Comparative Analysis of Lead Ions and Ammonium Salts in Malachite Sulfurization-Assisted Flotation Based on Surface Layer Durability

Abstract

Sulfurization-assisted flotation is a key process that uses sulfur compounds to modify mineral surfaces, enhancing hydrophobicity and flotation efficiency, especially for copper oxide minerals. This study introduced the preliminary activation of malachite utilizing a combination of Pb²⁺ and NH₄⁺ ions in sulfurization systems, significantly improving flotation recovery. Flotation tests and surface analysis techniques were employed to examine the effects of Pb²⁺ and NH₄⁺ ions on malachite’s flotation behavior and the stability of its sulfurized surface layer. The results showed that, after activation with Pb²⁺ and NH₄⁺ at optimal reagent concentrations, malachite’s flotation recovery reached 94.6%, compared to 68.13% with traditional sulfurization. Atomic force microscopy (AFM) revealed significant changes in malachite’s surface morphology, with a dense, cloud-like sulfide film forming that contained more sulfur than in direct sulfurization, enhancing the durability of the sulfurized surface. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis confirmed increased sulfide ion adsorption on the surface compared to traditional sulfurization. The Pb²⁺ + (NH₄)₂S + Na₂S system generated numerous active sites from copper-sulfide species, promoting the growth of sulfurized phases. FT-IR analysis showed stable Cu-S species on the malachite surface, improving SBX adsorption and flotation performance. Contact angle measurements indicated that the activation systems significantly improved surface hydrophobicity, with the copper-sulfide film achieving a contact angle of 95.29°, demonstrating superior durability and mineral recovery compared to traditional sulfurization. Thus, the activation of Pb²⁺ and NH₄⁺ ions offers a promising solution for sulfurization-assisted flotation, enabling more efficient and sustainable recovery of malachite ore, with improved sulfide layer durability and enhanced hydrophobicity.

1. Introduction

Global copper demand has surged in recent years, driven by the growth of industrial revolutions in electronics, construction, and renewable energy [1]. Copper is extensively used in modern technologies, including electrical wiring, solar panels, electric vehicles, and other systems integral to urban infrastructure. However, declining ore grades and the depletion of natural deposits are raising concerns about the long-term sustainability of the demand–supply balance [2,3]. Currently, the primary raw material for copper smelting is copper-sulfide ore, mined and processed to extract copper, but, as industrialization increases worldwide, copper demand is outpacing supply [4,5]. This challenge has driven a focus on sustainable solutions such as enhanced recycling and alternative sources such as recycled scrap, mine tailings, low-grade ores, and industrial by-products, as well as advanced smelting technologies like oxygen-enriched and flash smelting, which help reduce greenhouse gas emissions and toxic by-products. Copper can be efficiently extracted from various copper oxide minerals, including azurite, chrysocolla, cuprite, tenorite, and malachite, found in copper deposits’ oxidation zones [6,7].

Malachite (Cu₂CO₃(OH)₂) is a common copper oxide mineral characterized by its naturally hydrophilic surface. It does not effectively respond to traditional copper sulfide flotation collectors like xanthate [8]. Easily recognized by its vibrant green color, malachite can occur in various forms, including botryoidal, fibrous, or stalactitic masses. While it serves as a secondary copper ore and contributes to copper extraction, its economic significance is generally lower than that of primary copper sulfide ores. Additionally, malachite has been historically valued as both a pigment and a gemstone, further enhancing its cultural and industrial importance. The mineral’s chemical properties, including its flotation behavior and solubility, are crucial factors in mineral processing and copper recovery methods.

Malachite surface sulfurization involves reacting with sulfur or sulfur-containing compounds to form a copper sulfide (Cu_xS) phase layer, which increases hydrophobicity and improves flotation [9,10]. The stability and durability of this sulfurized layer are essential for ensuring the long-term success of sulfurization-assisted flotation, as factors like pH and temperature influence the sulfurization process. The hydrolyzed species of Na₂S in solution—H₂S, HS⁻, and S₂⁻—vary depending on the pH range [11]. In weak alkaline solutions (pH 9–10), malachite efficiently sulfurizes with HS⁻, the predominant hydrolyzed form of Na₂S [12]. The sulfurization typically occurs under controlled conditions of temperature, pressure, and sulfur source, which affect the composition, morphology, and crystallinity of the resulting copper sulfide. These copper sulfide phases can have different stoichiometries, such as Cu₂S or CuS, each with unique properties [13]. Gaining a deeper understanding of the mechanisms behind malachite surface sulfurization, including the diffusion of sulfur species and the formation of copper sulfide, is essential for optimizing the process for specific uses. This process offers valuable insights into malachite’s transformation and paves the way for designing advanced materials with tailored properties for technological applications. Numerous researchers have conducted in-depth studies on the sulfurization treatment of malachite flotation. Liu et al. [14] examined the sulfurization products djurleite (Cu₃₁S₁₆) and anilite (Cu₇S₄), both members of the chalcocite group. They suggested that Cu₂(OH)₂CO₃ on malachite surfaces transforms into Cu_2−xS during sulfurization. In this process, sulfide ions serve as both a reductant and a sulfidizing agent, promoting the heterogeneous nucleation and growth of Cu_2−xS phases.

Ammonium-based salts in particular have been explored as potent catalysts of copper oxidation and are widely used in the industrial processing of malachite [15]. Researchers have not studied the combination of lead ions and ammonium salts (e.g., ammonium chloride, ammonium nitrate, and ammonium sulfide) in sulfurization-assisted flotation to enhance the flotation performance of various minerals, including malachite. While the potential synergy between lead ions and ammonium salts in sulfurization processes has not been thoroughly investigated, this approach could provide valuable insights into enhancing the flotation of copper-bearing minerals. After preliminary treatment, Liu et al. [16] also discovered that ammonium sulfate promotes the sulfurization process. They found that using the correct amounts of ammonium sulfate and sodium sulfide reduces the excess sulfide ions that inhibit the reaction with copper oxide minerals. Liu et al. [17] found that treating malachite surfaces with ammonium sulfate [(NH₄)₂SO₄] activated the formation of Cu(II)-NH₃ complexes between NH₄⁺ and Cu²⁺. These complexes then interacted with S(II) species to produce copper sulfide, which was more readily adsorbed onto the malachite surface compared to the copper sulfide colloid formed by the direct reaction between Cu²⁺ and S(II). In addition, Han et al. [18] studied the effect of Pb(II) on sodium-sulfide-sulfurized malachite through micro-flotation. They demonstrated that lead ions enhance the collector’s adsorption on the malachite surface. They also found that Pb(II) alters the chemical environment of the surface, leading to the formation of Pb-S and S species (S₂⁻ and S_n²⁻), which improve sulfurization. Muanda et al. [19] examined the impact of lead nitrate conditioning, combined with sodium silicate (Na₂SiO₃), on the flotation of copper–cobalt oxide ores to enhance the recovery of valuable metals. They considered four factors: the amount of Pb(NO₃)₂, its conditioning process, its addition to the fraction, and the sulfurized dosage. The study concluded that the use of lead ions can significantly improve the recovery of copper and cobalt.

Although sodium sulfide is commonly used as a sulfurizing reagent to improve malachite floatability, flotation recovery is higher when lead ions and ammonium salts are combined. However, the synergistic effects of Pb²⁺ and NH₄⁺ ions on malachite sulfurization flotation have not been extensively studied. This study examines the durability of the sulfurized layer and the floatability of malachite treated with a preliminary combination of Pb²⁺ and NH₄⁺ ions to prevent degradation during sulfurization. The influence of these reagents on flotation behavior and surface morphology was investigated using micro-flotation tests, atomic force microscopy (AFM), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), field-emission scanning electron microscopy–energy dispersive X-ray spectroscopy (FESEM–EDS), Fourier-transform infrared (FT-IR) spectroscopy, and contact angle measurements. The study aims to assess the changes in the durability of the sulfurized surface layer on the malachite surface through flotation experiments, thereby promoting sulfide layer development and enhancing its stability.

2. Experimental

2.1. Materials and Reagents

The malachite sample used in this study was sourced from the Dongchuan region in Yunnan, China, and was separately crushed and ground. Dry-sieved samples were used to collect target size fractions ranging from −75 to +38 μm which were then used in experiments such as micro-flotation tests and FESEM-EDS, XPS, AFM, and ToF–SIMS analyses. X-ray fluorescence (XRF) spectroscopy was employed to determine the elemental composition of the malachite samples, and the results are presented in

Table 1. XRF analysis results of Malachite composition by element.

Elements	Cu	Fe	Mn	Al2O3	SiO2	CaO	MgO	Others
Wt. (%)	55.78	0.12	0.40	0.53	2.18	0.69	0.34	39.96

. The primary elements and compounds identified were Cu, Fe, Mn, Al₂O₃, SiO₂ CaO, and MgO. Quantitative analysis revealed that the sample contained 55.78 wt.% Cu, confirming its purity. The Lead nitrate (Pb(NO₃)₂), ammonium nitrate (NH₄NO₃), ammonium chloride (NH₄Cl), ammonium sulfide ((NH₄)₂S), hydrochloric acid (HCl), sodium hydroxide (NaOH), xanthate, and sodium sulfide (Na₂S) were of analytical grade and purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). All solutions used in this study were prepared using ultrapure water.

2.2. Flotation Experiments

The micro-flotation experiments were conducted using a 40 mL XFGII flotation cell with an agitation speed of 1200 r/min to assess the separation efficiency of the malachite sample, as shown in Figure 1. Each test involved adding 2 g of malachite to the flotation cell, then adding 38 mL of deionized water (18.25 mΩ·cm) and adjusting the pulp pH to 9 ± 0.05 using NaOH or HCl.

The sample was then conditioned with the appropriate reagents, including sulfurization-assisted flotation reagents (3 min), sulfurizing agents (5 min), xanthate collectors (2 min), and MIBC frothers (2 min). These reagents were selected based on their ability to bind the malachite particles selectively. The flotation process was then conducted for 4 min. Finally, the foam was collected manually. After drying and weighing, the flotation recovery and grade were calculated based on the mass of the collected froth and tailings.

2.3. AFM Measurements

The AFM (Bruker, Billerica, MA, USA) instrument was employed to examine the durability and stability of the sulfurized surface layer that could form on the malachite surface. Before testing, the following AFM preparation steps were performed: sample preparation, instrument setup, probe selection and calibration, and selection of the appropriate scanning mode. Sample preparation proceeded as follows: Clean cubic malachite samples were placed into a chemical laboratory glass beaker. Deionized water (18.25 mΩ·cm) was added throughout the tests, and the pulp pH was adjusted to around 9. Fresh concentrations of lead ions, ammonium salts, and sodium sulfide were prepared and added to each experiment as required and then conditioning was performed for 10 min. Ultimately, after the test’s completion, the samples were washed and dried before the AFM measurements.

2.4. ToF-SIMS Measurements

ToF-SIMS measurements were carried out using an ION-TOF IV instrument manufactured by ION-TOF GmbH (Münster, Germany). A pulsed Bi₃⁺ primary ion beam with an energy of 30 keV was used for surface analysis. Additionally, pure, solid malachite samples were manually selected, cut into cubes, polished, and prepared for characterization and testing. For the test, the malachite samples were reacted with reagents (e.g., sodium sulfide, Pb²⁺, and NH₄⁺ ions, as required in the test) at a pH of around 9. Furthermore, the primary ions were scanned over a selected sample area, and the emitted secondary ions were detected in both positive and negative modes. The analysis was performed under ultra-high vacuum conditions. The sample test area was 500 × 500 µm for each sample. Ion images were constructed by correlating the intensity of selected ions with the spatial distribution on the sample surface. Finally, 2D distributions of elements were analyzed using Rv 5.0 software.

2.5. FESEM-EDS Measurements

The FESEM-EDS tests were employed to examine the preliminary treatment of malachite, specifically focusing on the combined effects of Pb²⁺ and NH₄⁺ ions during sulfurization flotation. This technique provided a detailed evaluation of the durability of the sulfurized surface layer. For sample preparation and the test methodology, we followed the same procedure as in the micro-flotation tests previously conducted. Before detection, all samples were coated with carbon and adhered to copper tape to enhance the conductivity of the malachite surface. The instrument was set up with an acceleration voltage of 15 kV for detection and operated at a 20 kV accelerating voltage with a 60 mA probe current. The FESEM (Nova Nano SEM 450, Hillsboro, OR, USA) tests were then applied to the prepared samples, followed by EDS (Genesis 2000, Mahwah, NJ, USA) scanning to analyze the elemental composition.

2.6. FTIR Measurements

FT-IR spectra were recorded using a Nicolet iS50 (Thermo Nicolet Corporation, Waltham, MA, USA). For the preparation step, 2 g of malachite particles finer than 74 μm was weighed and placed into a volumetric flask for further experiments. Deionized water was added, and the mixture was stirred. Then, NaOH and HCl solutions were applied to adjust the pH to around 9. The reagents were freshly prepared for subsequent tests, including sodium sulfide, Pb²⁺, NH₄⁺ ions, and NaBX. After stirring for 10 min, the samples were filtered and then dried under a vacuum at around 40 °C. The spectrometer was used in the 4000–400 cm⁻¹ range with a resolution of 4 cm⁻¹. The background spectrum was recorded prior to each measurement and subtracted from the sample spectrum to remove atmospheric interference. The spectra were analyzed in both absorbance and transmittance modes, with characteristic peaks identified using reference spectra. Peak intensities and positions were used to evaluate chemical composition and molecular structure changes.

2.7. Contact Angle Measurements

Contact angles of the malachite surfaces were measured using a JY–82 analyzer (Shanghai JY Instrument Co., Ltd., Shanghai, China). For preparation, cubic malachite samples with smooth surfaces were cleaned with deionized water, dried, and placed on the contact angle stage. Each sample was treated under different flotation conditions (e.g., Pb²⁺, NH₄NO₃, NH₄Cl, (NH₄)₂S, and Na₂S) to reveal changes in hydrophobicity. A small droplet (2–3 µL) of deionized water was applied to the surface using a syringe, and the contact angle was recorded by measuring the droplet’s shape at the contact point with the solid surface. Each test was repeated at least three times to ensure measurement accuracy.

3. Results and Discussion

3.1. Effect of Xanthate and Lead Ions on Single-Flotation Recovery

Figure 2a shows the relationship between malachite flotation recovery and the concentrations of SBX in various sulfurization systems. At an SBX concentration of 0 mol/L, sodium-sulfide-treated malachite exhibited low recovery (3.9%). As SBX concentration increased to 25 × 10⁻⁴ mol/L, recovery rose to 68.13%, indicating enhanced malachite and sodium sulfide interaction. In the Pb²⁺ and Na₂S, recovery also increased with higher SBX concentrations, reaching 82.07%, which was attributed to improved surface hydrophobicity from metal sulfide layer formation. When the malachite surface was activated with lead ions and typical ammonium salts, recoveries reached 76.52%, 87.53%, and 93.7%, respectively, within the same SBX range, due to more effective adsorption of residual sulfur species. Beyond this range, the recovery showed only slight increases, suggesting diminishing returns from further increases in SBX concentration.

Figure 2b illustrates the relationship between malachite flotation recovery and Pb²⁺ concentration across various ammonium salts (NH₄NO₃, NH₄Cl, (NH₄)₂S) in sulfurization processes. In the Pb²⁺ + Na₂S system, recovery was initially low due to insufficient surface activation and limited formation of metal sulfide species. As the Pb²⁺ concentration increased, more Pb-S and Cu-S compounds formed on the malachite surface, enhancing its hydrophobicity and improving collector adsorption, which led to a gradual rise in recovery, peaking at 80.19% at 2 × 10⁻³ mol/L. However, beyond this concentration, recovery slightly declined to 76.4% at 2.5 × 10³ mol/L, likely due to over sulfurization. Excess Pb²⁺ can form a dense or overly thick sulfurized layer that inhibits collector interaction and reduces flotation efficiency. With the NH₄NO₃ and Na₂S system, the maximum recovery (92.4%) was also achieved at 2 × 10⁻³ mol/L Pb²⁺. At the same Pb²⁺ concentration, the NH₄Cl + Na₂S and (NH₄)₂S + Na₂S systems yielded recoveries of 89.45% and 94.6%, respectively, both outperforming the Pb²⁺ + Na₂S system alone. This suggests that the initial activation with Pb²⁺ and NH₄⁺ ions on the particle surfaces enhances mineral separation in sulfurization-assisted flotation, significantly improving mineral floatability. The improved performance is attributed to the forming of reactive sulfide species and stable Cu/Pb sulfide films on the malachite surface. However, excessive Pb²⁺ concentrations may lead to over-sulfurization, creating a thicker sulfurized layer and an overabundance of sulfur ions in the solution, which consumes the added collector and destabilizes the sulfur layer, resulting in poor sulfurization.

3.2. AFM Analysis

AFM images of the 2D and 3D surface morphology and height of malachite particles, both before and after treatment with Pb²⁺ and various NH₄⁺ ions through the sulfurization process, are shown in Figure 3a–e. AFM images and cross-sectional height measurements, acquired through NanoScope Analysis software v1.40r1, are often used to examine changes in mineral morphology following the addition of flotation reagents [20,21,22,23]. In the region with a scanning range of 5.0 μm × 5.0 μm, the cross-sectional height profile of malachite particles treated with sodium sulfide revealed the formation of regular flake-like layers on the surface, with an average roughness (Ra) of 53.2 nm, as seen in Figure 3a. The obtained morphology image effectively illustrates the intensity of the sulfurization reaction [24]. Furthermore, as shown in Figure 3b, introducing lead ions into the solution before sulfurization resulted in more prominent peaks and columnar protrusions, indicating the formation of Pb-S and Cu-S compounds. The surface roughness (Ra) increased by 57.9 nm compared to the sodium-sulfide-only treatment. This increase is attributed to the aggregation of protrusions driven by Pb²⁺ ion interactions. In the malachite sulfurization system containing Pb²⁺ + NH₄NO₃ shown in Figure 3c, the Ra of the modified malachite surface increased to 71.43 nm, with additional flake layers indicating greater copper sulfide formation than traditional sulfurization. In the malachite sulfurization system containing Pb²⁺ + NH₄Cl (Figure 3d), the Ra of the modified malachite surface further increased by 75.8 nm compared to the previous treatment. Numerous prominent peaks and columnar protrusions were observed on the surface, indicating the continued formation of copper sulfide products on the particle surface. In the sulfurization system containing Pb²⁺ + (NH₄)₂S (Figure 3e), the Ra of the modified malachite surface increased to 78.12 nm, with aggregated protrusions due to the chemical interactions of Pb²⁺ and ammonium sulfide ions. This combination created an optimal environment for enhanced Lead and copper sulfide formation, leading to more favorable sites for copper–sulfur compounds than direct sulfurization or other treatments. The increased copper sulfide formation in the Pb²⁺ + (NH₄)₂S system is due to the direct involvement of sulfide ions, whereas these ions are absent in the Pb²⁺ + NH₄NO₃ and Pb²⁺ + NH₄Cl systems, resulting in fewer copper–sulfur species on the surface.

3.3. ToF-SIMS Analysis

ToF-SIMS is a highly sensitive surface analysis technique that investigates and identifies the interactions between flotation reagents and malachite particle surfaces based on ion fragment characteristics [25,26]. To further explore the capture mechanism of the preliminary treatment with Pb²⁺ and various NH₄⁺ ions in the malachite sulfurization system, ToF-SIMS was performed to analyze the surface characteristics of malachite particles before and after treatment by detecting ionic fragment images (Cu⁺, S⁻, S₂⁻, and Pb⁺), as shown in Figure 4. In the malachite sulfurization system (Figure 4a), the Cu⁺ cation showed a copper distribution with a signal intensity of approximately 1.174 × 10⁵. At the same time, the anions (S⁻, S₂⁻) indicated sulfur distribution on the particle surfaces, with corresponding signal intensities of 1.125 × 10⁵ and 1.134 × 10⁵, respectively, suggesting effective sodium sulfide adsorption on the malachite surface. The small Pb⁺ intensity of about 1.859 × 10⁴ may have resulted from contaminants introduced during the treatment with sodium sulfide. Following Pb²⁺ and Na₂S treatment (Figure 4b), signal intensities rose to 4.268 × 10⁵ for Cu⁺, 3.621 × 10⁵ for S⁻, and 2.718 × 10⁴ for Pb⁺, while S₂⁻ slightly decreased to 1.051 × 10⁵. This suggests enhanced surface reactions driven by Pb²⁺, promoting the formation of metal–sulfur species such as Cu-S and Pb-S. When the malachite contained the Pb²⁺ + NH₄NO₃ system (Figure 4c), the Cu⁺ signal intensity increased to 5.009 × 10⁵, while the sulfur and lead anions (S⁻, S₂⁻, and Pb⁺) had intensities of 3.906 × 10⁵, 2.178 × 10⁵, and 3.942 × 10⁴, respectively. This indicates that the preliminary treatment with Pb²⁺ and NH₄NO₃ ions enhanced the interaction between the malachite surface and the sulfurizing agents, leading to more effective sulfide ion adsorption than direct sulfurization.

In the system containing Pb²⁺ + NH₄Cl (Figure 4d), the distribution of Cu⁺, Pb⁺, and sulfur anions (S⁻, S₂⁻) increased significantly, with copper and lead reaching approximately 7.127 × 10⁵ and 5.011 × 10⁴, and sulfur intensities of 4.284 × 10⁵ and 2.722 × 10⁵. This indicates an effective sulfurization process where copper from malachite is converted into copper sulfide, with sulfur species evolving during the reaction. The increase in Pb⁺ may be due to the greater formation of Pb-S species on the malachite surface, resulting in higher signal intensity. In the malachite sulfurization system containing Pb²⁺ + (NH₄)₂S (Figure 4e), the Cu⁺ and Pb⁺ signal intensities increased to 6.431 × 10⁵ and 7.363 × 10⁴, respectively, while sulfur anion distributions (S⁻, S₂⁻) were detected at 5.062 × 10⁵ and 3.187 × 10⁵, respectively, indicating effective Na₂S and collector adsorption. Compared to previous systems with Pb²⁺ and different ammonium salts, this system showed more favorable conditions for forming abundant sulfur species. The ToF-SIMS results also confirmed that the malachite surface in the Pb²⁺ + (NH₄)₂S + Na₂S system produced significant Pb and S components, with abundant Pb-S and Cu-S components providing active sites for xanthate adsorption.

The normalized intensities of copper and sulfur-containing ion fragments are shown in Figure 5. For malachite treated with Na₂S (Figure 5a), the normalized intensities of Cu⁺, S⁻, and S₂⁻ were 0.03287, 0.01897, and 0.02479, respectively, with no Pb⁺ detected on the mineral surface. After treatment with Pb²⁺ and Na₂S (Figure 5b), Pb⁺ intensity was 0.02168, confirming interaction between lead ions and the surface. Cu⁺, S⁻, and S²⁻ intensities were 0.02907, 0.02146, and 0.01981, respectively. With the Pb²⁺ + NH₄NO₃ + Na₂S system (Figure 5c), S⁻ and S₂⁻ intensities increased to 0.02634 and 0.03281, while Cu⁺ decreased to 0.02848. The normalized intensity of Pb⁺ was recorded at 0.0206. This suggests that sulfurization is enhanced by forming more stable Cu-S and Pb-S species, due to the catalytic role of Pb²⁺ and NH₄⁺ ions. In the Pb²⁺ + NH₄Cl + Na₂S system (Figure 5d), intensities of S⁻, S₂⁻, and Pb⁺ rose further to 0.03115, 0.03427, and 0.02451, respectively, while Cu⁺ declined to 0.02384 due to the excess sulfide ions in the sulfurization system. The higher intensities of S⁻, S₂⁻, and Pb⁺ are due to the sulfide reaction with Pb²⁺ and copper ions, leading to the formation of lead and copper sulfides. The decreased Cu⁺ intensity further supports greater conversion of Cu⁺ to CuS with sulfide. Following treatment with the Pb²⁺ + (NH₄)₂S + Na₂S system (Figure 5e), S⁻, S₂⁻, and Pb⁺ intensities increased to 0.03408, 0.03189, and 0.02785, respectively, and Cu⁺ further decreased to 0.03189. These results indicate that Cu-S and Pb-S species form more readily than in other systems. Compared to conventional sulfidation (only Na₂S treatment), without competing Pb²⁺ ions, sulfide ions preferentially react with Cu, resulting in higher Cu⁺ and S²⁻ intensities and lower free S⁻ levels. The increased presence of lead-containing components on the surface suggests improved durability of the sulfurized layer and better flotation recovery, which is consistent with the AFM results.

3.4. FESEM-EDS Analysis

FESEM-EDS was used to analyze the mineral’s surface morphology and elemental composition, providing high-resolution imaging and accurately identifying and quantifying the elements [27,28]. This technique helped us to investigate the sulfurization-assisted flotation behavior of the malachite. Additionally, the changes in the malachite surface morphology and chemical composition before and after preliminary treatment with flotation reagents in the malachite sulfurization system were investigated, as illustrated in Figure 6 and Figure 7 [29]. In the malachite sulfurization system, at 50,000× magnification (Figure 6a and Figure 7a), only a few flake-like particle products with uniform distributions were observed on the mineral surface. The elemental composition showed Cu (67.38%), O (31.57%), and S (0.5%), indicating that the flake-like product contains copper–sulfur compounds. Following Pb²⁺ and Na₂S treatment, cloud-like dense layers appeared on the surface, likely due to Pb-S compound formation. The surface contained 1.2% Pb, 0.8% S, 68.26% Cu, and 29.74% O. Compared to sulfurization alone, Pb²⁺ improved surface modification and enhanced hydrophobicity (Figure 6b and Figure 7b). When Pb²⁺ + NH₄NO₃ was added (Figure 6c and Figure 7c), numerous clustered or scaly-like products appeared, covering different parts of the malachite surface, with increased Cu (69.48%), Pb (1.68%), and S (1.12%) and decreased O (27.84%). This suggests that Pb²⁺ and NH₄NO₃ promoted copper–sulfur product formation and reduced oxygen-containing species due to sulfurization. With the Pb²⁺ + NH₄Cl system (Figure 6d and Figure 7d), more clustered and scaly products formed, covering the malachite surface, and elemental compositions were Cu (60.01%), Pb (1.35%), S (1.86%), and O (36.78%). These results suggest that Pb²⁺ and NH₄Cl enhanced the formation of sulfur compounds on the malachite surface, and the decrease in Cu content may be due to the conversion of copper into copper sulfide.

In the Pb²⁺ + (NH₄)₂S system (Figure 6e and Figure 7e), the malachite surface morphology changed markedly, with the products changing from clustered to dense cloud-like structures that almost completely covered the malachite surface. Furthermore, the dense cloud-like products were likely composed of Pb-S and Cu-S species. EDS surface scan results showed a significant increase in O (34.46%) and S (2.11%), especially compared to Figure 7a. The sulfur content was notably higher than in Figure 7a (1.61%), Figure 6b (0.99%), and Figure 6c (0.25%), suggesting that a significant number of Pb²⁺ + (NH₄)₂S components participated in the reaction, enhancing the sulfurization-assisted flotation process and improving recovery. Overall, this generated favorable conditions for the efficient flotation recovery of malachite, enabling the collector to improve the flotation process. In contrast, the Pb and Cu content decreased by 1.24% and 62.19%, respectively, compared to Figure 7b, as they were incorporated into lead–sulfur and copper sulfide compounds, which altered the surface composition.

3.5. FT-IR Analysis

Fourier-transform infrared spectroscopy (FTIR) is a valuable technique for studying the adsorption behavior of mineral surfaces by analyzing infrared absorption, revealing molecular vibrations and changes in surface chemistry [30]. In the FTIR spectra of (NH₄)₂S, Pb(NO₃)₂, NH₄Cl, and NH₄NO₃ (Figure 8), absorption peaks at 3439, 3442, and 3446 cm⁻¹ are coupled with -OH groups. Peaks at 2395, 2076, 2345, 2082, 2751, and 2921 cm⁻¹ are linked to the stretching vibrations of N-H bonds, particularly from ammonium ions (NH₄⁺). The peak at 1853 cm⁻¹ is likely related to N=O stretching vibrations from NO₃⁻ groups in Pb(NO₃)₂, and the signal at 1635 cm⁻¹ corresponds to N-H bending vibrations from NH₄⁺. Peaks at 1021 cm⁻¹ and 1096 cm⁻¹ are associated with symmetric stretching of the NO₃⁻ ions [31]. The bands at 674 cm⁻¹, 688 cm⁻¹, and 992 cm⁻¹ are attributed to S-N stretching vibrations in (NH₄)₂S.

In the FTIR spectra of Na₂S and malachite treated with Na₂S (Figure 9), the 3416 cm⁻¹ peak corresponds to the O-H stretching vibration, and the peaks at 2924 cm⁻¹ and 2852 cm⁻¹ are due to C-H stretching vibrations, possibly from minor hydrocarbon contamination. The 1636 cm⁻¹ peak corresponds to H-O-H bending vibrations from water. For malachite treated with Na₂S, the 2922 cm⁻¹ and 2845 cm⁻¹ characteristic peaks are assigned to alkyl groups’ C-H stretching vibrations, while the weak 567 cm⁻¹ peak suggests Cu-S stretching vibrations, indicating the formation of a copper sulfide (CuS) layer. The 1629 cm⁻¹ peak is due to water-bending vibrations [32], and the 1396 cm⁻¹ band corresponds to the antisymmetric stretching of (CO₃)²⁻ [33].

Figure 10 shows the FTIR spectra of malachite treated with Pb²⁺ and various ammonium salts. In the Pb²⁺ + NH₄NO₃ + Na₂S system, where the peak at 3436 cm⁻¹ corresponds to the O-H stretching vibration. The bands at 2921 cm⁻¹ and 2846 cm⁻¹ are attributed to C-H stretching vibrations, while the 1635 cm⁻¹ and 1400 cm⁻¹ peaks are associated with N-H bending vibrations from the ammonium ions. In the Pb²⁺ + NH₄Cl + Na₂S system, new peaks at 814 and 567 cm⁻¹ correspond to C=O and Cu-S stretching vibrations, respectively. Simultaneously, another new peak at 1805 cm⁻¹ represents the N=O stretching vibrations from nitrate groups in Pb(NO₃)₂, confirming its presence on the malachite surface and supporting enhanced sulfurization-assisted flotation. Additionally, compared to the malachite Pb²⁺ + NH₄NO₃ + Na₂S system, the peaks at 1049 cm⁻¹ and 1101 cm⁻¹ correspond to C=S vibrations on the malachite surface. In the Pb²⁺ + (NH₄)₂S + Na₂S system, compared to the direct sulfurization system, the peaks for C-H stretching vibrations at 2852 cm⁻¹ and 2924 cm⁻¹ are stronger in the FTIR spectrum of malachite. The intense bands at 567 cm⁻¹, 1805 cm⁻¹, and 814 cm⁻¹ are coupled with Cu-S, N=O, and C=O stretching vibrations, respectively. FTIR analysis confirms the formation of copper–sulfur species on the sulfurized malachite surface after activation with Pb²⁺ and NH₄⁺ ions, which enhances the adsorption of SBX during sulfurization, improving the hydrophobicity and floatability of the mineral surface. These results align with flotation test findings.

3.6. Contact Angle Analysis

The contact angle technique is used to assess the hydrophobicity of mineral surfaces by measuring the angle between the surface and the tangent of a water droplet [34,35]. The contact angle of the malachite surface under different flotation reagents is shown in Figure 11. For malachite treated with the Na₂S system, the contact angle of 82.95° is attributed to the copper-sulfide film formed on the surface, indicating a moderate water-repellent tendency, though the surface is not fully hydrophobic. After treatment with Pb²⁺ and Na₂S, the contact angle increased to 88.61° due to the combined effect of sulfurization and lead activation, which lowers the surface energy of malachite. Lead ions facilitate the formation of hydrophobic metal sulfide layers—primarily Pb-S and Cu-S—resulting in greater hydrophobicity than Na₂S alone. Treatment with the Pb²⁺ + NH₄NO₃ + Na₂S system raised the contact angle further to 92.57°, likely due to enhanced copper–sulfur film formation promoted by Pb²⁺ and NH₄NO₃. Use of the Pb²⁺ + NH₄Cl + Na₂S system increased the angle to 94.37°, showing an 11.42° increase compared to direct sulfurization. This enhancement is likely due to sulfurization-assisted flotation with Pb²⁺ and ammonium chloride. Finally, the Pb²⁺ + (NH₄)₂S + Na₂S system yielded the highest contact angle of 95.29°, a 12.34° increase, likely due to the development of larger copper sulfide crystals (Cu₂S), which enhance the durability of the sulfurized layer and improve surface hydrophobicity.

Based on the above analysis, the promotion mechanism of lead ions and various ammonium salts in enhancing the durability of the sulfurized surface layer is illustrated in Figure 12. In the direct sulfurization system, sodium sulfide interacts with malachite, forming a thin copper sulfide (CuS) layer that may consist of uniform flake-like particles or irregular products with good stability. However, when lead ions (Pb²⁺) and ammonium salts (NH₄⁺) are used in the preliminary treatment, dense clusters of copper sulfide (CuS) and lead sulfide (PbS) form on the malachite surface. Ammonium salts, such as NH₄NO₃, NH₄Cl, and (NH₄)₂S, promote sulfur–metal ion interactions, while lead ions further enhance the stability of the sulfurized layers. These layers act as a protective barrier, improving the malachite surface’s hydrophobicity and durability compared to direct sulfurization.

4. Conclusions

This study investigated the effect of preliminary activation of malachite by Pb²⁺ and various NH₄⁺ ions via sulfurization-assisted flotation systems. The adsorption of copper sulfide and the durability of the sulfurized surface layer were also assessed using various techniques, leading to the following conclusions:

1.

Micro-flotation results showed a 94.6% recovery at optimal SBX concentrations, which was further improved by the Pb²⁺ + (NH₄)₂S + Na₂S system, indicating superior flotation performance. Excessive Pb²⁺ caused over-sulfurization, reducing reactivity and efficiency. Controlling Pb²⁺ activation and SBX concentration is key for optimal recovery.

2.

The ToF-SIMS analysis showed higher signal intensities for Cu⁺ and sulfur anions, suggesting more effective sulfide ion adsorption on the surface compared to traditional sulfurization. The Pb²⁺ + (NH₄)₂S + Na₂S system produced significant Pb-S and Cu-S species, generated more active sites for xanthate-assisted adsorption, and promoted the growth of sulfurized species.

3.

FESEM-EDS analysis revealed significant changes in the malachite surface morphology after sulfurization. The Pb²⁺ + NH₄⁺ system formed dense Pb-S and Cu-S structures with increased sulfur content, enhancing sulfurization and flotation recovery. In contrast, the Pb and Cu contents decreased due to the formation of lead–sulfur and copper-sulfide compounds.

4.

The contact angle measurements showed that the activation systems significantly enhanced surface hydrophobicity, with the highest contact angle resulting from the excellent durability and hydrophobicity of the copper-sulfide film, compared to traditional sulfurization.

5.

FTIR analysis confirmed the formation of stable Cu-S species on the malachite surface after activation with Pb²⁺ and NH₄⁺ ions, which enhances SBX adsorption and improves hydrophobicity and floatability. Key FTIR peaks for Cu-S and N=O further indicate enhanced sulfurization-assisted flotation.