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Article

Effect of an Environment-Friendly Depressant on the Flotation of Bastnaesite and Fluorite

by
Chunlei Guo
1,2,*,
Shaochun Hou
1,2,
Weiwei Wang
1,2 and
Qiang Li
1,2
1
National Key Laboratory of Baiyunobo Rare Earth Resource Researches and Comprehensive Utilization, Baotou 014030, China
2
Baotou Research Institute of Rare Earths, Baotou 014030, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(2), 165; https://doi.org/10.3390/min14020165
Submission received: 30 November 2023 / Revised: 30 January 2024 / Accepted: 30 January 2024 / Published: 2 February 2024
(This article belongs to the Section Mineral Processing and Extractive Metallurgy)
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Abstract

:
To overcome the difficulty of separating bastnaesite from fluorite through the flotation technique, the present study examined the suitability of sodium alginate (SA) as a depressant in the flotation process. The effect of SA on the flotation separation of bastnaesite and fluorite was evaluated using micro-flotation tests, zeta potential measurements, adsorption density measurements, Fourier infrared spectroscopy, and X-ray photoelectron spectroscopy. The micro-flotation results showed that SA exerted a strongly detrimental effect on fluorite flotation, while slightly affecting bastnaesite flotation. The surface chemistry results revealed that the -COO- and HO- functional groups in SA coordinated with Ca2+ on the fluorite surface, which induced hydrophilicity and hindered adsorption in the subsequent octylhydroxamic acid as a collector. However, the interaction of SA with the bastnaesite surface was marginal and did not affect the anchoring of the collector on the surface of bastnaesite. Based on these results, the present study proposes a possible model for the interaction of SA on the surfaces of the two minerals, laying a foundation for the flotation separation of bastnaesite from fluorite with SA as an environmentally benign depressant.

1. Introduction

The significance of rare earth, a collective term for 17 metal elements (including lanthanides, scandium, and yttrium) has been fully recognized and affirmed owing to its extensive applications as a lubricant for electronics, lasers, superconductors as well as in nuclear industry and other advanced technologies [1,2,3]. In addition, it is required for enhancing the tactical performance of steel and alloys used in the manufacture of airplanes, tanks, and missiles. Rare earth elements are usually extracted from bastnaesite ([Ce,La]CO3F), which is coeval with fluorite (CaF2), dolomite, calcite, and other gangue minerals [2,3,4,5,6]. However, their flotation separation is extremely challenging.
The problem of separating bastnaesite from fluorite through flotation is attributed to their similarities, as both are micro-soluble salt minerals with similar surface chemical properties. While using hydroxamic acids, such as benzyl hydroxamic acid [7], salicyl hydroxamic acid [8,9], and naphthalene hydroxamic acid [10], as collectors, fluorite is inevitably entrained into the rare earth concentrates. In the flotation of rare earth ores, researchers have focused mainly on studying and developing collectors. As a result, reports on the depressants of fluorite are scarce. Water glass, as a commonly used inhibitor, provides limited inhibition of fluorite, especially to attain high-grade rare earth concentrates (REO > 58%), which requires that the CaO content should preferably be reduced to less than 5%. Apart from water glass, other depressants, including ethylene diamine tetraacetic acid [4], polyepoxysuccinic acid [11], xanthanate gum [5], and sodium lingosulfonate [12], have been reported for the separation of rare earth minerals through floatation. It has been demonstrated that these depressants, which contain abundant carboxyl and hydroxyl groups, can form complexes with calcium ions on the fluorite surface, hindering the adsorption of the collector and exerting only a marginal impact on bastnaesite. However, these depressants are expensive and may cause environmental problems; therefore, they have not been validated for application in rare earth flotation separation, and suitable depressants for flotation separation of bastnaesite from fluorite remain to be further investigated.
Sodium alginate (SA, (C6H7O6Na)n), is mainly found in the cell walls of brown algae [13]. It is a natural, non-toxic, non-hazardous, and biodegradable linear anionic polysaccharide that is widely used in the fields of food industry water treatment and pharmaceuticals [14,15]. SA belongs to linear block polyanionic polymers, composed of α-L-guluronic acid (G) and β-D-mannuronic acid (M) monomer units with different ratios and connected in different orders (Figure 1) [14,15]. The application of SA in flotation has been studied in detail. Gu et al. adopted SA to inhibit talc, which led to a reduction in the dosage of the inhibitor carboxymethyl cellulose (CMC) and an increase in the recovery of copper-nickel sulphide [16]. Using SA as an inhibitor, Pan et al. achieved the flotation separation of chalcopyrite from chlorite and serpentine, lithium pyroxene from feldspar, galena from chalcopyrite, and scheelite and apatite from calcite [17,18,19]. Fu et al. implemented SA in the reverse flotation of hematite to improve the grade of iron ore concentrate [20]. These studies underscore the ability of SA, attributed to its structure comprising numerous HO- and -COO- groups, to form complexes with most of the divalent or multivalent metal ions, such as Ca2+, Fe2+, Zn2+, Cu2+, Co2+, and Pb2+, leading to the phenomena of gelling and cross-linking, which result in the formation of water-insoluble alginate. However, studies on SA in the flotation of rare earth minerals are scarce.
In this study, the effect of SA on the flotation separation of bastnaesite from fluorite was investigated to confirm whether SA can be used as a selective depressant for fluorite in flotation separation. The adsorption behavior of SA on the two minerals was systematically investigated through adsorption density measurements, zeta potential measurements, Fourier infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS), based on which a possible adsorption model was proposed.

2. Materials and Methods

2.1. Samples and Reagents

Highly pure minerals of bastnaesite and fluorite were obtained from the Weishan mine in Shandong Province and Bayan Obo mine in Inner Mongolia Autonomous Region, China, respectively. The samples were crushed and sieved, followed by rigorous shaking using a shaker and a magnetic separator to further improve the purity. Particles sized −75 + 38 μm were used for micro-flotation and adsorption experiments. In addition, finer particles with a size of −38 μm were further ground to obtain −5 μm sized particles for zeta potential, FTIR, and XPS tests. X-ray diffraction (X’Pert PRO, Panalytical, Almelo, The Netherlands) and chemical composition data of the samples are shown in Figure 2 and Table 1 and Table 2. The results indicated that the purity of the bastnaesite and fluorite samples exceeded 95%, while the bastnaesite samples contained a small amount of parisite.
SA and octylhydroxamic acid (OHA) of analytical purity were purchased from Macklin Biochemical Technology Co., Ltd. (Shanghai, China). Sodium hydroxide (NaOH), hydrochloric acid (HCl), and sodium chloride (NaCl), all of analytical grade, were procured from Kemiou Chemical Reagent Co., Ltd. (Tianjin, China) for pH adjustment and background electrolyte for zeta potential determination. Deionized water with a resistivity of 18.3 MΩ·cm was used in all experiments and tests.

2.2. Micro-Flotation Tests

Micro-flotation tests were performed on an XFG-type mechanically agitated flotation machine (Jinlin Exploration Machinery Factory, Changchun, China). Briefly, 2 g of both minerals was added to a plexiglass tank containing 40 mL of water, and the mixture was agitated for 3 min. Subsequently, the depressant and collector were added successively at 3-min intervals, and the pH was adjusted with NaOH or HCl solution. The pulp was stirred for 3 min before turning on the air supply at a flow rate of 0.02 m3/h, and the froth product was scraped for 4 min. Finally, the concentrates and tailings (residual in the tank) were filtered and dried, respectively. Based on the solid weight of the two products, the average and standard deviation of the recoveries from three parallel sets of tests were calculated and presented.

2.3. Adsorption Density Measurements

The total organic carbon (TOC) analyzer (Multi N/C 2100S, Analytik Jena AG, Jena, Germany) was employed to determine the adsorption density using the solution depletion method. First, the calibration lines were constructed with correlation coefficients of 0.9999 and 1.0000 for total carbon and total inorganic carbon, respectively. For measurements, the temperature of the cracking furnace as well as the heating and cooling temperatures were set at 800 °C, 50 °C, and 5 °C, respectively, and the oxygen flow rate was 180 mL/min. Next, the sample (2 g) was placed in a beaker loaded with 40 mL of water, followed by agitation for 10 min and the addition of various concentrations of SA solutions. The mixture was stirred at the desired pH for 2 h and then filtered through a centrifuge. The TOC contents in the filtrate and the SA stock solution were measured separately. Finally, the average TOC content and the relative standard deviation of the three measurements were calculated.

2.4. Zeta Potential Measurements

The zeta potential of the minerals was measured by means of a Zetasizer Nano ZS90 (Malvern Instrument Co., Malvern, Worcs, UK). The suspension was obtained by adding 50 mg of −5 μm minerals to 100 mL of 1.0 × 10−3 mol/L NaCl solution as the background electrolyte. After adding the desired reagents, the pH of the suspension was adjusted using HCl and NaOH solutions, followed by stirring for 20 min. Finally, approximately 2 mL of the mixture was extracted to measure the zeta potential of the minerals. The results are expressed as the average and standard deviation of the values obtained through three independent measurements.

2.5. FTIR

FTIR was conducted using a Bruker-Tensor 27 FTIR spectrophotometer (Bruker Technology Co., Ltd., Ettlingen, Germany) in the wave number ranges of 600 and 4000 cm−1. For sample preparation, 1.5 g of the mineral samples was added to a beaker with the desired concentration of SA solution (25.0 mg/L). Subsequently, the pH of the mixture was adjusted to the optimum level for micro-flotation using HCl and NaOH solutions. The mixture was stirred for 1 h, and the suspension was filtered and rinsed three times with deionized water of the same pH. Then, the filter cake was dried naturally and stored in a desiccator until analyzed. For measurements, KBr discs with 0.5 wt % mineral samples were analyzed, and pure KBr discs were used as a reference.

2.6. XPS

XPS was acquired on a Thermo ESCALAB 250XI spectrometer (Thermo Fisher Scientific Co., Ltd., Waltham, MA, USA) with an Al Kα radiation source. Survey scans of the samples were recorded using an electron analyzer with 100 eV passage energy in steps of 1.0 eV and high-resolution scans at a constant pass energy of 20 eV in steps of 0.1 eV over a defined range of binding energies. The C1s spectrum at 284.80 eV was applied to the nuclear calibration of all measured spectra. The mineral samples were prepared in the same way as for the FTIR measurements.

3. Results and Discussion

3.1. Micro-Flotation Results

The flotation performance of bastnaesite and fluorite was initially investigated as a function of pH and the concentration of OHA as a collector. Figure 3 depicts the effect of pH on the flotation behavior of bastnaesite and fluorite, indicating that the recoveries of both bastnaesite and fluorite enhanced with increasing pH until the pH values were less than 9.0, but decreased sharply in the strongly alkaline pH range. Therefore, the pH value was set to 9.0 in subsequent experiments, which is consistent with the conclusions reported in the literature [4,5,8]. Furthermore, the results indicated that the recovery of bastnaesite was slightly superior to that of fluorite in the set pH range and that OHA is an effective collector for bastnaesite. In addition, the gap of recoveries between bastnaesite and fluorite was not found to be affected by the adjustment of the pH value.
The effect of OHA concentration on the recoveries of bastnaesite and fluorite is illustrated in Figure 4. The recovery of bastnaesite and fluorite increased with increasing concentrations of OHA. However, when the OHA concentration exceeded approximately 7.5 × 10−4 mol/L, the growth rate tended to decline. Suitable concentrations of OHA (1.57 × 10−4 mol/L) resulted in recovery rates of over 95% for bastnaesite and fluorite, with the recoveries of both mineral samples being almost indistinguishable at the same OHA concentrations. These results showed the indispensable role of adding a specific depressant in the flotation separation of bastnaesite and fluorite.
Figure 5 shows the effect of SA concentration on the flotation performance of bastnaesite and fluorite. With an increase in SA concentration, the recovery of bastnaesite was reduced marginally compared to that of fluorite, which dropped dramatically, especially when the SA concentration exceeded 25.0 mg/L. At the SA concentration of 25.0 mg/L, the recovery of fluorite decreased from 95.6% to 15.0%, while that of bastnaesite remained around 80.0%, indicating a disparity in the recovery of two minerals >65%. These results underscored the suitability of SA for the flotation separation of bastnaesite from fluorite.

3.2. Adsorption Density Results

The addition of SA induced its adsorption on the surface of bastnaesite and fluorite (Figure 6), which, in turn, caused a significant variation in their recoveries. The adsorption density of SA on the surface of fluorite gradually increased with the increasing SA concentration. However, the increase in the adsorption density of SA on the fluorite surface was significant only when the SA concentration exceeded 26.0 mg/L. Moreover, under identical concentration conditions, the adsorption density of SA on the surface of fluorite was high compared to bastnaesite, which indicated a stronger interaction of SA with fluorite. Considering the abundance of HO– and –COO– groups in SA [15,17], the higher adsorption density suggested that SA rendered the mineral more hydrophilic and less recoverable.

3.3. Zeta Potential Analysis

To examine the mechanism of the interaction of SA with bastnaesite and fluorite, shifts in the zeta potential of the bilayer on the mineral surface were determined, as shown in Figure 7 and Figure 8. For pure fluorite (Figure 7), the zeta potential decreased progressively with the increasing pH, with an isoelectric point (IEP) of approximately 8.4, which aligns with the results of other researchers [5,21,22,23]. In conjunction with micro-flotation results, these results indicated that the fluorite surface was negatively charged at an optimum flotation pH value of 9.0.
In the presence of SA (Figure 7), the zeta potential of fluorite shifted remarkably in the negative direction compared with that of pure fluorite; however, the offset diminished with the increasing pH, indicating that SA adsorption occurred on the fluorite surface. Furthermore, regardless of the presence of chemisorption between the fluorite surface and SA, the electrostatic attraction between the positively charged fluorite surface and the anionic SA component caused a significant shift in the zeta potential of fluorite at pH values below the IEP of fluorite. Conversely, the electrostatic repulsion between the negatively charged fluorite surface and the anionic SA component reversed the effect at pH values above the IEP of fluorite. From another perspective, the deviation of the zeta potential of fluorite at a pH higher than the IEP revealed the existence of SA chemisorption on the fluorite surface.
In the presence of OHA (Figure 7), the zeta potential of fluorite exhibited a maximal negative shift similar to the trend observed in the presence of SA. In addition, it has been documented that OHA interacts with the fluorite surface through chemisorption and electrostatic adsorption [4].
The minimal zeta potential for fluorite was observed in the presence of both OHA and SA (Figure 7). The offset of the zeta potential was not significantly altered from the scenario in the presence of SA alone, most notably in alkaline conditions. However, the negative shift in the zeta potential was smaller than that in the presence of OHA alone. Hence, when SA was preferentially attached to the fluorite surface to make it negatively charged, the addition of OHA inhibited its further attachment on the surface of fluorite due to electrostatic repulsion or the spatial steric effect [18]. Consequently, with the adsorption of SA, the surface of fluorite became hydrophilic, and the micro-flotation recovery was diminished.
The zeta potential of the pure bastnaesite gradually decreased with the increasing pH (Figure 8), with the IEP being close to 9.1, which aligns with the results reported in previous studies [4,24,25,26,27]. Combined with the micro-flotation results, these results indicated that the surface of bastnaesite was slightly positively charged at an optimum flotation pH of 9.0.
In the presence of SA (Figure 8), the zeta potential of bastnaesite deviated considerably from that of pure bastnaesite. However, the shift was mitigated under strongly alkaline conditions. Again, this was indicative of the adsorption of SA on the surface of bastnaesite and the similarity of the interaction between SA and bastnaesite to that of fluorite with SA, as evident from the observed adsorption density.
In the presence of OHA (Figure 8), the zeta potential of bastnaesite shifted markedly in contrast to pure bastnaesite and bastnaesite with SA, especially at pH higher than 5.0. Nevertheless, the shift was similarly reduced under alkaline conditions. This result implied that the interaction of OHA with the bastnaesite surface stemmed from physical and chemical adsorption [28,29,30].
The shift in the zeta potential of bastnaesite in the presence of SA and OHA versus the zeta potential of pure bastnaesite and bastnaesite with SA is depicted in Figure 8. This implied that the adsorption of SA on the surface of bastnaesite was relatively weak and that OHA could attach further. However, the gap was reduced at a pH of 9, where the recovery of bastnaesite was optimal compared with that in the presence of OHA. Therefore, the recovery of bastnaesite decreased slightly.

3.4. FTIR

To elucidate the mechanism of SA adsorption on the surface of bastnaesite and fluorite, the FTIR spectra of minerals in the presence and absence of SA were recorded (Figure 9 and Figure 10). For SA, as shown in Figure 9, the broad band at 3341 cm−1 corresponded to the stretching vibration of the hydroxyl (C–OH) group, and the absorption peaks at 2851 cm−1 and 2921 cm−1 corresponded to the symmetric and antisymmetric stretching vibrations of the –CH– group, respectively [13]. The bands at 1583 cm−1 and 1413 cm−1 were ascribed to the antisymmetric and symmetric vibrations of the –COO– group; the weak signal at 1317 cm−1 was ascribed to the stretching vibration of the –COOH group; and the peak at 1025 cm−1 was attributed to the stretching vibration of the C–O–C group [15,31]. These results affirmed that the main functional groups in SA molecules are –COO–, –OH, and C–O–C, which are strongly hydrophilic due to their polar nature.
For pure bastnaesite, the bands at 1425 cm−1, 1073 cm−1, 854 cm−1, and 728 cm−1 were ascribed to the anti-symmetric stretching, symmetric stretching, out-of-plane bending, and in-plane bending vibrations of the –CO32− group, respectively (Figure 9) [5,32]. Additionally, no noticeable shifts in the characteristic peaks or the new absorption peaks were observed for bastnaesite following SA treatment. These results, along with the adsorption density results, suggest that SA interacts with bastnaesite through physisorption, mainly involving hydrogen bonding or electrostatic interaction [31].
For pure fluorite, a characteristic peak was observed at 1033 cm−1 (Figure 10) [33]. With SA treatment, distinct absorption bands were observed for fluorite, which were attributed to the C–OH, –CH–, and –COO– groups in the SA molecule. The absorption peak of the –CH– group at 2921 cm−1 suggested the adsorption of SA on the fluorite surface. Furthermore, the characteristic peaks of the –COO– groups were shifted to 1604 cm−1, and 1392 cm−1, respectively, which resulted from the chemical interaction of the carboxyl group in the SA structure with the fluorite surface. In addition, the band of fluorite deviated to 1065 cm−1. Thus, the results indicated that SA interacted with the fluorite surface through chemisorption, which conferred hydrophilicity to the fluorite. Overall, the results of FTIR corroborated the micro-flotation test results.

3.5. XPS Analysis

To further examine the interaction between SA and minerals, XPS was performed, which helps in understanding the changes in the atomic content and chemical environment of mineral surfaces. The results are presented in Figure 11 and Table 3. The fluorite surface was relatively rich in calcium (Table 3), which facilitated its easy recovery by fatty acid-based collectors [34,35]. The low contents of C and O on the fluorite surface inevitably resulted in environmental contamination [5,8]. With SA treatment, the C and O contents increased by 14.11% and 8.25% to 18.86% and 12.44%, respectively. These results affirmed the occurrence of SA chemisorption on the surface of fluorite. In striking contrast, C and O exhibited higher contents on the surface of the bastnaesite. The cationic Ce content was only 4.62%, and in that case, it was the source of inferior floatability compared to fluorite. With SA treatment, the contents of individual elements did not exhibit significant variations, reflecting the relatively weak adsorption of SA on the surface of bastnaesite. The results were consistent with those of the FTIR analysis.
The high-resolution XPS spectra of C1s, O1s, Ce3d, and F1s of bastnaesite in the absence and presence of SA treatment are depicted in Figure 12. In the absence of SA, the C1s spectrum (Figure 12a) consisted of four portions of peaks at 284.80 eV, 286.60 eV, 287.95 eV, and 289.57 eV attributed to the C–C, C–O–C, C–O=C, and bastnaesite components, respectively [4,5,8]. The peaks at 531.50 eV and 533.42 eV (Figure 12b) were assigned to the C–O and C=O components, respectively [36,37]. The Ce3d region (Figure 12c) of bastnaesite had spin-orbit components that were well separated into Ce3d5/2 and Ce3d3/2 units, and the peaks at 885.78 eV and 904.39 eV were the primary line of bastnaesite [21,38,39]. Since Ce atoms held only one 4f electron, the inner electrons of Ce3d5/2 and Ce3d3/2 were ionized when forming bastnaesite with F, C, and O atoms, which induced splitting of the characteristic peaks of Ce, leading to matching peaks of Ce3d5/2 and Ce3d3/2 at 882.30 eV and 900.35 eV, respectively. In addition, the F1s peak (Figure 12d) was located at 684.45 eV [36]. In the presence of SA, the binding energies of C1s, O1s, Ce3d, and F1s of bastnaesite all exhibited a very slight shift, indicating that SA did not bind to the surface of bastnaesite.
The high-resolution XPS spectra of C1s, O1s, Ca2p, and F1s of fluorite in the absence and presence of SA are shown in Figure 13. In the absence of SA, the C1s peaks (Figure 13a), accounting for inescapable contamination, were located at 284.80 eV, 286.47 eV, and 289.08 eV, which were attributed to the C–C, C–O–C, and C–O=C components, respectively [4,8,36]. In the presence of SA, the C1s binding energies in the C–O–C component underwent a significant shift, reducing to 288.65 eV. The altered binding energy signified a change in the chemical environment of C atoms in the C–O–C composition. In the absence of SA, the O1s peaks (Figure 13b) comprised C=O, C–O, and Ca–O components at 531.54 eV, 533.04 eV, and 530.35 eV, respectively [8]. In the presence of SA, these peaks were maintained at 531.91 eV, 533.39 eV, and 530.94 eV. The pronounced chemical shift of the O1s component indicated that the O atoms in the SA molecular structure bonded to the surface of fluorite, signifying the adsorption of SA on the fluorite surface. In the absence of SA, the peaks of Ca2p (Figure 13c) split into two spin-orbit splitting peaks, namely Ca2p3/2 and Ca2p1/2, at 348.56 eV and 352.10 eV, respectively [40,41,42]. In the presence of SA, the binding energies of the two peaks were off-shifted to 348.12 eV and 351.66 eV, respectively. Combined with the changes in the binding energies of the O1s peaks, these results indicated that the O atoms in the SA molecular structure were chemically bound to the Ca atoms on the fluorite surface. In contrast, the binding energies of the F1s peaks (Figure 13d) remained markedly unchanged in the absence and presence of SA, indicating that SA did not interact with F atoms on the fluorite surface.

3.6. Discussion

The results of flotation experiments and surface analysis clearly indicate that SA chemically interacts with the fluorite surface but interacts weakly with the bastnaesite surface. These results are consistent with those of Chen et al., who investigated the flotation separation of calcite and fluorite from scheelite; apatite from calcite; and chalcopyrite from galena [18,19,31]. A possible model for the interaction of SA with fluorite and bastnaesite is illustrated in Figure 14. Under alkaline conditions, chemically active functional groups in SA, such as –COO–, and HO–, coordinate with the mineral surface to form a cross-linked network structure, resulting in the formation of a hydrogel that boosts the hydrophilicity of the mineral surface [43].
In the flotation systems for fluorite or bastnaesite, the crystal structure of the mineral is a key factor that affects the ability of SA to exert selective inhibition. Bastnaesite crystals are arranged hexagonally, with Ce and F atoms in the same plane and alternating CeF2+ and CO32− layers [44]. The (100) plane is the most stable cleavage plane, where the coordination number of Ce atoms is reduced from nine to five. In contrast, fluorite crystals are attributed to a cubic crystal system, with Ca atoms closely packed along the planes and F atoms filling the tetrahedral interstitial positions [45,46]. The (111) plane is the most stabilized cleavage plane of fluorite, wherein the coordination number of Ca atoms is reduced from eight to four. The ionic radii of Ce3+, Ca2+, F−, and O2− are 1.03 Å, 0.99 Å, 1.36 Å, and 1.40 Å, respectively [47,48,49], indicating that Ce3+ and Ca2+ and F− and O2− exhibit almost similar ionic radii. When SA interacts with the fluorite surface, the surface Ca2+ ions leave approximately 4/8 of the space for the attachment of O2− ions in SA. Conversely, when SA interacts with the bastnaesite surface, the surface Ce3+ ions leave 4/9 of the space. In addition, F− and Ce3+ ions in the surface of bastnaesite are in the same plane, which negatively affects the adsorption between Ce3+ ions and the –COO– groups in SA. As a result, the –COO– groups in SA can coordinate with Ca2+ on the fluorite surface but are unable to coordinate with Ce3+ on the bastnaesite surface due to electrostatic repulsion and steric hindrance, as shown in Figure 14.
The ability of metal ions to complex SA is another important factor affecting the selective adsorption of SA in this flotation system. Both bastnaesite and fluorite are slightly soluble in water, generating free metal ions. These metal ions are readily hydrolyzed to form metal hydroxides that reattach to the surface of the respective minerals [18]. Because of the different electronic structures, these metal ions or their metal hydroxides have different complexing abilities than SA. It has been found that the carboxylatec omplexation equilibrium constant (lgK) of calcium ions is much larger than that of rare earth ions [50,51], which means that SA is more readily adsorbed on the surface of fluorite compared to bastnaesite. As a result, the hydrophilicity of SA results in the depression of fluorite, while exerting only a slight effect on bastnaesite.

4. Conclusions

In the flotation of bastnaesite and fluorite, SA, as an environmentally friendly depressant, exerted selective depressing effects. The gap in recovery reduction between fluorite and bastnaesite was more than 65% under desirable conditions, that is, pH of about 9, SA concentration of 25 mg/L, and OHA concentration of 1.57 × 10−3 mol/L. From the results of adsorption density, zeta potential measurement, FTIR, and XPS, it can be concluded that SA is chemically anchored to the fluorite surface and impedes the subsequent adsorption of OHA as a collector and that SA interacts weakly with bastnaesite, exerting no effect on the adsorption of OHA.

Author Contributions

C.G.: Methodology, Writing—original draft. S.H.: Visualization. W.W.: Methodology, Resources. Q.L.: Software. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of Inner Mongolia Autonomous Region (2023LHMS05050) and the National Key Research and Development Program of China (2022YFC2905300) projects.

Data Availability Statement

Data are contained within the article.

Acknowledgments

We would like to thank the Physical and Chemical Testing Center of Baotou Research Institute of Rare Earths and Science Compass for providing FTIR and XPS testing services.

Conflicts of Interest

The authors declare no conflicts of interest.

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Minerals 14 00165 g001
Figure 1. Chemical building blocks of sodium alginate.
Figure 1. Chemical building blocks of sodium alginate.
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Figure 2. X-ray diffraction pattern of bastnaesite and fluorite samples.
Figure 2. X-ray diffraction pattern of bastnaesite and fluorite samples.
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Minerals 14 00165 g003
Figure 3. Recovery of bastnaesite and fluorite as a function of pH (c (OHA) = 1.57 × 10−3 mol/L).
Figure 3. Recovery of bastnaesite and fluorite as a function of pH (c (OHA) = 1.57 × 10−3 mol/L).
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Figure 4. Recovery of bastnaesite and fluorite as a function of OHA concentration (pH = 9.0).
Figure 4. Recovery of bastnaesite and fluorite as a function of OHA concentration (pH = 9.0).
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Figure 5. Recovery of bastnaesite and fluorite as a function of SA concentration (c (OHA) = 1.57 × 10−3 mol/L; pH = 9.0).
Figure 5. Recovery of bastnaesite and fluorite as a function of SA concentration (c (OHA) = 1.57 × 10−3 mol/L; pH = 9.0).
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Figure 6. Absorption density of SA on the bastnaesite and fluorite surfaces as a function of concentration.
Figure 6. Absorption density of SA on the bastnaesite and fluorite surfaces as a function of concentration.
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Figure 7. Zeta potential of fluorite with the treatment of different reagent schemes as a function of pH (c (OHA) = 1.57 × 10−3 mol/L; c (SA) = 25.0 mg/L).
Figure 7. Zeta potential of fluorite with the treatment of different reagent schemes as a function of pH (c (OHA) = 1.57 × 10−3 mol/L; c (SA) = 25.0 mg/L).
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Figure 8. Zeta potential of bastnaesite with the treatment of different reagent schemes as a function of pH (c (OHA) = 1.57 × 10−3 mol/L; c (SA) = 25.0 mg/L).
Figure 8. Zeta potential of bastnaesite with the treatment of different reagent schemes as a function of pH (c (OHA) = 1.57 × 10−3 mol/L; c (SA) = 25.0 mg/L).
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Figure 9. FTIR spectra of bastnaesite in the absence and presence of SA.
Figure 9. FTIR spectra of bastnaesite in the absence and presence of SA.
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Figure 10. FTIR spectra of fluorite in the absence and presence of SA.
Figure 10. FTIR spectra of fluorite in the absence and presence of SA.
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Figure 11. The survey scan XPS spectra of bastnaesite and fluorite in the absence and presence of SA.
Figure 11. The survey scan XPS spectra of bastnaesite and fluorite in the absence and presence of SA.
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Figure 12. The high-resolution XPS spectra of (a) C1s, (b) O1s, (c) Ce3d, and (d) F1s of bastnaesite in the absence and presence of SA.
Figure 12. The high-resolution XPS spectra of (a) C1s, (b) O1s, (c) Ce3d, and (d) F1s of bastnaesite in the absence and presence of SA.
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Figure 13. The high-resolution XPS spectra of (a) C1s, (b) O1s, (c) Ca2p, and (d) F1s of fluorite in the absence and presence of SA.
Figure 13. The high-resolution XPS spectra of (a) C1s, (b) O1s, (c) Ca2p, and (d) F1s of fluorite in the absence and presence of SA.
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Figure 14. Schematic diagram of the adsorption model of SA on the surface of bastnaesite and fluorite.
Figure 14. Schematic diagram of the adsorption model of SA on the surface of bastnaesite and fluorite.
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Table 1. Chemical composition of the bastnaesite samples (wt %).
Table 1. Chemical composition of the bastnaesite samples (wt %).
ElementsREOCaOFTFeAl2O3REE Distribution
Y2O3La2O3CeO2Pr6O11Nd2O5
Content70.942.307.770.26<0.050.1831.4048.954.5113.13
ElementsSm2O3Eu2O3Gd2O3Tb4O7Dy2O3Ho2O3Er2O3Tm2O3Yb2O3Lu2O3
Content1.140.220.42<0.10<0.10<0.10<0.10<0.10<0.10<0.10
Table 2. Chemical composition of the fluorite samples (wt %).
Table 2. Chemical composition of the fluorite samples (wt %).
ElementsFCaOBaOREOTFeMgO
Contents46.4568.260.981.510.19<0.10
Table 3. Atomic content of bastnaesite and fluorite surfaces before and after SA treatment.
Table 3. Atomic content of bastnaesite and fluorite surfaces before and after SA treatment.
MineralAtomic Content (at, %)
C1sO1sF1sCa2pCe3d
Fluorite4.75%4.19%37.70%53.36%
Fluorite + SA18.86%12.44%27.54%41.16%
Bastnaesite47.16%41.78%6.43% 4.62%
Bastnaesite + SA45.22%44.76%5.34% 4.67%
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Guo, C.; Hou, S.; Wang, W.; Li, Q. Effect of an Environment-Friendly Depressant on the Flotation of Bastnaesite and Fluorite. Minerals 2024, 14, 165. https://doi.org/10.3390/min14020165

AMA Style

Guo C, Hou S, Wang W, Li Q. Effect of an Environment-Friendly Depressant on the Flotation of Bastnaesite and Fluorite. Minerals. 2024; 14(2):165. https://doi.org/10.3390/min14020165

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Guo, Chunlei, Shaochun Hou, Weiwei Wang, and Qiang Li. 2024. "Effect of an Environment-Friendly Depressant on the Flotation of Bastnaesite and Fluorite" Minerals 14, no. 2: 165. https://doi.org/10.3390/min14020165

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