Mechanistic insights into the synergistic depression of pyrite by H₂O₂ and Fe³⁺ in low-alkalinity Cu–S flotation separation

Conventional lime depressants used in copper sulfide flotation separation are limited by persistent challenges of scaling, corrosion, and compromised target-metal recovery, which necessitates the development of efficient and green alternatives. This study demonstrates the synergistic depression of pyrite by H₂O₂/Fe³⁺ under low-alkalinity conditions. The complementary action pathways were systematically elucidated by multiscale characterization techniques including mono- and mixed-mineral flotation tests and surface and solution analysis. Flotation test results showed that the combined depressant system H₂O₂/Fe³⁺ enabled efficient separation of chalcopyrite and pyrite. Under the optimal mixed-mineral separation conditions of 0.025vol% H₂O₂ and 2 × 10⁻⁵ mol/L Fe³⁺, the artificial mixed-mineral flotation test yielded a copper concentrate with a chalcopyrite grade of 30.51wt%, while chalcopyrite recovery remained stably above 88%. Investigations into the depression mechanism and surface hydrophobicity revealed that H₂O₂ selectively oxidized disulfide ( \mathrmS_2^2- ) to sulfate ( \textSO_4^2- ) while facilitating Fe²⁺ conversion to Fe³⁺, generating hydrophilic Fe–SO₄/Fe–OOH/Fe–OH coatings that disrupted natural surface natural hydrophobicity. Simultaneously, Fe³⁺ hydrolyzed to hydroxyl complexes (Fe(OH)₂⁺ and Fe(OH)₃), which electrostatically adsorbed onto and chemically bonded to H₂O₂-oxidized pyrite surfaces, forming dense hydrophilic layers. The faster oxidation of pyrite resulted from its fundamental structural properties, specifically its high surface electronic activity and relatively weak Fe–S bonds, which collectively rendered it more susceptible to H₂O₂ attack, unlike chalcopyrite with its stable lattice and strong covalent Cu–S bonds. Consequently, the robust covalent Cu–S bonds of chalcopyrite effectively resisted oxidation, while its limited Fe³⁺ adsorption capacity favored the adsorption of sodium ethyl xanthate (SEX) at copper-active sites. As a result, the H₂O₂/Fe³⁺ system exerted only minimal depression on chalcopyrite, providing a sound theoretical basis and a practical technical strategy for the selective separation of copper-sulfide ores. Furthermore, the findings of this study contribute to the development of low-alkalinity, high-selectivity sulfide-ore processing methods, demonstrating considerable potential for industrial application.