Inhibiting Mechanism of High pH on Molybdenite Flotation. An Experimental and DFT Study

Abstract

The inhibiting mechanism of high pH on the molybdenite flotation was studied using an experimental and DFT method. The experimental results found that adverse effects of pH on molybdenite flotation should be attributed to the adsorption of OH⁻ on molybdenite [100] surface (MS100). The DFT results show the adsorption energy of H₂O/OH⁻ to molybdenite [001] surface (MS001) and MS100 is −8.61/288.30 kJ·mol⁻¹ and −226.81/−302.44 kJ·mol⁻¹. These indicate that H₂O is weakly adsorbed on MS001, while OH⁻ is not. Both H₂O and OH⁻ can be adsorbed onto MS100. The adsorption energy of OH⁻ to MS100 is much stronger than that of H₂O. The results of state density and charge transfer of the adsorption of OH⁻ on MS100 further show that OH⁻ can be chemically adsorbed on MS100 through the bonding of the O atom of OH⁻ and the Mo atom of MS100. This causes a significant reduction in the MS100 hydrophobicity and deteriorates the fine molybdenite flotation.

1. Introduction

pH is one of the essential affecting factors in the flotation process [1]. The flotation process can be carried out more efficiently with the appropriate pH. Park et al. showed that with increasing pH of the pulp, the flotation recovery of molybdenite continues to decrease [2]. Qiu and You also suggested that the pulp’s pH will influence the collector’s adsorption behavior on the molybdenite surface, thus affecting its flotation performance [3,4]. However, in the absence of a collector, the flotation recovery of molybdenite still continuously decreases as the pulp’s pH increases [5]. This indicates that the inhibitory role of high pH on molybdenite flotation does not only reduce the adsorption behavior of the collectors on the molybdenite surface but also changes the surface properties of the molybdenite.

Lin and Hao found that these changes in the molybdenite surface properties are mainly reflected in the increasing electronegativity and deteriorating hydrophobicity of the molybdenite particles as the pulp pH increases [6,7]. They speculated that alkaline conditions may improve the oxidation performance of molybdenite surfaces and cause molybdenite particles to be more hydrophilic and have lower floatability. However, the mechanism of the effect of pH on the molybdenite flotation under collectorless conditions has not been reported. From another point of view, pH also represents the hydroxide ion (OH⁻) concentration, indicating that OH⁻ concentration in pulp may also affect molybdenite flotation.

Molybdenite belongs to a hexagonal crystal system and has a typical layered structure [8,9]. The surface anisotropy of molybdenite on a macroscopic scale has been known for a long time [10,11]. This means that the molybdenite [001] surface (MS001) is a nonpolar surface with strong hydrophobicity, while MS100 is a polar surface with strong hydrophilicity [12]. The difference between MS001 and MS100 is caused by the fact that MS001 breaks molecular bonds while MS100 breaks covalent bonds during the grinding process [13]. Therefore, the adsorption behavior of OH⁻ on the MS001 and MS100 is bound to differ. Moreover, the adsorption mechanism of OH⁻ on the MS001/MS100 has not been reported.

In this paper, the effect of pulp pH on molybdenite flotation is first investigated by zeta potential and contact angle, after which the inhibiting mechanism of high pH on MS001/MS100 is analyzed by the density functional theory (DFT). The adsorption energies, bond lengths, Mulliken bond population, and electronic properties (including difference densities, the density of states, and Mulliken charge population) of H₂O and OH⁻ on MS001/MS100 are calculated. The competitive adsorption behaviors between H₂O and OH⁻ on MS001/MS100 are discussed. Finally, the inhibiting mechanism of OH⁻ on the MS001/MS100 is analyzed at the molecular and atomic levels.

2. Materials and Methods

2.1. Materials and Reagents

The pure molybdenite sample (PMS) used in this study was purchased from Huadong Ye’s Stone Specimen Company in Guangdong, China. A portion of bulk molybdenite crystals with a complete crystal structure was used to prepare contact angle test samples. They were flaked and polished to obtain the sample of MS001 and MS100, respectively (the MS001 and MS100 are shown in Figure 1).

The remaining molybdenite crystals were used to prepare samples for flotation and zeta potential tests. They were first crushed and ground in a porcelain ball mill. Then, the PMS was screened to obtain the −0.038 mm fraction for tests and analyses. This material assayed 59 wt% Mo. The XRD result of the purified molybdenite sample shows 99 wt% molybdenite, as shown in Figure 2.

Pine oil was used as frother in molybdenite flotation. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were used to adjust the pH of the water. Synthetic water was used in the experiment. It was prepared by dissolving NaOH and HCl in distilled water.

2.2. Flotation Tests

PMS flotation was conducted in an RK/FGC 40 mL hitch groove flotation cell at a rotating speed of 1750 r/min. In each set of experiments, 2.0 g of −0.038 mm PMS was dispersed in 40 mL synthetic water with different pH values and reacted for 2 min by magnetic stirring. Then, it was poured into a flotation machine, and a certain amount of frother (25 mg/L for all flotation tests) was added, and the flotation time was 3 min. The product was collected, filtered, dried, and weighed. Each test was repeated five times, and the average flotation recovery was reported.

2.3. Zeta Potential Measurements

Zeta potential was measured with a zeta potential analyzer (Nano plus, Micromeritics Instrument Corp., USA). The suspension containing less than 5% PMS was conditioned for 5 min in a solution of pH = 4~12. The suspension was then transferred to a sample vessel for the zeta potential measurement at room temperature (20 °C). The measurement was repeated five times, and the average zeta potential was reported.

2.4. Contact Angle Measurements

The contact angle was measured using a contact angle tester (JY-82C, Chengde Dingsheng Testing Equipment Co Ltd., Chengde, China). The sample of MS001 and MS100 was soaked for 10 min in a solution of pH = 4~12. The sample was then transferred to the viewing platform. Finally, one drop of water was dropped onto the sample’s surface for the contact angle measurement at room temperature (20 °C). The measurement was repeated five times, and the average contact angle was reported.

2.5. Simulation Details

The adsorption model of water molecules and OH⁻/H⁺ on the molybdenite surface (white is the H atom, red is the O atom) is shown in Figure 3. All density functional theory (DFT) calculations were regulated using the Cambridge Serial Total Energy Package (CASTEP) Module in Materials Studio 6.0. The simulation details are consistent with the simulation process mentioned in our published article on minerals engineering [14]; moreover, it can also be found in the Supplementary Materials.

3. Results and Discussion

3.1. Flotation Test

The effect of pH on the flotation of molybdenite was studied, and the results are shown in Figure 4. As can be seen in Figure 4, with the increase in the pulp pH, the recovery curve of molybdenite flotation in the absence of collector tends to be flat and then decreasing. The result is consistent with the results of Chander et al. in the presence of a collector [15]. These indicate that high pH has an inhibiting effect on molybdenite flotation regardless of the presence or absence of a collector. It is well known that high pH equates to high OH⁻ concentrations. Therefore, there may be significant OH⁻ adsorbed on the molybdenite surface at high pH pulp. It can cause a reduction in the floatability of the molybdenite and thus deteriorate the molybdenite flotation.

3.2. Zeta Potential Measurements

The effect of pH on the zeta potential of molybdenite is shown in Figure 5. As the pH of the pulp increases, the zeta potential of the molybdenite surface tends to decrease continuously. This result supports the view that the adsorption of OH⁻ on the molybdenite surface causes a deterioration in the flotation of molybdenite. Moreover, this is also consistent with the Eh–pH diagram for molybdenum in a Mo–H₂O system [16]. However, the decrease in the zeta potential of the molybdenite caused by an increase in pH does not indicate that it caused a decrease in the molybdenite floatability.

3.3. Contact Angle Measurements

The contact angle of MS001/MS100 with pH ≥ 7 is shown in Figure 6.

The results of Figure 6 found that as the pH of the pulp increases, the contact angle of MS001 tends to remain constant, whereas the contact angle of MS100 tends to decrease. This indicates that alkaline conditions have no significant effect on the hydrophobicity of MS001 while causing a decrease in the hydrophobicity of MS100. Therefore, the inhibitory role of high pH on molybdenite flotation may be due to the changing of the MS100 properties. This result is in agreement with the anisotropies of the molybdenite surface [17]. In other words, MS001 is a nonpolar surface with stable surface properties; MS100 is a polar surface with unstable surface properties. The alkaline conditions are favorable to the changing of the MS100 properties. This is the reason for the more hydrophilic and lower floatability of MS100 in alkaline conditions.

3.4. Inhibiting Mechanism of High pH on Molybdenite Flotation

To further study the inhibiting mechanism of high pH on molybdenite flotation, DFT was used to analyze the interaction of water (H₂O) and hydroxide/hydrogen ions (OH⁻/H⁺) with the molybdenite surface at the atomic and electronic levels.

3.4.1. Adsorption Energy of H₂O and OH⁻/H⁺ on MS001/ MS100

The adsorption energies of H₂O and OH⁻/H⁺ on MS001/MS100 are shown in

Table 1. The adsorption energy of H₂O and OH⁻/H⁺ with MS001/MS100.

Adsorbate	Surface	E surface + ion/kJ·mol−1	E ion/kJ·mol−1	E surface/kJ·mol−1	∆E/kJ·mol−1
H2O	001	46,312.59	471.93	5093.40	−8.61
	100	46,286.81	471.93	15,270.84	−226.81
OH−/H+	001	46,309.51	471.93	5093.40	288.30
	100	46,287.59	471.93	15,270.84	−302.44

.

The results in Table 1 show that the interaction energies of H₂O with MS001/MS100 are −8.61/−226.81kJ·mol⁻¹, respectively. The interaction energies of OH⁻/H⁺ with MS001/MS100 are 288.30/−302.44kJ·mol⁻¹, respectively. The adsorption energy shows that MS001 is strongly hydrophobic and MS100 is strongly hydrophilic. According to the Mulliken charge population of the atoms on the MS001 and MS100 (0.2 negative charges on the S-plane and 0.12 positive charge on the Mo-plane), OH⁻ is easily adsorbed on the Mo-plane, and H⁺ is easily adsorbed on the S-plane. Therefore, OH⁻/H⁺ is more easily adsorbed on MS100 than water molecules, enhancing its hydrophilicity.

3.4.2. Electronic Properties of H₂O and OH⁻/H⁺ at MS001/MS100

The electronic properties of H₂O and OH⁻/H⁺ at MS001/MS100 can reveal their adsorption mechanism on molybdenite surfaces at the molecular and electronic levels. It mainly includes the bond lengths, Mulliken bond population, differential density, density of states, and Mulliken charge population.

(1)

Analysis of Water Bonding on the Surface of Molybdenite

The stable adsorption configurations of H₂O in MS001/MS100 are shown in Figure 7 and Figure 8, respectively. To quantitatively characterize the adsorption configuration of H₂O on the MS001/MS100, the Mulliken bond populations and their corresponding bond lengths after stable adsorption of H₂O on the MS001/MS100 are listed in

Table 2. The adsorption energy of H₂O and OH⁻/H⁺ with MS001/MS100.

The Adsorption Configuration	Bond	Population	Length/(nm)
H2O–MS001	H1-S	0.00	2.94672
	H2-S	0.00	2.93816
H2O–MS100	O-Mo	0.10	2.25993
	H1-Mo	0.16	2.70418
	H2-Mo	0.17	2.78119
	H1-S1	0.09	2.34405
	H1-S2	0.02	2.76602
	H1-S3	0.01	2.98579

.

As shown in Figure 7a, when the H₂O is adsorbed on the MS001, the water molecules are far away from the MS001. As shown in Figure 7b, there is essentially no electron cloud density between the hydrogen atom and the S atom of MS001. Figure 7c shows that there are essentially no electrons around the S atom of MS001. There is only weak electron aggregation between the O and H atoms in the water molecule. Combining the results in Table 2 shows that the H atoms of the water molecules are weakly adsorbed to the S atoms on MS001. The corresponding bond lengths are 2.94672 nm and 2.93816 nm, respectively. The corresponding Mulliken bond population is zero. This is consistent with the results for the adsorption energy of water with the MS001 in Table 1 (∆E = −8.61 kJ.mol⁻¹).

Figure 8a shows that when water molecules are adsorbed on the MS100, the water molecules are essentially located between the Mo-plane and the S-plane of MS100. Combining the results in Table 2 shows that the H atoms of H₂O are located almost between the Mo and S surfaces of the MS100. The O atom of H₂O may be bonded to the Mo atom on MS100. H₁ forms a strong hydrogen bonding interaction with S₁ (with a sizeable covalent population). The corresponding bond lengths are 2.25993 nm and 2.34405 nm. The corresponding Mulliken bond population value is 0.10 and 0.09, respectively.

As shown in Figure 8b, there is a high electron cloud density between the O and H atoms of the water molecule and the Mo atoms of the MS100. The corresponding population values are −0.16 and −0.17. A high charge density also exists between the H₁ atom and the S₁ atom.

Figure 8c shows that the electron cloud is located between the water molecules and the MS100. The electron aggregation occurs between the Mo atoms of MS100 and the O atoms in the water and between the hydrogen bonds formed by the H₁ and S₁ atoms. These indicate that the O/H atoms of the water molecule interact with the Mo/S atoms of MS100, respectively.

(2)

Analysis of OH⁻/H⁺ Bonding on the MS100

The results of Table 1 show that the interaction energies of OH⁻/H⁺ with MS001 are 288.30kJ·mol⁻¹. This indicates that OH⁻/H⁺ exhibits no adsorption on MS001. Therefore, only OH⁻/H⁺ bonding on the MS100 needs to be analyzed.

The Mulliken bond populations and their corresponding bond lengths after stable adsorption of OH⁻/H⁺ on the MS100 are listed in

Table 3. Mulliken population and bond length of OH⁻/H⁺ adsorbed on molybdenite surface.

The Adsorption Configuration	Bond	Population	Length/(nm)
OH−/H+–MS100	O-Mo	0.44	1.90191
	H1-Mo	0.27	2.62442
	H2-S1	0.72	1.36465
	H2-S2	0.01	2.60470
	H2-S3	0.00	2.59003

and Figure 9.

Figure 9a shows that OH⁻ and H⁺ can adsorb to the MS100 by bonding to the Mo and S atoms on MS100, respectively. Figure 9b shows a clear charge transfer when the O atom of OH⁻ is bonded to the Mo atom on the MS100. The Mo atom loses its charge point, and the O atom in OH⁻ gains electrons. It is evident from Figure 9c that the O atom in OH⁻ is surrounded by a cloud of electrons. The electrostatic adsorption between the OH⁻ and Mo atoms of MS100 enhances the adsorption of OH⁻ to the Mo-plane of MS100. The S atoms of MS100 exhibit a negative charge, which favors the adsorption of H⁺ or cations on the S-plane of MS100. The results in Table 3 further show that the O atom in OH⁻ can strongly bond with the Mo atom of the Mo-plane, with a population value of 0.44 and bond lengths of 1.90191 nm. The H₂ atom can also strongly bond with the S₁ atom of the S-plane, with a population value of 0.72 and bond lengths of 1.36465 nm. These indicate that strong adsorption of OH⁻/H⁺ occurs with MS100. This is consistent with the calculation results of adsorption energy (∆E(100)-OH⁻/H⁺) = −302.44 kJ·mol⁻¹, showing the strong hydrophilicity of MS100.

(3)

State Density and Charge Transfer of H₂O Adsorption Process

To study the variation of density of states and charge transfer during the adsorption of H₂O on the MS001/MS100, the S atom on MS001 and the H atom in H₂O molecules were selected as the research objects. The S atom on the S-plane of MS100 (MS100-S₁ in the differential density map) and the H atom in the water molecule (H₁) were selected as the research objects (bond population 0.09, interatom distance 2.34405 nm) to analyze the changes in state density and bonding before and after adsorption. Figure 10 shows the density of states before and after the interaction of the H atoms in water with the S atoms on the molybdenite surface.

As shown in Figure 10, after the adsorption of water molecules on MS100, the H atoms interact with the S atoms, causing a decrease in the density of electronic states near their Fermi energy levels. The overall density of states moves towards lower energy levels. This indicates that the energy of the electron orbitals decreases after the adsorption of water molecules on the MS100. Then, the system becomes stable. Meanwhile, between −7.5 and −7.1 eV are the bonding interactions between the H and S atoms, and between 5 and 8 are their antibonding interactions. Both of these interactions are weak, but the bonding interaction is stronger than the antibonding interaction. Therefore, the interaction between water molecules and molybdenite is in the form of a bonding interaction, with a bonding population of 0.09.

Table 4. The charge transfer during the adsorption of H₂O on the MS001.

Atomic	State	s	p	d	Population	Charge/(e)
H1	Before	0.47	0.00	0.00	0.47	0.53
	After	0.51	0.00	0.00	0.51	0.49
H2	Before	0.47	0.00	0.00	0.47	0.53
	After	0.51	0.00	0.00	0.51	0.49
S	Before	1.86	4.16	0.00	6.02	0.02
	After	1.85	4.18	0.00	6.03	0.03

and

Table 5. The charge transfer during the adsorption of H₂O on the MS100.

Atomic	State	s	p	d	Population	Charge/(e)
O	Before	1.89	5.18	0.00	7.07	1.07
	After	1.85	4.99	0.00	6.83	0.83
H1	Before	0.47	0.00	0.00	0.47	0.53
	After	0.57	0.00	0.00	0.57	0.43
H2	Before	0.47	0.00	0.00	0.47	0.53
	After	0.54	0.00	0.00	0.54	0.46
S1	Before	1.88	4.32	0.00	6.20	0.20
	After	1.86	4.42	0.00	6.28	0.28
S2	Before	1.86	4.17	0.00	6.03	0.03
	After	1.85	4.17	0.00	6.03	0.03
S3	Before	1.86	4.17	0.00	6.03	0.03
	After	1.85	4.15	0.00	6.00	0.00
Mo	Before	2.44	6.26	5.12	13.82	0.18
	After	2.43	6.29	5.04	13.76	0.24

show the Mulliken charges of the bonding atoms involved before and after water adsorption on the MS001/MS100, respectively.

As shown in Table 4, when the water molecules are adsorbed on the MS001, the density of states of the S atoms on the MS001 remains essentially unchanged. There is no significant charge transfer from the MS001. Combined with the results in Figure 10, there is no charge transfer occurring in the vicinity of the O atom in the water molecule by aggregating only a small number of electrons. This suggests that water molecules do not adsorb stably or weakly on the MS001.

Table 5 shows that the 1s orbital population of the H₁ atom in the water molecule increases by 0.1, and the positive charge decreases by 0.1 e. The 3s orbital population of the S₁ atom on the S-plane of MS100 decreases by 0.02, and the 3d orbital population increases by 0.1, increasing the total charge by 0.08 e. This indicates that the H₁ atom loses electrons, and the S₁ atom gains electrons. The result is consistent with the results of Figure 10. These indicate that water molecules can undergo stable adsorption on the MS100.

(4)

State Density and Charge Transfer of OH⁻ Adsorption Process

The results of Table 1, Figure 8, and Figure 9 show that OH⁻/H⁺ exhibits no adsorption on MS001. Therefore, only the adsorption of OH⁻ on MS100 needs to be investigated. The O atom in OH⁻ and the Mo atom on the Mo-plane of MS100 were selected to analyze the density of states of the atoms before and after adsorption. The results are shown in Figure 11.

As shown in Figure 11, both the 4d orbital of the Mo atom and the 2p orbital of the O atom have a high density of states near the Fermi energy level. This indicates that both are highly active. After the interaction between the Mo atom and the O atom, the density of states of the 2p orbital of the O atom decreases and moves to the lower energy level, while the density of states of the 4p and 4d orbitals of the Mo atom decreases and the 4d orbital moves to the higher energy level, resulting in increased antibonding. The intervals of −8.6 to −7.5 eV/−6.1 to −2.5 eV are the bonding interactions between the 2p orbital of the O atom and the 4s/4d orbitals of the Mo atom, respectively.

The OH⁻ ions can adsorb onto the Mo-plane of MS100 by chemisorption and form a hydroxy-molybdenum compound with the Mo atoms. It will cause a drop in the potential of the MS100. This result is consistent with the effect of OH⁻ ion concentration on the zeta potential of molybdenite particles.

To obtain the charge transfer of atoms during the adsorption of OH⁻ on MS100, the Mulliken charge population of the directly bonded atoms was analyzed. The results of the analytical calculations are shown in

Table 6. The charge transfer during the adsorption of OH⁻ on the MS100.

Atomic	State	s	p	d	Population	Charge/(e)
O	Before	1.89	5.18	0.00	7.07	1.07
	After	1.86	4.93	0.00	6.78	0.78
H1	Before	0.47	0.00	0.00	0.47	0.53
	After	0.57	0.00	0.00	0.57	0.43
H2	Before	0.47	0.00	0.00	0.47	0.53
	After	0.95	0.00	0.00	0.95	0.05
S1	Before	1.89	4.32	0.00	6.20	0.20
	After	1.85	4.22	0.00	6.08	0.08
S2	Before	1.86	4.17	0.00	6.03	0.03
	After	1.86	4.20	0.00	6.05	0.05
S3	Before	1.86	4.17	0.00	6.03	0.03
	After	1.85	4.23	0.00	6.09	0.09
Mo	Before	2.44	6.26	5.12	13.82	0.18
	After	2.43	6.32	4.82	13.57	0.43

.

As shown in Table 6, the 1s state of the H atom in OH⁻ gains 0.1e, and the 2p state of the O atom loses 0.25e. The 4s state of the Mo atom loses 0.01e; the 4d state loses 0.3e; the 4p state gains 0.06e; the Mo atom loses a total of 0.25e. This indicates that in the process of OH⁻ adsorption on MS100, in addition to the formation of hydrogen bonds between the H atoms and the S atoms of MS100, there may also be electrostatic adsorption when the OH⁻ is near the Mo-plane of MS100.

4. Conclusions

This paper studied the effect of pH on the flotation of molybdenite by an experimental and DFT method at the molecular and atomic levels and discussed the effect mechanism. The main conclusions are summarized as follows:

(1)

With increasing pH of the pulp, the curve of molybdenite flotation recovery first tends to flatten out and then gradually decreases after pH above 7, the curve of the zeta potential of the molybdenite surface tends to decrease continuously, and the contact angle of MS100 tends to increase and then decrease. These indicate that the effect of pH on molybdenite flotation should be attributed to the adsorption behavior of the OH⁻ on MS100. Both H₂O and OH⁻ can be adsorbed onto the MS100.

(2)

The adsorption energy of the H₂O/OH⁻ to the MS001 and MS100 is −8.61/288.30 kJ·mol−1 and −226.81/−302.44 kJ·mol⁻¹. This indicates that H₂O is weakly adsorbed on the MS001, while OH⁻ is not. The adsorption energy of the OH⁻ to the MS100 is much stronger than that of H₂O.

(3)

The adsorption of H₂O on MS100 is through electrostatic interaction/hydrogen bond between the O/H atom of H₂O and the Mo/S atom of MS100. However, the adsorption of OH⁻ on MS100 is achieved in two steps: the OH⁻ ion is physically adsorbed on the MS100 by electrostatic interaction/hydrogen bonding between the O/H atom of OH⁻ and the Mo/S atom of MS100; then, the OH⁻ ion is further chemically adsorbed on the MS100 through the bonding of the O atom of OH⁻ and the Mo atom of MS100, and reacts to form a hydroxy-molybdenum compound. This causes a significant reduction in the MS100 hydrophobicity and deteriorates the fine molybdenite flotation.