calcite

Applied Surface Science 435 (2018) 752–758

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Selective adsorption of benzhydroxamic acid on fluorite rendering selective separation of fluorite/calcite Wei Jiang, Zhiyong Gao ∗, Sultan Ahmed Khoso, Jiande Gao, Wei Sun ∗, Wei Pu, Yuehua Hu School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China

a r t i c l e

i n f o

Article history: Received 10 July 2017 Received in revised form 29 September 2017 Accepted 11 November 2017 Available online 13 November 2017 Keywords: Fluorite Benzhydroxamic acid Flotation Collector Crystal chemistry

a b s t r a c t Fluorite, a chief source of fluorine in the nature, usually coexists with calcite mineral in ore deposits. Worldwide, flotation techniques with a selective collector and/or a selective depressant are commonly preferred for the separation of fluorite from calcite. In the present study, an attempt was made to use benzhydroxamic acid (BHA) as a collector for the selective separation of fluorite from calcite without using any depressant. Results obtained from the flotation experiments for single mineral and mixed binary minerals revealed that the BHA has a good selective collecting ability for the fluorite when 50 mg/L of BHA was used at pH of 9. The results from the zeta potential and X-ray photoelectron spectroscopy (XPS) indicated that the BHA easily chemisorbs onto the fluorite as compared to calcite. Crystal chemistry calculations showed the larger Ca density and the higher Ca activity on fluorite surface mainly account for the selective adsorption of BHA on fluorite, leading to the selective separation of fluorite from calcite. Moreover, a stronger hydrogen bonding with BHA and the weaker electrostatic repulsion with BHA− also contribute to the stronger interaction of BHA species with fluorite surface. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Fluorite (CaF2 ), which is a major source of fluorine in the Earth, has been considered as an important strategic mineral in recent years [1]. In natural ore deposits, fluorite is mostly associated with calcite (CaCO3 ) mineral [2]. Around the world, froth flotation method is commonly used for the separation of fluorite from calcite. Conventional collectors, like fatty acids and their salts, are mainly utilized for the flotation of fluorite ores [3]. Pugh and Stenius showed that the maximum flotation response for calcite and fluorite with sodium oleate as a collector can be correlated directly with the minimum interfacial tension of the air/solution interface based on solution chemistry calculations [4]. Marinakis and Shergold found that sodium silicate can depress fluorite and calcite by preventing oleate species from reacting with surface sites [5]. Gao et al. recently reported a novel reagent schedule including sulfoleic acid (SOA) as a collector, citric acid (CA) as a depressant and sodium fluoride (NaF) as a regulator to achieve the selective separation of calcite from fluorite [2]. It should be noted that the separation of fluorite from calcite by flotation technique is quite difficult due to

∗ Corresponding authors. E-mail addresses: [email protected] (Z. Gao), [email protected] (W. Sun). https://doi.org/10.1016/j.apsusc.2017.11.093 0169-4332/© 2017 Elsevier B.V. All rights reserved.

their similar surface properties, and hence similar interactions with fatty acid collectors [6–8]. Some efforts were made to sort out some new selective collectors to achieve the selective separation of calcite and fluorite, as fatty acid collectors possess poor selectivity and low watersolubility at a low temperature. Filippova et al. stated that the sulpho-succinamate or alkyl hydroxamate can perform moderate collecting difference in the flotation behaviour of fluorite and calcite [9]. Schubert et al. utilized oleyl sarcosine as a collector and quebracho as a depressant to achieve effective separation of fluorite from calcite [10]. Xu et al. showed that cyclohexene formic acid as a collector and water glass as a depressant could achieve the satisfactory separation [11]. However, the extremely high dosage of quebracho or water glass for the separation may cause the fine particles to well disperse in the tailing pond, and thus cause a serious problem for wastewater treatment. Some researchers have tried to use the mixture of collectors, such as anionic-cationic or anionic-nonionic, for the separation of fluorite from calcite. Helbig et al. indicated that anionic-cationic mixing collectors of sodium N-dodecanoyl sarcosine and dodecylammonium chloride can improve the floatability of fluorite, however the selectivity of the collector was not clearly elaborated [12]. Filippova et al. found that two combinations of anionic/nonionic reagents (di-2-ethylhexyl phosphoric acid/octylphenol ethoxylate and alkyl hydroxamate/iso-tridecanol) at pH 8 can float fluorite

W. Jiang et al. / Applied Surface Science 435 (2018) 752–758

effectively while calcite was not floated by these collectors [9]. However, it’s a pity that they had not investigate further the separation performance of those collectors for mixed binary minerals and/or run-of-mine. In recent years, hydroxamate types of collectors have attracted more and more attention for their better selectivity and moderate collecting power. Deng et al. conducted the pure mineral flotation tests at pH 10 and found that N-(6-(hydroxyamino)-6-oxohexyl) decanamide (one kind of amide hydroxamate) exhibits a superior affinity to scheelite against calcite [13]. Recently, benzhydroxamic acid (BHA) has been used widely as a collector for the cassiterite [14] and tungsten minerals (scheelite and wolframite) [15] flotation in the presence of activator Pb2+ both in the laboratory and in dressing plants. It is interesting to note that in either cassiterite or tungsten mineral flotation systems, calcite is the main gangue mineral. These reports have proved that BHA have a weak interaction with calcite, and it might be served as a better candidate collector for the separation of fluorite/calcite. In this work, BHA was utilized as a collector to separate the fluorite from calcite through flotation tests using single mineral and mixed binary minerals. The mechanism of the selective separation was analysed through zeta potential measurements, X-ray photoelectron spectroscopy (XPS) detections and crystal chemistry analysis. 2. Materials and methods 2.1. Samples and reagents Representative samples of fluorite and calcite were collected through hand picking from the Shizhuyuan Mine, Chenzhou, Hunan, China. The received samples were analysed through the Xray diffraction (XRD) for their purity. The XRD results revealed that the purity of fluorite and calcite is greater than 97%, as shown in Fig. 1. After that, samples were ground in a pottery ball mill, in order to obtain −75 + 38 ␮m size fraction for the micro-flotation. The received undersize fraction (i.e. −38 ␮m) was further ground in an agate mortar to obtain particles with size less than 5 ␮m for zeta potential and XPS measurements. BHA with 98% grade was obtained from Tokyo Chemical Industry. Pine oil was provided by Qingdao Ruchang Mining Industry Co., Ltd. The pH was adjusted with hydrochloric (HCl) and sodium hydroxide (NaOH) solutions. Deionized (DI) water with a resistivity of more than 18 M * cm was used throughout the experiments. 2.2. Flotation experiments The XFG Flotation machine (Exploring Machinery Plant, Changchun, China) with a 40 mL plexiglass cell was used for flotation tests. The impeller speed was set at l650 r/min, since the flotation results showed that the speed from 1250 to 2000 r/min exerts slight influence on the flotation response for two minerals. The XFG machine configuration has been reported in previous publication [2]. For single mineral, the flotation suspension was prepared by adding 2 g of single mineral, while for mixed binary minerals, the suspension was made by mixing fluorite and calcite at a certain mass ratio into 35 mL of DI water. The pulp was conditioned for 2 min followed by 3 min conditioning after adjusting the suspension pH to a desired value. After conditioning, the collector was added and agitated for 3 min. After that, Pine oil (0.1 ␮L) was added and conditioned for 1 min. Finally, the froth collection was performed for 5 min. For single mineral flotation, the products were collected, dried, weighed and the recovery was determined. For mixed minerals

(a)

6000

753

{111}

4000

2000

0 75-0363>Fluorite - CaF2

10

20

30

40

50

60

70

Two-Theta (deg)

5000

(b)

{104}

4000 3000 2000 1000 0 05-0586>Calcite - CaCO3

10

20

30

40

50

60

70

Fig. 1. XRD spectrums of powdered fluorite (a) and calcite (b) minerals used for flotation tests.

flotation, the concentrate and tail were assayed for CaF2 and CaCO3 content. The average recovery and standard deviation were also calculated and reported. 2.3. Zeta potential measurements Zeta potential measurements were carried out at a temperature of 20 ◦ C by using a Nano-ZS90 zeta potential analyzer (Malvern Instruments, UK). The suspension was prepared by adding 0.02 g of mineral samples into 40 mL KCl (0.01 mol/L) solution. Then BHA was added and the pH was adjusted and measured. The suspension was continuously agitated by magnetic stir for 10 min. After that, the supernatant liquid was sucked out and used for other tests. At least 3 measurements were made for a certain experimental condition, and the average value of zeta potential was determined simultaneously. 2.4. XPS measurements In present study, VG ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, USA) was used for the determination of XPS spectra of fluorite and calcite powders for both untreated and treated with BHA as a collector. The Al K␣ X-rays meter with an energy of 20 eV for narrow scans was employed for observations. The pressure in the analysis chamber was more than 5 × 10−10 mbar. Samples having particle size less than 5 ␮m were conditioned in BHA solution at pH of 9. The mineral suspension was than magnet-

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Fig. 2. Structures of different crystal surfaces in one unit cell (Ca = little green, F = light blue, O = red, C = grey; a: fluorite {111} surface and b: calcite {104} surface). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

ically stirred for 30 min, rinsed with DI water for three times, dried in vacuum oven at 40 ◦ C, and subsequently tested by the meter at ambient temperature. It should be noted here that the reduplicative rinsing before the XPS measurements assures the wash-off of any loosely adsorbed BHA from the mineral surface [13,16].

2.5. Computational simulation The XRD spectrums of pure fluorite and calcite minerals are shown in Fig. 1. Fig. 1 shows that the {111} surface for fluorite and {104} surface for calcite are the dominant cleavage planes and the most commonly exposed planes for the two minerals during crushing and grinding. Previous reports have proved that the surface reactivity (such as wettability, dissolution, adsorption, catalysis, etc.) and flotation behaviour of mineral particles are mainly governed by the characteristics of the most commonly exposed surface(s) of the mineral [17–19], especially the properties of active atom(s) on the surface(s). Using the Building Surface module in Material Studio 7.0 [20], the fluorite {111} surface and calcite {104} surface were generated, and their surface structures and the binding states of surface atoms are shown in Fig. 2. It should be noted that using different TOP values in Build Surface module will generate different surface structures of the same miller index. The criterion for the determination of TOP values in the present study is to assure the intactness of CO3 groups on the top layer of the surface cell, i.e., the C O bonds must not be broken during generating the calcite {104} surface [19]. As the fluorite {111} surface consists of planes of calcium ions in the hexagonal array with a layer of fluorine ions from both above and below, the fluorite {111} surface is terminated with fluorine atoms and just below the surface are seven-coordinate calcium atoms [21]. The structures of BHA and its anionic species BHA− were optimized by Dmol3 module in MS using GGA – PBESOL functional, DNP basis set, all electron core treatment, basis file of 3.5, multipolar expansion of hexadecapole, global orbital cutoff of 3.7 Å, and occupation of fermi and SCF tolerance of 1.0 × 10−6 eV/atom [22]. Other parameters were default settings. The optimized geometry structures of BHA and BHA− are shown in Fig. 3. The distance between two O atoms are 2.506 Å and 2.877 Å for BHA and BHA− , respectively. The calculated dihedral angles of O N C O in BHA and BHA− are −3.975◦ and 0.059◦ , respectively, which means that the O, N, C, O atoms in −C( O)-NH O group of BHA− lay almost on a plane [13] (Fig. 3).

3. Results 3.1. Single mineral flotation experiment results The flotation behaviour of fluorite and calcite using BHA as collector was evaluated by the single mineral flotation tests. Fig. 4(a) shows the effect of pH on the BHA flotation of fluorite and calcite. The recovery of fluorite promptly increases as pH value increases from 6 to 8.5, and then remains constant about 95% at pH 8.5–10. However, the calcite recovery does not exceed 20% in the total pH range. Solution with pH of 9 was selected for the following experiments. Fig. 4(b) shows the effect of BHA concentration on the flotation performance of fluorite and calcite at pH of 9. The flotation recovery of fluorite increases dramatically with an increase of BHA dosage. The maximum recovery of more than 95% is obtained at BHA concentration of 30 mg/L. The recovery of calcite doesn’t change obviously with a recovery below 20%. The results obtained here indicate that BHA shows a good flotation collection for fluorite. Hence, BHA collector with dosage of 50 mg/L was chosen for the subsequent experiments. 3.2. Mixed binary minerals flotation experiment results A flotation test on mixed binary minerals of fluorite and calcite with 50 mg/L of BHA at pH of 9 was carried out to evaluate the separation efficiency of BHA. The mixed binary minerals were prepared by mixing fluorite and calcite particles at a mass ratio of 5:5, 6:4, 7:3, 8:2 and 9:1. The flotation results presented in Fig. 5. show that the grade and recovery of CaF2 in the concentrate increase gradually with the increasing proportion of fluorite in binary minerals mixture. The concentrate with a grade of 89.50% CaF2 and recovery of 60.08% is obtained when the mass ratio of fluorite and calcite is 7: 3. The results indicated that using BHA as the collector, a high separation efficiency and selectivity between calcite and fluorite can be achieved with a fluorite concentrate having low calcite contamination. 3.3. Zeta potential results The zeta potentials of fluorite and calcite in the absence and presence of BHA in solution as a function of pH are illustrated in Fig. 6. In the absence of BHA, the IEPs (isoelectric points) of 8.15

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Fig. 3. Optimized geometries of (a) BHA and (b) BHA− (O = red, H = white, N = blue).

Grade of CaF2 in concentrate Recovery of CaF2 in concentrate

(a)

Recovery (%)

80

80

Fluorite Calcite

60 40

c (BHA)= 50 mg/L pH=9

60

BHA concentration: 50 mg/L

40

20 20

0

6

7

8

9

10

11

pH

0 5:5

100

6:4

7:3

8:2

9:1

Fig. 5. Flotation results of mixed binary minerals of fluorite and calcite at different mass ratios using BHA collector.

(b)

80

Fluorite Calcite

60

Fluorite Fluorite+BHA Calcite Calcite+BHA

20

pH = 9

10

40

0 -10

20

-20

0

0

20

40

60

80

-30 5

6

7

8

9

10

11

12

Fig. 4. Effect of solution pH (a) and BHA dosage (b) on fluorite and calcite flotation.

and 9.17 are obtained for fluorite and calcite, respectively, which are in agreement with the previous reports [2,6,17,23–25]. In the presence of BHA, the zeta potential of fluorite shifts towards more negative values at pH 8–10 by 10 mV. For calcite, in the absence and presence of BHA, the zeta potential changes slightly. 3.4. XPS analysis results The XPS tests were performed to investigate the adsorption mechanism of BHA on mineral surface. The detailed values of binding energy and percentage of each composition element on the

Fig. 6. Effect of the addition of BHA on the zeta potential of fluorite and calcite at different pH values.

fluorite and calcite surfaces before and after the treatment with BHA are presented in Table 1. The chemical shift of C, O or Ca on the calcite surface before and after BHA adsorption is negligible, indicating that BHA species hardly absorb on calcite surface. After interacted with BHA, the chemical shift of Ca, C and O on fluorite surface reaches −0.33, −0.38 and 0.93 eV, respectively, which is indicative of an obvious transformation of the chemical environment on fluorite surface. The N element was also detected on fluorite surface after interaction with BHA.

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Table 1 Binding energy and percentage of component elements on fluorite and calcite surface. C

O

Ca

N

F

Binding Energy (eV)

Fluorite Fluorite + BHA Chemical shift Calcite Calcite + BHA Chemical shift

285.41 285.03 −0.38 284.8 284.8 0

531.52 532.45 0.93 531.25 531.22 −0.03

348.23 347.9 −0.33 346.96 346.94 −0.02

– 399.37 – – – –

684.87 684.85 −0.02 – – –

Percentage (%)

Fluorite Fluorite + BHA Percent change Calcite Calcite + BHA Percent change

7.95 28.13 20.18 39.34 38.13 −1.21

20.2 17.03 −3.17 45.48 46 0.52

30.55 19.14 −11.41 15.18 15.87 0.69

– 0.51 0.51 – – –

41.31 35.19 −6.12 – – –

a

Ca 2p 3/2 348.02

fluorite

Ca 2p 1/2 351.59

Ca 2p 3/2 347.86

fluorite+BHA Ca 2p 1/2 351.45

354

352

350

348

346

Binding Energy (eV) Ca 2p 3/2 347.10

b calcite

Ca 2p 1/2 350.65

Ca 2p 3/2 347.10

calcite + BHA Ca 2p 1/2 350.64

354

352

350

348

346

Fig. 7. The Ca 2p XPS peaks on fluorite (a) and calcite (b) before and after BHA treatment.

W. Jiang et al. / Applied Surface Science 435 (2018) 752–758

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Table 2 Distribution and activity of Ca atoms on the commonly exposed surfaces of fluorite and calcite [18,19,26,27]. Mineral surface

Surface unit cell area, nm2

Number of Ca atoms on each unit cell area

Ca density, ␮mol/m2

Ca element percentage, %

Distance of the two adjacent Ca atoms, nm

Broken bonds number of each Ca atom

Dissociative energy for each dangling bond, kJ/mol

Calcite {104} Fluorite {111}

0.404 0.129

2 1

8.22 12.87

20 33.33

0.405 0.386

1 1

383.3[27] 529[27]

The Ca 2p XPS peaks of fluorite and calcite before and after BHA treatment are shown in Fig. 7. Before BHA treatment, the Ca 2p 1/2 and 3/2 peaks of fluorite appear at 348.02 eV and 351.59 eV, respectively. After interacted with BHA, the two peaks on fluorite surface shift to smaller values by 0.16 eV and 0.14 eV, respectively. On the other hand, on calcite surface before and after interacted with BHA, the two peaks always emerge near 347.10 eV and 350.65 eV. 3.5. Crystal chemistry calculation results As shown in Fig. 2, on the fluorite {111} surface in one unit cell, there are one Ca atom and two F atoms; while on the calcite {104} surface in one unit cell, there are two Ca atoms and two carbonates. So the Ca density and Ca element percentage on each surface can be calculate based on the Eqs. (1) and (2), respectively: Ca density = (number of Ca atoms on the surface of 1 unit cell)/(surface area of 1 unit cell ∗ NA), where NA is Avogadro constantof6.022 × 1023 mol−1

(1)

Ca element percentage = (number of Ca atoms on the surface of 1 unit cell)/(total number of atoms on the surface of 1 unit cell)

(2)

The other characteristics of active Ca atoms on fluorite {111} surface and calcite {104} surface were also analysed, and are summarized in Table 2. 4. Discussions Table 2shows that the Ca density and Ca element percentage are determined to be 12.87 ␮mol/m2 and 33.33% on fluorite {111} surface respectively while 8.22 ␮mol/m2 and 20% on calcite {104} surface [28], respectively, which are supported by the XPS results given in Table 1. Table 1 shows that the Ca element percentage on fluorite surface is 30.55% as compared to 15.18% on calcite surface. The chemical shift of Ca 2p peaks after the adsorption of BHA as shown in Fig. 7(a) implies that BHA chemisorbs onto fluorite surface. A higher Ca element percentage on fluorite surface can accommodate more BHA species to adsorb, resulting in a shift towards negative side of fluorite potential,since BHA− is the predominant species at pH 8.1–10 at a total BHA concentration of 4 × 10−4 mol/L (50 mg/L) [29]. In addition to the Ca atom density and Ca element percentage, the activity of Ca atom also plays an important role in the BHA adsorption on calcic mineral surfaces. Each calcium atom is seven-coordinated to fluorine atoms on fluorite {111} surface but eight-coordinated in the bulk fluorite crystal [21,26]. While for calcite, each Ca atom is five-coordinated to oxygen atoms on {104} surface and six-coordinated on the bulk crystal [19,26]. It is obviously clear that each Ca atom on the surface of calcite {104} or fluorite {111} has only one dangling bond, as shown in Table 2. The difference lies to the dissociative energy of the dangling bond since Ca-O bonds for calcite {104} surface and Ca-F bonds for fluo-

Fig. 8. A proposed schematic chemisorption model of BHA on fluorite {111} surface.

rite {111} surface were broken when the two surfaces were created [19]. The dissociative energy of Ca-F bond is 529 kJ/mol as compared to 383.3 kJ/mol for Ca-O bond [27]. It is worth mentioning here that a larger dissociative energy of the dangling bond implies a more reactive Ca atom. Accordingly, a more reactive Ca atom on fluorite surface can interact more strongly with BHA. On the other hand, the O element percentage on calcite surface is two times more than that on fluorite surface, as shown in Table 1. This higher percentage of negatively charged O atoms will lead to a higher magnitude of repulsion and wearker interaction between calcite surface and BHA− . Moreover, BHA molecule is also a main form at pH 8.1–10 at a total BHA concentration of 4 × 10−4 mol/L (50 mg/L) [29]. Apart from the chemisorption, BHA can also adsorb on calcic minerals through hydrogen bonding, i.e., N H· · ·Fs or O H· · ·Fs on fluorite surface and N H· · ·Os or O H· · ·Os on calcite surface [19,26]. As the F atom has the largest electronegativity and its radius is smaller than O atom, the hydrogen bond of N H· · ·Fs or O H· · ·Fs on fluorite surface is much stronger than that of N H· · ·Os or O H· · ·Os on calcite surface. However, compared to the chemisorption of BHA with fluorite surface, the hydrogen bonding is much weaker, which is supported by a slight chemical shift of F before and after BHA treatment. The most important advantage of BHA adsorption may lie in that the adsorption of molecular BHA can increase the adsorption of anionic species BHA− due to shielding of electrostatic head–head repulsion by intrusion of nonionics between BHA− [9,17,30]. A stronger interaction of BHA species with fluorite surface results in a much bigger chemical shift of Ca compared to that on calcite surface as shown in Table 1 and Fig. 7. Furthermore, Table 2 also shows that the distance of the two adjacent Ca atoms on fluorite surface is 3.86 Å which is longer than the O O distance of −C( O)-NH O- group in BHA− . This implies that the two O atoms in −C( O)-NH O- group of BHA- can’t bond with two adjacent Ca atoms to form a six-membered ring. Based on the structure chemistry analysis, a five-membered ring with less inner stress is more stable [13]. So the most possible schematic chemisorption model of BHA on fluorite {111} surface is proposed in Fig. 8. The −C( O)-

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NH O- group in BHA- reacts with Ca atom on fluorite surface to form BHA-Ca surface complexes with a stable five-membered ring structure. 5. Conclusions In the present study, selective separation of fluorite and calcite were successfully carried out by using BHA as a collector. Results obtained from the single mineral and mixed binary mineral flotation experiments showed that BHA exhibited selective collecting ability to fluorite. The zeta potential and XPS measurements further confirmed that BHA had a prior affinity to fluorite surface. A higher Ca density and Ca activity on fluorite surface mainly accounted for the selective adsorption of BHA on fluorite, hence, rendering selective separation of fluorite from calcite. In addition, a stronger hydrogen bonding with BHA and the weaker electrostatic repulsion between BHA− because of BHA intrusion also contributed to the stronger interaction of BHA species with fluorite surface. Acknowledgements The authors acknowledge the National Natural Science Foundation of China (51774328, 51404300), the financial support from the Innovation-driven Program of Central South University of China (2017CX007), the National 111 Project (B14034), and the Innovation Program for Postgraduate Students of Central South University (2017zzts192). References [1] D.E.E. Industrie, F. Isi, Critical Raw Materials for the EU Report of the Ad-hoc Working Group on Defining Critical Raw Materials, 2010. [2] Y. Gao, Z. Gao, W. Sun, Y. Hu, Selective flotation of scheelite from calcite: a novel reagent scheme, Int. J. Miner. Process. 154 (2016) 10–15. [3] Y. Hu, Z. Xu, Interactions of amphoteric amino phosphoric acids with calcium-containing minerals and selective flotation, Int. J. Miner. Process. 72 (2003) 87–94. [4] R. Pugh, P. Stenius, Solution chemistry studies and flotation behaviour of apatite, calcite and fluorite minerals with sodium oleate collector, Int. J. Miner. Process. 15 (1985) 193–218. [5] K.I. Marinakis, H.L. Shergold, Influence of sodium silicate addition on the adsorption of oleic acid by fluorite, calcite and barite, Int. J. Miner. Process. 14 (1985) 177–193. [6] Z. Gao, D. Bai, W. Sun, X. Cao, Y. Hu, Selective flotation of scheelite from calcite and fluorite using a collector mixture, Miner. Eng. 72 (2015) 23–26. [7] Q. Shi, G. Zhang, Q. Feng, L. Ou, Y. Lu, Effect of the lattice ions on the calcite flotation in presence of Zn(II), Miner. Eng. 40 (2013) 24–29. [8] K. Fa, A.V. Nguyen, J.D. Miller, Interaction of calcium dioleate collector colloids with calcite and fluorite surfaces as revealed by AFM force measurements and molecular dynamics simulation, Int. J. Miner. Process. 81 (2006) 166–177.

[9] I.V. Filippova, L.O. Filippov, A. Duverger, V.V. Severov, Synergetic effect of a mixture of anionic and nonionic reagents: ca mineral contrast separation by flotation at neutral pH, Miner. Eng. 66-68 (2014) 135–144. [10] H. Schubert, H. Baldauf, W. Kramer, J. Schoenherr, Further development of fluorite flotation from ores containing higher calcite contents with oleoylsarcosine as collector, Int. J. Miner. Process. 30 (1990) 185–193. [11] X.U. Hai-Feng, Z. Hong, W. Shuai, Z.Q. Huang, Z. Gang, Synthesis of novel cyclohexene carboxylic acid and its flotation performance for fluorite ore, Chin. J. Nonferrous Metals 24 (2014) 2935–2942. [12] C. Helbig, H. Baldauf, J. Mahnke, K.W. Stöckelhuber, H.J. Schulze, Investigation of Langmuir monofilms and flotation experiments with anionic/cationic collector mixtures, Int. J. Miner. Process. 53 (1998) 135–144. [13] L. Deng, G. Zhao, H. Zhong, S. Wang, G. Liu, Investigation on the selectivity of N −((hydroxyamino)-alkyl) alkylamide surfactants for scheelite/calcite flotation separation, J. Ind. Eng. Chem. 33 (2016) 131–141. [14] M. Tian, Y. Hu, W. Sun, R. Liu, Study on the mechanism and application of a novel collector-complexes in cassiterite flotation, Coll. Surf. A Physicochem. Eng. Aspects 522 (2017) 635–641. [15] H. Han, Y. Hu, W. Sun, X. Li, C. Cao, R. Liu, T. Yue, X. Meng, Y. Guo, J. Wang, Fatty acid flotation versus BHA flotation of tungsten minerals and their performance in flotation practice, Int. J. Miner. Process. 159 (2017) 22–29. [16] K. Wang, Q. Liu, Adsorption of phosphorylated chitosan on mineral surfaces, Coll. Surf. A 436 (2013) 656–663. [17] Z. Gao, W. Sun, Y. Hu, New insights into the dodecylamine adsorption on scheelite and calcite: an adsorption model, Miner. Eng. 79 (2015) 54–61. [18] Z. Gao, Y. Hu, W. Sun, J.W. Drelich, Surface-Charge anisotropy of scheelite crystals, Langmuir 32 (2016) 6282. [19] Z. Gao, C. Li, W. Sun, Y. Hu, Anisotropic surface properties of calcite: a consideration of surface broken bonds, Coll. Surf. A Physicochem. Eng. Aspects 520 (2017) 53–61. [20] Z.Y. Gao, W. Sun, H.U. Yue-Hua, Mineral cleavage nature and surface energy: anisotropic surface broken bonds consideration, Trans. Nonferrous Metals Soc. China 24 (2014) 2930–2937. [21] N.H.D. Leeuw, J.A. Purton, S.C. Parker, G.W. Watson, G. Kresse, Density functional theory calculations of adsorption of water at calcium oxide and calcium fluoride surfaces, Surf. Sci. 452 (2000) 9–19. [22] W. Jiang, Z. Gao, W. Sun, J. Gao, Y. Hu, A density functional theory study on the effect of lattice impurities on the electronic structures and reactivity of fluorite, Minerals 7 (2017) 160. [23] J.D. Miller, K. Fa, J.V. Calara, V.K. Paruchuri, The surface charge of fluorite in the absence of surface carbonation, Coll. Surf. A Physicochem. Eng. Aspects 238 (2004) 91–97. [24] J. Wang, Z. Gao, Y. Gao, Y. Hu, W. Sun, Flotation separation of scheelite from calcite using mixed cationic/anionic collectors, Miner. Eng. 98 (2016) 261–263. [25] H. Yuehua, A. Ruan Chi, Zhenghe xu, solution chemistry study of salt-type mineral flotation Systems: Role of inorganic dispersants, Ind. Eng. Chem. Res. 42 (2003) 1641–1647. [26] Z.Y. Gao, W. Sun, H.U. Yue-Hua, X.W. Liu, Anisotropic surface broken bond properties and wettability of calcite and fluorite crystals, Trans. Nonferrous Metals Soc. China 22 (2012) 1203–1208. [27] Y.R. Luo, Comprehensive Handbook of Chemical Bond Energies, 2007. [28] Y. Lu, J. Drelich, J.D. Miller, Oleate adsorption at an apatite surface studied by ex-situ FTIR internal reflection spectroscopy, J. Coll. Interface Sci. 202 (1998) 462–476. [29] Y.D. Gao, X.Y. Qiu, Q.M. Feng, Flotation solution chemistry study on scheelite with benzyl hydroximic acid as collector, Nonferrous Metals (2003). [30] H. Sis, S. Chander, Reagents used in the flotation of phosphate ores: a critical review, Miner. Eng. 16 (2003) 577–585.