Interaction and Inhibition Mechanism of Sulfuric Acid with Fluorapatite (001) Surface and Dolomite (104) Surface: Flotation Experiments and Molecular Dynamics Simulations

Abstract

The natural wettability of apatite and dolomite and the effect of sulfuric acid (H₂SO₄) and sodium oleate (NaOl) on the floatability and wettability of both minerals were studied using single-mineral flotation and contact angle measurement. The flotation experiments demonstrated that adding NaOl, apatite, and dolomite had good floatability. After adding H₂SO₄, the floatability of apatite decreased significantly. H₂SO₄ effectively inhibits apatite flotation. Contact angle measurements show that the use of H₂SO₄ induces a significant difference in surface wettability between apatite and dolomite. The moderate addition of H₂SO₄ can increase the contact angle of dolomite. In order to study the selective inhibition mechanism of H₂SO₄ in phosphorite flotation, molecular dynamics simulations (MDSs) were conducted to investigate the interaction between H₂SO₄ and fluorapatite and dolomite at the atomic–molecular level. The results of MDSs reveal that H₂SO₄ interacts with Ca sites on both fluorapatite and defective dolomite surfaces, hindering the interaction of NaOl with Ca sites on both mineral surfaces. SO₄²⁻ ions cannot prevent the interaction of oleate ions with Mg sites on dolomite surface. It is worth mentioning that SO₄²⁻ ions occupy the defective vacancies formed due to the dissolution of CO₃²⁻ on the surface of dolomite and interact with Ca sites. The remaining H₂SO₄ is subsequently adsorbed onto the surface of dolomite. Experimental and simulation results show that, due to the interaction of H₂SO₄ and NaOl, the surface of apatite can still undergo hydration forming a water molecule layer and maintaining a macroscopic hydrophilic property. In contrast, the oleate ions form an adsorption layer on dolomite transitioning it from a hydrophilic to a hydrophobic state. During the phosphate flotation process, the addition of an appropriate amount of sulfuric acid can further diminish the hydration of the dolomite surface, so that the surface of dolomite is more hydrophobic.

1. Introduction

Phosphate ore is a crucial raw material for the manufacturing of phosphoric acid. It is primarily utilized in the production of fertilizers and various other phosphorus-containing products, including yellow phosphorus, food additives, and phosphorus-based chemical compounds [1]. With the rapid growth in the world’s population, the demand for phosphate ore in both agricultural production and industry has been steadily increasing. It is reported that approximately 82% of the world’s phosphate ore is used to produce phosphate fertilizers, while the remaining portion is used in industries such as metallurgy, materials, pharmaceuticals, and light industry [2,3]. China is rich in phosphate ore resources, with phosphate ore reserves ranking second in the world [4].

However, it is important to note that phosphate rock resources are characterized by their abundance rather than their richness in China. Most of these resources consist of apatite [5], a mineral often associated with gangue minerals like dolomite. Apatite exhibits a complex structure, fine disseminated particle size, and poses challenges in terms of dissociation [6,7]. The separation of apatite from dolomite is notably challenging, primarily due to their similar surface physical and chemical properties [8].

Due to the low grade of apatite, it cannot be directly used to produce phosphate industrial products, and it requires enrichment through a mineral processing process to meet production requirements. At present, many separation methods are being applied to phosphate ore dressing, but froth flotation remains the most widely used process [9].

In flotation systems, the adsorption of agents onto the mineral surface plays a crucial role in mineral separation. Fatty acid reagents are commonly used as collectors in phosphate ore flotation. However, a challenge arises because both apatite and gangue minerals (such as dolomite, calcite) are calcium minerals, resulting in a similar exposure of their active sites in solution. This similarity leads to poor selectivity of these fatty-acid-type collectors in the flotation of calcium-bearing minerals [10,11]. Directly adding fatty acid collectors cannot effectively separate apatite from dolomite. Therefore, inhibitors must be introduced to facilitate the separation of apatite from other calcium-containing gangue minerals through flotation. In order to achieve efficient separation between apatite and carbonate minerals, extensive research has been conducted on inhibitors.

Liu et al. [12] used 2-phosphobutane-1,2,4-tricarboxylic acid (PBTCA) as an inhibitor to separate apatite and calcite. The flotation results showed that PBTCA reduced the adsorption of NaOl on the surface of calcite, achieving effective separation of apatite and calcite. Wang et al. [13] utilized carboxymethyl chitosan as a selective inhibitor to separate apatite and calcite. Their research found that the -COO⁻ of the inhibitor was adsorbed on the surface of calcite through chemical chelation at the Ca site, while carboxymethyl chitosan was adsorbed on the surface of apatite through hydrogen bonding. Both apatite and carboxymethyl chitosan were negatively charged, resulting in strong electrostatic repulsion and steric hindrance between carboxymethyl chitosan and apatite. This made it more difficult for carboxymethyl chitosan to adsorb on the surface of apatite. Zhong et al. [14] used sodium alginate as an inhibitor to separate apatite and dolomite. The flotation results showed that, within the pH range of 8–11, sodium alginate could completely inhibit dolomite, while having a minimal effect on apatite flotation.

Depending on the type of accompanying gangue minerals, different flotation processes are used to separate phosphate rock from these gangue minerals. For phosphate rock with gangue minerals mainly consisting of dolomite, reverse flotation is the most effective and widely used method of separating dolomite from phosphate rock [15]. In practical production, H₂SO₄ is commonly used as an inhibitor, and then fatty acid collectors are used in weakly acidic environments to separate carbonate gangue minerals such as dolomite from apatite [6]. Currently, there is a greater focus on researching the interaction between H₂SO₄ and apatite [16,17], but the interaction mechanism between H₂SO₄ and dolomite remains unclear. In recent years, computer simulation technology has advanced rapidly, and using molecular simulation to characterize the adsorption process and interface interactions of reagents on mineral particle surfaces has proven to be an effective research method [18]. For instance, Xie et al. [19] used DFT (density functional theory) to study the effects of carbon chain length, carbon chain configuration, and the number of C=C double bonds on the surface of fluorapatite (FAP). Their results showed that the fatty acid collector formed a stable adsorption configuration on the FAP surface, with the O of the fatty acid collector undergoing chemical adsorption with Ca1 on the FAP (001) surface. The H of the fatty acid collector underwent hydrogen bond adsorption with O on the FAP (001) surface. Molecular simulation has the capability to depict phenomena that are not observable in experiments at the molecular atomic scale, thus providing a better explanation for certain macroscopic experimental results. In addition, the results of molecular simulation can offer valuable guidance for our experimental research.

In this paper, the interactions between H₂SO₄ and the surfaces of apatite and dolomite were elucidated at the molecular–atomic level through molecular dynamics simulations using fluorapatite (001) and dolomite (104) as the objects of this study, which revealed the mechanism of selective inhibition of H₂SO₄ and elucidated the interactions between H₂SO₄, NaOl, and apatite and dolomite.

2. Materials and Methods

2.1. Mineral Samples

The experimental ore sample is a medium–low-grade calcium–magnesium phosphate ore taken from a phosphate mine in Guizhou, China. As can be seen from Figure 1, the valuable mineral in the original ore is fluorapatite (Ca₅F(PO4)₃), while dolomite (CaMg(CO₃)₂) is the associated vein mineral.

Following the initial ore crushing, representative single-mineral samples of apatite and dolomite were selected. The chemical multi-element analysis results of these single minerals of apatite and dolomite, as listed in

Table 1. Single-mineral chemical multi-element analysis/%.

Element	P2O5	CaO	MgO	SiO2	Fe2O3	Al2O3	K2O	Na2O	Others
Apatite	37.35	46.31	0.31	0.77	0.09	1.17	0.01	0.06	13.93
Dolomite	0.08	26.54	20.83	2.81	0.02	0.75	0.00	0.09	48.88

, indicate that the apatite contains 37.35% P₂O₅ and 0.31% MgO, while dolomite contains 20.83% MgO. The purity of the apatite and the dolomite was calculated to be 94.31% and 95.81%, respectively, which met the requirement for the subsequent tests.

2.2. Contact Angle Measurement

Large pieces of collophane and dolomite samples were selected and initially polished using sandpaper. Subsequently, the polished specimens were immersed in distilled water and subjected to ultrasonic cleaning to ensure that the polished surfaces were free from contaminants.

For contact angle measurements, the sessile drop method was employed. The test specimen was immersed in a reagent solution of the desired concentration for 10 min and then dried. The sessile drop contact angle was then measured. Three measurements were carried out for each reagent solution condition, and the average value was reported.

2.3. Single-Mineral Flotation

The single-mineral flotation experiments involving apatite and dolomite were conducted using an XFGC II inflatable hanging cell flotation machine manufactured by Jilin Prospecting Machinery Factory, China. The mixing speed was set at 1992 RPM. In each test, 2 g of individual minerals and 40 mL of deionized water were placed into a flotation cell. Prior to adding the reagents, the pulp was stirred for 30 s to ensure the proper dispersion of mineral particles in the deionized water. Subsequently, the inhibitor was added, followed by a 30 s conditioning period, after which the collector was introduced before initiating the flotation process. The floated product was manually skimmed off from the top of the cell. The resulting flotation process lasted for 5 min. The products within the flotation cell and the floated products were then filtered, dried, and weighed to calculate the flotation recovery. Each flotation condition experiment was repeated three times under identical experimental conditions, and the average value was determined.

2.4. Molecular Dynamics Simulation

2.4.1. Simulation Method

The Forcite module in MS software was used for MDS calculations. For all simulation calculations, the COMPASSII force field, NVT ensemble, and Velocity Scale thermostat methods were used. Atom-based and Ewald methods were used to calculate van der Waals and electrostatic interactions, with an accuracy of Fine. During dynamic simulation calculations on the system, the lower layer of fluorapatite and dolomite were immobilized, and a simulation time step of 1.0 fs was used to calculate the motion equations. The total simulation time for each simulation process was 500 ps, and after the simulation was completed, the final 100 ps trajectory file was selected for the calculation and analysis of the relative concentration distribution.

2.4.2. Simulation Model

The crystal models of both the minerals were sourced from the AMCSD database. The original lattice parameters for fluorapatite were a = b = 9.397 Å and c = 6.878 Å, while the original lattice parameters for dolomite were a = b = 4.815 Å and c = 16.119 Å. After optimization, the lattice parameters for fluorapatite became a = b = 9.444 Å and c = 6.887 Å, and, for dolomite, they were a = b = 4.823 Å and c = 16.046 Å. These optimized lattice parameters closely matched the original lattice parameters. To prepare the system, the fluorapatite and dolomite mineral crystals were cleaved along their most common cleavage planes, expanding the supercell. The Amorphous Cell module was used to construct the solution model of the pharmaceutical solution. In addition, the Build Layers tool was used to construct the pharmaceutical solution model and incorporated fluorapatite or dolomite to form the fluorapatite–/dolomite–pharmaceutical solution system. The number of water molecules in the solution model was 2000; the number of H₂SO₄ molecules was 100; and the number of NaOl molecules was 20. The optimized configurations for sulfate and oleic acid ions are shown in Figure 2.

To prevent any effects from periodic boundary conditions, a vacuum section with a thickness of 20 Å was added at the top of the system. Subsequently, the constructed system was geometrically optimized and dynamically simulated using the Forcite module to obtain the lowest-energy-state configuration. The constructed model is shown in Figure 3.

3. Results and Discussion

3.1. Study on Flotation Behavior of Apatite and Dolomite

The impact of various dosages of NaOl on the flotation ratio of apatite and dolomite is depicted in Figure 4. From the effect of NaOl dosage on the floatation ratio of apatite and dolomite shown in Figure 4a, it follows that the floatation ratio of apatite and dolomite increases with an increase in NaOl dosage, which indicates that NaOl has good collection performance for apatite and dolomite. The agent has no selectivity for apatite and dolomite. When the dosage of NaOl was 150 mg/L, the floatation ratio of apatite and dolomite exceed 95%. Therefore, the appropriate dosage of NaOl was chosen as 150 mg/L. When the dosage of NaOl was below 150 mg/L, the floatation ratio of apatite was higher than that of dolomite. This could be due to the superior floatation properties of apatite compared with dolomite. When the dosage of NaOl exceeded 150 mg/L, the floatation ratio of apatite showed a downward trend.

The floatation ratio of apatite and dolomite under different H₂SO₄ dosages with 150 mg/L NaOl is shown in Figure 4b. It can be seen that, without the addition of H₂SO₄, the floatation ratios of apatite and dolomite were very close, with both having higher floatation ratios of over 90%. The addition of H₂SO₄ resulted in a decrease in the floatation ratio of apatite, while the floatation ratio of the dolomite was essentially unchanged. As the dosage of H₂SO₄ increased, there was a continued decrease in the floatation ratio of apatite. In the presence of 110 mg/L H₂SO₄, the floatation ratios of apatite and dolomite were 6.38% and 91.73%, which represents a decrease of 86.16% and 1.93% compared that with the absence of H₂SO₄. The decreasing trend in the apatite flotation ratio slows down when the H₂SO₄ dosage exceeds 100 mg/L. The apatite flotation ratio remained essentially unchanged as the H₂SO₄ dosage continued to increase. The effect of H₂SO₄ on the floating ratio of dolomite was slight, and the floating ratio of dolomite remained above 90%.

3.2. Effect of Reagent on Wettability of Mineral Surface

Figure 5 shows the effect of H₂SO₄ and NaOl on the wettability of apatite and dolomite. Without adding any reagents, the contact angle of apatite was 56° and that of dolomite was 61°. The natural contact angles of apatite and dolomite show that they have good hydrophilicity. The natural contact angle of apatite, tested before, is about 10°. The contact angle of apatite tested in this study is greater than the theoretical value for apatite or dolomite. The apatite and dolomite used in the experiment were derived from sedimentary phosphate rock; in the complex sedimentary environment, some minerals, such as fluorapatite and dolomite, frequently undergo intricate ion exchange, organic matter or other impurity deposition, and infection. These conditions may be responsible for the increased contact angle of both minerals [20].

As shown in Figure 5a. The contact angle of apatite and dolomite increased with an increasing dosage of NaOl when only NaOl was added. This shows that the surface of apatite and dolomite changed from hydrophilic to hydrophobic under the action of NaOl. The hydration of the surface of apatite and dolomite can be improved by NaOl. When the dosage of the collector was 150 mg/L, the contact angle of the apatite was 92°, and the contact angle of the dolomite was 85°. Compared with the situation without the addition of the reagent, the contact angle of apatite and dolomite increased by 36° and 24°. When the dosage of NaOl was more than 150 mg/L, the contact angle of the apatite decreased. The main reason is that NaOl forms a large cylindrical micelle adsorption layer with a bimolecular layer on the surface of apatite when the amount of NaOl exceeds a certain concentration. The formation of multilayered large cylindrical micelles led to a decrease in the hydrophobicity of the collophane surface [21].

Figure 5b shows that, at a NaOl dosage of 150 mg/L, the contact angle of apatite decreased as the H₂SO₄ dosage increased, while the contact angle of dolomite increased slightly. At a dosage of 110 mg/L of H₂SO₄, the contact angles of collophane and dolomite were 23.3° and 88°, respectively. The contact angle difference between both minerals was 64.7°. The adsorption of NaOl on the surface of apatite was hindered by H₂SO₄. As a consequence, the surface of apatite remained hydrophilic. Following the adsorption of H₂SO₄ on the surface of dolomite, NaOl could still adsorb on the surface of dolomite to render it hydrophobic. The interaction of H₂SO₄ and NaOl can selectively regulate the hydration of apatite and dolomite. It is worth noting that the contact angle of dolomite was found to increase slowly as the H₂SO₄ dosage was increased during the test. The contact angle of dolomite was the largest at a H₂SO₄ dosage of 100 mg/L. When the H₂SO₄ dosage was further increased, the contact angle of dolomite decreased, but it was still higher than the previous contact angle. The contact angle of dolomite can be increased by adding H₂SO₄, and the hydrophobicity of dolomite can be improved. Zou et al. [17] also observed the same phenomenon in the contact angle test in their study, in which an increase in H₂SO₄ dosage increased the contact angle of dolomite, but they did not provide a reasonable explanation. The test results are consistent with the contact angle test results in Zou’s research.

3.3. Molecular Dynamics Simulation of Fluorapatite–/Dolomite–Agent Solution System

Figure 6 shows the molecular dynamics configuration of the fluorapatite–/dolomite–H₂SO₄ system. The CO₃²⁻ on the surface of dolomite is unstable in acid solution. In H₂SO₄ solution, CO₃²⁻ on the surface of dolomite will dissolve into CO₂ and H₂O. There will be defects on the surface of dolomite [22]. The adsorption of SO₄²⁻ on the perfect dolomite surface and the defective dolomite surface was compared. According to the equilibrium configuration and bond population calculated using the system’s molecular dynamics, theO atoms in SO₄²⁻ react with the Ca atoms on the fluorapatite surface to form Ca-O bonds with partial bond lengths of 1.817 and 1.868 Å. As shown in the black circle in Figure 6a. On the perfect dolomite surface, SO₄²⁻ did not bond with Ca/Mg sites, and SO₄²⁻ did not adsorb on the dolomite surface. On defective dolomite surfaces, SO₄²⁻ interacted with Ca sites with partial Ca-O bond lengths of 1.856 and 2.109 Å. As shown in the black circle in Figure 6c. The simulation results of the MDS indicate that SO₄²⁻ interacted with the Mg sites. Nevertheless, MgSO₄ displayed a high solubility [17,22]. In the real solution, the Mg present on the surface of dolomite reacted with SO₄²⁻ to produce MgSO₄, which then dissolved.

The SO₄²⁻ interacted with fluorapatite and defective dolomite but not with the perfect dolomite surface. According to the adsorption configuration of SO₄²⁻ on the surface of defective dolomite, SO₄²⁻ mainly occupied the defect position produced by the dissolution of CO₃²⁻ on the surface of dolomite and acted on the Ca site, the remaining SO₄²⁻ was adsorbed on the dolomite surface. The SO₄²⁻ adsorption amount on the dolomite surface was lower than that on the fluorapatite surface. The interaction between H₂SO₄ and dolomite was further demonstrated by molecular dynamics simulations. The SO₄²⁻ formed hydrogen bonds with water molecules in the solution, with some bond lengths of 1.906, 1.995, and 2.223 Å. Zou et al. [17] used TOF-SIMS to measure the surface of H₂SO₄ treated dolomite and also found the presence of SO₄⁻ on the surface, indicating that SO₄²⁻ can adsorb on the surface of dolomite. TOF-SIMS is a highly sensitive technique to determine the 2D/3D distributions of chemical fragments on a solid surface. The molecular dynamics simulation results are consistent with their test results.

The interfacial adsorption energies of fluorapatite and dolomite with H₂SO₄ were calculated, and the results are listed in

Table 2. Fluorapatite–/dolomite–H₂SO₄ interface adsorption energy.

System	Eads/kcal·mol−1
Fluorapatite–H2SO4	−117,830.60
Perfect dolomite–H2SO4	1.90
Defective dolomite–H2SO4	−100,334.92

. The adsorption energy at the fluorapatite–H₂SO₄ interface was −117,830.60 kcal·mol⁻¹; at the perfect dolomite–H₂SO₄ interface, it was 1.90 kcal·mol⁻¹, and at the defective dolomite–H₂SO₄ interface it was −100,334.92 kcal·mol⁻¹. The adsorption energies at the fluorapatite–/defective dolomite–H₂SO₄ interface were all negative, and the adsorption energy at the perfect dolomite–H₂SO₄ interface was positive. It indicates that H₂SO₄ can spontaneously adsorb on the surfaces of fluorapatite and defective dolomite. Cao et al. [23] studied the interactions of individual sulfate molecules with dolomite by using the density flood theory and also found that sulfate adsorption at the surface of perfect dolomite is not an energy-favorable process. The results of the present study are in agreement with the results obtained by them. The SO₄²⁻ does not interact with perfect dolomite. In actual flotation, perfect crystal surfaces are rare after the ore has been crushed, milled, and subjected to the action of H₂SO₄ in the flotation process. Therefore, defective dolomite surfaces were chosen for all subsequent simulations.

Figure 7 shows the configuration of the fluorapatite–/dolomite–H₂SO₄ and NaOl solution system after the molecular dynamics calculations were completed. According to the kinetic calculation equilibrium configuration and bond population analysis, it can be seen that the Ca site on the surface of fluorapatite was not bonded with the oleate ions. The main components on the surface of fluorapatite were SO₄²⁻ and H₂O. A large number of oleate ions were adsorbed on the surface of the dolomite, and the Mg-O bonds were formed by the interaction of oleate ions with Mg atoms on the surface of the dolomite. Some of the Mg-O bonds were 1.610 and 1.680 Å in length, and as shown in the black circle in Figure 7b. The surface of dolomite was mainly composed of SO₄²⁻ and oleate ions.

As shown in

Table 3. Adsorption energy of fluorapatite–/dolomite–oleate ion interface.

System	Eads/kcal·mol−1
Fluorapatite–oleate ion	4312.55
Defect dolomite–oleate ion	−30,891.66

, the interfacial adsorption energies of fluorapatite–/dolomite–oleate ions were 4312.55 and −30,891.66 kcal·mol⁻¹, respectively. The interfacial adsorption energy of fluorapatite–oleate ions also provided further evidence that oleate ions did not interact with the fluorapatite surface. The interaction of oleate ions with the Ca sites on the fluorapatite surface was prevented by the interaction of H₂SO₄ with the Ca sites. The interaction of oleate with the Mg sites on the dolomite surface was not affected by H₂SO₄.

The trajectory files obtained after the molecular dynamics simulation can be calculated and analyzed for the relative concentrations and root mean square displacements (MSD) of water molecules and agent molecules on the mineral surface, which can be used to characterize the adsorption structure, kinematic properties, and adsorption tightness of agent molecules on the mineral surface [24,25,26].

Figure 8 shows the relative concentration profiles of SO₄²⁻ in the H₂SO₄ solution on the surface of the fluorapatite and dolomite along the z-axis, which were calculated from the trajectory files at the end of the simulation. The crystal thicknesses of fluorapatite and dolomite were 1.87 and 1.71 nm. The analysis of the relative concentration distribution showed that the first peak of SO₄²⁻ on the surface of fluorapatite appeared at 2.05 nm along the z-axis on the surface of fluorapatite, which indicates that sulfate adsorbed on the surface of fluorapatite. On the dolomite surface, the first peak of the SO₄²⁻ concentration distribution curve appeared at 1.64 nm along the z-axis. The first peak of SO₄²⁻ concentration appeared in the dolomite crystal, and there were two obvious peaks for SO₄²⁻. It is further demonstrated that SO₄²⁻ first occupied the defective vacancies created by the dissolution of CO₃²⁻ and bound to the exposed Ca sites. The remaining SO₄²⁻ was adsorbed on the dolomite surface. The relative concentration distribution curve of water molecules can be used to characterize the intensity of hydration and the thickness of the hydration film formed on the mineral surface [24]. There was a peak in the relative concentration distribution curve of water molecules after the action of H₂SO₄. This demonstrates that hydration was present on the surfaces of fluorapatite and dolomite and that both minerals were capable of forming hydrated films on their surfaces.

Figure 9 shows the relative concentration distribution curves of SO₄²⁻, oleate ions, and water molecules in H₂SO₄ and NaOl solutions on the surface of both minerals along the z-axis direction. The peak of the relative concentration distribution curve of oleate ions on the dolomite surface was very high. Combined with the analysis of the adsorption configuration, it can be seen that the hydrophobic end of the hydrophobicity of the oleate ions faced upward after interacting with the Mg sites on the dolomite surface. In the presence of high concentrations of oleate ions in the solution, the oleate ions interacted with the Mg sites on the dolostone surface to form an adsorbent layer, rendering the dolostone hydrophobic.

The relative concentrations of water molecules on the surface of the fluorapatite peak are shown in Figure 9. The fact that the water molecules were denser on the surface of fluorapatite is evident from the adsorption configuration of both systems. The results showed that the surface of fluorapatite was still hydrated after interaction with H₂SO₄ and NaOl. The relative concentration distribution curve of water molecules on the surface of dolomite had no peak value, and there was no hydration on the surface of dolomite after the adsorption of oleate ions. The relative concentration distribution curves explain the decrease in the floatation ratio and contact angle of apatite after the addition of H₂SO₄ in the single-mineral flotation and the contact angle measurement. The simulation results were in agreement with the results of single-mineral flotation and contact angle measurement. In the reverse flotation of phosphate rock, the interaction between oleate ions and the surface of apatite was hindered by H₂SO₄ to keep it hydrophilic, and the apatite particles in the pulp were hydrophilic and sank. The interaction between oleate ions and Mg sites on the surface of dolomite was not affected by H₂SO₄. A large amount of oleate ion was adsorbed on the surface of dolomite, which made the surface of dolomite have good hydrophobicity. The dolomite particles adhered to bubbles and floated up; the flotation models of apatite and dolomite are presented in Figure 10.

Figure 11 shows the relative concentration distribution curves of water molecules on the dolomite surface along the z-axis direction for different amounts of H₂SO₄. The relative concentration of water molecules on the dolomite surface was larger when the sulfate was less, and the relative concentration of water molecules on the surface decreased on continuing to increase the amount of H₂SO₄. Combining the equilibrium adsorption configuration analysis, when the solution is free of H₂SO₄, all the exposed Ca/Mg sites on the dolomite surface will interact with oleate ions, forming a tight adsorption layer on the surface. In the presence of H₂SO₄ in the solution, the SO₄²⁻ interacts with the Ca sites on the surface of dolomite. After the sulfate ions are adsorbed on the surface of dolomite, the active sites on the surface of dolomite decrease, and the adsorption layer formed by the action of oleate ions becomes looser. Some water molecules can break through the adsorption layer of oleate ions and adsorb on the surface of dolomite. Water molecules will interact with SO₄²⁻ to adsorb onto the surface of the dolomite and water molecules will also interact with SO₄²⁻ adsorbed onto the surface to form hydrogen bonds if the amount of H₂SO₄ in the solution is low. When there is more H₂SO₄ in the solution, the defect space on the dolomite surface is completely occupied, SO₄²⁻ is adsorbed on the dolomite surface, and the SO₄²⁻ adsorbed on the dolomite surface hinders the interaction between water molecules and Ca sites on the dolomite surface; the water molecules can only interact with surface-adsorbed SO₄²⁻ radicals. This may be the reason why the contact angle of dolomite increased with increasing H₂SO₄ content and decreased again with increasing H₂SO₄ content.

4. Conclusions

NaOl enhances the floatability and hydrophobicity of apatite and dolomite, but it does not show selectivity between them. H₂SO₄ effectively inhibits apatite, leading to significant differences in the floatability and wettability of both minerals. Following the sulfuric acid treatment, the contact angle of dolomite slightly increases, and H₂SO₄ can interact with dolomite to slightly enhance its hydrophobicity.

The SO₄²⁻ ions can adsorb on the surface of fluorapatite and defective dolomite simultaneously. Initially, SO₄²⁻ ions occupy the defect position after CO₃²⁻ dissolution on the surface of dolomite and then interact with Ca sites. The interaction of SO₄²⁻ ions with the fluorapatite and dolomite surfaces hinders the interaction of the oleate ions with the Ca sites on the surface but does not hinder the interaction of oleate ions with the Mg site on the surface of the dolomite. The simulation results revealed the reasons for the significant differences in the floatability and wettability of the two minerals after the addition of H₂SO₄ and NaOl. The adsorption of H₂SO₄ on the surface of apatite hindered the adsorption of NaOl, maintaining hydration on the surface to form a water molecular layer and keeping it hydrophilic. After the action of H₂SO₄, NaOl can still interact with dolomite. The formation of an oleate ion adsorption layer on the surface of dolomite hindered the formation of the water molecular layer and made it hydrophobic. The concentration of water molecules on the surface of dolomite can be slightly increased after the interaction between SO₄²⁻ ions and the surface of the dolomite, which can well explain the reason for the increase in the contact angle of dolomite after adding H₂SO₄.