Gemini trisiloxane surfactant: Synthesis and flotation of aluminosilicate minerals

Minerals Engineering 56 (2014) 145–154

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Gemini trisiloxane surfactant: Synthesis and flotation of aluminosilicate minerals Zhiqiang Huang, Hong Zhong ⇑, Shuai Wang, Liuyin Xia, Gang Zhao, Guangyi Liu ⇑ School of Chemistry and Chemical Engineering, Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, Central South University, Changsha 410083, China

a r t i c l e

i n f o

Article history: Received 3 September 2013 Accepted 5 November 2013 Available online 10 December 2013 Keywords: Reverse flotation Aluminosilicate minerals Diasporic bauxite ores Gemini trisiloxane surfactant Density functional theory (DFT) calculation

a b s t r a c t In this paper, an amino-trisiloxane Gemini cationic surfactant, butane-1,4-bis(dimethyl-(3-(3-aminopropyl trisiloxane-3-yl)-propyl)-ammonium bromide) (BBAB) was prepared, and its flotation potential was assessed on kaolinite, pyrophyllite and illite minerals. The flotation results showed that changes to the pulp pH did not significantly influence BBAB’s collecting power in floating three aluminosilicate minerals. In addition, BBAB was a stronger collector than the conventional monomeric surfactant dedecyl trimethylammonium bromide (DTAB) or dodecylamine (DDA). The results of FTIR spectra, zeta potential measurements and density functional theory (DFT) calculations indicated that the interaction of BBAB with the three aluminosilicates was mainly through electrostatic attraction. BBAB’s unique properties, such as high positive grouping Mulliken charge of –CH2N+(CH3)2(CH2)4(CH3)2N+CH2–, and ‘‘parachute’’ shape structure of [(CH3)3SiO]2SiCH3, resulted in superior collecting powers for the aluminosilicate minerals. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction

2. Experimental section

The reverse flotation process has been proved to be a very effective desilication technology for bauxite resources (Smith and Akhtar, 1976; Iwasaki, 1983; Yoon and Shi, 1989; Yoon et al., 1992; Hu, 2003; Liu and Liu, 2005; Zhang et al., 2006). In Chinese diasporic bauxite resources, the main gangue minerals are kaolinite, pyrophyllite and illite (Huang et al., 2013). In order to remove these aluminosilicate minerals from bauxite ores, various cationic surfactants have been investigated in the reverse flotation process in recent years (Jiang et al., 2001; Cao et al., 2001; Hu et al., 2003; Cao et al., 2004; Zhao et al., 2003a,b, 2007; Liu et al., 2007, 2009, 2011; Zhong et al., 2008; Guan et al., 2009; Xia et al., 2009a,b,c, 2010). However, a failure of these cationic surfactants has been their inability to deliver strong flotation of aluminosilicate minerals at mild and strongly alkaline (high pH) conditions. In order to improve the floatability of these three aluminosilicate minerals in the alkaline pulp, a novel amino-trisiloxane Gemini cationic surfactant, butane-1,4-bis(dimethyl-(3-(3-aminopropyl trisiloxane-3yl)-propyl)-ammonium bromide) (BBAB) has been conceived, and its preparation and flotation response to kaolinite, pyrophyllite and illite investigated. The interaction of Gemini trisiloxane surfactant and the three mineral surfaces was evaluated by infrared surface analysis and zeta potential measurement. Moreover, the density functional theory (DFT) calculation was adopted to infer BBAB’s structure–property relationship.

2.1. Minerals and reagents

⇑ Corresponding authors. Tel./fax: +86 731 88830654. E-mail addresses: [email protected] (H. Zhong), [email protected] (G. Liu). 0892-6875/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2013.11.006

Hand-picked kaolinite, pyrophyllite and illite were obtained from the geological museum of China, Qing-Tian and Ou-hai of Zhe-jiang province in China respectively. They were drily ground to fine particles by using an agate mortar and pestle and the 0.076 mm mineral particles were selected for flotation experiments. The purity of the three mineral particles was over 90% based on mineralogical analysis, X-ray diffraction, and chemical analysis. The chemical analysis results were listed in Table 1. Gemini trisiloxane surfactant butane-1,4-bis(dimethyl-(3-(3aminopropyl trisiloxane-3-yl)-propyl)-ammonium bromide) (BBAB), as a collector, was synthesized in our laboratory. The structures of products were confirmed by IR and NMR. IR spectra were recorded as a thin neat film on an AVATAR360 FT-IR (Nicolet, USA) instrument. 1H and 13C NMR spectra were recorded on a Bruker AvanceIII (Switzerland), FT NMR 400 MHz system as a solution in CDCl3 and CD3OD, using tetramethylsilane (TMS) as an internal standard. Solutions of HCl and NaOH were used to adjust the pH of the system and distilled water was used in all tests.

2.2. Micro-flotation Flotation tests were carried out with a XFG5-35 flotation machine with a 40 mL plexiglass cell, and the impeller speed was fixed at l650 r/min. 3 g of 0.076 mm pure mineral samples were placed in the cell, which was then filled with 30 mL distilled water.

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Table 1 Chemical analysis of the pure minerals (wt.%). Composition

Kaolinite

Pyrophyllite

Illite

Al2O3 SiO2 TFe TiO2 CaO MgO K2O Na2O Others Total

37.66 44.22 0.54 0.22 0.074 0.093 0.064 0.049 17.08 100.00

31.10 61.57 0.18 0.19 0.81 0.013 0.049 0.12 5.97 100.00

32.93 50.86 0.29 0.36 0.80 0.30 7.52 0.18 6.76 100.00

After adding the desired amount of collector, the suspension was agitated for 3 min, and the pH was adjusted to a desired value with hydrochloric acid or sodium hydroxide solution before flotation. The flotation was conducted for 6 min. The products and tails were weighed separately after filtration and drying. The results were given in recovery (i.e. weight percentage) of mineral floated. 2.3. FTIR-spectrum The infrared spectra of samples were recorded by Nicolet AVATAR360 FT-IR spectrometer (USA) using the KBr technique. The mineral samples used for this purpose were ground in an agate mortar and pestle to less than 5 lm. To obtain the required sample,

50 mg of mineral sample below 5 lm were added to 30 mL aqueous solution with or without 2.0  10 4 mol/L BBAB at pH 8 and 25 °C, stirred for 30 min, filtered, and then dried in a vacuum oven at 35 °C for 24 h. Tablet was prepared by mixing approximate 5 mg the tested sample with about 200 mg KBr, and then the infrared spectra were immediately recorded from 400 cm 1 to 4000 cm 1. 2.4. Zeta potential measurement Zeta-potentials of minerals were measured using a Brookhaven Zetaplus zeta potential analyzer (USA). The mineral samples used for this purpose were ground to less than 5 lm in an agate mortar and pestle. 50 mg of the mineral samples below 5 lm were added to 30 mL aqueous solution with or without 2.0  10 4 mol/L BBAB, stirred for 5 min and the pH values were adjusted and measured. All measurements were conducted in a 0.1 mol/L KNO3 background electrolyte solution. The agitated suspension was sampled to record the zeta-potential. The results presented were the average of three independent measurements with a typical variation of ±2 mV. 2.5. DFT calculation Calculations on geometric structure and atomic charges of collectors were made using Gaussian 03 and Chemoffice 2005. The

Scheme 1. Synthetic route for Gemini trisiloxane surfactant BBAB.

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initial molecular modeling of the Gemini trisiloxane group was optimized by the MM2 and PM3 methods in Chemoffice. The obtained geometries were further optimized and calculated with DFT methods at the B3LYP/6-31G (d) level.

3.1.2. Characterization of product Intermediary characterization: (3-(N,N-dimethylamino)-propyl)-3- aminopropyl-trisiloxane (3), a colorless transparent liquid, yield 77.34%. 400 MHz 1H NMR (CDCl3, TMS): d 2.56–2.62 (m, 2H), 2.49–2.54 (m, 2H), 2.20–2.32 (m, 2H), 2.18 (s, 6H), 2.01 (m, 1H), 1.22–1.70 (m, 2H), 1.45 (t, 2H), 0.27–0.70 (m, 2H), 0.048 (s,18H), -0.005 (s, 3H). 400 MHz 13C NMR (CDCl3): d 58.10, 53.16, 48.24, 45.57, 28.22, 23.56, 15.14, 1.86, 0.39. IR (KBr): 3291.57, 2954.44, 2856.58, 1463.30, 1410.00, 1257.65, 1041.83, 850.01, 740.78 cm 1. Product characterization: Butane-1,4-bis(dimethyl-(3-(3-aminopropyl trisiloxane-3-yl)-propyl)-ammonium bromide) (5, BBAB), a light yellow solid, yield 66.51%. 400 MHz 1H NMR (CD3OD, TMS): d 4.347–4.668 (m, 4H), 3.812–4.187 (m, 4H), 3.695–3.913 (m, 4H), 3.227–3.428 (m, 4H), 2.627–3.015 (m, 4H), 2.303–2.472 (m, 4H), 2.227 (s, 12H), 1.775–1.980 (m, 2H), 1.391–1.674 (m, 4H), 0.356– 0.661 (m, 4H), 0.082 (s, 36H), 0.010 (s, 6H). 400 MHz 13C NMR (CD3OD): d 60.38, 58.06, 51.37, 45.47, 21.04, 15.01, 14.81, 14.18, 1.96, 0.09, 0.70. IR (KBr): 3295.14, 2951.16, 2922.68, 2860.32, 1468.96, 1412.47, 1262.58, 1042.05, 854.67, 736.19 cm 1.

3. Results and discussion 3.1. Synthesis and characterization of BBAB 3.1.1. Synthesis of BBAB Gemini trisiloxane surfactant BBAB was prepared by a cobromination protocol (Scheme 1). (3-(N,N-Dimethylamino)-propyl)3-aminopropyl-(methyl)dimethoxyl silane (1; 26.11 g, 0.1 mol) was slowly added to the stirred suspension of hexamethylditrisiloxane (2; 81.82 g, 0.5 mol) in the presence of a phase-transfer catalyst tetramethylammonium hydroxide (1 mL, 25%) under inert conditions (nitrogen atmosphere). After addition, the reaction mixture was stirred for 8 h at 90 °C. Hexamethylditrisiloxane was then removed from the crude reaction mixture under reduced pressure in a rotary flash evaporator at 65 °C for 1 h. The resulting compounds (3-(N,N-dimethylamino)-propyl)-3- aminopropyl-trisiloxane (3; 22.00 g, 0.050 mol) were heated with 1,4-dibromobutane (4; 6.13 g, 0.0275 mol) in methyl alcohol (60 mL) at 70 °C for 2 days. Methyl alcohol was removed at 65 °C for 1 h using a vacuum rotary flash evaporator under reduced pressure. The crude solid product was washed with acetic ether (30–40 mL) to remove the excess 1,4-dibromobutane and vacuum-dried at 40 °C in a rotary evaporator to get the final compound butane-1,4-bis(dimethyl-(3-(3-aminopropyl trisiloxane-3yl)-propyl)-ammonium bromide) (5, BBAB).

(a)

3.2. Micro-flotation tests Fig. 1 shows the effect of increasing pH on the floatability of the three aluminosilicate minerals by using BBAB, DTAB or DDA as a collector at 2.0  10 4 mol/L collector concentration (CC = 2  10 4 mol/L). The recovery–pH curves showed the recovery of the three aluminosilicate minerals dropped with increasing pulp pH values when conventional cationic surfactant DTAB or DDA was used. This was consistent with previous observations when

(b)

100

100

80

Recovery/%

Recovery/%

80

60

40

40

BBAB DTAB DDA

20

0

60

20

0 0

2

4

6

8

10

12

14

BBAB DTAB DDA

0

2

4

6

(c)

8

10

12

14

pH

pH 100

Recovery/%

80

60

40

20

0

BBAB DTAB DDA

0

2

4

6

8

10

12

14

pH Fig. 1. Recovery of kaolinite (a), pyrophyllite (b) and illite (c) as a function of pH by using BBAB, DTAB or DDA as a collector (CC = 2.0  10

4

mol/L).

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using cationic amines as collectors (Hu et al., 2003; Liu et al., 2007; Zhao et al., 2007; Zhong et al., 2008; Xia et al., 2009a,b). However, increasing pulp pH had minor influence on BBAB’s collecting abillity for aluminosilicates, even at pH values over 10. Compared with DTAB and DDA, BBAB had superior collecting ability, e.g. at pH 8.0, the flotation recoveries of kaolinite, pyrophyllite and illite were about 98%, 96% and 91% for BBAB, whereas, only about 71%, 76% and 63% for DTAB, and 56%, 65% and 55% for DDA respectively. The collecting ability of the three cationic surfactants to aluminosilicates followed the order as: BBAB > DTAB > DDA. The flotation responses of the aluminosilicate minerals as a function of the initial concentration of collectors were presented in Fig. 2, and the pulp pH maintained at 8. As can be observed from Fig. 2, the recoveries of the three aluminosilicates increased with increasing collector amounts until a maximum was reached. It also indicated that the Gemini trisiloxane collector BBAB was more efficient than DTAB and DDA, particularly when the dosage of collectors was below 3  10 4 mol/L.

The bending and stretching vibration of N–C bonds of BBAB appeared at 1467.18 cm 1 for kaolinite, 1466.17 cm 1 for pyrophyllite and 1464.22 cm 1 for illite (Pan, 1994), respectively. Fig. 3 also indicated the characteristic peaks of BBAB had insignificant changes after it interacted with the three aluminosilicates.

3.4. Zeta potential measurement Zeta potential of the aluminosilicate minerals as a function of pH in the absence and presence of BBAB were investigated, and the results were shown in Fig. 4. Fig. 4 showed that the isoelectric point (IEP) of kaolinite, pyrophyllite and illite was 3.3, 3.1 and 2.5 respectively, which was in accordance with those previously reported (Zhao, 2003b; Guan et al., 2009; Xia et al., 2009c). The zeta potential of kaolinite, pyrophyllite and illite showed a pronounced shift towards positive zeta potentials in the presence of BBAB, indicating that Gemini trisiloxane collector molecules positively charged were adsorbed onto aluminosilicate minerals through electrostatic force and the electrostatic force was definitely the main mechanism in the flotation. In addition, after BBAB adsorption, all three aluminosilicates became highly positively charged in the pH range tested (2–12). The reason was that BBAB formed aggregates (hemi-micelles) on the surface of minerals because of hydrophobic interactions between the hydrocarbon tails through London dispersion forces, which was in accordance with those previously reported (Gaudin and Fuerstenau, 1955; Somasundaran et al., 1964; Fuerstenan et al., 1964; Somasundaran and Fuerstenau, 1966; Gao et al., 1987; Li and Linda, 2005; Jiang et al., 2013).

3.3. FTIR-spectrum analysis The FTIR spectra of aluminosilicates before and after interaction with BBAB were presented in Fig. 3. The spectra of kaolinite, pyrophyllite and illite were consistent with those previously reported (Wen, 1989; Zhao et al., 2003a; Zhong et al., 2008). After interaction with BBAB, new bands at around 2924.42 and 2855.56 cm 1 for kaolinite, around 2926.34 and 2855.62 cm 1 for pyrophyllite, and around 2925.26 and 2856.91 cm 1 for illite were assigned to the stretching bands of –CH3 and –CH2 groups in BBAB.

(b) 100

80

80

Recovery/%

100

60

40

0

60

40

20

BBAB DTAB DDA

20

0

1

2

Dosage/1

3

4

5

0

6

BBAB DTAB DDA

0

1

2

10 mol/L

(c)

3

Dosage/1

-4

4

10-4mol/L

100

80

Recovery/%

Recovery/%

(a)

60

40

BBAB DTAB DDA

20

0

0

1

2

Dosage/1

3

4

5

6

-4

10 mol/L

Fig. 2. Recovery of kaolinite (a), pyrophyllite (b) and illite (c) as a function of reagent dosage at pH 8.

5

6

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Pyrophyllite

Kaolinite

Kaolinite with BBAB 2855.56 2924.42

4000

3500

3000

2500

Pyrophyllite with BBAB 1466.17

1467.18 2855.62 2926.34

2000

1500

Wavenumbers/cm

1000

500 4000

3500

3000

-1

2500

2000

Wavenumbers/cm

1500

1000

500

-1

Illite

Illite with BBAB 1464.22

2856.91 2925.26

4000

3500

3000

2500

2000

Wavenumbers/cm

1500

1000

500

-1

Fig. 3. FTIR spectra of aluminosilicates before and after interaction with BBAB.

3.5. Properties of cationic groups by DFT calculation The optimized geometries of BBAB2+, DTAB+ and DDA+ cationic group were shown in Fig. 5. Some selected atomic charges, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies of these species were described in Table 2. Table 2 indicated that the HOMO values of BBAB2+, DTAB+ and DDA+ cation were very low, thus their electron-donating ability was very weak. Their HOMO were mainly constituted by Px or Pz-orbits of carbon atoms which had fully filled valence orbitals and had no chance to offer P-orbital electrons to other atoms. The LUMO values of BBAB2+, DTAB+ and DDA+ were also low and the LUMO were mainly composed of an S-orbit of N, C and H atoms, indicating that LUMO cannot accept feedback electrons to form pbonds. Therefore, BBAB2+, DTAB+ and DDA+ had difficulty forming a covalent bond with the aluminum atom on the mineral surfaces, consistent with the above FTIR spectra results (Fig. 3). But the Mulliken charges of the cationic group -CH2N+(CH3)2(CH2)4(CH3)2N+CH2–, CH2N+(CH3)3 and CH2N+H3 were 1.7324, 0.8561 and 0.8155, respectively, indicating that BBAB2+, DTAB+ and DDA+ cations were easily adsorbed on the negative charge sites of mineral surface through electrostatic attraction, consistent with the above zeta potential results (Fig. 4). 4. Discussion Kaolinite [Al4Si4O10(OH)8], pyrophyllite [Al2Si4O10(OH)2] and illite [K1 x(H2O)x{Al2[AlSi3O10](OH)2 x(H2O)x}] are layer-structured

aluminosilicates, which can form two different planes (basal planes and edges planes) with different properties during crushing. The basal planes displaying an inert siloxane structure are constantly negatively charged both in acidic and in alkaline media, due to the isomorphous substitution of Al3+ for Si4+ and Mg2+/ Fe3+ for Al3+ (Rand and Melton, 1977; Newman, 1987; Herrington et al., 1992; Brady et al., 1996). However, the atomic structure of the edges planes on kaolinite, pyrophyllite and illite is totally different from that of the basal planes (Hu et al., 2005; Zhong et al., 2008; Guan et al., 2009). At the edges, the tetrahedral silica sheets and octahedral alumina sheets are destroyed, resulting in a surface of broken Si–O and Al–O covalent bonds. The charging mechanism of the edges can be described with the traditional model for oxides and silicates (Scheme 2). Depending on solution pH, the edges is positively charged due to H+ ion adsorption in acidic solution or negatively charged by adsorption of OH or by dissociation of H+ in alkaline solution. As the cationic collector BBAB, DTAB and DDA are introduced into pulp of pH less than the isoelectric point (IEP) of the aluminosilicates, their cations might be electrostatically attracted to the permanent negative basal planes on the aluminosilicate surfaces; In a pH range of IEP < pH < 12, the surface of aluminosilicates would be negatively charged, and thus cations of BBAB, DTAB and DDA might be attracted to the aluminosilicate surfaces by electrostatic forces too. The electrostatic effect is directly proportional to the grouping charge of collector, thereby the collecting ability of these collectors for aluminosilicates was in order as BBAB2+ > DTAB+ > DDA+ (Table 2), consistent with the above flotation test results (Fig. 1 and Fig. 2).

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80 60 60 40

- potential/mV

- potential/mV

40 20 0 -20 -40

0

2

4

6

0 -20 -40

Kaolinite Kaolinite with BBAB

-60

20

Pyrophyllite Pyrophyllite with BBAB

-60 8

10

12

0

14

2

4

6

8

10

12

14

pH

pH 80

- potential/mV

60 40 20 0 -20 -40

Illite Illite with BBAB

-60 0

2

4

6

8

10

12

14

pH Fig. 4. The zeta potential of aluminosilicates as a function of pH in the absence and presence of BBAB.

At pH < 7, the basal planes of kaolinite, pyrophyllite and illite are negatively charged, and the edges are positively charged (Hu et al., 2005; Guan et al., 2009; Liu et al., 2009; Jiang et al., 2013). The positively charged BBAB, DTAB and DDA molecules adsorbed onto the negatively charged basal planes, with self-aggregation between basal planes and the edge planes. The adsorption of BBAB, DTAB and DDA at the basal planes but not the edges made the aluminosilicates strongly hydrophobic and likely to form electrostatic-aggregates. Therefore, better floatability of kaolinite, pyrophyllite and illite was achieved in acidic solution (Hu et al., 2005). In the pH range of 7 < pH < 12, both edges and basal planes are negatively charged, thus the kaolinite, pyrophyllite and illite particles were readily dispersed in the pulp. The compensatory ions (neutralising the negative charge caused by isomorphous substitution of Si4+ by Al3+ in the surface of aluminosilicates) such as K+, Na+, Mg2+ and H+ could dissolve in aqueous solution, and the solubilities of those ions increased with increasing pH. As a result, the adsorption competition between BBAB, DTAB and above compensatory ions onto the aluminosilicate surfaces weakened the adsorption ability of BBAB2+, DTAB+ cations and then decreased the flotation response. In alkaline pH pulp, cationic DDA+ might react with free hydroxyls [OH–] to form free dodecylamine and water (Liu et al., 2009, 2011). Neutral dodecylamine could not interact electrostatically with negatively charged surfaces. Moreover, the OH– ions in aqueous solutions could also bind dodecylamine molecules through the formation of hydrogen bonds, which weakened the hydrogen bond interaction between N–H groups of dodecylamine molecules and oxygen atoms in aluminosilicate surfaces and further decreased the adsorption of dodecylamine on the sur-

faces of the three aluminosilicates. As a result, using DDA as a collector, the recoveries of kaolinite, pyrophyllite and illite in the alkaline pH pulp were, therefore, far less than in the acidic pulp. BBAB is different from DTAB and DDA, it consists of an inorganic siloxane backbone with pendant organic methyl groups. Both the nature of this backbone and the pendant organic groups contribute to the surface activity of BBAB. The prime role of the backbone is to present the available organic groups to their best advantage and this it does by virtue of its unique backbone flexibility (Michael, 1980). In most hydrocarbon systems, the bond angles are very fixed and steric packing considerations often prevent the available methyls from adopting lowest surface energy orientations. The siloxane backbone, because of the electron orbital interactions between silicon and oxygen, was a much more extended and flexible chain system than C–C–C or C–O–C backbone (Michael, 1980). The selected optimized bond lengths and bond angles of the BBAB2+, DTAB+ and DDA+ cationic group was given in Table 3, and some chain structure parameters for pure compounds reported by Leslie (1985) are also shown in Table 4. The theoretically obtained results by DFT calculation are similar to those previously reported. Compared to the carbon–carbon and carbon–oxygen situation, the siloxane has the flattest angle and the longest bond length, and the wide variability of the Si–O–Si bond angle also contributes to the backbone flexibility. When rotation around a bond occurs, the potential energy usually varies as a function of a rotation angle, however, the rotation about the siloxane bond was nearly free (Tobolsky, 1960). When BBAB is added to the water, it was predicted that all the Si–O bonds would align with the water surface. This was due to the Si–O bond being 50% ionic by Pauling electronegativity calculations, and polar interactions between these

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BBAB

2+

DTAB+

DDA+ Fig. 5. Optimized geometries of BBAB2+, DTAB+ and DDA+ cations at the B3LYP/6-31G (d) level.

Scheme 2. The charging mechanism of the aluminosilicate edges.

Table 2 The frontier orbital eigenvalues and some selected Mulliken charges of the BBAB2+, DTAB+ and DDA+ cationic group at B3LYP/6-31G(d) level. Reagents

Eigenvalues/a.u.

Mulliken charges/electron

HOMO

Atomic charges

LUMO

BBAB2+

0.2279

0.0365

1

DTAB+

0.3493

0.1134

13

Groups

N: 0.8372, 8N: 0.8372, 9C: 0.2773,

N: 0.3697, H:0.2249 13 N: 0.7318,

63

H:0.2085,

14

C: 0.3299,15C: 0.3351,

64

H:0.1937,

16

C: 0.3351,

43

65

H:0.2193

H:0.2202,

45

H:0.2176,

–CH2N+(CH3)2(CH2)4 (CH3)2N+CH2– :1.7324 –CH2N+(CH3)3:0.8561

49

DDA+

0.3516

0.1664

39

H:0.4289,

40

H:0.4289,

41

H:0.4334,

37

H:0.2314

–CH2N+H3:0.8155

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bonds and the water provided the driving force for this highly ordered configuration. Furthermore, the side away from the water phase was dominated by methyl groups of hydrophobic character, and the BBAB’s methyl groups could also act like ‘‘parachute’’ and would offer significant contributions in inducing hydrophobicity when adsorbed on aluminosilicate interfaces. However, DTAB and DDA contain –C12H25 hydrophobes, which consist primarily of -CH2– groups, and these pack loosely at the air/water interface. The ‘‘parachute’’ shape structure of BBAB was contrasted with that of DTAB and DDA, which was listed in Fig. 6. It is a well-known result of Zisman’s studies that –CH2– groups have an intrinsically higher surface energy than methyl groups (Shafrin and Zisman, 1960; Zisman, 1964), thus BBAB could achieve a lower surface tension in water than DTAB and DDA. Previous research indicated that trisiloxane surfactants could reduce the surface tension of water down to the low value of 16–20 mN/m (Milton and Song, 1996; Svitova et al., 1996, 1998; Wagner et al., 2000;

Table 3 Selected optimized bond lengths and bond angles of the BBAB2+, DTAB+ and DDA+ cationic group. Reagents

Bond length (Å)

Bond angle (deg)

BBAB2+

45

41

DTAB+ DDA+

Si–39O:1.67417, 37 Si–38O:1.65404 1 C–2C:1.53199, 2 C–3C:1.53425 1 C–2C:1.53190, 2 C–3C:1.53418

Si–38O–37Si:155.2964, Si–39O–37Si:151.3031 1 C–2C–3C:113.17154, 2 C–3C–4C:113.55190 1 C–2C–3C:113.17657, 2 C–3C–4C:113.47554 45

Table 4 Bond lengths and bond angles for selected pure compoundsa.

a

Compound

Bond length/Å

Bond angle/deg

Hexamethyldisiloxane Dimethyl ether Propane

Si–O 1.63 C–O 1.42 C–C 1.54

Si–O–Si 130 C–O–C 111 C–C–C 112

All values from Leslie (1985).

(b)

(c)

(a)

Fig. 6. The ‘‘parachute’’ shape structures of (a) Gemini trisiloxane surfactant BBAB and the structures of hydrocarbon surfactant (b) DTAB and (c) DDA, and the flotation mechanism for aluminosilicate minerals.

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Churaev et al., 2001a,b; Tatiana et al., 2001; Chengara et al., 2002; Wu and Milton, 2002; Nieholas et al., 2006; Jovana et al., 2010), however, DTAB and DDA could only reduce the surface tension of water down to 39 mN/m (Rosen et al., 1999) and 31 mN/m (Monte and Oliveira, 2004) respectively. A low surface tension of water was beneficial to the formation of stable and smaller bubbles in water (Srdjan, 2007; Xia et al., 2009b). Moreover, as a result of addition of surfactants BBAB, DTAB and DDA, the air bubbles formed under the surface of water were partly lined with a monomolecular sheath of BBAB, DTAB and DDA molecules that reduced the air–water interfacial tension (Fig. 6). This allowed each bubble with its lining to approach other bubbles without coalescing. Because BBAB was much more efficient than DTAB and DDA at decreasing the air–water interfacial tension, the resistance to bubble coalescence and the stability of froth formed by BBAB were better than that by DTAB and DDA. When collectors BBAB, DTAB and DDA are added to the pulp, because of electrostatic attraction between the collector polar portions and aluminosilicate surface sites, the collectors might adsorb on the particles with their non-polar ends orientated towards the bulk solution, thereby converting the surfaces of aluminosilicates from a hydrophilic condition to a hydrophobic condition, which created the required condition for attachment to air bubbles (Fig. 6). The trisiloxane group occupied a larger volume than the C12H25 group (Hill, 1998), and this increased the collision probability between the hydrophobic aluminosilicates and bubbles. Meanwhile, the hydrophobic aluminosilicates attached to bubbles via collector’s non-polar groups orientated towards the air (Fig. 6), and the surface energy of BBAB’s non-polar groups was lower than that of DTAB and DDA due to its unique ‘‘parachute’’ shape structure, thus the attachment between aluminosilicates and froth became more stable by using BBAB as a collector compared with DTAB and DDA. 5. Conclusions In this study, a new Gemini surfactant BBAB bearing the aminotrisiloxane as the headgroup was synthesized. Micro-flotation tests indicated that increasing pulp pH had minor influence on its collecting performance as the floatability of kaolinite, pyrophyllite and illite varied slightly with its change even for pulps with pH > 10, however, the floatability of aluminosilicates decreased in alkaline condition when using the conventional cationic collector DTAB or DDA. The collecting power on aluminosilicates increased according to the following order: BBAB > DTAB > DDA. About 98% of kaolinite, 96% of pyrophyllite and 91% of illite were recovered under the test condition pH 8 and BBAB 2  10 4 mol/L. The interaction between the collector BBAB and the three aluminosilicate surfaces consisted of a dominant electrostatic attraction force and this was confirmed by FT-IR spectra and zeta potential measurements. Using the density functional theory (DFT), we concluded that the theoretically obtained results were consistent with the flotation tests, FTIR spectra studies and zeta potential measurements, showing BBAB to be superior than DTAB and DDA as a collector agent. Acknowledgments The authors express their appreciation for the support of the National High-Tech Research and Development Program of China (863 Program) (Grant No. 2013AA064102), the Hunan Provincial Innovation Foundation For Postgraduate (Grant Nos. CX2013B044, CX2012B120), the National Natural Science Foundation of China (Grant No. 51004114), the Foundation for the Author

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of National Excellent Doctoral Dissertation of China (2007B52) and the Program for New Century Excellent Talents in Chinese University (NCEP-08-0568). This project is also supported by High Performance Computing Center of CSU, China. References Brady, P.V., Cygan, R.T., Nagy, K.L., 1996. Molecular controls on kaolinite surface charge. J. Colloid Interf. Sci. 183, 356–364. Cao, X.F., Hu, Y.H., Jiang, Y.R., Li, H.P., 2001. Flotation mechanism of aluminium silicate minerals with N-dodecyl-1,3-diaminopropane. Chin. J. Nonferrous Metal. 11, 693–696 (in Chinese). Cao, X.F., Hu, Y.H., Xu, J., 2004. Synthesis of c-alkoxy-propylamines and their collecting properties on aluminosilicate minerals. J. Cent. South Univ. Technol. 11, 280–286 (in Chinese). Chengara, A., Nikolov, A.D., Wasan, D., 2002. Surface tension gradient driven spreading of trisiloxane surfactant solution on hydrophobic solid. Colloid Surf. A: Physicochem. Eng. Aspects 206, 31–39. Churaev, N.V., Ershov, A.P., Esipova, N.E., 2001a. Application of a trisiloxane surfactant for removal of oils from hydrophobic surfaces. Langmuir 17, 1349– 1356. Churaev, N.V., Esipova, N.E., Hill, R.M., Sobolev, V.D., Starov, V.M., Zorin, Z.M., 2001b. The superspreading effect of trisiloxane surfactant solutions. Langmuir 17 (5), 1338–1348. Fuerstenan, D.W., Healy, T.W., Somasundaran, P., 1964. The role of hydrocarbon chain of alkly collectors in flotation. Trans. AIME 229, 321–325. Gao, Y.Y., Du, J.Z., Gu, T.R., 1987. Hemimicelle formation of cationic surfactants at the silica gel–water interface. J. Chem. Soc. 83 (8), 2671–2679. Gaudin, A.M., Fuerstenau, D.W., 1955. Quartz flotation with cationic colleclors. Trans. AIME 202, 958–962. Guan, F., Zhong, H., Liu, G.Y., Zhao, S.G., Xia, L.Y., 2009. Flotation of aluminosilicate minerals using alkylguanidine collectors. Trans. Nonferrous Met. Soc. China 19, 228–234. Herrington, T.M., Clarke, A.Q., Watts, J.C., 1992. The surface charge of kaolin. Colloids Surf. 68, 161–169. Hill, R.M., 1998. Superspreading. Curr. Opin. Colloid Interface Sci. 3, 247–254. Hu, Y.H., 2003. Progress in flotation de-silica. Trans. Nonferrous Met. Soc. China 13, 656–662. Hu, Y.H., Cao, X.F., Li, H.P., Jiang, Y.R., Du, P., 2003. Synthesis of N-decyl-l, 3diaminopropanes and its flotation properties on aluminium silicate minerals. Trans. Nonferrous Met. Soc. China 13, 417–420. Hu, Y.H., Sun, W., Jiang, H., Miller, J.D., Fa, K.Q., 2005. The anomalous behavior of kaolinite flotation with dodecyl amine collector as explained from crystal structure considerations. Int. J. Miner. Process. 76, 163–172. Huang, Z.Q., Zhong, H., Wang, S., Xia, L.Y., Liu, G.Y., 2013. Comparative studies on flotation of aluminosilicate minerals with Gemini cationic surfactants BDDA and EDDA. Trans. Nonferrous Met. Soc. China 23, 3055–3062. Iwasaki, I., 1983. Iron ore flotation, theory and practice. Miner. Eng. 35, 622–631. Jiang, H., Hu, Y.H., Qin, W.Q., Wang, Y.H., Wang, D.Z., 2001. Mechanism of flotation for diaspore and aluminum-silicate minerals with alkylamine collectors. Chin. J. Nonferrous Metal. 11, 688–692 (in Chinese). Jiang, H., Liu, G.R., Hu, Y.H., Xu, L.H., Yu, Y.W., Xie, Z., Chen, H.C., 2013. Flotation and adsorption of quaternary ammonium salts collectors on kaolinite of different particle size. Int. J. Mining Sci. Technol. 23, 249–253. Jovana, R., Khellil, S., Martin, E.R., 2010. Dynamics of trisiloxane wetting: effects of diffusion and surface hydrophobicity. J. Phys. Chem. C 114 (32), 13620–13629. Leslie, S., 1985. Tables of Interatomic Distances and Configuration in Molecules and Ions. Spec. Publ. No. 11. Chemical Society, London. Li, Z.H., Linda, G., 2005. Surface configuration of sorbed hexadecyltrimethylammonium on kaolinite as indicated by surfactant and counterion sorption, cation desorption, and FTIR. Colloid Surf. A: Physicochem. Eng. Aspects 264, 61–67. Liu, J.R., Liu, X.G., 2005. Application of treating middle and low grade bauxite by oredressing bayer process in alumina production. Light Metals 4, 11–14 (in Chinese). Liu, G.Y., Zhong, H., Hu, Y.H., Zhao, S.G., Xia, L.Y., 2007. The role of cationic polyacrylamide in the reverse flotation of diasporic bauxite. Miner. Eng. 20, 1191–1199. Liu, C.M., Hu, Y.H., Cao, X.F., 2009. Substituent effects in kaolinite flotation using dodecyl tertiary amines. Miner. Eng. 22, 849–852. Liu, C.M., Hu, Y.H., Feng, A.S., Guo, Z.X., Cao, X.F., 2011. The behavior of N,N-dipropyl dodecyl amine as a collector in the flotation of kaolinite and diaspore. Miner. Eng. 24, 737–740. Michael, J.O., 1980. The surface activity of silicones: a short review. Ind. Eng. Chem. Prod. Res. Dev. 19 (1), 97–103. Milton, J.R., Song, L.D., 1996. Superspreading, skein wetting, and dynamic surface tension. Langmuir 12 (20), 4945–4949. Monte, M.B.M., Oliveira, J.F., 2004. Flotation of sylvite with dodecylamine and the effect of added long chain alcohols. Miner. Eng. 17, 425–430. Newman, A.C.D., 1987. Chemistry of Clays and Clay Minerals. Longman Group UK Limited, London. Nieholas, A.A.R., Zhengeheng, Z., Sehneider, Y., 2006. Synthesis and characterization of tetra- and trisiloxane-containing o1igo(ethylene glycol) highly conducting electrolytes for lithium batteries. Chem. Mater. 18, 1289–1295.

154

Z. Huang et al. / Minerals Engineering 56 (2014) 145–154

Pan, Z.L., 1994. Crystallography and Mineralogy. Beijing Geology Press, pp. 164–182. Rand, B., Melton, I.E., 1977. Particle interactions in aqueous kaolinite suspensions. J. Colloid Interf. Sci. 60 (2), 308–320. Rosen, M.J., Mathias, J.H., Davenport, L., 1999. Aberrant aggregation behavior in cationic Gemini surfactants investigated by surface tension, interfacial tension, and fluorescence methods. Langmuir 15, 7340–7346. Shafrin, E.G., Zisman, W.A., 1960. Constitutive relations in the wetting of low energy surfaces and the theory of the retraction method of preparing monolayers. J. Phys. Chem. 64 (5), 519–524. Smith, R.W., Akhtar, S., 1976. Cationic flotation of oxide and silicates. In: Fuerstenau, M.C. (Ed.), Flotation-A.M. Gaudin Memorial, vol. 1. AIME, p. 87. Somasundaran, P., Fuerstenau, D.W., 1966. Mechanisms of alkylsulfonate adsorption at the alumina-water interface. J. Phys. Chem. 70, 90–97. Somasundaran, P., Healy, T.W., Fuerstenau, D.W., 1964. Surfactant adsorption at the solid–liquid interface-dependence of mechanism on chain length. J. Phys. Chem. 68, 3562–3566. Srdjan, M.B., 2007. Handbook of Flotation Reagents: Chemistry, Theory and Practice: Flotation of Sulfide Ores. Elsevier Science & Technology Books. Svitova, T., Hoflhlann, H., Hill, R.M., 1996. Trisiloxane surfactants: surface/interfacial tension dynamics and spreading on hydrophobic surfaces. Langmuir 12, 1712– 1721. Svitova, T., Hill, R.M., Smirnova, Y., 1998. Wetting and interfacial transitions in dilute solutions of trisiloxane surfaetants. Langmuir 14, 5023–5031. Tatiana, F.S., Hill, R.M., Radke, C.J., 2001. Spreading of aqueous trisiloxane surfactant solutions over liquid hydrophobic substrates. Langmuir 17, 335–348. Tobolsky, A.V., 1960. Properties and Structure of Polymers. Wiley, New York, p. 67. Wagner, R., Wu, Y., Perepelittchenko, L., 2000. Silicon-modified surfactants and wetting: 4. Spreading behaviour of trisiloxane surfactants on energetically different solid surfaces. Appl. Organometal. Chem. 14, 177–188. Wen, L., 1989. Mineral Infrared Spectroscopy. Chongqing University Press, Chongqing. Wu, Y.F., Milton, J.R., 2002. Superspreading of trisiloxane surfactant mixtures on hydrophobiesurfaces: 2. Interaction and spreading of aqueous trisiloxane

surfactant-N-alkyl-pyrolidinone mixtures in contact with polyethylene. Langmuir 18, 2205–2215. Xia, L.Y., Zhong, H., Liu, G.Y., Wang, S., 2009a. Utilization of soluble starch as a depressant for the reverse flotation of diaspore from kaolinite. Miner. Eng. 22, 560–565. Xia, L.Y., Zhong, H., Liu, G.Y., Huang, Z.Q., Chang, Q.W., 2009b. Flotation separation of the aluminosilicates from diaspore by a Gemini cationic collector. Int. J. Miner. Process 92, 74–83. Xia, L.Y., Zhong, H., Liu, G.Y., Huang, Z.Q., Chang, Q.W., Li, X.G., 2009c. Comparative studies on flotation of illite, pyrophyllite and kaolinite with Gemini and conventional cationic surfactants. Trans. Nonferrous Met. Soc. China 19, 446– 453. Xia, L.Y., Zhong, H., Liu, G.Y., 2010. Flotation techniques for separation of diaspore from bauxite using Gemini collector and starch depressant. Trans. Nonferrous Met. Soc. China 20, 495–501. Yoon, R.H., Shi, J., 1989. Processing of kaolin clay. In: Samsoundaran, P. (Ed.), Advances in Mineral Processing. SME, pp. 366–379. Yoon, R.H., Nagaraj, D., Wang, S., Hilderand, T., 1992. Beneficiation of kaolin clay by froth flotation using hydroxamate collectors. Miner. Eng. 5, 457–467. Zhang, H.L., He, J.H., Zhang, Y., 2006. Status and countermeasure of alumina industry in China. Light Metals. 2, 3–7 (in Chinese). Zhao, S.M., Wang, D.Z., Hu, Y.H., Liu, B.D., Xu, J., 2003a. The flotation behaviour of N(3-aminopropyl)-dodecanamide on three aluminosilicates. Miner. Eng. 16, 1391–1395. Zhao, S.M., Wang, D.Z., Hu, Y.H., Bao, X.S., Xu, J., 2003b. Flotation of aluminosilicates using N-(2-aminoethyl)-1-naphthaleneacetamide. Miner. Eng. 16, 1031–1033. Zhao, S.G., Zhong, H., Liu, G.Y., 2007. Flotation effect of quaternary ammonium salt collector on aluminosilicate minerals. Metal. Mine 2, 45–47 (in Chinese). Zhong, H., Liu, G.Y., Xia, L.Y., Lu, Y.P., Hu, Y.H., Zhao, S.G., Yu, X.Y., 2008. Flotation separation of diaspore from kaolinite, pyrophyllite and illite using three cationic collectors. Miner. Eng. 21, 1055–1061. Zisman, W.A., 1964. Relation of the equilibrium contact angle to liquid and solid constitution. Adv. Chem. Ser. 43 (1), 1–51.