Advanced collector-free flotation of typical sulfide minerals using a novel heterocyclic depressant

•A thiazole depressant CSD7 is developed for the collector-free flotation of molybdenum-bearing minerals.•CSD7 selectively chelates with the surfaces of chalcopyrite, galena and pyrite by the bonds of −S−M2+ and =O−M2+.•Under alkaline or acidic conditions, the depression effect of CSD7 is weakened by the conversion from C=O to C−O.•The molybdenite surface is covered with S atoms, with a strong hydrophobicity, limiting the adsorption of CSD7.•CSD7 significantly changes the electrochemical properties of the chalcopyrite, galena and pyrite surfaces.