Effect of sodium pyrophosphate on the flotation separation of calcite from apatite

Journal Pre-proofs Effect of sodium pyrophosphate on the flotation separation of calcite from apatite Zuchao Pan, Yunfan Wang, Qian Wei, Xintao Chen, Fen Jiao, Wenqing Qin PII: DOI: Reference:

S1383-5866(19)33857-2 https://doi.org/10.1016/j.seppur.2019.116408 SEPPUR 116408

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

28 August 2019 20 November 2019 7 December 2019

Please cite this article as: Z. Pan, Y. Wang, Q. Wei, X. Chen, F. Jiao, W. Qin, Effect of sodium pyrophosphate on the flotation separation of calcite from apatite, Separation and Purification Technology (2019), doi: https:// doi.org/10.1016/j.seppur.2019.116408

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© 2019 Published by Elsevier B.V.

Effect of sodium pyrophosphate on the flotation separation of calcite from apatite Zuchao Pan a, Yunfan Wang a, Qian Wei a, Xintao Chen b, Fen Jiao a, *, Wenqing Qin a, * a School

of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China

b Department

of Infrastructure Engineering, Melbourne University, Melbourne 3010, Australia

* Corresponding author: [email protected](F. Jiao), [email protected] (W. Qin).

Abstract In this study, the separation of calcite and apatite was achieved by the depressant sodium pyrophosphate (SPP). Microflotation results showed that when using 20 mg/l sodium oleate (NaOL) as the collector, apatite could be significantly depressed by 120 mg/l SPP, and calcite remained floatable at pH 6.5-9.3. Relatively more Ca2+ was selectively dissolved from the surface of apatite by pyrophosphate, and the dissolution of calcium on the surface of calcite was inhibited. For calcite, the dissolution of Ca2+ decreased first but then increased with increasing SPP dose, and the adsorption amount of SPP illustrated an opposite tendency. With respect to apatite, the dissolution of Ca2+ was similar to that of calcite, but pyrophosphate failed to adsorb onto apatite. The solution chemistry of pyrophosphate at different pH values and in the presence of Ca2+ was discussed. The XPS results revealed that both SPP and NaOL could be adsorbed on the surface of calcite, while NaOL was rarely adsorbed on the surface of apatite. Moreover, the Ca/P ratio of apatite decreased after adding SPP. Zeta potential measurements showed that NaOL could still be adsorbed on the negatively charged calcite surface by chemical adsorption with surface calcium. Regarding apatite, the adsorption of NaOL was hindered by the decrease of active calcium sites and negative surface charge. The differences in polyphosphate concentration and pH should be responsible for the divergent existing theories of the interaction of polyphosphate and calcium minerals. Keywords: apatite; calcite; sodium pyrophosphate; depression mechanism; flotation

1. Introduction Phosphorus resources are essential for agriculture, and more than 90% of phosphorus resources have been consumed as fertilizers for food growth [1]. The natural loss of phosphorus in soil requires the constant artificial addition of phosphate [2]. Due to the continuous consumption of phosphorus resources, it is necessary to improve the separation efficiency of phosphate ores. Additionally, calcareous phosphate ore is an important natural resource of phosphorus, in which apatite is the major phosphate mineral, while calcite and dolomite are the main gangue minerals [3,4]. Reverse flotation is a common method for the separation of apatite from calcareous phosphate ore [5–7]. Since the surface properties of calcium-bearing minerals in phosphate rock are similar, the reaction behavior of several minerals with fatty acid collectors is almost the same. [8,9]. Therefore, a highly selective depressant is vital in reverse flotation [5,10].

Polyphosphates (such as polyphosphates, metaphosphates, and pyrophosphate) can form stable soluble complexes with metal ions [11–13]. Additionally, they can be used in mineral processing as a dispersant [14,15] and depressant in the separation of calcium-bearing minerals [16–19]. Sodium pyrophosphate has also been employed as one of the combined depressants in galena [20,21]. The ability of polyphosphates to complex metal ions is well understood, but their depression mechanisms are diverse. Jiwei Lu et al. [22] found that hexametaphosphate dissolved Mg ions to form soluble complexes from the surface of serpentine into the solution, and the negatively charged serpentine dispersed. Yanfei Chen et al. [23] concluded that pyrophosphate is selectively adsorbed on calcium sites of apatite and dolomite rather than complexing with magnesium, so it can depress apatite more effectively than dolomite. Gao et al. [16] found that hexametaphosphate could be selectively adsorbed on calcite rather than on scheelite and prevent collectors from being adsorbed. Li and Lu [24] carried out a comprehensive study and concluded that “the depression of the calcium minerals (calcite, scheelite, and fluorite) is only related to the selective dissolution of calcium ions by the phosphates, but not related to the adhesion of the phosphates on the mineral surface”. Feng et al. [25] had the same conclusion in a study of calcite and hexametaphosphate. The separation of limonite from calcite was achieved by adding sodium pyrophosphate to depress the flotation of limonite, and sodium pyrophosphate had little effect on calcite [26]; however, there is no in-depth explanation of this phenomenon. In addition, polyphosphate depressants are usually simplified as orthophosphate in some studies [15,16,21,22]. However, polyphosphates are stable in neutral and alkaline solutions at room temperature but are degraded at a higher temperature [27]. Although polyphosphates, including orthophosphates, are strongly complex, they have quite different depression performances [28,29]. In this study, the flotation behavior of calcite and apatite with SPP as a depressant and NaOL as a collector was studied through pure mineral flotation. Additionally, the solution chemistry of pyrophosphate was also discussed. The mechanism of SPP on calcite and apatite was studied based on ion concentration measurements, X-ray photoelectron spectroscopy analysis, and zeta potential determination. We found that the dissolution of calcium ions could be inhibited or promoted at different concentrations of SPP. A model for the separation of apatite and calcite using SPP as a depressant was proposed.

2. Materials and methods 2.1 Materials The resistivity of deionized water used in the experiment was over 17 MΩ×cm. AR-grade sodium pyrophosphate (SPP, Na4P2O7·10H2O), sodium oleate (NaOL, C17H33COONa), and NaCl (Sinopharm Chemical Reagent Co., Ltd) were used for all experiments. The high-purity natural calcite (CaCO3) and apatite (fluorapatite, Ca5(PO4)3F) used in the experiment were larger than 98% and 95% (P2O5, 39.27%) purity, respectively, as determined by both chemical analysis and X-ray diffraction analysis (XRD). Fig. 1 shows the X-ray diffraction (XRD) patterns of calcite and apatite samples. Calcite and apatite were crushed and ground using a porcelain ball mill and then sieved in deionized water to obtain a particle size range of 38~75 μm for the flotation and adsorption experiments. The preparation of mineral samples was free from metal ion contamination. The 38 μm samples were ground to 2 μm for XRD and zeta potential measurements.

(a)

(b)

Fig. 1. XRD of (a) calcite and (b) fluorapatite

2.2 Flotation test Single mineral flotation was performed on an XFG flotation machine at a 1700 r/min stirring speed. The order of adding reagents was as follows: first, 2 g mineral and 30 ml water were added to the flotation cell; after stirring for 1 min, SPP was then added and stirred for 2 min. For the next step, HCl or NaOH was added and stirred for 2 min, and the stable pH was recorded. Finally, NaOL was added and conditioned for 2 min. The foam was scraped and collected for 5 min. The flotation recovery was calculated by dividing the weight of the foam produced by the total weight (2.0 g).

2.3 XPS analysis X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi Thermo Fisher, USA) combined with a monochromatic Al Kα source was used for XPS measurements. The energy scan step for narrow measurement was 0.05 eV. Binding energy was corrected for charging effects by referencing C 1s at 284.8 eV. The samples were prepared with the flotation process and rinsed three times with a NaOH solution at pH 8.8. Finally, they were dried at 40°C in the oven.

2.4 Ion concentration measurement The concentration of calcium ions and phosphorus in the pulp was determined by the residual concentration method. First, 2.0 g 38~75 μm minerals was added to the flotation cell with 30 ml water. Then, after stirring at 1700 r/min for 1 min, a certain amount of sodium pyrophosphate was added, stirring at 1700 r/min for another 8 min. The suspensions were allowed to settle for 1 min; then, the supernatant liquid was filtered with a 0.22 μm membrane. Finally, the concentration of calcium and phosphorus in the supernatant was determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Spectro Blue). Every sample was tested three times. The pH of calcite was not adjusted during the whole stirring process, and the pH of apatite was adjusted so that the two minerals had the same pH.

2.5 Zeta potential measurement A zeta potential analyzer (Malvern Zetasizer Nano) was used to analyze the zeta potential of the minerals with different amounts of added agents. A 40 mg sample was added to a 100 ml beaker with 50 ml of a 0.01 mol/l NaCl supporting electrolyte. The pH was adjusted with HCl or NaOH from 7 to 12. The condition time was the same as that of the flotation test. The pulp was precipitated for 2 min to remove large particles. Three measurements were repeated under each condition.

3. Results and discussion 3.1 Flotation results 100

100 (a)

(b) 80 calcite+NaOL apatite+NaOL

60

Recovery (%)

pH=9.0±0.2

40

calcite+SPP+NaOL apatite+SPP+NaOL calcite+NaOL apatite+NaOL

40

10

20

30

40

0

50

6

7

8

100

9

10

11

40 (c) calcite+SPP+NaOL apatite+SPP+NaOL

60 NaOL: 20 mg/l pH=8.5±0.2

40

30

P2O5 grade (%)

80

NaOL: 20 mg/l

grade recovery

100

150

200

Concentration of SPP (mg/l)

SPP: 120 mg/l pH= 8.5±0.2

73.43

80

26.37 60

20

19.90 40

10

20

50

12

100 (d)

Recovery (%)

SPP: 120 mg/l

pH

Concentration of NaOL (mg/l)

0 0

NaOL: 20 mg/l

20

20 0 0

60

20.17

20

0

P2O5 recovery (%)

Recovery (%)

80

0

Without SPP

With SPP

Fig. 2. Recovery of calcite and apatite as a function of (a) NaOL concentration, (b) pH and (c) SPP concentration; (d) recoveries and grades of P2O5 in the sink products. Fig. 2(a) shows the relationship between the NaOL concentration and the recovery of calcite and apatite. As the amount of NaOL increased, the recovery of both minerals increased gradually. When the amount was 20 mg/l, the recovery of calcite reached a maximum. Both minerals had good floatability in the absence of depressants. Fig. 2(b) shows the relationship between the pH and the recovery of calcite and apatite in the presence or absence of SPP. In the absence of SPP, the recoveries of both minerals were above 80.7%, and it was difficult to separate them using NaOL as the only collector. When SPP was added before NaOL, the recovery of apatite was less than 15% at pH 6.5-11.0. However, in the range of pH 6.5-9.3, the recovery of calcite remained at a high level. When it was larger than 9.3, the recovery of calcite decreased sharply. This result indicates that apatite and calcite could be separated using SPP as a depressant in the pH range of 6.5-9.3. Fig. 2(c) shows the relationship between the flotation recovery and SPP concentration. The recovery of calcite was above 95% in the whole tested range. The recovery of apatite decreased with increasing SPP concentration and significantly decreased when the concentration was over 90 mg/l, and the optimal dosage of SPP was 120 mg/l. Under these conditions, calcite and apatite could be efficiently separated by flotation.

Fig. 2(d) depicts the separation results of artificial mixed binary minerals with a mass ratio of calcite to apatite of 1:1. When the depressant SPP was not added, the grade and recovery of P2O5 in the sink product (concentrate of apatite) were 19.90% and 20.17%, respectively, using 20 mg/l NaOL as the collector. Since the grade of P2O5 in the artificial mixed minerals was 19.64%, the separation of apatite from calcite was insignificant. However, when SPP was added to the mixed mineral samples, the grade and recovery of P2O5 reached 26.36% and 73.43%, respectively. Thus, the grade and recovery of P2O5 in the sink product was greatly improved. This result indicates that the depressant SPP can achieve the flotation separation of calcite from apatite.

3.2 XPS analysis Further studies were conducted using XPS, which could provide elemental information 10 nm below the surface of the samples. The elemental composition, chemical state, surface element concentration and other information of the sample surface could also be obtained[30]. Table 1 shows the atomic concentrations (fluorine was omitted) and Ca/P atomic ratios on the samples. It should be noted that the presence of C on each sample, partially from atmospheric contamination or adsorption from the pulp, could affect the quantitative analysis. For calcite, P appeared on the surface after SPP treatment, indicating that pyrophosphate absorbed onto the calcite. Continuing to treat calcite with NaOL increased the concentration of C by approximately 10%, which originated from the carbon chain of NaOL. For apatite, after treatment with SPP followed by that with NaOL, the C concentration fluctuated slightly. Thus, the relative P concentration varied by approximately 0.6%. Therefore, it was difficult to distinguish whether the excessive P came from pyrophosphate. Nevertheless, we could infer that NaOL was hardly adsorbed on the surface of apatite. Table 1 Atomic concentrations on the sample surfaces Samples

Atomic（%）

Ratio

C 1s

O 1s

P 2p

Ca 2p

Ca/P

Calcite

39.92

45.20

—

14.88

—

Calcite+ SPP

40.60

43.97

0.44

14.98

—

Calcite+ SPP+ NaOL

49.55

37.16

0.43

12.86

—

Apatite

22.05

50.84

10.50

16.61

1.58

Apatite+ SPP

25.60

46.97

11.13

16.29

1.46

Apatite + SPP+ NaOL

25.99

46.96

10.90

16.15

1.48

Table 2 Binding energies and chemical shifts for the elements of calcite and apatite Samples

Binding energy (eV)

Chemical shifts (eV)

O 1s

P 2p

Ca 2p

O 1s

P 2p

Ca 2p

Calcite

531.28

——

346.84

——

——

——

Calcite+ SPP

531.29

133.53

346.99

0.01

——

0.15

Calcite+ SPP+ NaOL

531.46

133.58

347.13

0.18

——

0.29

Apatite

531.44

133.55

347.56

——

——

——

Apatite+ SPP

531.43

133.61

347.59

-0.01

0.06

0.03

Apatite + SPP+ NaOL

531.42

133.59

347.58

-0.02

0.04

0.02

In the experiments, the Ca/P atomic ratio of pure apatite was 1.58, which was lower than the theoretical value of 1.67. The difference between the theoretical and experimental values may be due to the impure apatite and the inherent error of atomic sensitivity factors. In addition, under the action of adventitious carbon, the inelastic scattering of calcium and phosphorus attenuates to different degrees. Regardless, the relative variation between the ratios was credible. The Ca/P atomic ratio decreased from 1.58 for pure apatite to 1.46 for the apatite treated with SPP. This decrease could be attributed to the increase of P or the decrease of Ca on the surface of apatite. In combination with the study discussed below in which the SPP was not adsorbed on the surface, it can be inferred that the decrease of Ca/P atomic ratio was induced by the decrease of Ca. This indicates that pyrophosphate dissolved Ca on the surface of apatite, while phosphate remained on the surface of apatite, thus reducing the Ca/P atomic ratio. Fig. 3 compares the narrow spectra of Ca and P on the samples. Table 2 lists the binding energies of each element in the sample in this experiment. For calcite treated with SPP, Ca 2p 1/2 and Ca 2p 3/2 increased by 0.15 eV, indicating that pyrophosphate was adsorbed on the calcite surface, but it was difficult to determine whether calcium was bound to pyrophosphate or physically adsorbed by only 0.15 eV shift. However, the binding energy of the O 1s of calcite is close to that of O 1s of calcium pyrophosphate [31], so the two chemical states of oxygen are indistinguishable. For the calcite treated with SPP and NaOL, the Ca 2p 1/2, Ca 2p 3/2 and O 1s values increased by 0.25 eV, 0.30 eV and 0.18 eV, respectively. These shifts were due to the existence of calcium oleate species, such as the calcium dioleate precipitant and oleate chemisorbed to surface calcium. For the apatite samples, the shifts of O 1s, Ca 2p and P 2p were negligible (nearly less than a scanning step of 0.05 eV), indicating that calcium oleate and pyrophosphate had no chemical adsorption or precipitation on the apatite surface. A previous study [23] inferred that pyrophosphate depresses apatite flotation by adsorption on the apatite surface. However, this conclusion is questionable because the fitting area of the P 2p peak of apatite and SPP analyzed by XPS was inconsistent with the P atomic percentage. Nevertheless, some of their flotation and zeta potential test results were consistent with ours.

(a) calcite+SPP+NaOL

experimental background

Ca 2p 3/2

Ca 2p 1/2 350.65eV

347.13eV

Intensity

calcite+SPP 346.99eV

350.52eV

calcite 346.84eV 350.37eV

354

352

350

348

346

344

Binding Energy (eV) (b) apatite+SPP+NaOL

Ca 2p 3/2

Intensity

Ca 2p 1/2 351.08eV

experimental background 347.58eV

apatite+SPP 347.59eV

351.09eV

apatite 347.56eV

351.08eV

354

352

350

348

346

344

Binding Energy (eV) (c)

P 2p

133.59eV

experimental background

apatite+SPP+NaOL

Intensity

133.61eV

apatite+SPP 133.55eV

apatite

138

136

134

132

130

Binding Energy (eV)

Fig. 3. XPS narrow spectra of (a) Ca on calcite (b) Ca on apatite and (c) P on apatite

3.3 Zeta potential analysis 10

(b) calcite calcite+NaOL calcite+SPP calcite+SPP+NaOL

-10 -20

-20

NaOL: 20mg/l SPP: 120mg/l

Zeta potential (mV)

Zeta potential (mV)

0

-10

(a)

-30 -40 -50

apatite apatite+NaOL apatite+SPP apatite+SPP+NaOL

NaOL: 20mg/l SPP: 120mg/l

-30 -40 -50 -60

-60 7

8

9

10

11

12

7

pH

8

9

10

11

12

pH

Fig. 4. Relationship between the zeta potential of (a)calcite and (b)apatite and pH. When particles move in an aqueous solution relative to the liquid phase under external force, the potential on the slipping plane is the zeta potential. Zeta potential plays an important role in characterizing the surface properties of particles. Fig. 4(a) depicts the relationship between the zeta potential of calcite and pH. The isoelectric point (IEP) of pure calcite was between 9.0 and 9.5, which was consistent with previous reports [32]. After adding NaOL, the zeta potential became more negative, indicating that oleate species with negative charge were adsorbed on the surface of calcite, which accounted for the floatability of calcite treated with NaOL. When only SPP was added, the zeta potential of calcite negatively shifted over the whole tested range. Combined with the XPS study, it can be inferred that pyrophosphate with a negative charge was adsorbed on the surface of calcite. After adding SPP, NaOL was added, and the zeta potential decreased by approximately 10 mV compared with that of SPP only at pH<9.3. This may be because negatively charged oleate continued to be adsorbed on the surface of calcite after the adsorption of pyrophosphate. Therefore, calcite adsorbed with a long hydrophobic chain of NaOL had good floatability. However, at pH>9.3, there was little negative shift in the zeta potential (<2 mV). This indicates that NaOL was no longer adsorbed on the surface of calcite. Fig. 4(b) depicts the relationship between the zeta potential of apatite and pH. in the whole range tested, pure apatite was negatively charged because the isoelectric point of apatite is at approximately pH 6 [33]. After adding NaOL, the zeta potential of apatite shifted negatively, indicating that the oleate species with negative charge were adsorbed on the surface of apatite. When SPP was added only, the zeta potential moved to the negative direction. The possible reasons for this were the selective dissolution of calcium ions or the adsorption of SPP. Furthermore, when NaOL was added after SPP addition, the zeta potential hardly moved (<1 mV), which was attributed to the lack of adsorption of oleate on the apatite surface. This was consistent with the low recovery of apatite.

3.4 Solution chemistry and ion concentration analysis Polyphosphates are formed by repeated tetrahedral [PO4] units [18], and pyrophosphate is the simplest polyphosphate. Although polyphosphates tend to hydrolyze into orthophosphate in water, they are stable in neutral and alkaline solutions

at room temperature [18,27]. Some data show that there is no obvious decomposition of pyrophosphate in water at 70℃ for 60 h as recorded at the National Center for Biotechnology Information (NCBI). Although the solubility product of Ca2P2O7 is 3×10-18, several studies [18,34] show that pyrophosphate complexes with calcium form soluble calcium-phosphate complexes. If the concentration of Ca2+ is higher than that of P2O74-, calcium precipitate will form, and if the concentration of free Ca2+ is lower than that of P2O74-, the precipitate will dissolve [13]. The formation constants of pyrophosphate species are shown in Table 3. The diagram in Fig. 5 shows the pyrophosphate component distribution as a function of pH (the Ca2+/P2O74- molar ratio is 0.5, the optimized state of flotation). Within the effective separation range (pH 6.5-9.3), pyrophosphate mainly exists in the form of HP2O73- and CaP2O74-. Above pH 10, P2O74- is the main free component. The species of pyrophosphate in solution may affect the ability to complex with calcium from the mineral surface. We infer that P2O74-, which accounts for a large proportion of pyrophosphate above pH 9.5, has a strong depressing effect on calcite, but other species at pH 6.5-9.3 have less impact on calcite flotation. Table 3 The formation constants of pyrophosphate species Reaction

Log of the formation constants

H++P2O74-=HP2O73-

9.25

H++HP2O73-=H2P2O72-

6.60

H++H2P2O72-=H3P2O7-

2.36

H++H

1.52

4-=H

3P2O7

4P2O7

Ca2++P2O74-=CaP2O72-

5.39

Ca2++HP2O74-=CaHP2O7-

3.32

100

6

species distribution (%)

80 60

CaHP O 2 7 2CaP O 2 7 4P O 2 7

5

7

3

1 2 3 4

1

H P O 4 2 7 H P O 3 2 7 2H P O 2 2 7 3HP O 2 7

2

40

7 6

4

20

5 0 1

3

5

7

9

11

13

pH

Fig. 5. Species distribution diagrams of 0.2 mmol/l SPP and 0.1 mmol/l Ca2+. Fig. 6(a) and (b) depict the relationship between the concentration and SPP dose in calcite and apatite pulp. When the SPP dose was above 0.13 mmol/l and 0.19 mmol/l for calcite and apatite, respectively, the concentration of residual pyrophosphate was always higher than that of calcium ions in the pulp. It can be concluded that pyrophosphate mainly

existed in the form of CaP2O72- and HP2O73- given excessive pyrophosphate in the pulp. Fig. 6(b) shows that the SPP concentration was slightly higher than that which was added, and the excess phosphorus was attributed to the dissolution of apatite. A previous study [24] concluded that calcite flotation is depressed at approximately pH 10, which may be because P2O74- has a stronger complexing ability and solubility than HP2O73-. However, a lower pH was not involved in that study. Fig. 6(c) and (d) depict the relationship between the adsorption or dissolution amount of the mineral surface and the SPP concentration. As shown in Fig. 6(c), when the dosage was less than 0.135 mmol/l (60 mg/L), the dissolved Ca2+ gradually decreased with the increasing SPP dose. This may be because the absorption of pyrophosphate on the calcite surface prevented the dissolution of calcium. When the concentration of SPP was greater than 0.135 mmol/l (60 mg/L), pyrophosphate adsorption decreased and calcium gradually dissolved at the same time. Combined with the analysis of Fig. 5, it can be inferred that the adsorbed pyrophosphate might exist as ≡CaP2O73- on the calcite surface. The active calcium sites available for NaOL adsorption were still high, leading to a high flotation recovery. From the trend, it can be expected that when the SPP concentration continued to increase beyond 1.35 mmol/l (600 mg/l), the dissolution of Ca2+ would continue to increase, and calcite flotation would also be depressed. In our exploratory tests, when the dose reached 2.70

1.2

SPP 2+

Ca

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0

(b)

1.2

2+

Ca

1.0

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 0.0

0.2

6

Amount (μmol/g)

5 4 adsorbed SPP 2+ dissolved Ca

2 1 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Concentration of SPP (mmol/l)

0.4

0.6

0.8

1.0

1.2

1.4

0.0

Concentration of SPP (mmol/l)

calcite surface

3

1.0

0.8

1.4

Dissolve amount (μmol/g)

(c)

1.2

SPP

Concentration of SPP (mmol/l) 6

1.4

apatite pulp

2+

1.2

1.4

concentration of Ca （mmol/l）

1.4

calcite pulp

Residual concentration of SPP (mmol/l)

(a)

2+

1.4

concentration of Ca （mmol/l）

Residual concentration of SPP (mmol/l)

mmol/l (1200 mg/l), calcite was completely depressed.

(d)

apatite surface

dissolved P 2+ dissolved Ca

5 4 3 2 1 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Concentration of SPP (mmol/l)

Fig. 6. (a) SPP and Ca2+ concentration in calcite pulp, (b) SPP and Ca2+ concentration in apatite pulp, (c) adsorbed SPP and dissolved Ca2+ on calcite surface, and (d) dissolved P and Ca2+ on apatite surface.

As shown in Fig. 5(d), the amount of dissolved Ca2+ is greater than that of dissolved phosphorus. As a result, NaOL was not adsorbed on the apatite lacking active calcium sites to depress apatite. In contrast with calcite, pyrophosphate was not adsorbed on the apatite surface but was able to selectively dissolve Ca2+. Bond length may be the crucial factor related to this observed difference. The Ca-O bond length in calcite (2.36 Å) is shorter than that in apatite (2.39/2.46 Å) [16,35,36]. It is generally believed that the shorter the bond, the higher the bond energy. Therefore, after cleavage and fracture, the Ca on the apatite surface is more easily dissolved by pyrophosphate. The difference in Ca-O bond length leads to different dissolution behaviors in the presence of pyrophosphate chelating agents. In addition, the amount of Ca2+ dissolved on the surface of calcite and apatite first decreased and then increased. However, the concentrations of SPP corresponding to the lowest points of dissolution were different. Calcium on the apatite surface could be selectively dissolved in large quantities at low doses, resulting in the depression of apatite, while this dose was not large enough to depress calcite flotation. Similarly, in the study of kaolin and sodium tripolyphosphate systems [15], it was found that the Ca2+ in the solution decreased first and then increased with increasing sodium tripolyphosphate. Therefore, it can be concluded that the relationship between SPP and the amount of calcium dissolved is complicated but related to SPP concentration. At low concentrations of SPP, calcium pyrophosphate could precipitate and would be adsorbed on the surface. Additionally, at high concentrations, soluble calcium pyrophosphate complexes form. Only when the selectively dissolved calcium reaches a certain level could flotation be inhibited. The purpose of flotation separation can be achieved by using the differences in the dissolution of different minerals. Eventually, according to the analysis above, we have drawn a possible model illustration, including the flotation separation results of artificially mixed ore, as shown in Fig. 7. Under optimal flotation conditions, NaOL and SPP can be adsorbed on the surface of calcite. Due to the adsorption of NaOL, the calcite surface hydrophobicity would be enhanced. Calcite particles were attached to bubbles and floated. The calcium on the apatite surface dissolved into the aqueous solution due to the chelating ability of pyrophosphate. The hydrophilic surface without NaOL adsorption causes the apatite to sink.

bubble

pulp

recovery 26.57% grade 11.51%

calcite

-

C17 H33COO 4-

P2 O7

2-

CO3

2+

Ca pulp

recovery 73.43% grade 26.37%

Feed P2 O 5 grade 19.64% pH=8.5± 0.2 NaOL 20mg/l SPP 120mg/l

3-

PO4 F

-

apatite

Fig. 7. Schematic diagram of the separation mechanism on calcite and apatite at pH 8.5 and separation results.

4. Conclusion Within the range of pH 6.5-9.3, the separation of apatite from gangue mineral calcite can be achieved by flotation in the case of SPP as a depressant (120 mg/l) and NaOL as a collector (20 mg/l). The different components of pyrophosphate and its complexing ability had significant influence on separation. The dissolution of calcium by SPP occurred in two stages. The dissolution of Ca2+ first decreased with increasing SPP dose (up to 0.13 mmol/l for calcite, 0.05 mmol/l for apatite). Furthermore, increasing the SPP dosage enhanced the dissolution of Ca2+. The separation of apatite and calcite was concerned with the selective dissolution of calcium. Under the conditions of optimal separation, the dissolution of Ca2+ on the calcite surface was inhibited. However, on the apatite surface, this process was promoted. SPP was adsorbed on the surface of calcite but did not inhibit the adsorption of NaOL. Moreover, due to the chemical adsorption of NaOL on the calcite surface, calcite was intended to be hydrophobic and floatable. The strong ability of SPP made calcium dissolve selectively from the surface of apatite, and phosphate was exposed on the surface. The decrease of active calcium sites, as well as the negatively charged surface, hindered the adsorption of NaOL.

5. Acknowledgements The authors gratefully acknowledge the financial support of this research by Provincial Science and technology leader (Innovation team of interface chemistry of efficient and clean utilization of complex mineral resources, Grant No. 2016RS2016); National Natural Science Foundation of China (Project No. 51604302 and No. 51574282); Key laboratory of Hunan Province for Clean and Efficiency Utilization of strategic Calcium-containing mineral Resources (No. 2018TP1002).

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Graphical abstract

bubble

pulp

recovery 26.57% grade 11.51%

calcite

-

C17 H33COO 4-

P2 O7

2-

CO3

2+

Ca pulp

recovery 73.43% grade 26.37%

Feed P2 O 5 grade 19.64% pH=8.5± 0.2 NaOL 20mg/l SPP 120mg/l

3-

PO4 F

apatite

-

Highlights Sodium pyrophosphate depresses apatite at pH 6.5-11.0. Sodium pyrophosphate depresses calcite at pH 9.3-11.0 but not at 6.5-9.3. Sodium pyrophosphate adsorbs on the surface of calcite but not apatite. The depression of pyrophosphate is related to the selective dissolution of Ca. Whether pyrophosphate promotes or inhibits Ca dissolution depends on the dosage.

Zuchao Pan: Conceptualization, Methodology, Investigation, Writing- Original draft preparation. Yunfan Wang: Methodology, Investigation. Qian Wei: Conceptualization, Methodology. Xintao Chen: Writing- Reviewing and Editing, Visualization. Fen Jiao: Project administration, Funding acquisition. Wenqing Qin: Supervision, Funding acquisition.