agglomerates that are mostly metallic. Here, we present the first quantitative analysis of those metallic products. Molten salt electrolysis of LHS-1 highland regolith simulant was carried out and the reduced simulant products were separated into size fractions. Residual oxygen content of the material was determined by inert gas fusion. Bulk composition, local phase compositions and particle morphology were assessed by Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Spectroscopy (EDX). The compositional data was analysed using Principal Component Analysis (PCA) and agglomerative clustering algorithms, to assess the interactions between the metallic elements in presence and to identify the main phases formed. Results were compared with the electrolysis of the major mineral component of highland regolith, anorthosite. The electrolysis was shown to extract 97–99 % of the oxygen present in the simulant feedstock, with an increased current efficiency for the anorthosite samples. Analysis of the reduced simulants showed a complex and heterogeneous mix of alloys. The main phases in presence (e.g. Al₂Si₂Ca, AlSiCa, Al-Si-Fe compounds) are shown to often occur within multiphase particles. It was concluded that mechanical methods (e.g. sieving, elutriation) are not suited to isolate specific metallic alloys from the products. Contamination of the metallic products by the reactor materials (i.e. Cr, Mn, Ni, Mo) was also evidenced, highlighting the need for further improvement of reactor designs.