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Chemical beneficiation of high ash Indian non-coking coal by alkali leaching under low-frequency ultrasonication Santosh Deb Barma, Sathish R, Prasanta Kumar Baskey, and Surendra Kumar Biswal Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03291 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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Chemical beneficiation of high ash Indian non-coking coal by alkali leaching under low-frequency ultrasonication Santosh Deb Barma1*, Sathish R2, Prasanta Kumar Baskey1 and Surendra Kumar Biswal1 1

Mineral Processing Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, INDIA-751013 2

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

Graphical Abstract:

1

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemical beneficiation of high ash Indian non-coking coal by alkali leaching under low-frequency ultrasonication Santosh Deb Barma1*, Sathish R2, Prasanta Kumar Baskey1 and Surendra Kumar Biswal1 1

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

2

Environment and Sustainability Department, CSIR - Institute of Minerals and Materials Technology, Bhubaneswar, Odisha-751013, India

*

To whom correspondence should be addressed. Email: [email protected] or

[email protected] ; Tel: +91 674 237 9379

2


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Chemical beneficiation of high ash Indian non-coking coal by alkali leaching under low-frequency ultrasonication

Abstract:

Coal mined from Talcher region of Odisha, India is known to be high ash, surface oxidised and non-coking in nature. It is quite challenging to beneficiate such low-grade coal by physical or physicochemical process due to its oxidised nature and presence of complex ash forming mineral-matter in its matrix. Chemical beneficiation is one of the alternative process to such problems. However, this chemical process consumes more chemicals, treatment time and energy which limits its application further. Therefore, an attempt has been made to chemically beneficiate this coal cost effectively with optimum chemicals, treatment time and energy. In the present study, an application of ultrasound at low frequency on alkali-acid leaching is employed to investigate on the demineralization of high ash Indian non-coking coal. The raw coal properties such as fixed carbon content, CHNS content, HGI, AFT and GCV were investigated before the experiments. The coal samples were leached with three different types of alkali namely, NaOH, KOH, and Na2CO3 followed by H2SO4 and HCl treatment, respectively. The quality of the treated coal was examined by proximate analysis and GCV measurement. The maximum ash removal was achieved on NaOH leached coal followed by 30% H2SO4 treatment with 73.91% demineralization and 57.21% fixed carbon. The raw and treated coal samples were characterized by FTIR, SEM and XRD to confirm the presence of oxygenated functional groups causing surface oxidation, surface modification by ultrasonication and formation of alkali aluminosilicates on the coal surface, respectively. The presence of trace elements in the alkali leachates released during ultrasonication was also determined by ICP-OES technique.

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Keywords: Non-coking coal; demineralization; alkali leaching; acid treatment; ultrasonication; beneficiation

1. Introduction India is the third largest coal producer after China and United State with a reserve of around 306.60 billion tonnes 1. This provides a unique opportunity to the energy sectors to utilise the readily available resources of coal to meet the country’s energy demands. Coal and its products are already playing an important role in fulfilling the energy needs for power generation, steel making, cement production and other domestic applications

2-5

. In India, the

reserves of good quality coal are limited and gradually declining over a period of time. This stimulates the utilization and application of inferior and low-grade coal. However, its applications are discouraged mostly due to the presence of high ash content 5. To make it available for end use, coal needs to be beneficiated to remove its ash forming mineral matters and associated sulphates. Since Indian coals are mostly of drift origin and have a poor washability characteristics, physical beneficiation technique is of limited use in removing such inorganic impurities 5. This attracts chemical beneficiation method as an alternative technique to achieve the coal demineralization effectively. 1.1 Conventional chemical method for coal cleaning Many researchers have reported coal cleaning by various chemical methods using different alkalis followed by acid treatment. For instance, Chi et al.

6

reported a two-step

chemical cleaning of bituminous coal with different hot alkali solutions followed by HNO3 acid washing. The lowest sulphur and ash content were achieved when leached with 1M Na2CO3 followed by HNO3 acid treatment. Wang et al.

4

7

achieved about 90% demineralization by


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leaching with NaOH at mild condition followed by HCl washing. Chriswell et al. 8 reported the effective leaching of pre-treated partial devolatilized coal using molten caustic liquid. Ash and total sulphur reduction of 95% and 90%, respectively were reported by them. About 70% carbonate removal was also achieved by pre-treating coal at 450 °C for 15 min under a nitrogen purge. Sharma and Gihar

9

reported maximum demineralization of 75.6%, when leached with

40% NaOH followed by 10% H2SO4. However, the optimum condition was attained by leaching with 20% NaOH followed by 10% H2SO4 treatment delivering 70.9% demineralization at 373 K under ambient pressure. An investigation reported by Wang et al.

10

showed the ash removal of

about 76% by leaching with 5% CaO at 340 °C for 120 min followed by HCl washing. Dash et al.

5

reported the reduction of ash content over 50% for various physically

beneficiated Indian coals by using NaOH treatment followed by HCl washing. The effect of various parameters such as reaction time, alkali concentrations, temperature, coal particle size and acid concentrations on the degree of demineralization was investigated. Dash et al.

2

also

investigated the alkali-acid leaching effect on Indian coal at elevated temperature and pressure and reported over 70% ash reduction along with significant reduction in the phosphorous content after acid treatment. Mukherjee and Borthakur

11

investigated the leaching of high sulphur sub-

bituminous coal using KOH followed by HCl treatment and reported the reduction in ash and total sulphur by 28-45% and 22-35%, respectively. An investigation by Mukherjee et al.

12

on

coal leaching using H2O2 in combination with H2SO4 showed 43% and 26% ash and organic sulphur reduction, respectively, with almost complete removal of pyritic and sulphate sulphur. Mukherjee

13

also reported a study on high sulphur coal using mixed alkali solution (1:1 KOH

and NaOH) followed by HCl treatment and achieved over 50% demineralization with 25% organic sulphur reduction under the optimum condition.

5


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1.2 Ultrasonic assisted chemical method for coal cleaning It is well explained in the literature about the conventional chemical leaching of coal using chemical reagents

1-13

. However, its commercialization is yet a big challenge due to the

high cost of chemical reagents and energy consumption 14. Although there are many techniques developed for the effective regeneration and recycling of reagents, techno-economic feasible processes are yet to emerge. The application of ultrasound in the conventional chemical leaching as an integrated system may be a good alternative in the present context. Few researchers have already studied the application of ultrasound in coal cleaning in the presence of different chemical reagents. The application of sonic energy to the caustic cleaning of coal was reported by Zaidi

15

with 59% reduction in total sulphur for low-rank coal. An investigation on the

feasibility of ultrasonic assisted coal cleaning using three different reagents (HCl, HNO3 and H2O2) at varying frequencies (25 kHz, dual 58/192 kHz and 430 kHz) were accounted by Ambedkar et al.

16

. This method showed over 90% demineralisation and desulphurization with

less treatment time, less reagent volume, and less reagent consumption. Ambedkar et al.

17

also

examined the effect of high frequency and intensity ultrasound in the absence of reagents on deashing and desulphurization of coal. Mi and Kang

18

reported coal desulphurization under the effect of microwave alone and

also in combination with ultrasound. Sahinoglu and Uslu

19

investigated on oil agglomeration

technique using ultrasonic treatment and reported pyritic sulphur and ash reduction with increased gross calorific value and combustible recovery. Saikia et al.

20

investigated on the

cleaning of high sulphur coal in water and mixed alkali media (1:1 KOH and NaOH) at a low energy frequency (20 kHz) and reported a reduction on ash, pyritic sulphur, sulphate sulphur and total sulphur with 87.52%, 83.92%, 12.50% and 18.80%, respectively. Saikia et al.

6


21

also

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reported a feasibility study on cleaning of Indian coals at 20 kHz and Brazilian 22 coal at 40 kHz in the presence of H2O2. 1.3 Ultrasound mechanism of coal cleaning According to the definition of sonic science, ultrasound is a sound wave with a frequency (above 20 kHz) higher than the audible limit of human hearing 3. The research based on the application of ultrasound in the cleaning of coal has gained an immense interest these days because of its effectiveness in demineralisation and desulphurization process

15-21

of ultrasound-based coal cleaning already exists in overseas power plants

14, 42-44

. The practice . However,

despite having high ash and sulphur coal in plenty, the commercialization of such advancement is not established in India. Some of the advantages of ultrasound based chemical leaching of coal over conventional methods are high degree of demineralization, less reagents, less energy consumptions and less treatment time 20. When ultrasound is exposed to a coal slurry, due to the ultrasonic vibration, two physical phenomena occur: acoustic cavitation and acoustic streaming. Acoustic cavitation is mostly predominant at a low frequency ranging between 20 to 40 kHz. An aqueous media undergoes alternate rarefaction and compression cycles under the influence of ultrasonic vibrations as shown in Figure 1. A bubble cavity is formed during rarefaction whereas bubble collapse during the compression cycle. The implosion of bubbles in the acoustic field results in the formation of microjets. The formation of microjets produces high local instantaneous temperature and pressure nearly equal to 5000 K and 500 atm., respectively, with enormous heating and cooling rates above 1010 K/s

20-24

. These microjets on exposure to coal particles create surface pitting

followed by particle fragmentation, then accelerate the reagents to diffuse into the interior of coal particles and activate the leaching reactions 17.

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Figure 1. Rarefaction and compression cycle showing growth and collapse of bubbles.

On the other hand, acoustic streaming occurs at a frequency above 400 kHz and 1 MHz in the ultrasonic and megasonic system, respectively. It is to be noted that the cavitation is omnidirectional while streaming is strongly unidirectional. The strong unidirectional streaming when formed, creates bulk motion of the liquid and generates a microstreaming. These effects are continuous and mainly responsible for dislodging the complex particles 16, 17. Both cavitation and streaming (coupled effect) are predominant at the high-frequency ultrasound (400 kHz), and leaching effect prevails

17, 20

. Due to the cavitation effect, water and alkali decompose into free

radicals as per the reactions 14, 20 : H2O → H+ + OHXOH → X+ + OH-

8


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where, X = Alkali. Cavitation results in the sonolysis of aqueous media which initiates the formation of OH- and H+ and produces H2O2 and hydrogen gas

23

. These products then participate in the secondary

reactions involving dissolved substances. The predominant back reactions occurring in the process may be represented as follows 20, 24-26 : OH- + OH- → H2O2; H+ + H+ → H2; H2O2 → OH- + OHH+ + OH- → H2O; H+ + O2 → HO2; HO2 + HO2 → H2O2 + O2 Ultrasound energy produces highly reactive species such as OH- radicals, H2O2 and ozone with high oxidation potentials when exposed to aqueous media. These radicals can enhance oxidation and reduction reactions. The oxidation process occurring due to ultrasound is termed as “Advanced Oxidation Process” and are strongly responsible for sulphur oxidation into sulphoxides, sulphones which may be removed by solvent extraction process 14. The available literature itself indicates the potential of ultrasound-assisted cleaning process as one of the promising technique in the field of sonochemical research. Various literature has been reported on conventional alkali leaching of coal, however, very few on ultrasound-assisted alkali leaching. Therefore, in this present study, an attempt has been made to study the effect of different alkali (NaOH, KOH, and Na2CO3) leaching on high ash coal under low frequency (40 kHz) ultrasound with varying concentration of acids (H2SO4 and HCl). The coal sample used in the present study is of high ash, low sulphur, surface oxidised and noncoking nature. Since the coal is of high ash with low sulphur content, the effort has been made to explain in details with regards to demineralization only. 2. Experimental work 2.1. Materials

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The coal sample obtained from Hingula mines of Talcher coalfield, India was used for conducting the present study. Chemicals such as NaOH, Na2CO3, KOH, HCl, H2SO4 and ashless filter paper (grade 41) with filtration speed rapidity of 200 mls/min were purchased from HiMedia. The ultrapure water of 18.20 MΩ cm resistivity, surface tension value of 72.16 mN/m, and a pH of 6.5-7.0 at room temperature (25 °C) was used throughout the experiments. 2.2. Sampling, analysis and characterization of coal The coal sample was prepared by jaw crushing followed by roll crushing to -3 mm particle size. It was further ground, screened to below 100 µm and used throughout the experiments. Proximate analysis was done by Proximate Analyser (TGA 601; LECO). The percentage of fixed carbon was calculated by difference. The carbon, hydrogen, nitrogen and sulphur analysis were done by CHNS Analyser (TruSpec®; LECO). The percentage of oxygen was calculated by difference. The gross calorific value was determined by Bomb Calorimeter (AC 500; LECO). Ash fusion temperature (AFT) was tested by Heating Microscope (Hesse Instruments) as per Indian Standard (IS 12891-1990). Hardgrove grindability index (HGI) was determined as per Indian Standard (IS 4433-1979). The chemical analysis of coal ash samples was analysed as per Indian Standard (IS 1355-1984). The sulphur content of the acid treated coal samples were determined using Carbon and Sulphur Analyser (EMIA-920V2, Horiba). The ash forming mineral-matter phases and oxygenated functional groups of raw and treated coal samples were characterized by XRD (X’Pert Pro; PANalytical) and FTIR (Spectrum-GX; Perkin Elmer), respectively. The surface morphology of ultrasonicated coal sample was characterized by SEM (JSM 6510; JEOL). The trace elemental analysis of alkali leachates was done by ICPOES (Optima 8300; Perkin Elmer).

10


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2.3. Beneficiation of coal by alkali leaching under ultrasonication followed by acid treatment In a 1 litre glass beaker, a freshly prepared 1M NaOH solution was added to 300 g of coal sample maintaining the solid/water ratio of 0.6 and placed in a centre position of the digital ultrasonicator bath (LMUC-16, LABMAN) of 16 L volume, 300 W power and 330×300×200 mm (L×W×H) tank dimension at 40±3 kHz frequency for 1 h maintaining bath temperature at 80 °C. After leaching, the coal slurry was filtered, washed with water till neutralization and oven dried. Then, 5 g of each oven dried NaOH leached coal samples were taken and treated with 10% and 30% HCl and H2SO4, respectively, in a hot plate at 90 °C for 1 h. The resultant coal slurry was filtered, washed with cold water till neutralisation and oven dried. The same procedure was carried out to leach coal using 1M KOH and 1M Na2CO3 followed by 10% and 30% of H2SO4 and HCl treatment, respectively. 3. Result and discussions 3.1 Physicochemical properties and ash composition of raw coal The physicochemical properties and ash composition of raw coal were determined and are summarized in Table 1. Besides low sulphur content, a higher percentage of ash forming mineral matters was observed which can be clearly seen from Table 1. These inorganic mineral matters are associated mostly with the clay particles and closely packed into the coal matrix. The AFT determination involves identification of four different temperature as shown in Table 1. These values are largely influenced by CaO content, Fe2O3 content and S/A weight ratio which minimizes the AFT value. The GCV value of raw coal is estimated to be 4423 cal/g which indicates low fixed carbon content due to the presence of inorganic impurities. Moreover, the lower value of HGI indicates hardness and less grindibility nature of coal sample. This prompts

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the need of removing ash forming mineral matters from the coal to make it commercially usable, marketable and environment friendly. Table 1. Physicochemical properties and ash composition of raw coal. Proximate analysis (air dried basis), % Moisture

7.88

Volatile matter

31.27

Ash

25.77

Fixed carbon (by difference)

35.08

Ultimate analysis (dry mineral matter free basis), % Carbon

78.00

Hydrogen

4.91

Nitrogen

1.87

Sulphur

0.69

Oxygen (by difference)

14.53

Ash fusion temperature (AFT), °C Deformation temperature

1271

Spherical temperature,

1485

Hemispherical temperature

1503

Flow temperature

1515

Gross calorific value (GCV), cal/g

4423

Hardgrove grindibility index (HGI)

56.49

Ash composition (wt. %)

12


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SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

TiO2

S/A

59.41

25.61

6.42

2.92

0.99

0.37

2.01

1.12

2.31

S/A = (SiO2/Al2O3 weight ratio) The ash composition of raw coal presented in Table 1 shows that the percentage of SiO2 is higher (constituting over 50% of the total ash) followed by Al2O3 with S/A weight ratio of 2.31. The S/A weight ratio indicates the presence of high amount of free silica, which may be of crystalline (quartz) or amorphous nature. These two together comprises about 85% of the total ash mostly responsible for the ash formation after combustion. Other constituents such as Fe2O3, CaO, MgO, Na2O, K2O and TiO2 were also detected which substantially contributes toward the ash formation. 3.2 SEM analysis of raw and ultrasonicated coal To have a better understandability of surface and fracture morphology, raw and ultrasonicated coal samples were analysed using SEM characterization technique. The SEM image in Figure 2(A) and (B) shows the surface texture of coal particles and associated impurities in raw coal. The ultrasonicated coal samples were prepared by subjecting to a low frequency (40 kHz) ultrasound in an aqueous media for 30 min, and 1 h. The corresponding SEM images are shown in Figure 2(C) and (D), respectively, which exhibits the particle breakage and widened crack formation. Moreover, the resultant widened crack formed on the coal surface, when exposed to the ultrasound waves, prevails the pitting and breaking effect further. This clearly describes the surface modification and successive particle breakage under the action of low-frequency ultrasound. The particle breakage at low frequency is more dominant comparative to high-frequency ultrasound as a result of acoustic cavitation and leads to more particle size reduction

16, 17

. The size reduction is a time-dependent phenomenon and progresses rapidly over

13


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a period of time as confirmed from Figure 2(C) and (D). The widely distributed inorganic impurities such as SiO2 and Al2O3 are held loosely and rigidly to the coal particle. It is possible that ultrasound may remove most of the loose matters during surface pitting and before crack formation 16, 17.

Figure 2. SEM morphology of raw and ultrasonicated coal samples. (A, B: raw coal; C: particles breakage after 30 min ultrasonication; D: crack formation after 1 h ultrasonication).

3.3 XRD characterization of coal samples To understand the different mineral matter phases quantitatively, the XRD characterization was conducted on the three different samples namely, raw coal, alkali leached coal and alkali leached coal followed by acid treatment. The various mineral phases identified in the raw coal are quartz (SiO2), kaolinite [Al2Si2O5(OH4)], pyrite (FeS2), calcite (CaCO3),

14


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fluorapatite

[Ca5(PO4)3F],

illite

[(K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]],

montmorillonite [(Na,Mg,Al)Si4O10(OH)2] and hematite (Fe2O3) as shown in Figure 3(A). In the raw coal, the predominance of quartz followed by kaolinite contribute towards the higher percentage of SiO2 and Al2O3 matters, respectively.

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Figure 3. XRD pattern of (A) raw, ultrasonic assisted alkali and (B) alkali-acid treated coal samples. (Q: quartz; K: kaolinite; M: montmorillonite; I: illite; F: fluorapatite; C: calcite; A: alkali aluminosilicate; P: pyrite; H: hematite).

The presence of Fe is mostly contributed by illite, hematite and pyrite mineral phases while some trace elements such as Na, Mg, Ca and K are also contributed by the associated mineral phases. During the leaching process, alkali penetrate into the coal particles through cracks due to ultrasound streaming, reacts with the inorganic matters and leach out SiO2 and Al2O3 matters in the form of alkali silicate and aluminate along with other associated impurities

3, 20

. The

continuous leaching results in the accumulation of excess alkali silicate and aluminate ions in the alkali solution that causes the precipitation of alkali aluminosilicate gel when exceeds above its solubility limit. The aluminosilicate gel remains in the coal surface after the water wash due to its highly insoluble nature and hence affects the ash reduction. This can be confirmed from the XRD patterns of alkali leached coal in Figure 3(A) which clearly shows a minor reduction in the peak intensity of mineral matter phases along with a new peak formation at 2θ=28° representing the alkali aluminosilicate compound formation over the coal surface. Such compound has an affinity to decompose and dissolve completely in acidic media under a specified conditions. Upon acidification of alkali leached coal samples, the peak intensity of alkali aluminosilicate along with other SiO2 and Al2O3 bearing mineral phase were found to be decreased as shown in Figure 3(B). Complete removal of impurities was not achieved due to the presence of some unliberated SiO2 and Al2O3 matters representing quartz and kaolinite mineral phase, respectively, which remains in the coal particles after the leaching process. 3.4 FTIR spectroscopy of coal samples

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It has been observed that freshly mined coal floats better than coal which has been exposed to the atmospheric oxygen for a prolonged periods

28

. Moreover, if the coal is of low

grade, its affinity towards surface oxidation is more than the high-grade coal. Since the present coal under study is of low grade, it is believed to be surface oxidised. The increase in coal hydrophilicity due to surface oxidation results in its poor floatability behaviour which affects the floatation performance

27, 28

. The FTIR spectra of raw coal in Figure 4 shows the presence of

oxygenated functional groups such as OH, COOH, and C-O-C attached on its surface and confirmed its oxidised nature. These oxygenated groups are hydrophilic in nature that changes the surface property of coal from hydrophobic to hydrophilic. To understand the rank dependence of coal hydrophobicity, on the basis of surface hydrophilic and hydrophobic, hydrophilicity index may be estimated as follows: Hydrophilicity index =

௄೔ (ு௅)೔ ௄ೕ (ுை)ೕ

(1)

where, (HL)i is a measure of the abundance of the hydrophilic functional group i and (HO)j is a measure of the abundance of the hydrophilic functional group j at the coal surface. Ki and Kj are corresponding coefficients and may be determined either theoretically or experimentally. If the aliphatic and aromatic CH groups are the only hydrophobic functional groups and the hydroxyl and carboxyl groups are the only hydrophilic groups, then the hydrophilicity index given in equation (1) can further be simplified as: Hydrophilicity index =

ఒ(ି஼ைைு)ାଶఒ(ିைு) ఒ(ோିு)ାఒ(஺௥ିு)

(2)

where, λ(–COOH), λ(–OH), λ(R–H) and λ(Ar–H) are the values of the absorption intensity as expressed by the Kubelka– Munk function for carboxyl, hydroxyl, aliphatic CH and aromatic CH groups 27, 28.

17


Energy & Fuels

Si-O Al-O

-OH

-OH

C-O-C COOH R-H

R-H

C-O-C Si-O

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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R-H

R-H -OH

-OH

Ar-H

R-H

R-H

1M Na2CO3+ 30% H2SO4

Ar-H R-H

-OH

Ar-H

Ar-H Al-O

Raw

1M KOH+ 30% H2SO4

C-O-C Si-O Al-O Ar-H

R-H C-O-C Si-O Al-O Ar-H

1M NaOH+ 30% H2SO4 4000

3500

3000

2500

2000

1500

1000

500

-1 Wavelength (cm ) Figure 4. FTIR spectra of raw and alkali-acid treated coal samples. In comparison to alkali-acid treated coal samples, the predominance of hydroxyl and carboxyl groups in raw coal are more as shown in Figure 4 in which the hydroxyl groups observed at 3681 and 3695 cm-1 and carboxyl group observed at 1605 cm-1 attributes to the hydrophilic oxygenated groups

27, 28

. Using FTIR band assignment data from Figure 4 and

equation (2), the quantitative value of hydrophilicity index of raw coal is calculated to be 1.44. In contrast, the FTIR spectra of NaOH, KOH and Na2CO3 leached coal after acid treatment shows a hydrophilicity index of 0.29, 0.32, and 0.34, respectively. This indicates the decrease of hydroxyl groups and nearly disappearance of carboxyl groups after alkali-acid treatment. Furthermore, an attainment of hydrophobicity can be confirmed by the appearance of aliphatic and aromatic CH

18


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groups after alkali-acid treatment which indicates an enhancement of fixed carbon content. The strong inorganic mineral peaks at 538 and 1004 cm-1 represent the presence of a higher percentage of silica while the peak at 686 cm-1 represents alumina bearing impurities, respectively

27

. Also, a sharp decrease in peak intensity of such silica-alumina impurities was

achieved after alkali-acid treatment. The C-O-C peak intensity is also found to increase for NaOH, KOH and Na2CO3 leached coal after acid treatment. 3.5 Demineralization study of alkali leached coal under ultrasonication followed by acid treatment The demineralization study conducted on ultrasonicated leached coal samples using different alkalis followed by acid treatment are discussed in this section. An attempt has also been made to study the demineralization of alkali leached coal samples without ultrasonication followed by acid treatment and compared with the ultrasonicated one. Data reported in Table 2 shows that the alkali leached coal without ultrasonication has an insignificant effect on demineralization compared to the ultrasonic leached coal. It can also be seen from Table 2 that after 10% H2SO4 treatment, the demineralization of NaOH, KOH, and Na2CO3 leached coal without ultrasonication are 53.07%, 32.26% and 42.70%, respectively. However, under the same operating conditions and alkali-acid dosage, the values of demineralization have been increased to 58.93%, 41.69% and 46.58%, respectively, when exposed to ultrasound. Table 2. Experimental results of different alkali leaching (at 80 °C for 1 h) followed by acid treatment (at 90 °C for 1 h) without and with ultrasound.

Alkali Properties, % concentration

Without ultrasound 10% H2SO4

With ultrasound 10% H2SO4

19


30% H2SO4

10% HCl

30% HCl

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Page 20 of 48

Ash

12.09

10.58

6.72

13.73

10.73

Fixed carbon

55.88

54.56

57.21

47.26

53.87

Reduction in ash content

13.67

15.18

19.04

11.03

15.03

Degree of demineralization

53.07

58.93

73.91

46.70

58.35

Ash

17.45

15.02

10.20

16.73

11.35

Fixed carbon

43.77

47.54

51.72

44.73

50.88


8.31

10.74

15.56

9.03

14.41


32.26

41.69

60.40

35.05

55.94

Ash

14.76

13.76

9.54

14.07

11.12

Fixed carbon

43.12

49.23

54.12

43.91

51.80


11.00

12.00

16.22

11.69

14.64


42.70

46.58

62.97

45.38

56.83

1M NaOH

1M KOH

1M Na2CO3

The lower ash percentage of alkali leached coal achieved under ultrasonication describes the potential of ultrasound in coal cleaning at a lower frequency (40 kHz). This is because the bubble cavitation is very severe at low frequency which results in pitting of the coal surface

17

.

Subsequently, surface pitting transforms to crack formation resulting in widening and deepening of coal surface cracks causing particles breakage under the continuous action of cavitation as witnessed from Figure 2(C) and (D). The ultrasound streaming acts as a driving force that pushes the alkali solution into the particle core and initializes the demineralization reactions. The increase in the degree of demineralization is directly proportional to the removal of mineral 20


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Energy & Fuels

matters present in coal. Upon application of alkali, the major mineral matters such as SiO2 and Al2O3 present in the coal dissociates into the alkali solution and forms alkali silicates and aluminates

11-13, 29-31

. The reactions involved in the leaching process and the formation of alkali

silicates and aluminates for different alkalis are presented in Table 3. Table 3. Reaction mechanism involved in alkali leaching of coal 13, 29, 30. Sl. No.

Alkali

1

NaOH

Reactions involved SiO2 + 2NaOH → Na2SiO3 + H2O Al2O3 + 2NaOH → 2NaAlO2 + H2O SiO2 + 2KOH → K2SiO3 + H2O

2

KOH Al2O3 + 2KOH → 2KAlO2 + H2O SiO2 + 2Na2CO3 → Na2SiO3 + CO2

3

Na2CO3 Al2O3 + 2Na2CO3 → 2NaAlO2 + CO2

As the leaching process advances, the concentration of silicate and aluminate ions present in the alkali solution increases and exceeds the solubility limit of alkali aluminosilicate. As a result, the alkali aluminosilicate precipitate out in the form of a gel by the following reaction: 5, 13 X2SiO3 + XAlO2 + XOH + H2O → [Xa(AlO2)b(SiO2)c.XOH.H2O] where, X = Alkali, and [Xa(AlO2)b(SiO2)c.XOH.H2O] = Alkali aluminosilicate gel. The continuous precipitation and accumulation of aluminosilicate gel onto the coal surface result in an increase in ash gain. Several authors have reported the increase in ash gain with an increase in alkali concentration. However, they have also reported the decrease in ash gain effectively with an increase in alkali concentration rendering a high degree of demineralization, provided a rigorous cleaning of alkali leached coal is required

21


9, 11, 13, 28

. This

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

demands the effective cleaning of alkali leached coal before combustion. Proper cleaning of aluminosilicate bearing coal particles before ash or proximate analysis is very crucial as the presence of such material may result in high ash gain or fusing with the base of the crucible (made of silica/quartz material) at high temperature as shown in Figure 5 (B-D). Normal water cleaning of alkali leached coal containing aluminosilicates was unsuccessful in the present study due to its poor solubility. The hot water cleaning is feasible with the coal containing low SiO2 and Al2O3 content and minimum alkali usage during leaching. The dosage of alkali concentration depends on the compositions of inorganic matter present in it. The lesser is the inorganic matters like SiO2 and Al2O3, the minimum is the alkali consumption and vice versa. However, treatment of alkali leached coal with mineral acid results in significant reduction in ash content. The reduction in ash content after acid treatment was found to increase with an increase in acid concentration in all the cases as shown in Figure 5(A). It is noteworthy to point out from Figure 5(A) that the demineralization of alkali leached coal treated with H2SO4 shows superiority in comparison with HCl treated coal. This indicates the effectiveness of treating alkali leached coal with H2SO4 over HCl in the present study. The dissociation of both the acid may be represented as H2SO4 → H+ + HSO4- ; HSO4- → H+ + SO42HCl → H+ + ClSince H2SO4 has a diprotic nature, dissociation of proton (H+) takes place in two steps while HCl dissociates in one step only. It is important to note that the effect of mineral acid or alkali on demineralization may vary from coal to coal depending on its nature.

22


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Energy & Fuels

Figure 5. Plot showing (A) effect of different alkalis on demineralization of coal after acid treatment; (B), (C) and, (D) represents fusing of NaOH leached, KOH leached, and Na2CO3 leached coal before acid washing during ash analysis at high temperature (above 800 °C). The results of demineralization study on alkali leached coal followed by acid treatment under ultrasonication are presented in Table 2. The NaOH leached coal has shown a higher degree of demineralization as compared to KOH and Na2CO3 leached coal when treated with H2SO4 and HCl. The maximum ash removal was achieved on NaOH leached coal followed by 30% H2SO4 treatment with 73.91% demineralization and 57.21% fixed carbon. This value is significantly higher than the demineralization value obtained among the other alkali-acid combinations. It can also be seen from Table 2 that the NaOH leached coal treated with 10%

23


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Page 24 of 48

H2SO4, and 30% HCl has shown nearly the same values in terms of ash content, fixed carbon content and degree of demineralization. In contrast, the Na2CO3 and KOH leached coal indicates insignificant demineralization in comparison with NaOH leached coal treated with varying percentage of acids. Conversely, the Na2CO3 leached coal has shown a better degree of demineralization comparatively with KOH leached coal when treated with H2SO4 and HCl. The minimum ash percentage of 9.54% and 10.20% were achieved on Na2CO3 and KOH leached coal treated with 30% H2SO4. In both the cases, the fixed carbon and degree of demineralization increases with increase in the percentage of acid. 3.6 Ash compositional analysis and GCV determination of ultrasonic assisted alkali leached coal followed by acid treatment The detailed ash compositional analysis and GCV measurement were conducted on NaOH, KOH and Na2CO3 leached coal treated with 30% H2SO4 and presented in Table 4. A significant increase in demineralization was observed after the removal of aluminosilicate compounds in all the cases. However, maximum demineralization was obtained on NaOH leached coal followed by 30% H2SO4 treatment. The effective demineralization results in increased GCV values which are certainly due to the increased fixed carbon content of treated coal. The GCV value of raw coal (4423 cal/g) after NaOH, KOH and Na2CO3 leaching followed by 30% H2SO4 treatment increases to 5789 cal/g, 5528 cal/g, and 5687 cal/g, respectively. Table 4. Ash compositional analysis and GCV values of ultrasonic assisted alkali leached coal followed by acid treatment. Ash composition (wt. %)

Alkali + 30% H2SO4

SiO2

Al2O3 Fe2O3 CaO

MgO

Na2O K2O

TiO2

S/A

1M NaOH

2.03

1.87

0.011 0.009 0.33

0.18

1.08

0.57

0.08

24


Sulphur GCV, (%) (cal/g) 0.74

5789

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Energy & Fuels

1M KOH

5.12

1.92

0.63

0.10

0.018 0.011 0.51

0.24

2.66

0.75

5528

1M Na2CO3

4.73

2.25

0.61

0.09

0.016 0.013 0.32

0.23

2.10

0.77

5687

S/A = (SiO2/Al2O3 weight ratio) It is clearly seen from Table 4 that there is a drastic reduction in other constituents apart from SiO2 and Al2O3. This is due to the fact that the alkali also reacts with oxides of calcium and magnesium and Fe2O3 forming soluble carbonates and ferrates, respectively 13. These carbonates and ferrates dissolve during acid treatment of coal. A slightly higher S/A weight ratio of KOH treated coal was also observed which possibly indicates the presence of free silica matters. A little gain in the sulphur content of coal samples was found after the H2SO4 treatment which indicates the absorption of the SO42- molecules on coal surface. Behera et al. 31 also reported the similar trend of marginal sulphur gain post NaOH-H2SO4 treatment of high ash coal. 3.7 Trace elemental analysis of the ultrasonic-assisted alkali leachates This study was conducted to understand the potential of ultrasound at low frequency (40 kHz) in releasing the trace elements present in the coal particles. The neutral leachate of raw coal ash and ultrasonic leachates of three different alkalis treated coal released during ultrasonic alkali leaching were analyzed using ICP-OES to confirm the presence of trace elements. The results in Table 5 clearly shows the predominance of Al, Fe, Ca, Mg and Ti accompanied by other trace elements in raw coal ash leachate and their removal in the subsequent ultrasonication process. Table 5. Trace elemental analysis of raw coal ash and ultrasonic alkali leachates. Elements (in ppm)

Raw coal ash leachate

NaOH leachate

KOH leachate

Na2CO3 leachate

Fe

39.77

19.92

18.91

17.93

25


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Page 26 of 48

Mg

4.846

2.74

2.12

2.48

Mn

0.408

0.124

0.135

0.149

Al

102.01

64.47

53.64

62.54

Ca

16.81

9.41

8.32

11.87

Cd

0.023

0.018

0.016

0.015

Cr

0.126

0.032

0.021

0.033

Cu

0.168

0.102

0.097

0.095

Ti

5.382

2.241

2.215

2.193

V

0.136

0.103

0.095

0.102

Zn

0.031

0.009

0.010

0.005

This is possibly because of surface pitting, and successive particle fragmentation occurs which allows the alkali to transport through capillary action into the core of coal particles. As a result, part of trace elements dissociated and dissolved in the alkali solution. 3.8 Feasibility of the process In this section, the feasibility and challenges in commercializing the ultrasound-based coal cleaning process have been discussed. The major challenges that industry could face during large-scale practice are the implementation of ultrasound processor and the regeneration of chemicals from spent liquor. Considering this two points, a comprehensive discussion has been made on the process viability in term of industrial aspects. 3.8.1. Implementation of ultrasound process on a large scale

26


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Energy & Fuels

In the past few decades, the application of ultrasound was limited to medical purpose and laboratory scale cleaning only. Despite its success and productive output, still high-intensity ultrasound is regarded as a laboratory technique and considered unfit for commercialization

32

.

The dynamic cleaning action of ultrasound has bound many researchers to investigate and explore its application in various fields

33-37

. To come up with the possible solutions, many

ultrasound based companies have explored the ultrasound chemistry from the root and explained the needs of large diameter horn that can generate higher ultrasound amplitude (above 70 µm) 32, 33

. This ultrasound amplitude of horn’s vibrating surface is the distance between its position in

the horn's fully extended and fully contracted states and measured in micron (µm). To be effective at a commercial scale, the ultrasound processor must generate ultrasound amplitudes of 70-100 µm 32, 33. Most of the conventional laboratory scale ultrasonicator uses the converging ultrasonic horn that can generate such amplitudes, but fails when upgrade to a large scale due to inability to generate amplitude greater than 25 µm. Moreover, overheating of transducers during continuous high-power operation and cavitation zone bypass in flow-through reactor chambers also reduces the uniform ultrasound treatment

32

. To overcome such limitations, the ultrasonic equipment

design has been proposed and well explained by Peshkovsky using large diameter horn (also known as Barbell Horn) that can generate high amplitude over 100 µm

32, 33

. Peshkovsky has

conducted the laboratory-, bench-, and industrial-scale experiments using LSP-500, BSP-1200, and ISP-3000 (Industrial Sonomechanics, LLC, New York) ultrasonic processors, respectively and successfully reported an overall process scale-up factor of 53.7 using Barbell Horn ultrasonic technology

33

. Many patents have been granted

38, 39

and few of them are being used by global

market players such as Hielscher Ultrasonics GmbH, Germany, and Industrial Sonomechanics

27


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LLC, New York in manufacturing high intensity ultrasound processor precisely for a various industrial process such as extraction, degassing , surface cleaning, homogenization, disintegration, particle size reduction, emulsification, water treatment and food processing

32-37

.

For instances, Genuine Bio-Fuel Inc., Florida, USA is one of the leading biodiesel production company that has adopted the Hielscher ultrasonic processor for large-scale production of biodiesel using various feedstock sources

40

. Due to the industry’s lack of familiarity with the

latest technology, ultrasonic equipment is still believed to be unfeasible for large scale. This negative reputation is rooted due to limitations of conventional ultrasonic processor designs, which have now been overcome with Barbell Horn Ultrasonic Technology

32, 33

. Hence, the

advancement in the sonochemical research and invention of the industrial grade ultrasonic processor indicates the possibilities of extension or implementation of such processor in coal leaching plant. 3.8.2. Recovery and regeneration of chemicals from spent liquor Undoubtedly, the chemical leaching of coal has proven to be a potential alternative to physical beneficiation process in term of ash reduction, enriching fixed carbon value, a substantial reduction in greenhouse gas emission during coal firing in a power plant, enhancement in efficiency of power generation during coal firing and elimination of the ash dams 41. Other various applications of clean coal include carbon anodes production, carbon steel alloy production, and reductant in silicon smelting, just to mention a few 41. As reported, the first chemical leaching plant of coal was operated in Germany with a capacity of producing 70,000 ton/year of low ash coal especially for the production of carbon electrodes for their aluminium industry. However, production was stopped after World War II and opted for petroleum coke to make carbon electrodes

41

. Since then, despite its valuable and productive output, its full-scale

28


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Energy & Fuels

commercialization is not established due to high chemicals cost and regeneration of reagents from spent liquor. However, the emerging of UCC Energy Pty. limited (a wholly own subsidiary of Yancoal International Limited) with patented technology for ultra clean coal (UCC) production has driven a breakthrough by building a pilot plant integrated with chemical regeneration unit in the year 2007-08

42, 43

. The plant has a nominal feed capacity of 350 kg/hr

and UCC output in the range of 1-2 kton/year situated in Cessnock, New South Wales, Australia. The plant was fully operational for 24 hr/day (4 days/week) during the year 2010-2012 attaining overall ash content of product below 0.2 % and found suitable for producing large quantities of UCC from different coal feeds 42, 43. The UCC pilot plant has been mothballed after its two years of successful operation. The mechanism used by the UCC Energy Pty. Limited is much similar to the well proven Bayer process of alumina production

42, 43

. This process uses NaOH, H2SO4,

and lime as basic reagents and works on the four stages namely, caustic digestion, acid soaking, caustic soda regeneration and acid causticization. The reaction mechanism involved in the four stages are as shown in Table 6. Table 6. Reaction mechanism involved in UCC production at UCC Energy Pty. Limited 42, 43.

SiO2 (s) + 2NaOH → Na2SiO3 (l) + H2O Caustic digestion 3Al2Si2O5(OH)4 (s) + 8NaOH → 2Na4Si3Al3O12(OH) (s) + 9H2O 2Na4Si3Al3O12(OH) + 13H2SO4 → 4Na2SO4 + 3Al2(SO4)3 + 6H2SiO3 + 8H2O Acid soaking

Fe2O3 + 3H2SO4 → Fe2(SO4)3 + 3H2O 2NaOH + H2SO4 → Na2SO4 + 2H2O Na2SiO3 + Ca(OH)2 (s) → CaSiO3 (s) + 2NaOH

Caustic soda regeneration Na2SO4 + Ca(OH)2 (s) + 2H2O → 2NaOH + CaSO4.2H2O (s)

29


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Page 30 of 48

Al2(SO4)3 + 3Ca(OH)2 (s) + 3H2O → 2Al(OH)3 (s) + 3CaSO4·H2O (s) H2SiO3 + Ca(OH)2 (s) → CaSiO3 (s) + 2H2O Acid causticization

Na2SO4 + Ca(OH)2 (s) + 2H2O → 2NaOH + CaSO4·2H2O (s) Fe2(SO4)3 + 3Ca(OH)2 (s) + 6H2O → 2Fe(OH)3 (s) + 3CaSO4·2H2O (s) H2SO4 + Ca(OH)2 (s) → CaSO4·2H2O (s)

Moreover, many researchers have also investigated and developed a process for possible regeneration and recycling of coal leaching based spent liquor

30, 42-44

. Waugh and Bowling

30

have developed a chemical leaching method integrated with the two-stage process of spent liquor regeneration. The first stage deals with coal leaching and subsequent filtration of the alkalized coal slurry to separate black liquor and coal cake. The black liquor containing Na2SiO3, an organic salt of coal and NaOH was regenerated by lime addition to the liquor. As a result, the silicate compounds react with lime thereby forming insoluble calcium silicate restoring the NaOH concentration. The second stage involves acidification of coal cake with SO2 and its subsequent filtration to separate white liquor and low ash coal. The white liquor containing sodalite was further limed to precipitate out the insoluble calcium salts restoring the NaOH concentration

30

. Sriramoju et al.

45

have reported about the regeneration of acid from spent

liquor by eliminating silicon compound by polycondensation process followed by subsequent removal of aluminium compound by treating with H2SO4. There are also several granted patents that deal with the recovery and regeneration of acid from coal leaching based spent liquor

46-48

.

This clearly shows the presence of the existing regeneration technology and can be implemented with some modification (if required) in the coal leaching process. Conclusions: 30


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Energy & Fuels

The present investigations on demineralization of high ash Indian non-coking coal under the action of low frequency (40 kHz) ultrasonication has proven to be the most promising and optimistic technique on a laboratory scale. This technique has shown its effectiveness over the conventional chemical process in terms of minimum treatment time and less consumption of reagents and energy with a higher degree of demineralization. The improvement in the fixed carbon content and GCV of coal increased drastically after each alkali leaching followed by acid treatment. The maximum degree of demineralization, fixed carbon content, and GCV were found to be 73.91%, 57.21% and 5789 cal/g, respectively, when leached with 1M NaOH followed by 30% H2SO4 treatment. Overall, this technique has the potential to replace the conventional chemical method and may be extended to large scale on trial basis. Reference [1] www.gsi.gov.in [2] Dash, P. S.; Lingam, R. K.; Santosh Kumar, S.; Suresh, A.; Banerjee, P. K.; Ganguly, S. Effect of elevated temperature and pressure on the leaching characteristics of Indian coals. Fuel 2015, 140, 302–308. [3] Balakrishnan, S.; Reddy, V. M.; Nagarajan, R. Ultrasonic coal washing to leach alkali elements from coals. Ultrason. Sonochem. 2015, 27, 235–240. [4] Nabeel, A.; Khan, T. A.; Sharma, D. K. Studies on the Production of Ultra-clean Coal by Alkali-acid Leaching of Low-grade Coals. Energy Sources, Part A 2009, 31(7), 594–601. [5] Dash, P. S.; Kumar, S. S.; Banerjee, P. K.; Ganguly, S. Chemical Leaching of High-Ash Indian Coals for Production of Low-Ash Clean Coal. Miner. Process. Extr. Metall. Rev. 2013, 34, 223–239.

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coking coal and its effects on cell flotation. Eur. J. Miner. Process. Environ. Prot. 2003, 3 (2), 167–176, 1303–0868. [29]

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Energy & Fuels

List of figures and tables Figures: Figure 1. Rarefaction and compression cycle showing growth and collapse of bubbles. Figure 2. SEM morphology of raw and ultrasonicated coal samples. (A, B: raw coal; C: particles breakage after 30 min ultrasonication; D: crack formation after 1 h ultrasonication). Figure 3. XRD pattern of (A) raw, ultrasonic assisted alkali and (B) alkali-acid treated coal samples. (Q: quartz; K: kaolinite; M: montmorillonite; I: illite; F: fluorapatite; C: calcite; A: alkali aluminosilicate; P: pyrite; H: hematite). Figure 4. FTIR spectra of raw and alkali-acid treated coal samples. Figure 5. Plot showing (A) effect of different alkalis on demineralization of coal after acid treatment; (B), (C) and, (D) represents fusing of NaOH leached, KOH leached, and Na2CO3 leached coal before acid washing during ash analysis at high temperature (above 800 °C).

Tables: Table 1. Physicochemical properties and ash composition of raw coal. Table 2. Experimental results of different alkali leaching (at 80 °C for 1 h) followed by acid treatment (at 90 °C for 1 h) without and with ultrasound. Table 3. Reaction mechanism involved in alkali leaching of coal. Table 4. Ash compositional analysis and GCV values of ultrasonic assisted alkali leached coal followed by acid treatment. Table 5. Trace elemental analysis of raw coal ash and ultrasonic alkali leachates. Table 6. Reaction mechanism involved in UCC production at UCC Energy Pty. Limited.

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Page 38 of 48

Table 1. Physicochemical properties and ash composition of raw coal. Proximate analysis (air dried basis), % Moisture

7.88

Volatile matter

31.27

Ash

25.77

Fixed carbon (by difference)

35.08

Ultimate analysis (dry mineral matter free basis), % Carbon

78.00

Hydrogen

4.91

Nitrogen

1.87

Sulphur

0.69

Oxygen (by difference)

14.53

Ash fusion temperature (AFT), °C Deformation temperature

1271

Spherical temperature,

1485

Hemispherical temperature

1503

Flow temperature

1515

Gross calorific value (GCV), cal/g

4423

Hardgrove grindibility index (HGI)

56.49

Ash composition (wt. %) SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

TiO2

S/A

59.41

25.61

6.42

2.92

0.99

0.37

2.01

1.12

2.31

S/A = (SiO2/Al2O3 weight ratio) 38


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Energy & Fuels

Table 2. Experimental results of different alkali leaching (at 80 °C for 1 h) followed by acid treatment (at 90 °C for 1 h) without and with ultrasound. Without ultrasound 10% H2SO4

With ultrasound 10% H2SO4

30% H2SO4

10% HCl

30% HCl

Ash

12.09

10.58

6.72

13.73

10.73

Fixed carbon

55.88

54.56

57.21

47.26

53.87


13.67

15.18

19.04

11.03

15.03


53.07

58.93

73.91

46.70

58.35

Ash

17.45

15.02

10.20

16.73

11.35

Fixed carbon

43.77

47.54

51.72

44.73

50.88


8.31

10.74

15.56

9.03

14.41


32.26

41.69

60.40

35.05

55.94

Ash

14.76

13.76

9.54

14.07

11.12

Fixed carbon

43.12

49.23

54.12

43.91

51.80


11.00

12.00

16.22

11.69

14.64


42.70

46.58

62.97

45.38

56.83

Alkali Properties, % concentration

1M NaOH

1M KOH

1M Na2CO3

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Page 40 of 48

Table 3. Reaction mechanism involved in alkali leaching of coal. Sl. No.

Alkali

Reactions involved SiO2 + 2NaOH → Na2SiO3 + H2O

1

NaOH Al2O3 + 2NaOH → 2NaAlO2 + H2O SiO2 + 2KOH → K2SiO3 + H2O

2

KOH Al2O3 + 2KOH → 2KAlO2 + H2O SiO2 + 2Na2CO3 → Na2SiO3 + CO2

3

Na2CO3 Al2O3 + 2Na2CO3 → 2NaAlO2 + CO2

Table 4. Ash compositional analysis and GCV values of ultrasonic assisted alkali leached coal followed by acid treatment. Ash composition (wt. %)

Alkali +

Sulphur GCV, (%) (cal/g)

30% H2SO4

SiO2

Al2O3 Fe2O3 CaO

MgO

Na2O K2O

TiO2

S/A

1M NaOH

2.03

1.87

0.57

0.08

0.011 0.009 0.33

0.18

1.08

0.74

5789

1M KOH

5.12

1.92

0.63

0.10

0.018 0.011 0.51

0.24

2.66

0.75

5528

1M Na2CO3

4.73

2.25

0.61

0.09

0.016 0.013 0.32

0.23

2.10

0.77

5687

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Energy & Fuels

Table 5. Trace elemental analysis of raw coal ash and ultrasonic alkali leachates. Elements (in ppm)

Raw coal ash leachate

NaOH leachate

KOH leachate

Na2CO3 leachate

Fe

39.77

19.92

18.91

17.93

Mg

4.846

2.74

2.12

2.48

Mn

0.408

0.124

0.135

0.149

Al

102.01

64.47

53.64

62.54

Ca

16.81

9.41

8.32

11.87

Cd

0.023

0.018

0.016

0.015

Cr

0.126

0.032

0.021

0.033

Cu

0.168

0.102

0.097

0.095

Ti

5.382

2.241

2.215

2.193

V

0.136

0.103

0.095

0.102

Zn

0.031

0.009

0.010

0.005

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Page 42 of 48

Table 6. Reaction mechanism involved in UCC production at UCC Energy Pty. Limited.

SiO2 (s) + 2NaOH → Na2SiO3 (l) + H2O Caustic digestion 3Al2Si2O5(OH)4 (s) + 8NaOH → 2Na4Si3Al3O12(OH) (s) + 9H2O 2Na4Si3Al3O12(OH) + 13H2SO4 → 4Na2SO4 + 3Al2(SO4)3 + 6H2SiO3 + 8H2O Acid soaking

Fe2O3 + 3H2SO4 → Fe2(SO4)3 + 3H2O 2NaOH + H2SO4 → Na2SO4 + 2H2O Na2SiO3 + Ca(OH)2 (s) → CaSiO3 (s) + 2NaOH

Caustic soda regeneration Na2SO4 + Ca(OH)2 (s) + 2H2O → 2NaOH + CaSO4.2H2O (s) Al2(SO4)3 + 3Ca(OH)2 (s) + 3H2O → 2Al(OH)3 (s) + 3CaSO4·H2O (s) H2SiO3 + Ca(OH)2 (s) → CaSiO3 (s) + 2H2O Acid causticization

Na2SO4 + Ca(OH)2 (s) + 2H2O → 2NaOH + CaSO4·2H2O (s) Fe2(SO4)3 + 3Ca(OH)2 (s) + 6H2O → 2Fe(OH)3 (s) + 3CaSO4·2H2O (s) H2SO4 + Ca(OH)2 (s) → CaSO4·2H2O (s)

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Energy & Fuels

Figures:

Figure 1. Rarefaction and compression cycle showing growth and collapse of bubbles.

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Figure 2. SEM morphology of raw and ultrasonicated coal samples. (A, B: raw coal; C: particles breakage after 30 min ultrasonication; D: crack formation after 1 h ultrasonication).

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Energy & Fuels

Figure 3. XRD pattern of (A) raw, ultrasonic assisted alkali and (B) alkali-acid treated coal samples. (Q: quartz; K: kaolinite; M: montmorillonite; I: illite; F: fluorapatite; C: calcite; A: alkali aluminosilicate; P: pyrite; H: hematite).

45


Energy & Fuels

Si-O Al-O

-OH

-OH

C-O-C COOH R-H

R-H

C-O-C Si-O

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 48

R-H

R-H -OH

-OH

Raw

Ar-H

R-H

R-H

1M Na2CO3+ 30% H2SO4

Ar-H R-H

-OH

Ar-H

Ar-H Al-O

1M KOH+ 30% H2SO4

C-O-C Si-O Al-O Ar-H

R-H C-O-C Si-O Al-O Ar-H

1M NaOH+ 30% H2SO4 4000

3500

3000

2500

2000

1500

-1 Wavelength (cm )

Figure 4. FTIR spectra of raw and alkali-acid treated coal samples.

46


1000

500

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Energy & Fuels

Figure 5. Plot showing (A) effect of different alkalis on demineralization of coal after acid treatment; (B), (C) and, (D) represents fusing of NaOH leached, KOH leached, and Na2CO3 leached coal before acid washing during ash analysis at high temperature (above 800 °C).

47


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Graphical abstract:

48


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Chemical Beneficiation of High-Ash Indian ... - ACS Publications

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