Effect of Acidified Sodium Silicate on the Flotation Separation of Microfine Apatite from Chlorite in Seawater

Abstract

In this work, selective flotation separation of microfine apatite and chlorite was achieved by using sodium oleate (NaOL) as a collector with a low dosage of acidified sodium silicate (ASS) as a depressant. The optimum ratio of sodium silicate to sulfuric acid for ASS was 5:3, and a good separation effect was also achieved in the mixed ore system. Compared to the deionized water system, the ions of Na⁺, Ca²⁺ and Mg²⁺ in seawater adsorbed on the surfaces of apatite and chlorite, which made the zeta potential of the two minerals shift positively. This presented a challenge to the selection of reagents for mineral separation. The addition of ASS changed the pH value of the pulp from weak alkalinity caused by seawater to weak acidity, which allowed the metal ions adsorbed on the mineral surface to desorb. Meanwhile, ASS can selectively adsorb on the desorbed chlorite surface in the form of Si(OH)₄, which hindered the action of NaOL, leading to the depression of chlorite. NaOL adsorbed well on the desorbed surface of apatite and increased the apatite particle size from 27 μm to 229 μm, with a hydrophobic agglomeration effect, thus enhancing the flotation of microfine apatite.

1. Introduction

Phosphorus, as one of the most abundant elements on earth and an irreplaceable element of life, is mainly extracted from phosphate ores [1,2]. As a one-time non-renewable valuable resource, phosphate ore resources are gradually being reduced due to continuous exploitation and utilization. Consequently, the efficient development and utilization of microfine, low-grade phosphate ores is becoming increasingly important [3,4]. In the natural environment, apart from their occurrence in various rare earth minerals, a significant proportion of rare earth elements occur isomorphically in phosphorus-bearing rocks. Apatite is a major phosphorus-bearing mineral, particularly in recent years, and it has been identified as a carrier of rare earth elements in deep-sea sediment [5,6]. These types of minerals also exist mainly in the form of microparticles, and carbonate minerals (such as dolomite and calcite) and silicate minerals (such as quartz and chlorite) are their main associated gangue minerals [7,8]. Therefore, the investigation of the separation of microfine apatite and its associated microfine gangue minerals (such as chlorite) in seawater systems is of significant importance.

Flotation has still been recognized as an efficient method for separating and recovering apatite from chlorite, which requires large amounts of water [9,10]. Seawater accounts for 97% of the Earth’s water resources and contains various ions, such as Na⁺, Mg²⁺, Ca²⁺, Cl⁻, ${\mathrm{SO}}_4^{2-}$, ${\mathrm{HCO}}_3^{-}$, etc. [11,12]. The use of seawater for mineral flotation processes has significantly addressed the issue of freshwater scarcity, particularly in the development of deep-sea mineral resources, which can significantly reduce transport and extraction costs at the same time [13,14,15]. However, the presence of various ions in seawater is considered detrimental to mineral flotation under alkaline conditions, especially for ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ [16,17,18]. Castro et al. found that the hydroxy complexes and the precipitates by ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ adsorbed on the molybdenite surface and then formed a hydrophilic coating that hinders the adsorption of a collector [19]. Thus, seawater flotation inevitably presents challenges in the separation of microfine apatite from chlorite.

In terms of flotation separation of apatite in deionized water, fatty acids or fatty acid derivatives, such as sodium oleate (NaOL), are generally used as collectors [20,21]. The main active site of apatite is the calcium ion, which is able to easily react with the carboxyl group of NaOL to form hydrophobic calcium oleate, leading to the efficient recovery of apatite [22,23]. Zhang et al. showed that NaOL can also achieve over 90% recovery of chlorite using NaOL as the collector [24]. The above investigations indicate that fatty acid-based reagents can easily chemically react with both apatite and chlorite. Therefore, the flotation separation of apatite from chlorite cannot be realized using NaOL only and exploring efficient depressants is of critical and practical importance.

The flotation and separation of chlorite in deionized water has been extensively studied, resulting in a relatively well-established reagent regime. Up to now, several depressants have been studied in the flotation separation of chlorite. Carboxymethyl cellulose (CMC) has been reported as a highly efficient depressant for chlorite [25]. Feng et al. reduced chlorite recoveries to less than 10% by using CMC as a depressant, owing to the fact that CMC adsorbed on the chlorite surface by both electrostatic interactions and hydrogen bonding, which increased the negative potential of the chlorite surface and reduced the adsorption of negatively charged collector molecules, such as xanthate, on the chlorite surface [25,26,27]. Sodium lignosulfonate also showed an excellent depressant effect on chlorite flotation [28]. However, the calcium ion sites on the apatite surface could react with CMC and sodium lignosulfonate to form a hydrophilic film, which depressed apatite flotation to some extent [29,30].

At present, the main selective depressant for aluminosilicate minerals is sodium silicate (SS), as the acid roots of silicate could be able to adsorb stably on the mineral surface [31]. SS exhibits better efficiency when it is used in conjunction with acid. Zhang improved the selective depression of quartz by mixing SS with sulfuric acid, mainly attributed to the fact that colloidal silica, the most active form of acidified sodium silicate (ASS), could well-adsorb on the silica–oxygen tetrahedral group of the quartz surface [32]. However, the adsorption of ASS on the surface of apatite was low, resulting in the selective separation of apatite from quartz [32]. Unfortunately, ASS was used in a large amount, and a further explanation of the selective mechanism of ASS was not provided. Moreover, no relevant studies have been carried out on the flotation separation of microfine apatite from chlorite, particularly in seawater.

To effectively separate microfine apatite from chlorite in seawater, in this study, the depression performance of ASS was investigated through flotation experiments using NaOL as a collector. ASS was a mixture of sodium silicate and sulfuric acid at a certain mass concentration. The depressant mechanism of ASS was revealed through different adsorption behaviors on apatite and chlorite. It is expected that our results will provide an effective way for the development and exploitation of phosphate resources, especially the flotation separation of microfine apatite from silicate minerals in seawater.

2. Materials and Methods

2.1. Pure Minerals and Reagents

High purity samples of apatite and chlorite for this study were obtained from Africa and the Zhejiang province in China. The phase analysis of the pure mineral samples was carried out by X-ray diffraction (XRD). The XRD results (Figure 1) showed that the purity of apatite and chlorite was high without impurities. The purity of apatite and chlorite was over 95%, which met the test requirements. After crushing and grinding, the samples were sieved to obtain the desired particle size. The results of laser particle size analysis of the samples indicated that 80% of the samples were less than 25 μm in size, as shown in Figure 2. The volumetric average particle size was less than 20 μm. The main chemical reagents employed in this study, including NaOL, NaCl, H₂SO₄, CaCl₂ and MgCl₂, were analytical grade. The SS used was a type of water-soluble silicate with a modulus of 2.5. The ASS was a mixture of SS and sulfuric acid. Deionized water was used for all tests.

2.2. Flotation Experiments

Before each experiment, a certain concentration of ASS and NaOL was prepared, and the artificial seawater solution was prepared according to

Table 1. Artificial seawater formula/(g/L) [33].

NaCl	MgCl2	MgSO4	CaCl2	NaHCO3	KCl	NaBr
26.60	2.628	3.186	1.26	0.51	0.715	0.216

. The pH of artificial seawater was about 8.1. Flotation experiments were carried out using an XFG flotation machine (Jilin Exploration Machinery Plant, Changchun, China) with a maximum volume of 40 mL. A pure apatite sample of 5.0 g was mixed with 40 mL of artificial seawater in the flotation cell, and the pulp was mixed at 1992 rpm for 2 min for each test. The depressant was then added to the pulp and conditioned for 3 min. NaOL was then added and conditioned for 3 min. After 4 min of flotation, the concentrate was obtained.

2.3. Zeta Potential Measurements

The zeta potential measurements were carried out by Nanoparticle Size and Zeta Potential Analyzer (Nano ZS90, Malvern Panalytical, Malvern, UK). All samples with a mass of 50 mg were ground to −5 µm in an agate mortar and then added to 40 mL of artificial seawater. H₂SO₄ and NaOH were used as pH adjusters. The reagent was then added. The addition order and mixing time of the reagents were the same as those for the flotation experiments. The pulp was stirred with a magnetic stirrer for 8 min, precipitated for 2 min to remove the large particles in the suspension, and then the supernatant of the suspension was extracted for zeta potential measurement. The zeta potentials of the samples were recorded under different pH conditions. By measuring three independent samples under the same conditions, the mean value of the zeta potential was obtained.

2.4. SEM Measurements

A field emission scanning electron microscope (FE-SEM, JSM-7900F) at 15 kV coupled with energy-dispersive X-ray spectroscopy (EDS, JEM-F200, JEOL Ltd., Tokyo, Japan) was used to measure the morphology and chemical composition of deionized water, seawater and seawater products from the interaction of ASS with minerals. A 5 g mineral sample was placed in a beaker, stirred according to different experimental conditions and dried in a freeze dryer.

2.5. Adsorption Test

The adsorption capacity of NaOL on the apatite and chlorite surfaces was determined using the Ultraviolet and Visible Spectrophotometer (UV-2600, Repligen Corporation, Waltham, MA, USA). For each measurement, 5.0 g of pure specimens and 40 mL artificial seawater were added into the beaker by stirring on a magnetic stirrer at a speed of 1000 rpm. The suspension was stirred for 2 min before the ASS was added. After that, the pulp was agitated for 3 min before NaOL was added to the system. The slurry was stirred for another 3 min and then centrifuged for 10 min. The supernatant was filtered through a 0.2 μm membrane before the subsequent measurement. First, standard curves of NaOL concentration and absorbency are shown. Then, the residual concentration method was used to measure the amount of NaOL adsorbed on the mineral surface. Each measurement was carried out three times, and the mean value was recorded. The adsorption ability of NaOL on the specimens can be calculated by Equation (1).

$$\tau =304.44({\mathrm{C}}_0-\mathrm{C})\mathrm{V}/\mathrm{m}$$

(1)

where $\tau$ (mg/g) was the adsorption capacity of NaOL on the mineral surface; C₀ (mol/L) and C (mol/L) represented the concentration of NaOL before and after adsorption, respectively; and V (mL) and m (g) were the pulp volume and the specimen quality, respectively.

2.6. FTIR Analysis

FTIR analysis was carried out on an infrared spectrometer (IRAffinity-1, Shimadzu Scientific Instruments, Kyoto, Japan) using the potassium bromide tablet method. Five grams of the mineral sample was dispersed in 40 mL of artificial seawater, and then flotation agents were added according to the experimental conditions. The pulps were then centrifuged and washed at least three times with artificial seawater and dried in a freeze dryer. The dried minerals were mixed with potassium bromide at a ratio of 1:100 and then ground to −5 µm in an agate mortar. Finally, a small amount of the mixture was put into a tablet presser to be pressed into a transparent tablet, and the infrared spectrum was recorded at 4000–450 cm⁻¹.

2.7. Observation of Hydrophobic Agglomerates

An optical microscope (OLYMPUS CX33, PLENIC-Pro, Hangzhou, China) was used to observe the agglomeration phenomenon and particle size of microfine apatite and chlorite before and after the addition of different reagents in seawater. The preparation conditions of the samples were similar to those in the micro-flotation experiments in each test. After stirring, 1 mL of the pulp was extracted with a pipette and placed on a glass slide so that the aggregates in the pulp could be distributed as much as possible in a single layer. The slide was placed under a light microscope, and the sample was observed at 100$\times$ magnification. Then, the particle size analysis of the mineral particle agglomerates was carried out using OPLENIC-Pro software (OLYMPUS CX33, OPLENIC-Pro, Hangzhou, China). Each group of experiments was sampled 10 times to ensure accurate data.

3. Results and Discussion

3.1. Microfine Apatite and Chlorite Flotation Experiments

The effect of NaOL dosage on the flotation recovery of apatite and chlorite was studied through single mineral flotation tests in seawater system, and the results are shown in Figure 3. It is evident from Figure 3 that the flotation recoveries of both apatite and chlorite were high. With increasing levels of NaOL dosage, the flotation recovery of apatite gradually increased, while the flotation recovery of chlorite changed little. The recoveries of apatite and chlorite reached 86.39% and 72.18%, respectively, when the NaOL dosage was 350 mg/L, where the recoveries of apatite and chlorite were the most different. The results indicated that it was difficult to separate apatite from chlorite without the addition of a depressant in a seawater system.

The effect of ASS (a mass concentration ratio of 5:3 of SS to sulfuric acid) on the flotation of apatite and chlorite in seawater was studied, and the results are shown in Figure 4. Figure 4 showed that the recovery of apatite decreased slightly as the metal ions on the apatite surface were completely dissociated, and the recovery of chlorite decreased rapidly to 9.07% at an ASS dosage of 30 mg/L and NaOL dosage of 350 mg/L. When the concentration of ASS increased to 100 mg/L, the pulp pH of apatite was 5.1, and that of chlorite was 7.1. The recovery of apatite was basically not affected, and the recovery of chlorite was not further reduced. When the ASS dosage was higher than 100 mg/L, the apatite recovery gradually decreased due to the pulp pH below 4, which was not conducive to NaOL adsorption. It was obvious that the separation of apatite from chlorite in artificial seawater could be realized at an ASS dosage of 100 mg/L. To further enhance the flotation separation effect, the NaOL dosage was increased to 450 mg/L. It can be seen from Figure 4 that the recovery of apatite was increased to 90.84%, and the recovery of chlorite was 9.34%.

To better confirm the impact of ASS on the flotation performance of these two minerals, the recoveries of apatite and chlorite as a function of different mixing ratios of SS and sulfuric acid were investigated, and the results are shown in Figure 5. It can be seen from Figure 5 that the optimum dosage to achieve selective depression varies at different ratios due to the different acidity of the system. When the ratio of SS to sulfuric acid was increased from 1:0 to 1:1, the recovery of apatite increased from 35.07% to 71.94%, and the recovery of chlorite decreased from 29.97% to 10.73%. By further increasing the mixing ratio to 5:3, apatite recovery increased to approximately 85% and chlorite recovery was maintained at around 9%. As the mixing ratio continued to increase to 2:1, there was no significant increase in the recovery of apatite. Therefore, it can be seen that the optimum ratio of SS to sulfuric acid is 5:3. SS and H₂SO₄ were mixed to form Si(OH)₄, which can stably adsorb on the surface of chlorite because it had the same silicon–oxygen tetrahedron structure as chlorite; however, it will not adsorb on the surface of apatite by electrostatic action.

In order to further verify the depressing impact of ASS in the flotation separation of apatite and chlorite in seawater, artificial mixed mineral experiments were carried out with a mass ratio of 1:1 of apatite to chlorite. As described in Figure 6, When the depressant ASS was not added, the recovery and grade of P₂O₅ were 67.54% and 17.3%, respectively, using 350 mg/L NaOL. Since the P₂O₅ grade of the original minerals mixture was 19.87%, the separation of apatite from chlorite was insignificant. When ASS was added to the mixed mineral samples, the recovery and grade of P₂O₅ reached 81.56% and 27.46%, respectively. Thus, the grade and recovery of P₂O₅ was greatly improved. This result indicated that the depressant ASS could achieve the flotation separation of chlorite from apatite in seawater.

3.2. Zeta Potential Measurement

The adsorption behavior of different flotation reagents might also change the zeta potential of minerals. In this study, the zeta potentials of apatite and chlorite were investigated as a function of pH under different conditions in artificial seawater. The results are demonstrated in Figure 7. As presented in Figure 7a, the isoelectric point (IEP) of pure apatite in deionized water was around pH 4.0, similar to the studies of Zhan and Kou et al. [34,35]. However, the zeta potential of apatite in seawater remained positive over the entire pH range and increased slightly with increasing pH value. It has been shown that positively charged metal ions in seawater adsorb on the surface of apatite [33]. With the addition of ASS, the surface of apatite became negatively charged.

In Figure 7b, the addition of seawater resulted in significant shifts in the IEP of chlorite, which moved from 3.2 in deionized water to 4.3 in seawater. It could be inferred that the adsorption of positively charged metal ions from seawater occurs on the surface of chlorite. The zeta potential of chlorite in seawater after the effect of the reagent showed the same trend as that of apatite, which first decreased and then gradually increased. Under alkaline conditions, the zeta potential of apatite and chlorite gradually increased, probably due to the presence of metal ions in the form of hydroxyl compounds, which are more likely to be adsorbed on the mineral surface. At the same time, hydroxyl compounds of metal ions reacted with SS, which reduced the adsorption of the negatively charged silicate component to the mineral surface [36].

3.3. SEM–EDS Analysis

To investigate the effects of seawater and the addition of ASS on the morphology and chemical composition of the two mineral surfaces and to further verify the mechanism of the zeta potential changes in Figure 7, SEM–EDS studies were carried out, and the results are shown in Figure 8 and Figure 9. As shown in Figure 8a,b and Figure 9a,b, the morphology of apatite and chlorite in deionized water and seawater hardly changed, with apatite showing a smooth block structure and chlorite showing a layered accumulation structure. EDS analysis results showed that compared with the deionized system, the contents of Na, Mg and Ca on the surfaces of apatite and chlorite in seawater have increased to varying extents. Figure 8c and Figure 9c indicated that with the addition of ASS, the morphology of apatite and chlorite remained essentially unchanged compared to the seawater system, but the elemental content of Na and Ca on the apatite surface was significantly reduced, and Mg was essentially absent. On the other hand, the Si content on the chlorite surface obviously increased, the Na content decreased and the Ca element disappeared. These results, therefore, further demonstrated that metal ions from seawater adsorbed on the surface of minerals, and the addition of ASS caused the metal ions to dissociate to different degrees, which was consistent with the results in Figure 7.

3.4. Adsorption Test

The results of flotation experiments in Figure 4 and Figure 6 indicated that ASS could achieve selective separation of apatite from chlorite in seawater. In order to understand the effect of ASS on the adsorption capacity of NaOL on the surfaces of apatite and chlorite, an adsorption capacity test was carried out at the NaOL dosage of 350 mg/L, and the result is shown in Figure 10. It can be seen from Figure 10 that without ASS, the amounts of NaOL adsorbed on the surfaces of apatite and chlorite were almost the same, which greatly benefited their high flotation recovery in Figure 3. As the dosage of ASS increased to 100 mg/L, the adsorption of NaOL on the surface of chlorite decreased from 2.27 mg/g to 0.52 mg/g, while the adsorption capacity of NaOL on the surface of apatite changed relatively little. Hence, the adsorption capacity of NaOL on apatite was almost five times higher than that on the surface of chlorite. The marked discrepancy in adsorption amount might result in the discrepant flotation recoveries of the two minerals.

3.5. FTIR Analysis

To further investigate the adsorption mechanism of ASS and NaOL on the two mineral surfaces, FTIR spectra were obtained. The results are shown in Figure 11 and Figure 12. Figure 11 demonstrates the IR spectra of NaOL and ASS. The IR spectra of NaOL are shown as line 1 in Figure 11. The bands at 2920 cm⁻¹ and 2850 cm⁻¹ were attributed to the stretching vibrations of -CH₃ and -CH₂- [37]. The peaks at 1560 cm⁻¹ and 1446 cm⁻¹ corresponded to the stretching vibration of -COO- and -CH₂- groups [37,38]. For ASS, in line 2, the stretching vibrations of O-Si-O were detected to varying degrees at 713 cm⁻¹, 1658 cm⁻¹, 993 cm⁻¹ and 1170 cm⁻¹ [39].

Figure 12 shows the IR spectra of apatite and chlorite treated with and without reagents. As seen in Figure 12a, the characteristic peaks at 1093 cm⁻¹ and 1047 cm⁻¹ were attributed to the asymmetric stretching vibration of P-O from the apatite, and the symmetric stretching vibration of P-O was observed at 603 cm⁻¹ and 576 cm⁻¹ [40,41]. Line 2 of Figure 12a shows the IR spectrum of apatite after the interaction with ASS, and the characteristic peak of ASS was not found. The IR spectrum of apatite interacting with NaOL is shown in line 3 of Figure 12a. The characteristic peaks of NaOL at 2856 cm⁻¹ and 2927 cm⁻¹ appeared. In addition, the stretching vibration band of -COO- was observed at 1622 cm⁻¹, indicating that NaOL was absorbed on the apatite surface. The IR spectra of apatite interacting with the addition of ASS and NaOL are shown in line 4. By comparing the spectra of lines 3 and 4, the characteristic adsorption bands of NaOL still appeared at 2927 cm⁻¹, 2856 cm⁻¹ and 1537 cm⁻¹. These results illustrated that ASS did not adsorb on the apatite surface and would not affect the adsorption of NaOL on the apatite surface.

Line 2 in Figure 12b showed the IR spectrum of chlorite after interaction with ASS. The characteristic O-Si-O peak of ASS was detected at 1641 cm⁻¹. The IR spectrum of chlorite after interaction with NaOL was shown as line 3 in Figure 12b. New bands of NaOL appeared at 2927 cm⁻¹, 2856 cm⁻¹ and 1622 cm⁻¹ in line 3 due to the stretching vibrations of the -CH₃, -CH₂- and -C=O- groups, which were slightly shifted. When treated with ASS and NaOL in sequence, the characteristic adsorption bands of NaOL disappeared, and the characteristic peak of ASS still occurred at 1656 cm⁻¹ in line 4 in Figure 12b, which illustrated that ASS adsorbed on the chlorite surface and interfered with the adsorption of NaOL in seawater.

3.6. Hydrophobic Agglomeration

In order to investigate whether there existed agglomeration behavior of microfine apatite and chlorite under the action of ASS and NaOL, the optical microscope images and particle size of the two microfine minerals before and after the action of different reagents were compared, and the results are shown in Figure 13. The optical microscopy images of Figure 13a clearly indicated that all the apatite fines were in a dispersed state in the form of their original size in the absence of reagents. When 100 mg/L ASS was added, the apatite particles appeared slightly agglomerated but mostly dispersed, with a maximum particle size of 69 $\mathsf{\mu }\mathrm{m}$. After the addition of 350 mg/L NaOL, there was an obvious agglomeration phenomenon. Furthermore, the agglomerates were observed to be exceedingly compact, and the particle size further increased to 229 $\mathsf{\mu }\mathrm{m}$. Figure 13b shows that the chlorite particle size increased from 24 $\mathsf{\mu }\mathrm{m}$ to 55 $\mathsf{\mu }\mathrm{m}$ at an ASS dosage of 100 mg/L. With the addition of NaOL, apparent agglomeration was not observed, and the size of the chlorite particles did not change significantly. Most of the chlorite particles were around 66 $\mathsf{\mu }\mathrm{m}$ in size, and the agglomerates were in a loose structure.

Hydrophobic agglomeration mainly depends on the hydrophobicity of mineral particle surfaces; the stronger hydrophobicity, the better agglomeration behavior and the more regular agglomerates [42,43]. When ASS was added, the microfine apatite and chlorite appeared partially agglomerated but with a looser structure, presumably due to the desorption of metal ions adsorbed on the mineral surface by the addition of ASS, which reduced the electrostatic repulsion between the particles [44]. The addition of NaOL resulted in significant agglomeration of microfine apatite with a regular and compact agglomerate structure. Hence, it might be further speculated that the addition of NaOL had a hydrophobic agglomeration effect on the microfine apatite treated with ASS, which further enhanced the flotation of microfine apatite. However, the adsorption of ASS on the chlorite surface made it hydrophilic, which further hindered the hydrophobic agglomeration of NaOL and depressed mineral flotation.

3.7. Discussion

Based on the flotation results, it can be inferred that in the absence of depressant, as the pH of artificial seawater was about 8.1, the NaOL could easily adsorb on both apatite and chlorite surfaces (Figure 10 and Figure 12), leading to their high flotation recovery in Figure 3. The presence of Na⁺, ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ in seawater adsorbed on the surface of apatite and chlorite, altering the surface electrical properties of the minerals (Figure 7, Figure 8b and Figure 9b).

As the ASS was added, the slurry was changed from an alkaline to a weakly acidic state, allowing ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ to exist mainly in free form and ASS mainly in the form of Si(OH)₄ [36]. The addition of ASS allowed the metal ions adsorbed on the mineral surface to dissociate (Figure 8c and Figure 9c), thus promoting the adsorption of NaOL. Si(OH)₄ could adsorb on the surface of chlorite very stably (Figure 10 and Figure 12b), owing to the reason that chlorite had the same silicon–oxygen tetrahedral structure as ASS, which reduced the number of available adsorption sites available for NaOL, leading to the depression of chlorite flotation (Figure 4). However, Si(OH)₄ did not adsorb on the apatite surface by electrostatic action, thus not affecting the adsorption of NaOL (Figure 10 and Figure 12a). The addition of NaOL could induce the appearance of microfine apatite agglomerates (Figure 13) and enhance microfine apatite flotation [43,45]. Therefore, ASS has a selective depressant effect on chlorite while eliminating the negative effects of calcium and magnesium ions in seawater on apatite flotation and promoting apatite flotation, as shown in Figure 14.

4. Conclusions

ASS exhibited excellent selective depression performance in the flotation separation of microfine apatite from chlorite in seawater when the optimum ratio of SS to sulfuric acid was 5:3. When the dosage of ASS was 100 mg/L, and the NaOL dosage was 450 mg/L, the selective separation effect was optimum, and the recoveries of apatite and chlorite in seawater were 90.84% and 9.34%, respectively. As the slurry changed from alkaline to weakly acidic conditions with the addition of ASS, the metal ions adsorbed on the surface of apatite were desorbed, which promoted the adsorption of NaOL. Moreover, ASS did not prevent the apatite surface from chemically reacting with NaOL, while it stably adsorbed on the chlorite surface, preventing the further adsorption of NaOL. The marked discrepancy in the adsorption amount of NaOL led to the discrepant flotation recoveries of the two minerals. The addition of NaOL had a hydrophobic agglomeration effect on the microfine apatite treated with ASS, which further enhanced the flotation of microfine apatite. Consequently, ASS could eliminate the detrimental effect of calcium and magnesium ions in seawater on apatite flotation, providing efficient chlorite depression, which achieved the selective separation of microfine apatite from chlorite in seawater. The results of this study could provide an effective way for the flotation separation of microfine apatite from silicate minerals in seawater.