Flotation mechanisms of molybdenite fines by neutral oils - Springer Link

International Journal of Minerals, Metallurgy and Materials Volume 25, Number 1, January 2018, Page 1 https://doi.org/10.1007/s12613-018-1540-8

Flotation mechanisms of molybdenite fines by neutral oils Qing-quan Lin1,2),Guo-hua Gu3), Hui Wang2), You-cai Liu2), Jian-gang Fu2), and Chong-qing Wang2) 1) Jiangxi Copper Technology Research Institute Co. Ltd., Jiangxi Copper Corporation, Nanchang 330096, China 2) School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China 3) School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China (Received: 3 February 2017; revised: 25 August 2017; accepted: 12 September 2017)

Abstract: The flotation mechanisms of molybdenite fines by neutral oils were investigated through microflotation test, turbidity measurements, infrared spectroscopy, and interfacial interaction calculations. The results of the flotation test show that at pH 2−11, the floatability of molybdenite fines in the presence of transformer oil is markedly better than that in the presence of kerosene and diesel oil. The addition of transformer oil, which enhances the floatability of molybdenite fines, promotes the aggregation of molybdenite particles. Fourier transform infrared measurements illustrate that physical interaction dominates the adsorption mechanism of neutral oil on molybdenite. Interfacial interaction calculations indicate that hydrophobic attraction is the crucial force that acts among the oil collector, water, and molybdenite. Strong hydrophobic attraction between the oily collector and water provides the strong dispersion capability of the collector in water. Furthermore, the dispersion capability of the collector, not the interaction strength between the oily collectors and molybdenite, has a highly significant role in the flotation system of molybdenite fines. Our findings provide insights into the mechanism of molybdenite flotation. Keywords: molybdenite fines; flotation; mechanisms; neutral oils; interfacial interaction; hydrophobic attraction

1. Introduction The exploitation of molybdenum ores has grown sharply due to the increased industrial demand for molybdenum metal over the past few years. Refractory molybdenum ores, such as finely disseminated and low-grade ores, have gradually grown in importance with the increasing depletion of accessible ores [1–2]. Thus, the flotation recovery of molybdenite fines has received increasing attention and has been the focus of many recent studies [3–7]. Molybdenite has a laminar crystalline structure that consists of a single sheet of molybdenum atoms sandwiched between two sheets of sulfur atoms [8–9]. Strong covalent bonds act within S–Mo–S layers, whereas only weak van der Waals forces act between adjacent S–S sheets. Hence, ground molybdenite particles exhibit two kinds of surfaces: faces and edges. The breakage of the van der Waals forces between faces renders molybdenite particles naturally floatable. However, the natural floatability of molybdenite decreases as its particle size decreases given the significant effects of the particle face/edge aspect ratio on the hydrophoCorresponding author: Guo-hua Gu

bicity of molybdenite surfaces [10–12]. Coarse molybdenite particles with high face/edge aspect ratios possess excellent natural floatability. By contrast, molybdenite fines have low aspect ratio and small mass; thus, they exhibit poor floatability and flotation kinetics [13–14]. The floatability of molybdenite can be enhanced by the addition of neutral oils, such as kerosene, diesel, transformer oil, and plant oils [1,15–17]. Many studies have been carried out using neutral oils as collectors to improve the flotation efficiency of molybdenite fines. Song et al. [4] studied the flotation of hydrophobic molybenite agglomerates under strong mechanical conditioning with the addition of a small amount of kerosene. Their results demonstrated that floc-flotation is considerably more effective than conventional flotation for the collection of molybdenite fines. Yang et al. [18–19] investigated the morphology and kinetics of hydrophobic molybdenite agglomerates in aqueous suspensions. They found that high stirring strength and kerosene addition not only accelerate agglomeration rate but also induce the formation of spherical and compact molybdenum agglomerates. Rubio et al. [20] investigated the flotation of

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© University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature 2018

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Int. J. Miner. Metall. Mater., Vol. 25, No. 1, Jan. 2018

copper/molybdenum sulfide fines using the emulsified oil extender technique and found that the metallurgical efficiencies of extender flotation with the addition of emulsified diesel are higher than those of conventional flotation. Fu et al. [5] examined the collecting ability of some neutral oils in recycling ultrafine molybdenite resources from waste tailings; they also discussed the relationships among oil amount, collected particle size, and agglomerate size. Most of these studies have focused on the optimization of flotation process conditions and increasing the agglomerate size of molybdenite fines in the flotation system. However, limited research has been conducted on the flotation mechanisms of molybdenite fines by neutral oils. In a previous study, we calculated the free energy of interfacial interaction among various minerals, collectors, water, and bubble; we also explained some phenomena in the molybdenite flotation system [21]. Nevertheless, the mechanism that underlies the interaction between molybdenite fines and different neutral oils in aqueous medium remains unclear. In the present study, we selected fine molybdenite particles with a size of −38 μm as the research object to investigate the adsorption mechanism of different neutral oils on molybdenite. We performed Fourier transform infrared (FT-IR) analysis and calculated extended Derjaguin-Landau-Verwey-Overbeek (EDLVO) potential energy profile. We evaluated the aggregation/dispersion behaviors of molybdenite in the presence of different neutral oils on the basis of turbidity measurements. We identified the force that dominates the interaction between molybdenite and the oily collector, as well as the effects of the dispersing capability of the collector on flotation efficiency. The results of this study provide insights into the flotation mechanism of molybdenite and can aid the selection of the appropriate oily collector for the flotation collection of molybenite fines.

2. Experimental 2.1. Materials and reagents The molybdenite sample used in the test was collected from Jiangxi, China. Chemical analysis revealed that the molybdenite sample contains 56.52wt% Mo and 37.80wt% S, indicating that the purity of molybdenite is 94.29%. The

XRD (X-ray diffraction) spectrum of the molybdenite sample is shown in Fig. 1. The samples for microflotation experiments were prepared by dry hand-grinding pure crystals in a porcelain mortar and by screening out −38 μm fractions. The size distribution of the −38 μm molybdenite sample is shown in Fig. 2, in which d10, d50, and d90 indicate particle sizes of 2.72, 10.15, and 32.13 μm, respectively.

Fig. 1. X-ray diffraction pattern of the molybdenite sample.

Fig. 2. Size distribution of the molybdenite particles used in microflotation tests.

The pH value of the suspension was adjusted using sodium hydroxide and sulfuric acid with analyzed purity. Kerosene, diesel oil, and transformer oil were used as collectors for molybdenite fines. The physicochemical properties of the oily collectors are shown in Table 1. All oils were industrial products. In addition, analytical-grade methyl isobutyl carbinol (MIBC) was used as a frother. Double-distilled water was used in all of the tests.

Table 1. Physicochemical properties of the oily collectors Oil type

Density / (kg·m−3, 20°C)

Kinematic viscosity / (mm2·s−1, 40°C)

Surface tension / (mN·m−1)

Fundamental components

Length of carbon chain

Kerosene

800

1.6

24.0

n-Alkanes

C12−C15

Diesel oil

840

3.7

27.2

n-Alkanes

C15−C18

Transformer oil

895

11.5

29.5

Cycloalkanes

C16−C23

Q.Q. Lin et al., Flotation mechanisms of molybdenite fines by neutral oils

The oily collector was added to the molybdenite suspensions in the form of oil emulsion, which was prepared as follows: The oil–water mixture (0.5 g oily collector and 99.5 mL distilled water) was first mechanically conditioned at 2000 r·min−1 for 10 min. Then, the conditioned mixture was subjected to ultrasonic treatment for 120 min using a KQ-250E ultrasonic cleaner at the frequency of 40 kHz and the potential of 250 W. 2.2. Microflotation test The microflotation test was conducted in a flotation cell with an effective volume of approximately 40 mL under an open-air condition and an impeller speed of 1900 r/min. First, 2.0 g of sample was cleaned through ultrasonic treatment prior to the flotation test. Then, the sample was mixed with 30 mL distilled water, and the suspension pH value was adjusted with H2SO4 and NaOH. The pH values were measured with a pH meter (PHS-3C). The collector was added in the form of an oil emulsion. The collector–suspension mixture was agitated for 3 min and then subjected to 1 min of frother conditioning and 3 min of flotation operation. The flotation concentrates and the tail were filtered, dried, and weighed. Hence, the mass of flotation products were used to calculate the recovery of molybdenite in microflotation tests. To assess the accuracy of flotation tests, the standard deviations of flotation recovery were found to be 2.0% after at least five tests in each condition, and the average values are reported in this work. 2.3. Turbidity measurement The turbidity of the molybdenite and reagent suspension was measured with a WGZ-3(3A) spectrometer. Briefly, 0.2 g molybdenite sample and 100 mL distilled water were mixed for 3 min with a magnetic stirrer. Meanwhile, the pH value of the suspension was adjusted with H2SO4 or NaOH. A certain amount of collector was then added to the slurry, which was then conditioned for 3 min. Then, the slurry was transferred to a graduated cylinder for 3 min of settling. Finally, 20 mL of the upper suspension was taken for turbidity measurements. 2.4. FT-IR spectroscopy Infrared spectra were recorded by a NEXUS-470 spectrometer. First, 1.0 g single mineral was mixed with 30 mL of reagent solution at pH 7 and then magnetically stirred at an agitation speed of approximately 1500 r/min. After 30 min of collector conditioning, the pulp was filtered and rinsed thrice using the corresponding pH buffer solution.

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Subsequently, the sample was dried in a vacuum desiccator at room temperature and then used for the collection of FT-IR spectra. The spectrum of the untreated single mineral powder was used as a reference. 2.5. Calculation of interfacial interactions We have previously demonstrated the calculation of surface energy in detail [21]. Based on the EDLVO theory, the total interaction between two kinds of materials (components) includes Lifshitz–van der Waals (LW), Lewis acid–base (AB), and electrostatic interactions [22–25]. Molybdenite is naturally hydrophobic, and the oily collectors used in its flotation process are neutrally charged. Hence, the electrostatic interaction between interfaces is relatively weak and can be assumed as negligible. The free energy of interfacial interaction can thus be calculated using the LW–AB approach, which is expressed by the following equation [22–23]: LW  AB LW AB GSW  GSW  GSW  2   S W



  S W



LW  SLW W 

(1)



LW AB where GSW is the free energy of LW–AB interaction between the solid surface and the liquid film surface or water (mJ·m−2), wherein subscript S indicates solid surface and LW subscript W indicates liquid film surface or water; GSW is the free energy of the LW interaction of the two compoAB nents (mJ·m−2); GSW is the free energy of AB interaction of the two components (mJ·m−2); and  is the surface energy (mJ·m−2) and superscripts LW, +, and − refer to Lifshitz–van der Waals, Lewis acid component, and Lewis base components, respectively. If water is taken as the third component, the interaction free energy of components 1 and 2 via component 3 can be calculated with the following equation [24]:

LW  AB LW AB G1W2  G1W2  G1W2

2



LW  1LW   W

 2 W 







   1   2   W  W



 

LW W   2LW 

 1 

  2   W   1  2   1 2  



(2) LW AB is the LW–AB interaction free energy of where G1W2 components 1 and 2 in aqueous phase (mJ·m−2) wherein subscripts W, 1, and 2 indicate water, component 1, and LW AB component 2, respectively; and G1W2 and G1W2 are the LW interaction free energy and the AB interaction free

4


energy of the two components in aqueous phase (mJ·m−2), respectively. In accordance with the EDLVO theory, the relationships between Gibbs function and action distance can be shown as Eqs. (3) and (4) [26–27]: A132 R1R2 (3) G LW  H    6 H  R1  R2  G AB  H   A132 



H H  2πR1R2 h0 G ABexp  0  R1  R2  h0 

A11  A33



A22  A33



and 2 in medium 3, mJ; A13 is the Hamaker constant between substance 1 and medium 3, mJ; Aii is the Hamaker constant of substance i in a vacuum, mJ; R1 and R2 are the particle radius of substance 1 and 2, respectively, m; H is the action distance between particles, nm; H0 is the minimum balance contact distance, 0.158 nm [28]; h0 is the decay length, 10 nm [26]; and  iLW is the LW component of substance i, mJ·m−2.

(4)

3. Results and discussion (5)

A13  A11 A33

(6)

Aii  24πH 02 iLW

(7)

where A132 is the Hamaker constant between substance 1

3.1. Microscopy images of oil emulsions The microscopy observation of three oil emulsions was conducted using an Olympus CX31 optical microscope. The results are shown in Fig. 3.

Fig. 3. Optical microscopy images of emulsified oily collectors: (a) 0.5wt% kerosene emulsion; (b) 0.5wt% diesel oil emulsion; (c) 0.5wt% transformer oil emulsion.

Fig. 3 shows that among the three neutral oily collectors, transformer oil achieves the best emulsifying effect under the same ultrasonic treatment without an emulsifier because of the following reasons. First, the maximum droplet size of transformer oil emulsions is approximately 20 μm, whereas those of kerosene and diesel oil emulsions are 50 and 40 μm, respectively. This result indicates that the droplet size of transformer oil emulsions is the smallest among those of the three emulsions. Second, the droplets of transformer oil emulsions are highly uniform in size compared with those of other emulsions. Hence, the dispersion of transformer oil in water medium is better than that of kerosene or diesel oil.

3.2. FT-IR analysis The FT-IR spectra of the three neutral oils are shown in Fig. 4(a). The FT-IR spectra of the three oils are very similar given the minor differences in their chemical components. The strong bands observed at 2955, 2924, and 2855 cm−1 in the FT-IR spectra of three neutral oils could be attributed to the C−H stretching vibration of −CH3 and −CH2− groups. Moreover, bands at 1460 and 1377 cm−1 can be attributed to −CH3 symmetric bending vibration, and the weak band at 725 cm−1 can be attributed to −(CH2)n− deformation vibration. The FT-IR spectra of untreated molybdenite samples and molybdenite samples treated with oily collectors are shown


in Fig. 4(b). The spectrum of untreated molybdenite presents characteristic bands at 1204, 1053, 596, and 461 cm−1. In the spectra of molybdenite samples conditioned with oily collectors, new bands are observed near 2922 and 1460 cm−1 due to −CH3 stretching vibration and −CH3 symmetric bending vibration, respectively. The lower shift value of infrared bands in the spectrum of untreated molybdenite than that of infrared bands in the spectrum of the oil-conditioned molybdenite indicates that the adsorption of oily collectors on molybdenite occurs mainly as physical

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adsorption. In addition, as observed from Fig. 4(b), the adsorption of transformer oil on molybdenite is more noticeable than that on kerosene and diesel oil. By contrast, the characteristic bands of kerosene are not clearly present in the spectrum of treated molybdenite. The sample preparation procedure for FT-IR measurements may have removed the film of kerosene on the molybdenite surface. Specifically, physically adsorbed kerosene on molybdenite might have been removed during washing prior to FT-IR measurement.

Fig. 4. Infrared spectra of collectors and mineral samples: (a) kerosene, diesel, and transformer oils; (b) untreated molybdenite samples and molybdenite samples treated with 200 mg·L−1 collector at pH 7.

3.3. Interfacial interactions between the oily collector and water Interfacial interactions were studied within the framework of EDLVO theory to understand the interactions between different oily collectors and the molybdenite surface. First, the free energy of the interfacial interaction between the oily collector and water was calculated using Eq. (1). The calculation results are listed in Table 2. Table 2. Free energy of the interfacial interaction between the oily collector and water mJ·m−2 Interaction free energy LW GSW G

AB SW

LW AB GSW

Transformer oil–water

Diesel oil–water

Kerosene–water

−64.51

−62.81

−59.85

−46.74

−39.46

−30.67

−111.25

−102.27

−90.52

The dispersion capability of the oily collector in aqueous solution is dependent on the interfacial interactions between the collector and water. As shown in Table 2, the free energy values of LW and AB interaction are negative, indicating that the van der Waals force caused by LW interaction and the hydrophobic attractive force caused by AB interaction are attraction forces. Hence, van der Waals and hydrophobic

attractive forces can cause the oily collector to disperse in the water medium. Comparing the total interaction free energy between three oily collectors and water reveals that the dispersion capability of the oily collector in aqueous medium follows the order of transformer oil > diesel oil > kerosene. The relationships between the LW or AB interaction function and action distance were calculated using Eq. (3) or Eq. (4) with the assumption that the mean radius of the oil droplets of the three collectors is 5 μm. The results are plotted in Fig. 5. The EDLVO potential energy profiles provided in Fig. 5 show that the AB interaction function is two orders greater in magnitude than the LW interaction function. This suggests that the hydrophobic attractive force caused by the AB interaction dominates the interaction between the oily collector and water. Moreover, hydrophobic attraction between the three oily collectors and water decreases in the order of transformer oil > diesel oil > kerosene. This result provides a good explanation for the differences in the emulsification effect of the three neutral oils. Given the more intensive hydrophobic attraction between transformer oil and water, the effectiveness of the emulsification of transformer oil is better than those of kerosene and diesel oil (see Fig. 3).

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Fig. 5. EDLVO potential energy (G) profiles of the interaction between the oily collector and water: (a) LW; (b) AB.

3.4. Interfacial interactions between molybdenite fines and the oily collector The free energy of the interfacial interaction between molybdenite fines and the oily collector in water was calculated using Eq. (2). The results are shown in Table 3 and indicate that the van der Waals force caused by LW interaction acts as an attractive force in aqueous medium. Moreover, hydrophobic attraction, which originates from the AB interaction, exists between molybdenite and the oily collector in water medium. Hence, van der Waals and hydrophobic attraction forces exert the molybdenite-collecting effect of neutral oils. Table 3. Free energy of the interfacial interaction between molybdenite and the oily collector in water medium mJ·m−2 Interaction free energy LW G1W2

MoS2– transformer oil

MoS2– diesel oil

MoS2– kerosene

−10.71

−9.84

−8.33

AB G1W2

−52.68

−60.12

−68.55

LW AB G1W2

−63.39

−69.96

−76.88

The relationships between the LW or AB Gibbs function and distance were calculated using Eq. (3) or (4) on the assumption that molybdenite particles and oil droplets are spheroidal particles with a radius of 5 μm. The Hamaker constant between molybdenite and the oily collector is provided in Table 4, and the EDLVO potential energy profiles are shown in Fig. 6. As shown in Table 4, the value of the Hamaker constant is greater than zero, and the Hamaker constant between molybdenite and transformer oil in water medium is larger than that between molybdenite and diesel oil or kerosene. As shown in Fig. 6, LW and AB Gibbs functions between molybdenite and water are lower than zero, indicating that van der Waals and hydrophobic attractive forces exist be-

tween the molybdenite and water interface. Thus, a hydrated film can form on the surfaces of molybdenite fines in the water medium. The formation of this hydrated film may be a reason for the declining floatability of molybdenite fines. Table 4. Hamaker constant between molybdenite and the oily collector Interface

Hamaker constant / (10−17 mJ) Vacuum

Water

Water–water

4.10

4.10

MoS2–MoS2

9.38

1.08

Transformer oil–transformer oil

8.98

0.94

Diesel oil–diesel oil

8.52

0.80

Kerosene–kerosene

7.73

0.57

MoS2–transformer oil

—

1.01

MoS2–diesel oil

—

0.93

MoS2–kerosene

—

0.78

In addition, Fig. 6 shows that the LW Gibbs function between molybdenite and the oily collector is negative and considerably lower than the AB Gibbs function. Thus, the van der Waals force between molybdenite and the oily collector surfaces acts as an attractive force, but is not the main force, between the two surfaces. By contrast, the strong hydrophobic attractive force from AB interaction has a highly significant role in molybdenite collection. Consequently, on the basis of the intensity of hydrophobic attraction, the interaction strength between the collectors and molybdenite follows the order of kerosene > diesel oil > transformer oil. Given that molybdenite has excellent natural floatability, the feasibility of using the interaction strength between oily collectors and molybdenite as a criterion for evaluating the flotation performance of the collector remains to be determined by flotation tests.


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Fig. 6. EDLVO potential energy (G) profiles of the interfacial interaction between molybdenite and the oily collector in aqueous medium: (a) LW; (b) AB.

3.5. Flotation behaviors of molybdenite 3.5.1. Effects of different oily collectors The effects of different oily collectors on the floatability of molybdenite as a function of pH value are shown in Fig. 7. In the absence of an oily collector, molybdenite fines exhibit poor floatability, which is mainly attributed to the small particle size and low face/edge ratio of molybdenite surfaces [4]. The addition of an oily collector can significantly increase the floatability of molybdenite fines. As shown in Fig. 7, transformer oil has superior molybdenite collection performance than kerosene and diesel oil and yields a high molybdenite flotation recovery that exceeds 86% in the tested pH range. When the amount of three oily collectors is fixed at 200 mg·L−1, the transformer oil emulsion exhibits the most extensive surface contact with molybdenite particles, thus intensifying the floatability of molybdenite and consequently resulting in the highest flotation recovery. Diesel oil provides a lower molybdenite recovery rate than transformer oil at the same reagent dosage and solution pH value. In addition, under alkaline pH conditions, difference in the floatability of molybdenite in transformer oil and diesel oil is in the range of 6%−9%. In the presence of kerosene, the flotation recovery of molybdenite decreases sharply with increasing solution pH value and drops to lower than 60% in the alkaline pH range. This phenomenon may be attributed to the large size of oil droplets in kerosene emulsions and the strong dispersion behavior of molybdenite particles under alkaline conditions. The turbidity of untreated molybdenite pulp and the molybdenite pulp treated with 200 mg·L−1 oily collector as a function of pH value is shown in Fig. 8. As shown in Fig. 8, untreated molybdenite pulp is highly turbid in the alkaline pH range, indicating the strong dispersibility of fine molybdenite particles in alkaline solution. This phenomenon may

Fig. 7. Effects of different collectors on the floatability of molybdenite as a function of pH value with [MIBC] = 12.5 mg·L−1.

Fig. 8. Turbidity of untreated molybdenite pulp and the molybdenite pulp treated with 200 mg·L−1 collector as a function of pH value.

be explained by the fact that the high negative values of zeta potential from molybdenite edges can generate strong electrostatic repulsion between particles under alkaline conditions [12]. Fig. 8 also demonstrates that the turbidity of molybdenite pulp decreases significantly upon treatment with

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200 mg·L−1 oily collector. The trend exhibited by this behavior opposes that by the floatability of molybdenite as shown in Fig. 7. In fact, the excellent aggregation of molybdenite particles corresponds to good floatability. The turbidity of molybdenite pulp is lowest in the presence of transformer oil compared with other oily collectors. This result suggests that excellent aggregation among molybdenite particles can be achieved with the addition of transformer oil. Thus, the good floatability of molybdenite fines can be obtained in the acidic and alkaline pH range with the addition of transformer oil. 3.5.2. Effect of collector dosage Fig. 9 shows the effect of collector dosage on the floatability of molybdenite at pH 7. The results indicate that the floatability of molybdenite increases dramatically as the collector dosage increases in the initial range of 0−200 mg·L−1. This increasing tendency, however, gradually slows down when the collector dosage exceeds 200 mg·L−1. At a high dosage of 400 mg·L−1, the flotation recoveries of molybdenite concentrates by kerosene, diesel, and transformer oils are approximately 63%, 84% and 89%, respectively. These results indicate that under the same reagent dosage and solution pH value conditions, the collection efficiency of three neutral oils for molybdenite decreases in the order of transformer oil > diesel oil > kerosene.


the floatability values of molybdenite almost reached the maximum values at the flotation time of 2 min. However, in the case of kerosene addition, the floatability of molybdenite reached plateaus after 2.5 min of flotation. This result suggests that the flotation speed of molybdenite is faster when diesel oil and transformer oil are used as collectors than when kerosene is used as a collector.

Fig. 10. Flotation kinetics of molybdenite fines using different collectors (collector dosage: 200 mg·L−1; MIBC dosage: 12.5 mg·L−1; solution pH: 7).

The flotation tests show that the collection efficiency of transformer oil for molybdenite fines is notably superior to that of kerosene and diesel oil. Thus, the dispersion capability of the oily collector in aqueous medium, not the interaction strength between the oily collector and molybdenite, is the crucial factor that influences the flotation of molybdenite fines.

4. Conclusions

Fig. 9. Effect of collector dosage on the floatability of molybdenite (MIBC dosage: 12.5 mg·L−1; solution pH: 7).

3.5.3. Flotation kinetics The flotation kinetics of molybdenite fines was also investigated using different oily collectors under the same conditions: collector dosage, 200 mg·L−1; MIBC dosage, 12.5 mg·L−1; and solution pH, 7. The results are shown in Fig. 10. As indicated in Fig. 10, the floatability of molybdenite was gradually enhanced with prolonged flotation time. When diesel and transformer oils were used as collectors,

(1) Microscopy observation of the three oil emulsions in aqueous medium shows that the dispersion performance of transformer oil is superior to that of kerosene and diesel oil. Interfacial interaction calculations indicate that the attractive hydrophobic force between the oily collector and water is the dominant force that causes the dispersion of the collector in the water medium. On the basis of the attractive hydrophobic force, the dispersion capability of three oily collectors in water is ranked as transformer oil > diesel oil > kerosene. (2) FT-IR measurements indicate that neutral oils and the molybdenite surface physically interact. Moreover, interfacial interaction calculations demonstrate that van der Waals and hydrophobic attractive forces exist between the oily collector and molybdenite in aqueous medium. The role of strong hydrophobic attraction is more significant than that of


van der Waals forces in the molybdenite-collecting effect of neutral oils. The interaction strength between the collectors and molybdenite can be ranked as kerosene > diesel oil > transformer oil. (3) The flotation behavior of molybdenite fines in the presence of different neutral oils over various pH ranges is closely correlated with the aggregation or dispersion behavior of molybdenite. Excellent aggregation among molybdenite particles can be achieved and the good floatability of molybdenite fines can be obtained in the presence of transformer oil over the wide pH range of 2−11. The results of flotation tests indicate that the dispersion capability of the oily collector in aqueous medium, not the strength of interaction between the collectors and molybdenite, is the crucial factor that determines the flotation of molybdenite fines.

[8]

[9]

[10]

[11]

[12]

Acknowledgements The authors gratefully acknowledge the financial support by the Fundamental Research Funds for the Central Universities of Central South University (No. 2016zzts103), the National Natural Science Foundation of China (No. 51374249), and the National Science-technology Support Plan (No. 2015BAB12B02), as well as the technical support provided by Guangdong Provincial Science and Technology Plan (No. 2013B090800016).

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