Ultrasound-promoter pre-treatment for enhancing the yield and

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Ultrasound-promoter pre-treatment for enhancing the yield and combustible matter recovery of high-ash oxidized coal flotation Santosh Deb Barma, S S Praneeth Tej, Boddepalli Ramya, and R Sathish Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01543 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 26, 2019

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Ultrasound-promoter pre-treatment for enhancing the yield and combustible matter recovery of high-ash oxidized coal flotation Santosh Deb Barma1*, S S Praneeth Tej1,2, Boddepalli Ramya1,3, R Sathish4

1Mineral

Processing Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, INDIA-751013

2Chemical

Engineering Department, GMR Institute of Technology, Rajam, Andhra Pradesh, INDIA-532127

3Chemical

Engineering Department, National Institute of Technology, Rourkela, Odisha, INDIA-769008

4Environment

& Sustainability Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, INDIA-751013

*

To whom correspondence should be addressed. Email: [email protected] or [email protected] ; Tel: +91 674 237 9449

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Abstract The nature of coal surface changes from hydrophobic to hydrophilic due to the adsorption of hydrophilic oxygenated functionalities upon weathering or natural oxidation. The presence of such oxygenated functionalities interferes the interaction between collector and coal particles, leading to the poor adsorption of collector over coal surface and therefore, reduce the flotability of coal. In order to enhance the flotability of oxidized coal, an attempt has been made to remove the oxygenated functional groups from oxidized coal by ultrasound, promoter and combined ultrasound-promoter pre-treatment. Ethanol was used as a promoter in this study. The optimized ultrasound pre-treatment time during flotation was found to be 5 min wherein maximum yield of 46.94% and combustible matter recovery of 60.39% were achieved. The effects of ethanol dosage and pre-treatment time on the flotability of oxidized coal were also studied in absence and presence of ultrasound. It was found that the combined ultrasoundethanol pre-treatment showed higher de-oxidizing potential than those of ultrasound or ethanol pre-treatment alone. Under the combined ultrasound-ethanol pre-treatment, the flotation concentrate showed maximum yield and combustible matter recovery of 65.92% and 87.55%, respectively, at 5 min ultrasonic pre-treatment time, 2.5 ml/kg ethanol dosage and 5 min ethanol pre-treatment time. FTIR analysis was also employed on the different flotation concentrates to compare the de-oxidizing potential of each pre-treatment mode.

Keywords: Ultrasound; Oxidized coal; Flotation; Promoter; Cavitation;


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1. Introduction Coal, as a primary fossil fuel, would remain as an important source of energy to many industrial sectors such as power, steel making, cement making, and so on1. During the mining and beneficiation of coal, large amount of coal fines generate, which could potentially act as a rich carbon source. The processing of coal fines by flotation process remains a widely accepted technique in many coal beneficiation plants due to its suitability, low cost and simple operation. Despite these advantages, the response of flotation process is inefficient when the surface of coal becomes oxidized. This abruptly affects the performance of flotation process, thereby reducing the flotability of coal 1-4. The oxidation of the coal refers to the adsorption of oxygenated functional groups such as hydroxyl (–OH), carboxyl (–COOH), carbonyl (–CO), ether (C–O–C), ketone (C=O) and methoxy (–OCH3) at the external surface of coal particles as well as internal surface of coal pores 5-7. Ambient and coal properties are the major two factors responsible for the oxidation of coal. Ambient properties consist of natural mass transport and diffusion of oxygen, ambient temperature, humidity and partial pressure of oxygen, whereas coal properties include history of coal oxidation or weathering, composition, particle size, porosity, moisture content and internal surface area 5. In general, the oxidized coal may be classified as weathered coal and high-temperature oxidized coal. The former case is based on the adsorption of atmospheric oxygen moieties from nature into the coal surface under the influence of wind, sunlight and precipitation. On the other hand, the latter case deals with the self-heating and spontaneous combustion of coal under hypoxic temperature 4. In practical situations, oxidation of coal takes place due to prolonged air exposure to the coal seams, stockpiled coal, during coal transportation and in the left coal of abandoned mines.


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The phenomena for the oxidation of coal is a complicated process which may be divided into four stages namely, (i) transportation of oxygen from atmosphere to the coal surface by convective mass transfer, (ii) diffusion of oxygen within the coal pores, (iii) chemical interaction between oxygen moieties and coal particles and (iv) release of heat and gaseous products such as CO2, CO and H2O 5. Note that, the degree of complexity of these four stages (i-iv) varies depending on the physical and chemical properties of coal. During the oxidation process of coal, peroxides or peroxygen (–O–O–) are believed to be the first intermediates formed due to the coupling of bi-radical oxygen molecules with a free carbon centre in the coal aliphatic or aromatic structure 5,6. Subsequently, abstraction of hydrogen atom by the peroxide radical from coal’s aliphatic or aromatic structure takes place, which leads to the formation of hydroperoxide (–O–O–H) along with the generation of a new carbon centre

7-9.

The radial

displacement of hydroperoxides occur then, leading to the formation of –OH and –C–O–C– groups 9. The so formed –OH species further undergo thermal decomposition and produce carbonyl groups. It was also reported that the thermolysis of the hydroperoxides results in the formation of carbonyl-containing products such as carboxylic acids and aldehydes 5. The decarboxylation or decarbonylation reactions were found to occur during oxidation process, which corresponds to the decomposition of carboxyl and carbonyl species and release of CO2, CO and H2O 10-12. A schematic mechanism of the coal oxidation process is shown in Fig. 1. Flotation process is one of the physicochemical methods for fine coal cleaning and its separation principle is based on the selective separation of hydrophobic carbonaceous matter from hydrophilic inorganic impurities 1. As a result of oxidation of coal, the coal particles become hydrophilic from hydrophobic state due to the presence of oxygenated functional complexes at both external surface as well as internal pores of coal 5. This modification in coal surface significantly obstructs the flotability and reduces the coal concentrate recovery 2. Using oily collector, the beneficiation of oxidized coal is very difficult in a conventional flotation due 4 ACS Paragon Plus Environment

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to poor selectivity of collector towards oxidized coal particles, and hence require upgradation either by introducing promoters or intensifying the process using some means to improve the collector selectivity.

Fig. 1. Coal oxidation mechanism by chemical adsorption/chemisorption. (Redrawn from reference 5) In the last few decades, many researchers have reported different methodologies in which different surfactant- and alcohol-based promoters were used to enhance the hydrophobicity of coal surface

2-4,13-15.

Among several promoters used during flotation process, alcohol-based

promoters were found to be the most promising. For instances, Jena et al.

13

reportedly used

commercial aliphatic alcohols such as ethanol and butanol to effectively de-oxidize the coal surface of Indian high-ash oxidized coal. Biswal and Acharya

14

also studied the effect of

different aliphatic alcohols (methanol, ethanol, butanol and octanol) in improving the flotability of oxidized coal. They found that pre-treatment of oxidized coal using ethanol showed significant removal of oxygenated functional groups than other aliphatic alcohols. Dey et al. 3 5 ACS Paragon Plus Environment

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used ethanol as a promoter during mill grinding and cell flotation to increase the hydrophobicity of oxidized coal. Gray et al.

15

conducted a study where the surface of the

oxidized coal was modified by methanol vapour. Methylation of oxidized coal was performed by treating coal surface with methanol vapour at 150-190 °C temperature for 1 h. The results showed that methanol-treated coals acquired lower induction time, more positive zeta potential and higher contact angle, indicating a significant enhancement in surface hydrophobicity of coal. Likewise promoters, studies on intensifying flotation process by pre-treating the coal slurry using ultrasound in removing the oxygenated functional groups from coal surface were also reported by several researchers 1,16-18. The implementation of ultrasound in coal slurry results in the generation of cavitation effect at millions of locations. During cavitation, the several cavity bubbles collapse and transmit equivalent microjets which promote surface pitting on the coal particles. This leads to the removal of hydrophilic layer from coal surface and enhancement in the surface hydrophobicity 1. The cavitation effects also induce sonicfragmentation of coal particles over a period of ultrasonic treatment time, which promotes aqueous grinding, detachment of impurities from coal particles and liberation of slimes

19-23.

The combination of these effects help in increasing reagent-particle interactions and finally improve the performance of the flotation process

24-28.

For instance, Xu et al.

16

investigated

the effect of ultrasonic pre-treatment (at 40 kHz frequency) on the flotation of oxidized coal using kerosene and octanol as collector and frother, respectively, and reported significant improvement in the flotability of oxidized coal at optimized conditions. Feng and Aldrich

29

studied the flotability of oxidized coal by employing ultrasonic pre-treatment and compared its performance to high-intensity conditioning (HIC) during the flotation process. Better performance was achieved for ultrasonicated coal over HIC in terms of demineralization


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efficiency, clean coal recovery and froth stability due to effective removal of oxidized layers during ultrasonic conditioning. Although several studies have been reported on the improvement of flotability of oxidized coal using alcohol-based promoters and ultrasonic pre-treatment alone, no studies have been reported which used both alcohol-based promoter and ultrasound in combination as pretreatment techniques. Also, the detailed mechanism on de-oxidation of oxidized coal using ethanol as promoter is lacking in most of the literature studies. Since both promoter and ultrasound pre-treatment can reduce the degree of oxidation of oxidized coal, it would be of great interest to investigate the de-oxidation effect using this dual pre-treatment method. It is expected that combined pre-treatment using ultrasound and promoter could intensify the deoxidation process significantly and restore the hydrophobicity of coal. Therefore, an effort has been made in this paper to study the role of an alcohol-based promoter such as ethanol and ultrasound pre-treatment on the flotability of oxidized coal. A comprehensive study has been presented on the ultrasound-ethanol interaction with the coal surface and their after-effect on the de-oxidizing potential. The yield and recovery of conventional flotation were compared to those of ultrasound, ethanol and ultrasound-ethanol pre-treated coals. The objective of the present study is to understand the synergistic effect of ultrasound-ethanol pre-treatment in removal of oxygenated functional groups and enhancing the hydrophobicity of oxidized coal to an optimal limit. 2. Materials and methods 2.1. Materials One of the high-ash oxidized coal obtained from Dakra mine of central coalfield, India was used to conduct the present study. The representative sample of raw coal (RC) was prepared by crushing followed by grinding the as-received coal sample below 150 µm and subsequently 7 ACS Paragon Plus Environment

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used in all the experiments. Commercial grade methyl isobutyl carbinol (MIBC), diesel, sodium hexametaphosphate (SHMP) and ethanol were used as frother, collector, dispersant/depressant and promoter, respectively, during flotation process. Normal tap water was used throughout the flotation experiments. 2.2. Characterization and analysis The properties of representative coal sample were studied by proximate analysis (Proximate Analyzer; TGA 601; LECO), ultimate analysis (CHNS Analyzer; TruSpec; LECO), size fractional analysis (using IS-460 Test Sieve), chemical compositional analysis (IS-1355), gross calorific value (GCV) measurement (Bomb Calorimeter; AC 500; LECO), free swelling index (FSI) measurement (IS 1353), and hardgrove grindibility index (HGI) measurement (IS 4433). The coal samples were characterized using X-Ray Diffractometer (XRD; X’Pert Pro; PANalytical) with Cu Kα radiation (λ=1.54 Å) operated at a tube current of 30 mA and a voltage of 40 kV. All the XRD patterns were recorded at 2θ from 10° to 80° at a scan speed of 2°/min. The surface morphology of coal samples was studied using Scanning Electron Microscope (SEM; EVO 18; CARL ZEISS), operated at a working distance (WD) of 9 mm and EHT voltage of 20 kV. Prior to SEM experiment, all the respective samples were sputtered with the gold-palladium layer to improve the surface conductivity. The functional groups of coal samples were examined over the wavelength range of 4000-400 cm-1 using Fourier Transform Infrared Spectrometer (FTIR; Nicolet 6700; Thermo Scientific) by applying KBr pellet technique under transmittance mode. 2.3. Flotation study 2.3.1. Flotation experiment and reagent optimization All the flotation experiments were carried out in a Denver D-12 sub-aeration flotation cell having volume capacity of 1.5 l. To optimize the reagent dosage for flotation, release analysis 8 ACS Paragon Plus Environment

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was carried out in a flotation cell using SHMP, diesel and MIBC as depressant/dispersant, collector and frother, respectively. The optimized dosage of reagents was deduced as: SHMP: 1 g/kg of feed, diesel: 0.2 ml/kg of feed, MIBC: 0.1 ml/kg of feed and this optimized reagent dosage was used in all the subsequent flotation experiments. Standard procedure was followed for performing all the flotation experiments. Initially, about 50 g of raw coal (RC) sample was taken in a flotation cell and mixed with water, maintaining the slurry concentration up to 40%. The slurry was conditioned with SHMP (1 g/kg of feed) for 2 min at an impeller speed of 1500 rpm. Thereafter, the slurry was conditioned with diesel (0.2 ml/kg of feed) for 2 min followed by MIBC (0.1 ml/kg of feed) for 2 min, maintaining impeller speed at 1500 rpm for each case. The final slurry concentration was brought down to 10% by adding make-up water into the flotation cell, and neutral pH value was maintained throughout the experiments. The flotation test was performed by opening the air valve and introducing the air at a constant flow rate (3 lpm) through sub-aeration system. The flotation products, i.e. concentrates and tailings were collected, then dried in an oven under mild conditions (to minimize the change in coal’s chemical structure) until complete dewatering and finally weighed in a weighing balance. 2.3.2. Ethanol/ultrasound/ultrasound-ethanol pre-treatment of coal slurry The flotation experimental procedures for all pre-treated coal slurries remain same as defined in section 2.3.1. The only difference is that, prior to addition of SHMP (depressant/dispersant), the slurry was pre-treated in a flotation cell using (i) ethanol (dosage: 0.5-2 ml/kg; pretreatment time: 1-15 min; impeller speed: 1500 rpm), (ii) ultrasound (pre-treatment time: 1-10 min) and (iii) ultrasound (optimized pre-treatment time: 5 min) followed by ethanol (dosage: 0.5-2.5 ml/kg and pre-treatment time: 1-15 min; impeller speed: 1500 rpm). For the purpose of


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ultrasound pre-treatment of coal slurry, ultrasonic processor (CPX-750; Cole-Parmer) was operated using 25 mm threaded probe diameter at 20 kHz frequency and 750 W power. 3. Results and discussion 3.1. Physical, chemical and mineralogical properties of raw coal To understand the nature of RC used in this study, some of the important properties were determined and discussed in this section. The results of proximate analysis, ultimate analysis, GCV, FSI and HGI are provided in Table 1. It can be seen that RC has a high ash and low fixed carbon content of 44.76% and 33.27%, respectively. This indicates that RC contains large amount of inorganic impurities (such as quartz, clay minerals, trace elements, etc.), and therefore, necessary to beneficiate RC prior to utilization. Total moisture content (both inherent and surface) in RC was found to be 3%. Note that, the inherent moisture content may act as a promoting agent or catalyst during coal oxidation. This is because inherent moisture helps in carrying oxygen to the adsorption sites and forming unstable intermediates at the early stage of coal oxidation 5. Ultimate analysis shows that oxygen percentage is over 50% of the total elemental composition of RC, revealing the oxidized nature of coal. FSI value indicates the absence of swelling properties, confirming the non-caking nature of RC. The GCV value (3580 kcal/kg) of RC is reasonably low and eventually fall under the G13 grade in Indian context 30. Table 1. Important properties of raw coal Proximate analysis

Ultimate analysis

GCV

(wt. %, AD)

(wt. %, DMMF)

(kcal/kg)

M

VM

A

FC

C

H

N

S

O

3.00

18.97

44.76

33.27

39.94

2.19

0.88

0.39

56.60

3580

FSI

HGI

0

87.96

M: moisture; VM: volatile matter; A: ash; FC: fixed carbon (calculated by difference); C: carbon; H: hydrogen; N: nitrogen; S: sulphur; O: oxygen (calculated by difference); AD: Air-dried basis; DMMF: dry-mineral-matterfree basis; GCV: gross calorific value; FSI: free swelling index; HGI: hardgrove grindibility index


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The different chemical compositions present in the ash of RC (on 100% ash basis) are summarized in Table 2. As revealed from Table 2 that majority of the total ash content (44.76%) in RC consisted of SiO2 followed by Al2O3, accounting over 90% of the total ash composition present in RC. A small percentage of Fe2O3, TiO2 and K2O along with some trace metal oxides are also detected in ash composition of RC. Table 2. Ash compositional analysis of raw coal

Ash composition (wt. %) Sample

RC

SiO2

Al2O3

Fe2O3

TiO2

K2O

Na2O

MgO

CaO

MnO2

Cr2O3

V2O5

P2O5

others

65.84

25.20

4.82

1.37

1.02

0.02

0.56

0.65

0.05

0.01

0.02

0.14

0.03

Fig. 2. Mineralogical properties of raw coal. (A) XRD pattern; Petrographic image of (B) fusinite with bogen structure and (C) resinite. The mineralogical properties of RC were studied using X-Ray Diffraction (XRD) technique and petrographic microscopy, and presented in Fig. 2. As shown in Fig. 2 (A), majority of the 11 ACS Paragon Plus Environment

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mineral phase are comprised of quartz (SiO2) and kaolinite [(Al2Si2O5(OH)4)]. While the presence of quartz is responsible for the major contribution of silica (SiO2) impurities, significant amount of alumina (Al2O3) is contributed by kaolinite in RC. Fig. 2 (B) and (C) shows the petrographic image of RC where the appearance of fusinite with bogen structure and resinite are detected. The petrographic study showed that the modal distribution of macerals is consisted of 50% vitrinite, 14% liptinite and 23% inertinite, whereas remaining 13% corresponds to the existence of mineral matter in RC. 3.2. Oxidized behaviour of raw coal Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM)/Energy Dispersive Spectroscopy (EDS) and flotation test were conducted on RC to understand the oxidized behaviour and its degree of oxidation. From Fig. 3 (A), preliminary study showed that the presence of hydroxyl (–OH) group is predominant in RC. The peak of –OH group was detected in the wavelength range of 3617-3695 cm-1. Higher peak intensity of –OH is attributed to the formation of large numbers of solid oxygenated complexes at both internal and external sites of coal particles. As reported, when coal is brought in contact with oxygen, the occurrence of coal oxidation can be observed from the consumption of oxygen by coal. Subsequently, interaction between coal and oxygen takes place due to physical adsorption followed by chemisorption. Chemisorption is comparatively very strong than physical adsorption, where particular sites at the internal surface of coal pores can attract oxygen molecules and bind them strongly by valance forces. This adsorption cycle is continuous and responsible for the existence of huge amount of oxygen species at both internal pores and external surface of coal 5. The presence of such oxygenated groups modify the coal surface from hydrophobic to hydrophilic significantly, and subsequently hamper the flotability during flotation process. On the other hand, the peak intensity of –CH3 at 2921 cm-1 and –CH2 at 2846


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cm-1 was found to be relatively small as compared to the –OH group. This indicates that the oxygenated species are firmly adsorbed onto the coal’s hydrocarbon structure. Fig. 3 (B) and (C) shows the SEM image of RC and its corresponding EDS characteristics, respectively. It can be seen from Fig. 3 (B) that there are extremely rugged surfaces along with surface deformation and irregularities in RC, indicating partial decomposition of carbonaceous organic matter due to surface oxidation 4. EDS study shown in Fig. 3 (C) further reveals that the oxygen content in RC is reasonably high, accounting 33% (by weight) of the total composition.

Fig. 3. Characterization of oxidized raw coal using (A) FTIR, (B) SEM and (C) EDS.


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To further confirm the oxidized behaviour of RC, preliminary flotation study was conducted as per the procedure defined in section 2.3.1 at optimized reagent dosage. The results of flotation are shown in Table 3. Table 3. Flotation results of raw coal Products

Y%

A%

FC %

CMR %

C

34.00

32.14

42.52

41.77

T

66.00

51.26

28.51

58.23

Total

100

44.76

33.27

100

C: flotation concentrate; T: flotation tailings; Y: yield; A: ash; FC: fixed carbon; CMR: combustible matter recovery

It can be seen from Table 3 that ash content of flotation concentrate reduced from 44.76% (RC) to 32.14% after flotation process. Conversely, fixed carbon content of concentrate product increased significantly from 33.27% (RC) to 42.52%. However, it is obvious from Table 3 that the yield of flotation concentrate is only 34%, whereas remaining 66% belongs to the flotation tailings. This is because oxygenated functionalities adsorbed on the coal particles are hydrophilic (water-loving) in nature due to which interaction between water-coal become dominant due to hydrogen bonding of oxidized functionalities with water molecules, and this reduces the tendency of collector to spread over the coal surface. Moreover, the combustible matter recovery in tailings was found higher than those of flotation concentrate. This indicates that the flotability of the RC was poor, and strongly attributed to the oxidation of coal particles which decreased the surface hydrophobicity as well as surface potential of coal particles. To overcome this, surface modification of coal particles by adding promoters or pre-treatment is always encouraged to enhance the flotability of oxidized coal. In the next section, the ability of ethanol as a promoter and its sequential combination with ultrasound pre-treatment during


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flotation are described in de-oxidizing the coal particles and enhancing the flotability of oxidized coal. 3.3. Flotation of oxidized coal 3.3.1. Effect of ethanol on flotability of oxidized coal The pre-treatment effect of ethanol as promoter during flotation of RC was studied and discussed in this section. The flotation experiments were performed at optimized reagent conditions with varying ethanol dosages (0.5-2 ml/kg) and pre-treatment times (1-15 min) separately. The results of flotation are discussed in term of ash, fixed carbon, yield and combustible matter recovery of flotation concentrate.

Fig. 4. Effect of ethanol dosages on ash, fixed carbon, yield and combustible matter recovery of flotation concentrates.


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A plot showing the effect of different ethanol dosages (0.5-2 ml/kg) on the flotability of RC at constant ethanol treatment time (5 min) under the optimized reagent conditions is presented in Fig. 4. It is obvious from Fig. 4 that, both the yield and combustible matter recovery of flotation concentrates increased significantly from 47.77% to 51.83% and 60.03% to 66.77%, respectively, with increasing ethanol dosage. Apparently, ash content decreased from 30.12% to 27.87%, whereas fixed carbon content increased from 44.33% to 46.18% with increasing dosage of ethanol. These results indicate significant enhancement in the flotability due to deoxidation of oxidized coal during ethanol pre-treatment under intense mechanical stirring.


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Fig. 5. Schematic mechanism showing de-oxidation of oxidized coal under ethanol pretreatment. The de-oxidation mechanism of the ethanol pre-treated coal is very complex and may be classified into two possible routes (case 1 and 2) as shown in Fig. 5. It is well-described above that the internal and external sites of oxidized coal contains hydrophilic layer due to the adsorption of oxygenated moieties during oxidation. To remove this oxygenated hydrophilic layer, RC was pre-treated with ethanol. Ethanol is a solvent which has both polar and non-polar group as shown in Fig. 5. In ethanol, the polar part is due to the presence of oxygenated functional group (–OH), whereas non-polar part is due to C–C and C–H bond (CH2CH3). Since ethanol has a polar group (–OH), it can interact with the polar sites (oxygenated functionalities) of oxidized coal by strong hydrogen bonding. Conversely, non-polar group of ethanol would interact by physical adsorption (due to van der Waals force) with carbonaceous sites on the coal surface by dispersing water molecules from coal surface. The actual interaction between non-polar group of ethanol and carbonaceous sites of coal depends on the degree of oxidation of the oxidized coal, i.e. the greater the oxidation of coal surface, the lesser the interaction between non-polar groups of ethanol and coal. Noteworthy to mention that, the hydrogen bonding between polar groups of ethanol and oxygenated functionalities of oxidized coal is stronger than the van der Waals interaction between non-polar groups of ethanol and carbonaceous sites of coal

31.

During mechanical conditioning by stirrer at 1500 rpm in

presence of ethanol, de-oxidation mechanism of coal takes place in two possible ways: (i) by the formation of hydrogen bonding between polar groups of ethanol and oxidized coal at the surface of coal while orienting non-polar tail of ethanol (hydrocarbon chain) towards the water (case 1) and (ii) by the formation of hydrogen bonding between polar groups of ethanol and oxidized coal at the surface of coal followed by detachment of hydrogen-bonded polar groups


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from coal surface under intense mechanical stirring (case 2). A schematic mechanism showing de-oxidation of oxidized coal under ethanol pre-treatment is shown in Fig. 5. In case 1, hydrogen-bonded polar groups are oriented towards coal surface while placing nonpolar tails towards water. This case is possible when coal is of low-grade containing large fraction of oxygen functional groups over its surface. This results in the existence of strong hydrogen bonding between –OH groups of coal and polar groups of ethanol, whereas poor interaction between carbonaceous sites of coal and non-polar tails of ethanol, leading to the orientation of non-polar tail of ethanol towards water. Upon addition of collector (diesel) during mechanical conditioning, the non-polar tails of ethanol would interact with the collector molecules since diesel is also non-polar in nature. These non-polar tails of ethanol are expected to play a significant role in bridging the interaction between coal and collector molecules and enhance the surface hydrophobicity of coal by effective collector absorption. In case 2, the dissociation and detachment of the hydrogen-bonded polar groups (between ethanol and coal) from oxidized coal takes place during intense mechanical stirring at higher rpm. Because polar groups of ethanol and oxidized coal are strongly attached through hydrogen bond, these hydrogen-bonded polar groups detached together from coal surface under prolong mechanical stirring, leading to the separation or carry-away of the oxygenated complexes from oxidized coal. These hydrogen-bonded polar groups would remain in the aqueous solution since these polar groups are hydrophilic in nature and have more affinity towards water. At the same time, the interaction between non-polar groups of ethanol and carbonaceous sites of coal also dissociated due to weak physical interaction under mechanical stirring. Also, the length of the hydrocarbon chain of non-polar groups in ethanol is very small and is expected to impart less hydrophobicity to the coal surface. Moreover, interaction between hydrocarbon chain of ethanol and carbonaceous part of coal is significantly weak due to short hydrocarbon tail of the ethanol, and easily miscible in aqueous solution under mechanical stirring. The removal of 18 ACS Paragon Plus Environment

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such polar groups from oxidized coal helps in restoring the hydrophobicity of coal surface. This encourage the effective interaction of collector (diesel) with the hydrophobic part of coal surface and enhance the flotability of oxidized coal significantly. The mechanism described in case 2 is found predominant in the present study which is revealed using FTIR analysis as described in section 3.4. The effect of ethanol pre-treatment times (1-15 min) on the flotability of RC at constant ethanol dosage (2 ml/kg) under the optimized reagent conditions is shown in Fig. 6. It can be clearly seen from Fig. 6 that there is a significant increase in ash, fixed carbon, yield and combustible matter recovery of flotation concentrate up to 5 min ethanol pre-treatment time beyond which these parameters become nearly constant. This could be due to the fact that de-oxidation of oxidized coal was weaker at lower ethanol pre-treatment time. However, with increasing ethanol pre-treatment time, de-oxidation of coal increased significantly and become constant after saturation.


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Fig. 6. Effect of ethanol pre-treatment time ash, fixed carbon, yield and combustible matter recovery of flotation concentrates. 3.3.2. Effect of ultrasound on flotability of oxidized coal The effect of ultrasound pre-treatment on the flotability of oxidized RC in absence of ethanol was studied and presented in this section. The coal slurry was pre-treatment with low-frequency (20 kHz) ultrasound at different ultrasound treatment times (1-10 min) prior to flotation. Plot in Fig. 7 shows ash, fixed carbon, yield and combustible matter recovery of flotation concentrates obtained during ultrasound pre-treated flotation at optimized reagent conditions. The results in Fig. 7 clearly shows a notable increment in yield and combustible matter recovery from 34% to 46.94% and 41.77% to 60.39%, respectively, with increasing ultrasonic pretreatment time up to 5 min. With increasing ultrasonication time (up to 5 min), yield and combustible matter recovery of flotation concentrate increased, which is certainly attributed to the enhancement in the cavitation effect with respect to ultrasonic-treatment time due to lowfrequency ultrasonication (20 kHz). Noteworthy to mention that, cavitation effect is very strong at low-frequency ultrasonication (preferably below 40 kHz) than high-frequency ultrasonication. Hence, at low-frequency ultrasonication, intensification in the cavitation effect resulted in the enhanced surface pitting (which removed the hydrophilic oxidized layer) followed by surface fracture on coal particles. This can be described further from SEM images as shown in Fig. 8, which clearly reveals the surface deformation of carbonaceous matter (Fig. 8 (A)) occurred during natural weathering or spontaneous heating in RC, and subsequent cleaning followed by fissure development in RC surface (Fig. 8 (B)) under low-frequency ultrasonication for 5 min. It is expected that deformed layers in oxidized surface coal would remove together with hydrophilic oxygenated functional groups during ultrasonication. Following removal of oxygenated groups during ultrasonication, the performance of flotation increased substantially due to the enrichment in surface hydrophobicity of coal particles, which 20 ACS Paragon Plus Environment

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improved the selectivity between collector and hydrophobic coal particles. Comparatively to ethanol pre-treatment, flotation concentrates showed lower yield and combustible matter recovery after ultrasound pre-treatment. This indicates that ethanol pre-treatment under mechanical stirring was more effective than ultrasound pre-treatment, which could be due to effective ethanol interaction with external surface or diffusion of ethanol into the internal pores of coal, leading to the formation of strong hydrogen bonding between the polar groups of ethanol and oxidized coal, and their efficient de-oxidation during mechanical stirring.

Fig. 7. Effect of ultrasound pre-treatment time on ash, fixed carbon, yield and combustible matter recovery of flotation concentrates. It was also observed that, beyond 5 min of ultrasonication, the yield and combustible matter recovery of flotation concentrate reduced drastically. From maximum yield (46.94%) and combustible matter recovery (60.39%), these values have been reduced to 42.86% and 54.62%, respectively, during 10 min ultrasonic pre-treatment. Simultaneously, ash and fixed carbon content were also found to be increased and decreased, respectively, at 10 min ultrasonic pre21 ACS Paragon Plus Environment

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treatment time. This may be attributed to the re-oxidation of coal surface due to prolonged ultrasonic treatment which restricted the coal flotability further

16.

Because cavitation effect

takes place at the millions of locations producing high local instantaneous temperatures and pressures approximately equal to 5000 K and 500 atm., respectively, there are possibilities that the coal surface would re-oxidize again since coal is a temperature sensitive material 1,16,18.

Fig. 8. SEM image of oxidized RC (A) before and (B) after ultrasound pre-treatment for 5 min. 3.3.3. Effect of ultrasound followed by ethanol pre-treatment on flotability of oxidized coal


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Various observations from above results showed that both ethanol and ultrasound pre-treatment have the potential to efficiently reduce the detrimental effect of coal oxidation during flotation. Although these pre-treatment methods prior to flotation showed appreciable results, it would be interesting to see if their sequential combination could bring positive impact on oxidized coal flotability or not. Therefore, flotation study was conducted at optimized reagent conditions on oxidized RC to explore the synergistic effect of ultrasound and ethanol pre-treatment on flotation performance in term of ash, fixed carbon, yield and combustible matter recovery. The flotation study was divided into two parts: firstly, to study the effect of varying ethanol dosages (0.5-2.5 ml/kg) at fixed ultrasound (5 min) and ethanol pre-treatment time (5 min) and secondly, to study the effect of varying ethanol pre-treatment times (1-15 min) at constant ultrasound (5 min) and ethanol dosage (2 ml/kg). Note that, optimized ultrasound pre-treatment time of 5 min was considered throughout the experiments since higher yield and combustible matter recovery was obtained under this condition as discussed in section 3.3.2.


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Fig. 9. Effect of ethanol dosage at fixed ultrasound (5 min) and ethanol pre-treatment time (5 min) on ash, fixed carbon, yield and combustible matter recovery of flotation concentrates. The effect of ethanol dosage at fixed ultrasound (5 min) and ethanol pre-treatment time (5 min) under optimized reagent conditions in term of ash, fixed carbon, yield and combustible matter recovery of flotation concentrates is shown in Fig. 9. It is obvious from Fig. 9 that, both yield and combustible matter recovery increased significantly with increasing ethanol dosage. Correspondingly, ash content decreased to 25.70% whereas fixed carbon content increased to 49.25% with increasing dosage of ethanol. Maximum yield and combustible matter recovery of 65.92% and 87.55%, respectively, were obtained at 2.5 ml/kg ethanol dosage. Comparatively to these results, ethanol pre-treatment at varying dosage without ultrasound (section 3.3.1) showed much lower yield (51.83%) and combustible matter recovery (66.77%) of flotation concentrate. This confirms that the combined effect of ultrasound and ethanol pretreatment has higher deoxidizing potential than individual ultrasound or ethanol pre-treatment. In the initial stage of ultrasound pre-treatment, intensified cavitation effect (at low-frequency ultrasound) helps in scrubbing the hydrophilic oxidized layer of oxygenated functionalities from coal surface and subsequently create cavities at the coal surface due to surface pitting effect 1. As a result, successive ethanol treatment of ultrasonicated coal not only interacts with the remaining oxygenated functionalities present over coal surface but also penetrates into the pores through cavities, which potentially drive-out the internal oxygen complexes under mechanical stirring. A schematic illustration showing ultrasound-ethanol pre-treatment effect during oxidized coal flotation process is presented in Fig. 10. Following combined ultrasoundethanol pre-treatment, flotability of RC enhanced drastically. From the yield and combustible matter recovery of 34% and 41.77%, respectively, without any pre-treatment, the yield and combustible matter recovery were increased to 65.92% and 87.55% respectively, under combined ultrasound-ethanol pre-treatment followed by flotation. 24 ACS Paragon Plus Environment

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Fig. 10. A schematic illustration of ultrasound-ethanol pre-treatment on flotability of oxidized coal. Fig. 11 shows the effect of ethanol pre-treatment time at fixed ultrasound (5 min) and ethanol dosage (2 ml/kg) under optimized reagent conditions in term of ash, fixed carbon, yield and combustible matter recovery of flotation concentrates. It was found that, with increasing ethanol pre-treatment time, the yield, fixed carbon and combustible matter recovery increased significantly particularly up to 5 min ethanol pre-treatment time beyond which the yield and combustible matter recovery becomes nearly constant. This may be attributed to the fact that, at lower ethanol pre-treatment time, de-oxidation effect was lesser. However, with increasing pre-treatment time, de-oxidation effects enhanced significantly and become constant after saturation.


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Fig. 11. Effect of ethanol pre-treatment time at fixed ultrasound (5 min) and ethanol dosage (2 ml/kg) on ash, fixed carbon, yield and combustible matter recovery of flotation concentrates. Furthermore, to understand the effect of ethanol pre-treatment time at fixed ultrasound (5 min) and ethanol dosage (2 ml/kg) on removal of inorganic impurities, XRD analysis of different flotation concentrates was conducted. Fig. 12 shows the XRD pattern of flotation concentrates obtained at different pre-treatment times (1-15 min). It can be seen from Fig. 12 that the peak intensity of each mineral phase (quartz and kaolinite) reduced significantly with increasing ethanol pre-treatment time. This indicates the removal of ash impurities from flotation concentrates during flotation and is strongly attributed to the ultrasonic pitting followed by defragmentation of coal particles, leading to the aqueous grinding and liberation of inorganic ash impurities. Following liberation of ash impurities from coal particles, effective interaction between ash impurities and SHMP (depressant agent) took place while reagent conditioning which not only depressed them during the flotation process but also enhanced the selectivity of collector with carbonaceous part of coal.


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Fig. 12. XRD patterns (A-D) of flotation concentrates after ultrasound-ethanol pre-treatment at different ethanol pre-treatment times: (A) 5 min ultrasound pre-treatment time + 2 ml/kg ethanol dosage + 1 min ethanol pre-treatment time; (B) 5 min ultrasound pre-treatment time + 2 ml/kg ethanol dosage + 3 min ethanol pre-treatment time; (C) 5 min ultrasound pre-treatment time + 2 ml/kg ethanol dosage + 5 min ethanol pre-treatment time and (D) 5 min ultrasound pre-treatment time + 2 ml/kg ethanol dosage + 10 min ethanol pre-treatment time. 3.4. FTIR analysis of flotation products It was found that pre-treatment of oxidized RC by ultrasound-ethanol combination followed by flotation showed greater performance than those of ethanol and ultrasound pre-treatment alone. To support these experimental results and quantitatively understand the effects of promotor, ultrasound and their combinations on removing the oxygenated functionalities from RC at the molecular level, FTIR analysis was employed on different flotation concentrates and discussed in this section.


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Fig. 13. FTIR spectrums of raw coal (RC) and flotation concentrates (A-H). RC: raw coal; A: 0.5 ml/kg ethanol dosage + 5 min ethanol pre-treatment time; B: 1 ml/kg ethanol dosage + 5 min ethanol pre-treatment time; C: 5 min ultrasound pre-treatment time; D: 10 min ultrasound pre-treatment time; E: 5 min ultrasound pre-treatment time + 1 ml/kg ethanol dosage + 5 min ethanol pre-treatment time; F: 5 min ultrasound pre-treatment time + 1.5 ml/kg ethanol dosage + 5 min ethanol pre-treatment time; G: 5 min ultrasound pre-treatment time + 2 ml/kg ethanol dosage + 5 min ethanol pre-treatment time; H: 5 min ultrasound pre-treatment time + 2.5 ml/kg ethanol dosage + 5 min ethanol pre-treatment time. The FTIR spectrums of flotation concentrate in Fig. 13 (A) and (B) shows the effect of ethanol dosages (0.5 and 1 ml/kg, respectively) at constant pre-treatment time (5 min) on the absorption band of oxygenated functionalities. Comparatively to FTIR spectrum of untreated RC (Fig. 13 28 ACS Paragon Plus Environment

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(RC)), some reduction in the peak intensity of –OH group was observed after ethanol pretreatment as shown in Fig. 13 (A, B). This attributes to the removal of oxygenated functionalities from oxidized coal by the polar groups of ethanol under mechanical stirring with increasing ethanol dosage. It can also be seen that reduction of –OH peak intensity (at 36143698 cm-1 wavelength) was higher for 1 ml/kg ethanol dosage than those of 0.5 ml/kg ethanol dosage during same pre-treatment time (5 min). This clearly indicates that dissolution of oxygenated functionalities was higher with increasing ethanol dosage, possibly due to the rise in ethanol’s polar groups and their strong interaction with the polar groups of oxidized coal by hydrogen bonding, and subsequent detachment of hydrogen-bonded polar groups during mechanical stirring. As compared to C=C peak intensity in oxidized RC (Fig. 13 (RC)), enhancement in the peak intensity of C=C after ethanol pre-treatment was also observed in both the case as shown in Fig. 13 (A) and (B). The FTIR spectrums of flotation concentrate in Fig. 13 (C) and (D) shows the effect of ultrasound pre-treatment times (5 and 10 min, respectively) in absence of ethanol on the absorption band of oxygenated functionalities. For 5 min ultrasonic pre-treatment of RC, it can be seen that there is a reduction in –OH peak intensity (at 3619-3702 cm-1 wavelength) compared to untreated RC spectrum (Fig. 13 (RC)). Increase in the sharpness of –CH2 and – CH3 peak intensity was also observed in Fig. 13 (C), which indicates the appearance of hydrocarbon groups after the removal of hydrophilic oxygenated groups due to ultrasonic pretreatment. Significant increase in the peak intensity of –OH group and decrease in the peak intensity of –CH2, –CH3 and C=C was also encountered when ultrasound pre-treatment was extended to 10 min. This may attributed to the re-oxidation of the coal surface under intense cavitation effect, which increased the hydrophilicity of coal particles due to the adsorption of –OH group over coal surface. Re-oxidation of coal surface occurs due to prolonged ultrasonic treatment as a result of the implosion of cavity bubbles at millions of locations, producing high 29 ACS Paragon Plus Environment

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local instantaneous temperatures and pressures approximately equal to 5000 K and 500 atm. as explained above. The FTIR spectrums of flotation concentrates in Fig. 13 (E-H) shows the effect of ultrasound (5 min) followed by ethanol pre-treatment with varying ethanol dosages (1-2.5 ml/kg) at constant ethanol pre-treatment time (5 min) on the absorption band of oxygenated functionalities. In comparison to individual ethanol and ultrasound pre-treatment, combined ultrasound-ethanol pre-treatment showed promising de-oxidizing potential. For instance, ultrasound pre-treated RC for 5 min followed by ethanol pre-treatment using 1 ml/kg dosage at 5 min (Fig. 13 (E)) showed higher degree of de-oxidation than those pre-treated with ethanol and ultrasound alone. With increasing dosage of ethanol from 1 ml/kg to 2.5 ml/kg, substantial deduction in the peak intensity of –OH group was found as shown in Fig. 13 (E-H). Higher reduction in –OH peak intensity was observed at 2.5 ml/kg ethanol dosage, signifying the maximum removal of oxygenated species under combined ultrasound-ethanol pre-treatment. Increase in the sharpness of –CH2, –CH3 and C=C (aromatic stretching) peak intensity was also observed after ultrasound-ethanol pre-treatment, indicating from the emergence of carbonaceous matter of coal after the removal of oxygen moieties. Overall, promising results were obtained under combined ultrasound-ethanol pre-treatment, which is attributed to the ultrasonic scrubbing of hydrophilic oxygenated moieties due to cavitation effect in the initial stage followed by subsequent dissolution of remaining oxygenated moieties by ethanol pretreatment. 4. Conclusion In this study, the effect of ultrasound, ethanol and combined ultrasound-ethanol as pretreatment method on the flotability of oxidized coal was investigated. The flotation study was conducted on one of the oxidized high-ash Indian coal at optimized reagent conditions (SHMP:


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1 g/kg of feed; diesel: 0.2 ml/kg of feed; MIBC: 0.1 ml/kg of feed). Comparatively to ultrasound pre-treatment, ethanol pre-treatment showed better flotability due to the effective removal of oxidized moieties, attaining a maximum yield and combustible matter recovery of 51.83% and 66.77%, respectively, in the flotation concentrate. However, the combined effect of ultrasound and ethanol pre-treatment showed higher de-oxidizing potential than those of ultrasound or ethanol pre-treatment alone. This is attributed to the ultrasonic pitting of the coal surface which removed the hydrophilic oxygenated layer in the initial stage followed by subsequent dissolution of remaining oxygenated moieties by ethanol treatment in the next stage. Maximum yield and combustible matter recovery of 65.92% and 87.55%, respectively, were obtained under combined ultrasound-ethanol pre-treatment in the flotation concentrate. FTIR analysis of flotation concentrates also revealed the same, which showed a drastic reduction in the adsorption band of –OH peak after the combined ultrasound-ethanol pretreatment. In overall, it can be stated that pre-treating oxidized coal prior to flotation using ultrasound followed by ethanol could restore the hydrophobicity of the coal particles and significantly improve the flotability of coal. Acknowledgements The authors would like to acknowledge the Director of the institute and Head of Mineral Processing Department, CSIR-IMMT, Bhubaneswar for their permission to communicate this work. The funding from the CSIR (Project No. OLP-79) is highly acknowledged. References [1] S. D. Barma, Ultrasonic-assisted coal beneficiation: A review, Ultrason. Sonochem. 50 (2019) 15–35. doi:10.1016/j.ultsonch.2018.08.016. [2] S. Dey, Enhancement in hydrophobicity of low rank coal by surfactants — A critical overview, Fuel Process. Technol. 94 (2012) 151–158. doi:10.1016/j.fuproc.2011.10.021. 31 ACS Paragon Plus Environment

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Ultrasound-promoter pre-treatment for enhancing the yield and

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