Electrolytic Extraction of Copper, Molybdenum and Rhenium from Molten Sulfide Electrolyte

The validity of the electrochemical series for metal sulfides decomposition in their standard state has been tested experimentally at 1500 K for La₂S₃, Cu₂S, MoS₂, and ReS₂ in a molten electrolyte with the following molar composition: (BaS)₅₄₋(Cu₂S)₃₁₋(La₂S₃)₁₅ (electrolyte B). Voltammetry measurements indicated the presence of faradaic reactions in the investigated electrolyte with and without the addition of MoS₂ and/or ReS₂. Electrolysis experiments showed that the addition of La₂S₃ to BaS-Cu₂S increases the faradaic efficiency for liquid copper production with respect to a previously studied (BaS)₅₄₋(Cu₂S)₄₆ electrolyte, and enabled isolation of elemental sulfur as the anodic product. Electrochemical measurements suggested the need to take into account the activity of dissolved Cu₂S in order to explain the observed cell voltage during electrolysis. Electrolysis in the presence and absence of ReS₂ and/or MoS₂ confirmed their relative stability as predicted by assuming decomposition in their standard states. Analysis of the metal products electrowon from an electrolyte containing Cu₂S, MoS₂, and ReS₂ indicated the simultaneous production of solid and liquid phases with nonequilibrium compositions.Office of Naval Research (Contract N00014-12-1-0521