Efficient Flotation Separation of Antimonate Minerals from Quartz Using Sodium Dodecyl Sulfonate as Collector

Abstract

The efficient separation of antimonate minerals from quartz remains a significant challenge in mineral processing due to their similar surface properties and strong hydrophilicity. This study explored the application of sodium dodecyl sulfonate (SDS) as a selective collector for antimonate–quartz flotation separation. Micro-flotation tests demonstrated that SDS achieved optimal recovery of antimonate minerals (90.25%) at pH 8 with a dosage of 70 mg/L, while quartz recovery remained below 10%. Contact angle measurements revealed a significant increase in the hydrophobicity of antimonate minerals after SDS treatment, whereas quartz remained highly hydrophilic. FTIR and XPS analyses confirmed the selective chemisorption of SDS on antimonate mineral surfaces through Sb-O-S bond formation, while negligible adsorption occurred on quartz. Adsorption isotherms further showed the higher SDS uptake on antimonate minerals compared to quartz. These findings collectively demonstrate the effectiveness of SDS as a selective collector for the flotation of antimonate minerals, providing a promising approach to enhancing the recovery of fine antimonate particles.

1. Introduction

Antimony represents a critical metal for modern industrial development due to its unique physicochemical properties, and it has been widely utilized in alloy production, military equipment, ceramic manufacturing, and flame retardants [1,2,3]. Naturally occurring antimony primarily exists as sulfide minerals [4,5,6], which readily oxidize to form various antimonate minerals owing to antimony’s strong oxygen affinity [7]. Currently, antimonate minerals constitute approximately 15% of global antimony resources, with estimated reserves of 300,000 tons. As easily floatable antimony sulfide deposits become increasingly depleted, the efficient exploitation of antimonate resources has gained paramount importance.

Gravity separation remains the dominant industrial process for antimonate ore beneficiation, leveraging the density contrast between antimonate minerals and gangue materials [8]. Current research on the gravity separation of antimonate ores mainly focuses on the development of new gravity separation equipment and systems. The use of the cross-belt chute yields favorable recovery of antimonate minerals, owing to its advantages of effectively recovering fine-grained minerals and high enrichment ratio [9]. Lu et al. [10] developed a reflux classifier integrated with an innovative pulsation-fluidized bed system, which achieved a separation efficiency as high as 70%. However, the relatively low hardness of antimonate minerals leads to significant fine particle generation during comminution processes. These fine particles typically report to tailings in gravity separation systems, resulting in substantial antimony losses. Consequently, developing effective flotation techniques for fine antimonate minerals recovery has become imperative.

Antimonate minerals exhibit strong hydrophilicity, primarily due to their tendency to undergo hydration and form a hydration layer on the surface. Furthermore, the limited number of exposed active sites further hinders the effective adsorption of collectors, making their collection particularly challenging. Consequently, recent research has focused on developing efficient reagents for the flotation of antimonate minerals. Xiao et al. [11] achieved 97.5% recovery of fine antimonate minerals at pH 4.0 using octyl hydroxamic acid emulsified with hydrocarbon oil. Wang et al. [12,13] explored the flotation behavior of dodecyl amine and sodium dodecyl sulfonate as collectors with Cu²⁺ activation and clarified their interaction mechanisms with the antimonate mineral surfaces. Deng et al. [14] found that Mn²⁺ exhibits a significant activation effect on the surface of antimonate minerals at pH 7. However, the feasibility of these collectors for the flotation separation of antimonate minerals from gangue minerals (primarily quartz) still remains unclear. Due to the similar surface properties of quartz and antimonate minerals, the investigations specifically targeting this separation have remained limited. Therefore, developing appropriate reagents for the efficient separation of antimonate minerals from quartz is essential.

Sodium dodecyl sulfonate (SDS), an anionic surfactant, offers distinct advantages through its combination of moderate surface activity and high selectivity [15,16,17]. Previous applications include hematite–quartz separation when combined with cationic collectors [18,19] and enhanced lepidolite recovery with octyl hydroxamic acid [20]. Fundamental studies have established SDS’s ability to chemically adsorb onto aluminosilicate minerals (kaolinite, talc, lepidolite) via interactions between sulfonate groups and surface metal atoms [21,22,23]. These characteristics position SDS as a promising candidate for antimonate–quartz separation.

This study systematically investigated SDS as a selective collector for antimonate minerals flotation. Micro-flotation experiments demonstrated SDS’s high selectivity, achieving >90% antimonate recovery with minimal quartz entrainment. Comprehensive surface characterization (contact angle measurements, FTIR spectroscopy, adsorption analysis, and XPS) revealed that the superior performance originated from chemisorption via Sb-O-S bond formation on antimonate surfaces, which enhanced surface hydrophobicity. These findings not only establish SDS as an effective collector for antimonate–quartz separation but also provide fundamental insights into surfactant-mineral interactions that could guide reagent development for other oxidized mineral systems.

2. Experimental

2.1. Materials and Reagents

High-purity antimonate minerals and quartz samples were procured from the Xikuangshan Mine located in Hunan Province, China. The raw materials underwent manual selection followed by sequential processing, including crushing, grinding, and dry sieving, to obtain specific particle-size fractions. Bulk samples of both minerals were reserved for contact angle measurements, while the −74 + 38 μm size fraction was employed for micro-flotation experiments, chemical analysis, phase characterization, and adsorption capacity tests. X-ray diffraction (XRD) analysis (Figure 1), combined with standard chemical multi-element analysis (

Table 1. Major chemical composition of antimonate mineral and quartz samples.

Minerals	Chemical Composition (wt.%)
	Sb	SiO2	CaCO3	Al2O3
Quartz	-	99.78	0.10	0.06
Antimonate mineral	71.65%	0.90	0.23	0.14

) and phase composition (

Table 2. Phase composition of antimonate mineral sample.

Phase	Antimonate	Stibnite	Antimony Bloom
Content, wt.%	98.46	0.49	1.05

) determinations, confirmed that both the antimonate minerals and quartz satisfied the purity requirements for subsequent experimental investigations. Additionally, the antimonate minerals existed in the form of cervantite (Sb₂O₄ and Sb₂O₅).

All experiments were performed using deionized water (resistivity = 18 MΩ·cm). Sodium dodecyl sulfonate (SDS, analytical grade, Aladdin, Shanghai, China) served as the collector, while pH adjustments were made using analytical grade NaOH and HCl solutions. All reagent solutions were prepared fresh according to experimental protocols.

X-ray diffractometer (D8-ADVANCE, Bruker, Karlsruhe, Germany) was run in continuous scanning mode with Cu Kα radiation (λ = 0.15406 nm, tube potential of 40 mV, and tube current of 40 mA), with a step size of 0.004°/step and a scanning speed of 5°/min (at 2θ = 20°). Phase analysis and identification were performed using JADE 6 software.

2.2. Micro-Flotation Experiments

Micro-flotation tests were conducted using an XFG-type flotation machine (Wuhan Exploration Machinery Co., Ltd., Wuhan, China) with a 40 mL cell operating at a constant impeller speed of 1890 rpm. For single mineral tests, 2.0 g of either antimonate minerals or quartz was mixed with 35 mL deionized water and agitated for 1 min to achieve slurry homogeneity. The pH was then adjusted to the desired value using NaOH or HCl solutions, followed by the addition of the SDS collector with 2 min of additional conditioning. Froth products were manually collected over 3 min of flotation time, with the remaining material designated as tailings. For flotation experiments with artificially mixed minerals, equal masses (1.0 g each) of antimonate minerals and quartz were combined in the flotation cell while maintaining identical procedural parameters. Recovery calculations for single mineral tests were based on mass balance, whereas mixed mineral tests incorporated grade analysis of both concentrate and tailing products to determine separation efficiency. The recovery (R, %) was calculated using the following equation:

$$R={\displaystyle \frac{m_C\ast {\gamma }_C}{m_C\ast {\gamma }_C+m_T\ast {\gamma }_T}}$$

(1)

where m_C and m_T represent the mass of the concentrate and the tailing (g), and γ_C and γ_T are the grade of Sb in the concentrate and the tailing (%).

2.3. Contact Angle Measurements

Surface wettability characteristics were evaluated using a JC2000C contact angle goniometer (Meter, Powercat Co., Shanghai, China). Mineral specimens were prepared by sectioning bulk samples to expose fresh surfaces, followed by epoxy mounting and sequential polishing down to a 5000-grit finish. After thorough rinsing with deionized water and vacuum drying, the samples were conditioned in SDS solutions at predetermined concentrations and pH values for 10 min with continuous stirring. Following triple-rinsing with deionized water and air-drying, the contact angles were measured via the sessile drop method after droplet stabilization.

2.4. FTIR Spectroscopy

Surface adsorption studies were performed using an IRAffinity-1 FTIR spectrometer (Shimadzu, Kyoto, Japan). Mineral samples (2.0 g) were conditioned in 35 mL deionized water with collector addition according to the flotation sequence, followed by 30 min reaction time. After filtering the suspension, the precipitate was washed three times with deionized water. The solids were subsequently vacuum-dried at 45 °C before being homogenized with KBr (1:100 mass ratio) for transmission-mode FTIR analysis using the KBr pellet technique.

2.5. Adsorption Capacity Measurements

The adsorption density of SDS on mineral surfaces was quantified via total organic carbon (TOC) analysis. Test suspensions containing 2.0 g mineral in 40 mL deionized water were pH-adjusted and conditioned with varying collector concentrations for 30 min. After centrifugation (9000 rpm, 15 min), the supernatant was analyzed for residual organic carbon content. The adsorption capacity (Q_e, mg/g) was calculated using the following equation [24]:

$$Q_e={\displaystyle \frac{(C_0-C_e)·V}{m}}$$

(2)

where C₀ and C_e represent initial and equilibrium concentrations (mg/L), respectively, V is solution volume (L), and m denotes mineral mass (g).

The Langmuir model was selected to analyze and fit the adsorption data, and it is expressed as follows:

$$Q_e={\displaystyle \frac{Q_0\ast K\ast C_e}{1+K\ast C_e}}$$

(3)

where K represents the affinity constant, and Q₀ represents the maximum adsorption amount.

2.6. XPS Analysis

Surface chemical characterization was conducted using a NEXSA spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with monochromatic Al Kα radiation (1486.6 eV). Sample preparation followed identical procedures as for FTIR analysis. The analysis chamber was maintained at 6 × 10⁻⁹ mbar, with survey scans collected at a pass energy of 50 eV and high-resolution regional scans at 20 eV. Spectral calibration referenced the C 1s peak at 284.8 eV, with data processing performed using Avantage software (v5.9921) incorporating Gaussian-Lorentzian peak fitting algorithms.

3. Results and Discussion

3.1. Micro-Flotation Performance

3.1.1. Single Mineral Flotation Experiment

Figure 2a presents the flotation recovery of antimonate minerals and quartz as a function of pH using 70 mg/L SDS as the collector. The antimonate minerals exhibited a characteristic bell-shaped recovery curve. Recovery gradually increased as the pH value rose from 2 to 8, with maximum recovery (81.77%) achieved at pH 8, and then gradually decreased as the pH value increased from 8 to 12. In contrast, quartz maintained consistently low recovery (<10%), with a stable trend across the entire pH range (2–12). The observed pH-dependent recovery profile of antimonate minerals, with the higher yield occurring at pH 8, can be explained by the following factors: (1) Studies have shown that as the pH decreases, the surface tension of SDS increases, which affects the stability of the foam [25]. Under acidic conditions, the instability of the foam reduces the probability of particle–bubble interactions, resulting in a decrease in flotation recovery. (2) Under alkaline conditions, the competitive adsorption between OH⁻ and anionic SDS molecules decreases collector efficiency through surface site competition.

The concentration-dependent flotation behavior (Figure 2b) revealed that antimonate minerals recovery increased from 66.15% to 81.77% as the SDS concentration rose from 30 to 110 mg/L at pH 8. Notably, recovery plateaued above 70 mg/L, suggesting that the optimal flotation recovery of antimonate minerals can be achieved with a relatively low reagent dosage of 70 mg/L. Quartz recovery remained below 10% throughout the tested concentration range, demonstrating excellent selectivity of SDS. These results suggest that effective separation can be achieved at moderate SDS dosage (70 mg/L) under mildly alkaline conditions.

3.1.2. Artificially Mixed Ore Flotation Experiments

To validate these findings in a more realistic system, artificially mixed ore flotation tests were conducted (Figure 3). Antimonate minerals recovery increased from 66.15% to 90.25% with increasing SDS concentration (30–70 mg/L), beyond which further dosage increases caused marginal recovery reduction while maintaining >80% recovery. This phenomenon can be attributed to the strong foaming properties of SDS, which produces a relatively sticky foam [26,27]. The SDS concentrations used in the micro-flotation experiments were well below the critical micelle concentration (CMC) of SDS (approximately 8 × 10⁻² mol/L [28]). Below the CMC, SDS molecules exist primarily as monomers. As the SDS concentration increases, its surface tension decreases [28,29] and foam stability improves. When the reagent dosage is high, the foam layer becomes thicker, leading to the potential entrapment of gangue minerals and subsequently affecting the flotation performance of antimonate minerals [30]. Quartz recovery remained consistently below 10%, confirming the selectivity observed in single mineral tests. The mixed ore results corroborate the collector’s effectiveness for antimonate–quartz separation while demonstrating practical applicability under optimized conditions.

3.2. Surface Wettability Analysis

The contact angle experiment was conducted using the sessile drop method, which was used to obtain static contact angles. The contact angle data were measured when the solid–liquid–gas three-phase system reached equilibrium. Wettability describes the mineral surface’s affinity for water, which plays a crucial role in determining its ability to attach to air bubbles during flotation. Contact angle measurement serves as a direct indicator of mineral surface wettability alterations, where the angle formed at the solid–liquid–vapor interface reflects the surface’s hydrophobic character [31,32]. This parameter exhibits an inverse relationship with surface wettability: larger contact angles correspond to hydrophobic surfaces with superior floatability, while smaller angles indicate hydrophilic surfaces with poor flotation response [33]. Figure 4 presents comparative contact angle data for antimonate minerals and quartz before and after treatment with 70 mg/L SDS at pH 8. The untreated minerals showed similar initial contact angles (antimonate: 28.41°; quartz: 31.43°), suggesting comparable native hydrophilicity. Following SDS treatment, a marked divergence in surface properties emerged: antimonate minerals exhibited a significant increase in contact angle (64.73°), while quartz displayed minimal change (34.13°). This pronounced differential response confirms that SDS successfully induced the wettability difference between antimonate minerals and quartz, thus contributing to their contrasting flotation behaviors.

3.3. FTIR Spectroscopic Analysis

Figure 5 presents the FTIR spectrum of SDS, showing characteristic vibrational modes of the sulfonate functional group. Specifically, the absorption peaks observed at 1221.01 cm⁻¹ and 1083.83 cm⁻¹ are attributed to the antisymmetric and symmetric stretching vibrations of the S=O bond, respectively [34]. Additionally, the peak at 720.09 cm⁻¹ corresponds to the stretching vibration of the S-O bond [34,35].

Figure 6 compares the FTIR spectra of antimonate minerals and quartz before and after SDS treatment. The untreated antimonate minerals spectrum (Figure 6a) showed no detectable SDS signatures. Following SDS exposure, two new absorption peaks appeared at 1059.83 cm⁻¹ and 720.72 cm⁻¹, which correspond to the symmetric stretching vibration of S=O and the stretching vibration of S-O, respectively. The observed frequency shifts, particularly the significant right shift of the symmetric S=O stretching mode, suggest an interaction between sulfonate groups and antimonate mineral surface sites [36,37]. This behavior of adsorption accounts for the stable surface modification observed in flotation and contact angle measurements. In contrast, quartz spectra (Figure 6b) showed no detectable SDS characteristic peaks post-treatment, indicating the negligible adsorption of SDS on quartz surface. These spectroscopic findings confirm the selective adsorption mechanism of SDS on antimonate mineral surfaces.

3.4. Surface-Adsorption Analysis

The adsorption isotherms of SDS on antimonate mineral and quartz surfaces at pH 8 are presented in Figure 7. By fitting the data with the Langmuir model, the affinity constants were obtained as 1.13 × 10⁻³ for antimonate minerals and 4.37 × 10⁻⁴ for quartz. The higher affinity constant indicates that SDS forms a more stable adsorption layer on the surface of antimonate minerals. The adsorption density on antimonate mineral surfaces exhibited a concentration-dependent increase from 0.079 mg/g to 0.32 mg/g across the tested SDS concentration range (30–160 mg/L). Correspondingly, the surface coverage increased from 21.95% to 88.93%. In marked contrast, quartz showed minimal SDS uptake (<0.075 mg/g) throughout the entire concentration series. The pronounced adsorption preference for antimonate mineral surfaces explains the excellent separation efficiency demonstrated in both single mineral and mixed ore flotation tests. These findings provide quantitative support for the proposed selective adsorption mechanism, where SDS molecules preferentially interact with specific surface sites on antimonate mineral, while showing negligible affinity for quartz.

3.5. XPS Analysis

X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical states of surface elements and elucidate the adsorption mechanism of SDS on mineral surfaces. This technique provides complementary information to the FTIR results, offering atomic-level insights into the surface interactions.

The XPS survey spectra (Figure 8) reveals distinct differences in surface composition after SDS treatment. For antimonate minerals (Figure 8a), the appearance of a S 2p peak at ~169 eV confirms successful SDS adsorption. In contrast, quartz spectra (Figure 8b) showed no detectable sulfur signal after treatment, consistent with the FTIR and adsorption results.

Table 3. Surface atomic concentrations of antimonate mineral and quartz before and after SDS treatment.

Sample	Collector	Relative Atomic Concentration (%)
		C	O	Si	Sb	S
Antimonate Mineral	-	12.59	12.88	-	74.53	-
	SDS	14.9	13.7	-	71.14	0.26
Quartz	-	7.47	80.55	11.98	-	-
	SDS	8.23	80.62	11.15	-	-

summarizes the quantitative changes in surface atomic concentrations. Antimonate minerals exhibited 0.26%, 0.82%, and 2.31% increases in S, O, and C contents after SDS treatment, primarily attributed to the adsorption of SDS on mineral surface. For quartz, the changes in the relative atomic concentrations of O and C were minimal, at only 0.07% and 0.76%, respectively.

To further investigate the changes in the chemical environment of the elements on the surface of antimonate minerals before and after SDS treatment, Figure 9 presents high-resolution spectra and peak deconvolution for Sb 3d_5/2, O 1s, and S 2p. As shown in Figure 9a, the untreated antimonate minerals displayed two fitted peaks at 530.33 eV and 529.87 eV, corresponding to Sb 3d_5/2 and O 1s, respectively, with this overlap being characteristic of antimonate minerals [38,39,40]. Following SDS treatment, these peaks shifted to 530.13 eV and 529.97 eV, representing a 0.2 eV shift for the Sb 3d_5/2 peak. This shift provides evidence for chemical adsorption of SDS at Sb surface sites through Sb-O-S bond formation, which modifies the local electronic environment of Sb atoms [41]. Figure 9b further reveals the appearance of characteristic S 2p doublet peaks at 169.59 eV and 168.38 eV, corresponding to the S 2p_1/2 and S 2p_3/2 of the −SO₃⁻ group of adsorbed SDS molecules. These findings align with both the survey scan results and the observed changes in surface atomic concentrations.

Figure 10 displays the corresponding high-resolution spectra for quartz surfaces. The Si 2p spectrum (Figure 10a) showed a single peak at 103.13 eV (SiO₂) that remained unchanged after SDS treatment. The O 1s region (Figure 10b) contained three components: 533.61 eV (OH⁻), 532.48 eV (Si-OH), and 531.00 eV (Si-O), exhibiting only minimal shifts (≤0.11 eV) post-treatment. Most significantly, no sulfur signal was detected in the S 2p region (Figure 10c) following SDS exposure. These results collectively indicate either no adsorption or only negligible physical adsorption of SDS on quartz surfaces.

3.6. Discussion

Based on the results of the micro-flotation experiments, contact angle measurements, FTIR spectroscopy, adsorption capacity measurements, and XPS, it can be concluded that SDS is an effective and selective collector for the separation of antimonate minerals from quartz. In contrast to approaches that depend on reagent combinations to depress quartz, SDS achieves efficient separation under milder pH conditions (neutral to slightly alkaline). A detailed comparison is provided in

Table 4. Comparison of flotation performance for antimonate minerals and quartz using different reagent systems under optimal pH conditions.

Reagent	PH	Sb Recovery	Quartz Recovery	Literature
SDS	8	90.25%	9.46%	This work
CuSO4, Thiourea, and Hydroxamic acid	4	87.80%	37.00%	Nonferrous Metals (Mineral Processing Section), (1991) 24–26. [42]

.

Sodium dodecyl sulfate (SDS) is a typical anionic surfactant that can dissociate in aqueous solution, exposing its sulfonate headgroup (−SO₃⁻) to the surrounding water phase. The sulfonate group is highly polar and electron-rich, enabling it to interact with electron-deficient sites on mineral surfaces. For antimonate minerals, the adsorption of SDS likely involves coordination bonding, where the electron-rich oxygen atoms of sulfonate groups donate lone pairs to the electron-deficient Sb centers, resulting in the formation of stable surface complexes. Despite the overall negative surface charge of the antimonate minerals under the studied pH ranges, such interactions between functional groups and surface metal sites can overcome electrostatic repulsion, enabling strong adsorption. In contrast, the quartz surface lacks such active sites, resulting in the weak adsorption of SDS. In the adsorption configuration of SDS on the antimonate mineral surface, the hydrophobic alkyl chains of SDS extend outward from the mineral surface, forming an ordered hydrophobic layer. This molecular arrangement alters the surface wettability of the antimonate minerals, enhancing their hydrophobicity. The adsorption mechanism of SDS on antimonate minerals is illustrated in Figure 11.

4. Conclusions

In this study, sodium dodecyl sulfate (SDS) was investigated as a novel and selective collector for the flotation separation of antimonate minerals from quartz, and the flotation performance and adsorption mechanism of SDS were systematically evaluated. The results demonstrated that SDS achieved excellent selectivity, with a recovery of 90.25% for antimonate minerals at pH 8 and minimal quartz entrainment (<10%). Contact angle measurements showed that SDS significantly increased the hydrophobicity of antimonate minerals while having negligible effect on quartz, thereby enhancing the wettability contrast between the two minerals. Spectroscopic analyses confirmed that SDS chemisorbs onto antimonate mineral surfaces through Sb-O-S bonding, whereas no significant adsorption occurred on quartz. The adsorption isotherms further indicated a markedly higher uptake of SDS on antimonate minerals (0.32 mg/g) compared to quartz (<0.075 mg/g). These findings highlight SDS as a highly selective collector for antimonate minerals flotation, addressing a critical challenge in processing oxidized antimony ores. However, SDS exhibits excessive foaming ability under neutral and alkaline conditions, which may limit its practical application. Future work should focus on the exploration of effective strategies to control the foaming properties of the SDS system.