Pressure leaching of a lead-zinc-silver concentrate with nitric acid at moderate temperatures between 130 and 170 °C

The aim of this investigation was to study the oxidative leaching of lead and silver from a lead-silver concentrate containing galena, sphalerite, pyrite and argentite in nitric acid solutions above the melting point of sulfur (119 °C). The elemental sulfur formed was expected to agglomerate, thereby slowing its oxidation to sulfate and inhibiting the precipitation of lead and silver sulfate.The experimental conditions employed 0.13-0.65 M nitric acid, which is 3 to 15 times the stoichiometric amount needed to form sulfur, at temperatures between 130 and 170 °C. Higher acid concentrations have a favorable effect on silver and lead dissolution at 130 °C with about 90% Ag and 80% Pb extracted into 0.65 M nitric acid within 90 min. However, an increase in temperature caused a decline in the extraction of both metals, due to the greater amount of sulfate produced. Pyrite was found to oxidize directly to akaganeite and sulfate ion and promotes the formation of plumbo- and argento-jarosite. X-ray diffraction analyses of the leach residues show the presence of anglesite (PbSO₄), plumbo-jarosite (PbFe₆(SO₄)₄(OH)₁₂), elemental sulfur (S-β) and akaganeite (FeO(OH)). Enhanced silver extraction was obtained when the oxidation was carried out in the presence of citrate ion which was added to complex with lead and silver ions.