Understanding Interactions between Clay and Model Coal Surfaces in

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Understanding interactions between clay and model coal surfaces in electrolyte solutions by QCM-D study Qian Chen, Tiantian Cao, Yong Xiong, Chen Wang, Zehui Lin, Zihui Chen, Shengming Xu, and Zhenghe Xu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02976 • Publication Date (Web): 25 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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Understanding interactions between clay and model coal surfaces in electrolyte solutions by QCM-D study Qian Chena, Tiantian Caoa, Yong Xionga, Chen Wangc, Zehui Linc, Zihui Chenc, Shengming Xua,*, Zhenghe Xub,c,*

a

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China

b

Department of Materials Science and Engineering, Southern University of Science and Technology,

Shenzhen 518055, China c Department

of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G

1H9, Canada

Abstract: Clay slime coating is known to have a negative impact on coal flotation. It is therefore of great importance to study interactions between clay and coal under flotation conditions. In this study, the effect of electrolytes on interactions of montmorillonite (MMT) clays with carbon or asphaltenes on sensor surfaces of quartz crystal microbalance with dissipation (QCM-D) as model coal was investigated. Ion beam sputtering deposition and spin coating methods were used to prepare carbon and asphaltene (a class of natural polyaromatic compounds) surfaces. The prepared surfaces were characterized by Raman spectroscopy and atomic force microscope (AFM). QCM-D was used to determine in situ interactions between MMT clays and model coal surfaces in electrolyte aqueous solutions. The MMT clays were shown to deposit significantly on the model coal surfaces only in the presence of NaCl or CaCl2 as a result of the compression of electrical double layers. Moreover the enhanced deposition by calcium ions is stronger than sodium ions. The results from this fundamental study provide a better understanding of the critical role of water chemistry in modulating interactions of clays with coal surfaces and scientific guidance to achieve better performance of fine coal flotation.

ACS Paragon Plus Environment

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1. INTRODUCTION Despite great efforts and tremendous investments on developing renewable energies in an effort to combat greenhouse gas emissions and provide security for future energy supplies, the fossil fuels (oil, natural gas and coal) remain a major source of energy for foreseeable future. A recent study predicted a total energy demand greater than 700 quadrillion BTU by 2040, among which 77% comes from fossil fuels with coal accounting for 25% of total energy demand. With the rapid depletion of high quality coal deposits, the utilization of low quality coal of high clay-type mineral matter content has become inevitable. In addition to shorter life time of combustion boilers, utilizing coal of higher mineral matter content would lead to low efficiency of energy utilization and cause severer air pollutions such as emission of fine particulates (PM2.5). It is therefore of great importance to improve coal quality by rejecting more mineral matters prior to its utilization.1 Flotation is considered to be the most versatile technology to physically upgrade coal, especially for fine coals. The presence of fine clays in the raw (run of mine) coal is known to pose a great challenge to fine coal flotation.2-5 The deleterious effect of clays on coal flotation has largely been attributed to slime coating of fine clays on naturally hydrophobic coal.6-8 Arnold and Aplan for example studied systematically the role of fine clays in coal flotation and reported serious depression of fine coal recovery by bentonite but not by kaolinite and illite clays.2 In a recent study, a novel zeta potential distribution measurement technique was used to determine the interactions between fine coal and various types of clays, which showed a significant slime coating of montmorillonite (MMT) clay but not kaolinite on fine coal, providing a direct evidence on the depression mechanism of fine coal flotation by slime coting of bentonite/MMT but not kaolinite on fine coal. An increase in flotation pH from 5 to 10 was shown to decrease MMT clay slime coating and hence improve fine coal flotation, illustrating the critical role of the sign and the value of coal surface charge and the type of clays in controlling the slime coating of clays on coal.9 The zeta potential distribution measurement technique was also applied to studying the slime coating phenomenon of


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clays on oppositely charged mineral particles and hence depression of fine sphalerite and bitumen flotation.9-11 More recently, the zeta potential distribution technique was used to study interactions between solids and air bubbles.12 Wang et al. detected the presence of clay coatings on coal surfaces by cyro-scanning electron microscopy (SEM) and attributed the observed low combustible recovery to clay slime coating on coal surfaces.5 Although the zeta potential distribution measurement and cyro-SEM techniques have been successfully used to clearly demonstrate the slime coating of fine clays on coal/mineral surfaces, none of these techniques provides the quantitative information of slime coating on the coal/mineral surfaces.13 Furthermore, cyro-SEM is an ex-situ technique which could lead to artifacts of the results, while the zeta potential distribution measurement for studying slime coating depends on the difference in zeta potential of interacting components in question, which may not always be possible. Such condition limits the use of zeta potential distribution measurement technique to study slime coating phenomena in systems of general practical importance. Recently, Deng et al. used quartz crystal microbalance with dissipation (QCM-D) technique to study interactions of gypsum and calcite with silica and sphalerite in gypsum/calcite-supersaturated solutions.14 Gypsum supersaturated solution was found to promote hetero-aggregation between silica and sphalerite, and homo-aggregation between silica particles, which could not be studied by zeta potential distribution measurement due to its inherent limitations.9,

15-16

The hetero-aggregation between

silica and sphalerite minerals was shown to be responsible for the reduced flotation recovery and selectivity in both laboratory flotation tests and commercial operations. Using QCM-D technique, Bakhtiari et al. studied slime coating of kaolinite and illite on bitumen surfaces, and demonstrated a critical role of humic acid in reducing slime coating of illite on bitumen surfaces.4 QCM-D is a rapid, highly sensitive and easy to use instrument to study deposition of nanoparticles on various solid sensor surfaces in complex aqueous solutions.17 It is the objective of this study to determine interactions (slime coating) of fine clays (MMT) on coal surfaces, focusing on the effect of water


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chemistry on slime coating of fine clays. To achieve this objective, two model coal surfaces (i.e., sputter-coated carbon on gold sensor (SCC) and spin-coated asphaltenes on silica sensor (SCA)) were prepared and fully characterized by Raman spectroscopy techniques and atomic force microscope (AFM), respectively.

2. EXPERIMENTAL 2.1 Materials CaCl2 (>99.5%, Sigma-Aldrich) was used for preparation of electrolyte aqueous solutions. Reagent-grade sodium hydroxide (Fisher Scientific) was used to control solution pH as desired. Asphaltenes were extracted from Athabasca coker feed bitumen, an extra heavy crude provided by Syncrude Canada, Ltd. The procedures for extracting asphaltenes were described in great detail elsewhere.18-19 HPLC grade toluene (Fisher Scientific) was used as solvent to prepare asphaltene in toluene solutions used in spin coating for preparation of SCA surfaces. Milli-Q water (resistivity=18.2 MΩ·cm) was used for preparation of solid suspensions and electrolyte solutions as needed. Montmorillonite (MMT) clays were obtained from Ward’s Natural Science (Rochester, NY). The average particle size (d50) of MMT clays was measured by a Mastersizer 3000 (Malvern Instruments, U.K.) to be 0.29 μm. MMT suspensions were prepared to 0.05 wt% in electrolyte aqueous solutions of different water chemistries. Prior to QCM-D measurement, all the suspensions were sonicated for 20 min. 2.2 Preparation of model coal surfaces An essential requirement to study slime coating of clays on coal surfaces by QCM-D method is to prepare coal surfaces on quartz crystal sensors. Recognizing carbon being the most abundant elements (~80%) and polyaromatic nature of carbon in coal,20 two types of model coal surfaces were prepared by ion sputtering for carbon surface (SCC) and spin coating for polyaromatic asphaltene surface (SCA). The quartz crystal oscillators (sensors) of AT-cut family with a diameter of 14 mm and a fundamental shear oscillation frequency of 5 MHz were used for preparing model coal surfaces. The


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process of model coal surfaces preparation and QCM-D experiments is schematically shown in Figure 1. 2.2.1 Ion beam sputtering deposition The SCC surface was prepared by ion beam sputtering deposition.21 High purity graphite (mass fraction: > 99.995%) was selected as a target material. The deposition chamber was set at a base pressure less than 5×10−4 Pa. The ion beam voltage and current were set at 7000 V and 370 mA, respectively. High-purity (99.999%+) Ar was introduced into the ionization chamber and the ionized gas was used as the sputtering ion source. The working pressure during the deposition was set at 1×10−3 Pa. The carbon films were deposited at ambient temperatures. 2.2.2 Spin coating Although the exact structure is largely unknown, the polyaromatic nature of coal could be better represented by asphaltene molecules.22-23 The SCA surface was prepared using well established procedures described elsewhere.10 Briefly, the silica-coated wafer was first treated with UV/ozone, followed by the exposure to dichlorodimethylsilane (DDMS, Fisher Scientific) vapors for 2.5 min. The physio-sorbed DDMS was transformed to be chemically bound in a vacuum oven at 80 °C for 24 h. The contact angle of the sensor increased from 20°to 100°after such treatment. This pretreatment to prepare a hydrophobic surface is necessary to ensure stable asphaltene films being subsequently coated on the sensor surfaces. In spin coating, 10 g/L asphaltenes in toluene solution was prepared and centrifuged at 20,000 g-force for 15 min with the centrifugation step repeated 5 times to remove solid particles (asphaltene aggregates). Two drops of the asphaltene solutions were added slowly (over 60 s) to the center of the sensor surface spinning at 2500 rpm on a spin coater (Laurell WS-400A-6NPP/Lite). The sensor was then rotated at 4500 rpm for an additional 40 s to form a uniform surface and remove any excess solvent. The resultant surface showed a mirror-like appearance, indicating a smooth coating.


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Figure 1. Schematic process of model coal surfaces preparation and QCM-D experiments

2.3 QCM-D experiments A detailed description of the QCM experimental protocols is available elsewhere. Deposition kinetics of clay particles onto two model coal surfaces in different aqueous solutions is investigated by QCM-D technique. A QCM-D system from Q-sense (Gothenburg, Sweden) was used in this study. The cell was mounted on a Peltier element with an accurate temperature control (±0.02 °C). To eliminate the concern of deposition due to gravity, the chamber was inverted as such that any deposition detected is a direct consequence of clay slime coating on model coal surfaces as a result of colloidal forces. All the QCM-D experiments were run at a suspension flow rate of 0.15 mL/min at 22 °C. The changes in frequency and dissipation were measured simultaneously at 5, 15, 25, 35, 45, 55 and 75 MHz. All the measurements started by running the background solution (i.e., electrolyte solutions). After establishing a stable baseline in the background electrolyte solution for 5 mins, the clay suspension (0.05%) was pumped through the flow module continuously until a stable QCM-D signal (both Δf and ΔD) was reached. The fluid was then switched to the background electrolyte solution to remove any loosely attached particles. Voight model available in QCM-D software (Q-tool) was used to determine the amount of mass deposited on two model


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coal surfaces (SCC and SCA). The effect of 500 ppm MMT on the density and viscosity of water could be considered negligible. Therefor the density and viscosity of bulk fluid were replaced by the density (1000 kg·m-3) and viscosity (0.894 mPa·s) of water with the density of adsorbed MMT layer being the density of solid montmorillonite (2600 kg·m-3). All the QCM-D experiments were repeated at least twice to ensure the reproducibility. The experimental variability was measured to be less than 10%. 2.4 Characterization of SCC and SCA surfaces Raman spectroscopy is an important tool to obtain information on microstructure of carbon-based materials.24-27 Raman spectroscopy measurements were therefore performed at room temperatures. Raman spectra were obtained at an excitation of wavelength 532 nm. The samples were scanned between 1000 cm-1 and 2000 cm-1 (the spectral region that provides the most valuable information on the microstructure of carbons), with a data acquisition time of 20 s. The static contact angles of water on hydrophobic carbon surfaces were determined using a 3 µL of distilled water sessile drop at 25 oC, with the contact angle being measured through the water phase by a goniometer (DSA30, KRÜSS, Germany). In order to obtain representative values of contact angle, the measurement was repeated six times for each single sample under the same conditions. The typical error in contact angle measurement for the surfaces and technique used in this study is ±3o. Streaming potential of two model coal surfaces (SCC and SCA) was determined by SurPASS Instrument (Anton Paar, USA). One piece of QCM-D sensor was loaded into an adjustable gap cell, bounded between two cylindrical holders at 100 μm apart. This sample holder cell was then connected to two electrodes that can direct the background electrolyte solution parallel to the sensor surface, during which separation of charges occurs. Charged species or functional groups present on the sample surface attract their respective counter-ions from the flowing background solution, and the subsequent changes in conductivity of the flowing solution were detected by the electrodes.


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3. RESULTS AND DISCUSSION 3.1 Characterization of deposited SCC and SCA films For the SCC film, the static contact angle of the sensor surface increased from 76°to 85° after carbon coating, illustrating the surface coverage of original gold sensor surfaces. For the SCA film, the surface remained hydrophobic with a static contact angle of 100°. Thickness of carbon and asphaltenes deposited on QCM-D sensors was obtained using the software of Q-sense 401 by stitching the data files determined by measuring resonance frequency of sensor oscillations in air, before and after ion sputtering or spincoating deposition.28 A clean gold/silica QCM-D sensor was mounted in the QCM-D measurement chambers. Crystal oscillations were recorded for 5 min with an air flow rate at ∼0.15 mL/min. After carbon/asphaltenes deposited on QCM-D sensor, crystal oscillation of the same sensor was recorded again for 5 min under the identical conditions. The frequency change before and after carbon/asphaltenes deposition on gold/silica sensor was determined, which was converted to the mass of carbon or asphaltenes deposited on the gold/silica sensor surfaces using Sauerbrey equation in Qtool 401 software (C= 17.7 ng·Hz-1·cm-2 at 5 MHz). The typical measurement error in the mass of carbon or asphaltenes deposited with this method is within ±1%. The measured mass was converted to thickness using an effective density ρSCC=2.1 g·cm-3 for carbon, and ρSCA=1 g·cm-3 for asphaltenes. The mean thickness of carbon/asphaltenes layer deposited is 120 nm and 200 nm for SCC and SCA, respectively. To compare the microstructure of two model coal surface with real coal, a Raman spectrometer from Horiba Jobin Yvon S.A.S. (France) was used to investigate the degree of ordering and crystallinity in these three carbonaceous materials. The Raman spectra of these carbonaceous surfaces over the spectral range of 1000-1800 cm-1 with λ0= 532 nm are shown in Figure 2. The spectrum of SCA features the similar shape to the common coal (lignite, bitumite, anthracite), exhibiting two broad and overlapping bands with the intensity maxima at ~1363 cm-1 and ~1602 cm-1. The band at 1602 cm-


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1

(G band) is attributed to stretching vibrations of sp2 bonds in hexagonal aromatic

molecules of graphitic carbon, while the band at 1363 cm-1 (D band) is attributed to disorder-allowed phonon vibrational modes.27,

29-30

Considering a strong signal

intensity of the two (G and D) bands, the spectra can be further deconvoluted to show an additional small band (D3) at ~1496 cm-1. The D3 band has been attributed to sp2bonds in amorphous carbons.24, 31 In contrast, the spectrum of SCC exhibits only a broad band which can also be deconvoluted into three bands centered at ~1372 cm-1, 1501 cm-1 and 1563 cm-1, corresponding to D, D3 and G bands, respectively.

Figure 2. Raman spectra of different carbonaceous samples (The black lines is the Raman data, and the red, blue and green lines represent the fitted curve). Spectroscopic parameters, such as band positions, bandwidth (full widths at half maximum, FWHM) of D, D3 and G bands, and intensity ratio of D and G bands (ID/IG) are summarized in Table 1. Qualitatively, the FWHM of the G band is a key parameter describing the structural disorder in amorphous carbon.29, 32 A narrow G band (i.e., small FWHM) indicates a highly crystalline material. The FWHM values of G band for SCA, lignite, bitumite and anthracite are comparable at 66 cm-1, 69 cm-1, 72 cm-1 and 83 cm-1, respectively. These values are much greater than the corresponding values of highly ordered pyrolytic graphite, indicating a less crystalline nature of SCA, lignite, bitumite and anthracite. In comparison, the FWHM value of G band for SCC (140 cm-


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1

) is much larger than the values for lignite, bitumite, anthracite and SCA, indicating

much less crystalline nature of coated SCC as anticipated. From the relative intensity of band D3 to D and G bands (ID3/(ID+IG)), we can further estimate the relative concentration of amorphous carbons in these five carbonaceous samples. The values of ID3/(ID+IG) in Table 1 show similar amorphous carbon content in lignite, bitumite, anthracite and SCA (9%, 3%, 7% and 9%, respectively), which is much smaller than the value for SCC (30%). Furthermore the relative intensity of D and G bands (ID/IG) represents the degree of disorder carbons in these five carbonaceous samples. These results collectively show that two model carbon surfaces have different degrees of disorder and crystallinity that could represent surfaces of real coal. The results indicate resemble of SCA to anthracite and SCC to bitumite. Therefore, these two carbon surfaces are suitable for studying slime coating of clays using QCM-D techniques to shed the lights on fine coal flotation. 3.2 Effect of NaCl on deposition of MMT clays on SCC and SCA surfaces. In coal flotation, the pH of pulp is normally set at ~8.5 to minimize negative effect of clays on coal recovery.5 Effect of electrolyte (NaCl) concentration at this pH on deposition of MMT clays on model coal surfaces (slime coating) was studied first. Representative frequency shifts normalized by corresponding over tone (Δf = Δf3/3) of the SCC and SCA are shown in Figure 3. The results showed that no MMT deposition on SCC and SCA without NaCl addition. Increasing the concentration of NaCl from 1 mM to 100 mM, the deposition of MMT on model coal surfaces was shown by Δf becoming increasingly more negative, more so for SCC surfaces than SCA surfaces. The maximum mass (mmax) of the MMT clay particles deposited on SCC and SCA surfaces from suspensions of different NaCl concentrations was obtained by analyzing the frequency and dissipation data with the Voigt model. The results in Figure 4 show a significant increase in mmax of MMT clay deposition from 0 to 18.73 µg/cm2 on SCC and 9.89 µg/cm2 on SCA, respectively, with increasing NaCl concentration from 1 mM to 100 mM.


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Table 1. Fitting parameters obtained from Raman spectra of the different carbonaceous materials Sample

Lignite

Bitumite

Anthracite

SCC

SCA

Band

Position (cm-1)

Bandwidth* (cm-1)

D

1380

268

D3

1532

60

G

1594

69

D

1362

221

D3

1512

43

G

1589

72

D

1369

233

D3

1508

55

G

1592

83

D

1372

485

D3

1501

259

G

1563

140

D

1363

212

D3

1496

42

G

1602

66

ID/IG

ID3/(ID+IG)

4.03

0.09

2.23

0.03

1.59

0.07

2.5

0.3

1.72

0.09

*full widths at half band maximum In the classical DLVO theory, the two fundamental colloidal forces, i.e., the attractive van der Waals forces and the electrostatic double layer force are included to describe the stability of a colloidal system. The attractive van der Waals forces are determined by the Hamaker constant of a given system and do not change substantially with changing electrolyte concentration and/or solution pH. On the other hand, the electrolytes (such as the type and concentration of electrolyte and/or solution pH) play an important role in controlling the electrostatic double layer force by changing the surface (often approximated by zeta) potential of two surfaces and double layer thickness.


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Figure 3. Frequency shift (Δf) obtained by QCM-D when MMT was deposited on SCC (a) and SCA (b) as a function of time at pH 8.5 and different levels of NaCl.

Figure 4. Maximum mass deposited (mmax) determined from the third overtone results during the deposition of MMT onto SCC and SCA over a range of NaCl concentrations at pH 8.5. Figure 5 shows zeta potential of SCC, SCA and MMT clay at pH 8.5 and various NaCl concentrations. Zeta potentials of SCC, SCA and MMT clay are all negative at pH 8.5, and become less negative with increasing NaCl concentration as a result of charge screening. The zeta potential value of SCC, SCA and MMT clay decreases for example from −81.6, −123.4 and −41.5 mV to −44.6, −90.1 and −15.3 mV, respectively, with increasing NaCl concentration from 0 to 100 mM. Therefore, the repulsive electrostatic


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double layer force between MMT clay particles and SCC or SCA surface is anticipated to decrease with increasing NaCl concentration in solution. As a result, more MMT clay particles deposit on SCC and SCA surface in higher NaCl concentration solution. It is interesting to note a much more negative zeta potential of SCA than SCC. A stronger deposition of MMT clays on SCC than SCA is anticipated as observed experimentally. It is clear that classic DLVO forces control the deposition of MMT clay particles on model coal surfaces.

Figure 5. Zeta-potential of SCC, SCA and MMT clay as a function of NaCl concentration at pH 8.5. 3.3 Effect of CaCl2 on deposition of MMT clays on SCC and SCA surfaces. To understand the adverse effect of calcium ions in fine coal flotation, the deposition of MMT clay particles on two carbon surfaces from suspensions with different calcium ion concentrations at pH 8.5 was studied. The results in Figure 6a show a decrease in Δf to −21 Hz with the addition of 0.05 mM CaCl2, indicating a strong deposition of MMT clays on the SCC surface. At higher CaCl2 concentrations (0.15, 0.45 and 1 mM CaCl2), increasingly more dramatic decrease in Δf was observed. In 1 mM CaCl2 solutions, for example, a decrease in frequency to lower than -200 Hz was observed, indicating an enhanced deposition of MMT clay particles on SCC with increasing CaCl2


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concentration. Figure 6b shows a similar effect of CaCl2 on deposition of MMT clays on SCA surfaces, although it is to a less extent to -150 Hz in 1 mM CaCl2 solutions.

Figure 6. Frequency shift (Δf) of SCC (a) and SCA (b) in MMT suspension of different levels of CaCl2 at pH 8.5 as a function time. The results in Figure 7 show an increase in the mmax of MMT clay particles deposited on the SCC surface from 1.59 µg/cm2 to 17.11 µg/cm2 with increasing CaCl2 concentration from 0.05 mM to 1 mM. For a given CaCl2 concentration the mmax of MMT clay particles deposited on the SCA surface is always lower than that on the SCC surface.

Figure 7. Maximum deposition (mmax) of MMT onto SCC and SCA determined from the third overtone at pH 8.5 as a function of CaCl2 concentration.


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To understand the observed deposition characteristics of MMT clays on model coal surfaces shown in Figure 7, zeta potential of SCC, SCA and MMT clay surfaces was measured in CaCl2 aqueous solutions. The results in Figure 8 show a decrease in the value of zeta potential with increasing the concentration of CaCl2. Therefore, the repulsive electrostatic double layer force between SCC or SCA surface and MMT clay particle is anticipated to decrease with increasing the CaCl2 concentration in solution. Based on more negative charge of SCA surfaces than SCC surfaces, one would anticipate a stronger deposition of MMT clays on SCC than on SCA, which is exactly what was observed. The results once again indicate critical role of decreasing electrostatic double layer forces in the presence of CaCl2 in promoting slime coating of clays on coal surfaces. It is interesting to note a more significant impact of CaCl2 than NaCl on zeta potential of MMT clay and model coal surfaces. As shown in Figure 8, as low as 0.15 mM of CaCl2 significantly decreased the zeta potential values of SCC, SCA and MMT clay to −44.2, −70.2 and −12 mV, indicating the stronger effect of charge neutralization from divalent cations as anticipated. This phenomenon was also observed in other studies.3334

In a natural aquatic environment, the monovalent (Na+) cation concentration is

usually below 10 mM in a typical aquatic environment, and the divalent (Ca2+) cation concentration is about 1 mM. According to the results in Figures 4 and 7, the deposited mass of MMT clays on the SCC and SCA surfaces in 10 mM NaCl solution is much less than that in 1 mM CaCl2 solution. Therefore, the deposition of MMT clay nanoparticles on fine coal particle (slime coating) in the natural aquatic environment is primarily controlled by divalent cations. 3.4 Effect of pH on deposition of MMT clays on SCC and SCA surfaces. By establishing the critical role of surface (approximated by zeta) potential in controlling slime coating of MMT on model coal surfaces, it would be interesting to investigate whether imposing more negative surface potential by increasing pH could alleviate the slime coating of MMT on model coal surfaces, i.e., using pH regulation as a method to suppress slime coating. The results in Figure 9 show that in the presence


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of 0.15 mM CaCl2, increasing suspension pH from 8.5 to 10 significantly suppressed the decrease in frequency for both model coal surfaces, indicating much less deposition of MMT clays on model coal surfaces from suspensions of higher pH. At a given suspension system, MMT showed a more significant deposition on SCC surface than SCA surfaces. In both cases, only a slight detachment of deposited MMT was observed with washing, indicating strong binding of MMT clays with model coal surfaces.

Figure 8. Zeta-potential of SCC, SCA and MMT clay as a function of CaCl2 concentration at pH 8.5.

Figure 9. Frequency shift (Δf) of SCC (a) and SCA (b) in MMT clay suspensions in the presence and absence of 0.15 mM CaCl2 at pH 8.5 and 10 as a function of time. As shown in Figure 10, the maximum deposition (mmax) of MMT particles on both SCC and SCA surfaces decreased with increasing pH from 8.5 to 10. Despite significantly


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less deposition of MMT clays on SCA than on SCC, the extent of suppression by increasing pH from 8.5 to 10 was found to be similar, indicating a similar effect of increasing pH on depressing MMT deposition on model coal surfaces. The results correspond well to the increased recovery of fine coal flotation at higher pH than at lower pH due to reduced slime coating, as reported earlier.7 Zeta-potential of SCC, SCA and MMT clay at pH 6.5, 8.5 and 10 is shown in Figure 11. It is evident that adding 0.15 mM Ca2+ significantly increased zeta potential (becoming less negative) for all three surfaces at all three pHs investigated. In the presence of 0.15 mM Ca2+, increasing pH from 8.5 to 10 made the surface of SCC and MMT more negative as shown in Figure 11 b, which would induce a stronger electrostatic repulsion between SCC and SCA and hence a reduced deposition of MMT on SCC as observed in Figure 10. Although increasing pH in the presence of 0.15 mM Ca2+ did not change the negative zeta potential of SCA substantially, one would still expect a decrease in deposition of negatively charged MMT on negatively charged SCA as observed in Figure 10 although to a less extent as compared with the case of MMT on SCC. This decrease also arises from to the increased electrostatic repulsion due to increasing negative zeta potential value of MMT with increasing pH from 8.5 to 10.

Figure 10. Maximum mass deposition (mmax) on SCC and SCA in the presence of 0.15 mM CaCl2 at pH 8.5 and 10.


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Collectively, the current study clearly demonstrates the important role in controlling electrostatic double layer forces in mediating advert effect of slime coating on fine coal flotation. The results from our study confirm that locking up calcium ions by either carbonate addition or use of calcium complex reagent such as citric acid to alleviate slime coating without affecting surface charge and hence hydrophobicity of fine coal would greatly enhance fine coal flotation.

Figure 11. Zeta-potential of SCC, SCA and MMT clay at pH 6.5, 8.5 and 10 in the presence of (a) no Ca2+, (b) 0.15 mM Ca2+.

4. CONCLUSIONS QCM-D is shown to be an ideal technique for studying the interactions encountered in fine coal flotation. Two model coal surfaces representing two different types of coal were prepared successfully by sputter coating of carbon (SCC) and spin coating of asphaltenes (SCA) on QCM-D sensor surfaces. Raman spectroscopy investigation revealed that the model coal surface with different degrees of disorder and crystallinity can be used to represent surfaces of different types of coal. The deposited mass of MMT clay particles on SCC and SCA surfaces was found to occur only in the presence of NaCl or CaCl2, as a result of screening electric double layers and hence suppressing electrostatic repulsion. Divalent cations were found to have a more profound effect than monovalent cations on MMT deposition on model coal surfaces, as predicted by the classical DLVO theory. Increasing pH suppressed deposition of MMT clays on both


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model coal surfaces as a result of increasing surface charges of both model coal surfaces and MMT clays, leading to a stronger electrostatic repulsion between MMT and model cola surfaces. The results from this study help to understand the clay slime coating in other carbonaceous materials processing, such as graphite suspensions and oil emulsions.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. [email protected]

ACKNOWLEDGMENTS The financial support for this project from the National Natural Science Foundation of China (GrantNumber 51274129) is very much appreciated.

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