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Selective flotation of scheelite from calcite using xanthan gum as depressant
Minerals Engineering 138 (2019) 14–23
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Selective flotation of scheelite from calcite using xanthan gum as depressant ⁎
⁎
T
Liuyang Dong, Fen Jiao , Wenqing Qin , Wei Liu 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
A R T I C LE I N FO
A B S T R A C T
Keywords: Scheelite Calcite Flotation XG Adsorption
Xanthan gum (XG), a clean, non-toxic macromolecular polysaccharide, was studied as a depressant in the flotation separation of scheelite from calcite. Its selective inhibition mechanism was investigated by adsorption experiments, fourier transform infrared spectroscopy (FTIR) analysis and X-ray photoelectron spectroscopy (XPS) analysis. It was found that the XG exhibited a strong selective depressive effect on calcite. However, XG had minimal effect on the flotation recovery of scheelite. Adsorption experiments indicated that the adsorption density of XG on the surface of calcite was much higher than that of scheelite. FTIR results indicated that XG was selectively chemisorbed on calcite surface, while sodium oleate (NaOL) can still be chemisorbed on scheelite surface in the presence of depressant XG. XPS results indicated that the carboxyl groups (eCOO−) of the XG can be chemically chelated with the calcium ions species on calcite surface. It was intuitively observed by atomic force microscopy (AFM) that the coverage and adsorption density of XG on calcite surface were higher than that of the scheelite surface. As such, the calcite surface adsorbed more depressant XG than that of scheelite, which hindered the further adsorption of the collector NaOL. This greatly deteriorated the flotation of calcite and achieved flotation separation between the two minerals.
1. Introduction Tungsten is an important non-ferrous metal with excellent melting point, stable chemical properties and high hardness (Gong et al., 2019; Kim et al., 2006). It is often used to make metal alloys, optical devices and superhard abrasives, and it has a wide range of uses in metallurgy, chemical, electronics, materials, machinery and many other fields. China is the world's leading tungsten producer, consumer and supply country (Castanho and Vieira, 1998; Emin et al., 2018; Xu et al., 2019). Wolframite [(Fe, Mn) WO4] and scheelite (CaWO4) are the two most important sources of tungsten-bearing minerals (Kotlov et al., 2007; Meng et al., 2015). The wolframite deposit has the advantages of good quality, high grade, easy mining, easy selection, convenient subsequent treatment and less environmental hazard. For wolframite deposit with higher grade, it can be recovered by simple and pollution-free gravity selective method (Harlaux et al., 2018; Singh and Pandey, 2015). After extensive mining, this wolframite deposit has been exploited and grade is decreasing. Deposits available for development are shrinking (Gong et al., 2019; Kumar et al., 2011). Therefore, the development and utilization of scheelite and mixed tungsten ore resources have received more attention. The scheelite deposits are complex in type and often coexist with other gangue minerals, such as various sulfide ore and ⁎
calcium-containing oxidized minerals (Li and Gao, 2018; Wang et al., 2019; Zhao et al., 2019a). Calcite (CaCO3), a mineral that often coexists with scheelite, is a low-value mineral that often needs to be removed (Hu et al., 2011). The ideal dissociation particle size of scheelite is mostly in the flotation particle size range. Therefore, in the process of separating scheelite from gangue minerals, flotation is the most commonly used method (Filippov et al., 2018). Flotation is a physico-chemical separation process that utilizes the difference in surface properties of the valuable minerals and the unwanted gangue minerals (Wills and Finch, 2016). Mineral surface wettability, surface lattice ion dissolution, surface electrical properties and solution chemical properties have an important impact on the flotation process (Gao et al., 2019; Liu et al., 2019b). Flotation reagents (such as surfactants) are often added during the flotation process to alter these properties and thereby alter the flotation performance of the mineral (Liu et al., 2019a; Xu et al., 2017). Flotation depressant is an important and widely used flotation reagent that reduces the flotation recovery of some minerals, and it inhibits different minerals to varying degrees to achieve flotation separation between minerals. Traditional scheelite flotation depressants are mainly inorganic depressants, mainly including sodium silicate (water glass), phosphates, fluorosilicates, etc (Bo et al., 2015; Dong et al., 2019; Gao et al., 2017; Yin and Wang,
Corresponding authors at: School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China. E-mail addresses: [email protected] (F. Jiao), [email protected] (W. Qin).
https://doi.org/10.1016/j.mineng.2019.04.030 Received 6 January 2019; Received in revised form 15 April 2019; Accepted 22 April 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.
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2. Materials and methods
2014). However, these depressants also have some shortcomings, such as large dosage, easy to be affected by pH, and narrow flotation range. The combination of metal ions and depressant to improve the depressive power and selectivity of the depressant has also been studied (Deng et al., 2018). In addition, organic depressants have also been widely used. Organic depressants mainly include small molecule organic depressant such as citric acid, tartaric acid, tannic acid, salicylic acid, lactic acid, etc (Guo et al., 2017; Zhang et al., 2017). However, the depressive power and selectively of small molecule organic depressants is usually weak, and it is often necessary to use them in combination to achieve the depressive effect. In recent years, macromolecular polymers have been found to have a depression effect on flotation. For example, calcium lignosulfonate and sodium alginate are used in the scheelite flotation to selectively depress calcite (Chen et al., 2018a; Chen et al., 2018b). Locust bean gum, guar gum and sesbania gum are also used as depressants in the flotation of minerals such as chalcopyrite, pyrite and scheelite (Feng et al., 2018; Zhao et al., 2018; Zhao et al., 2017). Xanthan gum (XG), a kind of plant gum, has been studied for its depressive effect on the flotation of pyrophyllite (Zhao et al., 2019b). However, its depression is rarely studied in the flotation separation of scheelite from calcite, and the depression mechanism still needs to be improved. XG is a water-soluble polymer polysaccharide, and its structure is composed of repeating pentasaccharide units (Fig. 1). XG is white or beige powder (Kalogiannis et al., 2003; Krstonošić et al., 2019). Because of its good gel thickening, emulsifying, stability, wettability, biocompatibility, non-toxic, low price, and biodegradable, xanthan gum is widely used in oil extraction, food, medicine, cosmetics and other industries (Shekarforoush et al., 2018; Veiga-Santos et al., 2005). The molecular structure of XG contains large amounts of hydrophilic groups of hydroxyl groups. The pyruvic acid group and the acetyl group of the side chain can interact to generate hydrogen bonds, so that it has good water solubility and can be dissolved even in cold water (Chen et al., 2019; Muljana et al., 2018). At the same time, this may also be the reason why XG has a depressive effect (Song et al., 2013). There is also large amount of free carboxyl groups on the molecular chain of XG, which can chelate with some metal ions such as calcium ions. This may be why xanthan gum can be adsorbed on mineral surfaces. In this study, the selectively depressive effect of XG on calcite during scheelite flotation was studied. The flotation recoveries of the two minerals in the absence and presence of XG were studied by microflotation experiments. The depressive mechanism of XG was uncovered by adsorption experiments, fourier transform infrared spectroscopy (FTIR) analysis and X-ray photoelectron spectroscopy (XPS) analysis. AFM was used to visually observe the adsorption morphology of XG on mineral surfaces. The results have important implications for the flotation separation of scheelite from calcite and similar calciumcontaining gangue minerals.
2.1. Pure mineral samples and reagents The bulk scheelite and calcite samples used in the experiment were collected from Guangxi and Hunan Province, China. The block sample was crushed with a hammer and the portion with high purity was selected by hand. Then the small block samples of high purity were then ground by a porcelain mill with a certain proportion of ceramic balls. Different proportions of ceramic balls of different sizes were used to ensure that the minerals cannot be over milled. The fixed grinding time was 15 s each time. The grinding products were then sieved using a standard sieve of 74 μm. The +74 μm samples were continued to be milled, and the −74 μm samples were sieved using a 37 μm standard sieve. The samples of −74 + 37 μm was obtained, which were used for microflotation experiments, adsorption experiments, and XPS analysis experiment. Samples with a particle size of less than 37 μm were further ground to below 5 μm in an agate mortar for XRD analysis, FTIR analysis and chemical analysis. The XRD of scheelite and calcite are shown in Fig. 2. Chemical analysis results showed that the purity of scheelite and calcite reached 96% and 98.91%. The results of XRD and chemical analysis indicated that both the minerals have a very high purity. The preparation process of the experimental sample is shown in Fig. 3. The depressant XG and collector NaOL were of analytical grade, and the purity was ≥98% and ≥99%, respectively. The NaOL was purchased from BSQ Chemistry Technique, Shanghai, China. Depressant XG was purchased from SHENGSIDE Industrial Co., Ltd, Henan, China. During the experiment, the slurry pH was adjusted using dilute solution of hydrochloric acid (HCl) (2 mol/L) and sodium hydroxide (NaOH) (1 mol/L). The water used in all run was ultrapure water having a resistivity of more than 18.0 MΩ × cm. 2.2. Experiment methods 2.2.1. Microflotation The flotation recovery of the two minerals was investigated by microflotation experiments. The recovery of scheelite and calcite was studied separately. The experiments were conducted in an XFG flotation machine (Jilin Exploration Machinery Plant, Changchun, China). For pH experiment, the concentrations of XG and NaOL used in the experiment were 20 mg/L and 4.0 × 10−5 mol/L, respectively. During the experiment, the rotation speed was fixed at 1500 rpm. In each test, 2.0 g of mineral sample and 35 ml of ultrapure water were added in the plexiglass cell of the flotation. It was then stirred for 1 min to fully disperse the slurry. The pH of the slurry was then adjusted and recorded using HCl or NaOH solution. Next, the flotation reagents were sequentially added into the slurry. The reaction time of each flotation reagent in the slurry was 3 min. Finally, the flotation time was also 3 min, and the concentrates and tailings products were collected separately. Then they were weighed separately and the recovery was calculated. Microflotation experiments under the same conditions were repeated three times, and the average value was reported in the flotation results. The standard deviation for each test was calculated and presented as the error bars. The adding order of the flotation reagents is shown in Fig. 4, and the recovery formula is as follows:
ε=
m1 × 100% m1 + m2
where ε is the recovery; m1 and m2 represent the weight of the concentrates and tailings (g). 2.2.2. Adsorption experiments The residual concentration method was used to measure the adsorption density of the depressant on the mineral surface. For the sample preparation process of the adsorption experiments, 2 g of the mineral sample and 40 ml of ultrapure water were added to a 100 ml
Fig. 1. The molecular structure unit of XG (Zhao et al., 2019b). 15
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7000
4000
C
S
6000
C-Calcite
S-Scheelite
3000
I (count/s)
S
4000 3000
S
2000
S
1000
S S
0 10
20
1000
S S
30
S S
40
2000
S S S S
50
60
CC C C C
C
S SS
0
70
10
20
30
2θ (degree)
40
C C C
50
60
70
2θ (degree) Fig. 2. XRD of scheelite and calcite.
Standard Line Fitted Line
Organnic carbon (mg/L)
6
R2=0.9975 4
Equation Plot Weight Intercept Slope Residual Sum of Squares Pearson's r R-Square(COD) Adj. R-Square
2
0
0
10
20
y = a + b*x B No Weighting 0.10126 0.22032 0.06122 0.99902 0.99804 0.99754
30
XG concentration (mg/L) Fig. 5. The linear relationship between the concentration of XG and the TOC in the supernatant. 100
Fig. 3. The preparation process of the pure mineral samples. 80
Pure Mineral (2.0g)
Recovery (%)
I (counts/s)
5000
Agitating 1min Adjusting pH 2min XG 3min
60
Scheelite+NaOL Scheelite+XG+NaOL Calcite+NaOL Calcite+XG+NaOL
40
NaOL 3min Flotation 3min
0
Concentrate Products
NaOL dosage=4.0×10 -5 mol/L XG dosage=20 mg/L
20
7
8
9
10
pH
Tailings
Fig. 6. Effect of depressant XG on the recoveries of minerals as a function of pH.
Fig. 4. Flowsheet of microflotation experiment.
16
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100
100
(b)
(a) 80
60
Recovery (%)
Recovery (%)
80
NaOL dosage=4.0×10-5 mol/L pH=8.0
40
40
XG dosage=20 mg/L pH=8.0
20
20
0
Scheelite+XG+NaOL Calcite+XG+NaOL
60
Scheelite+XG+NaOL Calcite+XG+NaOL
0 0
4
8
12
16
20
4
8
12
16
20
-5
XG Concentration (mg/L)
NaOL Concentration (10 mol/L)
Fig. 7. Effect of XG (a) and NaOL (b) concentration on the recoveries of scheelite and calcite.
erlenmeyer flask, and then the pH of the slurry was adjusted and the depressant XG was added. The bottle mouth of the erlenmeyer flask was sealed with a plastic wrap. The erlenmeyer flask was then placed in a constant temperature shaking box (25 °C) for two hours to saturate the XG on the mineral surface. The slurry was centrifuged for 20 min at 2000 rpm. The amount of total organic carbon (TOC) in the supernatant was then tested using a TOC-LCPH Analyzer (Shimadzu, Kyoto, Japan). The amount of the XG in the supernatant was converted by the TOC according to the standard curve (Fig. 5). From Fig. 5, the concentration of XG and TOC are very linearly related, and the linear correlation coefficient is 0.9975. The amount adsorbed on the mineral surface can then be obtained by subtracting the amount of XG in the supernatant from the total amount of XG. According to the formula, the adsorption density of the XG on the mineral surface can be finally calculated. Adsorption experiments under the same conditions were repeated three times, and the final average value was given. The standard deviation for each test was calculated and presented as the error bars. The calculation formula of the adsorption density is as follows:
Adsorption density (mg/m2)
2.5
Scheelite Calcite
2.0
1.5
1.0
0.5
0.0
0
5
10
15
20
XG concentration (mg/L)
V (Co − C ) mA
Fig. 8. Adsorption density of XG on scheelite and calcite as a function of XG concentration.
where Γ is the adsorption density of XG (mg/m2), C0 and C are the total and supernatant concentration (mg/L); V is the slurry volume (L); m is the amount of sample (g); A is the specific surface area of the mineral powder (m2/g).
XG 2333.7
2.2.3. FTIR analysis This experiment was conducted using a Spectrum One FTIR in transmission mode. The instrument was manufactured by Shimadzu Corporation, Kyoto, Japan. The infrared spectrum of the reagent and mineral was measured by the method of tableting with potassium bromide (KBr). The preparation process of the mineral sample for FTIR analysis was as follows: 50 mg of the mineral sample (−2 μm) was dispersed in 100 ml of ultrapure water. Then the slurry pH was adjusted, and an appropriate dosage of flotation reagent was added. After stirring for 30 min on a magnetic stirrer, it was repeatedly washed with ultrapure water of the same pH and suction filtered through a microporous membrane. The samples dried at a constant temperature (35 °C) in a vacuum oven and used for detection. The wave number range was 4000–400 cm−1. The spectrum of potassium bromide was first measured. 100 mg of the KBr powder and 1.0 mg of the sample were uniformly tableted to measure the spectrum of the sample.
1707.8 1639.4 2932.9 1416.9
3358.1 400 0
350 0
1025.4 300 0
250 0
200 0
150 0
100 0
50 0
-1
Wavenumbers (cm ) Fig. 9. FTIR spectrum of XG.
2.2.4. XPS analysis The XPS experiment was carried out using ESCALAB 250Xi (Thermo 17
(a)
(b)
440.5
876.2
Calcite
Scheelite Scheelite+XG
711.3
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1431.7
Calcite+XG 2314.9
Scheelite+XG+NaOL
Calcite+XG+NaOL 2309.9 814.8
2950
1712.8
2850.3
2923.6 3000
1688.6
2900
2850
2800
1720
4000
3500
3000
1710.2 1692.6
2750
2500
2000
1500
1000
4000
500
3500
1700
1680
3000
1660
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Wavenumbers (cm-1)
Fig. 10. FTIR spectra of scheelite (a) and calcite (b) before and after the reaction of reagents.
further depressant concentration experiment. The effect of XG and NaOL concentration on the flotation recoveries of the minerals is shown in Fig. 7. As shown in Fig. 7(a), with the increase of XG concentration, the recovery of calcite decreased sharply, while the recovery of scheelite did not change much. When XG concentration was increased to 20 mg/L, calcite was depressed (1.79%) while scheelite still reported to the concentrate (81.32%). It was shown that the depressant XG was more selective of calcite over scheelite. As shown in Fig. 7(b), with the increase of NaOL concentration, the recovery of scheelite increased sharply at first, then increased slowly, and finally remained almost unchanged (over 83%), while the recovery of calcite increased slowly. When the NaOL concentration increased to 2 × 10−4 mol/L, calcite was still depressed (40.53%). When the concentration was 4 × 10−5 mol/L, the difference in recovery between scheelite and calcite was the largest.
Fisher, USA) with monochromatic Al Ka X-ray source at 150 W. The energy step size of survey spectra and high-resolution spectra were 1.00 eV and 0.05 eV, respectively. The sample used for XPS detection had the same particle size and reagent system as the microflotation (Section 2.2.1). The sample was filtered and washed repeatedly with ultrapure water for the purpose of washing away the weak physical adhesion of the reagent on the mineral surface. It was then dried in a vacuum oven and then used for testing. 2.2.5. AFM imaging The surface morphology of minerals before and after the reaction of XG was observed by atomic force microscopy. The instrument model is MultiMode 8. The probe material is Silicon Tip on Nitride Lever and the probe model is SCANASYST-AIR. It is difficult for a scheelite crystal to naturally dissociate into a flat surface (Gao et al., 2016). To get a flat surface, it needs to polish scheelite with a polishing machine (Tegramin-25, Struers). The samples were immersed in ultrapure water for several cleanings. The scheelite and calcite samples were then soaked in the XG solution for 3 min. It was then removed with tweezers and repeatedly rinsed with ultrapure water to remove the XG adhering to the mineral surface. Then the samples were blown dry with ultrapure nitrogen for testing. The measured AFM images were processed by software Nanoscope 7.3. For ease of comparison, the height ruler of the image was unified (from −10 nm to 10 nm).
3.2. Adsorption experiment results The adsorption density of the flotation reagent on the mineral surface directly affects the flotation behavior of the mineral. The adsorption density can be expressed by the adsorption amount of the flotation reagent on the unit area of the mineral. The adsorption density of the depressant XG on the two mineral surfaces as a function of XG concentration at a fixed pH 8 was shown in Fig. 8. It was shown that the adsorption density of XG on the surface of the two minerals increased with the initial concentration of XG. However, the increase of the two curves gradually slowed down. This indicated that as the concentration of XG increased, the adsorption density of XG on both minerals increased sharply at first, then increased slowly, and finally remained almost unchanged. In addition, the adsorption density of XG on the calcite surface was greater than that of the scheelite surface over the concentration range of the study. This indicated that the depressive effect of XG on calcite was more intense than scheelite. These are all consistent with the results of the microflotation experiment.
3. Results and discussion 3.1. Microflotation results The flotation results of the two minerals under the depressant XG and the collector NaOL system are shown in Fig. 6. When the depressant XG was not added, it was shown that both two minerals maintained excellent recovery (over 80%). The recovery of calcite (over 90%) was even better than that of scheelite (about 80%), and they all remained a recovery of over 80% throughout the pH range of the study. It was difficult to separate the two minerals by flotation without the addition of a depressant. After the addition of depressant XG, the recovery of calcite is drastically reduced in the pH range of 6.5–8.5 (from about 90% to less than 10%). However, the addition of depressant XG almost did not impair the recovery of the scheelite. About 80% of the scheelite can still be floated in the pH range of 7.5–9.5 in the presence of depressant XG. The large difference in recovery of the two minerals in the presence of the depressant XG provided excellent conditions for flotation separation between each other. A fixed pH of 8.0 was selected for
3.3. FTIR analysis FTIR analysis was used to explain the adsorption mechanism of flotation reagents on mineral surfaces. In our previous studies, we found that NaOL had significant characteristic peaks at 2921.6 cm−1, 2851.7 cm−1, 1447.4 cm−1 and 1559.3 cm−1, respectively (Dong et al., 2019). The first two peaks were caused by the stretching vibration of the eCH2- and eCH3 groups. The latter two peaks were attributed to the symmetrical and asymmetric vibrations of the carboxyl groups. The 18
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Experimental line Fitting Line Background Hydrocarbons Carbon oxides CO32- of calcite C-OH O-C-O/O=C-O
(a)
289.36
286.85
286.24
284.77
285.41
531.35 531.14
Calcite+XG
532.74
Counts/s
Experimental line Fitting Line Background Hydrocarbons Carbon oxides CO32- of calcite
(b)
531.79
Calcite+XG
289.29
284.79
Experimental line Fitting line Background Oxygen in CO32Carbon oxides
531.14
531.79
Calcite
294
285.40
292
290
288
Calcite
286
284
282
536
534
532
Binding energy (eV)
530
Binding energy (eV)
Experimental line Fitting line Background Ca 2p 3/2 in CaCO3 Ca 2p 3/2 in Ca-COOR Ca 2p 1/2 in CaCO3 Ca 2p 1/2 in Ca-COOR
(C)
Calcite+XG 346.89
346.69
350.43 350.22
Counts/s
Counts/s
Experimental line Fitting line Background Oxygen in CO32Carbon oxides -COO-OH/-COOCH3
Experimental line Fitting line Background Ca 2p 3/2 in CaCO3 Ca 2p 1/2 in CaCO3
346.89
Calcite
350.43
354
352
350
348
346
344
Binding energy (eV) Fig. 11. Fitting peaks of C 1s (a), O 1s (b) and Ca 2p (c) on calcite surface before and after adding XG.
19
528
526
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Fig. 12. AFM images (left-height; right-3D) of scheelite in the absence (a) and presence (b) of XG.
dramatic change in the adsorption peak of the carboxyl group (from 1688.6 cm−1 to 1639.4 cm−1) on the surface of calcite indicated that the carboxyl group (eCOO−) had chemical chelation with the calcium ion species on calcite surface. In the presence of depressant XG and collector NaOL, the scheelite surface had new absorption peaks at 2923.6 cm−1 and 2850.3 cm−1. The two peaks were shifted from the adsorption peaks of methylene and methyl groups in NaOL at 2921.6 cm−1 and 2851.7 cm−1. This indicated that NaOL was still chemisorbed on scheelite surface in the presence of depressant XG. However, in addition to the characteristic peaks of XG at the 1710.2 cm−1 and 1692.6 cm−1 on calcite surface, no other absorption peaks appeared. This indicated that the pre-adsorption of XG hindered the adsorption of NaOL on the calcite surface. FTIR analysis results further confirmed the selective depressive effect of XG on calcite.
infrared spectrum of XG was shown in Fig. 9. It was shown that the characteristic peak at 3358.1 cm−1 was assigned to the stretching vibration of eOH (Inamuddin, 2019). The peaks at 2932.9 cm−1 and 2333.7 cm−1 were contributed by the stretching vibration of eCH2- and eCH (Inamuddin, 2019; Yang et al., 2018). The peak at 1707.8 cm−1 was due to stretching vibrations of carbonyl (C]O) of the acetyl groups. The characteristic peaks at 1639.4 cm−1 and 1416.9 cm−1 were all attributed to the stretching vibration of carboxyl (De Falco et al., 2018; Kang et al., 2019; Zheng et al., 2019). The absorption band at 1025.4 cm−1 was assigned to the stretching vibration CeOeC groups (Inamuddin, 2019; Kang et al., 2019). The changes in the infrared spectra of the minerals before and after the reaction of the flotation reagent are shown in Fig. 10. As depicted in Fig. 10, bare scheelite had two significant characteristic peaks at 814.8 cm−1 and 440.5 cm−1, which belonging to the stretching vibration and bending vibration peaks of WeO (Abdalla et al., 2018). The stretching vibration peak of CeO in calcite appeared at 1431.7 cm−1, and the bands at 876.2 cm−1 and 711.3 cm−1 were contributed by the deformation vibration of CeO in calcite (Azizi and Larachi, 2017). After the interaction of the depressant XG and the minerals, no significant new absorption peak appeared on scheelite surface. However, two significant new absorption peaks at 1712.8 cm−1 and 1688.6 cm−1 appeared on the calcite surface, which were shifted from the characteristic peaks of the acetyl groups and carboxyl group in XG at 1707.8 cm−1 and 1639.4 cm−1, respectively. This indicated that the adsorption of the depressant XG on scheelite surface was very weak, whereas XG had a strong chemisorption on calcite surface. The
3.4. Surface analysis XPS analysis further revealed the adsorption mechanism of depressant XG on calcite. It mainly explored the changes in the relevant chemical environment of specific elements on the surface of calcite, and determining the functional groups that acted of the depressant. The high-resolution spectra of C1s, O 1s and Ca 2p of calcite are shown in Fig. 11. At the same time, the spectra of C1s, O 1s and Ca 2p of calcite before and after the reaction of XG were separated and fitted. As shown in Fig. 11(a), bare calcite not only contained separated peak of its own CO32− at 289.29 eV, but also contained separated peaks at 284.79 eV and 285.40 eV (Jiao et al., 2019). The latter two peaks were due to the 20
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Fig. 13. AFM images (left-height; right-3D) of calcite in the absence (a) and presence (b) of XG.
3.5. AFM images of XG on mineral surfaces
hydrocarbons and carbon oxides contaminants on calcite surface (Deng et al., 2017; Deng et al., 2018). After the reaction of calcite and XG, in addition to the separated peaks of CO32−, hydrocarbons and carbon oxides at 289.36 eV, 284.77 eV and 285.41 eV, two new separated peaks can be observed at 286.24 eV and 286.85 eV. The two peaks were assigned to CeOH and OeCeO/O]CeO in XG molecule, respectively (Jin et al., 2016). This indicated that XG had a strong adsorption on calcite surface. As shown in Fig. 11(b), the high-resolution spectra of O 1s of bare calcite had two separated peaks at 531.14 eV and 531.79 eV (Jiao et al., 2019). The two peaks were derived from the CO32− of calcite and the contamination of carbon oxides (Deng et al., 2018). In the presence of XG, two new separated peaks can be observed at 531.35 eV and 532.74 eV. The two peaks were attributed to eCOO− and eOH/eCOOCH3 of XG, respectively. This indicated that the chemical environment of oxygen on calcite surface had changed greatly. It was further proved that the chemical adsorption of XG occurred on calcite surface. As can be seen from Fig. 11(c), the spectra of Ca 2p of bare calcite had double peaks at 346.89 eV and 350.43 eV, which were attributed to the Ca 2p 3/2 and Ca 2p 1/2 in CaCO3 itself (Liu et al., 2017b). After the interaction of calcite and XG, two separated double peaks can be observed. In addition to the double peaks of CaCO3 at 346.89 eV and 350.43 eV, the other double peaks were observed at 346.69 eV and 350.22 eV. The double peaks were attributed to Ca 2p 3/ 2 and Ca 2p 1/2 in Ca-COOR (Liu et al., 2017a). This indicated XG had strong adsorption on calcite surface. It was once again demonstrated that the carboxyl groups (eCOO−) of the XG molecule and the calcium ions species on calcite surface were chelated. XG was mainly adsorbed on calcite surface by chemical chelation, thereby inhibiting calcite.
The surface morphology of scheelite before (a) and after (b) the reaction of depressant XG is shown in Fig. 12. The scan range was 3.0 μm × 3.0 μm. The left graph represented a height image, and the height ruler ranged from −10 nm to 10 nm, meaning that different colors represent different heights. The graph on the right was a 3D image that visually reflected the surface morphology of the mineral. The scheelite exhibited a relatively flat surface after polishing (Fig. 12(a)). The gully left by polishing can be clearly seen. A point of bulging on the image may be an inevitable, slight contamination during the sample preparation process. After the reaction of scheelite and depressant XG (Fig. 12(b)), XG exhibited sparse and irregular point adsorption on the surface of scheelite. It can be seen from the 3D image that XG was very sparsely adsorbed on the surface of scheelite. This indicated that the adsorption of the depressant XG on scheelite surface was relatively weak. Fig. 13 shows the surface morphology of calcite before (a) and after (b) the reaction of depressant XG. The scan range was also 3.0 μm × 3.0 μm. From Fig. 13(a), both the height image (left) and the 3D image (right) indicated that the naturally dissociated calcite surface has a flat surface and the sample preparation process was not contaminated. The height ruler was the same as the scheelite images (from −10 nm to 10 nm). After the reaction of calcite and depressant XG (Fig. 13(b)), it is shown that XG exhibited a dense agglomerate adsorption on the surface of calcite. The adsorption morphology of XG even became reticulate adsorption. The adsorption density and adsorption height of XG on the surface of calcite were significantly 21
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increased compared with scheelite surface. This indicated that the adsorption capacity of XG on the surface of calcite was more intense than that of scheelite.
using acidified sodium silicate as depressant. Miner. Eng. 80, 45–49. Castanho, J.M., Vieira, M.T., 1998. The influence of the interstitial element on tribological behaviour of tungsten coatings. Surf. Coat. Technol. 102, 50–62. Chen, C., Wu, L., Perdjon, M., Huang, X., Peng, Y., 2019. The drying effect on xanthan gum biopolymer treated sandy soil shear strength. Constr. Build. Mater. 197, 271–279. Chen, W., Feng, Q., Zhang, G., Liu, D., Li, L., 2018a. Selective flotation of scheelite from calcite using calcium lignosulphonate as depressant. Miner. Eng. 119, 73–75. Chen, W., Feng, Q., Zhang, G., Yang, Q., 2018b. Investigations on flotation separation of scheelite from calcite by using a novel depressant: sodium phytate. Miner. Eng. 126, 116–122. De Falco, F., Gentile, G., Avolio, R., Errico, M.E., Di Pace, E., Ambrogi, V., Avella, M., Cocca, M., 2018. Pectin based finishing to mitigate the impact of microplastics released by polyamide fabrics. Carbohydr. Polym. 198, 175–180. Deng, R., Hu, Y., Ku, J., Zuo, W., Yang, Z., 2017. Adsorption of Fe(III) on smithsonite surfaces and implications for flotation. Colloids Surf., A 533, 308–315. Deng, R., Yang, X., Hu, Y., Ku, J., Zuo, W., Ma, Y., 2018. Effect of Fe(II) as assistant depressant on flotation separation of scheelite from calcite. Miner. Eng. 118, 133–140. Dong, L., Jiao, F., Qin, W., Zhu, H., Jia, W., 2019. Selective depressive effect of sodium fluorosilicate on calcite during scheelite flotation. Miner. Eng. 131, 262–271. Emin, S., Altinkaya, C., Semerci, A., Okuyucu, H., Yildiz, A., Stefanov, P., 2018. Tungsten carbide electrocatalysts prepared from metallic tungsten nanoparticles for efficient hydrogen evolution. Appl. Catal. B 236, 147–153. Feng, B., Peng, J., Zhang, W., Ning, X., Guo, Y., Zhang, W., 2018. Use of locust bean gum in flotation separation of chalcopyrite and talc. Miner. Eng. 122, 79–83. Filippov, L.O., Foucaud, Y., Filippova, I.V., Badawi, M., 2018. New reagent formulations for selective flotation of scheelite from a skarn ore with complex calcium minerals gangue. Miner. Eng. 123, 85–94. Gao, Y., Gao, Z., Sun, W., Yin, Z., Wang, J., Hu, Y., 2017. Adsorption of a novel reagent scheme on scheelite and calcite causing an effective flotation separation. J. Colloid Interface Sci. 512, 39–46. Gao, Z., Fan, R., Ralston, J., Sun, W., Hu, Y., 2019. Surface broken bonds: an efficient way to assess the surface behaviour of fluorite. Miner. Eng. 130, 15–23. Gao, Z., Hu, Y., Sun, W., Drelich, J.W., 2016. Surface-charge anisotropy of scheelite crystals. Langmuir 32, 6282–6288. Gong, D., Zhou, K., Peng, C., Li, J., Chen, W., 2019. Sequential extraction of tungsten from scheelite through roasting and alkaline leaching. Miner. Eng. 132, 238–244. Guo, W., Feng, B., Zhong, Z., Weng, C., Luo, X., 2017. Research Progress on the Depressants for Flotation Separation of Scheelite and Calcium-bearing Gangue. 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Kalogiannis, S., Iakovidou, G., Liakopoulou-Kyriakides, M., Kyriakidis, D.A., Skaracis, G.N., 2003. Optimization of xanthan gum production by Xanthomonas campestris grown in molasses. Process Biochem. 39, 249–256. Kang, M., Oderinde, O., Liu, S., Huang, Q., Ma, W., Yao, F., Fu, G., 2019. Characterization of Xanthan gum-based hydrogel with Fe3+ ions coordination and its reversible solgel conversion. Carbohydr. Polym. 203, 139–147. Kim, G.-S., Lee, Y.J., Kim, D.-G., Oh, S.-T., Kim, D.-S., Kim, Y.D., 2006. The behavior of tungsten oxides in the presence of copper during hydrogen reduction. J. Alloys Compd. 419, 262–266. Kotlov, A., Jönsson, L., Kraus, H., Mikhailik, V., Nagirnyi, V., Svensson, G., Zadneprovski, B.I., 2007. Excited states of molybdenum oxyanion in scheelite and wolframite structures. Radiat. Meas. 42, 767–770. Krstonošić, V., Milanović, M., Dokić, L., 2019. Application of different techniques in the determination of xanthan gum-SDS and xanthan gum-Tween 80 interaction. Food Hydrocolloids 87, 108–118. Kumar, A., Singh, K., Pandey, O.P., 2011. Direct conversion of wolframite ore to tungsten carbide nano particles. Int. J. Refract Metal Hard Mater. 29, 555–558. Li, C., Gao, Z., 2018. Tune surface physicochemical property of fluorite particles by regulating the exposure degree of crystal surfaces. Miner. Eng. 128, 123–132. Liu, C., Zhang, W., Song, S., Li, H., 2019a. A novel method to improve carboxymethyl cellulose performance in the flotation of talc. Miner. Eng. 131, 23–27. Liu, C., Zhu, G., Song, S., Li, H., 2019b. Flotation separation of smithsonite from quartz using calcium lignosulphonate as a depressant and sodium oleate as a collector. Miner. Eng. 131, 385–391. Liu, X., Li, C., Luo, H., Cheng, R., Liu, F., 2017a. Selective reverse flotation of apatite from dolomite in collophanite ore using saponified gutter oil fatty acid as a collector. Int. J. Miner. Process. 165, 20–27. Liu, X., Luo, H., Cheng, R., Li, C., Zhang, J., 2017b. Effect of citric acid and flotation performance of combined depressant on collophanite ore. Miner. Eng. 109, 162–168. Meng, Q., Feng, Q., Shi, Q., Ou, L., 2015. Studies on interaction mechanism of fine wolframite with octyl hydroxamic acid. Miner. Eng. 79, 133–138. Muljana, H., Kleopas Sugih, A., Prima Kristijarti, A., Karlus, R., Kurnia, R., Evan, C., Picchioni, F., 2018. Acetylation of xanthan gum in densified carbon dioxide (CO2).
3.6. Depression mechanism of XG Because of the differences in the crystal structure and the solution chemical behavior of the two minerals, the dominant ionic components of the mineral surface in the slurry are also different (Dong et al., 2019). For scheelite, anionic tungstate ions (WO42−) are the dominant component. However, calcite surface is mainly determined by cationic calcium ions (Ca2+). The XG molecular contains a large amount of free carboxyl groups (eCOO−). These carboxyl groups (eCOO−) can be chemically chelated with calcium ion species on the mineral surface. At the same time, the anions WO42− or CO32− of the mineral surface can hinder the occurrence of chelation by electrostatic repulsion or steric hindrance (Chen et al., 2018b). Therefore, XG has more chelation with calcium ions on calcite surface than on scheelite surface. Moreover, the chains of XG macromolecules produce hydrophobic associations within and between molecules, which also increases the adsorption of XG on the mineral surface. In addition, hydrogen bonds effect also play an important role in the adsorption process due to the presence of hydroxyl groups and carboxyl groups in the XG molecule. More depressant XG is adsorbed on calcite surface, which hinders further adsorption of the collector NaOL. Therefore, the floatability of calcite deteriorates severely. The addition of XG selectively inhibits the flotation of calcite. 4. Conclusions XG has excellent selective inhibition effect on calcite during scheelite flotation. When the dosages of depressant XG and the collector NaOL are 20 mg/L and 4.0 × 10−5 mol/L, the recovery of calcite is drastically reduced to only 1.79%, while scheelite still has a very high flotation recovery of 81.32% at pH 8. The adsorption density of the depressant XG on the calcite surface is much larger than that of the scheelite surface. The depressant XG selectively chemisorbs on the surface of calcite, while NaOL can still chemisorb on the surface of scheelite with the pre-addition of depressant XG. The free carboxyl groups in the XG molecule can be more chemically chelated with calcium ion species on calcite surface. It was more directly observed by AFM that the adsorption of XG on the calcite surface was more dense than that of scheelite surface. These all prove that the calcite surface adsorbs more depressant XG than that of scheelite, which hinders the further adsorption of the collector NaOL. As a result, this creates a huge difference in the floatability of the two minerals, which allows flotation separation between the two minerals. 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); and the Key laboratory of Hunan Province for Clean and Efficiency Utilization of strategic Calcium-containing mineral Resources (No. 2018TP1002). References Abdalla, M.A.M., Peng, H., Younus, H.A., Wu, D., Abusin, L., Shao, H., 2018. Effect of synthesized mustard soap on the scheelite surface during flotation. Colloids Surf., A 548. Azizi, D., Larachi, F., 2017. Surface interactions and flotation behavior of calcite, dolomite and ankerite with alkyl hydroxamic acid bearing collector and sodium silicate. Colloids Surf., A 537. Bo, F., Luo, X., Wang, J., Wang, P., 2015. The flotation separation of scheelite from calcite
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mixed collectors on oxide and silicate minerals. Adv. Colloid Interface Sci. 250, 1–14. Yang, X., Nisar, T., Hou, Y., Gou, X., Sun, L., Guo, Y., 2018. Pomegranate peel pectin can be used as an effective emulsifier. Food Hydrocolloids. Yin, W.-Z., Wang, J.-Z., 2014. Effects of particle size and particle interactions on scheelite flotation. Trans. Nonferr. Metals Soc. China 24, 3682–3687. Zhang, C., Wei, S., Hu, Y., Tang, H., Gao, J., Yin, Z., Guan, Q., 2017. Selective adsorption of tannic acid on calcite and implications for separation of fluorite minerals. J. Colloid Interface Sci. 512, 55–63. Zhao, C., Sun, C., Yin, W., Luo, B., 2019a. An investigation of the mechanism of using iron chelate as a collector during scheelite flotation. Miner. Eng. 131, 146–153. Zhao, K., Gu, G., Wang, X., Yan, W., Hu, Y.H., 2018. The effect of depressant sesbania gum on the flotation of a talc-containing scheelite ore. J. Mater. Res. Technol. Zhao, K., Wang, X., Yan, W., Gu, G., Wang, C., Wang, Z., Xu, L., Peng, T., 2019b. Depression mechanism of pyrophyllite by a novel polysaccharide xanthan gum. Miner. Eng. 132, 134–141. Zhao, K., Yan, W., Wang, X., Hui, B., Gu, G., Wang, H., 2017. The flotation separation of pyrite from pyrophyllite using oxidized guar gum as depressant. Int. J. Miner. Process. 161, 78–82. Zheng, M., Lian, F., Xiong, Y., Liu, B., Zhu, Y., Miao, S., Zhang, L., Zheng, B., 2019. The synthesis and characterization of a xanthan gum-acrylamide-trimethylolpropane triglycidyl ether hydrogel. Food Chem. 272, 574–579.
Mater. Today: Proc. 5, 21551–21558. Shekarforoush, E., Ajalloueian, F., Zeng, G., Mendes, A.C., Chronakis, I.S., 2018. Electrospun xanthan gum-chitosan nanofibers as delivery carrier of hydrophobic bioactives. Mater. Lett. 228, 322–326. Singh, H., Pandey, O.P., 2015. Novel process for synthesis of nanocrystalline WC from wolframite ore. Ceram. Int. 41, 10481–10487. Song, Z.G., Ling, P.X., Zhang, T.M., 2013. Production and development of xanthan gum and its research progress in medical field. Pharm. Biotechnol. Veiga-Santos, P., Oliveira, L.M., Cereda, M.P., Alves, A.J., Scamparini, A.R.P., 2005. Mechanical properties, hydrophilicity and water activity of starch-gum films: effect of additives and deacetylated xanthan gum. Food Hydrocolloids 19, 341–349. Wang, R., Wei, Z., Han, H., Sun, W., Hu, Y., Wang, J., Wang, L., Liu, H., Yang, Y., Zhang, C., He, J., 2019. Fluorite particles as a novel calcite recovery depressant in scheelite flotation using Pb-BHA complexes as collectors. Miner. Eng. 132, 84–91. Wills, B.A., Finch, J.A., 2016. Chapter 12 – Froth flotation. In: Wills, B.A., Finch, J.A. (Eds.), Wills' Mineral Processing Technology, eighth ed. Butterworth-Heinemann, Boston, pp. 265–380. Xu, G.L., Guterl, J., Abrams, T., Wang, H.Q., Zhang, P.F., Elder, J.D., Unterberg, E.A., Thomas, D.M., Guo, H.Y., Ye, M.Y., 2019. Study of DIII-D tungsten erosion processes by using a carbon–tungsten mixed material model. Nuclear Mater. Energy 18, 141–146. Xu, L., Tian, J., Wu, H., Lu, Z., Sun, W., Hu, Y., 2017. The flotation and adsorption of
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Selective flotation of scheelite from calcite using xanthan gum as depressant ⁎
⁎
T
Liuyang Dong, Fen Jiao , Wenqing Qin , Wei Liu 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
A R T I C LE I N FO
A B S T R A C T
Keywords: Scheelite Calcite Flotation XG Adsorption
Xanthan gum (XG), a clean, non-toxic macromolecular polysaccharide, was studied as a depressant in the flotation separation of scheelite from calcite. Its selective inhibition mechanism was investigated by adsorption experiments, fourier transform infrared spectroscopy (FTIR) analysis and X-ray photoelectron spectroscopy (XPS) analysis. It was found that the XG exhibited a strong selective depressive effect on calcite. However, XG had minimal effect on the flotation recovery of scheelite. Adsorption experiments indicated that the adsorption density of XG on the surface of calcite was much higher than that of scheelite. FTIR results indicated that XG was selectively chemisorbed on calcite surface, while sodium oleate (NaOL) can still be chemisorbed on scheelite surface in the presence of depressant XG. XPS results indicated that the carboxyl groups (eCOO−) of the XG can be chemically chelated with the calcium ions species on calcite surface. It was intuitively observed by atomic force microscopy (AFM) that the coverage and adsorption density of XG on calcite surface were higher than that of the scheelite surface. As such, the calcite surface adsorbed more depressant XG than that of scheelite, which hindered the further adsorption of the collector NaOL. This greatly deteriorated the flotation of calcite and achieved flotation separation between the two minerals.
1. Introduction Tungsten is an important non-ferrous metal with excellent melting point, stable chemical properties and high hardness (Gong et al., 2019; Kim et al., 2006). It is often used to make metal alloys, optical devices and superhard abrasives, and it has a wide range of uses in metallurgy, chemical, electronics, materials, machinery and many other fields. China is the world's leading tungsten producer, consumer and supply country (Castanho and Vieira, 1998; Emin et al., 2018; Xu et al., 2019). Wolframite [(Fe, Mn) WO4] and scheelite (CaWO4) are the two most important sources of tungsten-bearing minerals (Kotlov et al., 2007; Meng et al., 2015). The wolframite deposit has the advantages of good quality, high grade, easy mining, easy selection, convenient subsequent treatment and less environmental hazard. For wolframite deposit with higher grade, it can be recovered by simple and pollution-free gravity selective method (Harlaux et al., 2018; Singh and Pandey, 2015). After extensive mining, this wolframite deposit has been exploited and grade is decreasing. Deposits available for development are shrinking (Gong et al., 2019; Kumar et al., 2011). Therefore, the development and utilization of scheelite and mixed tungsten ore resources have received more attention. The scheelite deposits are complex in type and often coexist with other gangue minerals, such as various sulfide ore and ⁎
calcium-containing oxidized minerals (Li and Gao, 2018; Wang et al., 2019; Zhao et al., 2019a). Calcite (CaCO3), a mineral that often coexists with scheelite, is a low-value mineral that often needs to be removed (Hu et al., 2011). The ideal dissociation particle size of scheelite is mostly in the flotation particle size range. Therefore, in the process of separating scheelite from gangue minerals, flotation is the most commonly used method (Filippov et al., 2018). Flotation is a physico-chemical separation process that utilizes the difference in surface properties of the valuable minerals and the unwanted gangue minerals (Wills and Finch, 2016). Mineral surface wettability, surface lattice ion dissolution, surface electrical properties and solution chemical properties have an important impact on the flotation process (Gao et al., 2019; Liu et al., 2019b). Flotation reagents (such as surfactants) are often added during the flotation process to alter these properties and thereby alter the flotation performance of the mineral (Liu et al., 2019a; Xu et al., 2017). Flotation depressant is an important and widely used flotation reagent that reduces the flotation recovery of some minerals, and it inhibits different minerals to varying degrees to achieve flotation separation between minerals. Traditional scheelite flotation depressants are mainly inorganic depressants, mainly including sodium silicate (water glass), phosphates, fluorosilicates, etc (Bo et al., 2015; Dong et al., 2019; Gao et al., 2017; Yin and Wang,
Corresponding authors at: School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China. E-mail addresses: [email protected] (F. Jiao), [email protected] (W. Qin).
https://doi.org/10.1016/j.mineng.2019.04.030 Received 6 January 2019; Received in revised form 15 April 2019; Accepted 22 April 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.
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2. Materials and methods
2014). However, these depressants also have some shortcomings, such as large dosage, easy to be affected by pH, and narrow flotation range. The combination of metal ions and depressant to improve the depressive power and selectivity of the depressant has also been studied (Deng et al., 2018). In addition, organic depressants have also been widely used. Organic depressants mainly include small molecule organic depressant such as citric acid, tartaric acid, tannic acid, salicylic acid, lactic acid, etc (Guo et al., 2017; Zhang et al., 2017). However, the depressive power and selectively of small molecule organic depressants is usually weak, and it is often necessary to use them in combination to achieve the depressive effect. In recent years, macromolecular polymers have been found to have a depression effect on flotation. For example, calcium lignosulfonate and sodium alginate are used in the scheelite flotation to selectively depress calcite (Chen et al., 2018a; Chen et al., 2018b). Locust bean gum, guar gum and sesbania gum are also used as depressants in the flotation of minerals such as chalcopyrite, pyrite and scheelite (Feng et al., 2018; Zhao et al., 2018; Zhao et al., 2017). Xanthan gum (XG), a kind of plant gum, has been studied for its depressive effect on the flotation of pyrophyllite (Zhao et al., 2019b). However, its depression is rarely studied in the flotation separation of scheelite from calcite, and the depression mechanism still needs to be improved. XG is a water-soluble polymer polysaccharide, and its structure is composed of repeating pentasaccharide units (Fig. 1). XG is white or beige powder (Kalogiannis et al., 2003; Krstonošić et al., 2019). Because of its good gel thickening, emulsifying, stability, wettability, biocompatibility, non-toxic, low price, and biodegradable, xanthan gum is widely used in oil extraction, food, medicine, cosmetics and other industries (Shekarforoush et al., 2018; Veiga-Santos et al., 2005). The molecular structure of XG contains large amounts of hydrophilic groups of hydroxyl groups. The pyruvic acid group and the acetyl group of the side chain can interact to generate hydrogen bonds, so that it has good water solubility and can be dissolved even in cold water (Chen et al., 2019; Muljana et al., 2018). At the same time, this may also be the reason why XG has a depressive effect (Song et al., 2013). There is also large amount of free carboxyl groups on the molecular chain of XG, which can chelate with some metal ions such as calcium ions. This may be why xanthan gum can be adsorbed on mineral surfaces. In this study, the selectively depressive effect of XG on calcite during scheelite flotation was studied. The flotation recoveries of the two minerals in the absence and presence of XG were studied by microflotation experiments. The depressive mechanism of XG was uncovered by adsorption experiments, fourier transform infrared spectroscopy (FTIR) analysis and X-ray photoelectron spectroscopy (XPS) analysis. AFM was used to visually observe the adsorption morphology of XG on mineral surfaces. The results have important implications for the flotation separation of scheelite from calcite and similar calciumcontaining gangue minerals.
2.1. Pure mineral samples and reagents The bulk scheelite and calcite samples used in the experiment were collected from Guangxi and Hunan Province, China. The block sample was crushed with a hammer and the portion with high purity was selected by hand. Then the small block samples of high purity were then ground by a porcelain mill with a certain proportion of ceramic balls. Different proportions of ceramic balls of different sizes were used to ensure that the minerals cannot be over milled. The fixed grinding time was 15 s each time. The grinding products were then sieved using a standard sieve of 74 μm. The +74 μm samples were continued to be milled, and the −74 μm samples were sieved using a 37 μm standard sieve. The samples of −74 + 37 μm was obtained, which were used for microflotation experiments, adsorption experiments, and XPS analysis experiment. Samples with a particle size of less than 37 μm were further ground to below 5 μm in an agate mortar for XRD analysis, FTIR analysis and chemical analysis. The XRD of scheelite and calcite are shown in Fig. 2. Chemical analysis results showed that the purity of scheelite and calcite reached 96% and 98.91%. The results of XRD and chemical analysis indicated that both the minerals have a very high purity. The preparation process of the experimental sample is shown in Fig. 3. The depressant XG and collector NaOL were of analytical grade, and the purity was ≥98% and ≥99%, respectively. The NaOL was purchased from BSQ Chemistry Technique, Shanghai, China. Depressant XG was purchased from SHENGSIDE Industrial Co., Ltd, Henan, China. During the experiment, the slurry pH was adjusted using dilute solution of hydrochloric acid (HCl) (2 mol/L) and sodium hydroxide (NaOH) (1 mol/L). The water used in all run was ultrapure water having a resistivity of more than 18.0 MΩ × cm. 2.2. Experiment methods 2.2.1. Microflotation The flotation recovery of the two minerals was investigated by microflotation experiments. The recovery of scheelite and calcite was studied separately. The experiments were conducted in an XFG flotation machine (Jilin Exploration Machinery Plant, Changchun, China). For pH experiment, the concentrations of XG and NaOL used in the experiment were 20 mg/L and 4.0 × 10−5 mol/L, respectively. During the experiment, the rotation speed was fixed at 1500 rpm. In each test, 2.0 g of mineral sample and 35 ml of ultrapure water were added in the plexiglass cell of the flotation. It was then stirred for 1 min to fully disperse the slurry. The pH of the slurry was then adjusted and recorded using HCl or NaOH solution. Next, the flotation reagents were sequentially added into the slurry. The reaction time of each flotation reagent in the slurry was 3 min. Finally, the flotation time was also 3 min, and the concentrates and tailings products were collected separately. Then they were weighed separately and the recovery was calculated. Microflotation experiments under the same conditions were repeated three times, and the average value was reported in the flotation results. The standard deviation for each test was calculated and presented as the error bars. The adding order of the flotation reagents is shown in Fig. 4, and the recovery formula is as follows:
ε=
m1 × 100% m1 + m2
where ε is the recovery; m1 and m2 represent the weight of the concentrates and tailings (g). 2.2.2. Adsorption experiments The residual concentration method was used to measure the adsorption density of the depressant on the mineral surface. For the sample preparation process of the adsorption experiments, 2 g of the mineral sample and 40 ml of ultrapure water were added to a 100 ml
Fig. 1. The molecular structure unit of XG (Zhao et al., 2019b). 15
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7000
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2θ (degree) Fig. 2. XRD of scheelite and calcite.
Standard Line Fitted Line
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Equation Plot Weight Intercept Slope Residual Sum of Squares Pearson's r R-Square(COD) Adj. R-Square
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10
20
y = a + b*x B No Weighting 0.10126 0.22032 0.06122 0.99902 0.99804 0.99754
30
XG concentration (mg/L) Fig. 5. The linear relationship between the concentration of XG and the TOC in the supernatant. 100
Fig. 3. The preparation process of the pure mineral samples. 80
Pure Mineral (2.0g)
Recovery (%)
I (counts/s)
5000
Agitating 1min Adjusting pH 2min XG 3min
60
Scheelite+NaOL Scheelite+XG+NaOL Calcite+NaOL Calcite+XG+NaOL
40
NaOL 3min Flotation 3min
0
Concentrate Products
NaOL dosage=4.0×10 -5 mol/L XG dosage=20 mg/L
20
7
8
9
10
pH
Tailings
Fig. 6. Effect of depressant XG on the recoveries of minerals as a function of pH.
Fig. 4. Flowsheet of microflotation experiment.
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100
100
(b)
(a) 80
60
Recovery (%)
Recovery (%)
80
NaOL dosage=4.0×10-5 mol/L pH=8.0
40
40
XG dosage=20 mg/L pH=8.0
20
20
0
Scheelite+XG+NaOL Calcite+XG+NaOL
60
Scheelite+XG+NaOL Calcite+XG+NaOL
0 0
4
8
12
16
20
4
8
12
16
20
-5
XG Concentration (mg/L)
NaOL Concentration (10 mol/L)
Fig. 7. Effect of XG (a) and NaOL (b) concentration on the recoveries of scheelite and calcite.
erlenmeyer flask, and then the pH of the slurry was adjusted and the depressant XG was added. The bottle mouth of the erlenmeyer flask was sealed with a plastic wrap. The erlenmeyer flask was then placed in a constant temperature shaking box (25 °C) for two hours to saturate the XG on the mineral surface. The slurry was centrifuged for 20 min at 2000 rpm. The amount of total organic carbon (TOC) in the supernatant was then tested using a TOC-LCPH Analyzer (Shimadzu, Kyoto, Japan). The amount of the XG in the supernatant was converted by the TOC according to the standard curve (Fig. 5). From Fig. 5, the concentration of XG and TOC are very linearly related, and the linear correlation coefficient is 0.9975. The amount adsorbed on the mineral surface can then be obtained by subtracting the amount of XG in the supernatant from the total amount of XG. According to the formula, the adsorption density of the XG on the mineral surface can be finally calculated. Adsorption experiments under the same conditions were repeated three times, and the final average value was given. The standard deviation for each test was calculated and presented as the error bars. The calculation formula of the adsorption density is as follows:
Adsorption density (mg/m2)
2.5
Scheelite Calcite
2.0
1.5
1.0
0.5
0.0
0
5
10
15
20
XG concentration (mg/L)
V (Co − C ) mA
Fig. 8. Adsorption density of XG on scheelite and calcite as a function of XG concentration.
where Γ is the adsorption density of XG (mg/m2), C0 and C are the total and supernatant concentration (mg/L); V is the slurry volume (L); m is the amount of sample (g); A is the specific surface area of the mineral powder (m2/g).
XG 2333.7
2.2.3. FTIR analysis This experiment was conducted using a Spectrum One FTIR in transmission mode. The instrument was manufactured by Shimadzu Corporation, Kyoto, Japan. The infrared spectrum of the reagent and mineral was measured by the method of tableting with potassium bromide (KBr). The preparation process of the mineral sample for FTIR analysis was as follows: 50 mg of the mineral sample (−2 μm) was dispersed in 100 ml of ultrapure water. Then the slurry pH was adjusted, and an appropriate dosage of flotation reagent was added. After stirring for 30 min on a magnetic stirrer, it was repeatedly washed with ultrapure water of the same pH and suction filtered through a microporous membrane. The samples dried at a constant temperature (35 °C) in a vacuum oven and used for detection. The wave number range was 4000–400 cm−1. The spectrum of potassium bromide was first measured. 100 mg of the KBr powder and 1.0 mg of the sample were uniformly tableted to measure the spectrum of the sample.
1707.8 1639.4 2932.9 1416.9
3358.1 400 0
350 0
1025.4 300 0
250 0
200 0
150 0
100 0
50 0
-1
Wavenumbers (cm ) Fig. 9. FTIR spectrum of XG.
2.2.4. XPS analysis The XPS experiment was carried out using ESCALAB 250Xi (Thermo 17
(a)
(b)
440.5
876.2
Calcite
Scheelite Scheelite+XG
711.3
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1431.7
Calcite+XG 2314.9
Scheelite+XG+NaOL
Calcite+XG+NaOL 2309.9 814.8
2950
1712.8
2850.3
2923.6 3000
1688.6
2900
2850
2800
1720
4000
3500
3000
1710.2 1692.6
2750
2500
2000
1500
1000
4000
500
3500
1700
1680
3000
1660
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Wavenumbers (cm-1)
Fig. 10. FTIR spectra of scheelite (a) and calcite (b) before and after the reaction of reagents.
further depressant concentration experiment. The effect of XG and NaOL concentration on the flotation recoveries of the minerals is shown in Fig. 7. As shown in Fig. 7(a), with the increase of XG concentration, the recovery of calcite decreased sharply, while the recovery of scheelite did not change much. When XG concentration was increased to 20 mg/L, calcite was depressed (1.79%) while scheelite still reported to the concentrate (81.32%). It was shown that the depressant XG was more selective of calcite over scheelite. As shown in Fig. 7(b), with the increase of NaOL concentration, the recovery of scheelite increased sharply at first, then increased slowly, and finally remained almost unchanged (over 83%), while the recovery of calcite increased slowly. When the NaOL concentration increased to 2 × 10−4 mol/L, calcite was still depressed (40.53%). When the concentration was 4 × 10−5 mol/L, the difference in recovery between scheelite and calcite was the largest.
Fisher, USA) with monochromatic Al Ka X-ray source at 150 W. The energy step size of survey spectra and high-resolution spectra were 1.00 eV and 0.05 eV, respectively. The sample used for XPS detection had the same particle size and reagent system as the microflotation (Section 2.2.1). The sample was filtered and washed repeatedly with ultrapure water for the purpose of washing away the weak physical adhesion of the reagent on the mineral surface. It was then dried in a vacuum oven and then used for testing. 2.2.5. AFM imaging The surface morphology of minerals before and after the reaction of XG was observed by atomic force microscopy. The instrument model is MultiMode 8. The probe material is Silicon Tip on Nitride Lever and the probe model is SCANASYST-AIR. It is difficult for a scheelite crystal to naturally dissociate into a flat surface (Gao et al., 2016). To get a flat surface, it needs to polish scheelite with a polishing machine (Tegramin-25, Struers). The samples were immersed in ultrapure water for several cleanings. The scheelite and calcite samples were then soaked in the XG solution for 3 min. It was then removed with tweezers and repeatedly rinsed with ultrapure water to remove the XG adhering to the mineral surface. Then the samples were blown dry with ultrapure nitrogen for testing. The measured AFM images were processed by software Nanoscope 7.3. For ease of comparison, the height ruler of the image was unified (from −10 nm to 10 nm).
3.2. Adsorption experiment results The adsorption density of the flotation reagent on the mineral surface directly affects the flotation behavior of the mineral. The adsorption density can be expressed by the adsorption amount of the flotation reagent on the unit area of the mineral. The adsorption density of the depressant XG on the two mineral surfaces as a function of XG concentration at a fixed pH 8 was shown in Fig. 8. It was shown that the adsorption density of XG on the surface of the two minerals increased with the initial concentration of XG. However, the increase of the two curves gradually slowed down. This indicated that as the concentration of XG increased, the adsorption density of XG on both minerals increased sharply at first, then increased slowly, and finally remained almost unchanged. In addition, the adsorption density of XG on the calcite surface was greater than that of the scheelite surface over the concentration range of the study. This indicated that the depressive effect of XG on calcite was more intense than scheelite. These are all consistent with the results of the microflotation experiment.
3. Results and discussion 3.1. Microflotation results The flotation results of the two minerals under the depressant XG and the collector NaOL system are shown in Fig. 6. When the depressant XG was not added, it was shown that both two minerals maintained excellent recovery (over 80%). The recovery of calcite (over 90%) was even better than that of scheelite (about 80%), and they all remained a recovery of over 80% throughout the pH range of the study. It was difficult to separate the two minerals by flotation without the addition of a depressant. After the addition of depressant XG, the recovery of calcite is drastically reduced in the pH range of 6.5–8.5 (from about 90% to less than 10%). However, the addition of depressant XG almost did not impair the recovery of the scheelite. About 80% of the scheelite can still be floated in the pH range of 7.5–9.5 in the presence of depressant XG. The large difference in recovery of the two minerals in the presence of the depressant XG provided excellent conditions for flotation separation between each other. A fixed pH of 8.0 was selected for
3.3. FTIR analysis FTIR analysis was used to explain the adsorption mechanism of flotation reagents on mineral surfaces. In our previous studies, we found that NaOL had significant characteristic peaks at 2921.6 cm−1, 2851.7 cm−1, 1447.4 cm−1 and 1559.3 cm−1, respectively (Dong et al., 2019). The first two peaks were caused by the stretching vibration of the eCH2- and eCH3 groups. The latter two peaks were attributed to the symmetrical and asymmetric vibrations of the carboxyl groups. The 18
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Experimental line Fitting Line Background Hydrocarbons Carbon oxides CO32- of calcite C-OH O-C-O/O=C-O
(a)
289.36
286.85
286.24
284.77
285.41
531.35 531.14
Calcite+XG
532.74
Counts/s
Experimental line Fitting Line Background Hydrocarbons Carbon oxides CO32- of calcite
(b)
531.79
Calcite+XG
289.29
284.79
Experimental line Fitting line Background Oxygen in CO32Carbon oxides
531.14
531.79
Calcite
294
285.40
292
290
288
Calcite
286
284
282
536
534
532
Binding energy (eV)
530
Binding energy (eV)
Experimental line Fitting line Background Ca 2p 3/2 in CaCO3 Ca 2p 3/2 in Ca-COOR Ca 2p 1/2 in CaCO3 Ca 2p 1/2 in Ca-COOR
(C)
Calcite+XG 346.89
346.69
350.43 350.22
Counts/s
Counts/s
Experimental line Fitting line Background Oxygen in CO32Carbon oxides -COO-OH/-COOCH3
Experimental line Fitting line Background Ca 2p 3/2 in CaCO3 Ca 2p 1/2 in CaCO3
346.89
Calcite
350.43
354
352
350
348
346
344
Binding energy (eV) Fig. 11. Fitting peaks of C 1s (a), O 1s (b) and Ca 2p (c) on calcite surface before and after adding XG.
19
528
526
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Fig. 12. AFM images (left-height; right-3D) of scheelite in the absence (a) and presence (b) of XG.
dramatic change in the adsorption peak of the carboxyl group (from 1688.6 cm−1 to 1639.4 cm−1) on the surface of calcite indicated that the carboxyl group (eCOO−) had chemical chelation with the calcium ion species on calcite surface. In the presence of depressant XG and collector NaOL, the scheelite surface had new absorption peaks at 2923.6 cm−1 and 2850.3 cm−1. The two peaks were shifted from the adsorption peaks of methylene and methyl groups in NaOL at 2921.6 cm−1 and 2851.7 cm−1. This indicated that NaOL was still chemisorbed on scheelite surface in the presence of depressant XG. However, in addition to the characteristic peaks of XG at the 1710.2 cm−1 and 1692.6 cm−1 on calcite surface, no other absorption peaks appeared. This indicated that the pre-adsorption of XG hindered the adsorption of NaOL on the calcite surface. FTIR analysis results further confirmed the selective depressive effect of XG on calcite.
infrared spectrum of XG was shown in Fig. 9. It was shown that the characteristic peak at 3358.1 cm−1 was assigned to the stretching vibration of eOH (Inamuddin, 2019). The peaks at 2932.9 cm−1 and 2333.7 cm−1 were contributed by the stretching vibration of eCH2- and eCH (Inamuddin, 2019; Yang et al., 2018). The peak at 1707.8 cm−1 was due to stretching vibrations of carbonyl (C]O) of the acetyl groups. The characteristic peaks at 1639.4 cm−1 and 1416.9 cm−1 were all attributed to the stretching vibration of carboxyl (De Falco et al., 2018; Kang et al., 2019; Zheng et al., 2019). The absorption band at 1025.4 cm−1 was assigned to the stretching vibration CeOeC groups (Inamuddin, 2019; Kang et al., 2019). The changes in the infrared spectra of the minerals before and after the reaction of the flotation reagent are shown in Fig. 10. As depicted in Fig. 10, bare scheelite had two significant characteristic peaks at 814.8 cm−1 and 440.5 cm−1, which belonging to the stretching vibration and bending vibration peaks of WeO (Abdalla et al., 2018). The stretching vibration peak of CeO in calcite appeared at 1431.7 cm−1, and the bands at 876.2 cm−1 and 711.3 cm−1 were contributed by the deformation vibration of CeO in calcite (Azizi and Larachi, 2017). After the interaction of the depressant XG and the minerals, no significant new absorption peak appeared on scheelite surface. However, two significant new absorption peaks at 1712.8 cm−1 and 1688.6 cm−1 appeared on the calcite surface, which were shifted from the characteristic peaks of the acetyl groups and carboxyl group in XG at 1707.8 cm−1 and 1639.4 cm−1, respectively. This indicated that the adsorption of the depressant XG on scheelite surface was very weak, whereas XG had a strong chemisorption on calcite surface. The
3.4. Surface analysis XPS analysis further revealed the adsorption mechanism of depressant XG on calcite. It mainly explored the changes in the relevant chemical environment of specific elements on the surface of calcite, and determining the functional groups that acted of the depressant. The high-resolution spectra of C1s, O 1s and Ca 2p of calcite are shown in Fig. 11. At the same time, the spectra of C1s, O 1s and Ca 2p of calcite before and after the reaction of XG were separated and fitted. As shown in Fig. 11(a), bare calcite not only contained separated peak of its own CO32− at 289.29 eV, but also contained separated peaks at 284.79 eV and 285.40 eV (Jiao et al., 2019). The latter two peaks were due to the 20
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Fig. 13. AFM images (left-height; right-3D) of calcite in the absence (a) and presence (b) of XG.
3.5. AFM images of XG on mineral surfaces
hydrocarbons and carbon oxides contaminants on calcite surface (Deng et al., 2017; Deng et al., 2018). After the reaction of calcite and XG, in addition to the separated peaks of CO32−, hydrocarbons and carbon oxides at 289.36 eV, 284.77 eV and 285.41 eV, two new separated peaks can be observed at 286.24 eV and 286.85 eV. The two peaks were assigned to CeOH and OeCeO/O]CeO in XG molecule, respectively (Jin et al., 2016). This indicated that XG had a strong adsorption on calcite surface. As shown in Fig. 11(b), the high-resolution spectra of O 1s of bare calcite had two separated peaks at 531.14 eV and 531.79 eV (Jiao et al., 2019). The two peaks were derived from the CO32− of calcite and the contamination of carbon oxides (Deng et al., 2018). In the presence of XG, two new separated peaks can be observed at 531.35 eV and 532.74 eV. The two peaks were attributed to eCOO− and eOH/eCOOCH3 of XG, respectively. This indicated that the chemical environment of oxygen on calcite surface had changed greatly. It was further proved that the chemical adsorption of XG occurred on calcite surface. As can be seen from Fig. 11(c), the spectra of Ca 2p of bare calcite had double peaks at 346.89 eV and 350.43 eV, which were attributed to the Ca 2p 3/2 and Ca 2p 1/2 in CaCO3 itself (Liu et al., 2017b). After the interaction of calcite and XG, two separated double peaks can be observed. In addition to the double peaks of CaCO3 at 346.89 eV and 350.43 eV, the other double peaks were observed at 346.69 eV and 350.22 eV. The double peaks were attributed to Ca 2p 3/ 2 and Ca 2p 1/2 in Ca-COOR (Liu et al., 2017a). This indicated XG had strong adsorption on calcite surface. It was once again demonstrated that the carboxyl groups (eCOO−) of the XG molecule and the calcium ions species on calcite surface were chelated. XG was mainly adsorbed on calcite surface by chemical chelation, thereby inhibiting calcite.
The surface morphology of scheelite before (a) and after (b) the reaction of depressant XG is shown in Fig. 12. The scan range was 3.0 μm × 3.0 μm. The left graph represented a height image, and the height ruler ranged from −10 nm to 10 nm, meaning that different colors represent different heights. The graph on the right was a 3D image that visually reflected the surface morphology of the mineral. The scheelite exhibited a relatively flat surface after polishing (Fig. 12(a)). The gully left by polishing can be clearly seen. A point of bulging on the image may be an inevitable, slight contamination during the sample preparation process. After the reaction of scheelite and depressant XG (Fig. 12(b)), XG exhibited sparse and irregular point adsorption on the surface of scheelite. It can be seen from the 3D image that XG was very sparsely adsorbed on the surface of scheelite. This indicated that the adsorption of the depressant XG on scheelite surface was relatively weak. Fig. 13 shows the surface morphology of calcite before (a) and after (b) the reaction of depressant XG. The scan range was also 3.0 μm × 3.0 μm. From Fig. 13(a), both the height image (left) and the 3D image (right) indicated that the naturally dissociated calcite surface has a flat surface and the sample preparation process was not contaminated. The height ruler was the same as the scheelite images (from −10 nm to 10 nm). After the reaction of calcite and depressant XG (Fig. 13(b)), it is shown that XG exhibited a dense agglomerate adsorption on the surface of calcite. The adsorption morphology of XG even became reticulate adsorption. The adsorption density and adsorption height of XG on the surface of calcite were significantly 21
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increased compared with scheelite surface. This indicated that the adsorption capacity of XG on the surface of calcite was more intense than that of scheelite.
using acidified sodium silicate as depressant. Miner. Eng. 80, 45–49. Castanho, J.M., Vieira, M.T., 1998. The influence of the interstitial element on tribological behaviour of tungsten coatings. Surf. Coat. Technol. 102, 50–62. Chen, C., Wu, L., Perdjon, M., Huang, X., Peng, Y., 2019. The drying effect on xanthan gum biopolymer treated sandy soil shear strength. Constr. Build. Mater. 197, 271–279. Chen, W., Feng, Q., Zhang, G., Liu, D., Li, L., 2018a. Selective flotation of scheelite from calcite using calcium lignosulphonate as depressant. Miner. Eng. 119, 73–75. Chen, W., Feng, Q., Zhang, G., Yang, Q., 2018b. Investigations on flotation separation of scheelite from calcite by using a novel depressant: sodium phytate. Miner. Eng. 126, 116–122. De Falco, F., Gentile, G., Avolio, R., Errico, M.E., Di Pace, E., Ambrogi, V., Avella, M., Cocca, M., 2018. Pectin based finishing to mitigate the impact of microplastics released by polyamide fabrics. Carbohydr. Polym. 198, 175–180. Deng, R., Hu, Y., Ku, J., Zuo, W., Yang, Z., 2017. Adsorption of Fe(III) on smithsonite surfaces and implications for flotation. Colloids Surf., A 533, 308–315. Deng, R., Yang, X., Hu, Y., Ku, J., Zuo, W., Ma, Y., 2018. Effect of Fe(II) as assistant depressant on flotation separation of scheelite from calcite. Miner. Eng. 118, 133–140. Dong, L., Jiao, F., Qin, W., Zhu, H., Jia, W., 2019. Selective depressive effect of sodium fluorosilicate on calcite during scheelite flotation. Miner. Eng. 131, 262–271. Emin, S., Altinkaya, C., Semerci, A., Okuyucu, H., Yildiz, A., Stefanov, P., 2018. Tungsten carbide electrocatalysts prepared from metallic tungsten nanoparticles for efficient hydrogen evolution. Appl. Catal. B 236, 147–153. Feng, B., Peng, J., Zhang, W., Ning, X., Guo, Y., Zhang, W., 2018. Use of locust bean gum in flotation separation of chalcopyrite and talc. Miner. Eng. 122, 79–83. Filippov, L.O., Foucaud, Y., Filippova, I.V., Badawi, M., 2018. 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Food Hydrocolloids 87, 108–118. Kumar, A., Singh, K., Pandey, O.P., 2011. Direct conversion of wolframite ore to tungsten carbide nano particles. Int. J. Refract Metal Hard Mater. 29, 555–558. Li, C., Gao, Z., 2018. Tune surface physicochemical property of fluorite particles by regulating the exposure degree of crystal surfaces. Miner. Eng. 128, 123–132. Liu, C., Zhang, W., Song, S., Li, H., 2019a. A novel method to improve carboxymethyl cellulose performance in the flotation of talc. Miner. Eng. 131, 23–27. Liu, C., Zhu, G., Song, S., Li, H., 2019b. Flotation separation of smithsonite from quartz using calcium lignosulphonate as a depressant and sodium oleate as a collector. Miner. Eng. 131, 385–391. Liu, X., Li, C., Luo, H., Cheng, R., Liu, F., 2017a. Selective reverse flotation of apatite from dolomite in collophanite ore using saponified gutter oil fatty acid as a collector. Int. J. Miner. Process. 165, 20–27. Liu, X., Luo, H., Cheng, R., Li, C., Zhang, J., 2017b. 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3.6. Depression mechanism of XG Because of the differences in the crystal structure and the solution chemical behavior of the two minerals, the dominant ionic components of the mineral surface in the slurry are also different (Dong et al., 2019). For scheelite, anionic tungstate ions (WO42−) are the dominant component. However, calcite surface is mainly determined by cationic calcium ions (Ca2+). The XG molecular contains a large amount of free carboxyl groups (eCOO−). These carboxyl groups (eCOO−) can be chemically chelated with calcium ion species on the mineral surface. At the same time, the anions WO42− or CO32− of the mineral surface can hinder the occurrence of chelation by electrostatic repulsion or steric hindrance (Chen et al., 2018b). Therefore, XG has more chelation with calcium ions on calcite surface than on scheelite surface. Moreover, the chains of XG macromolecules produce hydrophobic associations within and between molecules, which also increases the adsorption of XG on the mineral surface. In addition, hydrogen bonds effect also play an important role in the adsorption process due to the presence of hydroxyl groups and carboxyl groups in the XG molecule. More depressant XG is adsorbed on calcite surface, which hinders further adsorption of the collector NaOL. Therefore, the floatability of calcite deteriorates severely. The addition of XG selectively inhibits the flotation of calcite. 4. Conclusions XG has excellent selective inhibition effect on calcite during scheelite flotation. When the dosages of depressant XG and the collector NaOL are 20 mg/L and 4.0 × 10−5 mol/L, the recovery of calcite is drastically reduced to only 1.79%, while scheelite still has a very high flotation recovery of 81.32% at pH 8. The adsorption density of the depressant XG on the calcite surface is much larger than that of the scheelite surface. The depressant XG selectively chemisorbs on the surface of calcite, while NaOL can still chemisorb on the surface of scheelite with the pre-addition of depressant XG. The free carboxyl groups in the XG molecule can be more chemically chelated with calcium ion species on calcite surface. It was more directly observed by AFM that the adsorption of XG on the calcite surface was more dense than that of scheelite surface. These all prove that the calcite surface adsorbs more depressant XG than that of scheelite, which hinders the further adsorption of the collector NaOL. As a result, this creates a huge difference in the floatability of the two minerals, which allows flotation separation between the two minerals. 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); and the Key laboratory of Hunan Province for Clean and Efficiency Utilization of strategic Calcium-containing mineral Resources (No. 2018TP1002). References Abdalla, M.A.M., Peng, H., Younus, H.A., Wu, D., Abusin, L., Shao, H., 2018. Effect of synthesized mustard soap on the scheelite surface during flotation. Colloids Surf., A 548. Azizi, D., Larachi, F., 2017. Surface interactions and flotation behavior of calcite, dolomite and ankerite with alkyl hydroxamic acid bearing collector and sodium silicate. Colloids Surf., A 537. Bo, F., Luo, X., Wang, J., Wang, P., 2015. The flotation separation of scheelite from calcite
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