Oxidized

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Interaction Forces between Paraffin/Stearic Acid and Fresh/Oxidized Coal Particles Measured by Atomic Force Microscopy Yaowen Xing,† Chenwei Li,† Xiahui Gui,*,‡ and Yijun Cao‡ †

School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China Chinese National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China

‡

ABSTRACT: Effective fresh coal flotation can be achieved using conventional hydrocarbon oily collectors while oxidized coal flotation can be enhanced using oxygen-containing fatty acid. Solid-state paraffin and stearic acid were selected to represent a hydrocarbon oil and fatty acid collector in the present study, respectively. The interaction forces between paraffin/stearic acid and fresh/oxidized coal particles were measured directly using atomic force microscopy (AFM) colloidal probe technique. Flotation experiments with hydrocarbon oil (dodecane) and fatty acid (oleic acid) were carried out to validate the AFM results. The results show that a pure attractive force was detected between fresh coal and paraffin while a slightly repulsive force before the jump-into contact was observed between fresh coal and stearic acid. Effective fresh coal flotation can be achieved by using both dodecane and oleic acid, while the recovery with dodecane was always higher than that of oleic acid. In contrast, a significant jump-into contact between oxidized coal particles and stearic acid was found while a monotonous repulsive force between oxidized coal and paraffin. 77.76% oxidized coal flotation recovery was obtained when 700 g/t oleic acid was used, much higher than that of dodecane. The outcome of the present study can give a basic understanding of unoxidized/oxidized coal−collector interactions and provide guidance for coal flotation engineering.

1. INTRODUCTION Froth flotation is an effective separation method for fine coal cleaning and upgrading that is based on the differences in surface hydrophobicity between organic matter and mineral matter.1−5 Collectors are added to the flotation pulp to make the coal particles more hydrophobic and separable from the pulp by their attachment to rising air bubbles, which form a particle-rich froth on the suspension surface. However, this is not the case for hydrophilic particles. The thinning rate of the thin liquid film between bubbles and hydrophilic particles is slow, preventing bubble−particle adhesion.6,7 As a result, they remain in the pulp and are removed from the flotation machine as tailings.8 The type of collector required in coal flotation depends on the rank of the coal and its hydrophobicity. In a common case, effective fresh coal flotation can be achieved using conventional hydrocarbon oily collectors that operate through hydrophobic bonding while the beneficiation of oxidized and low-rank coal is difficult to achieve using oily collectors because of the presence of a high number of oxygen functional groups on coal surface.9−13 Oxygen-containing groups such as phenolic and carboxyl groups have a detrimental effect on the floatability of the coal.10,14−18 Bubble particle attachment was denied due to the existence of these hydrophilic sites on the coal surface. Therefore, it speculated that reverse flotation may be an effective way for oxidized coal. However, Miller et al.19 found that the adsorption density of depressant dextrin also decreased with an increased level of oxidation. Many studies suggest that oxidized coal flotation can be enhanced by using oxygencontaining fatty acid as collectors.20−23 The most representative work was done by Fuerstenau and his co-workers.20,21 A series of nonionic surfactants containing oxygenated and aromatic © XXXX American Chemical Society

functional groups, i.e., tetrahydrofurfuryl esters (THF), were used as collectors to enhance low-rank/oxidized coal flotation. Three interaction patterns, hydrogen bonding of oxygen atoms in the polar part of the THF molecule with oxygenated surface sites on the coal, hydrophobic bonding of the aliphatic hydrocarbon chain with the hydrophobic sites on the coal surface, and Pi-bonding of the benzene ring on the hydrocarbon chain of the collector with aromatic sites on the coal surface, were proposed.21 High oxidized coal flotation recovery was mainly attributed to the hydrogen bonding function. Since then, the hydrogen bonding between an oxidized coal surface and the polar oxygen-containing collector is widely accepted to be the intensification mechanism for oxidized coal flotation.23,24 Recently, Gui et al.25 calculated the interfacial interaction energy between oxidized coal and dodecane or α-furanacrylic acid to identify their interaction mechanisms from a thermodynamic perspective. A new absorption pattern was proposed, in which water molecules in a hydration film around the oxidized coal act as a hydrogen bonding bridge for αfuranacrylic acid absorption on the oxidized coal surface. The interactions between fresh/oxidized coal and different types of collectors are complex and not well understood. In recent years, rapid development of advanced analytical methods such as atomic force microscopy (AFM) has enabled basic theoretical research on mineral processing to be performed in the nanoscale.26 For example, AFM colloidal probe technique has made it possible to measure the interaction forces between various micrometer-sized particles and surfaces.27−29 It has also Received: October 31, 2016 Revised: February 2, 2017 Published: February 2, 2017 A

DOI: 10.1021/acs.energyfuels.6b02856 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Particle Size Distributions of the Coal Samples positive cumulative size, mm +0.500 −0.500 −0.250 −0.125 −0.074 −0.045 total

+ + + +

negative cumulative

mass fraction, %

ash, %

mass fraction, %

ash, %

mass fraction,%

ash, %

4.28 42.17 19.25 9.34 6.00 18.95 100

8.70 16.02 18.15 19.45 21.66 24.95 18.47

4.28 46.45 65.70 75.04 81.05 100.00

8.70 15.34 16.17 16.58 16.95 18.47

100.00 95.72 53.55 34.30 24.96 18.95

18.47 18.90 21.18 22.88 24.16 24.95

0.250 0.125 0.074 0.045

Figure 1. AFM images of paraffin (a) and stearic acid (b) substrates. Henan Province, China. A wet screening test of the coal samples was conducted with a set of standard sieves with mesh sizes of 0.500, 0.250, 0.125, 0.074, and 0.045 mm. The particle size distributions are shown in Table 1. The main size fraction was 0.250−0.500 mm with a 42.17% yield. It should be noted that the 0.250−0.074 mm fraction, the optimal size range for flotation, was only 28.59%. Oxidized coal was prepared by hydrogen peroxide oxidation method. The fresh coal samples were immersed into 30% peroxide solutions for 6 h in an agitator with a 400 rpm rotatation speed. It should be noted that large amounts of heat and gases were released in the initial stage of oxidation, indicating that the H2O2 solution method was quick and effective in preparing the oxidized coal.9 Analytical pure paraffin, stearic acid, dodecane, and oleic acid were purchased from Sinopharm Chemical Reagent Co., Ltd. The paraffin substrate for AFM experiments was prepared using section-cutting while stearic acid substrate was prepared by the pellet method. To evaluate their surface topography, the prepared substrates were imaged using AFM, as shown in Figure 1. The surface roughness, Ra, of paraffin and stearic acid was 10.1 and 38.1 nm for a given 4 μm2 surface area, respectively. It should be noted that the high surface roughness of the collector substrates makes the quantitative analysis of interaction forces between coal particles and collector substrates with the standard Derjaguin− Landau−Verwey−Overbeek (DLVO) theory of colloid stability difficult. Consequently, the force curves are only qualitatively compared, and we have restricted our discussion to qualitative speculations. This procedure has been successfully used before to study coal particle−bubble interaction, coal−-kaolinite interaction, and the presence of nanobubbles on ZnS surfaces and/or slime removal from ZnS surfaces after methanol treatment.26,37,38 2.2. Methods. 2.2.1. AFM Interaction Force Measurements. To measure the interaction force between fresh/oxidized coal and different types of collectors, solid-state paraffin and stearic acid were selected to represent the hydrocarbon and fatty acid and cut/

been extensively used recently to study interparticle and bubble−particle interactions in flotation systems.30−34 However, the interaction forces between coal particles and collectors have not been reported in the literature due to the difficulty in preparing a well-defined solid-state collector colloid probe. The interaction forces between the calcium dioleate collector sphere and fluorite surfaces were measured successfully using AFM by Fa et al.35,36 Long-range attractive force and adhesion force were found between the calcium dioleate sphere and the fluorite surface. Based on this, AFM experiments were designed to measure the interaction forces between fresh/oxidized coal and different types of collectors in the present study. Solid-state paraffin and stearic acid were selected to represent hydrocarbon and fatty acid collector and cut/compressed into substrates, respectively. A micrometer-scale coal particle was attached to the end of a tipless triangular cantilever using a micromanipulator. Then, the interaction forces between paraffin/ stearic acid and fresh/oxidized coal particles were measured directly using AFM. The surface properties of fresh/oxidized coal were examined by scanning electronic microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Fresh/oxidized coal flotation experiments with hydrocarbon oil (dodecane) and fatty acid (oleic acid) collectors were carried out to validate the AFM results. The aim of the present study was to give a basic understanding of unoxidized/oxidized coal−collector interactions and provide guidance for coal flotation engineering.

2. EXPERIMENTAL SECTION 2.1. Materials. Fresh fine coal samples were collected from the Xuehu Coal Preparation Plant coal storage yard in Yongcheng City, B

DOI: 10.1021/acs.energyfuels.6b02856 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels compressed into a substrate, respectively. Multimode 8 AFM instrument (Bruker, Karlsruhe, Germany) operating in contact mode in fluid was used for the force measurements. A HA-C/tipless canilever (NT-MDT Spectrum Instruments, Moscow, Russia) with 0.65 N/m nominal spring constant was selected to prepare the coal colloidal probe. A fresh coal particle with diameter of approximately 35 μm was collected and attached to the end of a tipless triangular cantilever using a micromanipulator. This procedure has been described in detail in the literature.27,28 Before the AFM experiments, the system sensitivity and cantilever constant were updated using Bruker Software. Interaction forces were obtained by recording the cantilever deflection via optical lever technique. Ten force curves were collected at different locations of the collector substrate using the offset function, and the general trend was presented. For the oxidized coal colloidal probe, the fresh coal particle attached on the cantilever was oxidized in situ by H2O2 solution for 6 h. Millipore ultrapure water was used in all AFM measurements. Unless otherwise specified, all experiments were conducted at room temperature and the average forces were calculated and presented. 2.2.2. SEM Analysis. An SEM system (FEI Quanta TM 250, Hillsboro, OR, USA) equipped with an energy dispersive X-ray spectrometer (EDS) was used to detect the surface properties of fresh/ oxidized coal samples. Fresh coal and oxidized coal samples of −0.045 mm were coated with a layer of gold to aid imaging. EDS map scanning analysis was conducted to qualitatively identify the surface elemental composition. This allowed surface topography and mineral impurities on the coal samples to be identified directly. 2.2.3. XPS Analysis. Interfacial properties play a crucial role in coal particle−collector interaction. Recently, XPS has emerged as an advanced interface chemistry analysis tool and has been used extensively in flotation and energy engineering applications.9 To obtain more comprehensive information about the fresh/oxidized coal surface properties, an XPS system (ESCALAB 250Xi, Thermo Scientific, Waltham, MA, USA) with Al Kα radiation (hν =1486.6 eV) and a 900 μm light spot size was used to characterize the surface chemistry changes before and after oxidation. Each XPS spectra data set was fitted using the XPS Peakfit software. It should be noted that binding energy calibration was performed by setting the C 1s hydrocarbon peak at 284.6 eV. 2.2.4. Flotation Experiments. Fresh/oxidized coal flotation experiments were carried out in an XFDIII 1 dm3 laboratory flotation machine. Hydrocarbon oil (dodecane) and fatty acid (oleic acid) were used as the collector, respectively. For fresh coal, the dosages of dodecane or oleic acid were 0, 100, 200, 300, and 400 g/t. For oxidized coal, the consumptions of dodecane or oleic acid were 100, 300, 500, 700, 900, and 1100 g/t, respectively. The frother octanol dosage and impeller rotation were kept constant at 100 g/t and 1800 rpm. The solid content and air flow rate were kept constant at 80 g/dm3 and 0.15 m3/h, respectively. The pulp was prewetted for 2 min. After the prewetting process, collector and frother were added to the pulp stepby-step. The collector and frother conditioning process were maintained at 2 and 0.5 min. At the end of the conditioning period, bubbles were induced forming froth on the upper pulp surface. The flotation process was maintained for another 3 min. The flotation concentrates and tailings were collected, filtered, dried at 80 °C, and weighed. For the ash analysis, the dried samples were first ground using a mortar and pestle. Then, approximately 1 g of sample was burned in an oven at 815 °C for 2 h. The leftover ash was weighed to calculate the ash content.

Figure 2. Interaction forces between paraffin/stearic acid and fresh coal.

flotation. Different force behaviors were observed for the coal− stearic acid system. A repulsive force was detected at about 75 nm distance. The repulsive force always increased with decreasing separation distance. Note that a long-range slightly repulsive force and then jump-into contact at different separation distances was always observed in the individual force curves. Jump-into behavior disappeared in the general trend force curve in Figure 2 due to the average effect during the data treatment. Three individual force curves between stearic acid and fresh coal are presented in Figure 3. The

Figure 3. Three individual force curves between stearic acid and fresh coal.

repulsive force may be due to the electrostatic interaction between the carboxyl group in the stearic acid molecule and the negative charged coal surface, while the attractive force could be attributed to a hydrophobic bonding between the hydrocarbon chain in stearic acid and the fresh coal surface. The above AFM force curves indicate that both the hydrocarbon oil and fatty acid could be used as collectors for fresh coal flotation. However, hydrocarbon oil may be the more effective one compared with fatty acid because of the absence of repulsive force. To explain and validate the AFM results, SEM, XPS tests and fresh coal flotation experiments with hydrocarbon oil (dodecane) and fatty acid (oleic acid) were carried out. The SEM-EDS results are shown in Figure 4. No obvious cracks

3. RESULTS AND DISCUSSION 3.1. Interaction Forces between Paraffin/Stearic Acid and Fresh Coal. The interaction forces between paraffin/ stearic acid and fresh coal particle are shown in Figure 2. A pure attractive force at 25 nm separation distance and then a jumpinto contact were observed between fresh coal and paraffin in deionized water. This may be attributed to the hydrophobic force between hydrophobic coal and paraffin. It indicates that hydrocarbon oil could be an effective collector for fresh coal C

DOI: 10.1021/acs.energyfuels.6b02856 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 4. SEM-EDS results of the fresh coal.

were found on the fresh coal surface. Minerals’ impurities always appeared to be gray-white in SEM images. Si, Al, Fe, and Ca were the main inorganic elements in the fresh coal. The XPS wide energy spectrum of fresh coal is shown in Figure 5.

Semiquantitative results for surface chemical composition based on XPS wide energy spectrum are presented in Table 2. The carbon and oxygen contents are 63.90% and 22.97%, respectively. The main inorganic mineral elements present were Si and Al, consistent with EDS results. The C ls peak of fresh coal is shown in Figure 6. The binding energy peaks at 284.60, 285.60, 287.60, and 289.10 eV represent the groups of C−C and C−H, C−O, CO, and OC−O, respectively. The peak split result of C is shown in Table 3. A small amount of oxygen-containing groups existed on the fresh coal surface while the content of C−C/C-H was as high as 83.31%. This indicates that fresh coal particles have a good hydrophobicity and floatability, and the C−C/C−H group plays the key role in the fresh coal−collector interaction. The effect of dodecane and oleic acid on fresh coal flotation is shown in Figure 7. A maximum combustible recovery of 92.35% with 51.41% ash recovery was obtained when 400 g/t

Figure 5. XPS wide energy spectrum of fresh coal. D

DOI: 10.1021/acs.energyfuels.6b02856 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 2. Semiquantitative Results of the Surface Chemical Composition of Fresh Coal type

C 1s, %

O 1s, %

Al 2p, %

Si 2p, %

N 1s, %

Na 1s, %

Ca 2p, %

fresh coal

63.90

22.97

4.95

6.02

1.53

0.31

0.32

3.2. Interaction Forces between Paraffin/Stearic Acid and Oxidized Coal. The interaction forces between paraffin/ stearic acid and oxidized coal particle are shown in Figure 8. In

Figure 6. Cls peak of fresh coal.

Table 3. Peak Split Result of Carbon of Fresh Coal type

C−C/C−H, %

C−O, %

CO, %

OC−O, %

fresh coal

83.31

8.31

5.30

3.08

Figure 8. Interaction forces between paraffin/stearic acid and oxidized coal.

contrast to the case of fresh coal, a significant jump-into contact between oxidized coal particles and stearic acid at 20 nm distance was found while a monotonous repulsive force existed between oxidized coal and paraffin. The hydrogen bonding between hydrophilic sites on the oxidized coal surface and stearic acid may be responsible for the attractive force. Due to the firm hydration film on the oxidized coal surface, the absorption of traditional hydrocarbon oil was denied. It should be noted that although a jump-into contact in the stearic acid− oxidized coal system was observed in Figure 8, the force in the range of 0−7 nm was still repulsive. This indicates that a special water film may exist between stearic acid and oxidized coal. The water molecules in the hydration film may act as the bridging role of hydrogen bonding for stearic acid absorption on the oxidized coal surface. This result is consistent with the speculation of Gui et al.25 Nevertheless, fatty acid collectors should be more effective for oxidized coal flotation than hydrocarbon oil. The SEM-EDS results, XPS wide energy spectrum, the C ls peak, semiquantitative results of surface chemical composition, and the peak split results of C for oxidized coal are shown in Figures 9, 10, and 11 and Tables 4 and 5, respectively. The surface physical chemical properties of oxidized coal were significantly different from that of fresh coal. Obvious cracks and more hydrophilic mineral impurities were found on the oxidized coal surface. Also, the content of the carbon element decreased from 63.90% to 51.12% while the oxygen content increased from 22.97% to 31.60% after H2O2 oxidation based on XPS wide energy spectrum. The contents of inorganic mineral elements also increased after oxidation. These findings indicate that carbon groups on the fresh coal surface were attacked by oxygen and then generated carbon dioxide during oxidation, leading to the exposure of mineral impurities and the formation of cracks. The C 1s peak split results show that C−O and CO groups increased from 8.31% to 12.34% and from

Figure 7. Effect of dodecane and oleic acid on fresh coal flotation.

dodecane was used. In contrast, 89.64% combustible recovery with 52.55% ash recovery was obtained when 400 g/t oleic acid was used. Effective fresh coal flotation can be achieved by using both dodecane and oleic acid, while the recovery achieved with dodecane was always higher than that with oleic acid. The fresh coal flotation results are consistent with the AFM force curves. Because of the absence of oxygen-containing groups on the fresh coal surface, hydrophobic bondings between C−C/C−H groups on the coal surface and hydrocarbon chains in collectors are the main driving force for collector adsorption. The polar carboxyl group in oleic acid was detrimental to fresh coal flotation due to the introduction of a repulsive electrostatic force. It should be noted that the gangue could be also collected by oleic acid due to the hydrogen bonding interaction and, thus, the flotation selectivity was deteriorated. For example, a lower combustible recovery with higher ash recovery was obtained using 400 g/t oleic acid compared with that of using 400 g/t dodecane. Conventional hydrocarbon oil is more suitable for fresh coal flotation than fatty acid. E

DOI: 10.1021/acs.energyfuels.6b02856 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 9. SEM-EDS results of the oxidized coal.

Figure 10. XPS wide energy spectrum of oxidized coal.

Figure 11. Cls peak of oxidized coal.

5.30% to 10.29% after H2O2 oxidation, respectively, while C− C/C−H groups decreased from 83.31% to 72.83%. This is consistent with the XPS wide energy spectrum. A hydrogen film F

DOI: 10.1021/acs.energyfuels.6b02856 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 4. Semiquantitative Results of the Surface Chemical Composition of Oxidized Coal type

C 1s, %

O 1s, %

Al 2p, %

Si 2p, %

N 1s, %

Na 1s, %

Ca 2p, %

others, %

oxidized coal

51.12

31.60

5.43

6.91

1.88

0.59

0.64

1.83

Conventional hydrocarbon oil is more suitable for fresh coal flotation than fatty acid. (2) A significant jump-into contact between oxidized coal particles and stearic acid at 20 nm distance was found while a monotonous repulsive force existed between oxidized coal and paraffin. Only 60.55% combustible recovery with 25.89% ash recovery was obtained even when 1100 g/t dodecane was used. However, the oxidized coal flotation could be enhanced by using oleic acid. Combustible recovery of 77.76% with 41.78% ash recovery could be obtained when 700 g/t oleic acid was used. Fatty acid collector is more suitable for oxidized coal flotation than hydrocarbon oil. (3) The present study has demonstrated that AFM can be used as a powerful tool to investigate the interaction between coal particles and different types of collectors in coal flotation engineering. The interaction forces between paraffin/stearic acid and fresh/oxidized coal particles were measured directly using AFM colloidal probe technique. However, the AFM force curves were only qualitatively compared and discussion was also limited to qualitative speculation. Further experiments should be carried out to quantitatively analyze the AFM results with advanced sample preparation method.

Table 5. Peak Split Result of Carbon of Oxidized Coal type

C−C/C-H, %

C−O, %

CO, %

OC−O, %

oxidized coal

72.83

12.34

10.29

4.55

formed on these polar hydrophilic sites leading to a poor hydrophobicity and floatability for oxidized coal. The effect of dodecane and oleic acid on oxidized coal flotation is shown in Figure 12. The oxidized coal flotation

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-13775989229. Fax: +86-051683884289. Email: [email protected].

Figure 12. Effect of dodecane and oleic acid on oxidized coal flotation.

ORCID

recovery was significantly lower than that of fresh coal. Only 60.55% combustible recovery with 25.89% ash recovery was obtained even when 1100 g/t dodecane was used. However, the oxidized coal flotation could be enhanced by using oleic acid. Combustible recovery of 77.76% with 41.78% ash recovery could be obtained when 700 g/t oleic acid was used. Further increasing its dosage, the combustible recovery did not change apparently while ash recovery increased dramatically. The oxidized coal flotation results are also consistent with the AFM force curves. Hydrogen bonding between the large number of C−O/CO groups on the oxidized coal surface and the carboxyl group in oleic acid was responsible for the high flotation recovery of oxidized coal. The adsorption of dodecane was prevented because of repulsive forces between dodecane and the oxidized coal surface. Fatty acid collector is more suitable for oxidized coal flotation than hydrocarbon oil. It should be noted that the dosage of fatty acid should be controlled precisely during the oxidized coal flotation in the case of recovery of gangue minerals such as silica and silicate.

Xiahui Gui: 0000-0001-9270-7756 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Nature Science Foundation of China (Grant 51574236) and the Fundamental Research Funds for Central Universities (Grant 2015XKMS095) for which the authors express their appreciation. Y. Xing and X. Gui also appreciate the financial support from the China Scholarship Council for their study in Max Planck Institute for Polymer Research.

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REFERENCES

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4. CONCLUSIONS (1) A pure attractive force at 25 nm separation distance and then a jump-into contact due to hydrophobic bonding were detected between fresh coal and paraffin in deionized water while a slightly repulsive force before the jump-into contact was always observed between fresh coal and stearic acid. A maximum combustible recovery of 92.35% with 51.41% ash recovery was obtained when 400 g/t dodecane was used. In contrast, 89.64% combustible recovery with 52.55% ash recovery was obtained when 400 g/t oleic acid was used. G

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DOI: 10.1021/acs.energyfuels.6b02856 Energy Fuels XXXX, XXX, XXX−XXX