Electrochemistry of Molten Sulfides: Copper Extraction from BaS-Cu₂S

Abstract

The electrolytic extraction of liquid copper at 1105 • C from a molten sulfide electrolyte composed of 57 wt% BaS and 43 wt% Cu 2 S was investigated. DC cyclic voltammetry, Fourier transformed AC voltammetry, and galvanostatic electrolysis revealed that the electrodeposition of copper is possible in the selected molten sulfide electrolyte. The half wave potential for the reaction on graphite was determined, and liquid copper of high purity was obtained by galvanostatic electrolysis. These preliminary results confirm that molten sulfides free of alkaline elements could be used as an electrolyte for faradaic applications, despite the semi-conducting nature of the melt. In addition to demonstrating the need for enhanced understanding of the transport properties of such electrolyte, the results show the critical impact of the cell design to improve the process faradaic efficiency. Sulfide-containing ores are the main raw material for copper extraction. The conventional chemical principle underlying metal extraction from such ore (smelting) is the selective oxidation of sulfide ions (S 2− ) by oxygen. The reaction 1 forms copper metal and sulfur dioxide (SO 2 ) as products, as written here for chalcocite (Cu 2 S):Such principle leads to a process characterized by large capital investments and significant environmental challenges. 1 This route requires handling SO 2 as a by-product, typically converted to sulfuric acid. To circumvent this issue, additional pyrometallurgical steps to convert SO x into elemental sulfur have been devised, using for example reduction or chlorination. Hydrometallurgy is an alternative to traditional smelting that does not involve SO 2 .3-5 It involves a succession of leaching, solvent extraction and finally electrowinning of Cu in an aqueous electrolyte. This route is also characterized by a relatively large footprint and capital cost. One of the limitations is inherited from the electrowinning/ refining steps, where the current density for copper electrodeposition is typically limited to 0.05 A.cm −2 . 6An alternative approach to avoid SO 2 formation is the direct decomposition of copper sulfide into copper and elemental sulfur, following reaction 2:At 1106• C, more than 20• C above copper melting point, reaction 2 is not spontaneous ( r G • = 90.5 kJ.mol −1 ) and would require a minimum amount of energy of 267 kJ.mol −1 (equivalent to 583 kWh.t Cu −1 ) a . 7 This reaction could therefore be driven by electricity, as practiced industrially for most metals, including copper and aluminum. In principle, electrolysis can also offer the selective recovery of multiple metals contained in the sulfides ores, for example elements more noble than copper, e.g. silver or molybdenum.The direct electrolysis of sulfides was proposed in concept by Townsend in a patent in 1906.2 Since then, the challenge remains in selecting a supporting electrolyte with an acceptable solubility for copper sulfide concentrates to guarantee large cathode current density, a requirement for tonnage production. Previous studi...

Experimentally Determined Phase Diagram for the Barium Sulfide-Copper(I) Sulfide System Above 873 K (600 °C)

“…Due to the relative abundance of Ba and S, its low cost, and unique chemical nature [2], barium sulfide (BaS) is an important sulfide compound for which usage is hindered by a lack of thermodynamic understanding of its chemical interactions with other sulfides [5][6][7][8][9], including Cu 2 S. Despite such uncertainty, its usage in combination in the solid-state with Cu 2 S has been put forth for new high temperature superconductors [5] photovoltaic materials [10], or thermoelectrics [11,12]. Independently [13], the addition of barium sulfide to copper (I) sulfide (Cu 2 S) at 56.8 mol% and 1379 K proved to form a possible electrolyte for liquid copper extraction via electrolysis, where the addition of BaS is thought to decrease the electronic conductivity of Cu 2 S. Recent results [14] with addition of La 2 S 3 to this composition ((BaS) 54 -(Cu 2 S) 31 -(La 2 S 3 ) 15 ) enabled the extraction of Mo, Re and Cu using direct electrolysis of such electrolyte with higher efficiency. The reason for such increase in efficiency has been attributed to the increased ionic nature of sulfides melts when the concentration in Cu 2 S is lower.…”

Electrochemical Study of a Pendant Molten Alumina Droplet and Its Application for Thermodynamic Property Measurements of Al-Ir

“…Open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), direct current (DC) linear sweep voltammetry (LSV), large amplitude Fourier transform alternating-current voltammetry (ACV) and potentiostatic and galvanostatic electrolysis measurements were performed. The details of the equipment and measurement methods are available elsewhere, 42 and only important variations are reported herein.…”

“…Further details regarding observation and analysis equipment can be found elsewhere. 42 Figure 2 shows a typical DC linear sweep voltammogram in the negative-going potential scan where the current rapidly decreases with fluctuations from the starting scan limit (E = 1.05 V). Very large current densities (>3.5 A cm −2 ) are observed in this region.…”

Electrochemical Engineering for Commodity Metals Extraction

“…Molten sulfides remain understudied, and recent efforts toward high-melting point quasi-binary and -ternary systems indicate a possible path forward to the direct decomposition of sulfides to metal and sulfur. 33 In this situation, the handling and selectivity of direct electrolysis for multiple metal species coming from the sulfide concentrates remain to be demonstrated, e.g., Fe but also Se, As, or Ag from chalcopyrite. It indeed remains to be demonstrated that Eq.…”

“…As known in HH cells as the anode effect, a low concentration of oxygen next to the anode may lead to the oxidation of another species, such as CFC in fluoride-based electrolytes. 33 The author has in particular studied such mass transfer issues in molten oxides, in the context of oxygen ions oxidation to oxygen gas, 34,35 showing that natural and bubble-induced convection are likely essential phenomena to support the range of anode current densities necessary for commodity metal extraction by electrolysis.…”