The Influence of Sodium Butyl Xanthate and Ammonium Dibutyl Dithiophosphate on the Flotation Behavior of Chalcopyrite and Bornite

Abstract

Chalcopyrite and bornite are typical copper sulfide minerals; however, the interaction mechanisms between these minerals and conventional collectors—sodium butyl xanthate (BX) and ammonium dibutyl dithiophosphate (ADD)—when used in combination remain unclear. To address this issue, this study integrated flotation tests, FTIR spectroscopy, adsorption measurements, and AFM to investigate the effects of BX, ADD, and their mixture on the floatability of the two minerals. Flotation results showed that chalcopyrite exhibited excellent floatability in all collector systems, with above 90% recovery. Within the pH range of 3 to 11, BX outperformed ADD in enhancing bornite flotation. The BX/ADD mixture at a mass ratio of 3:1 had no significant effect on chalcopyrite recovery but significantly improved bornite recovery to 84.20% at pH 9. FTIR confirmed chemical adsorption of both collectors on the mineral surfaces. AFM further visualized stronger adsorption of BX on both chalcopyrite and bornite. Compared with single BX or ADD alone, the BX/ADD mixed collector exhibited more effective adsorption on chalcopyrite and bornite. The study provides guidance for reagent selection in the industrial flotation of chalcopyrite and bornite.

1. Introduction

Owing to its excellent ductility, thermal conductivity, electrical conductivity, and corrosion resistance, copper has established its strategic position as a core material in modern industry [1,2]. The proven global reserves of copper ore resources are substantial, yet their grades are generally low, and they are often closely associated with a variety of other minerals [3]. Therefore, these resources must be processed via mineral processing technology to be utilized efficiently and economically. Among various mineral processing methods, flotation has become one of the most widely used technologies for treating sulfide copper ores (such as chalcopyrite and bornite), thanks to its high separation efficiency and flexible process configuration [4,5].

Among common copper sulfide minerals, bornite (Cu₅FeS₄) is an economically important secondary copper mineral with a high theoretical copper content of approximately 63.3% [6,7,8,9]. However, compared to the extensively studied chalcopyrite, bornite presents more significant challenges in industrial flotation processes. Its surface is more susceptible to oxidation during grinding and pulp conditioning, leading to the formation of hydrophilic species that severely depress its floatability [10]. Additionally, the fast flotation kinetics of bornite often result in mechanical entrainment of gangue minerals, reducing concentrate grade. These inherent difficulties make the efficient and selective recovery of bornite, particularly from complex ores where it coexists with chalcopyrite, a persistent industrial challenge.

The effectiveness of flotation largely depends on the selection and application of flotation reagents, particularly collectors, which promote the selective attachment of target minerals to air bubbles [5,11]. Among collectors for sulfide mineral flotation, conventional ones such as sodium butyl xanthate (BX) and ammonium dibutyl dithiophosphate (ADD) have been extensively studied due to their unique chemical properties and application performance [12,13]. As a classic xanthate-based collector, BX exhibits advantages including good selectivity and strong collecting ability and is especially suitable for the flotation of sulfide ores (e.g., copper, lead, zinc ores) [14,15]. The thiol groups in its molecule can form stable complexes with metal ions on the surface of sulfide minerals, thereby significantly improving the floatability of the minerals. ADD, on the other hand, is a dithiophosphate-based collector. Its molecular structure contains thiophosphate groups, which can undergo stronger chemical adsorption with the surface of sulfide minerals and demonstrate excellent collecting efficiency, particularly in the processing of complex sulfide ores [14,16,17].

The interfacial interaction mechanisms between xanthate-based collectors and copper sulfides have been extensively investigated [12,18,19,20,21,22]. For instance, Hepel and Pomianowski constructed an electrochemical equilibrium diagram of a Cu–ethyl xanthate–water system at 25 °C via thermodynamic calculations and explored the application of sodium ethyl xanthate in natural copper ore flotation [20]. Roos et al. studied the interfacial interactions in a chalcopyrite–xanthate flotation system, revealing that chalcopyrite reacts directly with xanthate ions to form CuEtX species at potentials above −300 mV [22]. Mustafa et al. further confirmed that the adsorption of xanthate on chalcopyrite surfaces involves the formation of cuprous xanthate [21]. Additionally, Hwang et al. investigated the interaction between potassium amyl xanthate (PAX) and chalcopyrite surfaces in the presence of NaCl and found that this process induces the formation of hydrophobic CuS, thereby improving flotation efficiency in low-PAX concentration systems [12].

By contrast, research on dithiophosphates—another key class of collectors for sulfide flotation—exhibits obvious limitations in targeting chalcopyrite and bornite. Industrial tests have demonstrated that Aerophine 3418A, a representative dithiophosphate collector, can increase chalcopyrite recovery by 8% at a dosage 50% lower than that of xanthates, while also showing excellent selectivity against pyrite and sphalerite [23]. Dhar et al. partially filled this gap by comparing the flotation performance of di-secondary butyl dithiophosphate (DBD) and xanthate in processing Nussir ore (which contains chalcopyrite and bornite) and proved that mixed collectors can enhance copper recovery to 98% [24]. Further mechanistic studies on dithiophosphates have mainly focused on specific conditions rather than general applicability. For example, Chander et al. found that under oxidizing and slightly acidic conditions, dithiophosphates adsorb on chalcopyrite surfaces in the form of (DTP)₂ dithiolates [25]. Monte et al. added that dithiophosphates (e.g., sodium diethyl dithiophosphate, DTF) exhibit weaker affinity for sulfide mineral surfaces than xanthates, but this conclusion was derived from gold–pyrite systems and cannot be extrapolated to chalcopyrite–bornite systems [26]. Collectively, these studies confirm the potential of dithiophosphates in copper sulfide flotation.

While the flotation behavior of chalcopyrite and bornite with individual collectors (BX and ADD) has been extensively studied, the synergistic effects of their combined use remain poorly understood. Addressing this gap is critical for industrial flotation, as optimized collector blends can reduce reagent consumption, lower operational costs, and enhance selectivity in processing complex ores. This study investigated the interaction mechanisms of the collectors BX and ADD and their mixture with the surfaces of pure chalcopyrite and bornite using a multi-technique approach. Flotation tests were employed to evaluate the performance of collectors by measuring mineral recovery. FTIR spectroscopy was utilized to identify the functional groups of adsorbed collectors, elucidating the nature of surface adsorption. Adsorption measurements quantified the collector uptake, establishing a quantitative correlation between adsorption density and flotation performance. AFM characterized the nanoscale surface morphology before and after collector treatment. These complementary techniques provided comprehensive insights from macroscopic flotation efficiency to microscopic interaction mechanisms. The aim of the study is to provide theoretical support for optimizing reagent systems in the flotation of chalcopyrite–bornite mixed ores and to serve as a reference for the efficient application of xanthate–dithiophosphate systems in sulfide copper ore flotation.

2. Materials and Methods

The pure mineral samples of chalcopyrite and bornite utilized in the experiments were collected from Tonglvshan Mine in Daye, Hubei Province, China. The purity of the mineral samples was determined through X-ray diffraction (XRD) analysis (Figure 1). The XRD patterns confirmed high mineralogical purity, with a chalcopyrite content of approximately 98.8% and a bornite content exceeding 90%, ensuring their suitability for fundamental flotation mechanism studies. These two samples were crushed, dry-ground, and screened to the required size fraction (38–74 μm). The d80 particle size was approximately 68 μm.

Technical-grade sodium butyl xanthate (BX) and ammonium dibutyl dithiophosphate (ADD) purchased from Hubei Jingjiang Collector Co., Ltd. (Changchun, China) and Hunan Mingzhu Flotation Reagents Co., Ltd. (Zhuzhou, China) were employed as collectors in the flotation experiments. The pH regulators NaOH and HCl were both analytical-reagent-grade. Deionized water (once-distilled) was used in all the experiments.

2.1. Characterization of Flotation Collectors

The collectors used in this study, BX (C₄H₉OCS₂Na) and ADD ([C₄H₉O]₂PS₂NH₄), were of technical grade. Their molecular structures are depicted in Figure 2. BX has a molecular weight of 172.23 g/mol and is characterized by its thiocarbonate (-OCS₂⁻) functional group, which acts as the active site for chemisorption onto sulfide mineral surfaces. ADD has a molecular weight of 283.38 g/mol and features a thiophosphate (P=S) group, known for forming strong surface complexes. Both reagents are highly soluble in water and undergo gradual decomposition under acidic conditions, which is why flotation is typically conducted in neutral or alkaline pulp environments.

2.2. Flotation Tests

Flotation tests were conducted using an XFG-type flotation machine (Jilin Provincial Prospecting Machinery Factory, Changchun, China). A mineral sample weighing 2.0 g was placed into a 40 mL flotation cell, followed by the addition of 35 mL of deionized water. The slurry was conditioned for 1 min. Subsequently, the pH was adjusted to the target value using HCl or NaOH solutions over a period of 2 min. Following pH adjustment, the collector was added and stirred for 3 min. Finally, the frother was introduced and stirred for an additional minute before flotation commenced. Flotation was carried out for 3 min with bubble scraping. The floated and sink products were filtered, dried, and weighed to calculate the mineral recovery. All flotation tests were conducted in three independent replicates (n = 3) for each experimental condition to ensure statistical reliability. The recovery values reported in the results represent the arithmetic mean, and the variability is presented as the standard deviation, which is shown by error bars in all relevant figures.

2.3. Fourier Transform Infrared Spectroscopy Measurements

The interaction between minerals and collectors was investigated using an IRAffinity-1 Fourier transform infrared (FTIR) spectrometer by comparing the infrared spectra of mineral samples before and after treatment with collectors. Specifically, 2 g of pure mineral sample was added to a conical flask containing 40 mL of deionized water. The pH of the suspension was adjusted to the desired value, followed by the addition of an appropriate concentration of the collector. The mixture was stirred magnetically for 3 min, then filtered and centrifuged. The collected solid was vacuum-dried at 30 °C for 12 h to obtain the sample for FTIR analysis. The KBr pellet method was employed for infrared transmission measurements. Before use, the KBr powder was heated and dried. The dried sample was mixed with KBr at a mass ratio of 1:100 and thoroughly ground in an agate mortar until the particle size was reduced to below 2 μm. The homogeneous mixture was then compressed into a transparent pellet for infrared spectral acquisition. The spectra were acquired over a wavenumber range of 4000 to 500 cm⁻¹.

2.4. Adsorption Measurements

The adsorption capacity of collectors on chalcopyrite and bornite was evaluated using ultraviolet spectroscopy. Initially, 2.0 g of the mineral was subjected to ultrasound treatment in a beaker, followed by the addition of deionized water in a volume equivalent to that used in the flotation process. The pH of the slurry was adjusted to 9, and the necessary reagents were introduced. After magnetic stirring for 3 min, the mixture was centrifuged to collect the supernatant for ultraviolet spectrophotometry analysis. Before measurement, standard concentration samples were prepared to establish a standard curve. Finally, the supernatant was analyzed using a UV-2600 spectrophotometer at wavelengths of 300 nm and 225.5 nm to determine the residual concentration of the collector. The measurements were performed with a scanning speed of ‘medium’ and a spectral bandwidth of 2 nm to ensure accuracy. The adsorption capacity of the collector on chalcopyrite and bornite was calculated based on the changes in collector concentration. Each adsorption measurement was conducted in triplicate (n = 3), and the results are presented as mean ± standard deviation.

2.5. Surface Image Analysis

The surface morphological changes in chalcopyrite and bornite before and after interaction with collectors were investigated using Atomic Force Microscopy (AFM). The experiments were conducted with a Multimode VIII AFM instrument (Bruker, Manning Park Billerica, MA, USA) operating in Peakforce mode. Prepared mineral samples were immersed in collector solutions at the corresponding concentrations for ten minutes, followed by rinsing with deionized water and drying with nitrogen gas. A probe model RTESP-300, made of silicon nitride with a spring constant of 40 N/m, was utilized during testing. Scanning was performed at a rate of 1 Hz over an area of 1.5 μm × 1.5 μm. All acquired data were processed using NanoScope Analysis 1.9 software.

3. Results and Discussion

3.1. Flotation Experiments

To evaluate the flotation performance of chalcopyrite and bornite when using the collectors BX and ADD at a dosage of 20 mg/L, single mineral flotation tests were conducted, and the results are shown in Figure 3. As presented in Figure 3, the flotation recovery of chalcopyrite remained above 90% regardless of whether BX or ADD was used as the collector. This excellent floatability indicated that both BX and ADD exhibited outstanding collecting ability for chalcopyrite, which was consistent with the regularity of xanthate-based collectors in chalcopyrite flotation reported in the literature [27,28,29,30]. Notably, under identical pH conditions, the flotation recovery of chalcopyrite when using ADD as the collector was consistently higher than that of BX.

For bornite flotation, when using BX as a collector, the flotation recovery of bornite was maintained at over 85% within the pH range of 3 to 7. This was mainly because BX can effectively chemically adsorb onto the surface of bornite in this pH range (e.g., forming hydrophobic products such as Cu(BX)₂), significantly enhancing its floatability. However, when the pH exceeded 7, the flotation recovery of bornite began to decline. The trend was particularly pronounced under strong alkaline conditions, where the competitive adsorption of OH⁻ for surface sites resulted in limited surface coverage, as described using the Langmuir model, which consequently reduced surface hydrophobicity and thereby led to a sharp drop in flotation recovery [31]. In contrast, when using ADD as a collector, the flotation recovery of bornite significantly increases with rising slurry pH, from 67.5% at pH 3 to 79.0% at pH 11. Thus, ADD collecting ability for bornite increased with rising pH. The flotation results in the figure indicate that in the bornite flotation system, the recovery using BX as a collector was consistently higher than that when using ADD.

At pH 9, chalcopyrite maintained high flotation recovery with either BX or ADD. In contrast, bornite exhibited a declining trend when using BX. Nevertheless, the recovery of bornite with BX remained as high as 85.75%. The influence of collector dosage on flotation recovery was subsequently investigated at pH 9, and the results are presented in Figure 4. It is important to note that the concentration ranges were selected specifically to elucidate the distinct flotation kinetics of each mineral. Chalcopyrite exhibits rapid saturation, achieving maximum recovery at low collector doses. In contrast, bornite requires a broader range to fully capture its continuous increase in recovery toward a saturation point. This design allows for a more accurate characterization of each mineral’s response, rather than a direct comparison of absolute recoveries at identical concentrations.

As shown in Figure 4a, throughout the entire range of collector concentrations tested, both BX and ADD exhibited strong collecting ability for chalcopyrite, with flotation recovery remaining stable and showing no significant fluctuations. In the flotation of bornite (Figure 4b), the flotation recovery initially increased gradually with the dosage of BX, after which the growth trend stabilized. Specifically, when the BX dosage increased from 5 mg/L to 20 mg/L, the recovery of bornite rose significantly, reaching a maximum of 85.05%. Further increases in BX dosage resulted in only slight fluctuations in recovery. This plateau, indicative of saturated active adsorption sites on the chalcopyrite surface, suggested that once the optimal dosage was exceeded, the additional collector molecules cannot enhance the surface hydrophobicity beyond the complete monolayer coverage [6,32]. In contrast, when ADD was used as a collector, the recovery of bornite increased with dosage, peaking at 80.05% at 40 mg/L, which was notably lower than the maximum recovery obtained with BX. A comprehensive analysis of Figure 3b revealed that under identical dosage conditions, the flotation recovery of bornite using BX consistently exceeded that achieved with ADD.

To further investigate the influence of mixed collector systems, the flotation recovery of chalcopyrite and bornite with a BX/ADD mixture was evaluated. The effect of the BX:ADD mass ratio was studied at a fixed total concentration of 20 mg/L and pH 9, as shown in Figure 5a. A clear trend emerged for bornite: the flotation recovery of bornite exhibited a clear trend: it monotonically increased with the proportion of BX in the mixed collector, reaching a maximum at a BX:ADD ratio of 5:1 and remaining high even with BX alone (ratio 1:0). This trend demonstrated that BX is the dominant contributor to bornite collection within the mixed system, a finding consistent with the single-collector tests (Figure 4b) where BX outperformed ADD. The comparable recovery achieved at the 5:1 ratio to that of pure BX is particularly significant. It indicates that a mixed system, predominantly composed of the more cost-effective BX, can achieve performance on par with the superior single collector, suggesting a potential pathway for industrial reagent cost optimization. Therefore, the role of ADD is synergistic, likely enhancing the adsorption density or stability of the collector layer rather than acting as the primary contributor to collecting power.

To further elucidate the impact of pH on flotation performance within the mixed collector, the study investigated the effects of the BX/ADD mixed collector across varying pH conditions. A fixed mass ratio of 3:1 was selected for this investigation, as it represented a composition where a significant synergistic improvement in bornite recovery was already established (see Figure 5a), allowing for a clear analysis of pH influence on a functionally effective mixture. The results are illustrated in Figure 5b. Within the pH range of 3 to 11, the flotation recovery of chalcopyrite consistently exceeded 90%, aligning with the exceptional floatability of chalcopyrite reported in prior tests utilizing single collectors and variable ratio mixed collectors. In contrast, the flotation recovery of bornite remained approximately 80% across the tested pH range, achieving a peak recovery of 84.20% at pH 9.

As summarized in

Table 1. Flotation recoveries of chalcopyrite and bornite under different collector systems at a fixed condition (pH = 9; total dosage = 20 mg/L).

Mineral/Collector System	BX	ADD	BX/ADD (3:1)
Chalcopyrite	93.1%	94.25%	94.7%
Bornite	85.75%	75.45%	84.2%

, the mixed BX/ADD (3:1) system achieved excellent and comparable recovery for chalcopyrite relative to single collectors, but its most pronounced benefit was observed for bornite. A direct comparison was conducted between the mixed collector and single-collector systems (BX or ADD alone) under identical experimental conditions, including collector dosage and pH range. The results revealed that the mixed collector demonstrated a superior collecting ability for bornite. This finding substantiates the assertion that the mixed system produces a synergistic effect in bornite flotation [33]. Consequently, the mixed collector effectively enhances the overall recovery of bornite.

3.2. Fourier-Transform Infrared Spectroscopy Measurements

To clarify the adsorption mechanisms and interaction characteristics of collectors on the surfaces of chalcopyrite and bornite, this study employed FTIR measurements for analysis. The results of the FTIR spectral analysis are presented in Figure 6.

Figure 6a presents the FTIR spectra of pure collectors. For BX, the characteristic absorption peaks at 2958 cm⁻¹ and 2871 cm⁻¹ were attributed to the asymmetric and symmetric stretching vibrations of CH₃ groups, respectively. The peaks at 2924 cm⁻¹ and 1459 cm⁻¹ were assigned to the stretching and bending vibrations of CH₂ groups. Additionally, the peak observed at 1390 cm⁻¹ may correspond to the symmetric stretching vibration of sulfate esters or the bending vibration of CH₃ groups. For ADD, the strong double absorption peaks at 1038 cm⁻¹ and 967 cm⁻¹ were characteristic of P-O-C stretching vibrations, whereas the peaks at 700 cm⁻¹ and 550 cm⁻¹ were attributed to P-S₂ stretching vibrations. Figure 6b,c show the FTIR spectra of chalcopyrite and bornite before and after interaction with the collectors, respectively. Comparative analysis revealed that the characteristic peaks of BX and ADD undergo significant shifts after adsorption on the surfaces of the two minerals, providing clear evidence of chemical interaction.

For chalcopyrite treated with BX, the characteristic peaks at 1105 cm⁻¹, 1046 cm⁻¹, and 850 cm⁻¹ shifted by 3 cm⁻¹, 6 cm⁻¹, and 14 cm⁻¹, respectively. After interaction with ADD, the characteristic double peaks corresponding to P-O-C stretching vibrations appeared at 1028 cm⁻¹ and 991 cm⁻¹, exhibiting significant shifts; meanwhile, the P-S₂ stretching peak at 682 cm⁻¹ shifted by 18 cm⁻¹. For bornite, treatment with BX caused the characteristic peaks at 1120 cm⁻¹, 1035 cm⁻¹, and 848 cm⁻¹, shifting by 12 cm⁻¹, 17 cm⁻¹, and 2 cm⁻¹, respectively. When treated with ADD, the absorption peaks at 3122 cm⁻¹ and 1406 cm⁻¹ shifted by 3 cm⁻¹ and 6 cm⁻¹, while the characteristic peaks of ADD at 1029 cm⁻¹, 1002 cm⁻¹, and 698 cm⁻¹ exhibited significant shifts. These spectral shifts collectively confirmed that both BX and ADD undergo chemical adsorption on the surfaces of chalcopyrite and bornite.

3.3. Adsorption Capacity Measurement

At a fixed pH of 9, the variation trends of the adsorption capacities of collectors (BX, ADD, and their mixture) on the surfaces of chalcopyrite and bornite with concentration are shown in Figure 7. Both collectors exhibited a consistent pattern within the tested concentration range (5–50 mg/L), displaying similar trends in adsorption capacity on chalcopyrite and bornite. However, they consistently demonstrated slightly higher adsorption capacities on chalcopyrite compared to bornite. Specifically, the average adsorption capacity of BX on chalcopyrite was approximately 0.04 mg/g higher than that on bornite (for instance, at 20 mg/L, the values were 0.35 mg/g and 0.31 mg/g, respectively). The average difference in adsorption capacity of ADD between the two minerals was about 0.03 mg/g (for example, at 30 mg/L, the values were 0.32 mg/g and 0.29 mg/g, respectively). This indicated that both BX and ADD exhibited stronger adsorption interactions with chalcopyrite than with bornite. The conclusion was highly consistent with previous flotation experimental results, which showed that when BX or ADD was used as the collector, the flotation recovery of chalcopyrite was consistently higher than that of bornite. The coincident adsorption curves of the BX/ADD mixture on chalcopyrite and bornite underscore a synergistic effect that diminishes the innate differences in surface chemistry between the two minerals, leading to unified collector adsorption behavior.

3.4. AFM Tests

AFM can directly visualize the adsorption morphology of collectors on mineral surfaces [34,35]. Figure 8 illustrates the surface morphologies of chalcopyrite after treatment with different reagents. Compared with the untreated chalcopyrite surface (Figure 8a), the surface height of chalcopyrite treated with BX increased, indicating that BX was adsorbed on the chalcopyrite surface (Figure 8b). For chalcopyrite treated with ADD, a large area of ADD was adsorbed on its surface (Figure 8c). Figure 8d shows that denser punctate adsorption appeared on the chalcopyrite surface treated with the BX/ADD mixed collector. Compared with samples treated with a single collector, the adsorption density on the sample surface treated with the mixed collector was significantly higher, indicating that synergistic adsorption occurred between BX and ADD on the chalcopyrite surface.

Figure 9 presents AFM 2D and 3D images of bornite surfaces under different reagent treatments. For the untreated bornite surface (Figure 9a), the 3D image exhibited a uniformly flat morphology. After treatment with BX (Figure 9b), the 2D image showed dispersed bright spots, and the 3D image presents protrusions, indicating that BX was adsorbed on the bornite surface. When treated with ADD (Figure 9c), the distribution characteristics of bright spots in the 2D image were obviously different from those in Figure 9b, and the 3D image showed a surface with protrusions of different densities, indicating that the adsorption behavior of ADD differed from that of BX. For the bornite surface treated with the BX/ADD mixed collector (Figure 9d), the 2D image showed denser and clustered bright spots, while the 3D image exhibited prominent and abundant protrusions. This indicated that the mixed collector exhibited more remarkable adsorption on the bornite surface, which reflected the synergistic adsorption effect.

The AFM results clearly demonstrated enhanced surface adsorption density when using the BX/ADD mixed collector on both chalcopyrite and bornite, suggesting synergistic adsorption behavior.

However, the flotation recovery data present a more nuanced picture: while this enhanced adsorption translated to a notable improvement in bornite recovery, it did not lead to a further significant increase in chalcopyrite recovery, which was already exceeding 90% with single collectors. The discrepancy between adsorption and recovery can be explained by the following mechanisms.

Firstly, the concept of a “hydrophobic saturation threshold” is critical [36,37]. When a mineral surface achieves a certain degree of hydrophobicity, additional collector adsorption may occur on already hydrophobic sites or form multilayers that do not contribute to further enhancing the bubble–particle attachment probability, which is the decisive step for flotation recovery. For chalcopyrite, the extensive adsorption of either single BX or ADD alone might have already rendered the surface sufficiently hydrophobic to reach near-maximum recovery. Secondly, the nature of the adsorbed layer is as important as its quantity. The synergistic adsorption might not only increase coverage but also alter the morphology and packing of the collector layer on the bornite surface.

The denser and clustered bright spots in Figure 9d could lead to a more robust or uniform hydrophobic film compared to the adsorption of single BX (Figure 9b) or ADD (Figure 9c). This improved quality of the hydrophobic layer could be the key reason for the mixed collector’s ability to maintain high bornite recovery over a wider pH range, effectively addressing the sensitivity issue observed with single BX at higher pH (as shown in Figure 3). For chalcopyrite, which is inherently more readily floatable, the benefit of such a microstructural improvement might be marginal.

Moreover, even in cases where the mixed collector does not yield a significant improvement in overall recovery, it may still enhance the stability and selectivity of the flotation process, offering operational advantages under varying pulp conditions [38]. For chalcopyrite, which already exhibits excellent floatability with single collectors, the primary benefit of the BX/ADD mixture might not be a higher recovery ceiling but rather a more robust collector layer that could ensure consistent performance despite fluctuations in plant conditions, such as pH or reagent dosage. The improved stability, as visualized by the denser and more uniform adsorption in AFM images, could be particularly valuable for the treatment of complex ores where selective separation from gangue minerals is critical. The synergistic adsorption likely creates a more stable interfacial environment, potentially reducing the unwanted activation of gangue minerals and improving concentrate grade.

In conclusion, the synergistic effect of the BX/ADD mixture is substantiated by the AFM findings. For chalcopyrite, the effect is primarily observed as enhanced surface adsorption without a corresponding recovery increase due to the achievement of hydrophobic saturation. For bornite, the synergy contributes to both increased adsorption density and potentially a more effective hydrophobic layer, resulting in improved flotation performance stability across pH conditions.

Comprehensive analysis of the flotation recovery, FTIR, and AFM results allows us to infer that chalcopyrite exhibited superior hydrophobicity compared to bornite across all collector systems, which fundamentally explains its higher flotation recovery. The stronger adsorption density of both BX and ADD on the chalcopyrite surface (as visualized by AFM images) and their more stable chemical adsorption (as confirmed by FTIR spectra) collectively contributed to the formation of a more stable and continuous hydrophobic film on chalcopyrite, thereby significantly enhancing its attachment efficiency to air bubbles. The inherently superior floatability of chalcopyrite over common gangue minerals has been extensively attributed in the literature to its surface chemistry, which favors the chemisorption of xanthate and dithiophosphate collectors [39,40]. Although direct contact angle measurements to quantitatively characterize hydrophobicity were not conducted in this study, the evidence provided by the aforementioned complementary techniques strongly supports this conclusion.

4. Conclusions

This study investigated the interactions of BX and ADD with chalcopyrite and bornite.

Flotation tests demonstrated that the BX/ADD mixed collector (3:1 mass ratio) achieved a synergistic effect, significantly enhancing bornite recovery and stability across a wider pH range, while maintaining the inherently high recovery of chalcopyrite. Furthermore, the synergistic adsorption suggests potential for improved process stability and selectivity, which are crucial for industrial flotation operations. This performance was attributed to an enhanced adsorption mechanism, as unequivocally confirmed by complementary analytical techniques: FTIR verified chemical adsorption, adsorption measurements quantified greater collector uptake on chalcopyrite, and AFM visualized denser surface coverage, particularly on bornite.

These findings provide practical guidance for reagent optimization in the industrial flotation of chalcopyrite–bornite ores, suggesting a viable strategy to reduce operational costs and improve separation efficiency through tailored reagent formulations.

It is important to note that this study, utilizing pure minerals, provides a fundamental understanding of the collector–mineral interactions; however, the performance in complex ore systems containing multiple sulfides and gangue minerals may vary due to unforeseen interactions. Therefore, future research should prioritize validating these promising results by (i) investigating the effects of pulp ions (e.g., Fe³⁺, S²⁻); (ii) conducting batch flotation tests on real ore samples; and (iii) optimizing reagent dosage across industrially relevant pH ranges.