During copper smeltingCopper Smelting, arsenicArsenic accumulates in copperCopper-based solid wastes such as flue dust, which is produced in large quantities and contains both valuable metalsMetal and high toxicity. Selective sulfideSulfide precipitationPrecipitation is commonly used to recover copperCopper from leachingLeaching solutions, but traditional sulfideSulfide agents (e.g., Na₂S, NaHS) hydrolyze rapidly in acidic conditions, releasing H₂S abruptly and causing issues like low separation efficiencyEfficiency, high reagent use, and toxicity risks. This study explores ironIron sulfidesSulfide as controlled-release sulfideSulfide agents. By modifying the structure of pyritePyrite, a reactive and slow-releasing sulfideSulfide material was developed, lowering the effective S2⁻ concentration and enabling selective precipitationPrecipitation of valuable metalsMetal. In a reducingReducing atmosphere, natural pyritePyrite and ironIron undergo a phase transformation sequence: pyritePyrite + ironIron → monoclinic Fe₁₋ₓS → monoclinic + hexagonal Fe₁₋ₓS → hexagonal Fe₁₋ₓS → troilite. XRD and SEMScanning Electronic Microscope (SEM)-EDS analysesAnalysis show that reactivity is influenced by crystal structureCrystal structure and internal morphology. Monoclinic Fe₁₋ₓS extended H₂S release from 3 to 30 min, controlled by diffusion and product layer resistance. A coordinated leachingLeaching andSlow-release sulfidation slow-release sulfideSulfide precipitationPrecipitation method was developed, achieving a copper sulfideCopper sulfides product with 40.56% Cu and only 1.87% As, enabling efficient separation and resource recoveryRecovery from copper smelting dustCopper smelting dust.