Study on the Inhibition and Activation of Pyrite Under Low Alkalinity Conditions Created by Hydrogen Peroxide and Lime

Abstract

High alkalinity facilitates copper–sulfur flotation separation but also leads to issues such as high reagent consumption, pipeline scaling, and gold loss in tailings. The ore from a copper mine in Serbia contains 2.86% copper, 1.64 g/t gold, and 20.39% sulfur, with copper occurring mainly in covellite and enargite. To achieve efficient separation and recovery of copper–sulfur, a systematic study was conducted using micro-flotation, Scanning Electron Microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and contact angle analysis to investigate the inhibition and activation patterns of pyrite under low and high alkalinity conditions. The results indicate that the combined use of hydrogen peroxide and lime as inhibitor enables efficient separation of pyrite and covellite under low-alkalinity conditions. This effect is attributed to its ability to enhance oxidation of the pyrite surface, which generates more hydrophilic substances. Under low-alkalinity conditions (slurry pH = 10) regulated with hydrogen peroxide and lime in a covellite flotation cycle, and under acidic conditions (slurry pH = 6) in the pyrite flotation cycle, satisfactory results are obtained in both flotation cycles in comparison with industrial data. The copper flotation index was similar, but pyrite and gold recovery increased by 2.3% and ~4%, respectively, over those using lime alone. This process reduced the activator dosage required for pyrite activation substantially, while improving gold recovery. Results demonstrate an efficient method for copper–sulfur separation and recovery, providing theoretical guidance or industrial production processes.

1. Introduction

Pyrite (FeS₂) is a common iron sulfide mineral and an important raw material for the preparation of sulfuric acid (H₂SO₄) and sulfur [1,2,3,4]. Covellite (CuS), an important raw material for copper smelting, is a widespread copper sulfide mineral accounting for approximately 70% of the world’s total copper reserves [5,6]. The intergrowth or association of covellite and pyrite is extremely common in nature [7]. The natural floatability of covellite is similar to but slightly higher than that of pyrite [8,9,10]. Under low-alkalinity conditions, efficient separation of the two minerals is difficult to achieve. Currently, the most commonly used method for effective separation is the inhibition of pyrite flotation in a high-alkalinity environment, which increases the difference in floatability between the two [11,12].

The efficient separation and recovery of pyrite and covellite is essential for resource utilization. Commonly used pyrite inhibitors include lime, cyanide, sulfur oxides, and oxidizing agents [13,14]. Lime inhibitors inhibit the flotation recovery of pyrite by adjusting the slurry pH. A higher pH increases the OH⁻ concentration and the hydrophilic hydroxide content on the surface of pyrite, thereby inhibiting the flotation recovery of pyrite. Although lime possesses many advantages in pyrite inhibition, the excessive lime addition often leads to scaling, solidification, and pipeline blockage. The dosage of lime is difficult to control, and an excessively high slurry alkalinity leads to high collector consumption, which is not favorable for comprehensive resource recycling. Many researchers have thus focused on inhibitors of pyrite in low-alkalinity media [15,16]. For pyrite containing the precious metal gold, floatation conditions also significantly influence gold recovery.

Oxidizing agents are attractive alternatives due to their wide availability, low cost, and good selectivity. They also exert a smaller impact on associated valuable metals and are suitable for use over a wider slurry pH range [17]. Therefore, they are widely applied in copper–sulfur separation. Hydrogen peroxide (H₂O₂), a clean and efficient oxidant with low human toxicity, has recently become more widely used in mineral processing. Khoso et al. [18] achieve the selective separation of chalcopyrite and pyrite using H₂O₂ at a pH of 9.0, with the recovery of chalcopyrite exceeding 84% and that of pyrite being lower than 24%. Infrared and X-ray photoelectron spectroscopy (XPS) analyses revealed that H₂O₂ treatment substantially altered the surface properties of pyrite, rendering it highly hydrophilic and inhibiting collector adsorption, thereby reducing its floatability. Wang et al. [19] have shown that the surface oxidation of pyrite is increased and dixanthogen formation is decreased under high-alkalinity slurry flotation. This leads to the reduced adsorption of xanthate, which in turn inhibits the flotation of pyrite. Ozun et al. [20] have reported that pyrite is a chemically active mineral whose surface oxidizes when exposed to the atmosphere and aqueous environments during mining and flotation. The oxidation process results in the creation of a hydrophilic surface, which affects collector adsorption and flotation effects. Luo et al. [21] have regulated the dissolved oxygen concentration in slurry by adjusting the relative proportion of O₂/N₂ during aeration. The generation of dixanthogen and avoidance of excessive iron oxide/hydroxide formation on the pyrite surface contribute to the regulation of pyrite recovery by flotation. Copper flotation is associated with various issues, such as a low ore grade, high pyrite content, and complex mineral dissemination relationships [22,23]. For pyrite, a carrier mineral for common gold monomers, the effects of cyanide and hydroxides must be minimized. In other words, efficient recovery of elemental gold can only proceed in low-alkalinity environments [24]. Yuan [25], Yang [26], and Khosol et al. [18] have systematically evaluated low-alkalinity flotation systems. Besides inhibiting the flotation performance of pyrite by adjusting the environment of the flotation slurry to low alkalinity, the researchers also introduced oxidizing agents to further enhance the inhibitory effect. In addition to oxidation treatment in a low-alkalinity solution environment, the oxidizing agent dosage was reduced to 1/3 of the original dosage. The efficient formation of a passivation layer on the pyrite surface was achieved, which significantly inhibited floatability and effectively reduced the sulfur grade by 10%–20% in the copper concentrate.

In this paper, the depression and activation mechanisms of pyrite under high-alkali and low-alkali conditions were systematically investigated. The research findings were successfully applied at a mine in Serbia, achieving effective copper and sulfur copper–sulfur separation and recovery. The low-alkali process significantly improved the production indicators of the beneficiation plant while reducing reagent consumption, providing valuable insights for similar mining operations.

2. Materials and Methods

2.1. Ore Sample

The main chemical composition, particle size distribution and liberation relationship analysis of ore sample are presented in

Table 1. Main Chemical Composition of Ore from the Upper Zone of a Copper-Gold Mine in Sebia (Unit: %, *: g/t).

Elements	Cu	Au *	S	As	Fe	Pb	SO3	Zn	SiO2
Contents	2.8	1.64	20.39	0.035	18.87	0.01	3.74	0.02	3.74

,

Table 2. Particle Size Distribution and Liberation Relationship Analysis of Copper Sulfides in Ore.

Copper–Sulfur Mineral Intergrowth Type	Yield (%)	Intergrowth Association		Copper Sulfide Particle Size Distribution (%)
		With Py	With Others	−10 µm	−20 + 10 µm	−38 + 20 µm	−74 + 38 µm	−150 + 74 µm	+150 µm
X = 100%	85.88	--	--	0.62	14.7	26.15	30.03	14.12	0.26
80% ≤ X < 100%	6.22	4.34	1.88	--	0.12	0.77	2.26	2.81	0.26
50% ≤ X < 80%	3.86	3.29	0.57	--	0.24	1.47	1.74	0.41	--
X < 50%	4.04	3.07	0.97	0.6	1.04	1.69	0.71	--	--
Sum	100.00	10.7	3.42	1.22	16.1	30.08	34.74	17.34	0.52

Note: X represents the proportion of the target mineral in the liberation particle; “Py” is the abbreviation for pyrite, used consistently throughout the text.

and

Table 3. Particle Size Distribution and Liberation Relationship Analysis of Pyrite.

Type of Pyrite Intergrowth	Yield (%)	Intergrowth Association		Pyrite Particle Size Distribution (%)
		With Cu	With Others	−10 µm	−20 + 10 µm	−38 + 20 µm	−74 + 38 µm	−150 + 74 µm	+150 µm
X = 100%	83.77	--	--	0.23	18.47	30.35	27.41	7.22	0.09
80% ≤ X < 100%	6.12	2.08	4.04	0.01	0.24	1.43	3.06	1.38	--
50% ≤ X < 80%	3.37	0.58	2.79	0.03	0.49	0.96	1.73	0.16	--
X < 50%	6.74	0.63	6.11	0.90	2.22	2.51	1.10	0.01	--
Sum	100.00	3.29	12.94	1.17	21.42	35.25	33.30	8.77	0.09

.

As shown in Table 1, the main valuable elements in the raw ore were copper, gold, and sulfur. Copper and gold grades were 2.8% and 2.16 g/t, respectively, whereas sulfur possessed a lower value.

Table 2 and Table 3 show the mineralogical results of the ore sample. As shown in Table 2, 85.88% of the copper sulfides was completely liberated at a particle size of 70% passing 75 µm. Rich intergrowths with 50% ≤ X < 100% accounted for 10.08%, whereas lean intergrowths (X < 50%) accounted for only 4.04%.

Results shown in Table 3 indicate that at a particle size of 70% passing 75 µm, 83.77% of the pyrite was completely liberated, with the vast majority being in the floatable particle size range. Rich intergrowths accounted for 6.12% of the pyrite. It is noted that the pyrite in <10 µm size fraction that is difficult to recover by flotation accounted for only 1.17%. Lean intergrowths accounted for 6.74% of the pyrite. Floatable pyrite accounted for 92.99% of the total pyrite (>10 µm and X > 50%).

At a particle size of 70% passing 75 µm, the concentrate obtained by gravity concentration was examined manually using an optical stereomicroscope (Nikon Corporation, Tokyo, Japan) and subjected to MLA testing. Six liberated native gold particles with a particle size of up to 17 µm were observed, with no encapsulated gold observed. Figure 1 shows the concentrated liberated native gold particles.

The results of a chemical phase analysis are summarized in

Table 4. Chemical Phase Analysis of Serbia Gold–Copper Ore.

Mineral Species	Mineral Content/%
Covellite (CuS)	11.49
Tennantite (Cu3AsS4)	0.89
Pyrite (FeS2)	39.31
Quartz (SiO2)	27.74

. Covellite is the main copper sulfide mineral contained in the ore sample, and the major gangue minerals include pyrite and quartz, with pyrite being the primary gold carrier. Therefore, it is necessary to perform the efficient separation and recovery of copper and sulfur.

2.2. Flotation Process of the Mine

At present, the preferential flotation process is adopted at processing plant (Figure 2). Lime is used as the pyrite inhibitor, Z200 is used as the collector, and the slurry pH is maintained at approximately 12. Current flotation indicators on the site are shown in

Table 5. Current Flotation indicators of the Serbia Copper Mine.

Product	Yield (%)	Grade (%)			Recovery (%)
		Cu	Au (g·t−1)	S	Cu	Au	S
Copper Concentrate 1	7.57	30.35	4.07	34.70	80.42	18.81	12.89
Copper Concentrate 2	4.76	8.39	4.62	36.80	13.97	13.42	8.58
Pyrite Concentrate	23.41	0.52	3.77	49.40	4.26	53.87	56.70
Tailing 2	14.53	0.096	0.78	7.61	0.49	6.92	5.42
Tailing 1	49.73	0.050	0.23	6.73	0.87	6.98	16.41
Feed	100.00	2.86	1.64	20.39	100.00	100.00	100.00
Copper Concentrate 1 + 2	12.33	21.88	4.28	35.51	94.38	32.23	21.47
Tailing 1 + 2	64.26	0.06	0.35	6.93	1.36	13.90	21.83

, the obtained copper concentrate has a copper grade of 21.88%, copper recovery of 94.38%, and gold recovery of 32.23%. The sulfur concentrate has a sulfur grade of 49.40%, recovery of 56.70%, and gold recovery of 53.87%. Major production issues are as follows: (1) high dosage of lime (approximately 5 kg/t), resulting in a sticky flotation foam and a high tendency for slurry pipeline blockage, and (2) low sulfur flotation recovery rate and low gold recovery during sulfur flotation. A low-alkalinity process, in which lime is partially replaced by an oxidizing agent, is an effective method to reduce the amount of lime used while increasing the recovery of gold during sulfur separation.

The tests were performed with pure pyrite mineral and raw ore sample from upper zone of the deposit from a cooper mine in Serbia.

2.3. Pure Mineral

Figure 3 shows the X-ray diffraction (XRD) spectrum of the pure pyrite mineral sample used as the raw material for single-mineral flotation testing. As shown in Figure 3, the spectrum is free of any obvious impurity peaks. Together with chemical analyses, it revealed that the purity of pyrite exceeded 98%; the spectrum indicates that the pyrite sample fulfills the requirement for single mineral flotation tests.

2.4. Reagents

Table 6. Main Chemical Reagents Used in the Experiment.

Chemical Formula	Content (wt.%)	Supplier	Function
CaO	98	Macklin Biochemical (Shanghai), Shanghai, China	Depressant
H2SO4	98.08	Sinopharm Chemical Reagent Co., Ltd., Shanghai, China	pH regulator
NaBX	—	Zhuzhou Flotation Reagents Co., Ltd., Zhuzhou, China	collector
MIBC	99	Shanghai EON Chemical Technology Co., Shanghai, China	frother
Z200(C3H7CH(CH3)C(S)NHC2H5)	—	Zhuzhou Flotation Reagents Co., Ltd., Zhuzhou, China	collector
H2O2	30	Macklin Biochemical (Shanghai), Shanghai, China	depressant

shows the reagents used in this study, including regulators, collectors, inhibitors, and frothers. To note, H₂SO₄, when used, was diluted to a 1% solution with distilled water.

2.5. Methods and Processes

Pure mineral flotation testing was performed using a hanging cell flotation machine(Shandong Xinhai Mining Technology & Equipment Co., Ltd., Yantai, China). During each test, 2.0 g of the mineral sample was weighed and rapidly put into the 40 mL hanging cell flotation machine using a solution at the corresponding pH. The slurry was stirred for 3 min, and the slurry pH was measured. Subsequently, the regulator, collector, and frother were added sequentially, and the slurry was mixed and reacted for 2, 2, and 1 min. Aerated flotation was performed for 3 min, and the concentrate and tailings were dried in an oven and weighed. The recovery rate (%) was calculated using the following equation:

$$\epsilon ={\displaystyle \frac{m_1}{m_1+m_2}}\times 100$$

(1)

ε—recovery rate (%),

m₁—the mass of the froth product (concentrate weight, g),

m₂—the mass of the in-cell product (tailing weight, g).

Raw ore flotation testing was performed using laboratory single-cell flotation machines (Shandong Xinhai Mining Technology & Equipment Co., Ltd., Yantai, China) with capacities of 1.5, 1, and 0.5 L. For each test, 500 g of the ore sample was weighed, and copper concentrate flotation and separation testing was performed based on the optimum reagent system and test procedure derived from single-mineral pyrite flotation testing.

2.6. Testing and Analysis

2.6.1. Scanning Electron Microscopy (SEM)

The surface morphology of pure natural pyrite samples was analyzed using SEM (FEI Quanta 200, FEI Company, Hillsboro, OR, USA). Pyrite samples (2 g, particle size range: −74 + 45 μm) were treated using the reagent addition sequence and reaction times used in flotation testing, subjected to solid–liquid separation, and vacuum-dried at a low temperature before use as a test material.

2.6.2. Contact Angle Measurement

The wettability of mineral surfaces was characterized by measuring the contact angle using a drop shape analyzer (DSA255; KRŰSS, Hamburg, Germany). For sample preparation, an appropriately sized pyrite sample block was obtained, and one surface was ground sequentially until smooth with coarse to fine sandpaper. Subsequently, surface impurities were washed with dilute sulfuric acid, and the surface of the pyrite sample was rinsed with deionized water. The sample was then treated using the reagent addition sequence and reaction times used in flotation testing, subjected to solid–liquid separation, and vacuum-dried at a low temperature. The contact angle was measured using the sessile drop method, and the measured contact angle data were analyzed using ImageJ.js software.

2.6.3. Process Mineralogy Parameter Testing Using a Mineral Liberation Analyzer (MLA)

The raw ore was crushed and ground to a fineness of 70% passing 75 µm. Subsequently, an appropriate amount of the copper ore sample was ground into polished thin sections, which were analyzed using an MLA (Zhejiang Lante Optics Co., Ltd., Jiaxing, China) to determine the particle size of copper sulfides and pyrite and their liberation and intergrowth relationships.

2.6.4. XPS Measurement

The spectra of Fe and S elements on the surfaces of pyrite from different ore sites were measured using XPS (PHI 5000 VersaProbe III, ULVAC-PHI, Inc., Chigasaki, Japan) to determine the surface properties and elemental contents of the target minerals.

Full-spectrum scans of pyrite from different ore sites, narrow spectra of Fe and S on the pyrite surface, and raw XPS surface elemental data were obtained. Avantage 5.9931 BPM Foundation software was used to fit the spectral data and analyze the binding energies and contents of Fe and S on the surface.

3. Results and Discussion

3.1. Effects of Different Inhibition Systems on Pyrite Flotation

3.1.1. Effect of Lime on the Floatability of Pyrite in the Z200 System

Pyrite that had been ultrasonically cleaned and re-slurried was subjected to pH adjustment using lime. The dosages of Z200 and the frother methyl isobutyl carbinol (MIBC) were 10⁻³ mol/L and 10⁻⁴ mol/L, respectively. The dosages were determined based on the results of previous conditional tests. Figure 4 shows the test process and results.

As shown in Figure 4, lime exerted a strong inhibitory effect on pyrite flotation in the Z200 system. An increase in pH from 5.27 to 12 led to a gradual reduction in concentrate recovery, with pyrite recovery decreasing from 91.4% to 4.26%. However, it is evident that effective depression of pyrite requires a higher pH (>11) level to achieve.

3.1.2. Effect of H₂O₂ on Pyrite in the Z200 System

Given that pyrite is easily oxidized and H₂O₂ exerts a strong oxidizing effect, a hydrophilic film is readily formed on the surface of pyrite when reacted with H₂O₂, thus producing an inhibitory effect. During this test, natural slurry (pH = 4.37) that had not been pH-adjusted was treated with H₂O₂ as the inhibitor, Z200 as the collector (dosage: 10⁻³ mol/L), and MIBC as the frother (dosage: 10⁻⁴ mol/L). Figure 5 shows the test process and test results, respectively.

As shown in Figure 5, the recovery rate decreased gradually from 89.79% to 62.51% when the H₂O₂ dosage was increased from 1 mg/L to 100 mg/L in the Z200 system. Recovery exceeded 80% at a H₂O₂ dosage of <10 mg/L but decreased to 62.5% when the H₂O₂ dosage was increased to 100 mg/L. These results indicated that the depression effect of standalone H₂O₂ on pyrite was not pronounced.

3.1.3. Effect of Combined Lime-H₂O₂ Inhibition on Pyrite in the Z200 System

To avoid excess alkalinity in the flotation solution system and excessive consumption of H₂O₂, lime + H₂O₂ was used as a combined inhibitor. Z200 was used as the collector (dosage: 10⁻³ mol/L), and MIBC was used as the frother (dosage: 10⁻⁴ mol/L). Figure 6 and Figure 7 show the test process and test results, respectively.

Figure 7 shows that a combination of lime and H₂O₂ exerted an excellent inhibitory effect. The flotation recovery of pyrite decreased significantly with an increase in the pH of the solution environment. Pyrite recovery also exhibited a certain decline with increasing H₂O₂ dosage; However, excessively high pH and H₂O₂ dosage will inhibit covellite as Figure 7 shows. Therefore, pH = 10 and H₂O₂ dosage = 50 mg/L were ultimately selected as the optimal conditions, with a corresponding pyrite recovery of 24.67%.

3.2. Activation of Depressed Pyrite

To recover the inhibited pyrite, the effects of different activators on pyrite were investigated systematically. Sulfuric acid and copper sulfate were used to investigate the effect of activator dosage on the floatability of the inhibited pyrite.

3.2.1. Sulfuric Acid Activation Test

Using NaBX as the collector, pyrite recovery was evaluated with respect to the sulfuric acid dosage (pH). First, slurry pH was adjusted to 12 using the combined lime + H₂O₂ inhibitor, with the H₂O₂ dosage being 50 mg/L. Sulfuric acid was added for pH (from 10 to 2) adjustment after stirring. A butyl xanthate concentration of 1 × 10⁻⁴ mol/L was adopted, and MIBC was used as the frother. Figure 8 shows the test results.

As shown in Figure 8, in the system consisting of NaBX as the collector and lime + H₂O₂ as the combined inhibitor, a gradual increase in sulfuric acid dosage led to an initial increase and subsequent decrease in pyrite recovery. Optimal activation of the inhibited pyrite was achieved at a slurry pH of approximately 6, with recovery reaching 80%. Therefore, reducing the pulp pH can restore the floatability of pyrite.

3.2.2. Copper Sulfate Activation Test

Using NaBX as the collector, the effect of copper sulfate dosage on pyrite recovery was investigated. First, slurry pH was adjusted to 10 using the combined lime + H₂O₂ inhibitor, with the H₂O₂ dosage being 50 mg/L. Copper sulfate is added at dosage from 1 to 10 × 10⁻⁴ mol/L. A NaBX concentration of 1 × 10⁻⁴ mol/L was adopted, and MIBC was used as the frother. Figure 9 shows the test results.

As shown in Figure 9, the recovery of pyrite inhibited via a combination of lime and H₂O₂ exhibited an initial increase and subsequent decrease with increases in the copper sulfate dosage when NaBX was used as the collector. This may be attributed to the large number of residual copper ions in the slurry. The consumption of NaBX by excessive copper sulfate led to a decrease in the NaBX concentration, resulting in a decline in collection effect and poorer activation by copper sulfate. A comparative analysis revealed that sulfuric acid provided a better activation effect than that of copper sulfate, because the recovery rate of pyrite obtained through sulfuric acid activation is much higher. Compared to the test results of sulfuric acid restoring pyrite floatability, the activation effect of copper sulfate is somewhat inferior; therefore, sulfuric acid is the more suitable reagent.

3.3. XPS Analysis

To further elucidate the intrinsic causes of pyrite inhibition by lime + H₂O₂, an XPS analysis was performed to evaluate the elemental components and chemical states of the surfaces of pure pyrite mineral treated with the combined inhibitor (lime + H₂O₂) and lime alone.

Table 7. XPS Analysis Results of Pyrite before and after Depressant Treatment.

Species		B.E. (eV)	Atomic Concentration (at. %)
			Blank	CaO	H2O2 + CaO
Fe 2p3/2	Fe(II)-S	707.2 ± 0.1	30.2	27.1	26.5
	Fe(III)-S	709.2 ± 0.3	5.7	4.3	4.1
	Fe(III)-O	710.3 ± 0.2	4.5	7.23	8.27
S 2p3/2	S22−	162.9 ± 0.1	21.3	20.8	19.4
	S2−	162.5 ± 0.2	10.5	9.2	9.6
	Sn2−/S0	164.3 ± 0.2	3.1	4.6	4.1
	SO42−	168.7 ± 0.1	1.2	1.8	2.6
	SO32−	166.5 ± 0.1	-	-	1.13
O 1s	O-H	532.8 ± 0.2	8.0	24.5	25.3
	Me-O	530.1 ± 0.2	2.3	12.8	13.6
Ca 2p	Ca-O	348.1 ± 0.2	-	0.1	0.1

and Figure 10 show the analysis results.

As shown in Table 7, the contents of O existing as O-H and Fe(III)-O on the surface of untreated pyrite were 8.0% and 4.5%, respectively. After lime treatment, the values increased considerably to 24.5% and 7.23%, respectively. Calcium ions were also detected on the pyrite surface. This suggested that the addition of lime caused the generation of a large number of hydroxides, which were co-adsorbed with calcium hydroxyl compounds to the pyrite surface and contributed to its hydrophilicity. These results are consistent with those reported by Duan Wenting et al. [27]. Upon treatment with the combined lime + H₂O₂ inhibitor, the surface atomic concentrations of pyrite underwent further change. First, the atomic concentrations of the sulfur element existing as S²⁻ and S₂²⁻ decreased slightly, while those of sulfur existing as sulfates and sulfites increased. In addition, O contents in the form of O-H and Fe(III)-O increased by 1.04% and 0.8%, respectively, compared with those for lime treatment alone. These results indicated that oxidation occurred on the pyrite surface, leading to an increase in hydrophilic substances. This oxidation reaction was an inevitable outcome of H₂O₂ addition, with a greater hydrophilic effect on the pyrite surface induced by the combined lime + H₂O₂ inhibitor than that induced by lime alone.

Figure 10 shows the narrow spectrum of Fe 2p_3/2 on the pyrite surface. The main peak at 707.2 ± 0.1 eV was attributed to fully coordinated low-spin Fe(II)-S from the bulk position, while the two small peaks at 706.3 ± 0.2 eV and 708.2 ± 0.2 eV were designated as multiple splitting peaks of Fe(II)-S [28]. The peak located at 709.3 ± 0.3 eV may be related to Fe(III)-S due to the simultaneous oxidation of Fe²⁺ to Fe³⁺ and reduction in S on the surface to S²⁻ (Equation (5)) [28,29,30]. Broad peaks with binding energies of 710.3 ± 0.2, 711.4 ± 0.2, 712.2 ± 0.2, and 713.2 ± 0.1 eV were attributed to the multiple splitting peaks of the oxygen or hydroxyl compounds of Fe³⁺ (i.e., FeOOH) [28,31,32,33].

Comparison of curves a and b in Figure 10 revealed that the Fe(III)-O peak on the pyrite surface was strengthened after lime treatment, indicating a significant increase in the hydroxyl oxides of iron. However, the binding energies of iron in various species remained almost unchanged compared with that of blank pyrite. These results suggested that the adsorption of lime on the surface of pyrite did not cause significant changes to the chemical environment of elemental iron, leading to a low degree of chemical reaction. However, different results were obtained when curves c and b were compared. The Fe(III)-O peak was further strengthened, and the inner electron binding energies of iron in both Fe(III)-O and Fe(III)-S were clearly shifted to the left and right. This suggested that significant changes occurred in the chemical environment of iron. Such a phenomenon can be attributed to the oxidative effect of H₂O₂ on the pyrite surface, with surface oxidation resulting in the conversion of the main surface iron species Fe(II)-S to Fe(III)-O (Equations (2)–(4)) [34].

$${\mathrm{Fe}}_{\mathrm{surface}}^{2+}+{\mathrm{S}}_{\mathrm{surface}}^{-} \to { \mathrm{Fe}}_{\mathrm{surface}}^{3+}+{\mathrm{S}}_{\mathrm{surface}}^{2-}$$

(2)

$$\mathrm{Fe}{\mathrm{S}}_2+{\displaystyle \frac{15}{4}}{\mathrm{O}}_2+{\displaystyle \frac{7}{2}}{\mathrm{H}}_2\mathrm{O} \to \mathrm{Fe}{\left(\mathrm{OH}\right)}_3+2\mathrm{S}{\mathrm{O}}_4^{2-}+4{\mathrm{H}}^{+}$$

(3)

$$\mathrm{Fe}{\mathrm{S}}_2+{\displaystyle \frac{15}{4}}{\mathrm{O}}_2+{\displaystyle \frac{5}{2}}{\mathrm{H}}_2\mathrm{O} \to \mathrm{FeOOH}+2\mathrm{S}{\mathrm{O}}_4^{2-}+4{\mathrm{H}}^{+}$$

(4)

Figure 11 shows the S 2p spectra of the pyrite surface, with binding energies of 162.5 ± 0.3, 162.8 ± 0.1, and 164.4 ± 0.2 eV for S 2p3/2 corresponding to monosulfides (S²⁻), disulfides (S₂²⁻), and hydrophobic polysulfides (S_n²⁻/S₀), respectively [35,36]. Sulfur oxide species, such as sulfites (SO₃²⁻) and sulfates (SO₄²⁻), exhibited S 2p_3/2 peaks at 166.55 and 168.7 ± 0.1 eV, respectively [35,37]. In untreated pyrite, the concentrations of SO₃²⁻ and SO₄²⁻ were 0% and 1.2%, respectively. After lime treatment, the SO₄²⁻ concentration increased slightly from 1.2% to 1.8%. Treatment with the combined lime + H₂O₂ inhibitor led to a further increase in the pyrite surface SO₄²⁻ concentration to 2.6%. In addition to the occurrence of new SO₃²⁻ species, the S 2p peak in S₂²⁻S_n²⁻/S₀ was shifted leftward. These results suggested further enhancement of the pyrite surface oxidation process (Equations (5) and (6)).

$${\mathrm{S}}_2^{2-}-{\mathrm{e}}^{-}(\mathrm{to} {\mathrm{Fe}}^{3+})+{\mathrm{H}}_2\mathrm{O}\to \mathrm{S}{\mathrm{O}}_3^{2-}+{\mathrm{Fe}}^{2+}+{\mathrm{H}}^{+}$$

(5)

$$2\mathrm{S}{\mathrm{O}}_3^{2-}+{\mathrm{O}}_2+2{\mathrm{H}}^{+}\to 2\mathrm{S}{\mathrm{O}}_4^{2-}+{\mathrm{H}}_2\mathrm{O}$$

(6)

Therefore, the XPS analysis results indicated that the addition of H₂O₂ increased the concentration of hydrophilic iron oxide/hydroxide species on the pyrite surface.

3.4. SEM Analysis

Using SEM, the microscopic morphology of mineral surfaces was observed directly, providing a basis for determining the effects of different inhibitor conditions on the pyrite surface.

From the surface morphologies of pyrite under different conditions shown in Figure 12, it can be seen that the raw pyrite surface was relatively uniform. A certain amount of small debris was attached to the surface, and there were no obvious traces of oxidation or other chemical reactions. Figure 12b shows the spectrum of pyrite that had been reacted with an aqueous lime solution at a certain concentration. Compared with the raw ore, the pyrite surface remained nearly unchanged except for the presence of two natural small holes. Figure 12c shows the surface morphology of pyrite after reaction with H₂O₂. A large number of corrosion pits appeared on the pyrite surface. This indicated the occurrence of a strong chemical reaction between the pyrite surface and H₂O₂, resulting in oxidation of the mineral surface. Figure 12d shows the surface morphology of pyrite after reaction with lime + H₂O₂. In addition to the presence of corrosion pits on the surface, small bubble-like substances were also observed, likely to be caused by hydroxide deposition on the surface. The above results demonstrated that the addition of H₂O₂ led to oxidation of the pyrite surface, and the combined use of H₂O₂ with lime also significantly influenced the surface morphology of the mineral.

3.5. Contact Angle Analysis

The effects of lime + H₂O₂ on pyrite floatability were investigated based on analyses of the contact angle of the pure pyrite mineral surface before and after treatment with the combined inhibitor.

Table 8. Changes in the Contact Angle of Pyrite Before and After Depressant Application.

Test Condition	Pyrite
	Appearance of Contact Angle	Contact Angle (°)
Fresh Surface		63.8
Lime		48.3
Lime + Hydrogen Peroxide		40.5

shows the results of contact angle measurement and analysis.

The contact angle of the surface of untreated pyrite was 63.8°, indicating good floatability. After lime treatment, the contact angle was reduced to 48.3°, indicating the enhancement of hydrophilicity. Treatment with the combined lime + H₂O₂ inhibitor led to a significant decrease in surface contact angle to 40.5°, indicating a considerable increase in hydrophilicity. When viewed in combination with the XPS analysis results, these findings demonstrated that the increase in the contents of oxides and hydroxides generated on the surface of pyrite resulted in increased hydrophilicity.

3.6. Validation Through Actual Ore Flotation Testing

Based on the optimal parameters for pyrite inhibition and activation determined from single-mineral laboratory flotation testing, copper ore from a mine in Serbia was used as the raw material, Z200 was used as the copper collector, lime + H₂O₂ was used as the combined inhibitor, sulfuric acid was used as the activator, and NaBX was used as the sulfur collector for validation of the feasibility of the process protocol described above for the comprehensive separation and recovery of pyrite and covellite. Figure 1 shows the test process.

The flotation test results shown in

Table 9. Flotation Test Results of a Copper–Gold Mine in Serbia.

Product	Yield (%)	Grade (%)			Recovery (%)
		Cu	Au (g·t−1)	S	Cu	Au	S
Copper Concentrate 1	7.42	31.12	4.13	34.6	80.74	18.69	12.59
Copper Concentrate 2	4.68	8.51	4.58	36.8	13.93	13.07	8.45
Pyrite Concentrate	24.35	0.47	3.92	49.4	4.00	58.20	58.99
Tailing 2	14.53	0.084	0.5	7.61	0.43	4.43	5.42
Tailing 1	49.02	0.05	0.19	6.05	0.91	5.61	14.55
Feed	100	2.86	1.64	20.39	100	100	100
Copper Concentrate 1 + 2	12.33	22.37	4.30	/	94.66	31.76	21.04
Tailing 1 + 2	63.55	0.06	0.26	6.41	1.34	10.04	19.97

indicated that a change in the pyrite inhibitor from lime alone to lime + H₂O₂ led to an increase in the copper concentrate grade from 30.35% to 31.12%, with recovery remaining almost unchanged. The recovery of pyrite increased from 56.70% to 58.99%, while that of gold increased by nearly 4%. Therefore, the use of the combined lime + H₂O₂ inhibitor effectively increased gold recovery with remarkable results.

4. Conclusions

• Micro-flotation test results indicate that the addition of H₂O₂ reduces the pH required for pyrite suppression from 12 to 10. The combined use of lime and H₂O₂ as an inhibitor system enables efficient separation of covellite from pyrite.
• An investigation into the mechanisms revealed that the added H₂O₂ oxidized and corroded the pyrite surface, leading to increased oxide and hydroxide generation. This contributed to enhanced surface hydrophilicity and thereby to effective pyrite inhibition.
• Pyrite that has been inhibited can partially regain its floatability upon the addition of sulfuric acid or copper sulfate, with sulfuric acid demonstrating greater effectiveness.
• The Serbian copper ore exhibits a high sulfur content. When lime is used alone for pyrite inhibition, pyrite and gold recovery rates remain low. However, employing lime and H₂O₂ as a combined inhibitor reduces lime consumption in industrial operations, maintains copper flotation performance, and increases pyrite recovery by 2.3% and gold recovery by nearly 4%.