anionic collectors on flotation and adsorption of muscovite

anionic collectors on flotation and adsorption of muscovite

Colloids and Surfaces A: Physicochem. Eng. Aspects 492 (2016) 181–189 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochem...

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Colloids and Surfaces A: Physicochem. Eng. Aspects 492 (2016) 181–189

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Synergistic effect of mixed cationic/anionic collectors on flotation and adsorption of muscovite Longhua Xu a,b,∗ , Yuehua Hu b,∗∗ , Jia Tian a , Houqin Wu a , Li Wang b , Yaohui Yang c , Zhen Wang a a Key Laboratory of Solid Waste Treatment and Resource Recycle Ministry of Education, Southwest University of Science and Technology, Mianyang, Sichuan, PR China b School of Resources Processing and Bioengineering, Central South University, Changsha, Hunan, PR China c Key Laboratory of Vanadium-titanium Magnetite Comprehensive Utilization, Ministry of Land and Resources, Institute of Multipurpose Utilization of Mineral Resources, CAGS, Chengdu, Sichuan, PR China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Mixed DDA/NaOL collectors better enhances the muscovite flotation.

• Adsorption of both DDA and NaOL are enhanced by co-adsorption in the mixed systems. • More pronounced synergism of DDA/NaOL at ␣DDA = 0.25 on muscovite is observed. • Results from contact angle and adsorption tests coincide with flotation tests.

a r t i c l e

i n f o

Article history: Received 18 September 2015 Received in revised form 4 November 2015 Accepted 5 November 2015 Available online 7 November 2015 Keywords: Synergism Mixed collectors Muscovite Adsorption Flotation

a b s t r a c t The synergistic effect of the mixed cationic dodecyl amine-hydrochloride (DDA) and anionic collector sodium oleate (NaOL) at the air/water interface was investigated by the surface tension measurements. Various physicochemical properties such as surface activity parameters (CMC,  CMC ,  max , Amin ), the micellar and interfacial compositions (X1m , X1 ) and interaction parameters (␤m , ␤ ) were evaluated according to the theory of regular solutions. It is observed that the mixed DDA/NaOL at ␣DDA = 0.5 exhibit a maximum synergistic interactions at the air/water interface. The flotation and adsorption of DDA/NaOL at the solid–liquid interface was investigated by flotation tests with muscovite, contact angle measurements and adsorption measurements. The flotation results indicate that the recovery of muscovite could be achieved a maximum value (98.45%) by the mixed DDA/NaOL at ␣DDA = 0.25 at pH 7.0. Contact angle measurements further confirm that the DDA/NaOL exhibit more superior hydrophobicity and collecting property than the individual DDA and NaOL. And a more pronounced synergistic effect of DDA/NaOL

∗ Corresponding author at: Key Laboratory of Solid Waste Treatment and Resource Recycle Ministry of Education, Southwest University of Science and Technology, Mianyang, Sichuan, PR China. ∗∗ Corresponding author. E-mail addresses: [email protected] (L. Xu), [email protected] (Y. Hu). http://dx.doi.org/10.1016/j.colsurfa.2015.11.003 0927-7757/© 2016 Published by Elsevier B.V.

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on muscovite surface at ␣DDA = 0.25 is observed, which could be attributed to a favourable arrangement of adsorbing species. The adsorption results show that the individual cationic DDA can be adsorbed strongly onto the muscovite, but no significant adsorption of anionic NaOL can be detected. However, in the mixed systems, the adsorption amounts of both DDA and NaOL are enhanced due to co-adsorption. It is proved that there is also a remarkable synergistic effect of mixed DDA/NaOL collectors at the solid–liquid interface. © 2016 Published by Elsevier B.V.

1. Introduction Solution properties of mixed surfactants are more interesting than pure surfactants, from both physicochemical properties and application points of view [1–3]. Among the various types of binary surfactant systems, the mixed cationic/anionic surfactants exhibit the largest synergistic effects originated from the strong interactions between two surfactant molecules with oppositely charged head groups [4–6]. Such synergetic effects lead to a reduction of the total amount of surfactant used in a particular application, which in turn reduces both the cost and environmental impact [7,8]. Hence, it is important for a wide range of surfactant-based phenomena such as foaming, emulsication, solubilization, detergency, etc [9]. The synergism can be obtained even if they were mixed unequimolarly in a large range of ratios [10,11]. However, the mixed surfactant systems have the limitation that they form precipitates or phase separation in aqueous solution, especially for equimolar mixtures [12,13]. This drawback limits the in-depth study of such systems and most studies focused on the relatively short-chain anionic/cationic mixed systems [14]. The adsorption of surfactants at the solid–liquid interface plays an important role in such areas of industry as ore flotation, paper manufacturing, petroleum recovery, and pharmaceutical production [15]. It is well-known that mixtures of cationic–anionic surfactants possess much higher surface activities than their individual components in the air/liquid or liquid/liquid interfaces. So, it is of importance to check if such a synergism in the interfaces of air/liquid or liquid/liquid still exists in solid/liquid interfaces. The adsorption of single surfactants at solid/liquid interfaces has been investigated intensively [16,17]. However, most of works focused on the use of single surfactants [15,18]. While the studies involving surfactant mixtures were mainly on the same type surfactants or ionic—nonionic surfactant mixtures [19–21]. Numerous researchers have evaluated cationic surfactant adsorption on silica and other negatively charged surfaces [22–25]. Similarly, a number of studies have evaluated the adsorption of anionic surfactants on positively charged surfaces [26–28]. To the best of our knowledge, investigation on the adsorption of cationic–anionic surfactant mixtures on minerals was rare. So far, Synergistic effects between collectors have been observed in industrial practice. The use of mixed cationic–anionic collectors for flotation separation problems has been reported [29–31]. However, the mechanism involved has not, as yet, been fully understood. The mechanism of action of these mixtures in flotation processes, their adsorption on minerals, however, still largely unidentified. In recent years, an increased amount of effort has been devoted for investigating the adsorption mechanism of mixed anionic/cationic surfactants [32–34]. In particular, A. Vidyadhar et al. investigated the adsorption mechanism of mixed cationic alkyl diamine and anionic sulfonate/oleate collectors in feldspar-quartz flotation system [35]. And our team early works focused on the investigation of co-adsorption of the mixed surfactants on muscovite by qualitative tests and molecular dynamics simulation [36–38]. Muscovite is mainly used in electrical technology due to its high insulation properties and is used in the manufacturing of roofing

Table 1 Chemical composition of pure minerals (wt.%). Mineral

Al2 O3

Na2 O

Fe2 O3

K2 O

SiO2

Muscovite

35.90

0.64

1.96

10.41

46.16

papers, lubricants, rubber goods, etc when it is grounded to fractionlets [39]. As a representative silicate mineral, the muscovite surface chemistry/physics is of particular interest to mineralprocessing engineering. Due to the negative charge surface over the whole pH range of 2–12, the muscovite mica would not be expected to respond to anionic collectors in the absence of an activator. Conversely, the mineral is readily recovered using cationic collectors, such as amines. At pH values greater than 5, many silicates are negatively charged. This means that selective separation of muscovite mica from these minerals, using a cationic collector, may not be possible. The objective of the present investigation is to understand the underlying synergistic effect of mixed cationic/anionic collectors on flotation and adsorption of muscovite. In this work, a cationic/anionic mixture of dodecyl amine-hydrochloride (DDA) with sodium oleate (NaOL) was used for elucidating their surface properties and possible synergic effects at the air/water interface by the surface tension measurements. Subsequently, the flotation performance and adsorption of binary cationic/anionic collectors (NaOL/DDA) on muscovite were investigated by flotation tests, contact angle and adsorption measurements. It will present relevant information concerning the interaction between DDA and NaOL collector on muscovite.

2. Materials and methods 2.1. Materials The muscovite sample was obtained from Hebei province in China. The sample was crushed, hand-selected, and ground in a porcelain mill with agate ball. The ground sample screened out the −74 + 38 ␮m fractions for the flotation and adsorption experiments. Chemical composition of muscovite used for the study is shown in Table 1. The specific surface area of the powder muscovite was determined by a N2 adsorption analysis by a Micromeritics ASAP 2010C instrument and was found to be 3.474 m2 /g. The point of zero charge (PZC) of the powder muscovite has been reported to be 1.3 [36]. Contact angle measurements were taken for the rock mineral samples, which were provided by the Emsdiasum Company of America. The cationic surfactant dodecylamine and anionic surfactant NaOL with 99% purity were obtained from Sinopharm Chemical Reagent Co., Ltd. The dodecyl amine-hydrochloride (DDA) solution was prepared by mixing equimolar mixtures of dodecylamine and hydrochloric acid (HCl). The mixed NaOL/DDA collectors were freshly prepared when used to avoid precipitation. HCl and NaOH were used for pH adjustment. The water for all experiments was deionized water (Resistivity = 18.3 M*cm).

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2.2. Surface tension measurements The surface tension was measured by the ring method using a K100 automatic tensiometer (Kruss Corporation, Germany). And the environmental temperature was maintained at 25 ◦ C during the measurements. The temperature was maintained by circulating thermostated water through a jacketed vessel containing the solution. The ring was cleaned by heating in alcohol flame. The measurements were taken until constant surface tension values, which indicated that the equilibrium had been reached. In all cases the measurements were done for NaOL, DDA and the mixed surfactant DDA/NaOL in the presence of a swamping amount of electrolyte (0.1 mol/L NaCl aqueous solutions), to ensure that the interaction parameter values were accurate. The samples were stabilized for 10 min in the instrument before measurements were taken. The accuracy of ␥ measurements was within ±0.1 mN/m, i.e., the standard deviation did not exceed ±0.1 mN/m. 2.3. Flotation tests Flotation tests were carried out in a 40 mL hitch groove flotation cell. Mineral particles prepared (2 g) were placed in a plexiglass cell, and then filled with deionized water. HCl or NaOH was added to adjust the pH. After adding the desired amount of reagents, the suspension was agitated for 3 min. The flotation was conducted for 4 min. The froth products and tails were weighed separately after filtration and drying, and the recovery was calculated based on the dry weight of the products. The results of each flotation test were measured tree times in the same experimental conditions. And the average was reported as the final value. The standard deviation, which is presented as an error bar, was obtained by using Origin 9.2 to calculate based on the three measurements. It should be noted that the methods of adding collectors (mixing the two collectors together and then adding into the pulp or adding them into the pulp, respectively) have no influence on flotation results. 2.4. Contact angle measurements DSA 30 from KRUSS, Inc., Germany was used to measure the contact angles on exposed mineral surface in 0.2 mM reagent solution by a sessile liquid drop technique. A hand-picked pure single crystal muscovite prepared to the exposed surface was polished by 10 ␮m alumina powder, washed thoroughly with water, conditioned in the reagent solution at 25 ◦ C for 30 min, and subjected to the contact angle measurement. The last process was repeated for the repolishing prior to each measurement. The pH of the solution was adjusted individually using HCl and NaOH. An average of five readings was taken for each measurement. And the standard deviation was obtained as mentioned previously.

Fig. 1. Standard curves of the linear relationship between OC and ON in DDA.

calculated by the measured amount of OC. In the case of mixed cationic/anionic collectors DAA/NaOL, total organic carbon (TOC) and total organic nitrogen (TON) are measured, respectively. Then, according to the corresponding relations in Fig. 1, OC of DAA can be obtained by the amount of ON. OC of NaOL can be gotten when OC of DAA is subtracted from TOC in DAA/NaOL. All the experiments were done at 25 ◦ C, and the pH was maintained constant at 7.0 ± 0.5. The experiments were repeated at least three times and the average data were plotted. The sample preparation process is as follows: the pure mineral particles (2.0 g) were placed in a Plexiglas cell (40 mL), which was filled with 35 mL of deionized water. A certain amount of collectors were added to the cell in the way mentioned previously for flotation tests and conditioned with stirring for 5 min. The conditioned pulp suspension was then added in a 50 mL vial. The vial was added with deionized water to make 50 mL. The adsorption samples were obtained by shaking the vials for 24 h in an air-thermostat at 25 ◦ C. The supernatants were measured by Elementar liquid TOCII. The amount of collector adsorbed on the muscovite particles was calculated as:  =

V (C0 − C) mA

(1)

where C0 and C are the initial and supernatant concentrations, respectively; V is the solution volume; m is the amount of the particles per sample; A is the mineral specific surface area. 3. Results and discussion 3.1. The flotation of mixed collectors (DDA/NaOL) on muscovite

2.5. Adsorption measurements Elementar liquid TOCII (German, Elementar Co.), which is often used to determine the total organic carbon and total organic nitrogen concentration in organic matter, was used to measure the adsorption amount of NaOL, DAA and DAA/ NaOL on muscovite. Some DAA standard solutions with known concentrations were measured to find the test range of 0 ∼ 500 mg/L by this instrument. Thus, as-prepared surfactant solutions should be diluted to the measurement range. In the experiment we discovered that there was a large deviation of DAA concentration calculated by organic nitrogen, due to low concentration of the organic nitrogen (ON) in the tested DAA solution. But it is also discovered that the concentration of organic carbon (OC) and ON in DAA has a good linear relationship, and the linear correlation coefficient is 0.999. The results are showed in Fig. 1. NaOL solution concentration is directly

The mole ratio of the mixed collectors (DDA/NaOL) was found to be important factors in the muscovite flotation tests. The flotation response of the minerals independently with the mole fraction of DAA (␣DDA ) is first assessed. Fig. 2 shows the flotation performances of mixed DDA/NaOL with various ␣DDA at pH 7.0. The recoveries are presented at 0.2 mM initial mixed collectors. The results show that the muscovite recovery sharply increase with increasing ␣DDA , reaching a maximum value (98.45%) at ␣DDA = 0.25 (the mole ratio of DDA:NaOL = 1:3). At ␣DDA > 0.25, the muscovite recovery becomes slowly decrease. There is a marked positive synergistic effect between DAA and NaOL on flotation of muscovite. The mixed collectors DAA/NaOL is beneficial for the flotation of muscovite, and the optimum molar ratio of DAA to NaOL is 1:3, confirming the similar behaviour in the contact angle measurement discussed in the later sections. Therefore, the mixed DDA/NaOL with molar

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Fig. 2. The effect of ␣DDA of mixed collectors (DDA/NaOL) on flotation recovery of muscovite.

ratio of 1/3 is selected for the following study on the wetting and adsorption. As we all know, the pH plays a critical role in flotation system by controlling both mineral solubility and acid-base property of mineral pulp [40]. Fig. 3a shows the flotation responses of muscovite mica as a function of pH with 0.2 mmol/L cationic collector DAA, anionic collector NaOL and mixed collectors DAA/NaOL (␣DDA = 0.25), respectively. Muscovite with NaOL is not floated in the studied pH range. With DDA, the flotation recovery of muscovite mica decreases with increasing in pH. The recovery of muscovite with DAA changed in the range of 93–70%. The recovery of muscovite using the mixed DDA/NaOL is insensitive to pH and remains above 90% in the investigated pH ranges. The recovery-collector concentration curves of muscovite at pH 7 are presented in Fig. 3b. It shows that the recovery of muscovite with NaOL is independent of the NaOL concentration and little muscovite is floated in the studied concentration range. With increased concentrations of DDA, there is a sharp increase in recovery in the low collector concentration range (<0.4 mM). When the DDA concentration is above 0.4 mM, a flat horizontal is presented and the recovery reaches the maximum value improved up to 95%. In case of DDA/NaOL, DDA/NaOL critical concentration of maximum muscovite flotation recovery is 0.2 mM. It is expected that DDA/NaOL lead to a reduction of the total dosage of surfactant used in the muscovite flotation. But with the concentration further increase the flotation recovery of muscovite decreases. The reason could be explained by exceeding the critical micelle concentration (CMC) of the DDA/NaOL. The CMC of the surfactant is 0.501 mM, as shown by the descriptions below and surfactant aggregates occur above this concentration.

Fig. 3. Flotation recoveries of muscovite with individual collector NaOL, DDA and mixed collectors DDA/NaOL as a function of (a) pH and (b) concentrations.

3.2. Surface properties of mixed collectors (DDA/NaOL) at the air/water interface The surface properties of the individual collector (NaOL and DDA) and binary collector mixtures (DDA/NaOL) were investigated by surface tension measurements. The surface tension (␥) versus log concentration plots for the individual collector at ␣DDA = 0, 1 and mixed collectors of DDA/NaOL at ␣DDA =0.25, 0.5, 0.75 are shown in Fig. 4. Table 1 lists the CMC values and surface tension at CMC ( CMC ). For mixed collectors systems, CMC and  CMC values are lower than those of single collectors, indicating that the mixed collectors show better surface activity than single collectors [41]. Moreover, CMC values decrease as ␣DDA increases up

Fig. 4. Surface tension vs. log concentration plots for individual collector NaOL, DDA and mixed collectors DDA/NaOL at various molar ratios.

to a minimum value at ␣DDA =0.5. Then it increases with increasing ␣DDA . The initial decrease of CMC values with increasing ␣DAA can be attributed to the neutralization of hydrophilic groups by their electrostatic interaction. However, when ␣DDA is larger than

L. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 492 (2016) 181–189 Table 2 Surface properties of DDA/NaOL, ␣DDA = 0, 0.25, 0.50, 0.75, and 1.00 in aqueous solutions at 25◦ C. ␣DDA

CMC (mM)

 CMC

 max

Amin

0 0.25 0.50 0.75 1.00

1.720 0.501 0.100 0.603 0.108

32.66 26.85 25.51 28.03 29.71

1.56 2.05 2.91 2.29 1.94

106.43 80.99 57.06 72.50 85.58

max

1 =− 2.303nRT

Amin =



∂ ∂ log C

 (2) T

1 NA

Table 3 Molecular interaction and synergism parameters for mixed collectors DDA/NaOL at ␣DDA = 0.25, 0.5, and 0.75, in 0.1 mol/L NaCl aqueous solutions. ␣DDA

0.500, the mole fraction of DDA exceeds that of NaOL, and the self-interaction of individual surfactant gradually dominates the formation of micelles. Thus, it causes an increase of CMC [42]. The saturation adsorption value ( max ) at the air/water interface is a measure of the effectiveness of the surfactant adsorption, since it is the maximum value which adsorption can attain [7]. The adsorption effectiveness is an important factor in determining properties like foaming, wetting, and emulsification [41]. From the surface tension curves (Fig. 1),  max may be calculated using Gibbs surface adsorption Eq. (2). The average minimum area per molecule (Amin) was obtained from Eq. (3).

(3)

where (∂/∂ log C) is the slope of the surface tension vs. the logarithm of total surfactant concentration plot at 25 ◦ C, T is absolute temperature (298.15 K), R is the universal gas constant(8.314J mol−1 K−1 ), NA is Avogadro constant (6.023 × 1023 mol−1 ).  max is in ␮mol/m2 , and Amin is in Å2 . Based on previous reports [43], n = 1 for cationic/anionic mixed systems, n = 2 for DDA and NaOL. The  max and Amin values, which can be considered as a sign of packing densities of collector molecules at the air/liquid interface, are presented in Table 2. A greater value of  max or a smaller value of Amin means denser arrangement of surfactant molecules at the air/water interface [7]. And therefore the orientation of the collector molecule at the interface is more perpendicular to the interface. From Table 2, the data suggest a higher compactness of aggregation for the mixed collectors of DAA/NaOL than the individual collector. Within the investigated ␣DDA range, there are the greatest  max and the smallest Amin values for the mixed collectors at ␣DDA = 0.5, which could be attributed to the strongest electrostatic attractive interaction in all molar fractions mixed systems.

3.3. Synergism of mixed collectors (DDA/NaOL) at the air/water interface The nature and strength of the interaction between two mixed surfactants can be determined by calculating the value of ␤ parameters, including ␤m and ␤␴ . ␤m is the interaction parameter for mixed micelle formation in an aqueous solution, and ␤␴ is the interaction parameter for mixed adsorption film formation at the air/water interface [7]. The ␤ value reflects the interaction between the head groups of two surfactants, but does not include the interaction between the hydrophobic alkyl chains of the surfactants. A negative ␤ value implies an attractive interaction, a ␤ positive value to incompatible surfactant species and repulsion among mixed species, while zero value indicates ideal mixing [44]. It can be generally obtained from the ␥ − logC plots of aqueous solutions of the individual collectors and their mixtures.

185

0.25 0.50 0.75

X1m

␤m

|In

0.36 0.52 0.54

−8.39 −20.49 −12.08

 m C

1

Cm

|

2

1.84



X1␴



0.48 0.55 0.58

−15.95 −25.15 −15.90

According to regular solution theory and Rubingh’s theory of nonideal mixing, the micellar interaction parameter (␤m ) is calculated from the following equations [45]:

 

Xm 1

2  ˛1 Cm  12

In

1 − Xm 1

2



In

ˇ

=

(1−˛1 )Cm 12

(



m

Xm Cm 1 1 1−Xm 1

)

m m In ˛1 Cm 12 /X1 C1



1 − Xm 1

=1

(4)

Cm 2



2

(5)

where X1m is the molar fraction of cationic collector (DDA) in the m are CMC for DDA, NaOL, and their mixed micelle, C1m ,C2m , and C12 mixture, respectively, at a molar fraction of DDA ␣1 . Similarly, the interfacial interaction parameter ␤␴ has been evaluated using Rosen’s equation. From the analogy with derivation of the Rubingh equation, the mole fraction of DDA (␣1 ) in solution is related to its mole fraction in mixed monolayer (X1 ), by the following equations:

 

1 − X1

X1

ˇ =



In ˛1 Cs12 /X1 Cs1

2



 

In (1 − ˛1 ) Cs12 / 1 − X1 Cs2

 In

2 

˛1 Cs12 X Cs 1 1

1 − X1

=1

(6)



2

(7)

s are the concentrations that cause the surface where C1s ,C2s , and C12 tension of water to decrease by 35mN/m for DDA, NaOL and their mixture, respectively, at a molar fraction of DDA ␣1 . Substituting m m s s s the CMC, Cm 1 , C2 , C12 , C1 , C2 and C12 values obtained from the surface tension analysis into the corresponding Eqs. (3)–(8), a series  m  of parameters, including Xm 1 , ␤ , X1 , ␤ and have been calculated and summarized in Table 3. As seen from Table 3, both the ␤m and ␤ values for the all mixed collectors are negative. These values decrease with increasing ␣DDA and show a minimum at ␣DDA = 0.500, showing the mixtures at ␣DDA = 0.5 in the three mixed systems have stronger attractive interaction in the micelle. Therefore, the interaction between DDA and NaOL in the mixed micelles and monolayer is stronger than the self-interaction of the two components before mixing. It should be noted that the ␤␴ values are more negative than ␤m for mixed collectors DDA/NaOL, suggesting that the attractive interaction in the mixed micelles of the two mixtures is weaker than in the mixed adsorption film at the air/water interface. In addition, Synergism in the mixed micelle formation exists when Cm 12 is less than that of individual surfactants. The conditions for synergism to exist in the mixture are as follows [46]: (a) ␤m m m m must be negative; (b) |␤m |>|In(Cm 1 /C2 )|. The |In(C1 /C2 )| values is 1.84 for the DDA/NaOL at ␣DDA = 0.25, 0.50, and 0.75, respectively. For all the mixed collectors, it is apparent that the |␤m | value is m higher than that of |In(Cm 1 /C2 )|. This result further supports the conclusion that there exists a synergistic interaction for the mixed collectors DAA/NaOL.

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Fig. 5. Species distribution diagrams of the mixed collector (␣amine = 0.25) as a function of pH at total initial concentrations of 0.2 mM.

Fig. 6. Surface tension of NaOL, DDA and DDA/NaOL (␣DDA = 0.25) solutions at 0.2 mM concentrations as a function of pH.

3.4. Solution chemistry of mixed collectors (DDA/NaOL) The amine and oleate are weak electrolyte surfactants and ionic or molecular form of surfactants predominates depending on the pH in aqueous solutions [47]. The species distribution diagrams of the mixed collector (␣amine = 0.25) as a function of pH at total initial concentrations of 0.2 mM are presented in Fig. 5. It is presumed that the solution chemistry of mixed amine-oleate molecule will be similar to the hydrolysis of oleic acid and the ionisation of amines in aqueous solutions. This should be the case if there is no strong bonding (chemical) between the carboxylic and amine groups of the amine-oleate molecule. The only possibility of the amine and oleate molecules forming amine-oleate compound appears to be through hydrogen bonding involving the two electronegative atoms of O and N of the respective functional groups. It is thus rational to assume that the speciation of amine-oleate in water will be similar to the respective amine and oleate speciation [48]. The distribution diagram illustrates that the pH substantially changes the form of amine and oleate in solution. The ionic forms RNH3 + and (RNH3 + )2 2+ are ionised below pH 10.5 and above which the molecular form predominates. Conversely, oleate is in undissociated form below pH 8.4 and above which the negatively charged oleate ions and dimmers is ionised. The pH value of the solution has a great effect on the surface tension. Surface tension of NaOL, DDA and DDA/NaOL (␣DDA = 0.25) solutions at 0.2 mM concentrations were measured, as shown in Fig. 6. It can be seen from Fig. 6 that the surface tension of NaOL and DDA solutions decrease with the increase of the pH values. The minimum surface tension of NaOL and DDA were obtained at pH 7.5 ∼ 8.0 and 10.0, respectively, where can associate to form ionic–molecular complexes (RCOOH·RCOO− and (RNH2 RNH3 )+ , respectively) shown in Fig. 5. But further increase in pH caused an increase in the surface tension. In the case of DDA/NaOL, the surface tension is insensitive to pH. It is not only lower than that of NaOL but also less than that of DDA below pH 8.0. It can be explained that the oleate molecules possibly remain at the surface associated with amine due to tail–tail bonds, even if the bonding between amine and carboxylate groups is detached due to the undissociated nature of oleic acid below pH 8.0. The neutral oleate in between the amine molecules screen out the electrostatic amine head–head repulsion and could increase the adsorption of amine due to lateral tail–tail bonds, besides forming a closely packed adsorbed layer enhancing the hydrophobicity below pH 8.0. And it is known that the lower of surface tension of a surfactant, the stronger of hydrophobicity of

Fig. 7. Contact angle and surface tension (␥) and vs. mole fraction of DAA for muscovite with 0.2 mM mixed collectors at pH 7.0.

it. Therefore, the flotation pH 7.0 for muscovite is selected, which can reduce the amount of acid or alkali in the practical flotation industry. 3.5. Wetting characteristics of muscovite with mixed collectors (DDA/NaOL) Contact angle (CA) were measured in order to investigate the wettability nature of mixed DDA/NaOL collectors on muscovite and thereby validate synergistic effect between NaOL and DDA on solid/liquid interface. Fig. 7 shows the contact angle of muscovite as a function of ␣DDA , and the corresponding surface tension of the solution is also presented. As can be seen, there is an increase in the contact angles of muscovite when DDA is added to a NaOL solution. In other words, combined use of the reagents enhances the hydrophobicity of mouscovite surfaces and produces greater contact angles. A more pronounced synergistic effect is also observed when the mixed collectors at ␣DDA = 0.25 are added in 1/3 mole ratio (DDA/NaOL), which isn’t consistent with the air/water interface (␣DDA = 0.5). This discrepancy could be attributed to a favourable arrangement of adsorbing species. This is interpreted in terms of a model in which the molecule of one surfactant occupies the center and the molecules of the component that represents the larger fraction occupies the corners of hexagons [49]. This phenomenon

L. Xu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 492 (2016) 181–189

Fig. 8. Contact angles of muscovite as a function of (a) concentration and (b) pH.

has been observed previously by E. Valdiviezo [50] for NaOL/DTAC on the fluorite/ aqueous solution interface. It is postulated that in this case it could also explain the increased hydrophobicity at the solid–liquid interface. The different collector solutions at 0.2 mM were allowed to adsorb previously on the muscovite surface, and corresponding contact angles with water were measured. As shown in Fig. 8a, there is a sharp increase in the contact angle at the low concentrations of DDA and DDA/NaOL (␣DDA = 0.25), however, the contact angle with the DDA/NaOL is much larger than that of DDA alone. The maximum contact angle of 92◦ is obtained at about 0.2 mM DDA/NaOL concentration but further increase in concentration caused a decrease in the contact angle. The variation of contact angle agrees well with the above-mentioned flotation result, which could be explained by exceeding the CMC. The smallest contact angles are observed with NaOL. It can be concluded that NaOL alone cannot render hydrophilic muscovite surface to hydrophobic, which is supported by the fact that NaOL alone do not exhibit any collector action on muscovite in flotation. These results confirm that the DDA/NaOL exhibit more superior hydrophobicity property than the individual DDA and NaOL. Fig. 8b shows the effect of pH on the contact angle of muscovite treated by NaOL, DDA, and the mixed DDA/NaOL (␣DDA = 0.25) at 0.2 mM concentration. The contact angle of muscovite with DDA increases gradually with increasing pH and maintains around 90◦ when the pH values are over 8, which is not consistent with the

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Fig. 9. Adsorption of DAA and NaOL on muscovite from mixed DDA/NaOL solutions (a) with constant initial DDA concentration of 0.1 mM at pH 7.0, (The black bar represents the adsorption of NaOL from single solution.); (b) at a feed molar ratio of 1:3 DDA-NaOL (␣DDA = 0.25) and pH 7.0. (The dotted lines represent the adsorption of DDA or NaOL from single solution.)

flotation results. This discrepancy could be attributed to the crystal structure anisotropy of muscovite [36]. It can result from a different particle size of a fine size fraction (−74 + 38 ␮m) in the flotation tests while the basal plane was used in the contact angle measurement. However, in case of the DDA/NaOL, pH has minor effect on the contact angle of muscovite. And the contact angles of muscovite remain about 92◦ in the investigated pH ranges. It is clear that combined use of the reagents enhances the hydrophobicity of the surfaces and produces greater contact angles. 3.6. Adsorption of mixed collectors (DDA/NaOL) on muscovite The adsorption of DDA and the corresponding adsorption of NaOL from the mixed DDA/NaOL solutions on muscovite are shown in Fig. 9a, with a constant feed concentration of DDA (0.1 mM) and Fig. 9b with a constant feed molar ratio of DDA/NaOL (␣DDA = 0.25). It should be noted that all adsorption studies of mixed anionic–cationic surfactant systems were performed such that equilibrium concentrations of anionic and cationic surfactants were in the monomer-only region of the precipitation phase boundary (i.e., below the CMC). At the pH 7.0 evaluated, muscovite is negatively charged, since its PZC is ∼1.3. As seen from Fig. 9a, it is expected that the NaOL molecule adsorption is considerably less on muscovite surfaces at the absence of DDA. This results from the fact that the negatively charged muscovite at pH 7.0 is unable to attract the anionic collector by physisorption, and there is no

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chemisorption between muscovite and NaOL. There are no Al atoms with broken bonds on the muscovite (001) surfaces [51], and the surface consists of Si and O atoms only. The Al atoms are present below the surface atomic layer and are in octahedral coordination, thereby making it least favorable for adsorption of NaOL. In Fig. 9a, with the increase of NaOL concentration, the adsorption of NaOL on the muscovite surface significantly increases in the presence of 0.1 mM DDA. As can be seen from the DDA-alone adsorption isotherm on muscovite (Fig. 9b), two distinct adsorption regions exist in the investigated concentration range, which coincides well with the first two region of the four-region model of an cationic surfactant adsorbed onto a negatively charged mineral oxide surface in the low concentrations [52]. In region I, surfactant monomers are electrostatically adsorbed to the negatively charged muscovite surface, with head-groups in contact with the surface. Hydrocarbon tail-groups may interact with any hydrophobic regions of the substrate. Region II involves strong lateral interaction between adsorbed monomers, resulting in the formation of primary aggregates. The adsorption of DDA and NaOL are enhanced with each other in the mixed systems in Fig. 9a and b, which could be attributed to the presence of the co-adsorption. That is, there is a synergistic effect of mixed DDA/NaOL collectors in the adsorption on the muscovite. For interpretation of the co-adsorption, the conceptual model is proposed that as NaOL adsorbs between the molecules of DDA, they reduce the electrostatic repulsion between the similarly charged head groups [53]. This reduction in lateral repulsion allows closer packing of the DDA, thus accounting for the increase in DDA adsorption observed in Fig. 9a and b.

4. Conclusion In the present work, the synergistic effect of mixed cationic/anionic collectors (DDA/NaOL) at various molar ratios on adsorption of muscovite was investigated for its application in flotation. The following are the main conclusions:

(1) The mixed DDA/NaOL collectors display a stronger collecting power for muscovite than the individual DDA and NaOL. It can be obtained a maximum recovery (98.45%) by the DDA/NaOL at ␣DDA = 0.25, which implies that there is the most significant positive synergistic effect between DAA and NaOL on flotation of muscovite. (2) The mixed surfactant systems DDA/NaOL have better surface activity than the individual collector. There are the greatest  max and the smallest Amin values for the mixed collectors at ␣DDA = 0.5, which could be attributed to the strongest electrostatic attractive interaction in all molar fractions mixed systems. ␤m and ␤␴ values are observed a minimum at ␣DDA = 0.500, showing the mixtures at ␣DDA = 0.5 have strongest attractive interaction in the micelle. The |␤m | values are higher m than that of |In(Cm 1 /C2 )|, which further indicates there is a positive synergistic interaction for the mixed collectors DAA/NaOL. (3) The flotation results are in good agreement with the species distribution diagrams of the mixed collector, the contact angle and adsorption results. And a more pronounced synergistic effect of DDA/NaOL on muscovite surface at ␣DDA = 0.25 is observed by the contact angle measurements, which could be attributed to a favourable arrangement of adsorbing species. (4) The adsorption results show there is also a synergistic effect of mixed DDA/NaOL collectors at the solid–liquid interface. The individual cationic DDA can be adsorbed strongly onto the muscovite, but no significant adsorption of anionic NaOL can be detected. However, in the mixed systems, the adsorp-

tion amounts of both DDA and NaOL are enhanced due to co-adsorption.

Acknowledgements The authors would like to thank the National Natural Science Foundation of China (Grant Nos. 51304162, 51504224 and 51504199), Foundation of Key Laboratory of Solid Waste Treatment and Resource Recycle Ministry of Education for Professional Innovation Research Team (No. 14tdgk03) for the financial support.

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