- Email: [email protected]
aqueous emulsion used as a collector in potash ore desliming flotation
Journal of Molecular Liquids 209 (2015) 611–616
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
A kerosene/aqueous emulsion used as a collector in potash ore desliming flotation Da Li a, Pingmei Duan a, Zhiping Du a, Fangqin Cheng a,⁎, Yunshan Guan b a State Environmental Protection Key Laboratory of Efficient Utilization Technology of Coal Waste Resources, Institute of Resources and Environment Engineering, Shanxi University, No. 92 Wucheng Road, Xiaodian District, Taiyuan 030006, PR China b School of Chemistry and Chemical Engineering, Qinghai University, No. 251 Ningda Road, Xining, Qinghai 810016, PR China
a r t i c l e
i n f o
Article history: Received 11 February 2015 Received in revised form 12 May 2015 Accepted 14 May 2015 Available online 3 June 2015 Keywords: Emulsion stability Nonionic surfactant Kerosene KCl saturated solution
a b s t r a c t The removal of slimes (water-insoluble minerals) from high slime-containing potash ores from Qinghai Province, PRC, was investigated using flotation. A higher slime flotation recovery was obtained when a surfactant (polyoxyethylene fatty acid ester)–kerosene water emulsion was used as the collector. The stabilities of the kerosene/aqueous emulsions in water and in KCl saturated solution were tested to understand the mechanism of the desliming flotation process. The results showed that the phase separation of the emulsion was pronounced at low surfactant concentration. Microscopic observation of the emulsion microstructure indicated that the droplets were spherical with the exception of samples with low surfactant concentration and that the droplet size increased with decreasing surfactant concentration. The emulsion droplets in the KCl saturated solution were smaller than those in pure water. Additionally, the emulsion was easier to stabilize in KCl because of the higher surface activity in KCl saturated solution; therefore, this emulsion can be used as a collector in desliming flotation. © 2015 Elsevier B.V. All rights reserved.
1. Introduction As an important element for the growth of crops, potassium is essential in agricultural production. In China, most potash fertilizer is produced in Qinghai Province. However, most potash ores contain a significant amount of slime (water-insoluble minerals) due to overmining. The insoluble slimes need to be removed before potassium chloride flotation to prevent the loss of amine collectors [1]. Much research on mineral desliming has been carried out [2–6]. As a separation process, flotation in which the collector plays a key role has been used in some plants [7–10]. Oily collectors (kerosene, fuel oil) can be used as flotation collectors for materials that are hydrophobic by nature, such as coal [11,12]. Oily collectors (kerosene, fuel oil) are water-insoluble nonionic collectors; thus, the flotation efficiency may be affected by the dispersibility of the oily collector in solution, and it is necessary to study the dispersibility of oily collectors. Colloidal dispersion systems such as emulsions have attracted considerable attention in many applications. Most previous works discussed the formation and stability of water-in-oil (w/o) emulsions with nonionic surfactants. For instance, Shinoda and Saito studied the effects of salt and temperature on the phase diagram and hydrophile– lipophile balance (HLB) of a water–cyclohexane system stabilized with a nonionic surfactant [13]. Uson et al. studied the phase behavior of a water–nonionic surfactant–oil system. The formation of a w/o ⁎ Corresponding author. E-mail address: [email protected] (F. Cheng).
http://dx.doi.org/10.1016/j.molliq.2015.05.027 0167-7322/© 2015 Elsevier B.V. All rights reserved.
nano-emulsion by a low-energy emulsification method was reported [14]. Mei et al. indicated the influences of different types of inorganic salts on the PIT and long-term stability of paraffin oil–water nanoemulsions with nonionic surfactants [15]. Jiang et al. showed the effects of inorganic salts such as NaCl, CaCl2, and Na2SO4 on the formation and stability of w/o emulsions [16]. In this work, the efficiency of desliming flotation using a kerosene/ aqueous emulsion as a collector in saturated brines was investigated. The stabilities of colloidal dispersion emulsions in water and KCl saturated solution were studied, and the effect of surfactant concentration on emulsion properties was investigated. Furthermore, the evolution of emulsion morphology was examined by optical microscopy to elucidate changes in the structural properties of the emulsions. 2. Experimental procedures 2.1. Materials The potash ore sample used in this study was taken from the Mahai Basin in Qinghai Province, PRC. Samples of the slimes (water-insoluble minerals) were prepared by dissolving the potash ore sample with MilliQ water (Milli Q, Millipore Corp.). The mean slime particle was 10.58 μm, and most of them were less than 20 μm in size (D90 was approximately 17.57 μm). Reagent-grade KCl and kerosene with purities of 99% were purchased from the TianJin HengXing Chemical Preparation Co., Ltd. The MR-1 surfactant (polyoxyethylene fatty acid ester), which is a nonionic,
612
D. Li et al. / Journal of Molecular Liquids 209 (2015) 611–616
non-APEO-type kerosene emulsifier, was purchased from Zibo Zhangdian Shuangyi Fine Chemical Company. All materials were used as received without further purification. MilliQ water (Milli Q, Millipore Corp.) was used for the preparation of all of the emulsions. In this study, the temperature (25 ± 0.1 °C) was maintained using a water bath controlled by a DC-2006 thermostat (Ningbo Xingzhi Biotechnology Co., Ltd). 2.2. Emulsion preparation To disperse kerosene, the emulsion was prepared using the MR-1 surfactant as an emulsifier and MilliQ water or KCl saturated solution as the solvent. Briefly, 3 g kerosene and 20 g surfactant solution were added into a beaker and then stirred with a magnetic stirrer at 40 revolutions per minute (rpm) for 30 min at 25 ± 0.1 °C. The surfactant concentrations (w/w) were selected as 10%, 5%, 2%, and 1%. 2.3. Microflotation tests The flotation response of the slime was determined using a 115-ml flotation column cell (20 × 220 mm) equipped with a magnetic stirrer. The schematic diagram of the flotation experiment is shown in Fig. 1. A saturated solution containing different volume fractions of kerosene or kerosene emulsion as the collector was first conditioned for 10 min and then for five additional minutes after the addition of the 2 g of slime. The samples were floated for 2 min using nitrogen gas at a flow rate of 20 cm3/min at 25 ± 0.1 °C. The flotation products were collected and rinsed using MilliQ water then dried in an oven at 105 °C for 2 h. Later, the flotation products were cooled to room temperature in air and weighed. The flotation recovery was then calculated.
2.5. Surface tension measurements Stock solutions of MR-1 were prepared with MilliQ water and KCl saturated solution. Surface tension was determined using the Du Noüy ring method at 25 ± 0.1 °C (Shanghai Fangrui, Inc.). All glassware used in the experiments was washed with chromic acid and rinsed with DI water to reduce contamination. The platinum ring (9.55 mm in diameter) was washed sequentially with acetone, methanol, and DI water followed by flame treatment to remove organic contamination. For each measurement, 20 ml samples of solution with known concentrations of surfactant were used. To avoid adsorption kinetic effects, measurements were performed 5–10 min after the addition of surfactant solution. The surface tensions were averaged over five measurements carried out at intervals of 60 s after 600 s of stirring. 2.6. Interfacial tension measurements The interfacial tension between kerosene and Millipore water or KCl saturated solution was measured at various MR-1 concentrations using the Du Noüy ring method. A 20-ml aliquot of surfactant solution with a known concentration was poured into the circular glass container, and the cleaned platinum ring was immersed into the aqueous phase. The kerosene was then introduced on the top of the aqueous phase. The interfacial tension was determined by measuring the force required to detach the ring from the interface at 25 ± 0.1 °C. The ring was cleaned by washing with Millipore water and acetone and then flamed to remove remaining acetone before each measurement. 3. Results and discussion 3.1. Microflotation tests
2.4. Emulsion characterization 2.4.1. Stability measurements Emulsion stability was studied using the bottle test method by observing the volume of sample phase separation. Freshly prepared emulsions were transferred into a 25-ml colorimetric tube sealed with a cap and stored at 25 ± 0.1 °C. The volume of phase separation from the emulsion was visually monitored as a function of time. The volume percentage of each phase in relation to the total volume was calculated.
Fig. 2 shows the slime flotation recovery using kerosene alone or MR-1 stabilized kerosene/aqueous emulsion as the collector in KCl saturated solution at 25 ± 0.1 °C. Clearly, the slimes can hardly be floated when kerosene alone is used as the collector. In contrast, slimes can be effectively floated using the MR-1 stabilized kerosene/aqueous emulsion. The flotation recovery was lower when the surfactant concentration was 2% and 1%. The kerosene collector is only efficient for desliming flotation when the kerosene is dispersed into fine emulsion droplets with the addition of the MR-1 surfactant in the saturated solution. With an increase in the collector concentration to a volume
2.4.2. Morphology measurements Images of emulsion morphology were obtained using an optical microscope linked to a digital video camera (Nikon, DS-Fj, Japan) at a magnification of 200×. A drop of freshly prepared emulsion was placed on a glass slide and gently covered with a cover slip.
Fig. 1. Schematic diagram of the microflotation experimental setup. (1, nitrogen cylinder; 2, surge flask; 3, flow meter; 4, magnetic stirring apparatus; 5, microflotation tube; 6, pumping device; 7, pump).
Fig. 2. Flotation recovery for slime minerals in KCl saturated solution with different volume fractions of kerosene or with MR-1 stabilized kerosene/aqueous emulsion as the collector (the surfactant concentrations (w/w) in the emulsions were 10%, 5%, 2%, and 1%).
D. Li et al. / Journal of Molecular Liquids 209 (2015) 611–616
(a) in water
613
(b) in KCl saturated solution
Fig. 3. The volume of emulsion phase separation of as a function of time with different surfactant concentrations [-■- 10%, -♦- 5%, -▲- 2% and -●- 1%]: (a) in water and (b) in KCl saturated solution.
fraction of 0.7%, the flotation recovery increases monotonically to nearly 100% when the surfactant concentration reaches 5%. 3.2. Emulsion stability The desliming efficiency is affected by the dispersibility of the kerosene; thus, it is necessary to study the emulsion properties. A preliminary study was carried out to test the stability of the kerosene/ aqueous emulsion by visually measuring the volume of sample phase separation at 25 ± 0.1 °C. Both water and a KCl saturated solution were used as an aqueous phase. The employed surfactant concentrations (w/w) were 10%, 5%, 2%, and 1%. The volume of sample phase separation is plotted as a function of time (t) in Fig. 3. With increasing MR-1 concentration, the emulsion stability increased obviously, regardless of whether the aqueous phase was water or KCl saturated solution. No phase separation was observed in the emulsions when the concentration of MR-1 was up to 5%. The results indicated that the surfactant concentration plays an important role in emulsion stability. The kerosene/ aqueous emulsions could be stable when the surfactant concentration reaches a certain value. At low surfactant concentrations of 2% and 1%, higher phase separation volumes were obtained in water (Fig. 3a) than in KCl saturated solution (Fig. 3b), indicating that the emulsions were easier to stabilize in KCl saturated solution. Fig. 4 shows images
10%
5%
2%
(a) in water
1%
of the emulsions for the bottle test after the 48-h preparation. At a surfactant concentration of 5%, phase separation was observed in the kerosene/water emulsion but not in the kerosene/KCl saturated solution. These results were in agreement with the microflotation results. 3.3. Emulsion morphology Emulsion morphology was observed by optical microscopy. Fig. 5 shows the microscopy images of the kerosene/aqueous emulsions in water and KCl saturated solution. The images indicate that the emulsion droplet size is dependent on surfactant concentration in both water and KCl saturated solution. A small emulsion droplet size was obtained when a high surfactant concentration was used. The emulsion droplet size in KCl saturated solution is smaller than that in water. Additionally, the microscopic observation of sample microstructure shows that at low surfactant concentration, spherical droplets with well-defined drop contours were formed. For these emulsions, closely packed aggregates of fine droplets formed around the larger droplets. The flocculation and aggregation of small droplets allow the creation of a protective layer for the large droplets. Moreover, the difference in emulsion morphology (the presence of flocculated droplets at low surfactant ratio) observed by optical microscopy affects the mean droplet size. Thus, the concentration of surfactant is the most important
10%
5%
2%
1%
(b) in KCl saturated solution
Fig. 4. Images of MR-1 stabilized kerosene/aqueous emulsions after 48 h with different surfactant concentrations.
614
D. Li et al. / Journal of Molecular Liquids 209 (2015) 611–616
10%
5%
2%
1%
2%
1%
(a) in water
10%
5%
(b) in KCl saturated solution Fig. 5. Optical microscopy images of emulsions with different surfactant concentrations.
parameter in the formation of stable emulsions and significantly influences the emulsion droplet number and size distributions.
surface pressure (ΠCMC) in the region of surface saturation could be obtained by Eq. (1) [21]:
3.4. Surface tension
Π CMC ¼ γ0 −γCMC
As is known, amphiphilic molecules in solution accumulate at interfaces and reduce the excess surface free energy. This spontaneous process (adsorption) proceeds until the interface is saturated with adsorbed surfactants. Excessive amounts of surfactant molecules in solution form aggregates (micelles) with different morphologies. The surface tension decreases continuously and then becomes constant along a wide concentration range. The point of break, when the constancy of surface tension begins, was taken as the critical micelle concentration (CMC) of the system. The surface tension was measured to evaluate the surface activity of the MR-1 surfactant. The surface tension (γ) is plotted against the MR-1 concentration in both water and in KCl saturated solution in Fig. 6. In dilute solutions, there is a linear relationship between the decrease in surface tension and the increase in the log of the surfactant concentration. The CMC was shown as sharp bends in the curves, and the surface tension remained constant after the CMC. The CMC values in water and in KCl saturated solution are 0.005% and 0.0003%, respectively. It is well established that the presence of electrolytes in the surfactant solutions reduces their CMC values in solution due to the charge screening effect, which leads to a reduction in the Debye length [17–20]. As such (i.e., with a decrease in the Debye length), the repulsive interactions between similarly charged head groups decrease and micelle formation is facilitated. This results in an increase in surface activity and the consequent lowering of the CMC value. The inorganic salts affect micelle formation through electrostatic interactions. As the structure of the micelle is controlled by the hydrated size of the counter ion, counter ions with higher hydrophilicity prefer to stay in the bulk of the micelle solution; therefore, they are less effective at screening the charges on the micelle surfaces. A thermodynamic evaluation of the MR-1 systems was also carried out, and various parameters were calculated. First, the
where γ0 and γCMC are the surface tension of the solvent and the mixture at the CMC, respectively. The respective decreasing and increasing values of γCMC and ΠCMC with increasing salt concentration indicate that the efficiency of the system increases with increasing salt concentration (Table 1). According to Gibbs' law, the adsorption of surfactants at a surface at equilibrium leads to a reduction of the surface tension. The saturation surface excess concentration, Γmax, and the minimum area, Amin, per surfactant molecule can be calculated using the Gibbs adsorption
ð1Þ
Fig. 6. Surface tensions vs. the concentration of MR-1 surfactant in aqueous solution at 25 °C (■: in Millipore water, ○: in KCl saturated solution).
D. Li et al. / Journal of Molecular Liquids 209 (2015) 611–616
615
Table 1 Various thermodynamic parameters for MR-1 systems at the air/aqueous interface evaluated on the basis of surface tension measurements (25 °C). Solution
CMC (%)
γCMC (mN m−1)
Γmax(×10−7 mol m−2)
Amin (nm−2)
ΔG0mic (K J mol−1)
ΔG0ad (K J mol−1)
Millipore water KCl saturated solution
0.005 0.0003
33.44 30.71
43.62 73.01
0.38 0.23
−13.13 −20.10
−22.05 −27.34
isotherm Eqs. (2) and (3) [21]. The standard free energy of the micelle and of adsorption can be calculated from Eqs. (4) and (5) [21]. Γ max ¼
1 ∂γ − 2:303RT ∂ lgc T
ð2Þ
In Eq. (2), R is the gas constant (8.314 J mol−1 K−1) and T is the temperature in Kelvin. The plot of the slope of the tangent at the given concentration of the surface tension (γ) versus log surfactant concentration was used to calculate Γmax. The value of Γmax increases in the KCl saturated solution (Table 1), indicating that more surfactant molecules are adsorbed at the interface. This occurs due to the decrease in repulsion among the head groups caused by the addition of salt. The minimum area per molecule, Amin, can then be evaluated as: Amin ¼ 1020 =ðNΓ max Þ
ð3Þ
where N is Avogadro's number. Amin decreases in the KCl saturated solution (Table 1) due to the progressive charge shielding and closer packing of the surfactant molecules at the interface. To quantify the effect of salts in the mixture on the micellization process, the standard Gibbs energy of micellization (ΔG0mic) and the standard Gibbs energy of adsorption (ΔG0ad) were calculated from Eqs. (4) and (5): ΔG0mic ¼ RT ln CMC
ð4Þ
ΔG0ad ¼ ΔG0mic −NΠ CMC Amin :
ð5Þ
The standard state for the adsorbed surfactant is a hypothetical monolayer at its minimum surface area per molecule but at zero surface pressure. The values of both ΔG0mic and ΔG0ad are negative (Table 1). It is the hydrophobicity that leads an amphiphile toward the air/water interface; this is the main cause of adsorption. All of the negative ΔG0ad values imply that the adsorption of the surfactant molecules at the air/water interface takes place spontaneously. Table 1 also shows that ΔG0ad values are greater in magnitude than ΔG0mic, confirming that the micellization
is secondary in nature with respect to surface adsorption; thus, work must be done to transfer molecules from the monomeric form to the micelle form. The values of ΔG0ad and ΔG0mic of the MR-1 surfactant in KCl saturated solution are less negative than those in water, leading to high surface activity. 3.5. Interfacial properties It is also well known that the interfacial tension decreases with increasing surfactant concentration and reaches a minimum value at the CMC. Under the CMC, surfactant molecules reach adsorption saturation and form a monolayer at the interfaces. When the surfactant concentration is higher than the CMC, the interfacial tension would remain constant because only the surfactant monolayer contributes to the reduction of interfacial tension. To evaluate the interaction between kerosene and surfactant, the interfacial properties of the kerosene/ aqueous system were examined. The interfacial tension of the kerosene/aqueous system was measured at different concentrations of the MR-1 surfactant in water and KCl saturated solution. The plots of interfacial tension against surfactant concentration are presented in Fig. 7. The results demonstrate that for the dispersions containing the MR-1 surfactant, the interfacial tension progressively decreases with increasing surfactant concentration in solution. The interfacial tension is lower in KCl saturated solution, which may be due to the compression of the electric double layer in salt solution. The results show that the CMCs are 0.1% in water and 0.01% in KCl saturated solution. The CMCs in both water and brine are much higher than those at the air/water surface; however, the kerosene/aqueous interfacial tensions at the CMC are much lower than those at the air/water surface. These results suggest that the surfactant molecules tend to adsorb at the interface to stabilize the emulsion. 4. Conclusion In this study, it was found that the kerosene/aqueous emulsion can be used as a collector in insoluble slime flotation in KCl saturated solution. At a 0.7% volume fraction of emulsion collector, the flotation recovery of the slime reached nearly 100% when the concentration of MR-1 in the emulsion was 5%. In the MR-1 stabilized kerosene/aqueous emulsions, increasing surfactant concentration resulted in the reduction of emulsion droplet size and generated spherical and well-defined droplets. Stability tests revealed that stable emulsions could be formed in the KCl saturated solution when the concentration of the MR-1 was above 5%. The surface tension results indicated that compared to in water, the surface tension was lower in the KCl saturated solution, and the values of ΔG0ad and ΔG0mic of the MR-1 surfactant were less negative, indicating high surface activity. The results reported herein contribute to a better comprehension of MR-1 stabilized kerosene/aqueous emulsions, which can be used as collectors for desliming flotation. Acknowledgments
Fig. 7. Interfacial tension vs. MR-1 surfactant concentration for the kerosene/aqueous solution of the MR-1 surfactant system.
The financial support was provided by the National Natural Science Foundation of China (No. 21376144), the International Science & Technology Collaboration Program of China (No. 2012DFA91500), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20121401110001).
616
D. Li et al. / Journal of Molecular Liquids 209 (2015) 611–616
References [1] S. Aliferova, S. Titkov, R. Sabirov, V. Novoselov, N. Panteleeva, Application of nonionic surface-active substances in combination with acrylamide flocculants for silicate and carbonate mineral flotation, Miner. Eng. 18 (2005) 1020–1023. [2] Y.H. Wang, Y.H. Hu, P.B. He, G.H. Gu, Bauxite selective experimental study take off the mud, Metal Miner. 4 (2004) 38–40. [3] D.Y. Wei, The development of potassium deposits in New Mexico, Geol. Chem. Miner. 23 (2001) 31–38. [4] M.R. Yalamanchili, J.D. Miller, Removal of insoluble slimes from potash ore by airsparged hydrocyclone flotation, Miner. Eng. 32 (1995) 169–177. [5] J.L. Wang, S. Lin, Removal of slime from potash ore and rock salt using magnetic separation, Met. Ore Dress. Abroad 11 (2007) 34–35. [6] D. Li, H.L. Bian, C.Q. Zhu, F.Q. Cheng, Study on desliming technology for slimecontaining potash ores, Inorg. Chem. Ind. 44 (2012) 49–52. [7] S. Titkov, Flotation of water-soluble mineral resources, Int. J. Miner. Process. 74 (2004) 107–113. [8] A. Mehdilo, H. Zarei, M. Irannajad, H. Arjmandfar, Flotation of zinc oxide ores by cationic and mixed collectors, Miner. Eng. 36–38 (2012) 331–334. [9] L.Y. Xia, H. Zhong, G.Y. Liu, Flotation techniques for separation of diaspore from bauxite using Gemini collector and starch depressant, Trans. Nonferrous Metals Soc. China 20 (2012) 495–501. [10] L.C. Zhou, Y.M. Zhang, Flotation separation of Xixia andalusite ore, Trans. Nonferrous Metals Soc. China 21 (2011) 1388–1392. [11] Y.T. Lv, J.C. Wang, Y.B. Lv, Emulsification and floatation of kerosene catch agent, Coal Sci. Technol. 32 (2004) 57–59. [12] H.A. Temel, V. Bozkurt, A.K. Majumder, Selective oil agglomeration of lignite, Energy Fuel 23 (2009) 779–784.
[13] K. Shinoda, H. Saito, The stability of O/W type emulsions as functions of temperature and the HLB of emulsifiers: the emulsification by PIT-method, J. Colloid Interface Sci. 30 (1969) 258–263. [14] N. Uson, M.J. Garcia, C. Solans, Formation of water-in-oil (W/O) nano-emulsions in a water/mixed non-ionic surfactant/oil systems prepared by a low-energy emulsification method, Colloids Surf. A 250 (2004) 415–421. [15] Z. Mei, J. Xu, D. Sun, O/W nano-emulsions with tunable PIT induced by inorganic salts, Colloids Surf. A 375 (2011) 102–108. [16] J. Jiang, Z. Mei, J. Xu, D. Sun, Effect of inorganic electrolytes on the formation and the stability of water-in oil (W/O) emulsions, Colloids Surf. A 429 (2013) 82–90. [17] S.S. Helvaci, S. Peker, G. Ozdemir, Effect of electrolytes on the surface behavior of rhamnolipids R1 and R2, Colloids Surf. B 35 (2004) 225–233. [18] S. Javadian, H. Gharibi, Z. Bromand, B. Sohrabi, Electrolyte effect on mixed micelle and interfacial properties of binary mixtures of cationic and nonionic surfactants, J. Colloid Interface Sci. 318 (2008) 449–456. [19] S. Chiruvolu, J.N. Israelachvili, E. Naranjo, Z. Xu, J.A. Zasadzinski, E.W. Kaler, K.L. Herrington, Measurement of forces between spontaneous vesicle-forming bilayers, Langmuir 11 (1995) 4256–4266. [20] M. Akramb, S. Yousufb, T. Sarwara, K. Din, Micellization and interfacial behavior of 16-E2-16 in presence of inorganic and organic salt counterions, Colloids Surf. A 441 (2014) 281–290. [21] M.J. Rosen, A.W. Cohen, M. Dahanayake, X.Y. Hua, Relationship of structure to properties in surfactants. 10. Surface and thermodynamic properties of 2-dodecyloxypoly(ethnoxyethanol)s, C12H25(OC2H4)xOH, in aqueous solution, J. Phys. Chem. 86 (1982) 541–545.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
A kerosene/aqueous emulsion used as a collector in potash ore desliming flotation Da Li a, Pingmei Duan a, Zhiping Du a, Fangqin Cheng a,⁎, Yunshan Guan b a State Environmental Protection Key Laboratory of Efficient Utilization Technology of Coal Waste Resources, Institute of Resources and Environment Engineering, Shanxi University, No. 92 Wucheng Road, Xiaodian District, Taiyuan 030006, PR China b School of Chemistry and Chemical Engineering, Qinghai University, No. 251 Ningda Road, Xining, Qinghai 810016, PR China
a r t i c l e
i n f o
Article history: Received 11 February 2015 Received in revised form 12 May 2015 Accepted 14 May 2015 Available online 3 June 2015 Keywords: Emulsion stability Nonionic surfactant Kerosene KCl saturated solution
a b s t r a c t The removal of slimes (water-insoluble minerals) from high slime-containing potash ores from Qinghai Province, PRC, was investigated using flotation. A higher slime flotation recovery was obtained when a surfactant (polyoxyethylene fatty acid ester)–kerosene water emulsion was used as the collector. The stabilities of the kerosene/aqueous emulsions in water and in KCl saturated solution were tested to understand the mechanism of the desliming flotation process. The results showed that the phase separation of the emulsion was pronounced at low surfactant concentration. Microscopic observation of the emulsion microstructure indicated that the droplets were spherical with the exception of samples with low surfactant concentration and that the droplet size increased with decreasing surfactant concentration. The emulsion droplets in the KCl saturated solution were smaller than those in pure water. Additionally, the emulsion was easier to stabilize in KCl because of the higher surface activity in KCl saturated solution; therefore, this emulsion can be used as a collector in desliming flotation. © 2015 Elsevier B.V. All rights reserved.
1. Introduction As an important element for the growth of crops, potassium is essential in agricultural production. In China, most potash fertilizer is produced in Qinghai Province. However, most potash ores contain a significant amount of slime (water-insoluble minerals) due to overmining. The insoluble slimes need to be removed before potassium chloride flotation to prevent the loss of amine collectors [1]. Much research on mineral desliming has been carried out [2–6]. As a separation process, flotation in which the collector plays a key role has been used in some plants [7–10]. Oily collectors (kerosene, fuel oil) can be used as flotation collectors for materials that are hydrophobic by nature, such as coal [11,12]. Oily collectors (kerosene, fuel oil) are water-insoluble nonionic collectors; thus, the flotation efficiency may be affected by the dispersibility of the oily collector in solution, and it is necessary to study the dispersibility of oily collectors. Colloidal dispersion systems such as emulsions have attracted considerable attention in many applications. Most previous works discussed the formation and stability of water-in-oil (w/o) emulsions with nonionic surfactants. For instance, Shinoda and Saito studied the effects of salt and temperature on the phase diagram and hydrophile– lipophile balance (HLB) of a water–cyclohexane system stabilized with a nonionic surfactant [13]. Uson et al. studied the phase behavior of a water–nonionic surfactant–oil system. The formation of a w/o ⁎ Corresponding author. E-mail address: [email protected] (F. Cheng).
http://dx.doi.org/10.1016/j.molliq.2015.05.027 0167-7322/© 2015 Elsevier B.V. All rights reserved.
nano-emulsion by a low-energy emulsification method was reported [14]. Mei et al. indicated the influences of different types of inorganic salts on the PIT and long-term stability of paraffin oil–water nanoemulsions with nonionic surfactants [15]. Jiang et al. showed the effects of inorganic salts such as NaCl, CaCl2, and Na2SO4 on the formation and stability of w/o emulsions [16]. In this work, the efficiency of desliming flotation using a kerosene/ aqueous emulsion as a collector in saturated brines was investigated. The stabilities of colloidal dispersion emulsions in water and KCl saturated solution were studied, and the effect of surfactant concentration on emulsion properties was investigated. Furthermore, the evolution of emulsion morphology was examined by optical microscopy to elucidate changes in the structural properties of the emulsions. 2. Experimental procedures 2.1. Materials The potash ore sample used in this study was taken from the Mahai Basin in Qinghai Province, PRC. Samples of the slimes (water-insoluble minerals) were prepared by dissolving the potash ore sample with MilliQ water (Milli Q, Millipore Corp.). The mean slime particle was 10.58 μm, and most of them were less than 20 μm in size (D90 was approximately 17.57 μm). Reagent-grade KCl and kerosene with purities of 99% were purchased from the TianJin HengXing Chemical Preparation Co., Ltd. The MR-1 surfactant (polyoxyethylene fatty acid ester), which is a nonionic,
612
D. Li et al. / Journal of Molecular Liquids 209 (2015) 611–616
non-APEO-type kerosene emulsifier, was purchased from Zibo Zhangdian Shuangyi Fine Chemical Company. All materials were used as received without further purification. MilliQ water (Milli Q, Millipore Corp.) was used for the preparation of all of the emulsions. In this study, the temperature (25 ± 0.1 °C) was maintained using a water bath controlled by a DC-2006 thermostat (Ningbo Xingzhi Biotechnology Co., Ltd). 2.2. Emulsion preparation To disperse kerosene, the emulsion was prepared using the MR-1 surfactant as an emulsifier and MilliQ water or KCl saturated solution as the solvent. Briefly, 3 g kerosene and 20 g surfactant solution were added into a beaker and then stirred with a magnetic stirrer at 40 revolutions per minute (rpm) for 30 min at 25 ± 0.1 °C. The surfactant concentrations (w/w) were selected as 10%, 5%, 2%, and 1%. 2.3. Microflotation tests The flotation response of the slime was determined using a 115-ml flotation column cell (20 × 220 mm) equipped with a magnetic stirrer. The schematic diagram of the flotation experiment is shown in Fig. 1. A saturated solution containing different volume fractions of kerosene or kerosene emulsion as the collector was first conditioned for 10 min and then for five additional minutes after the addition of the 2 g of slime. The samples were floated for 2 min using nitrogen gas at a flow rate of 20 cm3/min at 25 ± 0.1 °C. The flotation products were collected and rinsed using MilliQ water then dried in an oven at 105 °C for 2 h. Later, the flotation products were cooled to room temperature in air and weighed. The flotation recovery was then calculated.
2.5. Surface tension measurements Stock solutions of MR-1 were prepared with MilliQ water and KCl saturated solution. Surface tension was determined using the Du Noüy ring method at 25 ± 0.1 °C (Shanghai Fangrui, Inc.). All glassware used in the experiments was washed with chromic acid and rinsed with DI water to reduce contamination. The platinum ring (9.55 mm in diameter) was washed sequentially with acetone, methanol, and DI water followed by flame treatment to remove organic contamination. For each measurement, 20 ml samples of solution with known concentrations of surfactant were used. To avoid adsorption kinetic effects, measurements were performed 5–10 min after the addition of surfactant solution. The surface tensions were averaged over five measurements carried out at intervals of 60 s after 600 s of stirring. 2.6. Interfacial tension measurements The interfacial tension between kerosene and Millipore water or KCl saturated solution was measured at various MR-1 concentrations using the Du Noüy ring method. A 20-ml aliquot of surfactant solution with a known concentration was poured into the circular glass container, and the cleaned platinum ring was immersed into the aqueous phase. The kerosene was then introduced on the top of the aqueous phase. The interfacial tension was determined by measuring the force required to detach the ring from the interface at 25 ± 0.1 °C. The ring was cleaned by washing with Millipore water and acetone and then flamed to remove remaining acetone before each measurement. 3. Results and discussion 3.1. Microflotation tests
2.4. Emulsion characterization 2.4.1. Stability measurements Emulsion stability was studied using the bottle test method by observing the volume of sample phase separation. Freshly prepared emulsions were transferred into a 25-ml colorimetric tube sealed with a cap and stored at 25 ± 0.1 °C. The volume of phase separation from the emulsion was visually monitored as a function of time. The volume percentage of each phase in relation to the total volume was calculated.
Fig. 2 shows the slime flotation recovery using kerosene alone or MR-1 stabilized kerosene/aqueous emulsion as the collector in KCl saturated solution at 25 ± 0.1 °C. Clearly, the slimes can hardly be floated when kerosene alone is used as the collector. In contrast, slimes can be effectively floated using the MR-1 stabilized kerosene/aqueous emulsion. The flotation recovery was lower when the surfactant concentration was 2% and 1%. The kerosene collector is only efficient for desliming flotation when the kerosene is dispersed into fine emulsion droplets with the addition of the MR-1 surfactant in the saturated solution. With an increase in the collector concentration to a volume
2.4.2. Morphology measurements Images of emulsion morphology were obtained using an optical microscope linked to a digital video camera (Nikon, DS-Fj, Japan) at a magnification of 200×. A drop of freshly prepared emulsion was placed on a glass slide and gently covered with a cover slip.
Fig. 1. Schematic diagram of the microflotation experimental setup. (1, nitrogen cylinder; 2, surge flask; 3, flow meter; 4, magnetic stirring apparatus; 5, microflotation tube; 6, pumping device; 7, pump).
Fig. 2. Flotation recovery for slime minerals in KCl saturated solution with different volume fractions of kerosene or with MR-1 stabilized kerosene/aqueous emulsion as the collector (the surfactant concentrations (w/w) in the emulsions were 10%, 5%, 2%, and 1%).
D. Li et al. / Journal of Molecular Liquids 209 (2015) 611–616
(a) in water
613
(b) in KCl saturated solution
Fig. 3. The volume of emulsion phase separation of as a function of time with different surfactant concentrations [-■- 10%, -♦- 5%, -▲- 2% and -●- 1%]: (a) in water and (b) in KCl saturated solution.
fraction of 0.7%, the flotation recovery increases monotonically to nearly 100% when the surfactant concentration reaches 5%. 3.2. Emulsion stability The desliming efficiency is affected by the dispersibility of the kerosene; thus, it is necessary to study the emulsion properties. A preliminary study was carried out to test the stability of the kerosene/ aqueous emulsion by visually measuring the volume of sample phase separation at 25 ± 0.1 °C. Both water and a KCl saturated solution were used as an aqueous phase. The employed surfactant concentrations (w/w) were 10%, 5%, 2%, and 1%. The volume of sample phase separation is plotted as a function of time (t) in Fig. 3. With increasing MR-1 concentration, the emulsion stability increased obviously, regardless of whether the aqueous phase was water or KCl saturated solution. No phase separation was observed in the emulsions when the concentration of MR-1 was up to 5%. The results indicated that the surfactant concentration plays an important role in emulsion stability. The kerosene/ aqueous emulsions could be stable when the surfactant concentration reaches a certain value. At low surfactant concentrations of 2% and 1%, higher phase separation volumes were obtained in water (Fig. 3a) than in KCl saturated solution (Fig. 3b), indicating that the emulsions were easier to stabilize in KCl saturated solution. Fig. 4 shows images
10%
5%
2%
(a) in water
1%
of the emulsions for the bottle test after the 48-h preparation. At a surfactant concentration of 5%, phase separation was observed in the kerosene/water emulsion but not in the kerosene/KCl saturated solution. These results were in agreement with the microflotation results. 3.3. Emulsion morphology Emulsion morphology was observed by optical microscopy. Fig. 5 shows the microscopy images of the kerosene/aqueous emulsions in water and KCl saturated solution. The images indicate that the emulsion droplet size is dependent on surfactant concentration in both water and KCl saturated solution. A small emulsion droplet size was obtained when a high surfactant concentration was used. The emulsion droplet size in KCl saturated solution is smaller than that in water. Additionally, the microscopic observation of sample microstructure shows that at low surfactant concentration, spherical droplets with well-defined drop contours were formed. For these emulsions, closely packed aggregates of fine droplets formed around the larger droplets. The flocculation and aggregation of small droplets allow the creation of a protective layer for the large droplets. Moreover, the difference in emulsion morphology (the presence of flocculated droplets at low surfactant ratio) observed by optical microscopy affects the mean droplet size. Thus, the concentration of surfactant is the most important
10%
5%
2%
1%
(b) in KCl saturated solution
Fig. 4. Images of MR-1 stabilized kerosene/aqueous emulsions after 48 h with different surfactant concentrations.
614
D. Li et al. / Journal of Molecular Liquids 209 (2015) 611–616
10%
5%
2%
1%
2%
1%
(a) in water
10%
5%
(b) in KCl saturated solution Fig. 5. Optical microscopy images of emulsions with different surfactant concentrations.
parameter in the formation of stable emulsions and significantly influences the emulsion droplet number and size distributions.
surface pressure (ΠCMC) in the region of surface saturation could be obtained by Eq. (1) [21]:
3.4. Surface tension
Π CMC ¼ γ0 −γCMC
As is known, amphiphilic molecules in solution accumulate at interfaces and reduce the excess surface free energy. This spontaneous process (adsorption) proceeds until the interface is saturated with adsorbed surfactants. Excessive amounts of surfactant molecules in solution form aggregates (micelles) with different morphologies. The surface tension decreases continuously and then becomes constant along a wide concentration range. The point of break, when the constancy of surface tension begins, was taken as the critical micelle concentration (CMC) of the system. The surface tension was measured to evaluate the surface activity of the MR-1 surfactant. The surface tension (γ) is plotted against the MR-1 concentration in both water and in KCl saturated solution in Fig. 6. In dilute solutions, there is a linear relationship between the decrease in surface tension and the increase in the log of the surfactant concentration. The CMC was shown as sharp bends in the curves, and the surface tension remained constant after the CMC. The CMC values in water and in KCl saturated solution are 0.005% and 0.0003%, respectively. It is well established that the presence of electrolytes in the surfactant solutions reduces their CMC values in solution due to the charge screening effect, which leads to a reduction in the Debye length [17–20]. As such (i.e., with a decrease in the Debye length), the repulsive interactions between similarly charged head groups decrease and micelle formation is facilitated. This results in an increase in surface activity and the consequent lowering of the CMC value. The inorganic salts affect micelle formation through electrostatic interactions. As the structure of the micelle is controlled by the hydrated size of the counter ion, counter ions with higher hydrophilicity prefer to stay in the bulk of the micelle solution; therefore, they are less effective at screening the charges on the micelle surfaces. A thermodynamic evaluation of the MR-1 systems was also carried out, and various parameters were calculated. First, the
where γ0 and γCMC are the surface tension of the solvent and the mixture at the CMC, respectively. The respective decreasing and increasing values of γCMC and ΠCMC with increasing salt concentration indicate that the efficiency of the system increases with increasing salt concentration (Table 1). According to Gibbs' law, the adsorption of surfactants at a surface at equilibrium leads to a reduction of the surface tension. The saturation surface excess concentration, Γmax, and the minimum area, Amin, per surfactant molecule can be calculated using the Gibbs adsorption
ð1Þ
Fig. 6. Surface tensions vs. the concentration of MR-1 surfactant in aqueous solution at 25 °C (■: in Millipore water, ○: in KCl saturated solution).
D. Li et al. / Journal of Molecular Liquids 209 (2015) 611–616
615
Table 1 Various thermodynamic parameters for MR-1 systems at the air/aqueous interface evaluated on the basis of surface tension measurements (25 °C). Solution
CMC (%)
γCMC (mN m−1)
Γmax(×10−7 mol m−2)
Amin (nm−2)
ΔG0mic (K J mol−1)
ΔG0ad (K J mol−1)
Millipore water KCl saturated solution
0.005 0.0003
33.44 30.71
43.62 73.01
0.38 0.23
−13.13 −20.10
−22.05 −27.34
isotherm Eqs. (2) and (3) [21]. The standard free energy of the micelle and of adsorption can be calculated from Eqs. (4) and (5) [21]. Γ max ¼
1 ∂γ − 2:303RT ∂ lgc T
ð2Þ
In Eq. (2), R is the gas constant (8.314 J mol−1 K−1) and T is the temperature in Kelvin. The plot of the slope of the tangent at the given concentration of the surface tension (γ) versus log surfactant concentration was used to calculate Γmax. The value of Γmax increases in the KCl saturated solution (Table 1), indicating that more surfactant molecules are adsorbed at the interface. This occurs due to the decrease in repulsion among the head groups caused by the addition of salt. The minimum area per molecule, Amin, can then be evaluated as: Amin ¼ 1020 =ðNΓ max Þ
ð3Þ
where N is Avogadro's number. Amin decreases in the KCl saturated solution (Table 1) due to the progressive charge shielding and closer packing of the surfactant molecules at the interface. To quantify the effect of salts in the mixture on the micellization process, the standard Gibbs energy of micellization (ΔG0mic) and the standard Gibbs energy of adsorption (ΔG0ad) were calculated from Eqs. (4) and (5): ΔG0mic ¼ RT ln CMC
ð4Þ
ΔG0ad ¼ ΔG0mic −NΠ CMC Amin :
ð5Þ
The standard state for the adsorbed surfactant is a hypothetical monolayer at its minimum surface area per molecule but at zero surface pressure. The values of both ΔG0mic and ΔG0ad are negative (Table 1). It is the hydrophobicity that leads an amphiphile toward the air/water interface; this is the main cause of adsorption. All of the negative ΔG0ad values imply that the adsorption of the surfactant molecules at the air/water interface takes place spontaneously. Table 1 also shows that ΔG0ad values are greater in magnitude than ΔG0mic, confirming that the micellization
is secondary in nature with respect to surface adsorption; thus, work must be done to transfer molecules from the monomeric form to the micelle form. The values of ΔG0ad and ΔG0mic of the MR-1 surfactant in KCl saturated solution are less negative than those in water, leading to high surface activity. 3.5. Interfacial properties It is also well known that the interfacial tension decreases with increasing surfactant concentration and reaches a minimum value at the CMC. Under the CMC, surfactant molecules reach adsorption saturation and form a monolayer at the interfaces. When the surfactant concentration is higher than the CMC, the interfacial tension would remain constant because only the surfactant monolayer contributes to the reduction of interfacial tension. To evaluate the interaction between kerosene and surfactant, the interfacial properties of the kerosene/ aqueous system were examined. The interfacial tension of the kerosene/aqueous system was measured at different concentrations of the MR-1 surfactant in water and KCl saturated solution. The plots of interfacial tension against surfactant concentration are presented in Fig. 7. The results demonstrate that for the dispersions containing the MR-1 surfactant, the interfacial tension progressively decreases with increasing surfactant concentration in solution. The interfacial tension is lower in KCl saturated solution, which may be due to the compression of the electric double layer in salt solution. The results show that the CMCs are 0.1% in water and 0.01% in KCl saturated solution. The CMCs in both water and brine are much higher than those at the air/water surface; however, the kerosene/aqueous interfacial tensions at the CMC are much lower than those at the air/water surface. These results suggest that the surfactant molecules tend to adsorb at the interface to stabilize the emulsion. 4. Conclusion In this study, it was found that the kerosene/aqueous emulsion can be used as a collector in insoluble slime flotation in KCl saturated solution. At a 0.7% volume fraction of emulsion collector, the flotation recovery of the slime reached nearly 100% when the concentration of MR-1 in the emulsion was 5%. In the MR-1 stabilized kerosene/aqueous emulsions, increasing surfactant concentration resulted in the reduction of emulsion droplet size and generated spherical and well-defined droplets. Stability tests revealed that stable emulsions could be formed in the KCl saturated solution when the concentration of the MR-1 was above 5%. The surface tension results indicated that compared to in water, the surface tension was lower in the KCl saturated solution, and the values of ΔG0ad and ΔG0mic of the MR-1 surfactant were less negative, indicating high surface activity. The results reported herein contribute to a better comprehension of MR-1 stabilized kerosene/aqueous emulsions, which can be used as collectors for desliming flotation. Acknowledgments
Fig. 7. Interfacial tension vs. MR-1 surfactant concentration for the kerosene/aqueous solution of the MR-1 surfactant system.
The financial support was provided by the National Natural Science Foundation of China (No. 21376144), the International Science & Technology Collaboration Program of China (No. 2012DFA91500), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20121401110001).
616
D. Li et al. / Journal of Molecular Liquids 209 (2015) 611–616
References [1] S. Aliferova, S. Titkov, R. Sabirov, V. Novoselov, N. Panteleeva, Application of nonionic surface-active substances in combination with acrylamide flocculants for silicate and carbonate mineral flotation, Miner. Eng. 18 (2005) 1020–1023. [2] Y.H. Wang, Y.H. Hu, P.B. He, G.H. Gu, Bauxite selective experimental study take off the mud, Metal Miner. 4 (2004) 38–40. [3] D.Y. Wei, The development of potassium deposits in New Mexico, Geol. Chem. Miner. 23 (2001) 31–38. [4] M.R. Yalamanchili, J.D. Miller, Removal of insoluble slimes from potash ore by airsparged hydrocyclone flotation, Miner. Eng. 32 (1995) 169–177. [5] J.L. Wang, S. Lin, Removal of slime from potash ore and rock salt using magnetic separation, Met. Ore Dress. Abroad 11 (2007) 34–35. [6] D. Li, H.L. Bian, C.Q. Zhu, F.Q. Cheng, Study on desliming technology for slimecontaining potash ores, Inorg. Chem. Ind. 44 (2012) 49–52. [7] S. Titkov, Flotation of water-soluble mineral resources, Int. J. Miner. Process. 74 (2004) 107–113. [8] A. Mehdilo, H. Zarei, M. Irannajad, H. Arjmandfar, Flotation of zinc oxide ores by cationic and mixed collectors, Miner. Eng. 36–38 (2012) 331–334. [9] L.Y. Xia, H. Zhong, G.Y. Liu, Flotation techniques for separation of diaspore from bauxite using Gemini collector and starch depressant, Trans. Nonferrous Metals Soc. China 20 (2012) 495–501. [10] L.C. Zhou, Y.M. Zhang, Flotation separation of Xixia andalusite ore, Trans. Nonferrous Metals Soc. China 21 (2011) 1388–1392. [11] Y.T. Lv, J.C. Wang, Y.B. Lv, Emulsification and floatation of kerosene catch agent, Coal Sci. Technol. 32 (2004) 57–59. [12] H.A. Temel, V. Bozkurt, A.K. Majumder, Selective oil agglomeration of lignite, Energy Fuel 23 (2009) 779–784.
[13] K. Shinoda, H. Saito, The stability of O/W type emulsions as functions of temperature and the HLB of emulsifiers: the emulsification by PIT-method, J. Colloid Interface Sci. 30 (1969) 258–263. [14] N. Uson, M.J. Garcia, C. Solans, Formation of water-in-oil (W/O) nano-emulsions in a water/mixed non-ionic surfactant/oil systems prepared by a low-energy emulsification method, Colloids Surf. A 250 (2004) 415–421. [15] Z. Mei, J. Xu, D. Sun, O/W nano-emulsions with tunable PIT induced by inorganic salts, Colloids Surf. A 375 (2011) 102–108. [16] J. Jiang, Z. Mei, J. Xu, D. Sun, Effect of inorganic electrolytes on the formation and the stability of water-in oil (W/O) emulsions, Colloids Surf. A 429 (2013) 82–90. [17] S.S. Helvaci, S. Peker, G. Ozdemir, Effect of electrolytes on the surface behavior of rhamnolipids R1 and R2, Colloids Surf. B 35 (2004) 225–233. [18] S. Javadian, H. Gharibi, Z. Bromand, B. Sohrabi, Electrolyte effect on mixed micelle and interfacial properties of binary mixtures of cationic and nonionic surfactants, J. Colloid Interface Sci. 318 (2008) 449–456. [19] S. Chiruvolu, J.N. Israelachvili, E. Naranjo, Z. Xu, J.A. Zasadzinski, E.W. Kaler, K.L. Herrington, Measurement of forces between spontaneous vesicle-forming bilayers, Langmuir 11 (1995) 4256–4266. [20] M. Akramb, S. Yousufb, T. Sarwara, K. Din, Micellization and interfacial behavior of 16-E2-16 in presence of inorganic and organic salt counterions, Colloids Surf. A 441 (2014) 281–290. [21] M.J. Rosen, A.W. Cohen, M. Dahanayake, X.Y. Hua, Relationship of structure to properties in surfactants. 10. Surface and thermodynamic properties of 2-dodecyloxypoly(ethnoxyethanol)s, C12H25(OC2H4)xOH, in aqueous solution, J. Phys. Chem. 86 (1982) 541–545.