DFT simulations of pyrite galvanic interactions with bulk, solid-solution and nanoparticle Au occurrences - Insights into gold cyanidation

Gold cyanidation is an important hydrometallurgical route central to which are the gold/mineral galvanic interactions which pose perennial challenges to understanding gold dissolution. In this study, Density Functional Theory simulations have been performed to unveil some important electronic features of gold/pyrite associations on gold cyanidation. Three typical gold/pyrite occurrences have been considered, namely, model_1) gold slab/pyrite slab association to emulate contactless liberated gold and pyrite with up to a few angstrom separations; model_2) gold-substituted pyrite solid-solution slab to emulate atomically dispersed gold in a sulfidic matrix; and model_3) pyrite slab with tethered gold nanocluster, in addition to isolated gold nanocluster and to pure pyrite slab. Gold/pyrite associations for all three studied models have been shown to induce electron transfer from gold to both pyrite's sulfur and iron atoms. Model_1 revealed that galvanic interactions between liberated gold and pyrite can occur up to 10 angstrom separation distance between the metal and the sulfide slabs. However, the efficacy of galvanic interactions decreased with increasing the inter-slab separation distances. Model_2 divulged that S-substitutive incorporation of Au in the pyrite structure resulted in expanded crystal dimensions as well as changes in electron properties of gold, iron and sulfur atoms in the solid-solution structure. Occurrence of gold nanocluster on pyrite surface (model_3) led to the formation of metal-metal Au-Fe bond and covalent Au-S bond. Sorption interactions of cyanide with gold and pyrite surfaces in presence of dioxygen were investigated with the three model variants. The DFT simulations showed that CN- preferentially adsorbed on gold instead of pyrite sites provided gold/pyrite galvanic interactions deprived gold from electron density at the expense of pyrite. The cyanide adsorption energies were found to follow the sequence: model_1 with substituted Au- > model_2 > model_3 with substituted Au+ > model_1 (short distance) > model_3 > pure pyrite (1 0 0) & model_1 (long distance contact) > isolated Au-9 nanocluster. As Au electron deficiency via galvanic interactions or gold oxidation was enhanced, gold-cyanide interaction energy increased portending the promotion of gold dissolution in cyanidation processes.