Use of Pb²⁺ as a Selective Activator in Selective Flotation Separation of Specularite, Aegirine, and Chlorite: A DFT Study

Abstract

Despite being one of the main sources of iron, specularite is often associated with gangue minerals such as aegirine and chlorite. Flotation separation is challenging in the mineral processing industry because of the similar surface properties of specularite, aegirine, and chlorite. This study investigates the role and selective activation mechanism of Pb²⁺ in the flotation separation of specularite, aegirine, and chlorite using micro-flotation experiments, solution chemistry calculations, zeta potential analysis, and the density functional theory (DFT). The results of the micro-flotation experiments show that the addition of lead ions can significantly improve the floatability of specularite, but has little impact on the floatability of aegirine and chlorite. Additionally, the solution chemistry calculations results show that PbOH⁺ is the main component of selectively activated specularite. The zeta potential analysis shows that Pb²⁺ is more inclined to adsorption on the surface of specularite, and that more collectors are adsorbed on the surface of specularite after the addition of Pb²⁺. Finally, the DFT calculations show that different chemical bonds are formed during the interaction between CuOH⁺ and the mineral surface, resulting in different adsorption energies.

1. Introduction

Iron has a wide range of industrial applications. In recent years, the demand for and consumption of high-quality iron ores have shown a continuous growth trend with the continuous development of the world economy, deepening resource development, and continuous expansion of the field of iron applications [1,2]. Consequently, high-grade and easily processed iron ore resources are gradually being exhausted, and the efficient development and utilization of low-grade, complex, and refractory iron ore resources has gradually attracted attention [3,4].

Specularite is one of the main sources of iron and an indispensable mineral resource [5]. Specularite is a common iron-bearing mineral with the chemical formula Fe₂O₃ and a cubic crystal system. Specularite is rich in impurities and is often associated with other gangue minerals such as aegirine and chlorite [6,7,8]. Aegirine (NaFeSi₂O₆) is a chain-like iron-containing silicate mineral with a monoclinic system comprising long columnar or needle-shaped crystals [9]. The chemical formula of chlorite can be expressed as Y₃[Z₄O₁₀](OH)₂·Y₃(OH)₆, which is a general term for monoclinic, triclinic, or orthorhombic layered silicate minerals. In the chemical formula, Y is mainly Mg²⁺, Fe²⁺, Al³⁺, and Fe³⁺; and Z is mainly Si²⁺ and Al³⁺, and occasionally Fe³⁺ or B³⁺. The commonly used term chlorite refers only to mineral species in which the main components are Mg and Fe [10]. Owing to the presence of iron in the lattice of aegirine and chlorite, the chemical properties of aegirine and chlorite surfaces are very similar to those of specularite, causing them to exhibit similar floatability during the flotation process. Therefore, the flotation separation of specularite, aegirine, and chlorite has always been a challenge that the mineral processing industry is committed to solving [11,12,13].

Numerous studies have shown that metal ions are often used as auxiliary reagents to improve the selectivity of the flotation reagents [14,15,16]. Our previous study showed that lead ions have a good activation effect on specularite, and the mechanism of action between lead ions and specularite in a sodium oleate system was studied using zeta potential and solution chemical analysis. The adsorption of lead ions and their hydroxyl complexes on the surface of specularite increases the surface potential, promotes the adsorption of sodium oleate on the surface of specularite, and exhibits a certain activation effect on specularite [3]. However, our previous research did not study the effect of lead ions on the flotation separation of specularite, aegirine, and chlorite, and it is difficult to explain the microscopic and deep mechanisms (including the adsorption configuration, chemical bond formation characteristics, and electron transfer) of the selective activation of specularite by lead ions using only test methods. Therefore, density functional theory (DFT) studies are required. DFT is a method used to study the electronic structure of a multielectron system that has been widely used in the flotation process [17,18,19]. Zhao et al. [20] used DFT to verify that the collection effect of cyclohexyl hydroxamic acid for scheelite flotation is better than that of BHA. Li et al. [21] calculated the influence of the structure of six chelating collectors on their collection ability using DFT, and the results showed that the electronic and spatial structures of the atoms are very important for their interactions with metals and minerals. Zhao et al. [22] studied the influence of different adsorption positions of HS⁻ on the surface structure and properties of a smithsonite (101) surface, and revealed the best adsorption site of HS⁻ on the smithsonite surface and its mechanism. The above research results show that the DFT theory has various surface science applications, can be used to explore the roles of reagents and mineral surfaces in the flotation process, and can be used to supplement and improve the interaction mechanism between reagents and mineral surfaces. Therefore, it was reasonable to use DFT calculations to simulate the mechanism of Pb²⁺ action on the surfaces of specularite, aegirine, and chlorite. This study used Pb²⁺ as an activator to explore its role and the selective activation mechanism of Pb²⁺ in the flotation separation of specularite, aegirine, and chlorite in a system employing NaOL as the collector. Moreover, the effects of the lead ions on the flotation separation of specularite, aegirine, and chlorite were investigated using micro-flotation experiments, solution chemistry calculations, zeta potentials, and density functional theory (DFT). This study provides a new perspective on the efficient flotation separation of iron ore and iron-containing gangue minerals, which is conducive to the efficient utilization of iron resources, thereby providing a theoretical basis for their promotion and application in industrialization.

2. Methods

2.1. Materials

The specularite, aegirine, and chlorite ore samples used in this investigation were obtained from mining locations in Yunnan, China. The ore samples were broken down into fragments having a size of 2 mm. To obtain pure samples, impure particles were manually removed under a microscope. The samples were then graded to produce two particle fractions (38–74 µm and 38 µm) by grinding them using a three-headed grinder. The 38–74 µm fraction was used for the micro-flotation experiments, whereas the 38 µm fraction was employed for the thorough mechanistic analyses. The X-ray diffraction (XRD) patterns of the pure samples are shown in Figure 1. The results of this study demonstrate the high purity of the samples, which allows them to be classified as pure individual minerals.

All the reagents used in the tests were of analytical grade. NaOL was used as the collector and lead nitrate (Pb(NO₃)₂) was used as the activator. The solution pH was modified using 0.1 mol/L NaOH or H₂SO₄ solutions.

2.2. Micro-Flotation Experiments

An RK/FGC5-35 flotation machine was used to conduct micro-flotation investigations on the single minerals under an air flow rate of 15 cm³/min and a stirring speed of 1600 rpm. The examination was conducted as follows: (1) the flotation tank was filled with 60 mL of deionized water, 2 g of the mineral sample, and pH-modifying solutions to change the pH of the pulp. (2) After stirring for two minutes, lead ions were added followed by NaOL. (3) A manual froth scraper was then used to collect the concentrate after agitation for 3 min. (4) Finally, the tailings and concentrate were filtered, dried, and weighed. The flotation recovery (%) of minerals was calculated as follows:

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

(1)

where m₁ and m₂ are the masses of the concentrate and tailings, respectively, and each test was repeated three times. To create samples for examination in future studies, the optimal depressant concentration was determined in the micro-flotation experiments.

Visual MINTEQ (version 3.1) software was used to calculate the hydrolytic component distribution of the lead ions in the pulp systems at different pH values. Visual MINTEQ software is widely used to simulate the equilibrium of ions and minerals in environmental water equilibrium solutions. It can theoretically calculate the interaction between chemical substances using thermodynamic data such as equilibrium constants and the Gibbs free energy to determine the morphological distribution of chemical substances through mass-action expressions, thereby predicting the adsorption mode of metal ions on the mineral surface.

2.3. Zeta Potential Experiments

The zeta potential of the specularite sample was assessed at 20 °C using a Malvin Zetasizer (Nano-ZS900; Malvin & Co., Malvern, UK). The typical background electrolyte, KCl, was used (5 × 10⁻³ mol/L). Each experiment involved the addition of 100 mg of a mineral sample to 100 mL of a potassium chloride solution and stirring the mixture using a magnetic stirrer. After adjusting the pH to the correct level, the solution was then treated for 6 min under a predetermined concentration of the reagent (copper ions or NaOL) and at a predefined pH level. Subsequently, the suspension was left standing to allow the larger particles to settle. The zeta potential at room temperature was then determined by pouring the suspension’s tiny mineral particles into a measurement device. At least three measurements were made, and the average zeta potential was calculated.

2.4. DFT Details

Density functional theory (DFT) computations of the flotation reagent adsorption systems on the mineral surfaces were performed using the CASTEP program in Material Studio 8.0 [23]. The electron exchange correlation was described using the Perdew–Burke–Ernzerhof (PBE) function and generalized gradient approximation (GGA), and the interaction between the ions and valence electrons was described using the ultrasoft pseudopotential (USPP). Brillouin zone integrations were performed using the Monkhorst–Pack technique with a (2 × 2 × 1) k-point grid and plane wave cutoff energy of 400 eV. The self-consistent field convergence criterion was set to 1.0 × 10⁻⁶ eV/atom. The Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm and the following convergence conditions were used for geometry optimization: a maximum atom displacement of 1 × 10⁻⁴ nm and total energy convergence within 1.0 × 10⁻⁵ eV/atom.

The surfaces of specularite (001), aegirine (110), and chlorite (001) were chosen as the research objects, and a (2 × 2 × 1) supercell model with periodic borders was built for the simulation to guarantee unrestricted adsorption. Furthermore, the crystal structure of specularite, as shown in Figure 2, was obtained by optimizing the above parameters, where a = b = 9.45 Å, c = 13.75 Å, α = β = 90°, and γ = 120°.

The crystal structure of aegirine, as shown in Figure 3, was obtained by optimizing the above parameters, where a = 10.58 Å, b = 13.06 Å, c = 15.90 Å, α = β = 90°, and γ = 100°.

The crystal structure of chlorite, as shown in Figure 4, was obtained by optimizing the above parameters, where a = 10.0 Å, b = 10.14 Å, c = 27.27 Å, α = β = 90°, and γ = 60°.

For structural optimization, improved metal ion ligands were added to the mineral surfaces to produce a stable adsorption structure. The exchange-correlation function, pseudopotential approach, cutoff energy, and convergence criteria utilized for calcite crystal calculations were applied for energy calculations and geometry optimization of the surface models and interaction systems. To analyze the interaction of the metal ion ligands with the mineral surface, E_interaction, which is related to the adsorption energy, was calculated as follows:

$$E_{interaction}=E_{total}-\left(E_1+E_2\right)$$

(2)

where E₁ is the single-point energies of the metal ion ligand, E₂ is the single-point energy of the mineral crystal, and E_total is the total energy of the interaction system. Therefore, a stable interaction was represented by a negative E_interaction. Moreover, a higher negative energy produces a stronger connection and better inhibitory effect [24,25].

3. Results and Discussion

3.1. Micro-Flotation Results

The effects of the activator concentration (1, 2, 5, 10, and 15 mg/L) and pulp pH on the flotation behavior of the three minerals using NaOL as the collector and Pb²⁺ as the activator are shown in Figure 5.

As shown in Figure 5a, under natural pH conditions (pH = 6–7), the recovery rates of specularite, aegirine, and chlorite without the addition of Pb²⁺ are 68.26, 2.07, and 3.16%, respectively. After the addition of Pb²⁺, the recovery rates of all three minerals increased with increasing Pb²⁺ concentration. When the Pb²⁺ concentration is more than 10 mg/L, the recovery rate of specularite stabilizes. As shown in Figure 5b, the maximum difference in the floatability of specularite and the two types of gangue minerals occurs when the Pb²⁺ concentration is 10 mg/L and the pulp pH is 8. At this point, the recovery rates of specularite, aegirine, and chlorite are 91.69, 10.25, and 5.78%, respectively. These results indicate that the addition of Pb²⁺ can significantly improve the floatability of specularite, but has little effect on the floatability of aegirine and chlorite. In other words, Pb²⁺ can selectively activate the specularite.

3.2. Solution Chemistry Calculations

To explore the morphology and content of the activator lead ions in the solution, Visual MINTEQ (Version 3.1) software was used to calculate the effect of the pH on the morphology and lead ion content in each component of the solution system, and the results are shown in Figure 6.

At a pH of 6, the component content is mainly PbOH⁺ and follows the order: PbOH⁺ > Pb²⁺ > Pb(OH)₂(aq) > Pb₂(OH)₃⁺ > Pb(OH)₃⁻, as shown in Figure 6; this indicates that PbOH⁺ may be the main component of the selectively activated specularite. Based on the calculated ion products, we infer that the chemical reactions occurring in solution are as follows:

$$\mathrm{Pb}{({\mathrm{NO}}_3)}_2\leftrightharpoons {\mathrm{Pb}}^{2+}+2{\mathrm{NO}}_3^{-}$$

(3)

$${\mathrm{Pb}}^{2+}+2{\mathrm{OH}}^{-}\leftrightharpoons \mathrm{Pb}{(\mathrm{OH})}_2\left(\mathrm{aq}\right)$$

(4)

$${\mathrm{Pb}}^{2+}+{\mathrm{OH}}^{-}\leftrightharpoons {\mathrm{PbOH}}^{+}$$

(5)

$$2{\mathrm{Pb}}^{2+}+3{\mathrm{OH}}^{-}\leftrightharpoons {\mathrm{Pb}}_2{(\mathrm{OH})}_3^{+}$$

(6)

$${\mathrm{Pb}}^{2+}+3{\mathrm{OH}}^{-}\leftrightharpoons \mathrm{Pb}{(\mathrm{OH})}_3^{-}$$

(7)

3.3. Zeta Potential Analysis

The adsorption of reagents can lead to changes in the charge of the specularite surface. Therefore, zeta potential testing was used to evaluate the charge changes on the surface of a mineral. Figure 7 shows the relationship between the zeta potential and pH of specularite, aegirine, and chlorite when Pb²⁺ was used as an activator.

Figure 7a shows that the isoelectric point (IEP) of specularite occurs at pH of 4.4. After adding NaOL, the surface potential of specularite decreases over the entire pH range, and the IEP shifts to 3.7. This is mainly because NaOL is a polar molecule that adsorbs and replaces water molecules on the mineral surface and arranges them directionally, thereby forming an additional adsorption dipole layer that changes the interphase potential difference between the remaining charges on the mineral surface. Consequently, a negative shift occurs in the mineral surface potential. After treatment with Pb²⁺, the potential of the specularite shifts significantly and positively over the entire pH range, with an IEP of 8.9, indicating that Pb²⁺ was adsorbed onto the surface of the specularite. After simultaneous treatment with NaOL and Pb²⁺, the zeta potential of specularite decreased significantly over the entire pH range. Additionally, its IEP shifted from 8.9 to 5.1, indicating that a large amount of NaOL was adsorbed onto the surface of the specularite after lead ion activation. Comparing the zeta potential changes of natural specularite and specularite treated with Pb²⁺ before and after NaOL treatment, the zeta potential difference of specularite treated with lead ions is greater than that of natural specularite, particularly in the pH range of 6–8. This indicates that more oleic acid species were adsorbed on the surface of specularite after lead ion treatment, which may have been due to the increase in the number of reaction sites on the surface of specularite after treatment with Pb²⁺. Consequently, the adsorption of oleic acid substances on the surface of the modified minerals was promoted and the hydrophobicity of the specularite particles was improved.

As shown in Figure 7b,c, the zeta potential changes of the aegirine and chlorite treated with sodium oleate or Pb²⁺ are relatively small over the entire pH range compared with those of specularite. This indicates that the addition of Pb²⁺ has a small impact on the adsorption quantity of sodium oleate on the surface of aegirine and chlorite.

3.4. DFT Results

Using the above research methods, the mechanism of action of lead ions and their effects on the adsorption of the NaOL collector on the mineral surfaces were analyzed. However, the underlying mechanisms remain unclear. Therefore, DFT was used to clarify the mechanisms of the interactions between the agents and mineral surfaces from an atomic point of view. The results of the solution chemistry calculations indicate that PbOH⁺ may be the main component of the selectively activated specularite. Therefore, this section discusses the adsorption configuration of PbOH⁺ on the mineral surface, further revealing the selective activation mechanism of metal ion activators on specularite.

Figure 8 and

Table 1. Bonding characteristics and adsorption energy of the interaction between PbOH⁺ and specularite.

Adsorption Energy (eV)	Bond Type	Bond Length (Å)	Population
−3.58	Fe1-O1	1.922	0.29
	Pb1-O2	2.647	0.05
	Pb1-O3	2.661	0.04

show the adsorption configuration and bonding characteristics of PbOH⁺ on the surface of specularite.

According to the calculation results, PbOH⁺ can interact with the O atom and the Fe atom on the surface of specularite to form three chemical bonds, Fe₁-O₁, Pb₁-O₂, and Pb₁-O₃, with bond lengths of 1.922, 2.647, and 2.661 Å, and bond populations of 0.29, 0.05, and 0.04, respectively. The adsorption energy of the interaction between PbOH⁺ and specularite is −3.58 eV, indicating that the reaction occurred spontaneously.

Figure 9 and

Table 2. Bonding characteristics and adsorption energy of the interaction between PbOH⁺ and aegirine.

Adsorption Energy (eV)	Bond Type	Bond Length (Å)	Population
−1.84	Pb1-O2	2.402	0.02
	Fe1-O1	1.965	0.20
	O3-H1	2.504	−0.02

present the adsorption configuration and bonding characteristics of PbOH⁺ on the aegirine surface.

According to the calculation results, PbOH⁺ can interact with the O and Fe atoms on the surface of aegirine to form two chemical bonds, Pb₁-O₂ and Fe₁-O₁, with bond lengths of 2.402 Å and 1.965 Å, and bond populations of 0.02 and 0.20, respectively. The adsorption energy of the interaction between PbOH⁺ and aegirine is −1.84 eV, which is lower than that of PbOH⁺ and specularite (−3.58 eV).

Figure 10 and

Table 3. Bonding characteristics and adsorption energy of the interaction between PbOH⁺ and chlorite.

Adsorption Energy (eV)	Bond Type	Bond Length (Å)	Population
−2.29	Pb1-O1	2.269	0.08

present the adsorption configuration and bonding characteristics of PbOH⁺ on the chlorite surface.

According to the calculation results, PbOH⁺ can interact with the O atom on the surface of chlorite, forming a Pb₁-O₁ bond with a bond length of 2.269 Å and bond population of 0.08. The adsorption energy of the interaction between PbOH⁺ and chlorite is −2.29 eV. These results indicate that the interaction strength between PbOH⁺ and the surface of chlorite is also lower than that of specularite, which may be the main reason why lead ions can selectively activate specularite and have a smaller effect on the flotation of aegirine and chlorite.

To further determine the electron transfer between atoms in the system during the surface bonding process of lead ions with specularite, aegirine, and chlorite, the Mulliken population and charge changes of the relevant atoms in each system were analyzed before and after the action of Pb ions.

The Mulliken populations of the related elements before and after the interaction between PbOH⁺ and the surface of the specularite are shown in Figure 11 and

Table 4. Mulliken population of related atoms before and after the interaction between PbOH⁺ and specularite.

Atom		Mulliken Population			Total	Charge/e
		s	p	d
Pb1	Before	1.96	1.57	10.01	13.53	0.47
	After	1.90	1.29	10	13.19	0.81
Fe1	Before	0.36	0.27	6.53	7.16	0.84
	After	0.27	0.24	6.46	6.98	1.02
O1	Before	1.89	5.06	0	6.95	−0.95
	After	1.89	5.08	0	6.97	−0.97
O2	Before	1.87	4.71	0	6.58	−0.58
	After	1.87	4.73	0	6.60	−0.60
O3	Before	1.87	4.72	0	6.59	−0.59
	After	1.87	4.73	0	6.60	−0.60

.

According to the results, there is a strong interaction between the Fe₁ and O₁ atoms, and between the Pb₁ and O₂ atoms, after the adsorption of PbOH⁺. This is accompanied by an evident shared electron behavior, indicating an interaction between the two forms of Fe-O and Pb-O bonds.

After the interaction between PbOH⁺ and the surface of specularite, the charge of the Fe₁ atom on the surface of specularite increased from 0.84 e to 1.02 e, an increase of 0.18 e, and its electron loss mainly occurs in the Fe 4s orbital. The charge of the O₁ atom in PbOH⁺ decreased from −0.95 e to −0.97 e, a decrease of 0.02 e, and its electrons are mainly located in the O 2p orbital. These results indicate that the charge in the Fe-O bond formed by the adsorption of PbOH⁺ on the surface of specularite is mainly transferred from the 4s orbital of the Fe₁ atom to the 2p orbital of the O₁ atom. Moreover, the charge of the O₂ atoms on the surface of specularite decreased from −0.58 e to −0.60 e, a decrease of 0.02 e. The charge of O₃ atom decreased from −0.59 e to −0.60 e, a decrease of 0.01 e, indicating that the electrons of the O atom on the surface of specularite are mainly located in the O 2p orbital. The charge of the Pb₁ atom is 0.81 e, and compared to that before adsorption, the electron loss is mainly in the Pb 6p orbital. Based on the charge analysis between the above atoms, when PbOH⁺ is adsorbed on the surface of the specularite, the charge of the Pb-O bond formed by the interaction is mainly transferred from the 6p orbital of the Pb₁ atom to the 2p orbitals of the O₂ and O₃ atoms.

The Mulliken populations of related elements before and after the interaction between PbOH⁺ and the aegirine surface are shown in Figure 12 and

Table 5. Mulliken population of related atoms before and after the interaction between PbOH⁺ and aegirine.

Atom		Mulliken Population			Total	Charge/e
		s	p	d
Pb1	Before	1.96	1.57	10.01	13.53	0.47
	After	1.92	0.98	10.01	12.91	1.09
Fe1	Before	0.28	0.35	6.55	7.18	0.82
	After	0.29	0.29	6.46	7.04	0.96
O1	Before	1.89	5.06	0	6.95	−0.95
	After	1.89	5.09	0	6.98	−0.98
O2	Before	1.87	5.04	0	6.91	−0.91
	After	1.87	5.06	0	6.93	−0.93

.

After the adsorption of PbOH⁺, the electronic ability of Fe₁ on the aegirine surface is significantly reduced. In addition, there is a clear shared electronic behavior between the Pb₁ atom in PbOH⁺ and the O₂ atom on the surface of aegirine, indicating that the two interacted to form Fe-O and Pb-O bonds.

After the interaction between PbOH⁺ and the surface of aegirine, the charge of Fe₁ on the surface of aegirine increases from 0.82 e to 0.96 e, an increase of 0.14 e, and its electron loss mainly occurs in the Fe 3d orbital. Additionally, the charge of the O₁ atom in PbOH+ decreases from −0.95 e to −0.98 e, a decrease of 0.03 e, and its electrons are mainly located in the O 2p orbital. These results indicate that the charge in the Fe-O bond formed by the adsorption of PbOH⁺ on the surface of aegirine is mainly transferred from the 3d orbital of the Fe₁ atom to the 2p orbital of the O₁ atom. The charge of the O₂ atom on the surface of aegirine decreases from −0.91 e to −0.93 e, a decrease of 0.02 e, indicating that the electrons of the O atom on the surface of specularite are mainly in the O 2p orbital. The charge of the Pb₁ atom is 1.09 e, and compared to that before adsorption, the electron loss is mainly in the Pb 6p orbital. Based on the charge analysis between the above atoms, when PbOH⁺ is adsorbed on the surface of aegirine, the charge in the Pb-O bond formed by this interaction is mainly transferred from the 6p orbital of the Pb₁ atom to the 2p orbital of the O₂ atom.

The Mulliken populations of the related elements before and after the interaction between PbOH⁺ and the chlorite surface are shown in Figure 13 and listed in

Table 6. Mulliken population of related atoms before and after the interaction between PbOH⁺ and chlorite.

Atom		Mulliken Population			Total	Charge/e
		s	p	d
Pb1	Before	1.96	1.57	10.01	13.53	0.47
	After	1.90	1.00	10.02	12.92	1.08
O1	Before	1.90	4.61	0	6.52	−0.52
	After	1.89	4.78	0	6.67	−0.67

.

After the adsorption of PbOH⁺, the electron acquisition ability of the O1 atom on the surface of chlorite is significantly reduced, and there is a clear shared electron behavior between the Pb₁ atom and O₁ atom on the surface of chlorite, further confirming the formation of the Pb-O bond.

After the interaction between PbOH⁺ and the surface of chlorite, the charge of the O₁ atom on the surface of chlorite decreases from −0.52 e to −0.67 e, a decrease of 0.15 e, indicating that the electrons of the O₁ atom on the surface of chlorite are mainly in the O 2p orbital. The charge of the Pb₁ atom in PbOH⁺ is 1.08 e, and compared to that before adsorption, the electron loss is mainly in the Pb 6p orbital. Based on the charge analysis between the above atoms, it can be concluded that when PbOH⁺ is adsorbed on the surface of the green mud, the charge in the Pb-O bond formed by the interaction is mainly transferred from the 6p orbital of the Pb₁ atom to the 2p orbital of the O₁ atom.

4. Conclusions

In this study, micro-flotation experiments, solution chemistry calculations, zeta potential analysis, and DFT were combined to study the role and selective activation mechanism of Pb²⁺ in the flotation separation of specularite, aegirine, and chlorite. The results of the micro-flotation experiments show that Pb²⁺ can significantly improve the flotation effect of specularite. After the addition of Pb²⁺, the recovery rates of specularite, aegirine, and chlorite are 90.43, 9.87, and 5.32%, respectively. The solution chemistry calculation results indicate that when the pulp pH is 8, PbOH⁺ is the main component of selectively activated mineral flotation. The zeta potential analysis results show that compared to aegirine and chlorite, Pb²⁺ is more inclined to adsorption on the surface of specularite, and after the action of Pb²⁺, more oleic acid species are adsorbed on the surface of specularite. This may be because the number of reaction sites on the surface of specularite increases after treatment with Pb²⁺, promoting the adsorption of oleic acid on the surface of the modified minerals. The DFT results show that PbOH⁺ can interact with the O and Fe atoms on the surface of specularite to form three chemical bonds, Fe₁-O₁, Pb₁-O₂, and Pb₁-O₃, with an adsorption energy of −3.58 eV. Additionally, PbOH⁺ can interact with the O and Fe atoms on the surface of aegirine to form two chemical bonds, Pb₁-O₂ and Fe₁-O₁, with an adsorption energy of −1.84 eV. Finally, PbOH⁺ can also interact with the O atoms on the surface of chlorite to form a Pb-O bond, with an adsorption energy of −2.29 eV. The adsorption energy of the interaction between PbOH⁺ and the surface of specularite is higher than that of aegirine and chlorite. This is the main reason why lead ions can selectively activate the flotation of specularite.