Separation of bastnäsite from fluorite using ethylenediamine tetraacetic acid as depressant

Minerals Engineering 134 (2019) 134–141

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Separation of bastnäsite from fluorite using ethylenediamine tetraacetic acid as depressant

T

Zhao Caoa,b,c,d, , Yongdan Caoa, Qiqi Qua, Jinshan Zhanga, Yufan Mud ⁎

a

Institute of Mining Engineering, Inner Mongolia University of Science and Technology, Baotou, China Guangdong Institute of Resources Comprehensive Utilization, Guangzhou, China c State Key Laboratory of Rare Metals Separation and Comprehensive Utilization, Guangzhou, China d School of Chemical Engineering, The University of Queensland, Brisbane, Australia b

ARTICLE INFO

ABSTRACT

Keywords: Bastnäsite Fluorite Flotation Depression EDTA

Bastnäsite is an important mineral resource in the production of rare earth materials and is usually beneficiated by flotation. The flotation of bastnäsite is problematic due to the competitive adsorption of hydroxamate collector on bastnäsite and its associated calcium-bearing gangue minerals such as fluorite. One strategy to solve this problem is to use effective depressants to depress the gangue minerals. However, the current depressants all have some drawbacks. In this study, the effect of ethylenediamine tetraacetic acid (EDTA) as a depressant was tested in the flotation of single mineral of bastnäsite and fluorite and their mixture. The mechanism underpinning the role of EDTA was investigated through theoretical thermodynamic calculation, zeta potential and Xray photoelectron spectroscopy (XPS) measurements. The results show that fluorite was significantly depressed, while the flotation of bastnäsite was almost unaffected when EDTA was present. The separation index between bastnäsite and fluorite increased from 1.18 to 12.66 with the increase of EDTA concentration from 0 to 7.79 kg/t. It was found that EDTA could dissolve the chemically adsorbed octyl hydroxamic acid (OHA) on fluorite through the formation of soluble Ca-EDTA complexes, whereas the chemically adsorbed OHA on bastnäsite was more stable and could not be transformed into Ce-EDTA spontaneously. Therefore, the flotation of fluorite was selectively depressed by EDTA. The results show that EDTA was a promising depressant for fluorite gangue mineral in bastnäsite flotation.

1. Introduction Nowadays, rare earth elements (REE) and compounds are of significant importance for a range of rapidly expanding industrial applications including catalysis, metallurgy, renewable energy, and special materials due to their unique electrical, magnetic, optical and chemical properties (Azizi et al., 2016). The REE, including the lanthanide-group fifteen elements and yttrium, can be divided into three groups, i.e., light, medium and heavy REE based on the different solubility of REE sulfates. Bastnäsite ((Ce,La)CO3), containing approximately 75% rare earth oxides (REO), is one of the most important light rare earth minerals (Jordens et al., 2014). Flotation is the common method to recover basetnaesite (Jordens et al., 2013; Yang et al., 2015; Yu and Aghamirian, 2016). Nonetheless, bastnäsite is often associated with calcium-bearing gangue minerals such as fluorite, and the separation of bastnäsite from fluorite by flotation is usually difficult due to their similar surface properties and floatability (Pradip and Fuerstenau, 1983,

⁎

1991; Zhang et al., 2013). Bastnäsite and fluorite both belong to semisoluble salt minerals and both contain fluorite ions in their crystal lattices. Metal ions dissolved from the surface of bastnäsite or fluorite can adsorb to the surface of the other mineral, leading to the similar surface properties. (Fuerstenau et al., 1992; Zhang, 2014). Therefore, research on high selective flotation reagents of bastnäsite over gangue minerals has attracted a lot of attention (Azizi et al., 2016; Liu et al., 2017; Pradip and Fuerstenau, 2013). Over the past decades, progress has been made in developing more selective collectors, such as hydroxamate, to replace less effective, fatty acid in rare earth ore flotation (Cui et al., 2012; Pavez et al., 1996; Pradip and Rai, 2003; Ren et al., 1997). However, the flotation of bastnäsite is still problematic due to the competitive adsorption of hydroxamate collector on bastnäsite and fluorite (Fuerstenau et al., 1992; Pradip and Fuerstenau, 2013), in which case, depressants are often applied to depress the gangue minerals (Pradip and Fuerstenau, 1991).

Corresponding author at: Institute of Mining Engineering, Inner Mongolia University of Science and Technology, Baotou, China. E-mail address: [email protected] (Z. Cao).

https://doi.org/10.1016/j.mineng.2019.01.030 Received 6 March 2018; Received in revised form 18 July 2018; Accepted 30 January 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.

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Typical depressants used in bastnäsite flotation include sodium silicate, sodium hexafluorosilicate, lignin sulfonate and sodium carbonate (Jordens et al., 2013; Pradip and Fuerstenau, 1991, 2013; Ren et al., 1997; Xia et al., 2015). In the dressing of Mountain Pass rare earth ore using fatty acids as collector, lignin sulfonate and sodium carbonate have been used as the depressant and pH modifier, respectively, in the separation of bastnäsite from associated calcite and barite gangue minerals. All these reagents need to be conditioned with high temperature steam (Pradip and Fuerstenau, 2013). In addition, as the depressant for gangue minerals, lignin sulfonate also affects bastnäsite flotation to a certain degree (Houot et al., 1991). In the processing of some other rare earth ores using hydroxamate as collector, sodium silicate has been commonly used as the depressant due to its higher affinity towards gangue minerals than that of rare earth minerals. Sodium silicate has also been applied to depress fluorite in scheelite and calcite flotation (Gao et al., 2015; Gao et al., 2016). However, a large amount of sodium silicate (nearly 25 kg/t) is usually required in order to get an acceptable REO grade, which may also depress rare earth minerals and cause significant reduction of REO recovery (Pradip and Rai, 2003; Zhang and Honaker, 2018). The use of sodium hexafluorosilicate is restricted because it is an environmental pollutant as well as a hazard to plant workers (Jordens et al., 2013). In summary, all the currently used depressants have some drawbacks or limitations. Therefore, it is of great importance to study new depressants to improve rare earth flotation efficiency. EDTA is a common complexing agent, which has often been used in sulphide minerals flotation to remove the hydrophilic metal hydroxide layers from mineral surface and thus restore the floatability of oxidized sulphide minerals (Nanthakumar and Kelebek, 2007; Wang and Forssberg, 1990). It has also been used in the separation of feldspar and quartz (Shehu and Spaziani, 1999), as well as, monazite and calcite (Zhang and Honaker, 2018). Due to its strong complexing ability with calcium ions (Arena et al., 1983), EDTA may be used to effectively depress alkaline earth minerals such as fluorite in RE flotation. In the current study, the effect of EDTA on the separation of bastnäsite and fluorite was studied and the depression mechanism of EDTA on fluorite was systematically investigated through thermodynamic calculation, zeta potential and X-ray photoelectron spectroscopy (XPS) measurements.

2.2. Methods 2.2.1. Flotation experiments Flotation experiments were carried out in an XFG flotation machine with a 40 mL plexiglass cell, at an impeller speed of 1800 rpm. In single mineral flotation experiments, 2.0 g bastnäsite or fluorite was dispersed in the cell with 40 mL distilled water. The pH of the mineral suspensions was adjusted by NaOH or HCl, and then OHA and EDTA (if needed) were added and conditioned for 2 min for each reagent, after that the pH was readjusted. The flotation lasted for 4 min. The recovery was calculated based on the dry mass percent of concentrate. In mixed minerals flotation, 3.0 g mixed minerals of bastnäsite and fluorite (mass ratio 1:1) were used. The other conditions were kept consistent with single mineral flotation experiments. All the concentrates and tailings were filtered, dried, weighed and assayed REO and CaF2 grades to calculate the bastnäsite and fluorite recoveries in concentrate. The results presented were the average values of three independent experiments under the same conditions. The selectivity index SI was adopted to evaluate the effect of EDTA on the separation of bastanesite from fluorite using the following equation (Salmani Nuri et al., 2016):

SI =

B C B T

×

F T F C

(1)

where SI was selectivity index; and were the recoveries of bastnäsite in concentrate and tailings, respectively; TF and CF were the recoveries of fluorite in tailings and concentrate, respectively. B C

B T

2.2.2. REO and CaF2 assay method For REO content assay in mixed mineral flotation tests, 0.1 g of sample was mixed with sodium hydroxide and sodium peroxide in a crucible and then heated at 750 °C in a muffle furnace to form a homogeneous melt. The melt was digested in hydrochloric acid and then determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The CaF2 content in all mixed mineral flotation products was calculated as follows: CaF2 content = 100% – (REO content of flotation product)/(REO content of bastnäsite sample)%, the REO content of bastnäsite sample used in this work was 71.2%.

2. Materials and methods

2.2.3. Zeta potential measurement Zeta potential measurements were conducted at 20℃ using a Brookhaven ZetaPlus Analyzer (Brookhaven Corporation, USA). The bastnäsite or fluorite samples for zeta potential measurements were ground to d95 around 5 μm with an agate mortar. 20 mg of the ground samples were added to 100 mL 1 × 10−3 mol/L KNO3 background electrolyte solution and conditioned without or with OHA or EDTA with known concentration individually or in combination. The pH of the mineral suspensions was adjusted by NaOH or HCl. The results presented were the average of three independent tests under the same condition.

2.1. Materials and reagents The bastnäsite sample was obtained from the Weishanhu Rare Earth Mine in Shandong Province, China. The bastnäsite sample was ground to −74 μm with a porcelain ball mill and then processed with a shaking table to remove the light mineral impurities such as calcite and silicate minerals. The heavy fraction containing primarily bastnäsite and barite was processed with a Frantz Isodynamic Separator at a magnetic field strength of 2.0 T to remove the remaining barite. The assay results showed that the purified bastnäsite sample contained 71.2% REO, 2.21% CaO, 0.82% SiO2 and 0.65% BaO, indicating a purity of approximately 94%. Fluorite sample with a purity of 98% was purchased from Linqu Geological Collection Market in Shandong Province, China. The X-ray diffraction (XRD) results indicated that both bastnäsite and fluorite samples were of high purity. The −74 + 38 μm fractions of bastnäsite and fluorite samples were used in the flotation tests. Research grade octyl hydroxamic acid (OHA) (obtained from Cytec, USA) was used as collector. Other chemicals of analytical grade were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used as pH regulators. Deionized water (resistivity 18.2 MΩ cm) was used in all experiments.

2.2.4. X-ray photoelectron spectroscopy (XPS) measurements The bastnäsite and fluorite samples at different flotation conditions for XPS analysis were taken from the flotation cell. The solid particles were dried in a vacuum desiccator before XPS measurements. The XPS characterization of samples was carried out by a Kratos Axis 165 spectrometer with monochromatized Al KαX-rays (1486.6 eV). Survey (wide) scans were taken at a pass energy of 160 eV, and multiplex (narrow) high resolution scans were taken at 20 eV. Survey scans were carried out over the 1200–0 eV binding energy range with 1.0 eV steps and a dwell time of 100 ms. Narrow high-resolution scans were performed with 0.05 eV steps and 250 ms dwell time. All measurements were carried out at pressures below 10−8 Torr. The resulting spectra 135

Minerals Engineering 134 (2019) 134–141

100

100

80

80

Flotation recovery (%)

Flotation recovery (%)

Z. Cao et al.

60

40

Bastnasite Fluorite

20

0

60

20

0

6

7

8

9

pH

Bastnasite Fluorite

40

10

11

12

0

200

400

600

OHA dosage (g/t)

800

1000

1200

Fig. 2. Recoveries of bastnäsite and fluorite as a function of OHA dosage in single mineral flotation at pH 9.

Fig. 1. Recoveries of bastnäsite and fluorite as a function of pH in single mineral flotation (OHA dosage 796 g/t).

100

were charge-corrected using C 1s as a reference with a binding energy of 284.8 eV. Data processing and fitting were carried out using CasaXPS.

80

Flotation recovery (%)

3. Results and discussion 3.1. Single mineral flotation of bastnäsite and fluorite in the absence and presence of EDTA Fig. 1 shows the variation of the recoveries of bastnäsite and fluorite in single mineral flotation with pulp pH. It can be seen that the recoveries of both bastnäsite and fluorite increased with the increase of pH to the maximum recoveries at around pH 9.0, beyond which the recoveries decreased with the further increase of pH to 12.0. Several other investigators have also reported that the characteristic maximum recoveries were obtained at around pH 9.0 when processing minerals such as bastnäsite (Jordens et al., 2014; Pradip and Fuerstenau, 1983), wolframite (Meng et al., 2015; Yang and Ai, 2016), monazite (Zhang and Honaker, 2017), ilmenite (Xu et al., 2015) and cassiterite (Wang et al., 2013) using hydroxamate as collector. It should be noted that the pKa of hydroxamic acid is close to 9 (Pradip, 1981). Both anionic and neutral hydroxamate are present in the solution at such a pH (Zhang et al., 2014). Pradip and Fuerstenau (1983) and Fuerstenau (2005) have suggested that multilayers of hydroxamate adsorption occur at the pKa of the hydroxamic acid as the hydroxamate anion adsorbs alongside the hydroxamic acid molecule. Besides that, hydroxamate generally functions best when metal hydroxyl species Ce(OH)2− are present as the predominate cerium species at pH 9 (Pradip and Fuerstenau, 1983). However, when the pH greatly exceeds the pKa, the electrostatic repulsion between the negatively charged mineral surface and the hydroxamate anion will inhibit the adsorption of hydroxamate collector on mineral surface and thus prevent successful flotation (Fuerstenau and Pradip, 2005). Fig. 2 shows the recoveries of bastnäsite and fluorite as a function of OHA dosage at pH 9.0 in single mineral flotation. Both bastnäsite and fluorite recoveries increased with the increase of OHA dosage and stabled when OHA dosage was over 796 g/t. However, the differences between bastnäsite and fluorite recoveries were always less than 20% over the whole range of OHA dosage. In order to further increase the flotation differences between bastnäsite and fluorite, effective depressants might be needed in the separation of these two minerals. Fig. 3 shows the recoveries of bastnäsite and fluorite as a function of EDTA dosage in single mineral flotation at pH 9.0 with 796 g/t OHA. The recovery of bastnäsite was almost unchanged with the increase of EDTA dosage with a recovery over 80% obtained at the whole range of EDTA dosage tested. On the contrary, the recovery of fluorite dropped

60

40

Bastnasite Fluorite

20

0

0

1

2

3

EDTA concentration (kg/t)

4

5

Fig. 3. Recoveries of bastnäsite and fluorite as a function of EDTA dosage in single mineral flotation at pH 9 (OHA dosage 796 g/t).

dramatically with the increase of EDTA dosage. When EDTA dosage was increased to 4.67 kg/t, bastnäsite recovery was still unaffected, whilst fluorite was fully depressed. The results show that EDTA might be able to separate bastnäsite from fluorite in mixed mineral flotation. 3.2. Mixed mineral flotation of bastnäsite and fluorite in the absence and presence of EDTA Fig. 4 shows the recoveries of bastnäsite and fluorite as a function of EDTA dosage in mixed mineral flotation with mass ratio 1:1 at pH 9.0 with 796 g/t OHA. The REO and CaF2 grade of the concentrate in mixed mineral flotation is presented in Fig. 5. It is evident from Fig. 4 that the recovery of bastnäsite was slightly affected and remained over 80%, whereas the recovery of fluorite was significantly reduced with the increase of EDTA dosage. From Fig. 5, it is found that the REO grade of the concentrate increased from 35% to 68%, whilst CaF2 grade decreased from 50% to 3.9% when the dosage of EDTA was increased from 0 to 7.79 kg/t. Fig. 6 shows the effect of EDTA dosage on selectivity index between bastnäsite and fluorite in mixed mineral flotation. It can be seen that when EDTA dosage was increased from 0 to 7.79 kg/t, the selectivity index between bastnäsite and fluorite was improved from 1.18 to 12.66. This would guarantee a desired separation of bastnäsite from the gangue mineral fluorite. 136

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100 40

80

Zeta potential (mV)

Flotation recovery (%)

20

60

40

Bastnasite Fluorite

20

-60

0

2

4

EDTA concentration (kg/t)

6

8

80

50 40

30

30

20

20 10

10

4

EDTA dosage (kg/t)

6

8

0

14 12

Selectivity index

10 8 6 4 2

2

4

EDTA dosage (kg/t)

pH

8

10

12

Zeta potential is the electrical potential for the interfacial double layer at the location of the slipping plane relative to a point in the bulk solution away from the interface (Fuerstenau et al., 1992; Jordens et al., 2014). The zeta potentials of bastnäsite as a function of pH in the absence and presence of 8 × 10−4 mol/L EDTA and 2.5 × 10−4 mol/L OHA individually or in combination is illustrated in Fig. 7. Isoelectric point (IEP), the pH value where the zeta potential is zero, is an important mineral property that can be used to characterize charging of the mineral surface (Liu et al., 2016). As shown in Fig. 7, in the absence of EDTA and OHA, the IEP value of bastnäsite was pH 6.8, which was similar to the results reported by Ren et al. (2000) and Houot et al. (1991). In the presence of 2.5 × 10−4 mol/L OHA, the zeta potentials of bastnäsite were negatively shifted and its IEP was reduced to pH 5.2 due to the adsorption of OHA through the chelation of hydroxamate anions with metal cations on bastnäsite surface, thereby neutralizing the surface electropositivity of bastnäsite (Liu et al., 2016). In the presence of 8 × 10−4 mol/L EDTA, the zeta potentials of bastnäsite were negatively shifted to a greater extent than those with OHA and the IEP was reduced to pH 3.3, which was probably because that the metal cations on bastnäsite were dissolved into aqueous solution through the formation of soluble metal-ligand complexes with EDTA. When both OHA and EDTA were present, the zeta potentials of bastnäsite were slightly decreased compared with those of bastnäsite conditioned with OHA only. It indicates that the adsorption of OHA on bastnäsite was almost unaffected by the addition of EDTA. Fig. 8 presents the zeta potentials of fluorite as a function of pH in the absence and presence of 2.5 × 10−4 mol/L OHA and 8 × 10−4 mol/L EDTA individually or in combination. It can be seen from Fig. 8 that in the absence of OHA and EDTA, the IEP of fluorite was around pH 7.4. Different IEP values for fluorite between 6 and 11 have been reported in the published literatures (Gao et al., 2015; Kosmulski, 2009; Zhang et al., 2013), which might be due to the different sources and impurity compositions of fluorite samples as well as the differences in solids concentration used in each study. Mineral dissolution and the resultant differences in potential determining ion content in the solution may have a significant effect on the zeta potential of minerals. The addition of 2.5 × 10−4 mol/L OHA decreased the zeta potentials of fluorite and reduced the IEP value to pH 4.3. It is

Fig. 5. REO (left y-axis) and CaF2 (right y-axis) grades of concentrate as a function of EDTA dosage in mixed mineral flotation of bastnäsite and fluorite (mass ratio 1:1) at pH 9 (OHA dosage 796 g/t).

0

6

3.3. Zeta potential measurements

Concentrate CaF2 grade (%)

Concentrate REO grade (%)

40

2

4

50

60

0

2

To understand the action mechanism of EDTA on the separation of bastnäsite from fluorite, zeta potential and XPS were conducted to investigate the effect of EDTA on the electrical property and components present on bastnäsite and fluorite surfaces.

60

Concentrate REO grade Concentrate CaF2 grade

70

Bastnasite Bastnasite+EDTA Bastnasite+OHA Bastnasite+OHA+EDTA

Fig. 7. Zeta potentials of bastnäsite as a function of pH in the absence and presence of 8 × 10−4 mol/L EDTA and 2.5 × 10−4 mol/L OHA individually or in combination.

Fig. 4. Recoveries of bastnäsite and fluorite as a function of EDTA dosage in mixed mineral flotation of bastnäsite and fluorite (mass ratio 1:1) at pH 9 (OHA dosage 796 g/t).

0

-20

-40

0

0

0

6

8

Fig. 6. Selectivity index of bastnäsite/fluorite as a function of EDTA dosage in mixed mineral flotation of bastnäsite and fluorite (mass ratio 1:1) at pH 9 (OHA dosage 796 g/t). 137

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15

Zeta potential (mV)

399.2 eV

400.5 eV

N 1s

(D) Bastnasite+OHA+EDTA

0

-15

399.1 eV

400.6 eV -30

-45

-60

(C) Bastnasite+OHA

Fluorite Fluorite+EDTA Fluorite+OHA Fluorite+OHA+EDTA 2

4

6

pH

8

10

12

(B) Bastnasite+EDTA

Fig. 8. Zeta potentials of fluorite as a function of pH in the absence and presence of 8 × 10−4 mol/L EDTA and 2.5 × 10−4 mol/L OHA individually or in combination.

proposed that hydroxamate anions were adsorbed on fluorite surface through the formation of metal-hydroxamate chelates which accounted for the negative shift of zeta potentials of fluorite. However, when treated with 8 × 10−4 mol/L EDTA, the zeta potentials of fluorite were decreased significantly, which is probably because that the metal cations were removed from fluorite by forming soluble metal-ligand complexes with EDTA. When conditioned with both OHA and EDTA, the zeta potentials of fluorite were also remarkably negatively shifted, the values of which were close to the zeta potentials of fluorite conditioned with EDTA only. This is probably due to that the metal-hydroxamate chelates were transformed into soluble metal-EDTA complexes and dissolved from fluorite surface when fluorite was conditioned with EDTA and the more negatively charged fluorine ions exposed on fluorite surface lowering its zeta potential.

(A) Bastnasite

406

404

402

400

398

396

Binding energy (eV) Fig. 9. N 1s XPS spectra recorded from the surfaces of bastnäsite in the absence (A) and presence of 8 × 10−4 mol/L EDTA (B) and 2.5 × 10−4 mol/L OHA (C) individually or in combination (D).

3.4. XPS characterization Bastnäsite and fluorite conditioned without or with 8 × 10−4 mol/L EDTA and 2.5 × 10−4 mol/L OHA individually or in combination were subjected to XPS analyses. The atomic concentrations on bastnäsite and fluorite surfaces under different conditions are listed in Table 1. The atomic concentration of Ce on bastnäsite reduced from 14.1% to 12.6%, whilst the atomic concentration of F increased from 16.0% to 17.3% with the addition of 8 × 10−4 mol/L EDTA. This may be due to that metal cations were partially dissolved from bastnäsite surface through the formation of soluble metal-ligand with EDTA, and thus more negatively charged fluorine ions were exposed. This is also confirmed by the negative shift of zeta potentials of bastnäsite in the presence of

EDTA as shown in Fig. 7. When conditioned with OHA, 1.6% N was detected on bastnäsite due to the adsorption of nitrogen-containing hydroxamate collector. The atomic concentration of N on bastnäsite in the presence of both OHA and EDTA was nearly same as that in the presence of OHA only, which indicates that the adsorption of hydroxamate on bastnäsite was almost unaffected by the addition of EDTA. However, the atomic concentration of N on fluorite was decreased from 1.2% when conditioned with OHA only, to 0.6% with the further addition of EDTA. This demonstrates that the adsorption amount of hydroxamate on fluorite was significantly reduced by the addition of EDTA. Fig. 9 displays the N 1s XPS spectra recorded from the surfaces of bastnäsite in the absence and presence of 8 × 10−4 mol/L EDTA and 2.5 × 10−4 mol/L OHA individually or in combination. It can be found that N 1s signal was not detected on bare bastnäsite or bastnäsite treated with EDTA. The N 1s binding energy peak appeared on OHA treated bastnäsite, which confirmed the adsorption of hydroxamate. The N 1s binding energy peak was de-convoluted into two peaks: one at around 399 eV and the other at above 400 eV. The peak at around 399 eV was attributed to nitrogen atoms in the deprotonated hydroxamate groups, R-CO-NH-O−, and the peak above 400 eV was due to nitrogen in neutral molecules, i.e., the protonated form of the hydroxamic acid, R-CO-NH-OH (Alagta et al., 2008; Folkers et al., 1995). Since deprotonation is the first step of the chemisorption of the hydroxamic acid on the mineral surface (Ni and Liu, 2012), the peak at around 399 eV was attributed to the chemically adsorbed OHA, while the peak at above 400 eV was caused by the physical absorption of OHA.

Table 1 The XPS atomic concentration of elements recorded from the surfaces of bastnäsite and fluorite in the absence and presence of 8 × 10−4 mol/L EDTA and 2.5 × 10−4 mol/L OHA individually or in combination. Samples

Atomic concentration (at. %) Ce

C

F

O

N

Bastnäsite Bastnäsite + EDTA Bastnäsite + OHA Bastnäsite + OHA + EDTA

14.1 12.6 13.7 14.0

41.3 38.9 39.6 38.7

16.0 17.3 15.1 15.9

28.6 31.2 30.0 29.9

– – 1.6 1.5

Fluorite Fluorite + EDTA Fluorite + OHA Fluorite + OHA + EDTA

Ca 29.7 24.4 29.9 25.8

C 25.1 24.1 22.2 23.1

F 37.9 42.7 38.8 42.1

O 7.3 8.8 7.9 8.4

N – – 1.2 0.6

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400.5 eV 399.0 eV

(D) Fluorite+OHA+EDTA

400.7 eV

changed, proving that the adsorption and configuration of OHA on bastnäsite were almost unaffected by the addition of EDTA. Fig. 10 displays the N 1s XPS spectra recorded from the surfaces of fluorite in the absence and presence of 8 × 10−4 mol/L EDTA and 2.5 × 10−4 mol/L OHA individually or in combination. No N 1s signal was found on the surface of bare fluorite and fluorite treated with only EDTA. Similar to bastnäsite, the N 1s peak on OHA treated fluorite was also split into two components caused by chemically adsorbed (around 399 eV) and physically adsorbed (above 400 eV) OHA. However, the proportion of physically adsorbed OHA (55%) was higher than that by chemical adsorption (45%), indicating that the chemical adsorption of OHA on fluorite was relatively weaker than that on bastnäsite. This is because that the hydroxamate group is highly specific to RE cations, Ce3+ of bastnäsite lattice, compared with alkaline earth cations, Ca2+ of fluorite lattice (Pradip and Fuerstenau, 1983). When conditioned with both OHA and EDTA, it is interesting to note that the proportion of chemically adsorbed OHA on fluorite (20%) was significantly reduced, suggesting that the chemically adsorbed OHA, in the form of metalhydroxamate chelate, can be removed from fluorite surface by the addition of EDTA. Therefore, fluorite was significantly depressed by EDTA, while bastnäsite was almost unaffected in flotation.

N 1s

399.2 eV

(C) Fluorite+OHA

(B) Fluorite+EDTA

3.5. Thermodynamic calculation The above study indicates that fluorite was selectively depressed in bastnäsite flotation using EDTA. It is proposed that EDTA was able to transform the chemically adsorbed OHA, Ca-hydroxamate chelate (denoted as Ca-OHA) on fluorite, into the soluble Ca-EDTA complexes, whereas the Ce-OHA chelate on bastnäsite resulting from the adsorption of OHA was more stable and cannot be dissolved by EDTA. In this part, the theoretical thermodynamic calculation was adopted to verify whether Ca, Ce-OHA chelates can be converted to Ca, Ce-EDTA complexes by EDTA. The conversion reactions of Ce-OHA to Ce-EDTA and Ca-OHA to CaEDTA were expressed in Eq. (2) and Eq. (3), respectively:

(A) Fluorite

406

404

402 400 Binding energy (eV)

398

396

Fig. 10. N 1s XPS spectra recorded from the surfaces of fluorite in the absence (A) and presence of 8 × 10−4 mol/L EDTA (B) and 2.5 × 10−4 mol/L OHA (C) individually or in combination (D).

20

0

G (kJ/mol)

CeY + 3OHA

(2)

Ca(OHA)2 + Y4

CaY 2 + 2OHA

(3)

where Y4− was shorted for the deprotonated EDTA ions and CeY− and CaY2− denoted to Ce-EDTA and Ca-EDTA complexes, respectively. The reaction equilibrium constants for Eq. (2) and Eq. (3) were described as follows:

10

-10

(1)

-20 -30

(1) Conversion of Ce-OHA to Ce-EDTA (2) Conversion of Ca-OHA to Ca-EDTA

-40 -50 -60

Ca(OHA)3 + Y4

4

6

pH

8

10

[CeY ][OHA ]3 KL (CeY ) = [Ce(OHA)3][Y4 ] KL (Ce(OHA)3)

(4)

K2 =

[CaY 2 ][OHA ]2 KL (CaY 2 ) = [Ca(OHA)2][Y4 ] KL (Ca(OHA) 2)

(5)

where KL (CeY - ), KL (Ce(OHA)3), KL (CaY 2 ) and KL (Ca(OHA) 2) were the complex stability constants 1011.98, 1012.8, 1010.4 and 102.4 for CeEDTA, Ce-OHA, Ca-EDTA and Ca-OHA, respectively (Hu et al., 2007; Zhang, 2014). The standard Gibbs free energy changes ( G ) for Eq. (2) and Eq. (3) were given as follows:

(2) 2

K1 =

12

Fig. 11. Gibbs free energy changes (ΔGθ) for conversion of Ce-OHA precipitate to Ce-EDTA complex (1), and Ca-OHA precipitate to Ca-EDTA complex (2) as a function of pH.

On the OHA-treated bastnäsite, the N 1s peak was comprised of 75% chemically adsorbed (deprotonated) and 25% physically adsorbed (protonated) OHA, indicating that the majority of OHA were chemically adsorbed on bastnäsite, which is also the main adsorption form of hydroxamate collector on pyrochlore (Ni and Liu, 2012), wolframite (Meng et al., 2015) and ilmenite (Xu et al., 2015). After being conditioned with OHA and EDTA, the component ratios of chemically adsorbed (68%) and physically adsorbed (32%) OHA were slightly

G1 =

RT ln K1

G2 =

RT ln K2

3 OHA Y4 -

(6)

2 OHA Y4 -

(7)

where R was the ideal gas constant; T was the thermodynamic temperature; OHA - and Y 4 - were the side reaction coefficients for OHA and EDTA, which were calculated by Eq. (8) and Eq. (9), respectively: OHA -

139

= 1 + [H+] K1H

(8)

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Z. Cao et al.

Fig. 12. Schematic representation of the variation of surface chemistry on fluorite (a) and bastnäsite (b) conditioned with OHA and EDTA.

Y4 -

= 1 + [H+] K2H1 + [H+]2 K2 +

2

+ [H+]3 K2

3

+ [H+]4 K2

4

+ [H+]5 K2

4. Conclusions

5

(9)

[H+]6 K2 6

In the current study, the effect of ethylenediamine tetraacetic acid (EDTA) as a depressant for fluorite in bastnäsite flotation was investigated through flotation, zeta potential and X-ray photoelectron spectroscopy (XPS) tests. The flotation results showed that fluorite was significantly depressed, while the flotation of bastnäsite was almost unaffected when EDTA was present. The separation index between bastnäsite and fluorite increased from 1.18 to 12.66 with the increase of EDTA concentration from 0 to 7.79 kg/t. The results of zeta potential and XPS tests, as well as theoretical thermodynamic calculation, showed that EDTA could dissolve the chemically adsorbed octyl hydroxamic acid (OHA) on fluorite through the formation of soluble Ca-EDTA complexes, whereas the chemically adsorbed OHA on bastnäsite was more stable and could not be transformed into Ce-EDTA spontaneously. As a result, the flotation of fluorite was selectively depressed by EDTA. The findings of this study showed that EDTA was a promising depressant for fluorite gangue mineral in the flotation of bastnäsite.

9.4

where K1H was the protonation constant for OHA ions, 10 ; K2H1, K2 2 , K2 3 , K2 4 , K2 5 and K2 6 were the accumulated protonation constants 1010.34, 1016.58, 1019.33, 1021.4, 1023 and 1023.9 for each protonating step of EDTA ions (Hu et al., 2007). The relationship between Gibbs free energy changes ( G ) and pH was calculated by combining Eqns. (8) and (9) with Eqns. (6) and (7), and the results are shown in Fig. 11. As shown in Fig. 11, the Gibbs free energy change G for conversion of Ce-OHA to Ce-EDTA was positive, whereas that for Ca-OHA to Ca-EDTA was negative at pH 9.0, indicating that the transformation of Ca-OHA to Ca-EDTA on fluorite was thermodynamically feasible, whilst Ce-OHA was more stable on bastnäsite and could not be transformed into Ce-EDTA spontaneously. The reactions occurring on fluorite and bastnäsite surfaces with the addition of OHA and EDTA is illustrated in Fig. 12. The adsorption of OHA on fluorite and bastnäsite includes physical absorption due to

Acknowledgments

hydrogen bonding, in the form of

The financial supports from National Natural Science Foundation of China (51764045), Inner Mongolia Young Science & Technology Talent Support Plan (NJYT-18-B08), Research Fund Program of State Key Laboratory of Rare Metals Separation and Comprehensive Utilization (No.GK-201804) and Outstanding Youth Science Foundation of Inner Mongolia University of Science and Technology (2017YQL05) are gratefully acknowledged. The first author gratefully acknowledges the scholarship provided by China Scholarship Council.

, and chemical adsorption in the form of , due to chelating reaction, which is formed by the replacement of the hydrogen atom of the hydroxamate group by metal cations on mineral surface and ring closure by the carbonyl oxygen atom (Zhang, 2014). The more negative surface potential after OHA adsorption is due to the electrical neutralization of OHA anions with metal cations on mineral surface, which is demonstrated by the results of zeta potential measurements shown in Figs. 7 and 8. When conditioned with EDTA, the chemically adsorbed OHA on fluorite is almost totally removed due to its transformation into soluble Ca-EDTA complexes, which is responsible for the depression of fluorite by EDTA. This also explains the distinct negative shift of zeta potential on the EDTA treated fluorite. However, the chemically adsorbed OHA on bastnäsite is more stable and not influenced by the addition of EDTA, which allows for high recovery of bastnäsite and selective depression of fluorite by using EDTA as the depressant.

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