Studies on the effect of addition of silver ions on the direct oxidation of pyrite

The nature of the reaction between Ag⁺ and pyrite in 0.25 M H₂SO₄ solutions has been investigated in order to determine whether Ag⁺ can enhance the ferric sulfate leaching of this mineral. Analysis of reacted pyrite particles using scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and low-angle X-ray diffraction (XRD) indicates that elemental silver and elemental sulfur are the primary surface species formed by this interaction. Rest potential measurements of a pyrite electrode immersed in a solution containing 10^-2 M Ag⁺ are also consistent with what is expected for the deposition of metallic silver. Furthermore, the XRD data reveal that, at the most, only minor amounts of Ag₂S are being produced. The presence of Ag₂O has also been detected, but this is due to oxidation of silver after the experiment is complete and while the particles are being transferred for surface analysis. When 1 M ferric sulfate is contacted with pyrite which has been pretreated in a AgNO₃ solution, most of the silver immediately redissolves and does not redeposit while ferric ions are present. This indicates that the kinetics of the transfer reaction between Ag⁺ and pyrite is slower than the reaction between Fe³⁺ and pyrite and suggests that Ag⁺ does not likely enhance the ferric sulfate leaching.