## 1 Introduction  
Molybdenum and copper are indispensable to modern industry, finding widespread applications in power transmission, electronics manufacturing, construction materials, and high-temperature alloys, owing to their outstanding mechanical strength, electrical conductivity, and corrosion resistance [1, 2, 3, 4]. More than 75% of the world's copper and a significant portion of molybdenum are extracted from porphyry copper deposits, which serve as the primary source for both metals [5, 6]. The recovery of copper and molybdenum from porphyry deposits commonly employs a two-step flotation strategy. Bulk flotation is initially conducted to generate a mixed concentrate, then followed by selective flotation, where molybden (MoS\({}_{2}\)) is floated and chalcopyrite (CuFe\({}_{2}\)) is depressed to achieve separation [7, 8, 9]. In practice, kerosene is commonly used as the collector, while depressants such as sodium sulfide or sodium thioglycolate (STG) are applied to selectively inhibit copper minerals [6; 10; 11]. The high efficiency and selectivity of the interaction between depressant and mineral emerges as the crucial point for successful chalcopyrite-molybdenum flotation separation.  
On the other hand, the presence of metal ions in the pulp has been confirmed exhibiting significant impacts on flotation performance [12; 13; 14]. For instance, in lead-zinc sulfide ones containing zinc sulfate and ferrous sulfate, the addition of lime is typically restricted because it induces the precipitation of metal hydroxides such as Zn(OH)\({}_{2}\), Fe(OH)\({}_{2}\), and Fe(OH)\({}_{3}\), which severely depress both sphalerite and galena [15; 16; 17]. Similarly, in alkaline seawater environments (pH > 9), Ca\({}^{2+}\) and Mg\({}^{2+}\) precipitation as insoluble carbonates and hydrophilic hydroxides on mineral surfaces adversely affect the flotation performance of chalcopyrite and molybdenume [18]. During the flotation of gold-bearing pyrite, hydroxide films from Cu\({}^{2+}\), Fe\({}^{2+}\), and Fe\({}^{3+}\) hinder collector adsorption [19], while Fe\({}^{2+}\) adsorption during smithsonite sulfidization produces hydrophilic FeOOH species that decrease reactive sites [20]. Moreover, ions like Cu\({}^{2+}\), Pb\({}^{2+}\), and Fe\({}^{3+}\) tend to adsorb onto molybden surfaces as hydroxy complexes, elevating zeta potentials and generating precipitates that inhibit molybden flotation [5; 21]. Although the effects of metal ions in various flotation systems have been extensively studied, relatively little attention has been paid to the specific impact of copper ions on the selective flotation of molybden in the presence of chalcopyrite, particularly under conditions involving STG as a depressant. This lack of understanding may hinder further optimization of copper-molybdenum separation processes.  
This study systematically investigates the effect of Cu\({}^{2+}\) (the primary metal ion in pulp) on the flotation behavior of chalcopyrite and molybden with kerosene as collector and STG as depressant. Flotation results demonstrated that Cu\({}^{2+}\) exhibited negligible influence on the depressant performance of STG for chalcopyrite, while significantly affecting the molybden flotation. Detailed mechanism analysis revealed that Cu\({}^{2+}\) was preferentially adsorbed onto the molybden surface as Cu(I), providing active sites for the chemisorption of STG molecules. This interaction formed a stable molybden-Cu(I)-STG surface complex, which significantly increased surface hydrophilicity and depressed molybden floatability. These findings clearly verify the influence of Cu\({}^{2+}\) on the copper-molybdenum separation, conducive to developing more efficient separation techniques.