Aqueous oxidation of galena and pyrite with nitric acid at moderate temperatures

The aim of the present work was to study the nitric acid leaching of galena and pyrite, above the melting point of sulfur (119°C), to determine under which conditions anglesite (PbSO ₄) and plumbojarosite (PbFe ₆(SO ₄) ₄(OH) ₁₂) are formed. Pure galena and pyrite particles, both separate and together, were leached in autoclave experiments with 0.13-0.91 M HNO ₃ solutions at temperatures ranging from 130 to 170°C. Additionally, particles of anglesite were leached with ferric nitrate-nitric acid solutions at temperatures in this same range to determine the conditions for jarosite formation. At 130°C, with galena alone, increased acid concentration had a favorable effect on the initial lead dissolution, however promoted the sulfur oxidation rate, which accelerated subsequent lead precipitation and surface passivation. Elevated temperatures did not result in higher lead extractions, probably due to a larger and more rapid sulfate production. On the other hand, with pure pyrite, higher acid concentrations promoted increased pyrite oxidation (100% transformation with 0.91 M after only 15 min) and initial iron dissolution. Comparison between galena leaching products in the presence and absence of pyrite show marked differences in the mechanism; while in the former case, highly oxidized products are formed, in the latter, only unoxidized galena is detected for the same conditions. The solid residues of the combined galena-pyrite leach were examined with X-ray diffraction and scanning electron microscope, finding evidence of both anglesite and plumbojarosite. This is probably due to the role of the oxidation by ferric ion under these conditions. It was proven that anglesite, in nitrated solutions, reacts with ferric ions to form Pb-jarosite, especially at the higher temperatures.