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Article

New Insights into the Depressive Mechanism of Sodium Silicate on Bastnaesite, Parisite, and Fluorite: Experimental and DFT Study

by
Jieliang Wang
1,2,3,
Wenda Lu
1,2,
Zhao Cao
1,2,3,*,
Xu Wu
1,2,3,*,
Peng Wang
1,2,3,
Xiaoping Wang
1,2,3 and
Wenli Liu
1,4
1
School of Mining and Coal Engineering, Inner Mongolia University of Science and Technology, Baotou 014010, China
2
Inner Mongolia Key Laboratory of Mining Engineering, Baotou 014010, China
3
Key Laboratory of Green Extraction & Efficient Utilization of Light Rare-Earth Resources, Inner Mongolia University of Science and Technology, Ministry of Education, Baotou 014010, China
4
Baotou Steel Group Baoshan Mining Limited Corporation, Bayan Obo, Baotou 014080, China
*
Authors to whom correspondence should be addressed.
Minerals 2024, 14(9), 870; https://doi.org/10.3390/min14090870
Submission received: 21 July 2024 / Revised: 21 August 2024 / Accepted: 22 August 2024 / Published: 27 August 2024
(This article belongs to the Special Issue Selected Papers from the 7th International Conference of Young Scholars in Mineral Processing)
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Abstract

:
The surface properties of bastnaesite and parisite are similar to their associated gangue mineral, fluorite, which makes the flotation separation of these two rare earth minerals from fluorite one of the industry’s most significant challenges. This study systematically investigates the inhibitory effects and mechanisms of sodium silicate (SS) on bastnaesite, parisite, and fluorite in an octyl hydroxamic acid (OHA) collector system through flotation experiments, various modern analytical methods, and DFT simulations. The flotation test results indicate that the inhibition effects of SS on the three minerals are in the order: fluorite > parisite > bastnaesite. Detection and analysis results indicate that SS forms hydrophilic complexes with Ca atoms on the surfaces of fluorite and parisite, enhancing surface hydrophilicity and inhibiting OHA adsorption, but its impact on bastnaesite is relatively minor. DFT simulation results show that OHA forms covalent bonds with metal ions on mineral surfaces, favoring five-membered hydroxamic-(O-O)-Ce/Ca complexes, and reacts more strongly with Ce atoms than Ca atoms. SS primarily forms covalent bonds with metal atoms on mineral surfaces via the SiO(OH)3− component, and OHA and SS compete for adsorption on the mineral surfaces. OHA has a stronger affinity for bastnaesite, whereas SS shows the highest affinity for fluorite, followed by parisite, and the weakest affinity for bastnaesite.

1. Introduction

Rare earth elements, including the lanthanides as well as scandium and yttrium, comprise seventeen metal elements in the periodic table and are nonrenewable strategic mineral resources [1]. They are widely used in various industries such as metallurgy, petrochemicals, defense, new materials, and new energy [2]. Bastnaesite and parisite are two primary types of rare earth fluorocarbonate minerals, both of which possess significant industrial value and market potential [1,3]. Bastnaesite (Ce,La)FCO3 and parisite Ca(Ce,La)2[CO3]3F2 typically coexist with the calcium-bearing mineral fluorite CaF2. The similarities in elemental compositions and surface physicochemical properties of these three minerals make their effective separation a major technological challenge in the rare earth mineral processing industry.
Currently, flotation is the primary method for separating bastnaesite, parisite, and calcium-bearing gangue minerals [1,4]. In past rare earth flotation studies, common collectors mainly include hydroxamic acids, fatty acids, and phosphonic acids [4]. However, although fatty acid collectors have strong capturing abilities for rare earth minerals and calcium-bearing gangue minerals, their selectivity is poor [5]. Meanwhile, phosphonic acid collectors have poor capturing abilities for rare earth minerals [6]. Due to the lone pairs of electrons in the hydroxamic acid collector chelate functional groups, these collectors can form more stable cyclic metal chelates with the metal active sites on the surface of rare earth minerals [7]. Therefore, hydroxamic acid collectors exhibit better-capturing abilities and selectivity for rare earth minerals, thus making them commonly used efficient collectors in rare earth flotation. Water glass (Na2O·nSiO2), with a composition similar to sodium silicate (SS), is a widely utilized inhibitor of calcium-bearing gangue minerals in rare earth flotation, exerting strong inhibitory effects on salt-type minerals such as fluorite [1,3,8,9,10], calcite [9,10,11], barite [11,12,13], and dolomite [14,15,16]. In the pulp, water glass hydrolyzes to form components H4SiO4 and SiO(OH)3−. The hydrophilic colloid H4SiO4 can adsorb onto the surface of calcium-bearing gangue minerals. SiO(OH)3−, on the other hand, tends to react with the calcium active sites on the surface of gangue minerals, forming hydrophilic metal silicate complexes that precipitate, thereby hindering the adsorption of collectors and inhibiting their flotation [3,14,17,18,19].
In recent years, density functional theory (DFT) simulations have gained extensive application in elucidating the adsorption mechanisms of flotation reagents on mineral surfaces [20,21,22,23]. Guo et al. [24,25] confirmed that 2-hydroxy-3-naphthalene hydroxamic acid, 1-hydroxy-2-naphthalene hydroxamic acid, CHA, and SPA have stronger adsorption capacity for bastnaesite than fluorite by combining experimental and DFT methods, and additionally, 1-hydroxy-2-naphthalene hydroxamic acid (1-OH-2-NHA) is more selective for minerals than 2-hydroxy-3-naphthalene hydroxamic acid (2-OH-3-NHA). Although SPA has strong capture ability, its selectivity towards minerals is not as good as that of CHA. Cao et al. [26] conducted a study on the adsorption behavior of sodium silicate (SS) on the surfaces of fluorite, calcite, and scheelite using first-principles calculations. Their results indicated that SS exhibits the strongest adsorption capacity on fluorite and the weakest on calcite. Xu et al. [27] synthesized new collectors for the bastnaesite-barite flotation system, including octyl dihydroxyamide acid (OTBHA), decyl dihydroxyamide acid (DCBHA), and dodecyl dihydroxyamide acid (DDBHA). Through DFT calculations and flotation experiments, it was found that, compared with OCBHA and DDBHA, DCBHA has stronger reactivity and better collecting ability for bastnaesite, while it has no collecting ability for barite. Liu et al. [28] found that the affinity of the collector N-[6-(hydroxyamino)-6-oxohexyl] octanamide (NHOO) to bastnaesite was stronger than that of fluorite, and the adsorption mechanism of the collector on the mineral surface was well explained by DFT. However, there are few reports on the study of the competitive adsorption mechanisms of water glass/SS and hydroxamic acid on the mineral surfaces of bastnaesite, parisite, and fluorite by DFT simulations.
Based on this background, this paper investigated the interaction mechanisms of OHA and SS with bastnaesite, parisite, and fluorite using microflotation, zeta potential tests, XPS tests, contact angle analysis, and DFT simulations. This work examined the fundamental reasons for the flotation behavior differences between bastnaesite, parisite, and fluorite in the SS and OHA reagent system, which provided a theoretical basis for enhancing the flotation separation of the two rare earth minerals and fluorite through mineral interface regulation.

2. Experiment

2.1. Materials and Reagents

Bastnaesite was sourced from the rare earth mines in Maoniuping, Sichuan, and Weishanhu, Shandong, respectively. Following manual separation, crushing, and grinding in a ceramic ball mill, purification was achieved through gravity and magnetic separation methods. This process yielded pure bastnaesite mineral with a purity exceeding 95% and pure parisite mineral with a purity exceeding 90%. Fluorite was sourced from a mine in Guangxi. The same process as purifying rare earth ores was employed to obtain fluorite with a purity exceeding 98%, The XRD patterns of pure bastnasite, parisite and fluorite samples are illustrated in Figure 1 [1,3,29,30].
Octyl hydroxamic acid (OHA) and sodium silicate (SS) were used as collectors with purity >98% from Aladdin Biochemical Technology Co., Ltd, Shanghai, China. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were chosen to function as pH-modifying agents. Deionized water with a resistivity of 18.2 MΩ × cm was used in all tests.

2.2. Microflotation Experiments

The flotation experiments were conducted using an laboratory flotation machine (XFGC II-35, Jilin Exploration Machinery Factory, Changchun, China)with a cell volume of 40 mL. A 2-g sample was placed in the flotation cell, and 30 mL of deionized water was added. The mixture was stirred at 1992 rpm for 1 min, followed by the addition of HCl and NaOH to adjust the pulp’s pH to the desired experimental value. Subsequently, preprepared reagents were added sequentially, with each addition followed by 2 min of stirring. The flotation process was then carried out by scraping for 4 min. The resulting foam and bottom products were filtered, dried, and weighed to calculate the flotation recovery rate. Each experiment was performed in triplicate, and the average value was recorded as the experimental result. The standard deviation of each experiment was calculated and is represented by error bars (representing ± 1σ) [31].

2.3. Zeta Potential Measurements

In each test, a 30-mg sample with a particle size of −5 μm was weighed and placed in a small beaker. Then, 50 mL of a 1 × 10−3 mol/L KCl electrolyte solution was added to form the slurry. A magnetic stirrer was used to mix the solution, and the pH was adjusted using NaOH and HCl. The required concentrations of OHA and SS were added, and the mixture was stirred for 5 min each time. After the reagents fully reacted with the sample, the solution was allowed to stand for 10 min. A disposable pipette was used to extract 2 mL of the upper clear solution, which was then placed in a potential measurement sample cell. The potential test was conducted using a potential analyzer (NanoBrook 90 Plus analyzer, Brookhaven Instruments Corporation, Holtsville, NY, USA). Each sample was measured three times, and the average of the three measurements was recorded as the test result. The standard deviation of each test was calculated and represented by error bars.

2.4. Contact Angle Tests

The contact angle measurements were performed at room temperature using contact angle measuring instrument (DSA25, KRÜSS GmbH, Hamburg, Germany). Initially, 2.00 g of the test sample was weighed and placed in a small beaker. The pH was adjusted to the specified value, and a specific concentration of OHA and SS was added, followed by stirring for 5 min. Subsequently, the test sample was filtered and dried. Boric acid served as the substrate, onto which the dried sample powder was deposited. The powder was then pressed into a compact sheet at 40 MPa pressure for 30 s. The contact angle tests were performed on the prepared samples. Each sample group underwent three independent measurements, and the average value was recorded as the test result.

2.5. XPS Measurements

The 1.00-g experimental sample was weighed and placed in a small beaker, to which 1 × 10−3 mol/L KCl electrolyte solution was added to adjust the slurry. The mixture was stirred with a magnetic stirrer, and the pH was adjusted using NaOH and HCl, after which specific concentrations of SS and OHA solutions were added sequentially. Each stirring time was 5 min, and the mineral sample was washed three times with deionized water. After filtration, the sample was dried in a vacuum environment at 50 °C, and an XPS test was performed [32]. The XPS test conditions were as follows: A X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) was used, hν = 1486.6 eV, spot area of 500 μm, vacuum degree of 5 × 10−8 Pa, full spectrum scanning step of 1.0 eV, and N1s high-resolution spectrum scanning step of 0.05 eV. Peak analysis and fitting were performed using CasaXPS Version 2.3.15, with a C1s correction value of 284.8 eV.

2.6. First-Principles Calculations

Using first-principles calculations based on density functional theory (DFT), the adsorption behavior of OHA and SS on the surfaces of bastnaesite (100), parisite (100), and fluorite (111) was studied. The unit cell models of bastnaesite, parisite, and fluorite used in the calculations were obtained from the American Mineralogist Crystal Structure Database (AMCSD). The structure optimization and property calculations for the collector OHA and the inhibitor SS were performed using the Dmol3 module of Materials Studio 2017. The Castep module was used to optimize the minerals and mineral surfaces and to calculate the geometric parameters, adsorption energy, and charge density difference of the adsorption of OHA and SS on the mineral surfaces.
During the calculations, the Dmol3 module employed the generalized gradient approximation (GGA) and PBE exchange functional [33], with the self-consistent field (SCF) convergence criterion fixed at 2 × 10−6 Ha. The convergence criteria for energy, maximum force, and maximum displacement were set to 1 × 10−6 Ha, 0.05 Ha/Å, and 0.002 Å, respectively. The Castep module used GGA and PW91 exchange functionals, with the SCF convergence precision set to Ultrasoft [25,34]. The convergence criteria were set to 2 × 10−5 Ha, the plane wave cut-off energy was set to 480 eV, and the k-point sampling density was 3 × 3 × 2 [1,35]. More stringent SCF and convergence criteria did not significantly alter the results.

3. Results and Discussion

3.1. Microflotation Results

Figure 2 illustrates the effects of pulp pH value, OHA, and SS dosage on the flotation recovery of bastnaesite, parisite, and fluorite. Figure 2a shows that the recovery of these minerals increases with rising pH, reaching maximum values at pH 9.5 of 93.86%, 94.39%, and 69.29% for bastnaesite, parisite, and fluorite, respectively. Figure 2b demonstrates that at pH 9.5, the recovery of the three minerals initially increases with the rise in OHA dosage and then stabilizes. When the OHA dosage reached 1 × 10−4 mol/L, the recovery for bastnaesite, parisite, and fluorite were 93.8%, 94%, and 78%, respectively, indicating that OHA had a superior collecting ability for bastnaesite and parisite compared to fluorite. Figure 2c shows that at pH 9.5 and an OHA dosage of 1 × 10−4 mol/L, the recovery of the three minerals decreases as the SS dosage increases. When the SS dosage was 105 mg/L, the recovery of bastnaesite, parisite, and fluorite dropped to 81.63%, 48.98%, and 16.84%, respectively, and bastnaesite exhibits the smallest reduction among the three. When the SS dose was increased to 200 mg/L, fluorite and parisite were severely inhibited, while bastnaesite still maintained a certain level of floatability. Therefore, SS had the strongest inhibitory effect on fluorite, followed by parisite and bastnaesite.

3.2. Zeta Potential Results

Zeta potential measurement is an effective method for evaluating the surface charge properties of minerals [36,37,38,39]. Figure 3 shows changes in zeta potentials of bastnaesite, parisite, and fluorite before and after treatment by SS and OHA. The isoelectric points of bastnaesite, parisite, and fluorite are 7.1, 6.2, and 7.25, respectively, which are consistent with previous research results [1,3,27]. The zeta potential of all three minerals shifted to more negative values after being treated with 105 mg/L of SS. At pH 9.5, the zeta potential shifts of parisite and fluorite (24.24 mV and 29.24 mV, respectively) were significantly greater than that of bastnaesite (9.37 mV), indicating that SS has a greater impact on the surface charge of fluorite, followed by parisite, and the least on bastnaesite. With the further addition of 1×10−4 mol/L OHA, the zeta potentials of bastnaesite, parisite, and fluorite continued to shift negatively, with bastnaesite exhibiting the greatest negative shift, parisite a significant shift, and fluorite a minimal shift. At pH 9.5, the zeta potential of bastnaesite shifted negatively by 9.31 mV, while parisite and fluorite shifted by 2.78 mV and 0.79 mV, respectively, indicating that bastnaesite still adsorbed a significant amount of OHA. The SS adsorbed on the surfaces of fluorite and parisite hindered the subsequent adsorption of OHA, meaning that SS had the strongest inhibitory effect on fluorite, followed by parisite, and the weakest effect on bastnaesite, which is consistent with the flotation experiment results.

3.3. Contact Angle Measurements

The size of the mineral contact angle reflects the wettability of the mineral surface. The larger the contact angle, the stronger the hydrophobicity and the better the floatability [40]. Figure 4 shows the contact angle changes of bastnaesite, parisite, and fluorite before and after interaction with SS and OHA. As can be seen, the contact angles of natural bastnaesite, parisite, and fluorite were 23.2°, 28.0°, and 21.9°, respectively, indicating that these minerals in their natural state were hydrophilic. After adding OHA, the contact angles of bastnaesite, parisite, and fluorite increased to 78.0°, 73.6°, and 51.0°, respectively, suggesting that OHA adsorbed on the surface of the three minerals and enhanced their hydrophobicity. The contact angle of bastnaesite after OHA and SS treatment decreased slightly to 61.1°, the contact angle of parisite decreased to 49.0°, and the contact angle of fluorite decreased significantly to 19.1°. Therefore, SS had a more selective inhibitory effect on parisite and fluorite, which is consistent with the results of flotation experiments and zeta potential measurements.

3.4. XPS Analysis

Figure 5 shows the Si 2p spectra of bastnaesite, parisite, and fluorite before and after treatment by SS. As depicted in Figure 5a, on the pure bastnaesite surface, the two peaks of 102.84 eV and 105.89 eV are assigned to La 4d 5/2 and La 4d 3/2, respectively. A weak Si 2p peak appears at a binding energy of 102.49 eV after SS treatment, indicating slight adsorption of SS on the bastnaesite surface. In Figure 5b, two La 4d peaks are observed on the surface of parisite at 102.86 eV and 106.02 eV, respectively. In the presence of SS, a Si 2p peak appears at a binding energy of 102.64 eV on the parisite surface, with significantly higher intensity than that on bastnaesite. This phenomenon likely results from calcium ions on the parisite surface, indicating a significant adsorption effect of SS on parisite. Figure 5c illustrates that the Si 2p peak appears at 102.35 eV on the surface of fluorite treated with SS, which is significantly stronger than that of bastnaesite and parisite. The results indicate that SS might adsorb on the mineral surface by reacting with calcium ions, thereby exerting an inhibitory effect on the minerals. The stronger the SS adsorption, the greater the inhibitory effect.
Figure 6 shows the N 1s spectra of bastnaesite, parisite, and fluorite before and after treatment with SS. As shown in Figure 6a, the N 1s spectra on the surface of OHA-treated bastnaesite can be deconvoluted into two peaks. The peak at 399.13 eV corresponds to the N in the OHA anion (R-CO-NH-O-), while the peak at 400.73 eV corresponds to the N in the OHA molecule (R-CO-NH-OH) [1,3,41]. Therefore, both chemical adsorption and physical adsorption of OHA occur on the surface of bastnaesite. After SS treatment, the N 1s deconvoluted peaks are located at binding energies of 400.79 eV and 399.47 eV, respectively. The amount of physically adsorbed OHA on the surface of bastnaesite decreases more significantly than the chemically adsorbed OHA, indicating that SS could inhibit the physical adsorption of OHA on the surface of bastnaesite. In Figure 6b,c, the N 1s spectra of parisite and fluorite treated with OHA were deconvoluted into two peaks, similar to the results for bastnaesite. Therefore, both physical and chemical adsorption of OHA occurred on the surfaces of these two minerals. After SS treatment, the amount of OHA on the surfaces of parisite and fluorite was significantly reduced through both chemical and physical adsorption, with the reduction in the N 1s peak being greater for fluorite than for parisite. The results show that the adsorption of SS on the surface of fluorite had the greatest inhibitory effect on the adsorption of OHA, followed by parisite, and the weakest effect on bastnaesite.

3.5. DFT Simulation

The above discussion illustrates the consistency of the results of flotation recovery, zeta potential, contact angle, and XPS test analysis. In this section, the interaction mechanisms of collector OHA and the inhibitor SS with three minerals will be further explained at the atomic level using DFT simulations.
The highest occupied molecular orbital (HOMO) of anionic flotation reagents reflects their ability to donate electrons, serving as an indicator of the reactivity of anionic flotation reagents [42,43,44,45]. The molecular electrostatic potential (MEP) diagram can visually represent the electronic distribution of flotation reagents by using different colors to indicate areas of positive and negative electrostatic potential [46,47]. Therefore, the Dmol3 module was employed to optimize the structures of the dominant components of OHA (OHA−) and SS (SiO(OH)3−, Si(OH)4) at pH 9.5, and to calculate the HOMO, MEP maps, and Mulliken charges in this section. The results are presented in Figure 7.
Figure 7a–i illustrate the optimized structures of OHA−, SiO(OH)3−, and H4SiO4, along with Mulliken charges, HOMO, and MEP maps. In the MEP maps, the red and blue regions represent negative and positive electrostatic potential areas, respectively. The HOMO of OHA− (Figure 7d) and its negative electrostatic potential region (Figure 7g) are primarily located around the oxygen atoms in the C=O and N-O groups. Mulliken charge analysis (Figure 7a) indicates that the negative charges around these oxygen atoms were −0.615 e and −0.628 e, respectively. Thus, it can be inferred that the oxygen atoms in the C=O and N-O groups of OHA− might interact with the surfaces of the three minerals through covalent bonding.
As shown in Figure 7h, the HOMO and negative electrostatic potential regions of SiO(OH)3− are also mainly distributed around the dehydrogenated oxygen atom, with a Mulliken charge of −1.025 e. Therefore, the dehydrogenated oxygen atom of SiO(OH)3− may similarly form covalent bonds with the metal ions on the mineral’s surfaces. For H4SiO4, the mobile electrons are uniformly distributed within its structure, as depicted in Figure 7f,i. Consequently, the negatively charged oxygen atoms in the H4SiO4 molecule are likely to engage in electrostatic adsorption with the metal active sites on the mineral surfaces, while the hydrogen atoms in the molecule might form hydrogen bonds with the negatively charged oxygen and fluorine atoms on the mineral surfaces. From a thermodynamic perspective, the physical adsorption forces (electrostatic and hydrogen bond adsorption) of H4SiO4 on the mineral surfaces are significantly weaker than the chemical adsorption forces of SiO(OH)3−. Furthermore, at a pH of 9.5, the higher concentration of SiO(OH)3− in the slurry indicates that SiO(OH)3− is the primary inhibitory component for these three minerals, consistent with previous research findings [26,48].
According to previous studies, the bastnaesite (100) surface, the parisite (100) surface, and the fluorite (111) surface are the most stable cleavage surfaces of these minerals [1,35]. Therefore, this study chose these three surfaces to cleave the mineral cells and construct the corresponding mineral surface models. The vacuum height selected for the crystal plane calculation was 20 Å to avoid mutual interference between the upper and lower parts of the crystal plane. The optimized bastnaesite (100) surface, parisite (100) surface, and fluorite (111) surface are shown in Figure 8. As can be seen, metal ions exposed on the surface of bastnaesite (100) are Ce3+, the metal ions exposed on the surface of parisite (100) are Ca and Ce sites, and the metal ions exposed on the surface of fluorite (111) are Ca sites. These metal ions exposed to the surfaces of the three minerals are the main active sites for the adsorption of the anionic collector OHA and the inhibitor SS.
To investigate the interaction between the anionic collector OHA and the (100) surface of bastnaesite, the (100) surface of parisite, and the (111) surface of fluorite, the collector was initially positioned differently on the mineral surfaces to calculate the adsorption energy and determine the most stable configuration. The adsorption energy calculation is given by Equation (1). Lower adsorption energy indicates a stronger interaction between the flotation reagent and the mineral surface. Charge density difference maps, which directly reflect the movement and distribution of electrons, were also calculated to assess the charge density difference after the reagent’s adsorption on the mineral surfaces [49].
The adsorption energy is calculated as follows [26,50]:
E ads = E total − E surface − E flotation   agent
In the formula, E ads is the adsorption energy, E total is the total energy of flotation reagent mineral, E surface is the surface energy of minerals, and E flotation   agent is the single point energy of the optimized collector or inhibitor.
Figure 9 shows that OHA adsorbs on the surface of bastnaesite in three different configurations: monodentate, chelate, and bridge. In the monodentate configuration, the O atom in the N-O group interacts with Ce sites on the surface of bastnaesite, with the optimized Ce-O bond length shortened from 3 Å to 2.48 Å. In the chelate configuration, the O atoms in both the C=O and N-O groups simultaneously adsorb on one Ce sites, resulting in two optimized Ce-O bond lengths of 2.36 Å and 2.49 Å, respectively. In the bridge configuration, the O atoms in the C=O and N-O groups adsorb onto two different Ce sites, with final O-Ce bond lengths of 2.54 Å and 2.43 Å, respectively. The order of the adsorption energies for these configurations is: monodentate coordination (−561.99 kJ·mol−1) > bridge coordination (−724.41 kJ·mol−1) > chelate coordination (−767.7 kJ·mol−1), indicating that the chelate configuration is the most stable. Additionally, the differential charge density diagram shows that the O atom in the OHA chelate configuration transfers more electrons to Ce sites, further confirming that the chelate configuration is the easiest to form and the most stable.
Figure 10 illustrates the five possible conformations of OHA on the parisite (100) surface. There are three adsorption configurations between OHA and Ce sites on the (100) surface of parisite, as shown in Figure 10a–c. In the monodentate configuration, the optimized Ce-O bond length was shortened from 3 Å to 2.17 Å. In the chelate configuration, the final Ce-O bond lengths were 2.43 Å and 2.57 Å, respectively. In the bridge configuration, the O atoms in the C=O and N-O groups were adsorbed with two different Ce sites, and the calculated bond lengths of the two O-Ce bonds were 2.54 Å and 2.11 Å, respectively. The order of the calculated adsorption energies from large to small is monodentate coordination (−600.86 kJ·mol−1) > bridge coordination (−635.73 kJ·mol−1) > chelate coordination (−674.59 kJ·mol−1). The analysis of bond length and adsorption energy indicates that the chelate coordination configuration formed by OHA with Ce sites on the (100) surface of parisite is more stable and easier to form. The obtained charge density difference diagram also reflects that the O atom of OHA in the chelate coordination configuration transfers more electrons to the surface of parisite, further confirming this result. As shown in Figure 10d,e, OHA with Ca sites on the (100) surface of parisite has two adsorption modes: monodentate coordination (−381.64 kJ·mol−1) and chelated coordination (−507.19 kJ·mol−1), with the latter being more stable. After comparison, it was found that the adsorption energy of OHA with Ce sites (−674.59 kJ·mol−1) was more negative than that with Ca sites (−507.19 kJ·mol−1). Thus, it can be concluded that OHA is more likely to adsorb onto Ce sites on the surface of parisite.
As shown in Figure 11, the Ca-O bond length in the monodentate configuration shortens from 3 Å to 2.23 Å after adsorption. In the bridge configuration, the two Ca-O bond lengths are 2.26 Å and 2.07 Å, respectively. The adsorption energies, in descending order, are monodentate coordination (−180.25 kJ·mol−1) > bridge coordination (−230.08 kJ·mol−1) > chelate coordination (−250.25 kJ·mol−1). Thus, the chelate configuration is the most favorable adsorption configuration for OHA− on the surface of fluorite. In summary, the five-membered hydroxamic-(O-O)-Ce/Ca chelate coordination formed between OHA and metal atoms on the three mineral surfaces is the most stable. Compared with parisite and fluorite, OHA exhibits a greater affinity for bastnaesite. These findings are consistent with the results of flotation experiments, zeta potential measurements, contact angle measurements, and XPS tests.
To explore the interaction mechanism between SS and mineral surfaces, the adsorption behavior of SiO(OH)3−, a key component of SS, on the surface of these three minerals was carried out by DFT simulation. The adsorption configuration of SiO(OH)3− on the mineral surfaces is shown in Figure 12.
As shown in Figure 12a, SiO(OH)3− could adsorb at the Ce site on bastnaesite (100) surface, forming a Ce-O bond with a bond length of 2.08 Å and an adsorption energy of −624.77 kJ mol−1. Figure 12b,c indicate that, for parisite, SiO(OH)3− adsorbs simultaneously on the Ca and Ce sites on its (100) surface, forming Ce-O and Ca-O bond lengths of 2.11 Å and 2.51 Å, respectively, with adsorption energies of −678.53 kJ·mol−1 and −480.79 kJ·mol−1, respectively. In Figure 12d, SiO(OH)3− forms a Ca-O bond with a bond length of 2.11 Å and an adsorption energy of −678.58 kJ mol−1 on the Ca site of the fluorite (111) surface. Accordingly, SiO(OH)3− has the strongest affinity for fluorite, followed by parisite and bastnaesite.
Based on the DFT calculations and the data presented in Table 1, it can be concluded that the adsorption energy of OHA on the surface of bastnaesite (−767.68 kJ mol−1) is significantly lower than that of SS (−624.77 kJ mol−1), indicating that OHA is more easily adsorbed on the surface of bastnaesite and that SS cannot inhibit bastnaesite flotation. The adsorption energies of OHA and SS on the Ce site on the surface of parisite are comparable (−674.59 kJ mol−1 and −678.58 kJ mol−1), indicating a competitive adsorption between the two at this active site. However, the adsorption energy of OHA on the Ca sites on the surface of parisite (−507.19 kJ mol−1) is higher than that of SS (−678.53 kJ mol−1), indicating that the presence of Ca sites allows SS to inhibit parisite. In addition, the adsorption energy of SS on the surface of fluorite (−678.58 kJ mol−1) is much lower than that of OHA (−250.25 kJ mol−1), indicating that SS tends to adsorb more on the surface of fluorite, thereby hindering the subsequent adsorption of OHA and exhibiting a stronger inhibitory effect of SS on fluorite.
Figure 13 shows a schematic diagram of the flotation mechanisms of bastnaesite, parisite, and fluorite in the SS and OHA systems. In the flotation process, the collecting effect of OHA on bastnaesite is stronger than the inhibiting effect of SS, making bastnaesite the easiest to flotation and recover. On the other hand, the inhibitory effect of SS on fluorite is stronger than the collecting effect of OHA, so fluorite is the most difficult to float, and most of it stays in the flotation tailings. Parisite is a special case where only a small amount of parisite becomes hydrophobic and floats due to the competing adsorption of OHA and SS on its surface, resulting in the loss of rare earth elements in the tailings.

4. Conclusions

The flotation behavior and mechanism of bastnaesite, parisite, and fluorite were systematically studied using OHA as a collector and SS as a depressant. The following conclusions were obtained:
  • OHA demonstrates a stronger collecting ability for bastnaesite compared to parisite, with the weakest collecting ability for fluorite. Conversely, SS exhibited the strongest depressing ability for fluorite, followed by parisite, and the weakest for bastnaesite.
  • SS has the strongest adsorption on fluorite, significantly hindering the adsorption of OHA. SS and OHA compete for adsorption on parisite surfaces. SS has weak adsorption on the surface of bastnaesite and has little effect on the adsorption of OHA.
  • OHA ions form covalent bonds with metal ions on the surfaces of the three minerals. The five-membered hydroxamic-(O-O)-Ce/Ca chelate coordination formed between OHA and metal atoms on the three mineral surfaces is the most stable. Compared with parisite and fluorite, OHA exhibits a greater affinity for bastnaesite. SS interacts with metal ions on the mineral surface primarily through the SiO(OH)3− component, and SiO(OH)3− has the strongest affinity for fluorite, followed by parisite and bastnaesite.

Author Contributions

J.W.: Conceptualization, Data curation, Investigation, Methodology, Software, Validation, Visualization, Writing—original draft. W.L. (Wenda Lu): Data curation, Investigation, Visualization. Z.C.: Conceptualization, Formal analysis, Project administration, Resources, Supervision. X.W. (Xu Wu): Conceptualization, Formal analysis, Funding acquisition, Project administration, Resources, Supervision, Writing—review and editing. P.W.: Software, Validation, Visualization. X.W. (Xiaoping Wang): Software, Investigation, Methodology. W.L. (Wenli Liu): Visualization, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are grateful for the financial support provided by the National Natural Science Foundation of China (Grant No. 52364034 and 52264033), the National Key Research and Development Program (Grant No. 2021YFC2901000 and 2022YFC2905302), the Science Foundation for Distinguished Young Scholars of Inner Mongolia Autonomous Region (Grant No. 2024JQ02), and the Inner Mongolia Autonomous Region Department of Education first-class discipline scientific research special project (Grant No. YLXKZX-NKD-037 and YLXKZX-NKD-004).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The author Wenli Liu was employed by the Baotou Steel Group Baoshan Mining Limited Corporation. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Minerals 14 00870 g001
Figure 1. XRD patterns of single minerals, bastnasite (a), parisite (b), and fluorite (c).
Figure 1. XRD patterns of single minerals, bastnasite (a), parisite (b), and fluorite (c).
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Figure 2. Effects of pH value (a), OHA (b), and SS (c) on the flotation recovery of bastnaesite, parisite and fluorite.
Figure 2. Effects of pH value (a), OHA (b), and SS (c) on the flotation recovery of bastnaesite, parisite and fluorite.
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Figure 3. Changes in zeta potentials of bastnaesite (a), parisite (b), and fluorite (c) treatment with SS and OHA or not.
Figure 3. Changes in zeta potentials of bastnaesite (a), parisite (b), and fluorite (c) treatment with SS and OHA or not.
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Figure 4. Contact angles of bastnaesite, parisite, and fluorite before and after treatment by SS and OHA.
Figure 4. Contact angles of bastnaesite, parisite, and fluorite before and after treatment by SS and OHA.
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Figure 5. Si 2p spectra of bastnaesite (a), parisite (b), and fluorite (c) before and after treatment with SS.
Figure 5. Si 2p spectra of bastnaesite (a), parisite (b), and fluorite (c) before and after treatment with SS.
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Figure 6. N 1s spectra of bastnaesite (a), parisite (b), and fluorite (c) before and after treatment with SS.
Figure 6. N 1s spectra of bastnaesite (a), parisite (b), and fluorite (c) before and after treatment with SS.
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Figure 7. DFT optimized structures, HOMO, and MEP maps of OHA− (a), SiO(OH)3− (b), H4SiO4 (c), and the HOMO of OHA− (d), SiO(OH)3− (e), H4SiO4 (f), and also the MEP maps of OHA (g), SiO(OH)3− (h), H4SiO4 (i). (N ≡ blue, O ≡ red, C ≡ gray, H ≡white, and Si ≡ orange).
Figure 7. DFT optimized structures, HOMO, and MEP maps of OHA− (a), SiO(OH)3− (b), H4SiO4 (c), and the HOMO of OHA− (d), SiO(OH)3− (e), H4SiO4 (f), and also the MEP maps of OHA (g), SiO(OH)3− (h), H4SiO4 (i). (N ≡ blue, O ≡ red, C ≡ gray, H ≡white, and Si ≡ orange).
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Figure 8. The optimized structure of bastnaesite (100) surface, parisite (100) surface, and fluorite (111) surface from different views. (O ≡ Red, C ≡ Gray, H ≡ White, Ca ≡ Green, Ce≡ yellow, F ≡ pale bluish green).
Figure 8. The optimized structure of bastnaesite (100) surface, parisite (100) surface, and fluorite (111) surface from different views. (O ≡ Red, C ≡ Gray, H ≡ White, Ca ≡ Green, Ce≡ yellow, F ≡ pale bluish green).
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Figure 9. OHA anion adsorption geometries on bastnaesite (100) surface in monodentate (a), chelate (b), bridge (c), and configurations. (N ≡ blue, O ≡ red, C ≡ gray, H ≡ white, Ca ≡ green, Ce ≡ yellow, F ≡ pale bluish green).
Figure 9. OHA anion adsorption geometries on bastnaesite (100) surface in monodentate (a), chelate (b), bridge (c), and configurations. (N ≡ blue, O ≡ red, C ≡ gray, H ≡ white, Ca ≡ green, Ce ≡ yellow, F ≡ pale bluish green).
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Figure 10. OHA anion adsorption geometries on Ce atoms of parisite (100) surface in monodentate (a), bridge (b), chelate (c), and configurations, and on Ca sites in monodentate (d) and chelate (e) configurations. (N ≡ blue, O ≡ red, C ≡ gray, H ≡ white, Ca ≡ green, Ce ≡ yellow, F ≡ pale bluish green.).
Figure 10. OHA anion adsorption geometries on Ce atoms of parisite (100) surface in monodentate (a), bridge (b), chelate (c), and configurations, and on Ca sites in monodentate (d) and chelate (e) configurations. (N ≡ blue, O ≡ red, C ≡ gray, H ≡ white, Ca ≡ green, Ce ≡ yellow, F ≡ pale bluish green.).
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Figure 11. OHA anion adsorption geometries on fluorite (111) surface in monodentate (a), chelate (b), bridge (c), and configurations. (N ≡ blue, O ≡ red, C ≡ gray, H ≡ white, Ca ≡ green, Si ≡ orange, F ≡ pale bluish green).
Figure 11. OHA anion adsorption geometries on fluorite (111) surface in monodentate (a), chelate (b), bridge (c), and configurations. (N ≡ blue, O ≡ red, C ≡ gray, H ≡ white, Ca ≡ green, Si ≡ orange, F ≡ pale bluish green).
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Figure 12. The adsorption model of SiO(OH)3− on bastnaesite (100) surface (a), Ce sites on parisite (100) surface (b), Ca sites on parisite (100) surface (c), and fluorite (111) surface (d). (O ≡ red, C ≡ gray, H ≡ white, Ca ≡ green, Si ≡ orange, Ce ≡ yellow, F ≡ pale bluish green.).
Figure 12. The adsorption model of SiO(OH)3− on bastnaesite (100) surface (a), Ce sites on parisite (100) surface (b), Ca sites on parisite (100) surface (c), and fluorite (111) surface (d). (O ≡ red, C ≡ gray, H ≡ white, Ca ≡ green, Si ≡ orange, Ce ≡ yellow, F ≡ pale bluish green.).
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Figure 13. Flotation mechanism model of bastnaesite, parisite, and fluorite in SS and OHA systems.
Figure 13. Flotation mechanism model of bastnaesite, parisite, and fluorite in SS and OHA systems.
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Table 1. Adsorption energies of OHA and SS on different mineral surfaces.
Table 1. Adsorption energies of OHA and SS on different mineral surfaces.
Mineral SurfaceActive SiteOHA Adsorption Energy (kJ mol−1)SS Adsorption Energy (kJ mol−1)
Bastnaesite (100) surfaceCe −767.68−624.77
Parisite (100)
surface		Ce−674.59−678.53
Ca−507.19−480.79
Fluorite (100)
surface		Ca−250.25−678.58
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Wang, J.; Lu, W.; Cao, Z.; Wu, X.; Wang, P.; Wang, X.; Liu, W. New Insights into the Depressive Mechanism of Sodium Silicate on Bastnaesite, Parisite, and Fluorite: Experimental and DFT Study. Minerals 2024, 14, 870. https://doi.org/10.3390/min14090870

AMA Style

Wang J, Lu W, Cao Z, Wu X, Wang P, Wang X, Liu W. New Insights into the Depressive Mechanism of Sodium Silicate on Bastnaesite, Parisite, and Fluorite: Experimental and DFT Study. Minerals. 2024; 14(9):870. https://doi.org/10.3390/min14090870

Chicago/Turabian Style

Wang, Jieliang, Wenda Lu, Zhao Cao, Xu Wu, Peng Wang, Xiaoping Wang, and Wenli Liu. 2024. "New Insights into the Depressive Mechanism of Sodium Silicate on Bastnaesite, Parisite, and Fluorite: Experimental and DFT Study" Minerals 14, no. 9: 870. https://doi.org/10.3390/min14090870

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