Electrochemical Selectivity for Metals in Molten Sulfides

Wagner, Mary-Elizabeth

Molten sulfides show promise as an alternative electrolyte for production of metals such as copper and silver. They provide a means to sidestep the current emissions- and waste- heavy processes used both in primary (e.g. from ore) and secondary (e.g. recycled product) streams by direct reduction of a metal's sulfide form, such as Cu₂S, to produce metal and sulfur. Experiments with molten sulfide electrolytes have shown high efficiency in the reduction of copper sulfides and iron sulfides to their metals. Data have also demonstrated high solubility of precious metals silver and gold, and stable electrochemical signals during reduction, evidence for a viable and streamlined method of precious metal refining and recycling.

The current work focuses on predicting which metal, and of what purity, is reduced from the electrolyte. Because the standard-state reduction potentials of copper and iron sulfides are very close to each other (less than 100mv), and because the reduction potentials of silver and gold sulfides are also hypothesized to be close in value with each other, there is a possibility of co-deposition occurring, compromising the purity of the final metal product. By carefully tailoring the activities of the electrolyte species and cathode products, it is possible to bias the reduction reaction towards one species over another. Therefore, the thermodynamics of both the electrolyte and metal product should be very well defined. This talk will discuss experiments in determining metal/sulfide equilibria and how that equilibrium relates to efforts in modeling the cathode product.