Enhanced sulfidization flotation mechanism of smithsonite in the synergistic activation system of copper–ammonium species

•Copper–ammonium species exhibited superior activation performance than individual copper ions.•The activity of sulfidization products on smithsonite surfaces enhanced after activation with copper–ammonium species.•Addition of copper–ammonium species significantly enhanced the surface hydrophobicity of smithsonite.•Smithsonite floatability greatly increased in the synergistic activation system of copper–ammonium species.