Selective Separation of Galena from Sphalerite with a Novel Depressant Sodium Thiophosphate: Flotation and Adsorption Mechanism

The selective flotation of galena from sphalerite is a persistent challenge in mineral processing due to their similar floatability. This study introduces sodium thiophosphate (Na₃PO₃S) as a novel and highly efficient depressant for sphalerite. The depression performance and underlying mechanisms were systematically investigated through microflotation tests, contact angle measurements, adsorption studies, microcalorimetry, surface analyses (Raman, ToF-SIMS, XPS), and Density Functional Theory (DFT) calculations. Flotation results demonstrated that Na₃PO₃S exhibits selective depression against sphalerite in the pH range of 4.0–11.5, while having a negligible effect on galena flotation. In a mixed mineral system at pH 8.5, a lead concentrate with a lead recovery of 93.59% was obtained, while zinc recovery was suppressed to just 8.36%. Contact angle and adsorption measurements revealed that Na₃PO₃S strongly adsorbs on the sphalerite surface, which enhances the hydrophilicity of sphalerite, while its adsorption on galena is weak and it can be replaced by DDTC. Spectroscopic analyses (Raman, ToF-SIMS, XPS) provided evidence that Na₃PO₃S forms ZnSPO₃^– by reacting with Zn sites on the sphalerite surface, which enhances the hydrophilicity of sphalerite and hinders the adsorption of the collector (DDTC). Microcalorimetry results revealed that the heat of adsorption on sphalerite was twice that on galena, amounting to 0.30 J/g. This aligns with DFT calculations, which showed that the adsorption energy of Na₃PO₃S on the sphalerite (1 1 0) surface (−291.085 kJ/mol) is substantially higher than that on the galena (1 0 0) surface (−186.783 kJ/mol), confirming the adsorption on sphalerite is more stable compared to galena. This work presents Na₃PO₃S as a promising and effective depressant for the selective and efficient separation of lead–zinc sulfide ores.