The Efficient Separation of Apatite from Dolomite Using Fucoidan as an Eco-Friendly Depressant

Abstract

The aim is to explore new depressants for achieving the efficient separation of apatite and dolomite. In this work, fucoidan (FD) was examined as an eco-friendly dolomite depressant to separate dolomite from apatite. The depression ability and adsorption mechanisms were investigated. The flotation results indicated that FD selectively depressed dolomite. The flotation difference between dolomite and apatite reached 70% approximately at an FD concentration of 75 mg/L. Meanwhile, the recovery and grade of P₂O₅ reached 89.84% and 32.88% and that of MgO decreased to 1.64% and 34.24% in the artificially mixed minerals test. Wettability, zeta potential, and Fourier transform infrared spectroscopy (FTIR) results revealed that FD tended to adsorb onto dolomite, impeding the interaction of sodium oleate (NaOL) with dolomite, but barely affected that on apatite. Microcalorimetry analysis indicated that the adsorption heat of FD on dolomite was much higher and less time was required to achieve equilibrium. X-ray photoelectron spectroscopy (XPS) results proved that the sulfonic acid radicals within FD chemically interacted with Mg atoms on dolomite while it weakly adsorbed on apatite.

1. Introduction

As important strategic resources to extract phosphorus, phosphate ores play an indispensable role in the production of fertilizers, which have also been widely applied in many fields, such as food, defense, medicine, and new energy [1,2]. With the continuous depletion of phosphate ores, the exploration and utilization of low-grade phosphate ores are imperative to meet the constantly increasing demand for phosphorus elements, but unfortunately, most of the low-grade phosphate ores have not been explored yet due to the undeveloped reagents and techniques [3,4,5]. One of the important reasons is that the apatite generally coexists closely with carbonate and silicate minerals in these ores, in particular with dolomite, making the efficient separation difficult.

Froth flotation, as the major method to beneficiate phosphate ores, enables the selective separation of phosphates from calcium-containing gangue minerals through direct or reverse flotation [6]. Through the action of some specific surfactants, the hydrophobicity of mineral particles can be intentionally adjusted, based on which the different minerals have been separated by their hydrophobicity properties [7,8]. Fatty acid collectors are commonly used in the flotation of phosphate ores because of their low cost and good availability [9]. However, it is unable to separate them efficiently without the aid of depressants, because they possess similar surface properties and the selectivity of fatty acids for calcium-containing minerals is poor. In practice, the reverse flotation techniques are commonly adopted to upgrade phosphate ores, in which apatite depressants such as H₂SO₄ and H₃PO₄ are used to depress apatite while dolomite is floated by fatty acid collectors [10]. However, these reagents have significant disadvantages in many aspects; they not only cause serious corrosion damage to the flotation equipment and pipelines but also lead to serious environmental pollution and eutrophication.

To overcome the shortcomings of reverse flotation, much attention has been paid to exploring dolomite depressants to substitute conventional apatite depressants in direct flotation. Wang et al. proposed that tamarind seed gum (TSG) interacted with the dolomite surface by chemical bonding between the -OH group and Ca and Mg sites [11]. Du et al. [12] noted that carboxymethyl cellulose (CMC) could selectively depress dolomite without affecting apatite, which preferred to adsorb onto the dolomite surface because the two O atoms in the carboxyl group could combine with Ca and Mg atoms to generate covalent and ionic bonds, respectively, creating more stable bridge adsorption. Yang [13] discovered that hydrolytic polymaleic anhydride (HPMA) could produce stronger chelation with Mg²⁺ than Ca²⁺, which was the main reason why HPMA selectively depressed dolomite but did not influence the flotation of apatite. These novel depressants show good selective depression effect on dolomite but have not been widely used in practice yet, probably due to their limitation of availability, cost, and environmental pollution. Therefore, it is still a great challenge to explore novel depressants that are more efficient, cheap, and biocompatible for the recovery of low-grade phosphate ores.

As a water-soluble sulfated polysaccharide, FD was derived from brown algae resources, which has gained significant attention in feed processing and medical research due to its antioxidant and anticancer properties [14,15,16]. Meanwhile, because of the abundant algae resources in the ocean, the production of FD is continuously guaranteed [17]. Figure 1 illustrates the molecular structure of FD. Within molecular FD, there are many multifunction hydroxyl and sulfate groups, which may easily interact with metal ions such as calcium and magnesium. Moreover, due to its good availability and non-toxicity, it may be a good choice to depress dolomite in the flotation separation of phosphate ores.

In this work, the biodegradable polysaccharide FD has been used as a dolomite depressant to upgrade phosphate ores through direct flotation. Furthermore, the inhibition mechanism of FD in the selective flotation system was uncovered by means of wettability characterization, Zeta potential measurement, FTIR detection, microcalorimetry determination, and XPS analysis. This work aimed to develop a novel dolomite depressant in the direct flotation of phosphate ores and shed some light on the development of a dolomite depressant in the direct flotation of phosphate ores.

2. Experiment

2.1. Materials and Reagents

Dolomite and apatite mineral samples from Guizhou province were selected manually first and then ground and sieved separately. Mineral samples between 38 and 74 μm were subject to XRD and flotation tests, and mineral sample fines smaller than 38 μm were used to investigate the flotation mechanism. The apatite sample had a P₂O₅ grade of 38.77% determined through the volumetric titration method using ammonium phosphomolybdate, and the dolomite sample had a MgO grade of 22.42% determined via the EDTA volumetric method. XRD results (Figure 2) of the samples matched well with standard reference cards of PDF#15-0876 and PDF#36-042.

Fucoidan (Macklin, China) with a molecular weight of 242.25 was tested as a dolomite inhibitor. The analytical grade of sodium oleate (NaOL, collector), HCl, and NaOH (pH modifier) was acquired from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ultrapure water with a resistivity of more than 18.2 MΩ·cm was used for flotation and mechanism study.

2.2. Flotation Tests

An XFG-type flotation machine equipped with a 40 mL plexiglas cell was used for micro-flotation tests. About 2.0 g samples were dispersed in 30 mL of ultrapure water for 2 min first; then, pH modifier FD and NaOL solutions were introduced sequentially at 5 min intervals. The specific flotation flowsheet is illustrated in Figure 3. After aeration, the floated minerals and sinking minerals were collected, weighted, and assayed to evaluate the flotation results. Each test was carried out three times and the average was reported as the ultimate result. In the case of mixed minerals, 1.6 g apatite and 0.4 g dolomite samples were mixed and adopted for flotation; the experiment was performed as illustrated in Figure 3.

2.3. Contact Angle Measurement

The wettability of minerals surface treated and untreated was determined by the captive bubble method using a contact angle goniometer (JC2000DM, Beijing Zhongyi Kexin Technology Co., Ltd., Beijing, China). The surfaces of apatite and dolomite lumps were first polished using sandpaper and a polishing cloth. After that, the polished surface of mineral samples was soaked in a container with 110 mL of various solutions. Then, an air bubble (around 5 μL) was generated and placed onto the mineral surface using a microsyringe. At equilibrium, the image was captured, and the contact angle results were determined. For each sample, three different locations were measured, and the mean value was reported.

2.4. Zeta Potential Determination

Malvern Zeta Sizer (Nano90, Malvern Panalytical, Malvern, UK) was employed to measure zeta potential using the Smoluchowski equation; 0.2 g of minerals were dispersed into 50 mL of 0.001 mol/L KNO₃ solution first and then the pulp pH was adjusted. After that, FD or NaOL solutions were introduced and stirred for 10 min. Finally, a small portion of the pulp was taken for zeta potential determination. To ensure accuracy and reliability, each sample was tested three times.

2.5. FTIR Determination

The FTIR measurement was carried out in transmission mode using a Nicolet 6700 spectrometer (Thermo, Waltham, MA, USA); 2.0 g of sample was uniformly dispersed and stirred in 70 mL solution (75 mg/L FD or 75 mg/L FD with 3 ×10⁻⁴ mol/L NaOL) for 5 min. After adsorption, the filtered samples were freeze-dried for FTIR tests.

2.6. Microcalorimetry Analysis

Microcalorimetry analysis was conducted using a Microcalorimeter (C80). About 100 mg of pure samples (−74 μm) and 1 mL of FD solution were placed in the upper and lower calorimetric cells, which were separated by membranes, respectively. After the film was pierced, the change in heat flow after the interaction between the reagent and minerals was recorded. The measurement was terminated, and the results were obtained once the signal stabilized. The specific surface areas of dolomite and apatite samples were determined using the BET method, which were 0.1827 m²/g and 0.3411 m²/g, respectively.

2.7. XPS Analysis

Escalab 250Xi photoelectron spectrometer (Thermo, Waltham, MA, USA) was used for XPS analysis. The samples were treated in the same manner as mentioned in Section 2.5. The C_1s peak at 284.80 eV was used as the standard to calibrate the spectra.

3. Results and Discussion

3.1. Micro-Flotation of Single Mineral

3.1.1. Influence of FD Concentration

The flotation performances as affected by FD dosage are illustrated in Figure 4. Without the addition of depressants, both apatite and dolomite floated well and it was hard to separate them. Once FD was employed, the floatability of dolomite dropped significantly; in contrast, the flotation of apatite remained around 90% as FD concentration increased. The flotation difference was expanded to 70% at an FD concentration of 50 mg/L, illustrating that FD may be a potential dolomite depressant capable of selectively depressing dolomite in direct flotation.

3.1.2. Effect of pH

Figure 5 shows the impact of pH on the flotation with the addition of FD. Dolomite was strongly depressed at pH less than 10, above which, it increased slightly. As for apatite, it was slightly depressed but still maintained high floatability at a weak alkaline medium. However, it regained its floatability as pH increased above 10. The optimized difference (approximately 70%) was gained at pH of 10, which provided an optimum separation window for these two minerals. These results reconfirmed that FD could be a good dolomite depressant to upgrade phosphate ores by means of direct flotation, which provided a good option for recovering apatite.

3.1.3. Effect of NaOL Concentration

Figure 6 shows the depression ability of FD as related to NaOL concentration. Dolomite was strongly depressed at NaOL dosage less than 3 ×10⁻⁴ mol/L, but it ascended significantly as NaOL concentration exceeded 3 ×10⁻⁴ mol/L. As for apatite, the flotation was almost not influenced by NaOL concentration. This phenomenon may be attributed to the fact that high NaOL concentration increased the possibility of NaOL species bonding with Ca and Mg sites on the dolomite surface, thereby weakening the depression ability of FD to dolomite. These results suggested that high collector concentration could worsen the separation performance of FD on dolomite and apatite.

3.1.4. Effect of Temperature

Figure 7 represents the influence of pulp temperature on flotation. The recovery of apatite remained around 90% in the whole temperature range. For dolomite, the floatability gradually increased as temperature increased from 10 to 50 °C, which may be attributed to the fact that high temperature promoted the adsorption of NaOL on the dolomite surface, which deteriorated the selectivity of FD [18].

3.2. Flotation of Artificially Mixed Mineral

To further confirm the selectivity of FD in flotation, the flotation of artificially mixed minerals was conducted; the results are shown in Figure 8. In the absence of FD, more than 70% dolomite was floated. But as FD was introduced, both the grade and recovery of MgO dropped significantly; meanwhile, the grade and recovery of P₂O₅ barely changed at an FD concentration of less than 75 mg/L. When FD concentration was 75 mg/L, the mixed ore achieved a better separation effect, at which the recovery and grade of P₂O₅ reached 89.84% and 32.88% and that of MgO decreased to 1.64% and 34.24%, respectively. These results suggested that FD was able to depress dolomite selectively using NaOL as a collector.

3.3. Contact Angle Results

Figure 9 indicates the wettability image of mineral surface conditioned in various solutions, and the results are shown in

Table 1. Contact angle results of dolomite and apatite.

Minerals	Various Conditions
	Ultrapure Water	3 × 10−4 mol/L NaOL	75 mg/L FD and 3 × 10−4 mol/L NaOL
Dolomite	28.58° ± 1.36°	73.67° ± 1.65°	30.08° ± 3.48°
Apatite	26.75° ± 0.94°	81.92° ± 0.72°	60.50° ± 3.36°

. For pure dolomite and apatite treated with ultrapure water, the contact angle was 28.58° and 26.75°, respectively. Upon the addition of 3 × 10⁻⁴ mol/L NaOL, the contact angle of the dolomite and apatite surface surged to 73.67° and 81.92°, respectively. After the addition of 75 mg/L FD and 3 × 10⁻⁴ mol/L NaOL, the contact angle of dolomite dramatically decreased to 30.08°. In comparison, the contact angle of apatite decreased slightly to 60.50°. The contact angle results proved that FD could greatly enlarge the hydrophobicity difference between apatite and dolomite in NaOL solutions, possibly due to the differential affinity of FD and NaOL toward the mineral surface.

3.4. Zeta Potential Results

Figure 10 presents the zeta potential of dolomite and apatite with different reagents. In Figure 10, after the introduction of FD of NaOL alone, the absolute value of zeta potential for both minerals increased dramatically, which means that both FD and NaOL could interact with apatite and dolomite individually without selectivity [19]. In Figure 10a, after the addition of both reagents, the zeta potential of dolomite approached the value conditioned by FD, which suggested that the adsorption of FD on dolomite was predominant. Compared with dolomite, the zeta potential for apatite (conditioned with FD and NaOL) was close to that conditioned with NaOL alone, as indicated in Figure 10b, which means that the adsorption of NaOL prevailed. These results reflect the preferential adsorption of FD and NaOL on dolomite and apatite surfaces, which confirmed the flotation results.

3.5. FTIR Analysis Results

Figure 11 displays the FTIR results under different experimental conditions. For dolomite and apatite treated with NaOL alone, the methylene and methyl groups originating from NaOL occurred around 2925 cm⁻¹ and 2854 cm⁻¹ [20]. Under the circumstance where both reagents were added, the -CH₂ and -CH₃ groups of NaOL vanished (Figure 11a), while they still existed on the apatite surface (Figure 11b). The results showed that FD blocked NaOL from interacting with the dolomite surface but did not influence the interaction of NaOL with apatite.

3.6. Microcalorimetry Determination

Figure 12 represents the adsorption heat of FD on apatite and dolomite. It is obvious that the reaction between reagent and mineral was exothermic. By comparing these results, the adsorption heat between FD and dolomite was −1.598 J/m² before it reached equilibrium at 0.71 h (Figure 12a). While adsorption heat between FD and apatite was −0.794 J/m², 8 h was required to achieve the equilibrium, as illustrated in Figure 12b. These results proved that the reaction between FD and dolomite was much stronger and faster than that of apatite [21].

3.7. XPS Analysis Results

Figure 13 displays the spectra of Ca_2p, O_1s, Mg_1s, and S_2p on dolomite surface. In Figure 13a, the peaks at 347.24 eV and 350.82 eV were associated with Ca_2p 3/2 and Ca_2p 1/2 in dolomite, respectively [22,23]. After the introduction of FD, a weak shift (0.03 eV) of Ca_2p was observed, indicating that Ca atoms did not take part in the reaction with FD [24,25]. In Figure 13b, the peak of CO₃²⁻ almost did not change after the addition of FD, indicating that CO₃²⁻ in dolomite did not interact with FD. In Figure 13c, the Mg_1s peak of dolomite moved downward by −0.21 eV after the addition of FD [26], which proved that Mg atoms in dolomite chemically interacted with FD [27]. In Figure 13d, the S_2p 3/2 and S_2p 1/2 peaks of -SO₃ appeared at 168.49 eV and 169.65 eV after adsorption [28].

Figure 14 shows the high-resolution spectra of apatite with or without FD. In Figure 14a, the two peaks corresponded to Ca_2p 3/2 (347.53 eV) and Ca_2p 1/2 (351.06 eV) shifted just by 0.03 eV after adsorption, which means that the Ca atoms may not bond chemically with FD. In Figure 14b–d, the O_1s, F_1s, and P_2p orbital almost did not move after the addition of FD, suggesting that FD may not interact with these atoms on the apatite surface. In addition, Figure 14e showed that the S_2p signal on apatite became very vague after the treatment with FD, proving that -SO₃ groups within FD had a weak affinity toward the apatite surface. Overall, the XPS results demonstrated that the -SO₃ groups in FD may strongly interact with Mg sites chemically on the dolomite surface but physically adsorbed onto the apatite surface, which altered the adsorption of NaOL and resulted in separation.

4. Conclusions

The flotation performance and depression mechanism of a new dolomite depressant FD were studied in this work. The flotation results indicated that FD exhibited good selectivity in the beneficiation of apatite from dolomite using NaOL as a collector. At an FD dosage of 75 mg/L, the flotation difference between dolomite and apatite reached approximately 70% in single minerals flotation tests. Meanwhile, the recovery and grade of P₂O₅ reached 89.84% and 32.88% and that of MgO decreased to 1.64% and 34.24% in the artificially mixed minerals test. Wettability, zeta potential, FTIR, and microcalorimetry results confirmed that FD had a stronger affinity toward dolomite compared with apatite. XPS analysis showed that FD could chemically adsorb onto dolomite due to the strong interaction between sulfonic acid radicals and Mg sites, but it physically adsorbed onto the apatite surface.