Interfacial interaction and molecular simulation insights into a novel green and efficient depressant for smithsonite–calcite flotation separation in a sodium oleate system

To improve the separation of smithsonite and calcite in the sodium oleate (NaOL) system, chondroitin sulfate A (CSA) was used as a green organic depressant for calcite. Micro-flotation tests showed good separation at 80 mg/L NaOL, 30 mg/L CSA, and pH 9. Under these conditions, the recovery difference between the two minerals reached 93.10 %age points. In the mixed-mineral test, the ZnCO₃ grade in the concentrate increased to 80.56 ± 2.5%, further showing the separation effect of CSA. Contact angle, zeta potential, and atomic force microscopy (AFM) results showed that CSA mainly changed the calcite surface through preferential adsorption, while its effect on smithsonite was weak. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), molecular dynamics (MD), and density functional theory (DFT) results further explained this selectivity. CSA had a stronger affinity for the exposed Ca sites on calcite, where it formed a hydrophilic surface layer and enhanced interfacial hydration. This limited the further adsorption of NaOL on calcite. In contrast, CSA interacted weakly with smithsonite, so its NaOL-induced hydrophobicity was largely retained. Compared with previously reported depressant systems, this work demonstrates that selective coordination with surface Ca sites and enhanced interfacial hydration can be integrated within a single metal-ion-free reagent. This finding provides a mechanism-based strategy for designing multifunctional depressants for carbonate mineral separation. These results show that CSA improves smithsonite/calcite separation by selectively regulating the calcite interface, and it has potential as a green depressant for carbonate mineral flotation.