Combined regulation of oxidized pyrite flotation by sodium sulfide and malic acid: Experimental investigation and DFT calculations

Surface oxidation of pyrite (FeS 2 ) during flotation produces hydrophilic Fe(III)-bearing species, substantially decreasing its floatability. In this study, a combined sodium sulfide (Na 2 S)-malic acid regulation system was developed to improve the flotation response of oxidized pyrite. Malic acid alone increased the flotation recovery from 16.67% to 73.90% at 25 mg/L, whereas 7.5 mg/L malic acid combined with 20 mg/L Na 2 S increased the recovery to 93.13%. Contact-angle measurements, Raman spectroscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and dynamic foam analysis were used to evaluate changes in wettability, surface chemistry, collector adsorption, and foam behavior. After the combined treatment, the relative surface oxygen content decreased from 38.32% to 32.36%, and the Fe 2p and S 2p spectra showed changes in the chemical environments of surface iron and sulfur. The Raman Tg band shifted to 427.79 cm −1 , while the maximum foam height during aeration increased from 7.0 mm for untreated oxidized pyrite to 40.3 mm after the combined treatment and the initial Sauter mean bubble radius increased from 94 to 150 µm. Unified PBE single-point calculations identified an intact monodentate configuration as the most stable of four tested malic-acid structures (Eads = -0.379 eV). A 2 × 2 × 1 k-point test changed the M1 adsorption energy by only 0.015 eV, and fixed-geometry PBE-D3(BJ) calculations strengthened the adsorption energies while retaining M1 as the lowest-energy tested configuration. The combined experimental and computational results support complementary Na 2 S-induced modification of oxidized surface species and malic-acid interaction with iron sites, while the calculations alone do not represent the full combined reagent pathway.