Ultrasonic-assisted flotation for enhancing the recovery of flaky graphite from low-grade graphite ore

Accepted Manuscript Ultrasonic-assisted flotation for enhancing the recovery of flaky graphite from low-grade graphite ore Santosh Deb Barma, Prasanta Kumar Baskey, Danda Srinivas Rao, Sachida Nanda Sahu PII: DOI: Reference:

S1350-4177(19)30430-4 https://doi.org/10.1016/j.ultsonch.2019.04.033 ULTSON 4573

To appear in:

Ultrasonics Sonochemistry

Received Date: Revised Date: Accepted Date:

20 March 2019 12 April 2019 21 April 2019

Please cite this article as: S.D. Barma, P.K. Baskey, D.S. Rao, S.N. Sahu, Ultrasonic-assisted flotation for enhancing the recovery of flaky graphite from low-grade graphite ore, Ultrasonics Sonochemistry (2019), doi: https://doi.org/ 10.1016/j.ultsonch.2019.04.033

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Ultrasonic-assisted flotation for enhancing the recovery of flaky graphite from lowgrade graphite ore Santosh Deb Barma*, Prasanta Kumar Baskey, Danda Srinivas Rao, and Sachida Nanda Sahu Mineral Processing 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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Ultrasonic-assisted flotation for enhancing the recovery of flaky graphite from lowgrade graphite ore Santosh Deb Barma*, Prasanta Kumar Baskey, Danda Srinivas Rao, and Sachida Nanda Sahu Mineral Processing Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, INDIA-751013

Abstract The feasibility of low-frequency ultrasound in enhancing the flotability of flaky graphite from low-grade graphite ore was investigated in this present study. This study involves a fundamental understanding of ultrasonic-assisted graphite flotation process and its relative comparison with the conventional flotation process. The flotation experiments were conducted in three stages viz., rougher, cleaner, re-cleaner stage during both convention and ultrasonicassisted flotation. It was found that the efficiency of ultrasonic-assisted flotation was significantly higher than conventional flotation. The yield, fixed carbon content and percentage recovery of the flotation concentrate products increased significantly under ultrasonic-assisted flotation. Furthermore, the ultrasonic mechanism and its effect responsible for the breakage of graphite-impurities locked particles and particle size reduction were also discussed comprehensively. The raw graphite (RG) and final flotation concentrate products of conventional and ultrasonic-assisted flotation process were characterized by Stereomicroscope, X-Ray Diffraction, Field Emission Scanning Electron Microscope and Raman Spectroscopy to understand the graphite liberation properties, mineralogical properties, surface properties and microcrystalline properties, respectively.

Keywords: Flaky graphite; ultrasound; flotation; cavitation

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1. Introduction Graphite is one of the naturally occurring allotropes of crystalline carbon with heterodesmic layered structure, consisting of six carbon atoms in hexagonal rings arranged in broadly spaced horizontal sheets [1,2]. It is known to exhibits the properties of both metal and non-metal, responsible for its profound applications in various industries [2-4]. While the metallic properties offer high thermal and electrical conductivity, the non-metallic properties offer lubricity, inertness, and high thermal resistance [3-5]. Due to these unique properties, graphite is used in several applications such as refractories, batteries, fuel cells, pencils, electrical goods, crucibles, brake linings, brushes, coating, electrodes, paints, welding rods, desulphurizing agent, etc. [2-10]. The occurrence of natural graphite is believed to be due to the metamorphosing of carbonaceous sediments as a result of the reaction between carbon compounds and hydrothermal or magmatic fluids underneath the earth crust [11-13]. Natural graphite may be found as an isolated scales, large masses or veins, distributed uniformly over the metamorphic rocks which are known to have composed of quartzite, limestone, gneiss, schist, pegmatites, etc. [3,4]. For the separation and segregation of graphite, the metamorphic rocks are crushed and ground to the desired size fractions. This results in the liberation of graphite particles along with a large number of inorganic impurities. In order to separate natural graphite from the associated impurities, flotation technique is commonly employed at an industrial scale [14-16]. Several studies have also been reported on the separation of natural graphite from the associated impurities using flotation process [3,6,10,11,14-18]. The separation mechanism of flotation is based on the difference in surface properties where hydrophobic materials are selectively separated from hydrophilic materials using reagents [19]. It was reported that the flotation process is well-responsive for graphite mineral 3

because of the fact that the graphite is naturally hydrophobic on the cleavage planes due to the low surface energy [3]. During the flotation process, the hydrophobic graphite floats at the airliquid interface and collected as a froth, whereas the hydrophilic gangue impurities remain in the suspension. Despite having natural hydrophobicity, flotability of graphite is generally enhanced by the addition of flotation reagents such as frother, collector and depressant/dispersant agent [3,16]. The amount of reagent dosage further depends upon the nature of impurities present in the graphite body. For commercial purpose, the natural graphite has been classified into three types, namely flaky graphite, vein graphite and microcrystalline graphite [4,17]. All these types of graphite have their own natural hydrophobicity, which further varies depending on the crystal properties and structures. It was also reported that the crystallinity of flaky and vein graphite is comparatively higher than the microcrystalline graphite [17]. Noteworthy to mention that not all types of graphite are suitable for all applications and therefore, their applications are target dependent where specific applications require one type in particular. The flotability of the flaky graphite is found better than other two types because of its higher crystallinity and bigger size of the crystal flakes. Their flotability nature was reported to be in the order: flaky graphite > vein graphite > microcrystalline graphite [5]. The flaky graphite occurs as an isolated, flat, plate-like particles, and may be classified on the basis of the flake size and graphitic carbon content. Generally, the graphite flake with a particle size of +150 µm is regarded as large flaky graphite while those with particle size -0.150 µm is regarded as fine flaky graphite. The industrial demand of the larger flaky graphite is significantly more than the smaller one due to its distinct properties such as excellent lubricity and higher thermal conductivity [5].

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As mentioned above that the flotability of flaky graphite is well responsive towards flotation process. However, in spite of having well flotability, it is barely possible to obtain the high purity flaky graphite by the conventional flotation [3-5,16,20]. This is because of the carry-over of the graphite-impurities locked particles toward the froth as well as cross-pollution effect between fine impurities and graphite flakes during the flotation process [15,20]. The cross-pollution effect refers to the adherence of ultrafine impurities over the graphite surface due to the greasy nature of graphite and their entrainment during sub-aeration air flow of flotation process [20]. It is important to note that obtaining high-grade graphite by conventional flotation depends on the many factors such as (i) nature of graphite crystallinity, (ii) percentage of impurities present in graphite ore, (iii) grade and fixed carbon content of graphite ore, (iv) pre-processing of graphite ore by grinding, scrubbing, screening, etc. and (v) number of stages involved in flotation process. Reportedly, the final purity of the graphite concentrate obtained by flotation may vary from 85-95% depending on the factors mentioned above and generally requires chemical treatment to enrich the purity of graphite concentrate further [3,4]. In order to overcome these challenges, the mechanical grinding process is commonly employed to break the locked particles and provide the attrition effect to scrub the fine impurities coated over the surface of graphite flakes [4,5]. Although the grinding process is quite effective, this process could potentially destroy the large flakes and reduce the size of the flakes [5]. Therefore, an alternative approach needs to be implemented to overcome such limitations without damaging the flakes. In this context, ultrasonic pre-treatment of graphite slurry prior to the flotation process is one of the potential alternatives. Due to its cleaner approach, ultrasound technique could be considered as one of the sustainable ways for enhancing the graphite flotation process from its natural ore. The incorporation of lowfrequency ultrasound into the slurry leads to the generation of strong cavitation effect that produces extremely fast microjets, streaming currents and enormous shear force [21-23]. All 5

these effects together could potentially participate in the scrubbing of the ultrafine impurities coated over the surface of graphite flakes and subsequently helps in the breaking of the locked particles. It is expected that the ultrasonic pre-treated graphite slurry upon subjecting to the flotation process could improve the flotation performance due to effective collector-graphite particles interaction and depressant-impurities interaction. Although many studies were reported on graphite beneficiation by conventional flotation process [3,5,14,15], no specific studies are reported on ultrasonic-assisted graphite flotation till date. Taking this into account, a feasibility study has been reported in this paper in improving the flotability of low-grade flaky graphite ore by flotation process under the effect of ultrasound. The present study is mainly emphasized to develop the fundamental understanding of ultrasonic-assisted graphite flotation process and its comparison with the conventional flotation process rather than enriching the low-grade graphite to high-purity graphite products. The principle and mechanism of ultrasonic-assisted graphite flotation process are also explained in results and discussions (section 3.3.1) based on the experimental observations of the present study. 2. Materials and methods 2.1. Materials The raw graphite (RG) sample of flaky origin received from Toamasina Province, Madagascar was used for conducting the present study. For preparing the representative sample, the as-received RG sample was initially crushed, ground, then screened below 300 µm, and used in the subsequent study. The commercially available diesel oil, Methyl Isobutyl Carbinol (MIBC), sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) were used as collector, frother, depressant/dispersant and pH regulator, respectively, during each flotation experiments. Normal tap water was used throughout the flotation experiments. 2.2. Analysis and characterization 6

The size fractional analysis for the representative RG sample was conducted using standard IS-460 test sieve. The photomicrographs of different size fractions of RG were obtained using stereomicroscope (Wild M7S; Wild Heerbrugg). Moisture, volatile matter, ash and fixed carbon of graphite samples were determined by proximate analysis. Ash compositional analysis of graphite samples was done by the standard wet chemical method. Sulphur analysis of graphite samples was done by carbon sulphur analyser (EMIA 920V2; Horiba). The graphite samples were characterized using X-Ray Diffractometer (XRD; X’Pert Pro; PANalytical) with Co Kα radiation (λ=1.79 Å) operated at a tube current of 30 mA and a voltage of 40 kV. All the XRD patterns were recorded at 2θ from 6° to 80° at a scan speed of 2°/min. The surface morphologies of graphite samples were studied using Field Emission Scanning Electron Microscope (FESEM; SUPRA GEMINI55; CARL ZEISS), operated at a working distance (WD) of 7.4 mm and accelerating voltage of 15 kV. Prior to FESEM experiment, all the respective samples were sputtered with the gold-palladium layer to improve the surface conductivity. The microcrystalline structure of graphite samples and their degree of ordering were investigated using Micro Raman Spectrometer (inVia; Renishaw) in the range of 100-3000 cm-1 operated at 100 mW with an excitation wavelength of 532 nm. Particle size distribution of flaky graphite concentrate samples was measured by particle size analyser (Hydro 2000MU; Malvern). 2.3. Conventional flotation The conventional flotation study was conducted in a Denver D-12 sub-aeration flotation cell having 7 l capacity. The flotation test consisted of three stages, viz., rougher stage, cleaner stage and re-cleaner stage. A schematic flowsheet showing different stages of conventional flotation process is shown in Fig. 1.

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Fig. 1. Three-stage conventional flotation process of raw graphite (RG) sample where A, B and C represents rougher stage, cleaner stage and re-cleaner stage, respectively. CC1: rougher concentrate, CC2: cleaner concentrate, CC3: re-cleaner concentrate, CT1: rougher tailings, CT2: cleaner tailings, CT3: re-cleaner tailings. In rougher stage, about 500 g of RG sample was taken in a flotation cell and mixed with water, maintaining the slurry concentration up to 40%. The slurry was then conditioned with Na2SiO3 (0.5 g/kg of feed) for 3 min at an impeller speed of 1500 rpm. Subsequently, the slurry was conditioned further with diesel oil (0.5 ml/kg of feed) for 3 min followed by MIBC (0.1 ml/kg of feed) for 3 min, maintaining impeller speed at 1500 rpm in both the cases. The final slurry concentration was brought down to 10% by adding make-up water into the flotation cell, and pH value of 8.5 was maintained throughout for all the experiments by adding 1 M NaOH solution dropwise to increase collector selectivity under alkaline condition [3]. 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 concentrate of rougher stage or rougher 8

concentrate (CC1) was collected and introduced to the cleaner stage of flotation. Again the flotation concentrate of cleaner stage or cleaner concentrate (CC2) was collected and further introduced to the re-cleaner stage of flotation as shown in Fig. 1. The flotation concentrate of re-cleaner stage or re-cleaner concentrate (CC3) was finally collected as a final product. During the cleaner and re-cleaner stage of flotation, impeller speed of 1500 rpm was maintained with no further addition of any reagents. 2.4. Ultrasonic-assisted flotation The ultrasonic-assisted flotation study was conducted using a similar procedure as defined in section 2.3 for conventional flotation. The only difference is that each feed samples were pre-treated with ultrasound at 40 kHz frequency for 5 min before introducing them to rougher, cleaner and re-cleaner stage of flotation. For the purpose of ultrasound pre-treatment, a digital ultrasonicator bath (LABMAN; LMUC-16) of 16 l volume, 300 W power and 330 ×300×200 mm (L×W×H) tank dimension was used. A schematic flowsheet showing different stages of ultrasonic-assisted flotation process is shown in Fig. 2.

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Fig. 2. Three-stage ultrasonic-assisted flotation process of raw graphite (RG) sample where A, B and C represents rougher stage, cleaner stage and re-cleaner stage, respectively. UC1: rougher concentrate, UC2: cleaner concentrate, UC3: re-cleaner concentrate, UT1: rougher tailings, UT2: cleaner tailings, UT3: re-cleaner tailings. 3. Results and discussion 3.1. Properties of raw graphite The components of proximate analysis such as moisture, volatile matter, fixed carbon, and ash content of RG are presented in Table 1. The higher ash content (80.43%) and lower fixed carbon content (10.79%) of RG sample in Table 1 indicate the low-grade nature of RG. The high-ash content in RG majorly comprised of complex silicate minerals followed by other clay minerals as discussed in the later section (section 3.4) of this paper. Some of the major inorganic impurities present in RG are SiO2, Al2O3, Fe2O3, CaO, MgO, TiO2, etc. as confirmed from chemical compositional analysis (Table 5). Table 1. Proximate and size fractional analysis of raw graphite (RG). Proximate analysis Moisture (%)

Volatile matter (%)

Ash (%)

Fixed carbon (%)

0.05

8.73

80.43

10.79

Size fractional analysis Size fraction (µm)

Weight (%)

Ash (%)

Fixed carbon (%)

+1000

6.90

89.75

2.71

-1000+850

3.34

85.53

9.54

-850+710

4.96

82.06

13.48

-710+600

3.34

82.48

13.29

10

-600+500

6.04

76.90

18.91

-500+300

12.51

74.39

21.35

-300+212

9.16

75.45

19.62

-212+150

3.67

76.51

17.86

-150+100

2.80

75.01

18.86

-100+75

1.73

78.71

13.61

-75+53

1.29

78.56

12.23

-53+45

1.94

72.06

16.79

-45

42.32

82.63

3.86

The size fractional analysis of RG sample with corresponding weight percentage, ash content and fixed carbon content of the respective size fractions are provided in Table 1. The ash analysis of different size fractions of RG shows that the ash content of all the size fractions is significantly higher (> 70%). This implies that the inorganic ash-forming mineral impurities are uniformly distributed throughout the structural matrix of RG sample. The fixed carbon content of the top course (+1000 µm) and bottom fine (-45 µm) fraction is comparatively lower than the intermediate size fractions, confirming the existence of more flaky graphite particles in the intermediate size fractions. The lower fixed carbon in + 1000 µm size fraction is due to the presence of large number of graphite-impurities locked particles and non-liberation of flaky graphite particles from those locked particles. Among all the size fractions, -500+300 µm size fraction showed a maximum fixed carbon content of 21.35% alongside moderate ash (74.39%) and weight percentage (12.51%). Higher fixed carbon content in -500+300 µm size fraction corresponds to the existence of more percentage of natural graphite flakes in the size range 500+300 µm of RG. Furthermore, for the bottom size fraction (-45 µm), maximum weight

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percentage (42.32%) along with lower fixed carbon content (3.86%) was observed, indicating the presence of large amount of ultrafine clay minerals in the RG. 3.2. Stereomicroscopic study of raw graphite From the above-mentioned size fractional analysis study (Table 1) of RG, it is confirmed that the flaky graphite particles are uniformly distributed in the size range of 850+710 µm to -53+45 µm. However, liberation of graphite flakes from the associated impurities and their breakage occurred during mechanical grinding cannot be commented from such size fractional analysis data. Therefore, to confirm the liberation characteristics of flaky graphite particles and breaking of graphite flakes during mechanical size reduction process, the structural characterization was conducted on different size fractions of RG sample (-850+710, -710+600, -600+500, -212+150, -150+100, and -75+53 µm) using Stereomicroscope. The Stereomicroscopic photomicrographs of respective size fractions of RG are shown in Fig. 3. As shown in Fig. 3, the graphite flakes (sliver-grey in colour) is widely disseminated over the inorganic impurities (crystal-white in colour) in all the size fractions of RG. From Fig. 3 (a-c), it can be seen that the large numbers of flaky graphite are embedded into the inorganic impurities in the form of locked particles for the larger size fractions (up to +500 µm). With decreasing size fractions (below +150 µm), the liberation of flaky graphite increases progressively due to the detachment of graphite grains from associated inorganic impurities. For -75+53 µm size fraction, the maximum liberation of graphite particles with an insignificant number of locked particles was observed. It also elucidates the possible breakage of large graphite flakes during mechanical grinding, leading to a maximum liberation of graphite particles at +53 µm size fraction. Noteworthy to mention that, although liberation of graphite particles increases with decreasing size fraction, it is expected that the larger flakes of graphite would break during the grinding or size reduction process [5]. 12

Fig. 3. Stereomicroscopic photomicrographs of various size fractions of RG sample. White arrow mark in all the figures shows the presence of graphite-impurities locked particles in the respective size fractions.

It is generally desired to obtain the maximum liberation of graphite particles from associated inorganic impurities without destroying the large flakes of graphite during the 13

grinding process. In actual practice, it is quite challenging to achieve the same because of the complex mineralogical characteristics of the locked particles where the impurities are strongly embedded between the graphite flakes. Therefore, optimum conditions need to be determined so as to obtain the satisfactory liberation of flaky graphite with minimal breakage of the flakes. Keeping a view of the adverse effect of mechanical grinding on the shape of graphite flakes, a process has been proposed in the next section (section 3.3) where RG was pre-treated with ultrasound during the flotation process for maximizing the recovery of flaky graphite particles. 3.3. Flotation study The effect of ultrasound and its mechanism on enhancing the flotability of RG are studied in this section. To understand the effect of ultrasonic pre-treatment on graphiteimpurities locked particles and its influence on the subsequent flotation process, the study was divided into two parts. The first part (section 3.3.1) consisted of studying the effect of ultrasonic pre-treatment on RG and flotation concentrates (rougher concentrate, i.e., UC1 and cleaner concentrate, i.e., UC2) obtained during the flotation process. Each flotation products of the three-stage flotation (both conventional and ultrasonic-assisted) were collected, and subjected to weight and proximate analysis to confirm the effect of ultrasound on the graphite flotability, and determine the weight percentage and fixed carbon value of each product individually for comparison. The second part (section 3.3.2) consisted of studying the effect of ultrasonic pretreatment on the overall flotation performance. Unlike in the first part (section 3.3.2) where each flotation products of the three-stage flotation were collected, and subjected to weight and proximate analysis, herein each flotation concentrates were fed directly to the next flotation stages without collection them so that there is no product loss. The obtained data were used to determine the yield and recovery of the overall flotation process. 14

3.3.1. Effect of ultrasonic pre-treatment on flotation products and its mechanism To understand the effect of ultrasonic pre-treatment on feed (RG) and subsequent flotation products (both concentrates and tailings), the flotation experiment was conducted in three stages viz., rougher, cleaner, re-cleaner stage as per the flowsheet provided in Fig. 3. The three-stage conventional flotation experiment was also conducted on RG sample for the comparative study as per the flowsheet provided in Fig. 2. The detailed operating experimental conditions for both conventional and ultrasonic-assisted flotation process are given in Table 2. Table 2. Operating experimental conditions for conventional and ultrasonic-assisted flotation process. Operating conditions

Conventional

Ultrasonic-assisted

flotation

flotation

Feed, g

500

500

Final slurry concentration, %

10

10

Ultrasonic pre-treatment time, min

NA*

5

Conditioning time (for each reagent), min

3

3

Sodium silicate (dispersant/depressant) dosage, g/kg 0.5

0.5

Diesel oil (collector) dosage, ml/kg

0.5

0.5

MIBC (frother) dosage, ml/kg

0.1

0.1

Air flow rate, lpm

3

3

Froth collection time, min

1

1

Slurry pH

8.5

8.5

NA*= not applicable Noteworthy to mention that the ultrasonic pre-treatment time of each feeds (RG, UC1 and UC2) was limited to 5 min to avail the optimum cavitation effect without damaging the 15

graphite flakes significantly. During the flotation experiments (conventional and ultrasonicassisted), each flotation products (concentrates and tailings) of the first-stage flotation were collected, dried in an oven until complete moisture removal, and finally subjected to weight and proximate analysis. The oven-dried concentrates of first-stage flotation (which remains after the utilization of some portions of flotation concentrates in proximate analysis) were introduced to second-stage flotation. Similarly, the second-stage flotation products were collected, dried in an oven, and subjected to weight and proximate analysis. The oven-dried concentrates of second-stage flotation (which remains after the utilization of some portions of flotation concentrates in proximate analysis) were introduced to third-stage flotation. Again, the third-stage flotation products were collected, dried in an oven, and subjected to weight and proximate analysis. The results of weight and proximate analysis of each flotation products are presented in Table 3 in term of weight percentage and fixed carbon content. Table 3. Weight, recovery, volatile matter and fixed carbon analysis of flotation products obtained during conventional and ultrasonic-assisted flotation process. Conventional flotation Rougher stage

Cleaner stage

Re-cleaner stage

CC1

CT1

CC2

CT2

CC3

CT3

Weight (%)

10

90

90

10

94.44

5.56

Recovery (%)

67.79

32.21

99.59

0.41

99.86

0.14

Volatile matter (%)

3.17

9.33

2.54

8.81

2.35

5.45

Fixed carbon (%)

73.10

3.86

80.89

3

85.53

2

Sample analysis

Ultrasonic-assisted flotation Rougher stage

Cleaner stage

Re-cleaner stage

UC1

UC2

UC3

Sample analysis UT1

16

UT2

UT3

Weight (%)

10.50

89.50

95.24

4.76

98.75

1.25

Recovery (%)

82.42

17.58

99.931

0.069

99.997

0.003

Volatile matter (%)

2.66

9.54

2.47

8.93

2.28

7.99

Fixed carbon (%)

84.72

2.12

88.89

1.23

90.01

0.23

It can be seen from Table 3 that the weight percentage and fixed carbon content of concentrate products (UC1/UC2/UC3) obtained during ultrasonic-assisted flotation are higher in comparison to the concentrate products (CC1/CC2/CC3) of conventional flotation. This enhancement in the weight percentage and fixed carbon content of concentrate products indicates the enrichment of flaky graphite in the concentrate products during each stage of the ultrasonic-assisted flotation process. Conversely, decrease in the fixed carbon content and weight percentage of tailings products (UT1/UT2/UT3) during ultrasonic-assisted flotation elucidates the higher extraction of flaky graphite from tailings. Significant reduction in the volatile matter content of concentrate products obtained during ultrasonic-assisted flotation was also observed from 8.73% (RG) to 2.66% (UC1), 2.47% (UC2) and 2.28% (UC3) possibly due to the removal of volatile matter contributing minerals such as carbonate minerals. Note that, the recovery percentage data shown in Table 3 indicates the flaky graphite recovery for each stage (rougher/cleaner/re-cleaner) only (not to be confused with overall recovery) and was determined with respect to feed stream of the individual stage. Recovery data shown in Table 3 indicates that the fixed carbon recovery of UC1 was significantly higher than CC1 followed by the recovery of UC2 and UC3 due to more graphite flakes extraction from locked particles under cavitation effect.

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Fig. 4. Fixed carbon content and weight percentage of flotation concentrate products obtained during conventional and ultrasonic-assisted flotation process. In order to compare the flotation efficiency of conventional flotation with ultrasonicassisted flotation, a statistical plot is presented in Fig. 4, showing fixed carbon content and weight percentage of concentrate products obtained under both the flotation processes. Fig. 4 clearly shows that both fixed carbon content and weight percentage of UC1 is higher than CC1, and considerably increases for each stage of flotation. The enhancement in the higher recovery of flaky graphite during ultrasonic-assisted flotation is attributed to the cavitation phenomena,

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responsible for the generation of micro-scrubbing effect on graphite flakes followed by breaking of graphite-impurities locked particles. The power estimation by calorimetry showed that the actual energy transferred to the graphite slurry during ultrasonication (at 40 kHz/300 W) was 16.72 W. The schematic illustration of ultrasonic mechanism responsible for the higher extraction of flaky graphite from RG sample is presented in Fig. 5. From section 3.1, it is clear that the RG sample used in this present study is of low-grade which contains large amount of ashforming inorganic impurities (such as SiO2, Al2O3, Fe2O3, etc.). These impurities are finely disseminated inorganic minerals, either loosely adhered to the graphite surface or strongly associated between the graphite flakes in the form of locked particles. During the ultrasonic pre-treatment of graphite slurry, slurry media experiences alternative rarefaction and compression cycle due to the ultrasonic destruction of the attractive forces of molecules in the solution phase [23]. Many small bubble cavities arise due to sudden pressure drop during rarefaction cycle, which then undergoes alternate cyclic growth, and finally collapse upon maturity during the compression cycle. The collapse of bubbles (termed as cavitation) takes place at millions of locations during the process, producing high local instantaneous temperatures and pressures approximately equal to 5000 K and 500 atm, respectively, with enormous heating and cooling rates above 1010 K/s [21,24,25]. Simultaneously, the cavitation effect also leads to the generation of microjets or shockwaves uniformly due to high-impact bubble collapse in the solution phase, which causes pitting effect on the graphite surface upon collision. This surface pitting effect helps in scrubbing the loosely-adhered fine clay minerals coated over the surface of flaky graphite. Consequently, the continuous surface pitting effect, then, results in the breaking of the graphite-impurities locked particles and separates the graphite flakes from the associated impurities. As a result, the immediate reagents conditioning of the ultrasonic pre-treated sample during flotation process significantly enhances the reagents 19

adsorption onto the graphite flakes, strengthen the reagent-particle interaction and finally improves the flotability of flaky graphite.

Fig. 5. Mechanism showing liberation of flaky graphite from graphite-impurities locked particles under ultrasound. In the present study, the ultrasonic pre-treatment time of each feeds (RG, UC1 and UC2) to flotation was limited to 5 min with the purpose of availing the optimum cavitation effect without much damaging the graphite flakes. To further confirm this, particle size distribution study of CC3 and UC3 was performed so as to compare the effect of ultrasonic-assisted flotation with the conventional flotation on the graphite flakes. It can be seen from Fig. 6 that the majority of the particle size of flaky graphite lies in the range of 150-260 µm for both CC3 and UC3. Comparatively to UC3, CC3 has a higher weight percentage of the flaky graphite in the particle size range of 200-230 µm. It is expected that some of the locked particles are present in the CC3 and may be responsible for showing the large particle size than UC3. Fig. 6 also reveals the occurrence of size reduction and generation of a small number of fine particles in UC3 in the particle size range of 200-230 µm and 50-75 µm, respectively. The size reduction is attributed to the separation of graphite flakes from locked particles and possible breakage of some larger graphite flakes due to the cavitation effect. On the other hand, the generation of fine particles in the range of 50-75 µm may be attributed to the liberation of fine impurities or 20

broken pieces of graphite flakes during the breakage of locked particles. However, it can be commented that the occurrence of size reduction of flaky graphite is minimum and does not affect the overall process significantly.

Fig. 6. Particle size distribution of CC3 and UC3. 3.3.2. Effect of ultrasonic pre-treatment on overall flotation performance The overall flotation performance of conventional and ultrasonic-assisted flotation was studied by conducting the flotation experiments on RG sample in three stages viz., rougher, cleaner, re-cleaner stage. Similar experimental conditions were maintained during both the flotation experiments as defined in Table 2. The intermediate concentrate products (CC1 and CC2) obtained during conventional flotation were directly fed to the flotation cell immediately without disturbing their concentration. Similarly, the intermediate concentrate products (UC1 and UC2) obtained during ultrasonic-assisted flotation were fed to the flotation cell immediately after pre-treating them with 40 kHz frequency ultrasound. It is to be noted that no 21

intermediate products obtained during both the flotation experiments were taken for any analysis to ensure no product loss and dilution. The comparative results of flotation experiments showing yield, fixed carbon content and recovery of all the products are presented in Table 4. Table 4 shows that the fixed carbon content of all the flotation products is in good agreement with the results reported in Table 3. It is evident from Table 4 that the efficiency of the conventional flotation is significantly lower than the ultrasonic-assisted flotation. The lower value of fixed carbon (85.53%) and poor percentage recovery (67.41%) of the CC3 product in Table 4 confirm the same. This may be attributed to the entrainment of the fine clays mineral and carry-over of some graphite-impurities locked particles in the float concentrate, or due to the cross-pollution effect between fine impurities and graphite flakes during conventional flotation [20]. Conversely, the efficiency of ultrasonic-assisted flotation enhanced significantly as compared to the conventional flotation. The yield, fixed carbon content and percentage recovery of the final concentrate (UC3) of ultrasonic-assisted flotation were found to be 9.88%, 90.01% and 82.36%, respectively. This is because of the effective scrubbing of the clay minerals and breaking down of the locked particles which results in the liberation of flaky graphite particles during ultrasonic pre-treatment. Due to the liberation of graphite flakes, the collector molecules effectively adhered to the flaky particles during the flotation process. This effect not only enhances the selectivity of the collector but also strengthen the collector-flaky graphite interaction significantly during flotation conditioning. The decrease in the fixed carbon content in all the tailing products of ultrasonic-assisted flotation was also observed which indicates significant ultrasonic extraction of flaky graphite from the associated inorganic impurities. The so-obtained final concentrate products (CC3 and UC3) mentioned in this section were used for the analysis and characterization purpose, which are described in the subsequent sections. 22

Table 4. Overall flotation performance in terms of yield, fixed carbon and recovery. Product

Yield (%)

Fixed carbon (%)

Recovery (%)

CC3

8.5

85.53

67.41

Conventional

CT1

90

3.86

32.21

flotation

CT2

1

3

0.28

CT3

0.5

2

0.09

Total

100

10.78

100

Product

Yield (%)

Fixed carbon (%)

Recovery (%)

UC3

9.88

90.01

82.36

Ultrasonic-assisted

UT1

89.50

2.12

17.58

flotation

UT2

0.50

1.23

0.06

UT3

0.13

0.23

0.003

Total

100

10.79

100

3.4. XRD study The crystalline mineral phases present in RG, ash of RG, CC3, and UC3 were studied by X-Ray Diffraction (XRD) technique using diffractometer, and their respective XRD patterns are shown in Fig. 7.

23

Fig. 7. XRD pattern of (a) RG, (b) ash of RG, (c) CC3 and (d) UC3. Q: quartz; G: graphite; K: kaolinite; M: muscovite. Fig. 7 (a) and (b) shows the comparative mineralogical characteristics of RG and its ash product, respectively, to identify the inorganic mineral phases in the presence and absence of combustible matters. As evident from Fig. 7 (a) and (b), the mineral phases existing in RG sample

are

mostly

quartz

(SiO2),

kaolinite

[(Al2Si2O5(OH)4)]

and

muscovite

[KAl2(AlSi3O10)(F,OH)2]. The higher quartzitic phase in RG indicates the presence of large amount of silica impurities, which alone constitutes over 50% of the total ash impurities. While Fig. 7 (a) confirms the predominance of quartz followed by kaolinite and muscovite mineral phase in RG, Fig. 7 (b) shows the intensification of all the inorganic mineral peaks along with the appearance of new peaks in the XRD pattern of its ash product. The intensification of the 24

XRD peaks in Fig 7 (b) corresponds to the liberation of inorganic impurities after the complete combustion of graphite. It can be seen from Fig. 7 (c) and (d) that the mineral peaks present in the XRD pattern of CC3 and UC3 sharply decreased after the flotation process. Comparatively to the XRD pattern of CC3, higher reduction in the peak intensity of mineral phase was observed in the XRD pattern of UC3. This attributes to the ultrasonic-breakage effect responsible for the disruption of physical bonds between flaky graphite and impurities, and effective separation of graphite flakes from its associated impurities during the subsequent flotation process. 3.5. Ash compositional study The chemical compositional study (on ash basis) of RG, CC3 and UC3 was conducted to examine the composition of inorganic impurities present in them. The ash compositional data is presented in Table 5. Table 5. Ash composition (on ash basis) and sulphur content of graphite samples. Ash composition (wt. %)

Sulphur

Sample SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

TiO2

V2O5

others

(%)

RG

44.361

25.834

6.615

0.427

0.684

0.636

0.362

1.054

0.217

0.240

0.0041

CC3

6.489

1.742

0.466

0.010

0.008

0.005

0.021

0.015

0.005

0.350

0.0023

UC3

4.797

0.949

0.144

0.003

0.002

0.001

0.007

0.003

0.002

0.322

0.0021

It can be seen that the majority of the inorganic impurities present in the ash composition of RG are silica (SiO2) followed by alumina (Al2O3). While the occurrence of SiO2 impurities is mostly due to the predominance of quartz mineral in graphite ore, the occurrence of Al2O3 impurities are contributed by muscovite and kaolinite minerals, as 25

confirmed from the XRD analysis (Fig. 7). Around 6% of Fe2O3 and 1% alkali metal oxides were also identified in RG. Table 5 shows that the inorganic mineral impurities present in RG were remarkably removed during both conventional and ultrasonic-assisted flotation process. However, a higher reduction in the composition of ash impurities was encountered during ultrasonic-assisted flotation. This is certainly due to the cavitation effect that leads to microscrubbing of loosely-bound ash impurities coated on graphite particles and breaking of graphite-quartz locked particles during ultrasonic pre-treatment, and subsequent enhancement in the flotation efficiency. It is also evident from Table 5 that the extant of SiO2 impurities is still dominant in CC3 and UC3 possibly due to the complex mineralogical behaviour of RG, which remains in the flaky graphite particles after the flotation process. Furthermore, the existence of sulphur was also tested in the RG, CC3 and UC3. It was found that the sulphur content in RG is considerably lower and further reduced after the flotation process. A slightly higher sulphur reduction was encountered in UC3 than CC3. This may be attributed to the oxidation of sulphur components due to the generation of highly reactive species such as OHand H2O2 during ultrasonication, which converts them into hydrophilic sulphoxides (S=O) and sulphones (-SO2) and inhibits the flotability of sulphur during flotation process [22,23]. 3.6. FESEM study The FESEM images of RG, CC3, and UC3 and their respective silicon (Si) elemental mapping are shown in Fig. 8. The FESEM image of RG in Fig. 8 (a) confirms the presence of graphite-impurities locked particles and uniformly distributed inorganic impurities throughout the sample. The graphite-impurities locked particles are majorly composed of quartzite minerals responsible for the significant contribution of silica impurities in RG sample. Inset in Fig. 8 (a) confirms the existence of a large amount of Si-bearing impurities in RG. During the three-stage conventional flotation process, most of the loosely bound inorganic impurities were removed from the CC3 while leaving some amount of Si-bearing locked particles in the 26

flotation concentrate. This can be confirmed from Fig. 8 (b), which clearly shows the presence of graphite flakes in CC3 sample in association with Si impurities (inset in Fig. 8 (b)).

Fig. 8. FESEM image of (a) RG (b) CC3, (c) UC3 along with an inset showing respective silicon (Si) mapping and (d) elemental carbon mapping of UC3. In contrast to conventional flotation, higher removal of Si impurities was achieved during ultrasonic-assisted flotation. The FESEM image in Fig. 8 (c) clearly indicates the appearance of uniformly distributed graphite flakes in UC3 sample, whereas inset in Fig. 8 (c) reveals the reduction in the concentration of Si impurities during ultrasonic-assisted flotation process. The elemental carbon mapping in Fig. 8 (d) further reveals the enrichment of carbon values in UC3 sample. An enrichment in carbon value corresponds to the ultrasonic pretreatment of RG before the flotation process which scrubs the loosely bound inorganic

27

impurities due to surface pitting effect and also breaks the locked particles over a period of time, resulting in the higher flotability of flaky graphite during flotation. 3.7. Raman spectroscopic study The Raman spectra and deconvolution characteristics of RG, CC3 and UC3 are shown in Fig. 9. It is well defined that the Raman shift in the range of 1000-1800 cm-1 and 2200-3000 cm-1 represents first-order and second-order Raman spectra, respectively [22]. From Fig. 9 (a), it can be seen that D and G band are present in the first-order Raman spectrum, whereas G/ and 2D band are present in the second-order Raman spectrum of the graphite samples. This indicates that these samples exhibit both crystallinities as well as structural disorder in the graphitic structure. The corresponding peak position of all the bands is presented in Table 6. The appearance of D band in RG (1350 cm-1), CC3 (1350 cm-1) and UC3 (1349 cm-1) is due to breathing mode of k-point photons of 𝐴1𝑔 symmetry showing structural defects [26], whereas the appearance of G band in RG (1580 cm-1), CC3 (1581 cm-1) and UC3 (1581 cm-1) corresponds to the vibration of sp2 carbon atoms in the two-dimensional hexagonal lattice and in-plane 4 crystal symmetry in the aromatic layers of stretching vibration of 𝐸2𝑔 graphite mode with 𝐷6ℎ

graphite crystallite [26,27]. The structural modification in the graphitic lattice of RG, CC3 and UC3 can also be identified from the appearance of G/ and 2D band in the second-order Raman spectra. While the presence of G/ peak in the region 2434-2435 cm-1 is attributed to the overtone of Raman inactive graphitic lattice vibration mode [28], the presence of 2D peak in the region 2709-2711 cm-1 (observed as twice the wavelength of D band) is due to overtone of D band and depends on strong stacking order of graphite [25-29].

28

Fig. 9. Raman spectra of graphite samples. (a) RG, CC3 and UC3 samples; (b-d) deconvolution of Raman spectra of RG, CC3 and UC3 samples. The deconvolution of the Raman spectrum of RG, CC3 and UC3 was also done to study the spectral parameters and structural information. Fig. 9 (b-d) and Table 6 shows the curve deconvolution characteristics and spectral parameters, respectively, for RG, CC3 and UC3 Raman spectra. It can be seen from Fig 9 (b-d) that the intensity of the D, G, G/ and 2D band of both CC3 and UC3 sharply increases as compared to the RG. This may be attributed to the increase in the crystallinity as well as defect in the graphitic lattice of the CC3 and UC3 after the removal of inorganic impurities during the flotation process. However, higher crystallinity and defect are observed in UC3 due to intense structural deformation in the crystal lattice of

29

graphite under cavitation effect during ultrasonic pre-treatment. The band area ratio (AD/AG) of all the spectra given in Table 6 confirms that UC3 has the high degree of defect in the crystal lattice and is attributed to the delocalized π states around the sp2 chains. Table 6. Deconvolution characteristics of Raman spectra of graphite samples. Peak position (cm-1)

FWHM (cm-1)

Sample

R2

2D

D

G

G/

2D

1580 2434

2711

33.05

16.51

23.87

52.32

0.15

0.921

1350

1581 2435

2712

29.43

17.02

25.02

55.59

0.24

0.934

1349

1581 2435

2709

32.31

17.61

24.18

60.67

0.38

0.955

D

G

RG

1350

CC3 UC3

G/

AD /AG

FWHM: full width at half maximum; AD: area of D band; AG: area of G band 4. Conclusions Herein, a feasibility study has been reported in improving the flotation performance of flaky graphite under the action of low-frequency ultrasound (40 kHz), and the obtained results are compared with the conventional flotation process. In this study, the performance of conventional flotation was found poor due to the entrainment of the fine clays mineral and carry-over of some graphite-impurities locked particles in the float concentrate, or due to the cross-pollution effect between fine impurities and graphite flakes during conventional flotation. Conversely, it was observed that the pre-treatment of graphite samples under cavitation effect leads to micro-scrubbing of loosely-bound ash impurities from graphite surface and breaking of graphite-impurities locked particles during ultrasonic pre-treatment and subsequently enhanced the flotation efficiency due to effective collector-graphite flake interaction. Under the ultrasonic-assisted flotation process, overall flotation performance was

30

found maximum where yield, fixed carbon content and percentage recovery of the final concentrate (UC3) were found to be 9.88%, 90.01% and 82.36%, respectively. The decrease in the fixed carbon content of all the tailing products of ultrasonic-assisted flotation was also observed which indicates the ultrasonic extraction of flaky graphite from the associated inorganic impurities. The particle size of the flaky graphite was also affected during ultrasonic pre-treatment due to cavitation effect, however, to a very insignificant limit and did not affect the overall process significantly. Acknowledgement The authors would like to acknowledge the Director, CSIR-IMMT, Bhubaneswar for his permission to communicate this work. The funding from the CSIR (Project No. OLP-79 and FTT Project No. MLP-24) is highly acknowledged. References [1] B. Kwiecińska, H.I. Petersen, Graphite, semi-graphite, natural coke, and natural char classification—ICCP system, Int. J. Coal Geol. 57 (2004) 99–116. [2] H. Li, Q. Feng, L. Ou, S. Long, M. Cui, X. Weng, Study on washability of microcrystal graphite using float–sink tests, Int. J. Min. Sci. Technol. 23 (2013) 855–861. [3] S.C. Chelgani, M. Rudolph, R. Kratzsch, D. Sandmann, J. Gutzmer, A review of graphite beneficiation techniques, Miner. Process. Extr. Metall. Rev. 37 (2016) 58–68. [4] H. Wang, Q. Feng, X. Tang, K. Liu, Preparation of high-purity graphite from a fine microcrystalline graphite concentrate: Effect of alkali roasting pre-treatment and acid leaching process, Sep. Sci. Technol. 51 (2016) 2465–2472.

31

[5] K. Sun, Y. Qiu, L. Zhang, K. Sun, Y. Qiu, L. Zhang, Preserving flake size in an African flake graphite ore beneficiation using a modified grinding and pre-screening process, Minerals. 7 (2017) 115. [6] B.C. Acharya, D.S. Rao, S. Prakash, P.S.R. Reddy, S.K. Biswal, Processing of low grade graphite ores of Orissa, India, Miner. Eng. 9 (1996) 1165–1169. [7] D. Sandmann, S. Haser, J. Gutzmer, Characterisation of graphite by automated mineral liberation analysis, Miner. Process. Extr. Metall. Rev. 123 (2014) 184–189. [8] B.G. Kim, S.K. Choi, H.S. Chung, J.J. Lee, F. Saito, Grinding characteristics of crystalline graphite in a low-pressure attrition system, Powder Technol. 126 (2002) 22–27. [9] B.G. Kim, S.K. Choi, C.L. Park, H.S. Chung, H.S. Jeon, Inclusion of gangue mineral and its mechanical separation from expanded graphite, Part. Sci. Technol. 21 (2003) 341– 351. [10] T. Wakamatsu, Y. Numata, Flotation of graphite, Miner. Eng. 4 (1991) 975–982. [11] N. Vasumathi, T.V. Vijaya Kumar, S. Ratchambigai, S. Subba Rao, G. Bhaskar Raju, Flotation studies on low grade graphite ore from eastern India, Int. J. Min. Sci. Technol. 25 (2015) 415–420. [12] M.E. Galvez, O. Beyssac, I. Martinez, K. Benzerara, C. Chaduteau, B. Malvoisin, J. Malavieille, Graphite formation by carbonate reduction during subduction, Nat. Geosci. 6 (2013) 473–477. [13] M. Bonijoly, M. Oberlin, A. Oberlin, A possible mechanism for natural graphite formation, Int. J. Coal Geol. 1 (1982) 283–312.

32

[14] W. Peng, Y. Qiu, L. Zhang, J. Guan, S. Song, W. Peng, Y. Qiu, L. Zhang, J. Guan, S. Song, Increasing the fine flaky graphite recovery in flotation via a combined multiple treatments technique of middlings, Minerals. 7 (2017) 208. [15] X. Bu, T. Zhang, Y. Peng, G. Xie, E. Wu, X. Bu, T. Zhang, Y. Peng, G. Xie, E. Wu, Multi-stage flotation for the removal of ash from fine graphite using mechanical and centrifugal forces, Minerals. 8 (2018) 15. [16] X. Lu, E. Forssberg, Flotation selectivity and upgrading of Woxna fine graphite concentrate, Miner. Eng. 14 (2001) 1541–1543. [17] W. Peng, C. Wang, Y. Hu, S. Song, Effect of droplet size of the emulsified kerosene on the floatation of amorphous graphite, J. Disper. Sci. Technol. 38 (2017) 889–894. [18] Q. Shi, X. Liang, Q. Feng, Y. Chen, B. Wu, The relationship between the stability of emulsified diesel and flotation of graphite, Miner. Eng. 78 (2015) 89–92. [19] A. Norori-McCormac, P.R. Brito-Parada, K. Hadler, K. Cole, J.J. Cilliers, The effect of particle size distribution on froth stability in flotation, Sep. Purif. Technol. 184 (2017) 240–247. [20] N. Aslan, F. Cifci, D. Yan, Optimization of process parameters for producing graphite concentrate using response surface methodology, Sep. Purif. Technol. 59 (2008) 9–16. [21] S.D. Barma, S. R, P.K. Baskey, S.K. Biswal, Chemical beneficiation of high-ash Indian noncoking coal by alkali leaching under low-frequency ultrasonication, Energy Fuels. 32 (2018) 1309–1319. [22] S.D. Barma, R. Sathish, P.K. Baskey, Ultrasonic-assisted cleaning of Indian low-grade coal for clean and sustainable energy, J. Clean. Prod. 195 (2018) 1203–1213. 33

[23] S.D. Barma, Ultrasonic-assisted coal beneficiation: A review, Ultrason. Sonochem. (2018), https://doi.org/10.1016/j.ultsonch.2018.08.016 [24] B. Ambedkar, R. Nagarajan, S. Jayanti, Ultrasonic coal-wash for de-sulfurization, Ultrason. Sonochem. 18 (2011) 718–726. [25] B. Ambedkar, R. Nagarajan, S. Jayanti, Investigation of High-Frequency, HighIntensity Ultrasonics for Size Reduction and Washing of Coal in Aqueous Medium, Ind. Eng. Chem. Res. 50 (2011) 13210–13219. [26] W. Peng, H. Li, Y. Hu, Y. Liu, S. Song, Characterisation of reduced graphene oxides prepared from natural flaky, lump and amorphous graphites, Mater. Res. Bull. 78 (2016) 119–127. [27] P. Dash, T. Dash, T.K. Rout, A.K. Sahu, S.K. Biswal, B.K. Mishra, Preparation of graphene oxide by dry planetary ball milling process from natural graphite, RSC Adv. 6 (2016) 12657–12668. [28] C.D. Elcey, B. Manoj, Graphitization of coal by bio-solubilization: structure probe by Raman spectroscopy, Asian J. Chem. 28 (2016) 1557–1560. [29] D. Kumari, L. Sheikh, S. Bhattacharya, T.J. Webster, S. Nayar, Two-dimensional collagen-graphene as colloidal templates for biocompatible inorganic nanomaterial synthesis, Int. J. Nanomed. 12 (2017) 3605–3616. Ultrasonic-assisted flotation for enhancing the recovery of flaky graphite from lowgrade graphite ore Santosh Deb Barma*, Prasanta Kumar Baskey, Danda Srinivas Rao, and Sachida Nanda Sahu Mineral Processing Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, INDIA-751013

34

Highlights: 

Understanding the flotability of low-grade graphite under the action of low-frequency ultrasound.



Ultrasonic pre-treatment helps in scrubbing of the ultrafine impurities and breaking of graphite-impurities locked particles.



The yield, fixed carbon and recovery of the flotation concentrates were increased significantly under ultrasonic-assisted flotation.



Overall flotation performance of ultrasonic-assisted flotation was found higher than conventional flotation.

35