An investigation of the leaching of Pt and Pd from cooperite, sperrylite and column bioleached concentrates in thiocyanate-cyanide systems

- Cyanide leaching of Pt and Pd from their minerals is enhanced in the presence of thiocyanate.
- Formation of mixed-ligand complexes strengthens the solubility of Pt and Pd in aqueous solution.
- A cocktail of CN−/SCN−/[Fe(CN)6]3 − shows superior leaching of Pt from pure compounds and PGM concentrate.

This study investigates the effectiveness of a combined thiocyanate-cyanide leach system in Pt and Pd containing minerals with a view to simplifying the traditional smelt-matte-refine process for PGE recovery by providing a route that is potentially more economical and environmentally more sustainable. Initial studies performed on the key Pt minerals, cooperite (PtS2) and sperrylite (PtAs2) in batch stirred tank reactors explored the effect of lixiviant concentration, temperature, pH, oxidants and the influence of a mixed lixiviant system on Pt extraction and compared this to individual systems. Overall, cooperite demonstrated improved Pt extraction in the mixed (SCN−/CN−/[Fe(CN)6]3 −) and SCN−/CN− systems, obtaining 54% and 56% Pt extraction, respectively. Sperrylite did not leach well but showed enhanced Pt extractions in the CN−/[Fe(CN)6]3 − and the mixed (SCN−/CN−/[Fe(CN)6]3 −) system achieving, 5.9% and 8.1% extraction, respectively.Subsequent column leach tests conducted on a low grade Merensky concentrate, revealed that a mixed lixiviant system comprising of SCN−, CN− and [Fe(CN)6]3 − again displayed enhanced extraction relative to a system consisting of either SCN−/CN− or CN−/[Fe(CN)6]3 −; achieving 47% Pt extraction over a 55 day period and close to 100% Pd extraction over 30 days. It is concluded that ferricyanide acts as an oxidant promoting the dissolution of Pt and Pd, whereas the formation of heterogeneous (mixed) ligand complexes containing both thiocyanate and cyanide ligands, is postulated to ensure enhanced stability of the Pt and Pd in the leach solution.