Chalcopyrite and molybdenite are vital strategic metal resources. Due to their close association in ores, flotation methods are commonly used for separation. The flotation separation method primarily employs the “copper depression and molybdenum flotation” process, enhancing the wettability difference between chalcopyrite and molybdenite through a chalcopyrite depressant. Traditional depressants often face challenges, including low selectivity, high dosage requirements, poor stability, and significant environmental pollution, highlighting the need for new, highly selective green reagents. This study introduces the novel chalcopyrite depressant 2-mercapto-6-methylpyrimidin-4-ol (MMO) for flotation separation. The influence of MMO on chalcopyrite and molybdenite flotation recovery was examined through microflotation experiments. Additionally, the effects of MMO and ethyl xanthate on surface wettability were assessed via contact angle measurements. The adsorption microstructure and interaction mechanism of MMO on chalcopyrite were elucidated using FT-IR, TOF-SIMS, and XPS analyses and DFT simulations. Results indicate that MMO enhances chalcopyrite hydrophilicity and exhibits a strong depressing effect on its flotation, while minimally impacting molybdenite recovery. Thus, it serves as an effective depressant. During adsorption, N and S atoms in MMO donate electrons to Fe and Cu ions, leading to triple bond adsorption and a stable chelate structure. These findings are crucial for achieving a greener and more efficient flotation separation of copper and molybdenum.

Molybdenum (Mo) is a strategic raw material for high-end equipment manufacturing, aerospace technologies, and advanced alloys, and approximately 50% of global molybdenum resources are hosted in porphyry Cu–Mo deposits. To address the long-standing challenge of selectively separating chalcopyrite and molybdenite by flotation, this study screened five sulfur-containing organic depressants and investigated their effects on the flotation responses of the two minerals, motivated by the strong affinity of sulfur donor atoms for surface Cu sites on chalcopyrite. The results indicate that thiomalic acid, 4-hydroxythiobenzamide, and 6-methyl-2-thiouracil markedly depress chalcopyrite flotation, whereas 2-(methylthio)acetic acid and N-phenylthiourea exert only minor effects. In contrast, none of the five reagents significantly affects the floatability of molybdenite. Among these depressants, thiomalic acid exhibited the best selectivity. In practical Cu–Mo bulk concentrate flotation, it showed a clear dosage advantage at low addition levels and improved Cu–Mo separation performance; at a Mo recovery of 76.09% and a Mo grade of 5.45%, Cu recovery was reduced to 9.54%. The adsorption mechanism of thiomalic acid on chalcopyrite was further investigated using FT-IR spectroscopy, X-ray photoelectron spectroscopy, and self-consistent charge density-functional tight-binding (SCC-DFTB) calculations. The results suggest that thiomalic acid interacts strongly with surface Cu sites on chalcopyrite via its S- and O-containing functional groups, likely increasing surface hydrophilicity and inhibiting collector adsorption (and subsequent bubble attachment), thereby contributing to selective chalcopyrite depression.