The conventional sulfidization–xanthate flotation of oxide copper minerals is often limited by reagent consumption and the mismatch between surface generation and sulfidization when sulfidization is carried out only during flotation conditioning. To address these limitations, this study proposes an in situ sulfidization strategy during the grinding stage and investigates the combined regulation of grinding media (conventional steel vs. 18% Cr cast iron) and atmosphere (ambient air vs. N₂ purging) on the flotation of a synthetic malachite–dolomite ore. Real-time pulp chemistry monitoring and ethylenediaminetetraacetic acid (EDTA) extraction indicate that the conventional grinding environment has two main disadvantages: Fe dissolution from steel media increases Fe-related surface contamination, while the air-ground pulp promotes the oxidation and consumption of active sulfidizing species. The combined use of high-Cr media and an N₂ atmosphere improved the chemical environment for grinding-stage sulfidization. Specifically, the high-Cr media reduced Fe release and associated surface contamination, while N₂ purging shifted the pulp to a lower-potential environment that was more favorable for preserving active sulfide species. Under standardized reagent conditions, the optimized in situ sulfidization protocol increased copper recovery from 29.79% to 57.47% and improved the concentrate grade from 9.71% to 11.11%. These results suggest that regulating the grinding environment can enhance in situ sulfidization flotation of malachite by reducing Fe interference and controlling pulp redox conditions, providing useful guidance for the efficient beneficiation of oxide copper minerals.