Modified floc-flotation in fine sericite flotation using polymethylhydrosiloxane

Modified floc-flotation in fine sericite flotation using polymethylhydrosiloxane

Accepted Manuscript Modified floc-flotation in fine sericite flotation using polymethylhydrosiloxane Jingjing Tian, Huimin Gao, Junfang Guan, Zijie Re...

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Accepted Manuscript Modified floc-flotation in fine sericite flotation using polymethylhydrosiloxane Jingjing Tian, Huimin Gao, Junfang Guan, Zijie Ren PII: DOI: Reference:

S1383-5866(16)30281-7 http://dx.doi.org/10.1016/j.seppur.2016.10.051 SEPPUR 13321

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

4 May 2016 7 September 2016 24 October 2016

Please cite this article as: J. Tian, H. Gao, J. Guan, Z. Ren, Modified floc-flotation in fine sericite flotation using polymethylhydrosiloxane, Separation and Purification Technology (2016), doi: http://dx.doi.org/10.1016/j.seppur. 2016.10.051

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Modified floc-flotation in fine sericite flotation using polymethylhydrosiloxane By Jingjing TIANa, Huimin GAO,* a, Junfang GUAN a, Zijie RENa a

School of Resources and Environment Engineering, Wuhan University of Technology, Wuhan 430070, China

* Corresponding author Prof. Huimin GAO NO.122 Luoshi Road Hongshan, Wuhan 430070, China E-mail: [email protected] Phone number: +86- 27- 87882128

Abstract This study exploited polymethylhydrosiloxane (PMHS), a heteropolar silane coupling agent, as a new extender oil in the floc-flotation of fine sericite using coconut oil amine (COA) as collector to produce a concentrate required by the cosmetic industry. In parallel, kerosene, a non-polar oil, as the traditional extender oil was examined for a comparison. By assessing true flotation and entrainment in the flotation of fine sericite sample, coupled with the froth stability measurement, the advantages of PMHS over kerosene in fine sericite flotation were examined. It was found that PMHS was more effective extender oil than kerosene in generating larger sericite flocs while producing a drier froth with less mechanical gangue entrainment. As a result, both sericite grade and recovery were improved significantly with the new extender oil. This study also found that PMHS anchored on sericite surface directly

through chemical adsorption while bridging sericite particles through the hydrophobic methyl siloxane chain, which is different from kerosene which adsorbed on collector-coated mineral surface through hydrophobic interactions. Therefore, PMHS functioned as both collector and extender oil, enhancing the floc-flotation of fine particles without the interaction of traditional collector and extender oil. This study provides a new direction to improve fine particle flotation by the modified floc-flotation. Keywords: Floc-flotation; extender oil; silane coupling agent; entrainment; froth stability

1 Introduction In the cosmetic products, mica has been an important ingredient as the solar protector, creams, powder, eye shades, lipsticks, blushes, eyeliner, hair and body glitter [1, 2], while embellishing the physical appearance and preserving the physico-chemical conditions of skin [1, 3, 4]. The cosmetic activities of mica are mainly controlled by its unique physical properties such as small particle size [5], high aspect ratio [6], large specific surface area [3] and smooth cleavage face. Other important physio-chemical properties including optimum dispersivity [7], antibacterial activity [5, 8, 9], and organoleptic characteristics (e.g., desirable refractive index and reflectance) of mica can be improved by organosiloxane modification, a silanization process where hydroxyl groups on mica attack and displace the alkoxy groups on the silane thus forming a covalent -Si-O-Si- bond across the interface with the catalysis of NaOH [10]. The methyl siloxane chain on mica not only increases the flatness and

smoothness of the surface, but also its hydrophobicity [11]. With the depletion of high-quality mica ores on a world scale, substitutes for cosmetic mica have been exploited [12]. Sericite may replace mica in the cosmetic industry due to its natural fine particle size, high aspect ratio, silky luster and exquisite feeling. Tian et al. [13, 14] studied the characteristics of several cosmetic sericite pigments and found that sericite pigments with a high purity had the physical and physico-chemical properties required by the cosmetic industry and the dispersivity, optical aspects and lubrication of these pigments could also be improved by organosiloxane modification. Sericite is often associated with silicate minerals at fine grain sizes, and fine grinding followed by flotation is required to purify sericite ores [9]. However, the flotation of sericite ores is challenging due to the generated fine particles which display poor flotation as a result of low bubble-particle collision efficiency with high mechanical entrainment [15, 16]. Floc-flotation is a method used to enhance fine particle flotation [17]. In floc-flotation, the interaction of collector and non-polar oil which is called extender oil (typically kerosene) contributes to the aggregation of fine particles by hydrophobic attraction. When fine particles are rendered hydrophobic after the adsorption of collector, the oil interacts firstly as droplets with the hydrophobic mineral surface, and then spreads out as an oil layer. This leads to the formation of aggregated hydrophobic particles and enhanced mineral flotation [18]. The more hydrophobic the collector-adsorbed particles, the stronger the hydrophobic aggregation of fine particles [19]. Luo [20]

demonstrated that floc-flotation improved the flotation of fine sericite. However, kerosene used in floc-flotation gives sericite an unfavorable ordor and color to the cosmetic products [21]. Hence, alternative extender oil with a desired flavor and color has to be identified to replace kerosene in the floc-flotation of fine sericite. Organosiloxane which has been used to improve the physico-chemical properties of cosmetic mica and sericite may work well as ideal extender oil in the floc-flotation of fine sericite. It is established that organosiloxane can adsorb on sericite surface through the silanization process [13, 14]. It contains a long hydrophobic methyl siloxane chain which may benefits particle hydrophobilization and aggregation simultaneously. In this study, the role of organosiloxane as a new extender oil in the floc-flotation of fine sericite was examined.

2 Experiments 2.1 Materials Sericite ore was supplied by Chuzhou Grea Minerals Co., Ltd., China. The ore was wheel-ground before flotation. The particle size distribution of ground sericite ore was measured by Leica DMLP Microscope (Leica Ltd., Germany). The result is shown in Fig. 1. 90 % particles are smaller than 11 μm, and 50 % particles are smaller than 8 μm. The mineral composition of the sericite ore was analyzed by X-ray Diffraction (XRD). The major gangue minerals are quartz and feldspar. The elemental compositions of the sericite ore analyzed by X-ray fluorescence (XRF) are shown in Table 1 with 54.46% SiO2, 25.85% Al2O3, 8.46% K2O and 1.86 % Na2O. The

elemental compositions of the ore were also examined by GKF-VI Rapid Multi-element Analyzer and similar results were obtained. In this study, the elemental compositions of flotation products were examined by the Rapid Multi-element Analyzer.

Cumulative distribution (%)

100 90 80 70 60 50 40 30

Cumulative distribution (%)

20 10 0

0

2

4

6

8

10

12

14

16

18

Size (m) Fig.1. The cumulative size distribution of ground sericite ore sample. Table 1 Key elements of the sericite ore (wt.%). Elements

SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

content

58.90

23.59

1.96

1.01

1.88

5.87

Elements

Na2O

TiO2

SiO2*

Al2O3*

K2O*

Na2O*

content

1.86

0.29

58.59

23.57

5.52

1.80

* The analysis results by GKF-VI Rapid Multi-element Analyzer The XRD and XRF analyses together indicate that the ore contains 66 wt.% sericite, 16 wt.% quartz and 18 wt.% feldspar. The quartz is the main harmful constituent in cosmetic products.

2.2 Reagents All reagents were used as received without further purification. The distilled coconut oil amine (COA, CH3·(CH2)n·NH2, n=7, 9, 11, 13, 15, 17) with a purity of 98% provided by Shangdong Huarun Oil Chemical Co. Ltd was used as the collector. It was also used as the frother due to its frothing ability. H2SO4 solution was used to adjust flotation pH. Acidic sodium silicate was used as the depressant and dispersant. It was prepared by adjusting the pH of the industrial sodium silicate solution to 4 with the addition of sulfuric acid (H2SO4) solution. These reagents were used in the previous study to upgrade sericite ores by flotation [22]. The aviation kerosene or polymethylhydrosiloxane (PMHS) was used as extender oil. Kerosene (a mixture of C9~C16 hydrocarbons) has a density of 0.79 g cm−3 with 1.8 mPa·s viscosity. PMHS (C3H9OSi·(CH4OSi)n ·C3H9Si, n=45~90)provided by Dow Corning Corporation has a density of 1.006 g cm−3 with 20 mPa·s viscosity. PMHS and ethanol were mixed at a ratio of 1:2 and the resultant solution standed for 48 h at room temperature to react and release hydrogen gas under the catalysis of NaOH before the use. Wuhan tap water was used throughout the study. Ethanol, H2SO4 and NaOH were supplied by Tianjin Chemical Corporation. Industrial terpenic oil was used as the frother when the flotation was conducted in the absence of collector COA.

2.3 Methods 2.3.1 Flotation The slurry for flotation was prepared in a JM50 colloid mill with a stirring speed of

2840 rpm. 50 g sericite sample combined with 1000 mL tap water was added to the mill. Then sodium silicate, H2SO4 solution, COA, and extender oil (kerosene or PMHS) were added to the slurry and 5 min conditioning was allowed for each reagent addition. The dosage of acidic sodium silicate and collector COA was 1200 g/t and 300 g/t, respectively. The pulp pH was adjusted to 2 which was used in the previous study [23]. The emulsified slurry was then transferred to a 1 L RK/FDIII-type laboratory agitair flotation cell. In the test without the addition of collector COA, 300 g/t frother (terpenic oil) was added in the flotation cell and conditioned for 5 min. Flotation was commenced after aeration. The froth depth was controlled at about 20 mm, and the froth was scraped every 30 s. Four concentrates were collected after cumulative times of 4, 8, 14 and 22 min into four beakers. Flotation products were dried and then assayed for SiO2、Al2O3、K2O and Na2O. 2.3.2 Fourier transform infrared measurement Flotation concentrates in the presence of COA, COA+1200 g/t PMHS and COA+1200 g/t kerosene were measured by a Fourier transform infrared spectrometer (Nicolet-IS-10) to determine the adsorption of COA, kerosene and PMHS on sericite surface in a wavenumber range between 400 and 4000 cm−1. 2.3.3 Contact angle measurement Contact angles of sericite particles were measured based on the Washburn method at room temperature up to 10 min periods. The Flotation concentrates in the presence of COA, COA+1200 g/t PMHS and COA+1200 g/t kerosene were used for measuring

the contact angle. The Washburn method makes use of capillary pressure to drive a liquid at an observable rate through a packed bed of particles in a capillary tube. The wetting velocity is then related to the advancing contact angle. Two approaches are commonly used, height versus time (height of the liquid front) and weight versus time (imbibition rate of measuring liquid). The method gives the advancing contact angle only. In this study, the height versus time approach was used. 1 g sample was packed in the quartz tube and liquid height gain in hexane and tap water was determined, followed by contact angle analysis. 2.3.4 Microscope analysis of flocs The samples collected before flotation by using a wide-bore pipette were placed on microslides which were gently shaken to spread the particles. The microslides were transferred to Leica DMLP microscope (Leica Ltd., Germany) to observe the shape and size of flocs formed before the flotation. The magnification time of microscope objective lens and ocular lens was 20 and 10, respectively. 2.3.5 Froth stability measurement Air recovery was determined in the same flotation cell by measuring the vertical froth rising velocity under agitated condition. Air recovery (α) is the fraction of air introduced in the pulp that overflows the cell as unbroken bubbles and is defined in equation (1) [24].



Qout   f  l  w  Q Q

(1)

where l and w are the length and width of the flotation cell walls (surrounding froth) respectively. Qout is the volumetric flowrate of air leaving the top surface of the froth, Q is the volumetric flowrate of air introduced into the cell, ζ is the fraction of air in the overflowing froth and is often considered as unity, and νf is the vertical velocity of the bubbles on top of the froth. The flotation cell used in the current study was equipped with a digital capturing camera. During experiments, the height of the froth overflowing the flotation cell lip was measured. The width of flotation cell lip w is also known and the fraction of air in the overflowing froth is approximately assumed as unity. With the input air volumetric flowrate known, the air recovery was calculated.

3 Results 3.1 Flotation performance The flotation of the sericite ore was conducted in the absence and presence of extender oil, kerosene or PMHS, with different dosages. Fig. 2 shows the cumulative sericite recovery as a function of the cumulative sericite grade. In the absence of any extender oil, sericite flotation was very poor with only 25.1% recovery at 83.5% grade at the end of flotation. The poor sericite flotation was expected due to fine sericite particles generated after grinding. In the presence of kerosene, sericite recovery increased significantly. At 600 g/t and 1200 g/t kerosene, sericite recovery was 45.5% and 50.6% at the end of flotation, respectively. The flotation rate constant was also improved in the presence of kerosene. This is reflected by the higher sericite recovery

of the first concentrate. However, the addition of kerosene decreased sericite grade which the grade-recovery curve shifted downwards. At the end of flotation, sericite grade was reduced to 80.1% at 600 g/t kerosene and 80.3% at 1200 g/t kerosene. It is interesting to find that the addition of PMHS increased both flotation recovery and grade. As can be seen from Fig. 2, at 600 g/t PMHS, sericite recovery increased to 46.4 % at the same grade at the end of flotation compared with the baseline flotation. At 1200 g/t PMHS, sericite recovery increased to 56.2% at a higher grade at the end of flotation. The addition of 1800 g/t PMHS further increased sericite recovery and grade to 61.9% and 85.4%, respectively. Obviously, PMHS is better extender oil than

Cumulative sericite grade (%)

kerosene in fine sericite flotation.

90

Without extender oil 600 g/t PMHS 1200 g/t PMHS

89 88

600 g/t kerosene 1200 g/t kerosene 1800 g/t PMHS

87 86 85 84 83 82 81 80 5

10 15 20 25 30 35 40 45 50 55 60 65

Cumulative sericite recovery (%) Fig.2. The cumulative sericite recovery as a function of cumulative sericite grade. The improved sericite flotation in the presence of extender oils may be associated with the formation of flocs which increase bubble-particle collision efficiency.

However, the presence of extender oils also changed gangue flotation indicated by the difference in grade. Fig. 3 shows the cumulative gangue recovery as a function of the cumulative water recovery in the absence and presence of extender oils. In the absence of any extender oil, a linear relationship occurred between gangue recovery and water recovery with the extrapolation of the line passing the origin. This indicates the entrainment of fine gangue minerals in flotation as demonstrated by Laplante et al. [25]. The addition of extender oil changed the gangue recovery-water recovery relationship. The addition of kerosene shifted the gangue recovery-water recovery curve upwards with a much steeper slope with increased gangue recovery and water recovery. The addition of 600 g/t and 1200 g/t PMHS reduced the slope of the gangue recovery-water recovery curve. The higher the PMHS addition, the more reduction the water recovery and gangue recovery. Compared with kerosene, PMHS produced

Cumulative gangue recovery(%)

much lower gangue recovery and water recovery. 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Without extender oil 600 g/t PMHS 1200 g/t PMHS 1800 g/t PMHS 600 g/t kerosene 1200 g/t kerosene

0

5

10

15

20

25

30

35

Cumulative water recovery(%) Fig. 3. The cumulative gangue recovery as a function of cumulative water recovery.

It is worth noting that water recovery changed after the addition of extender oils. Water recovery is often associated with froth stability. Therefore, the addition of extender oils may modify floth stability which affects the entrainment of fine gangue minerals in sericite flotation. Fine gangue minerals may also be recovered through entrapment in the formed flocs. However, this may not be a primary mechnism for gangue recovery in this study since PMHS reduced gangue recovery compared with the baseline flotation.

3.2 The formation of sericite flocs Fig.4 shows the flocs formed in the presence of PMHS and kerosene. The average size of sericite flocs in the presence of PMHS is about 100 μm (Fig.4(a)),while the average size of sericite flocs in the presence of kerosene is less than 60 μm (Fig.4(b)). Therefore, PMHS promoted larger sericite flocs than kerosene, which explains the higher sericite recovery achieved in the presence of PMHS. Kerosene is a mixture of C9~C16 hydrocarbons, while PMHS is a silane coupling with 45~90 methyl siloxanes Therefore, PMHS has a longer hydrophobic chain than kerosene, which makes it easier to form larger compact funicular flocs by catching more fine sericite particles. The microscopic images also show that the flocs in the presence of either kerosene or PMHS were loosely formed and away from other gangue mineral particles. The entrapment of gangue minerals in sericite flocs may be insignificant also given the aeration and agitation in the flotation process.

Fig.4. Microscopic images (200×magnifications) for sericite flocs in the presence of PMHS (a) and kerosene (b). The FTIR spectra of flotation concentrates in the absence and presence of extender oil kerosene and PMHS are shown in Fig. 5 (a), (b) and (c), respectively. Fig. 5(a) shows a clear band at around 3623 cm−1 which is attributed to the stretching vibration of the Al–OH groups. The band at around 1020 cm−1 is attributed to the in-plane stretching vibration of the Si (Al)–O or Si–O–Si (Al) groups. The vibration bands of the cations and the translation band of the OH groups are all below 600 cm−1, corresponding to the two absorption bands at 523 cm−1 and 470 cm−1 [26]. All these characteristic banks of sericite occurred on the concentrate obtained from baseline flotation in the absence of extender oils. However, –CH stretching group at 2923 and 2848 cm-1, –CN stretching group at 2364 cm-1 with a shoulder at 2335 cm-1 attributed to COA were not observed in the FTIR spectra. Wang et al. [23] also demonstrated that COA was

adsorbed on sericite surface by physical adsorption. Fig. 5 (b) shows that the characteristic adsorption peaks on sericite concentrate in the presence of kerosene remained unchanged in the presence of kerosene. It is known that kerosene does not have a chemical interaction with mineral surface. In floc-flotation, kerosene associates with collector by hydrophobic interactions, leading to the formation of hydrophobic flocs. By contract, Fig. 5(c) shows that the flotation concentrate in the presence of PMHS displayed vibration bands around 2974 cm-1 and 2850 cm-1 which are assigned to the antisymmetry stretching vibration and symmetrical stretching vibration of the PMHS, respectively. This suggests the chemical adsorption of PMHS on sericite surface. Gao et al. [7] also found that the tightly anchored organosiloxane on sericite surface formed an organic anti-water film which is important for the dispersity of sericite in organic emulsions.

0 4000

3500

520

820 794 744 685 932 820 794 744 685 1021 932 820 794 744 685

1021

3000 1500 -1 Wavenumber (cm )

1000

520 470

20

1624

40

1021 932

60

3628

1624

3628

80

3628

Transparence (%)

100

1624

2974 2850

120

500

Fig. 5. FTIR spectra of sericite concentrate in the presence of COA (a), COA+ 1200 g/t kerosene (b), and COA +1200 g/t PMHS (c).

The FTIR measurements indicate that the role of kerosene and PMHS in sericite flotation is differently. While both kerosene and PMHS work as an extender oil to form flocs through the hydrophobic chains, kerosene does not adsorb on sericite surface but PMHS anchors on sericite surface through chemical adsorption. To further explore the role of kerosene and PMHS in sericite flotation, contact angles of flotation concentrates in the presence of COA, COA+PMHS and COA+kerosene were measured. The experimental results are presented in Table 2. COA concentrate has the contact angle of 36°, and the COA+kerosene concentrate has the contact angle of 37°. Obviously, the addition of kerosene examed in this study only affected sericite surface hydrophobicity slightly. However, the contact angle of COA+PMHS concentrate is about 90°which means that the sericite particle surfaces are highly hydrophobic and may be subjected to hydrophobic interactions. It is documented that fine particles aggregated through hydrophobic forces and the aggregation was strongly affected by surface hydrophobicity. The greater the surface hydrophobicity, the stronger the aggregation [27]. As a result, the hydrophobic forces through the silanization process of PMHS may promote the aggregation of fine sericite particles resulting in the increased flotation of sericite particles. Table 3. Contact angle measurement (°). Flotation condition

COA

COA+PMHS

COA+kerosene

Contact angle

36

95

37

3.3 The froth stability measurement In the flotation tests, it was observed that PMHS produced drier froths which may benefit the reduction of gangue recovery. Therefore, air recovery which indicates the froth stability was measured. Table 3 showed the air recovery before the flotation in the absence and presence of 1200 g/t kerosene, and 1200 g/t PMHS. In the absence of any extender oil, the air recovery was high, about 40%. This is due to the strong frothing ability of collector COA with an amine group and a long hydrocarbon chain. In the presence of extender oil kerosene, the air recovery increased to about 50%. Obviously, the interaction of hydrophobic chains between kerosene and collector COA extended the chain length and enhanced the froth stability resulting in an increase in water recovery and mineral recovery. In the presence of extender oil PMHS, the air recovery decreased to about 34%. In fact, PMHS is also used as an antifoaming agent inducing local film thinning along the air/water interface [28].

Table 3. Air recovery (%) in the presence of different extender oils. Flotation condition

COA

COA + kerosene

COA + PMHS

Air recovery

43.1

50.4

34.3

4 Discussion This study confirms the poor flotation of fine sericite with a low recovery although a very stable froth was produced by collector COA. The addition of traditional extender oil kerosene indeed produced sericite flocs and increased sericite flotation recovery. Fig. 6(a) shows the schematic diagram of the action of extender oil kerosene in the

flotation. Based on FTIR measurements, collector COA bonded to sericite surface through physical adsorption and kerosene did not adsorb on sericite surface directly. Like the traditional theory proposed by Song et al.[18, 19], to explain the floc-flotation, kerosene as a non-polar oil associated with the hydrocarbone chain of collector

COA

through

hydrophobic

interactions,

enhanced

the

surface

hydrophobicity and bridged fine sericite particles resulting in increased sericite flotation. Unfortunately, the interaction between kerosene and COA increased froth stability leading to a higher water recovery and gangue recovery. Fig. 6(b) shows the schematic diagram of the action of extender oil PMHS in the flotation. PMHS as a heteropolar silane coupling agent adsorbed on sericite surface through a covalent -Si-O-Si- bond [10]. The long hydrophobic methyl siloxane chain bridged fine sericite particles and formed large flocs. The strong adsorption of PMHS on sericite surface and the strong hydrophobic interaction of methyl siloxane chains may allow the flocs to sustain the agitation and aeration in the flotation process. Meanwhile, PMHS reduced the froth stability leading to a lower water recovery and gangue recovery. Therefore, PMHS is superior to kerosene in the floc flotation of fine sericite as extender oil.

Fig.6. The chematic diagram of the action of kerosene (a) and PMHS (b). Fig. 6 suggests that PMHS might function as a collector to adsorb on sericite surface and also an extender oil to bridge fine sericite particles while the collecting ability of COA might be masked by PMHS in the flotation. To confirm this hypothesis, flotation was conducted in the absence of COA with PMHS being both collector and extender oil while terpenic oil was used as frother. Sericite grade and recovery at the end of flotation are shown in Fig. 7. For a comparison, sericite grade and recovery from the flotation with extender oil kerosene were also shown in Fig. 7. As can be seen, in the absence of COA, PMHS still produced much better flotation than

85

40

80 75

sericite grade (%) sericite recovery (%) 35

70

30

65

25

60

20

55 50

15

45

10

40 5

35 30

Sericite recovery (%)

Sericite grade (%)

kerosene in terms of both sericite grade and recovery.

0 PMHS

Kerosene

Extender oil

Fig.7. Sericite recovery and sericite grade in the end of flotation in the absence of

COA with PMHS as both collector and extender oil.

5 Conclusions In the floc-flotation of fine sericite to produce a cosmetic product, the traditional oil extender kerosene increased sericite recovery due to the formation of sericite flocs. However, kerosene produced overly stable froth by interacting with the collector leading to a high mechanical entrainment and a low concentrate grade. The current study indicated that PMHS as extender oil produced better floc-flotation with both sericite recovery and grade improved significantly. PMHS functioned as both collector and extender oil. It anchored on sericite surface through a covalent -Si-O-Si- bond while bridging sericite particles together by its hydrophobic methyl siloxane chain so that larger and stronger flocs formed. Meanwhile, PMHS produced a drier froth reducing the mechanical entrainment of fine gangue minerals.

Acknowledgements The

authors

greatly

appreciate

financial

support

from

the

National

Youth Science Fund Project of China (No.51504137) as well as discussions and suggestions from Yongjun PENG from University of Queensland.

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Highlights     

PMHS promoted sericite flocs and enhanced sericite flotation. PMHS, a heteropolarsilane coupling agent, was a superior extender oil. Direct anchor of PMHS on sericite surface benefited the aggregation of sericite paticles. PMHS decreased froth stability and mechanical gangue entrainment. PMHS functioned as the collector and extender oil simultaneously.