Flotation separation of fluorite from calcite using sulfonated lignite as depressant

Journal Pre-proofs Flotation separation of fluorite from calcite using sulfonated lignite as depressant Yanfang Cui, Fen Jiao, Qian Wei, Xu Wang, Liuyang Dong PII: DOI: Reference:

S1383-5866(19)35302-X https://doi.org/10.1016/j.seppur.2020.116698 SEPPUR 116698

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Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

18 November 2019 4 February 2020 9 February 2020

Please cite this article as: Y. Cui, F. Jiao, Q. Wei, X. Wang, L. Dong, Flotation separation of fluorite from calcite using sulfonated lignite as depressant, Separation and Purification Technology (2020), doi: https://doi.org/ 10.1016/j.seppur.2020.116698

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Flotation separation of fluorite from calcite using sulfonated lignite as depressant Yanfang Cui; Fen Jiao*; Qian Wei*; Xu Wang; Liuyang Dong School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China Key Laboratory of Hunan Province for Clean and Efficient Utilization of Strategic Calcium-containing Mineral Resources, Central South University, Changsha 410083, China

ABSTRACT In this study, sulfonated lignite (SL) was studied as the depressant in the separation of fluorite and calcite with sodium oleate (NaOL) as the collector. The flotation recovery of fluorite and calcite with and without depressant SL was investigated by micro-flotation test. The mechanism of the depressant was studied through Fourier transform infrared spectroscopy (FTIR) analysis, contact angle analysis and X-ray photoelectron spectroscopy (XPS) analysis. The micro-flotation experiment showed that sulfonated lignite had an effective depression effect on calcite. FTIR results illustrated that sulfonated lignite was selectively absorbed on the calcite surface and NaOL could still be absorbed on fluorite surface with depressant sulfonated lignite. Contact angle analysis demonstrated that depressant sulfonated lignite could significantly increase the hydrophilicity of calcite surface with NaOL as the collector. However, the surface of fluorite was still relatively hydrophobic with depressant SL and collector NaOL. XPS results illustrated that sulfonated lignite was adsorbed on the calcite surface through the interaction of the carboxyl groups of the depressant with the Ca atom on the calcite surface. Therefore, sulfonated lignite could be used as a potential depressant in the flotation system of fluorite and calcite. Keywords: Fluorite; Calcite; Flotation; Sulfonated Lignite; Adsorption

1. Introduction Fluorite (CaF2) is one of the most important fluorine-bearing minerals and it is `

widely used in metallurgy, chemistry, ceramics, optics and other industries [1, 2]. Fluorite resources are also used as important strategic reserve supplies in many developed countries [3, 4]. There are abundant fluorite resources for production and consumption in China. With the constant exploitation of fluorite resources, an increasing amount of low-grade minerals have to be processed [5]. The most commonly used method to improve the content of fluorite is froth flotation [6-8]. However, fluorite usually coexists with some other gangue minerals, like dolomite, barite and calcite, etc. [9, 10]. The gangue minerals need to be removed during the flotation of fluorite. During the flotation slurry, many ions dissolve from the mineral surface and undergo many reactions, so the calcium-containing minerals have the similar physical and chemical properties and the separation of fluorite and calcite has always been a challenge [11-14]. Flotation is a process to selectively separate different minerals by using the differences in the mineral surface properties [7]. A depressant is a kind of regulator in the flotation that it is used to increase the hydrophilicity of gangue minerals. Depressants mainly include inorganic depressants and organic depressants [15-17]. Water glass or modified water glass is the most widely applied inorganic depressant in fluorite flotation [2, 18-20]. However, the dosage of water glass is high due to its low price and weak inhibition, which can make the water treatment difficult in subsequent. Therefore, organic depressants and the combination of different depressants have also been studied widely to improve the depressive ability and selectivity. For example, tannin and starch were used as depressants in fluorite flotation to depress calcite [5, 21, 22]. Locust bean gum was an effective depressant on the flotation of sphalerite [23]. In these studies, macromolecular organic depressants exhibit advantages such as extensive sources, environment-friendly and good stability. Therefore, it is essential to develop new macromolecular organic depressants in the flotation system. Sulfonated lignite is a derivative of lignite humic acid. Sulfonated lignite is a brown-black powder and has good water solubility. It is widely used as the fluid loss control agent, mud treatment agent and absorbent [24-26]. Sulfonated lignite

contains a large number of functional groups such as phenolic hydroxyl group, carboxyl group, hydroxyl group and sulfonic acid group [27, 28]. The free carboxyl groups in sulfonated lignite could chelate with metal ions and adsorb on the mineral surface, thereby the hydrophilicity on the mineral surface increased. Therefore, sulfonated lignite can be considered as a depressant in separation. In this study, sulfonated lignite was studied as a new depressant in the separation of fluorite and calcite with sodium oleate (NaOL) as a collector. Micro-flotation tests of the two minerals were studied to show the depressing effect of sulfonated lignite on fluorite and calcite. The mechanism of sulfonated lignite was investigated by Fourier transform infrared spectroscopy (FTIR) measurement, contact angle measurement and X-ray photoelectron spectroscopy (XPS) measurement.

2. Materials and methods 2.1. Materials and reagents The pure mineral samples of fluorite and calcite were purchased from Guangxi Province and Hunan Province, China. The minerals were crushed and manually selected to remove impurities. And then, the mineral particles were ground and screened by using a standard screen. The proportion of -74+37 μm was used for micro-flotation experiments and XPS measurements. The proportion of -37 μm was further ground to less than 2 μm and used for FITR tests and chemical tests [29, 30]. Fig. 1 showed the XRD of fluorite and calcite. The results of chemical tests indicated that the sample purities of fluorite and calcite were 99.71% and 99.85%, respectively. The results of XRF and XRD showed that the two mineral samples had a high purity. In this experiment, NaOL was used as the collector and obtained from BSQ Chemistry Technique, Shanghai, China. Sulfonated lignite (SL) was used as the depressant and purchased from Luoyang Zhenbei industry and trade co. LTD, Henan, China. Hydrochloric acid (HCl) solution and sodium hydroxide (NaOH) solution were used as regulators to adjust the slurry pH, and the concentrations of HCl and NaOH were 1 mol/L. All the reagents in the experiments are analytical grade. Ultrapure

water was used in all tests with a resistivity of over 18.0 MΩ×cm. 25000

30000 F

25000

F- Fluorite

(a)

I (count/s)

I (count/s)

20000 15000 10000 F F

20

30

40

C-Calcite

(b)

15000

10000

5000

5000 0 10

C

20000

50 60 2θ(degree)

F

70

C

F

F

80

0 90

10

20

C CC C C C

30

40

C C CC

50

60

C CC

70

80

90

2θ(degree)

Fig. 1. XRD spectra of fluorite (a) and calcite (b)

2.2. Methods 2.2.1 Micro-flotation tests The micro-flotation tests of fluorite and calcite were carried out in an XFG flotation machine. The rotation speed was controlled at 1602 rpm. For each test, 2g of the pure mineral samples were added to 40 mL flotation cell and agitated to obtain the uniform slurry. Then, the pH regulator (HCl or NaOH), depressant (SL) and collector (NaOL) were added in order (Fig. 2). Finally, the froth products and tailings products were separately collected, filtered, dried, and weighed to calculate the flotation recovery. The flotation recovery was calculated using the following equation: ε=

m1 m1 + m2

× 100%

Where ε represents the recovery; m1 and m2 represent the weight of the froth products and tailings products (g). Each test was repeated three times to ensure accuracy. The average value of recovery and the standard deviation of parallel results were calculated and presented.

Pure Mineral (2.0g) Agitating 1min Adjusting pH 2min SL 3min NaOL 3min Flotation 3min

Froth Products

Tailings Products

Fig. 2. Flowsheet of micro-flotation experiment

2.2.2. FTIR measurements The FTIR was carried out with an infrared analyzer (Shimadzu Corporation, Kyoto, Japan). The mineral samples of fluorite and calcite were ground to -2 μm. 1.0 g of the mineral sample was added to 40 mL flotation cell, and the pH was adjusted to about 8.5 like in the flotation test. Then, the depressant and the collector were added and the slurry was reacted for 20 min. The concentration of SL was 0.5 g/L, and the concentration of NaOL was 2.5 g/L. The samples were washed three times with the same pH, and the samples were used for FTIR measurement after dried in a vacuum oven.

2.2.3. Contact angle measurement The contact angle measurement was conducted by using a contact angle meter (JY-82C, Chengde Dingsheng testing equipment co. LTD, China) to characterize the wettability of mineral surface with different flotation reagents. Before contact angle measurement, the polished mineral sample was treated in the corresponding flotation reagent solutions at pH 8.5 for 5 min, and then washed using ultrapure water, dried and measured. A sessile drop method was utilized to determine the contact angle of the surface mineral samples. Each condition was measured at least three times, and an

average value was reported as the final result. 2.2.4. XPS measurements The XPS was conducted using the X-ray Photoelectron Spectroscopy with a type of K-Alpha+ (Thermo Fisher Scientific, USA). The pressure of vacuum was set to 2×10-7 mba and the X-ray source was monochromatic Al Kα with the energy of 1486.6 eV, 6 mA×12 KV. The accuracy of the XPS measurement is 0.1eV. The mineral samples of the XPS measurement were consistent with the flotation test process. Then the samples were filtered and dried for XPS test. The results were collected and processed by using XPS Peakfit software. The standard carbon 1s spectrum photoelectron peak was based on the binding energy of 284.8 eV.

3. Results and discussion 3.1. Micro-flotation results The effect of pH on the recovery of fluorite and calcite with and without depressant is presented in Fig. 3. The NaOL was used as the collector and the concentration of NaOL was 50 mg/L. It can be seen from Fig. 3, the recovery of fluorite and calcite had always kept an excellent recovery (above 70%) over the whole pH range. The results indicated that the separation of fluorite and calcite was impossible in the absence of depressant due to the similar floatability. With the depressant SL, the floatability of calcite was significantly reduced. When the slurry pH was 8.5, the recovery of calcite was about 20%. However, under the same flotation conditions, the recovery of fluorite still kept about 80%. The floatability difference between fluorite and calcite made the separation of fluorite and calcite possible with the depressant SL. Fig. 4 presents the effect of depressant concentration on the recovery of the two minerals with 50 mg/L NaOL at pH 8.5. It can be seen from Fig. 4 that SL had a selective depression effect on calcite flotation. The floatability of calcite reduced

sharply as the SL concentration increased, but the floatability of fluorite changed relatively little. The recovery of calcite reduced from 62% to 15% when 9 mg/L SL was added. While for fluorite, the recovery only dropped from 86% to 76%. The micro-flotation results illustrated that fluorite and calcite are able to be separated at an SL concentration of 10 mg/L. 100

Recovery (%)

80 NaOLdosage=50 mg/L SL dosage=10 mg/L

60

Calcite+NaOL Fluorite+NaOL Calcite+SL+NaOL Fluorite+SL+NaOL

40 20 0

6

8

10

12

pH

Fig. 3. Effect of pH on the recovery of fluorite and calcite with and without depressant SL 100

Recovery (%)

80 NaOL concentration=50 mg/L pH=8.5

60 40 20 0

Calcite+SL+NaOL Fluorite+SL+NaOL

0

2

4

6

8

10

SL concentration (mg/L)

Fig. 4. Effect of SL concentration on the recovery of fluorite and calcite

3.2. FTIR analysis The FTIR mainly studies the composition and structure of material molecules based on the absorption of different wavelengths of infrared light. The FTIR tests

were conducted to study the adsorption of different reagents on the two mineral surfaces by means of an infrared spectrometer. The results of the FTIR spectra are depicted in Figs. 5-6. Fig. 5 shows the FTIR spectra of SL and NaOL. From Fig. 5(a), the peak at 1706.69 cm-1 was contributed to the stretching vibration band of carbonyl (C=O) [31, 32]. The peaks at 1576.22 cm-1 and 1383.94 cm-1 were assigned to the C=O asymmetric stretching and symmetric stretching in COO− groups, respectively [31, 33]. The peak at 1032.94 cm-1 was contributed to the vibration of -SO3H and the O-H and C-O stretching vibration of phenolic hydroxyl [32, 34]. As seen in Fig. 5 (b), the stretching bands of -CH2- and -CH3 appeared at 2921.21 cm-1 and 2852.2 cm-1. The peaks at 1557.19 cm-1 and 1460.41 cm-1 were attributed to the stretching vibrations of -COO- [35]. The FTIR spectra of fluorite with different reagents at pH 9 are illustrated in Fig. 6 (a). As seen in Fig. 6 (a), there were no new significant characteristic peaks observed after reacting with depressant SL. It illustrated that the adsorption between depressant SL and fluorite surface was very weak. After fluorite was treated by depressant SL and collector NaOL together, new bands appeared at 2927.41 cm-1 and 2856.06 cm-1. The bands were attributed to the shift of stretching vibrations of the -CH2- and -CH3 at 2921.21 cm-1 and 2852.2 cm-1 [36]. The results illustrated that the collector could be adsorbed on the surface of fluorite after adding the depressant SL. Fig. 6 (b) presents the FTIR spectra of calcite with different reagents at pH 9. As seen from Fig. 6 (b), three new peaks appeared at 1739.48 cm-1, 1583.27 cm-1 and 1016.30 cm-1 after the addition of depressant SL. The peak at 1739.48 cm-1 was possibly due to the stretching vibration of carbonyl groups in SL, and the peaks at 1583.27 cm-1 and 1016.30 cm-1 were shifted from the C=O asymmetric stretching and the vibration of -SO3H at 1576.22 cm-1 and 1032.94 cm-1[32]. The results illustrated that there was a chemical adsorption between the depressant SL and the calcite surface. There is no new peak appeared when the depressant SL and the collector NaOL are present together. These results illustrated that the pre-adsorption of SL could hinder the adsorption of the collector on the calcite surface.

(a)

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NaOL

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2921.21 2852.2

1557.19 1460.41

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913.23 1032.94

1706.69 1576.22 1383.94

SL

500 4000

3500

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2000 -1

Wavenumber/cm

Fig. 5. The FTIR spectra of NaOL and SL

1014.37

1739.48 1583.27

Calcite+SL+NaOL

1741.41

2927.41 2856.06

1544.70 1459.85

Fluorite+SL+NaOL

4000

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711.60

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Calcite+SL

2514.72

Fluorite+SL

875.52

(b)

Calcite

1016.30

(a)

2979.48 2873.42

Fluorite

1500

1000

500

-1

Wavenumbers (cm )

Fig. 6. FTIR spectra of minerals with and without different reagents

3.3. Contact angle analysis The hydrophilic or hydrophobic of the mineral surface is usually represented by the wettability. Wettability is closely related to the floatability of mineral, which is generally measured by the contact angle. In generally, the larger the contact angle is, the weaker the hydrophilicity of the mineral surface will be [37, 38]. The contact angles of the fluorite surface under different reagents at pH 9 were depicted in Fig. 7. As shown in Fig. 7, the contact angle of fresh fluorite was 60.65°, indicating that the natural fluorite surface had a degree of hydrophilicity. The contact angle of fluorite dramatically increased to 98.34° after treatment with the NaOL. The results suggested that collector adsorbed on the mineral surfaces and increased the

floatability of the fluorite surface. A slightly decreased contact angle to 88.68° was observed with depressant SL and collector NaOL, suggesting that SL had a little effect on the wettability of the fluorite surface with collector NaOL. Fig. 8 shows the contact angle of the calcite surface with and without SL and NaOL at pH 9. From Fig. 8, the contact angle of the pure calcite surface was 53.59°, indicating that calcite had a poor natural floatability. After NaOL adsorption, the hydrophobicity of the calcite surface increased, from 53.59°to 73.35°, as shown in Fig. 8 (b). This means NaOL could adsorb on the calcite surface and make the hydrophobicity and floatability of calcite increase. After treatment with SL and NaOL, the contact angle decreased from 73.35° to 42.59° on the surface of calcite, illustrating that SL had a strong depressing effect on calcite. Thus, calcite had a poor floatability with SL and NaOL. The contact angles results indicate how SL and NaOL affect the surface wettability of fluorite and calcite. The results were consistent with the micro-flotation test, which SL can depress calcite but not fluorite.

Fig. 7. Contact angles of fluorite (a), fluorite + NaOL (b), fluorite + SL+ NaOL (c)

Fig. 8. Contact angles of calcite (a), calcite + NaOL (b), calcite + SL+ NaOL (c)

3.4. Surface analysis The interaction mechanism of depressant SL on the calcite surface was further revealed by XPS analysis. It mainly utilized to observe the changes of the chemical state of surface elements. The relative atomic concentrations of calcite surface

elements before and after the treatment with SL were measured and shown in Table 1. Table 1 showed that a new element S 2p appeared and the relative atomic concentration was 0.33%, indicating that SL adsorbed on the calcite surface. To obtain the specific changes of the chemical environment on the calcite surface elements in the presence of SL, the narrow scan of Ca 2p and C1s were further characterized and depicted in Fig 9. Fig 9 (a) shows that the fitting peaks of calcite contained one double peak at 346.68 eV and 350.28 eV, which could be attributed to the Ca 2p 3/2 and Ca 2p 1/2 in CaCO3 [36]. When depressant SL was added in flotation slurry, new fitted peaks can be observed at 347.64 eV and 351.10 eV. These peaks were assigned to the Ca 2p 3/2 and Ca 2p 1/2 in Ca-COOR on the surface of calcite. This illustrated that SL had an adsorption on calcite by the carboxyl group of the depressant SL with the Ca atom. The fitting peaks of C1s were depicted in Fig 9. From Fig. 9(b), pure calcite had three peaks at 284.78 eV, 285.48 eV and 289.68 eV and these peaks were attributed to hydrocarbons, carbon oxides and carbon in the carbonate of the calcite [35, 39, 40]. A new peak appeared at 288.98 eV after treatment with SL and it was contributed to Ca-COOR on the calcite surface. These results illustrated that SL had an adsorption on the calcite surface. Table 1 Atomic relative concentrations of elements on the samples Samples Element (atomic %) Ca (2p) C (1s) O (1s) Calcite 14.49 40.17 45.34 Calcite + SL 13.81 42.21 43.65

S (2p) —

0.33

Experimental line Fitting line Background Ca 2p 3/2 in CaCO3

Experimental line Fitting line Background Hydrocarbons Carbon dioxide 2CO3 of calcite

(a) 347.64

Ca 2p 1/2 in CaCO3 Ca 2p 3/2 in Ca-COOR Ca 2p 1/2 in Ca-COOR

346.78

Ca-COOR

350.28

351.1

289.89

(b) 284.78 288.98 285.68

Experimental line Fitting line Background Ca 2p 3/2 in CaCO3

Counts/s

Counts/s

Calcite+SL

346.68

Experimental line Fitting line Background Hydrocarbons Carbon dioxide 2CO3 of calcite

Ca 2p 1/2 in CaCO3

350.28

Calcite

354

352

350 348 Binding energy (eV)

346

344

296

294

284.78 289.68

285.48

292

288 290 Binding energy (eV)

286

284

282

Fig. 9. Fitting peaks of Ca 2p (a) and C 1s (b) on calcite surface with and without SL

4. Conclusions SL was used as the depressant in the separation of fluorite and calcite and the mechanism was studied. The micro-flotation results showed that SL could selectively depress calcite and has a little effect on the flotation of fluorite. FTIR results illustrated that sulfonated lignite was selectively absorbed on the calcite surface and collector can still absorb on the fluorite surface with the depressant SL. Contact angle measurement demonstrated that SL decreased the hydrophobicity of calcite. XPS tests demonstrated that carboxyl groups of SL could be reacted with calcium ion species of calcite surface. Therefore, the separation flotation of fluorite and calcite can be effectively achieved by using SL as the depressant.

Acknowledgements The authors gratefully acknowledge the financial support of this research by Pro vincial Science and technology leader (Innovation team of interface chemistry of effic ient and clean utilization of complex mineral resources, Grant No. 2016RS2016); Nati

onal Natural Science Foundation of China (Project No. 51604302 and No. 51574282) ; and the Key laboratory of Hunan Province for Clean and Efficiency Utilization of stra tegic Calcium-containing mineral Resources (No. 2018TP1002).

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Author statement: Yanfang Cui: Methodology, Investigation, Writing-Original Draft; Fen Jiao: Conceptualization, administration: Qian Wei: Supervision, Project; Xu Wang: Resources; Liuyang Dong: Writing- Reviewing and Editing.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Highlights 

Sulfonated lignite (SL) is a promising depressant in the flotation separation of fluorite from calcite and the depressive mechanism of sulfonated lignite on calcite was studied.

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Sulfonated lignite was selectively absorbed on calcite surface and sodium oleate (NaOL) can still be absorbed on fluorite surface in the presence of depressant sulfonated lignite.

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Sulfonated lignite was adsorbed on calcite surface by the bonding of the carboxyl groups of the depressant with the Ca atom on calcite surface

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Flotation separation of scheelite and calcite can be realized with sulfonated lignite.