Depressing mechanisms of sodium bisulphite in the collectorless flotation of copper-activated sphalerite

The effect of sodium bisulphite on the collectorless flotation of copper-activated sphalerite has been studied at pH 9 with nitrogen purging. X-ray photoelectron spectroscopy (XPS), time of flight secondary ion mass spectroscopy (ToF-SIMS) and ion chromatography (IC) have been used to identify a mechanism of interaction of sulphite ions with sphalerite particles. The results indicate that sodium bisulphite acts as an effective depressant for the collectorless flotation of copper-activated sphalerite and has a specific effect on the surface chemistry of sphalerite particles under the experimental conditions studied. It is suggested that sulphite ions react with the surface of copper-activated sphalerite and subsequently decompose the hydrophobic sulphur-like species responsible for flotation. Most likely, sulphite ions specifically interact with the reduced coordination sulphur atoms associated with copper activation and report into solution as a thiosulphate, which is then oxidised to sulphate. At the same time, zinc hydroxide is formed at the sphalerite surface. The consequent reduction in surface hydrophobicity explains the depression of copper-activated sphalerite in the presence of sulphite.