Electrochemical behavior of Sc (III) and preparation of Al-Sc alloys in NaF-AlF3-BaF2 by a floating molten aluminum cathode

Abstract

Scandium is one of the key alloying elements for aluminum. This study presents an innovative upward-flow liquid aluminum cathode molten salt electrolysis method for producing composition stability Al-Sc alloys. A system NaF-AlF₃-BaF₂ (molar ratio of NaF to AlF₃ = 2.2) containing 25 wt% BaF₂ was used as the electrolyte to achieve the floating molten aluminum cathode. The electrodeposition behavior of Sc at a tungsten cathode was analyzed in the electrolyte and experiments were carried out to produce Al-Sc alloys by the proposed process. The results show that the reduction of Sc (III) at 1273 K involved a one-step reaction with three-electron transfer. The process was limited by the diffusion of Sc (III) ions from the electrolyte to the electrode interface with a diffusion coefficient of 1.943 × 10^–5 cm²/s. SEM-EDS and XRD analyses revealed that the early electrolysis products contained Al₃Sc phase and W-Sc enrichment zones, while the final cooled products exhibited Sc in the surrounding Al layer, which primarily precipitates via aluminothermic reduction. Spherical products with the maximum scandium content of 3.68 wt% were obtained under a current density of 1.0 A/cm² at 1273 K.