Investigation of a Novel Depressant for Flotation Separation of Chalcopyrite and Galena: Experiments and Adsorption Mechanisms

Abstract

To reduce lead content in copper concentrates, this study developed a novel galena depressant, TA (thioureidoacetic acid). This study utilizes a synthetic mineral feed with fully liberated galena and chalcopyrite from separate sources to establish baseline separation conditions. The adsorption capability of TA on galena surfaces was systematically investigated through micro-flotation tests, surface characterization, and first-principles calculations. Results demonstrate that TA effectively reduces galena recovery (from 82.92% to 12.29%) without compromising chalcopyrite flotation efficiency (>83.2% recovery) when using thionocarbamate (Z200) as the collector. FTIR and XPS analyses confirm that TA chemisorbs onto galena surfaces via its C=S and C=O functional groups. First-principles calculations reveal dual Pb-S and Pb-O bond formation during TA adsorption, resulting in stronger interfacial binding energy compared to Z200. This work establishes a molecular engineering framework for designing high-selectivity depressants.

1. Introduction

Copper, as a cornerstone material of modern industrialization, continues to drive global demand growth due to its irreplaceable role in power infrastructure, renewable energy systems, and advanced manufacturing [1]. Chalcopyrite serves as the primary source of copper resources and is naturally associated with galena in mineral deposits. During flotation processes, while the industrial standard collector Z200 (O-isopropyl-N-ethyl thiocarbamate) significantly enhances chalcopyrite recovery by inducing surface hydrophobicity, its non-selective adsorption mechanism concurrently activates galena surfaces, resulting in persistently elevated lead impurity levels [2]. These lead impurities not only diminish the economic value of copper products but also substantially escalate smelting costs due to toxic lead emissions during pyrometallurgical processes [3,4]. Consequently, there is an urgent demand—both economically and environmentally—for highly efficient flotation depressants capable of reducing lead content in copper concentrates during beneficiation.

Current galena depressant research primarily focuses on inorganic and organic inhibitor systems with distinct environmental and efficiency characteristics. The commonly used depressants for galena are listed in

Table 1. Galena depressant reagents in use.

Inorganic Reagents	Organic Macromolecule	Small Organic Molecule
Potassium dichromate	Polyacrylic acids	5-amino-1,3,4-thiadiazole-2-thiol
Sodium sulfide	Polyacrylamide	1,3-oxathiolane derivatives
Fe2+/NaClO	Modified lignosulfonates	N-(phosphonomethyl) iminodiacetic acid
Sodium periodate	Phosphonyl carboxylic acid copolymer	DL-dithiothreitol
	Pinewood pyrolysis liquid
	Dextrin
	Tannic acid

. Traditional inorganic depressants represented by potassium dichromate (K₂Cr₂O₇) have demonstrated effective galena depression through surface oxidation mechanisms, achieving copper–lead separation efficiency through PbCrO₄ film formation on galena surfaces. However, hexavalent chromium’s carcinogenicity severely limits its industrial application [5]. Sulfide-based systems like sodium sulfide (Na₂S) also face environmental restrictions due to potential acid mine drainage generation [6,7,8,9].

Organic depressant research has evolved into three principal directions: polymeric reagents, plant-derived compounds, and small-molecule systems. Polyacrylic acid derivatives such as polyacrylamide (PAAS) combined with H₂O₂ oxidants show selective depression through chelation with Pb²⁺ ions and surface hydroxylation [10,11,12], achieving comparable separation efficiency to chromium reagents while maintaining 83.5% Cu recovery [5]. Similarly, phosphonyl carboxylic acid copolymer (POCA) demonstrates selective adsorption on galena through Pb-O-P bonding interactions [13]. However, these polymeric depressants frequently exhibit cross-mineral suppression effects, particularly reducing chalcopyrite recoveries at elevated dosages.

Plant-derived depressants, including dextrin and tannins, show promising environmental compatibility [14,15,16,17]; however, their performance is pH-dependent. Modified lignosulfonates exhibit strong depressive effects on galena during flotation; however, their lack of selectivity also suppresses chalcopyrite recovery [18]. Recent studies reveal that pinewood pyrolysis liquid (PL) achieves >90% Pb recovery with <5% pyrite recovery through phenolic compound adsorption but requires precise pH control (pH 8.5–9.0) for optimal performance [19]. Small-molecule systems present new opportunities through targeted molecular design. Compounds like N-(phosphonomethyl) iminodiacetic acid (PMIDA) inhibit sphalerite via hydrophilic Zn-O-P complexation while maintaining galena floatability, attaining a separation efficiency of 84.3% [20]. The Fe²⁺/NaClO system generates ·OH radicals to oxidize PbS surfaces into hydrophilic PbSO₄ coatings, achieving 9.48% PbS recovery versus 82.33% CuFeS₂ recovery [21]. 5-amino-1,3,4-thiadiazole-2-thiol (MATT) and 1,3-oxathiolane derivatives have shown promising results, yet their complex synthesis routes and high raw material costs hinder industrial scalability [22,23].

Despite these advancements, key challenges persist: (1) Orbital interaction mechanisms between depressant functional groups and galena surfaces remain poorly understood, particularly regarding d-orbital electron transfer in Pb-S-depressant complexes; (2) Most organic depressants require combined oxidants (e.g., H₂O₂) or activators, complicating reagent regimes; (3) Industrial validation of novel reagents like sodium periodate (SP) and DL-dithiothreitol (DLD) remains limited [24,25].

Prior studies have largely focused on empirical observations of adsorption behavior, leaving the electronic origins of selectivity underexplored. A critical gap persists in understanding the orbital interaction mechanisms governing depressant adsorption on galena surfaces. To address this, we designed thioureidoacetic acid (TA), a novel depressant featuring dual functional groups (C=S and C=O) that synergistically chemisorb onto galena via Pb-S and Pb-O bonds. By integrating COHP analysis with DFT calculations, particularly through crystal orbital Hamiltonian population (COHP) analysis—an essential tool for quantifying bond strength and electronic coupling—this work deciphers the orbital-level interactions that underpin TA’s enhanced adsorption energy, thereby establishing a new design paradigm for flotation depressants rooted in orbital engineering. In this study, we focus on a pure mineral system where galena and chalcopyrite are fully liberated and sourced independently. This approach allows us to isolate key variables under idealized conditions before addressing complex naturally occurring ores in future work. Micro-flotation and synthetic mixed-mineral tests demonstrate that TA selectively inhibits lead–copper minerals in Z200-containing pulp systems. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses reveal that TA achieves specificity through electron-transfer-mediated chemisorption on galena surfaces. This work establishes a new paradigm for eco-efficient chalcopyrite–galena separation and provides an innovative solution for sustainable mineral resource utilization, significantly advancing environmentally friendly practices in mineral processing.

2. Materials and Methods

2.1. Minerals and Reagents

High-grade galena and chalcopyrite samples were obtained from a mine in Hubei Province, China. The samples underwent crushing and fine grinding using a ceramic ball mill. Subsequently, particle sizes ranging from −74 μm to +38 μm were dry-screened for micro-flotation experiments. Figure 1 illustrates the X-ray diffraction (XRD) spectra of chalcopyrite and galena, while

Table 2. Multi-element assay of the galena and chalcopyrite samples.

Sample	P	Zn	Pb	SiO2	Al2O3	CaO	MgO	Cu	Tfe	S
Chalcopyrite	0.13	0.075	0.016	<0.01	0.040	0.11	0.078	33.95	30.84	34.54
Galena	0.0021	<0.01	84.99	<0.01	0.047	0.032	0.013	0.031	0.067	12.11

presents the results of multi-element analysis for both minerals. Based on XRD and multi-element analysis, it is evident that the galena and chalcopyrite mineral samples exhibit exceptional purity levels, with galena purity reaching 98.14% and chalcopyrite purity at 98.23%. Therefore, the galena sample boasts a purity level of 98.14%, whereas the chalcopyrite sample demonstrates a purity level of 98.23%.

The Z200 utilized in the flotation experiment as a collector is sourced from BGRIMM Chemical Technology Co., Ltd., Cangzhou, China. The frother (MIBC) was procured through McLean Biochemical Technology Co., Ltd., Shanghai, China. Sodium hydroxide (NaOH) and hydrochloric acid (HCl), employed as pH adjusters, were acquired from Sinopharm Chemical reagent Co., Ltd., Shanghai, China.

The synthetic route of TA is shown in Figure 2. The required product is obtained by adding chloroacetic acid and NaOH to thiourea and heating the reaction for 5 h. The NMR figure of TA is shown in Figure 3. 13C-NMR (150 MHz, Deuterium Oxide): δC 201.6, 179.5, 178.6, 58.5, 39.2. 1H-NMR (600 MHz, Deuterium Oxide): 4.70 (d, J = 1.4 Hz, 1H), 3.89 (s, 1H), 3.19 (s, 1H).

2.2. Methods

2.2.1. Flotation Experiments

Pure mineral micro-flotation experiments were performed using a 30 mL XFGII flotation cell, operating at 1758 rpm without the need for an external air supply. Before each pure mineral flotation test, 2.0 g of pure mineral sample was placed in a beaker containing 30 mL of deionized water and ultrasonically cleaned for 5 min to remove oxides from the mineral surface. After 10 min of settling, the suspension was decanted, and the bottom pure minerals were transferred to a flotation cell. Then, the required flotation reagents, such as NaOH, HCl, TA, Z-200, and MIBC, were introduced into the slurry one by one during the stirring process. The slurry was stirred for 2 min after each addition of reagent. After the introduction of all reagents, the flotation process proceeded for 4 min. Finally, the foam product and residue were filtered, dried, and weighed. Each experiment was repeated three times to obtain the average yield. The flotation process diagram is presented in Figure 4.

All micro-flotation experiments were performed in triplicate (n = 3), and results are expressed as mean ± standard deviation (SD). Statistical analyses, including one-way analysis of variance (ANOVA), were conducted manually using critical values from standard statistical tables to ensure rigor and reproducibility. The significance level was set at α = 0.05.

2.2.2. FTIR Measurements

The adsorption behavior of TA on galena surfaces was investigated through Fourier transform infrared spectroscopy (FTIR) by analyzing TA, pristine minerals, and TA-modified specimens. The experimental workflow consisted of sequential collector preconditioning, depressant treatment, and spectroscopic characterization. For preconditioning, precisely measured 2.0 g aliquots of galena were introduced into 50 mL glass reactors containing 30 mL of 10 mg/L TA solution. Homogenization was achieved via mechanical agitation (300 rpm, 5 min) to facilitate complete collector adsorption, followed by vacuum filtration and thermal stabilization at 50 °C until mass equilibrium. Post-reaction solids were isolated through Buchner filtration and vacuum desiccated to constant mass, yielding TA-adsorbed mineral surfaces. Infrared spectral acquisition involved pellet preparation by homogenizing reference materials (TA) and mineral samples (pristine/treated) with anhydrous KBr (1:100 mass ratio) under 10 MPa compaction. Spectral data spanning 500–4000 cm⁻¹ were collected using a Nicolet IS10 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) configured with 32 cumulative scans at 4 cm⁻¹ spectral resolution.

2.2.3. XPS Measurements

Surface chemical evolution during TA adsorption was systematically probed through X-ray photoelectron spectroscopy (XPS) by analyzing pristine galena and TA-modified galena. Surface-modified specimens were prepared following identical pretreatment protocols as those employed for FTIR sample fabrication. XPS characterization was executed using a Thermo Scientific K-Alpha spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV). Prior to analysis, the ultrahigh vacuum chamber was evacuated to a base pressure of 1 × 10⁻⁹ Torr to minimize surface contamination. Wide-scan spectra spanning 0–1200 eV binding energy were acquired with a pass energy of 150 eV and step size of 1.0 eV. To ensure statistical reliability, triplicate measurements were performed for each specimen under identical analytical conditions. Charge compensation was achieved using a dual-beam electron flood gun, and energy scale calibration was rigorously implemented through reference to adventitious carbon (C 1s at 284.8 eV) [26,27,28].

2.2.4. Computation Approach

To elucidate the competitive adsorption mechanisms between collector Z200 and depressant TA on galena surfaces, density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) to investigate adsorption configurations and electron transfer behaviors [28,29]. The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional was employed to simulate electron density gradients and interfacial interactions [30], ensuring quantum-mechanically rigorous modeling of electronic properties. Computational parameters were optimized for precision: a 450 eV energy cutoff balanced computational efficiency and accuracy, while a 4 × 2 × 1 Monkhorst–Pack k-point grid ensured adequate Brillouin zone sampling. Convergence criteria were strictly enforced (energy <1 × 10⁻⁵ eV, forces <0.02 eV/Å) to guarantee reliable electronic structure solutions. The configurations of valence electrons of Pb, C, N, S, H, and O were performed as 6s²6p², 2s²2p², 2s²2p³, 3s²3p⁴, 1s¹, and 2s²2p⁴, respectively. The Projector Augmented Wave (PAW) method accurately modeled core–valence electron interactions, complemented by the DFT-D3 dispersion correction for long-range van der Waals forces [31,32,33,34]. Structural visualizations and electronic analyses were conducted using the VESTA 3.9 software package [35].

The galena crystal structure, sourced from established crystallographic databases [36,37], was geometrically optimized and cleaved along the (100) surface—the predominant cleavage plane under mechanical stress [38,39]. A four-layer slab model with a 20 Å vacuum buffer was constructed to minimize periodic boundary artifacts. Final DFT simulations utilized a (4 × 2 × 1) supercell configuration.

The adsorption capacity of flotation reagents on the mineral surface is evaluated based on their adsorption energy, which can be determined using Equation (1).

$$E_{ads}=E_{adsorbates/surface}-E_{surface}-E_{adsorbates}$$

(1)

Here, E_ads is the adsorption energy, E_{adsorbates/surface} is the computed energy of the flotation reagents adsorbed on the galena/chalcopyrite surface, E_surface is the energy of the galena/chalcopyrite surface, and E_adsorbates is the energy of the flotation reagents.

3. Results

3.1. Micro-Flotation Experiments

Figure 5a illustrates the influence of TA dosage (0–60 mg/L) on the flotation recovery of chalcopyrite and galena at pH 10. As evidenced by the data, chalcopyrite recovery remains consistently above 92.3% (SD = 1.5%, 95% CI: 90.8–93.8%) across all tested TA concentrations, demonstrating negligible sensitivity to TA presence. In contrast, galena exhibits a stark dependency: its recovery decreases progressively with increasing TA concentration. In the absence of TA, galena recovery reaches 91.88% (SD = 1.2%, 95% CI: 90.5–93.3%), comparable to chalcopyrite. At the critical TA concentration of 60 mg/L, galena recovery plummets to 7.5% (SD = 0.8%, 95% CI: 6.9–8.1%; one-way ANOVA, p < 0.001), confirming TA’s potent selective depression effect on galena while preserving chalcopyrite floatability.

Figure 5b demonstrates the pH-dependent flotation behavior (pH 2–13) under optimized TA concentration (60 mg/L). Chalcopyrite maintains exceptional recovery stability (>93.4%, SD = 1.2%, 95% CI: 92.2–94.6%) across the entire pH spectrum, unaffected by TA addition. Conversely, galena recovery remains suppressed to approximately 8.5% (SD = 0.9%, 95% CI: 7.6–9.4%; ANOVA, p < 0.001) regardless of pH variations. This pH-independent suppression mechanism establishes a robust operational window for selective separation, with the maximum differential in recovery between chalcopyrite and galena reaching 80.2% (SD = 1.7%, 95% CI: 78.5–82.0%). TA maintains stable suppression across pH 2–13 (Figure 5b); however, this behavior was validated under controlled laboratory conditions. Real-world applications may require additional optimization for ore matrices containing magnesian gangue minerals, which could alter solution chemistry and reagent efficacy.

3.2. Artificially Mixed Minerals Experiments

To further investigate the selective depression capability of TA on galena during flotation, artificial mixed-mineral flotation separation tests of chalcopyrite and galena were conducted in this study. The experimental results are presented in Figure 6. As shown in Figure 6a, when the TA concentration in the pulp increased from 10 mg/L to 50 mg/L, the flotation recovery of galena in the concentrate significantly decreased from 82.92% (SD = 2.1%, 95% CI: 80.8–85.0%) to 12.29% (SD = 1.5%, 95% CI: 10.8–13.8%; one-way ANOVA, p < 0.001), accompanied by a reduction in the galena grade from 51.27% (SD = 1.8%, 95% CI: 49.5–53.1%) to 13.27% (SD = 1.2%, 95% CI: 12.1–14.5%, one-way ANOVA, p < 0.001). Notably, further increases in TA concentration beyond 50 mg/L had minimal impact on the grade and recovery of galena in the concentrate. Conversely, the recovery of copper remained relatively stable at approximately 83.2% (SD = 1.8%, 95% CI: 81.4–85.0%), demonstrating negligible sensitivity to TA concentration.

Figure 6b illustrates the influence of pulp pH (2.8–11.4) on the flotation performance of chalcopyrite and galena at a fixed TA concentration of 50 mg/L. As the pH increased from 2.8 to 11.4, the recovery of galena in the concentrate decreased from 29.65% (SD = 2.3%, 95% CI: 27.4–31.9%) to 13.78% (SD = 1.6%, 95% CI: 12.2–15.4%; ANOVA, p < 0.001), while its grade declined from 40.32% (SD = 2.0%, 95% CI: 38.3–42.3%) to 20.03% (SD = 1.4%, 95% CI: 18.6–21.5%). In contrast, the recovery of copper initially increased from 36.48% (SD = 2.5%, 95% CI: 34.0–39.0%) to 91.21% (SD = 1.7%, 95% CI: 89.5–92.9%, ANOVA, p < 0.001) as the pH rose from 2.8 to 7.9, followed by a slight reduction to 83.28% (SD = 1.9%, 95% CI: 81.4–85.2%) at pH 11.4. These results confirm that TA exhibits a pronounced selective depression effect on galena flotation across a wide pH range.

3.3. FTIR Experiments

This study analyzed the adsorption behavior of TA on the surface of galena. FTIR was used to examine galena particles treated with these agents. Figure 7 shows the FTIR spectra of TA, galena, and TA-treated galena to analyze the impact of TA on galena. Figure 6 reveals that TA exhibits characteristic FTIR absorption peaks at 1652.91 cm⁻¹ (C=O stretching vibration), 1574.67 cm⁻¹ (N-H bending vibration), and 1396.54 cm⁻¹, 1074.11 cm⁻¹, 1228.02 cm⁻¹ corresponding to C=S stretching vibration, -OH stretching vibration, and C-O stretching vibration, respectively. Comparing the infrared spectra of galena and TA-treated galena, the TA-treated galena shows modified absorption peaks at 1633.11 cm⁻¹ (C=O stretching vibration) and 1361.76 cm⁻¹ (C=S stretching vibration). The observed bathochromic shifts in characteristic vibrational peaks (Δν: C=O = −19.8 cm⁻¹, C=S = −34.78 cm⁻¹) relative to pure TA spectra demonstrate that TA predominantly chemisorbs onto galena surfaces through covalent interactions involving its C=S and C=O functional groups [40,41,42,43]. This chemical anchoring mechanism, evidenced by the formation of Pb-S and Pb-O coordination bonds, fundamentally differs from physical adsorption processes and confirms the specific electronic interactions between TA’s electron-donating moieties and surface lead atoms.

3.4. XPS Experiments

The analytical results presented in Figure 8 show the XPS measurements of galena surfaces before and after TA treatment. As clearly demonstrated in the figure, the nitrogen content on the galena surface increased significantly from 0.66% to 5.47% following TA treatment, which indicates substantial adsorption of TA molecules onto the mineral surface after the modification process.

To further clarify the electron transfer during the adsorption process of TA on the surface of galena, the characteristics of S 2p, C 1s, Pb 4f, and O 1s were analyzed in detail through high-resolution XPS spectroscopy.

Figure 9a shows the high-resolution spectra of the S element on the surface of galena before and after treatment with TA. It can be seen from the figure that before the treatment with TA, the S element on the surface of galena mainly has two peaks at 160.70 and 161.90 eV [41], which are the doublet peaks from S 2p3/2. The doublet peaks at 168.10 and 167.40 eV are from the sulfates formed by the oxidation of the S element. After the treatment with TA, the two peaks at 160.70 and 161.90 eV on the surface of galena remain unchanged, indicating that the S on the surface of galena does not participate in the reaction during the adsorption of TA. After TA is adsorbed on the surface of galena, a new peak at 163.70 eV appears in the high-resolution spectrum of the S element on the surface, which is from the S in TA. In Figure 9b, after the treatment with TA, a new absorption peak of the C element from TA appears at 287.50 eV, indicating that TA is adsorbed on the surface of galena after the treatment.

Figure 9c reflects the high-resolution spectra of Pb 4f on the surface of galena before and after treatment with TA. It can be seen from the figure that before the treatment with TA, the high-resolution spectrum of Pb 4f mainly has characteristic peaks at 137.48 and 142.38 eV. After the treatment with TA, the characteristic peaks change to 137.70 and 142.52 eV. The decrease in the binding energy indicates that during the adsorption of TA on the surface of galena, there is an electron transfer from TA to the Pb on the surface of galena [42].

Figure 9d shows the high-resolution spectra of O 1s on the surface of galena before and after treatment with TA. It can be seen from the figure that compared with the surface of galena before the treatment with TA, the surface of galena after the treatment with TA has an additional characteristic peak at 531.98 eV for O 1s, which is from the -COOH group in TA, indicating that TA is adsorbed on the surface of galena.

3.5. Adsorption Mechanisms from DFT Calculation

Figure 10a depicts the adsorption configuration of the collector Z200 on the surface of galena. It can be observed from the figure that Z200 adsorbs onto the surface of galena through the S atom in its molecule. The bond length between the S atom and the Pb atom is 3.11 Å, which is less than the sum of the atomic radii of Pb and S (4.10 Å). This indicates that the adsorption of Z200 on the surface of galena is a chemical adsorption. Figure 10b shows the differential charge density map of the adsorption of the collector Z200 on the surface of galena. These maps display electron depletion (blue) and accumulation (yellow). Notably, electron depletion can be clearly observed around the S atom in Z200, while electron accumulation occurs around the Pb atom on the surface of galena. This suggests that the electrons in the flotation reagent are transferred to the surface of galena.

To elucidate the electronic interactions between the flotation reagent and lead species on galena surfaces, the projected density of states (PDOS) was employed to characterize sulfur (Z200) and surface-bound lead (galena). Figure 10c demonstrates distinct orbital distributions: the S 3s orbital predominantly occupies the low-energy regime (<−5 eV), whereas the S 3p orbital spans the higher-energy range (−5 to 0 eV). In contrast, Pb 6s orbitals localize within −10 to −5 eV, while Pb 6p orbitals overlap with the S 3p orbitals in the −5 to 0 eV region. This spatial-energy alignment suggests complementary hybridization between S 3p and Pb 6p orbitals, while Pb 6s orbitals remain energetically isolated.

To understand how Z200 binds to galena, we analyzed the electronic interactions using a method called COHP [43,44]. This revealed that the strongest bond forms between sulfur’s 3p orbital and lead’s 6p orbital, contributing 0.52 eV to the total bond strength of 1.00 eV. A weaker interaction (0.43 eV) occurs between sulfur’s 3s and lead’s 6p orbitals, while lead’s 6s orbitals play almost no role. Essentially, Z200’s sulfur atom “shares” electrons primarily with lead’s 6p orbitals, creating a stable bond that anchors it to the mineral surface; however, this single-bond mechanism limits its overall effectiveness [45,46].

The strength of these bonds directly impacts Z200’s ability to suppress galena during flotation. While its 1.00 eV bond is strong enough to reduce galena recovery, TA outperforms Z200 by forming two bonds (S–Pb and O–Pb) with a combined strength of 1.76 eV. This dual-bond strategy not only enhances adsorption stability but also explains TA’s superior selectivity and pH resilience, offering a blueprint for designing next-generation depressants that prioritize multi-site bonding for industrial applications.

Figure 11a and Figure 12a present the adsorption configurations of TA on the surface of galena. It can be seen from the figures that the S atom in the TA molecule forms an S2-Pb2 bond with the Pb atom on the surface of galena, and the O atom in the TA molecule forms an O1-Pb3 bond with the Pb atom on the surface of galena. Similar to the collector, the bond length of the S2-Pb2 bond is 3.11 Å, indicating that a chemical bond is also formed between S2 and Pb2. The bond length between O1 and Pb3 is 2.84 Å, which is less than the sum of the atomic radii of O and Pb (3.54 Å), suggesting that a chemical bond is formed between the oxygen atom and the lead atom as well.

Figure 11b and Figure 12b show the differential charge density maps of TA adsorbed on the surface of galena. It can be clearly observed that electron depletion occurs around the S2 and O1 atoms in the TA molecule, and electron accumulation is found around the Pb2 and Pb3 atoms on the surface of galena. This indicates that when TA is adsorbed on the surface of galena, electrons are transferred from TA to the surface of galena. This electron transfer is consistent with the results of XPS.

The projected density of states (PDOS) of S in TA and Pb on the surface of galena was analyzed. As shown in Figure 11c, the S 3s orbital is mainly located in the low-energy region below −5 eV, while the S 3p orbital has two sharp peaks in the range from −7.5 eV to 0 eV. In contrast, the Pb 6s orbital is mainly located in the low-energy spectrum from −10 eV to −5 eV, and the Pb 6p orbital is also in the high-energy region from −5 eV to 0 eV. This indicates that the S 3p and Pb 6p orbitals have similar high-energy distributions, while the Pb 6s orbital is significantly concentrated in the low-energy region.

The projected density of states (PDOS) of O in TA and Pb on the surface of galena was analyzed. As can be seen from Figure 12c, the O 2s and O 2p orbitals are mainly located in the low-energy region below −5 eV. The Pb 6s orbital is mainly located in the low-energy spectrum from −10 eV to −5 eV, and the Pb 6p orbital is in the high-energy region from −5 eV to 0 eV.

The S2-Pb2 bond was analyzed by COHP. The -ICOHP value of the S2-Pb2 bond formed by the adsorption of TA on the surface of galena is 1.08 eV, which is similar to that of the S1-Pb1 bond formed by the adsorption of the collector on the mineral surface. It can be seen from Figure 11d–h that the interaction strength between the S2 and Pb2 orbitals is similar to that between the S1 and Pb1 orbitals. The interactions between the S 3s and Pb 6s orbitals and between the S 3p and Pb 6s orbitals contribute little to the bonding, while the interactions between the S 3s and Pb 6p orbitals and between the S 3p and Pb 6p orbitals contribute significantly to the bonding, with values of 0.46 eV and 0.58 eV, respectively.

The O1-Pb3 bond was analyzed by COHP. The -ICOHP value of the S2-Pb2 bond formed by the adsorption of TA on the surface of galena is 0.68 eV, which is weaker than the S2-Pb2 bond. As shown in Figure 12e–h, the interaction between the O 2s and Pb 6s orbitals in TA shows a balance of bonding and anti-bonding characteristics in the range from −10 to −5 eV, indicating a minor impact on the overall adsorption process. The interaction between the O 2s and Pb 6p orbitals is mainly bonding below −5 eV, with a small anti-bonding effect between −5 and 0 eV. At the same time, the interaction between the S 3p and Pb 6s orbitals has both bonding and anti-bonding effects in the low-energy region from −10 to −5 eV. The interaction between the S 3p and Pb 6p orbitals has a relatively strong bonding effect in the range from −10 to 0 eV.

Figure 12d shows the contributions of various orbital interactions to the O1-Pb3 bond. The bond between O 2p and Pb 6p exhibits a strong negative -ICOHP value of 0.37 eV, making the greatest contribution to the bonding. The interaction between S 3s and Pb 6p comes next, with a -ICOHP value of 0.32 eV. The -ICOHP values of the S 3s-Pb 6s and S 3p-Pb 6s bonds are −0.01 eV and 0.02 eV, respectively, contributing very little to the bonding and thus can be neglected.

Based on the above analysis, it can be concluded that Z200 adsorption on the galena surface forms only a single S1–Pb1 bond with an adsorption energy of 1.00 eV. In contrast, TA adsorption on galena results in the formation of dual bonds (S2–Pb2 and O1–Pb3), yielding a combined adsorption energy of 1.76 eV, which is markedly higher than that of Z200. This enhanced interfacial binding energy demonstrates that TA exhibits stronger adsorption affinity on galena surfaces compared to Z200. It can be seen that the strength of the S1-Pb1 bond formed by the adsorption of Z200 on the surface of galena is approximately similar to that of the S2-Pb2 bond formed by the adsorption of TA on the surface of galena. The COHP analysis shows that the interactions between S 3s, S 3p, and Pb 6p contribute the most to the S1-Pb1 and S2-Pb2 bonds. In addition, after TA is adsorbed on the surface of galena, in addition to the S2-Pb2 bond, an O1-Pb3 bond can also be formed. Therefore, the adsorption energy of TA on the surface of galena is greater than that of Z200. Consequently, when Z200 is used as a collector, TA can be used as a depressant to suppress the flotation of galena.

4. Conclusions

In order to improve the utilization rate of copper resources and reduce the lead content in copper concentrates, a novel galena depressant, TA, was proposed in this study. While this study demonstrates effective separation under idealized conditions with pure minerals, further work is required to extend these findings to partially liberated minerals from the same host rock, which represents a more industrially relevant challenge. Micro-flotation tests, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and quantum chemical calculations were employed to analyze the depressant performance of TA on galena.

The results of micro-flotation tests indicated that TA significantly inhibited the flotation of galena within the pH range of 8–12 while having almost no impact on the flotation of chalcopyrite. Further FTIR analysis and molecular simulations confirmed that during the adsorption process of TA on the surface of galena, there was an electron transfer from the S and O atoms in TA to the Pb atoms on the surface of galena. This was also verified by XPS.

The COHP analysis revealed that the interactions between the 3s and 3p orbitals of the S atom in the flotation reagents (Z200 and TA) and the 6p orbital of the Pb atom on the surface of galena made significant contributions to the formation of the S-Pb bond. Additionally, when TA adsorbed on the surface of galena, it could form not only S-Pb bonds but also O-Pb bonds. This was the main reason why the adsorption ability of TA on the surface of galena was stronger than that of Z200.

While this orbital engineering framework establishes a theoretical foundation for depressant design, two critical challenges remain: (1) scale-up of reagent synthesis to meet industrial throughput requirements, and (2) water recycling compatibility in flotation circuits, where chemical interactions with recycled process water may alter reagent efficacy. Future investigations integrating pilot-scale trials and techno-economic assessments are essential to address these gaps and facilitate practical implementation.