Removal of Pyrrhotite from Nickel Concentrate in a Sulfuric Acid Leaching System

Innovative hydrometallurgical processing methods for extracting nickel from sulfidic ores are being pursued as more environmentally friendly alternatives to pyrometallurgical methods. However, sulfidic ore concentrates contain significant amounts of the reactive iron sulfide mineral, pyrrhotite, that presents legacy issues of acid rock drainage from tailings. This study shows how a pre-leaching step of sulfuric acid at elevated temperatures shows great selectivity for pyrrhotite. Air-sparging further improves the leaching kinetics, even though oxidative leaching of pyrrhotite produces sulfur passivation layer on the surface of the pyrrhotite particles. We find that a sulfuric acid leach at 70 °C with air-sparging removes over 90% of the pyrrhotite from Ni concentrates within one hour, while leaching less than 5% of the Ni from the pentlandite ore. The addition of vanadium(II) sulfate to this leaching process under ambient condition shows complete leaching of pyrrhotite within 10 min. The mineralogy of the leached solid residues is consistent with high selectivity of pyrrhotite over pentlandite during both acid and reductive leaching. The main component of the leached solid residue is pentlandite with less than 10% of the original pyrrhotite remaining in the post-leaching solid. The pentlandite remains unchanged before and after leaching. The selective removal of pyrrhotite in the sulfuric acid leaching system can be completed in hours at 70 °C and in a reducing vanadium sulfate system, and it can be completed in just a few minutes at room temperature. This streamlines the extraction process for nickel from pentlandite and demonstrates environmentally friendly advantages compared with pyrometallurgy.