Oily Bubble Flotation of Coal Macerals of Shendong Jurassic Coal

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Introduction
Differentiated upgrading and classification of coal are essential prerequisites for the green and efficient utilization of coal in China.It is also crucial to achieve high value-added and low-carbon utilization of coal resources.As an important component of coal production and supply in China, approximately 90% of Jurassic coal is classified as low-rank bituminous coal, which is an ideal steam coal and an excellent choice for chemical raw materials.However, the high inertinite content (often exceeding 35%) [1] of Jurassic coal leads to poor coal caking and slurry properties, and degrades the liquefaction and gasification properties of coal [2].Therefore, effective separation and enrichment of coal macerals is necessary for the green and efficient utilization of Jurassic coal [3].
The methods used for the separation of coal macerals include specific gravity, centrifugation, and flotation [4,5].Dormans et al. [6] proposed a specific gravity method for separating coal macerals.Based on this method, Dyrkacz and Horwitz [7] further developed the density gradient centrifugal separation technique (DGC), which is currently the commonly used method for separating coal macerals.At the optimal crushing particle size, Men [8] obtained enriched products with a vitrinite content higher than 85% via float-sink tests.Gagarin [9] proposed a distribution function that simulates the separation of coal macerals by density.Ma and Tao [10] reported a better vitrinite recovery of 71.81% using enhanced gravity separation under the optimal experimental conditions.Flotation is a critical method with prospects for industrial application in the separation and enrichment of coal macerals.Several studies have examined the flotation separation and enrichment of coal macerals via conventional flotation (air flotation), and it was found that vitrinite-rich concentrates can be obtained to a certain extent [11][12][13][14].Despite much progress being made, techniques for separation and enrichment of coal macerals still face several limitations, such as low separation efficiency, low content of separated macerals, and discontinuous separation processes.
Recently, a new method of using oily bubbles as flotation carriers for mineral particles has been gradually adopted by researchers.For example, Liu et al. [15,16] used reactive oily bubbles, i.e., bubbles coated with a thin layer of oil containing a surfactant-modified collector, as flotation carriers for the extraction of oil sands.Wallwork et al. [17] used a collector (oil) flash evaporation device to conduct an oily bubble separation test on difficult-to-float oxidized oil sands, and found that the recovery of oily bubble separation increased by nearly 80%.Yu and Wang [18] took a homemade oily bubble flotation system with hydrocarbon flash evaporation to study the flotation behavior of Shenfu low-rank coal and obtained a low-ash concentrate with a yield higher than 70%.In our previous studies [19][20][21], it was found that the reactive oily bubble flotation method can significantly reduce reagent consumption and improve the collection ability and flotation selectivity of low-rank bituminous coal.Similarly, Wang et al. [22,23] proved that reactive oily bubbles can indeed improve the quality and flotation performance of low-rank coal.The findings of Zhu et al. [24] indicate that reactive oily bubbles modified by an appropriate concentration and type of surfactant can help improve the flotation efficiency of low-rank coal.Zhou et al. [25][26][27] reported that oily bubble flotation can effectively improve the flotation recovery of bastnaesite, apatite, dolomite, quartz, and collophane.Ramirez et al. [28] also demonstrated the positive effect of oily bubbles on the flotation recovery of molybdenite under experimental circumstances.Wang et al. [29] also proved that oily bubbles are more hydrophobic than air bubbles, which is conducive to strengthening the bubble-particle attachment and inhibiting the bubble-particle detachment in the flotation column.However, it is still unclear whether oily bubble flotation can promote the flotation performance of coal macerals.
In this work, coal macerals of two Shendong Jurassic coals were enriched using the density gradient centrifugation method.A comparative study of the conventional flotation and oily bubble flotation of the enriched coal macerals was also conducted.The possible mechanism through which oily bubble flotation improves the recovery of coal macerals was explained according to the results of induction time measurements, and wettability and surface group analyses.

Sample Preparation
The lump coal used in the test was collected from the 5 −2 coal seam (designated DLT 5 −2 ) of the Daliuta mine and the 2 −2 coal seam (BLT2 −2 ) of the Bulianta mine in the Shendong Mining Area, northwest China.The lump was carefully crushed and screened to prepare coal samples with a size less than 0.25 mm for coal maceral enrichment and flotation tests.In this study, centrifugally separated vitrinite-rich coal and inertinite-rich coal were labelled 'V' and 'I', respectively.
Based on the float-sink enrichment results shown in Figure 1, the tail throwing method was adopted to recover the vitrinite-rich coal and inertinite-rich coal, and the enriched centrifuge products were re-enriched.The final enriched products were quantitatively analyzed using a DM4500P type polarization microscope (Leica Microsystems, Germany).The polarization microscope results are presented in Table 1, where the counts are the statistical quantities obtained by the point measurement method.After undergoing two rounds of enrichment, the enrichment rates of the vitrinite-rich and inertinite-rich samples improved significantly, with both exceeding 84%, thereby satisfying the basic requirements for the subsequent single-mineral flotation tests.The proximate analysis of enriched coal macerals consists of the determination of moisture, ash, volatile matter, and fixed carbon contents.Those were determined using a muffle furnace according to the China National Standard (GB/T 212).The ultimate analysis of coal includes the determination of the contents of carbon, hydrogen, nitrogen, sulfur, and oxygen.It was carried out using an elemental analyzer and a sulfur meter according to China National Standards (GB/T 211, 212, 214 and 30733).The proximate and ultimate analysis results of the enriched coal macerals are shown in Table 2, where M, A, V, and FC are the moisture, ash, volatile matter, and fixed carbon contents, respectively (air-dry basis), and C, H, O, N, and St are the carbon, hydrogen, oxygen, nitrogen, and total sulfur contents, respectively (dry basis).An examination of the data presented in Table 2 shows that for the DLT5 −2 and BLT2 −2 coals, both the C and H contents of the vitrinite-rich component are greater than those of the inertinite-rich coal, while the O content of the vitrinite-rich component is relatively low.

Fourier Transform Infrared (FT-IR) Analysis
The surface functional groups of the vitrinite-rich and inertinite-rich materials were analyzed using a Nicolet iS5 Fourier transform infrared spectrometer (FT-IR) manufactured by Thermo Fisher Scientific, Inc. (Waltham, MA, USA).The scanning range was from 4000 cm −1 to 400 cm −1 , the number of scans was 32, the resolution was 4 cm −1 , and the accuracy was less than 0.1 cm −1 .The substrate was spectrally pure potassium bromide (KBr).The tablet diameter is 13 mm, and the pressure is 20 MPa.The experimental data were plotted after baseline correction, peak finding, and normalization.

Contact Angle Measurements
A JC2000D contact angle measuring instrument was adopted to examine the wettability of the vitrinite-rich and inertinite-rich coal samples by employing the droplet method.The sample was compressed into a tablet under a pressure of 25 MPa for 2 min.The diameter and thickness of the tablet were 13 and 2 mm, respectively.A detailed description of the contact angle measurement process can be found in previous reports [19,35].In this study, three droplets, namely, water, diesel oil, and modified diesel oil, were taken as the wettability liquids.The average value and the standard deviation from three repeated tests were reported.

Induction Time Measurements
Induction times between bubbles and coal macerals were measured using a 2017EZ-B+ Induction timer, as shown in Figure 2, manufactured by Oil Sands Environmental Development & Services, Inc. (Edmonton, AB, Canada).For each measurement, coal maceral (0.5 g) was stirred and settled in a beaker containing deionized water, and the suspension was removed.Following this, the coal slurry was transferred to a meas-urement vessel pre-filled with two-thirds deionized water.This process resulted in the formation of a flat particle bed with uniform thickness at the bottom of the vessel (shown in Figure 3).The amplitude of the bubble motion was set as 3 V, the times to approach and retraction were fixed on 10 ms, and the air/oily bubble size was kept at 1.3 mm.During the measurement, air needs to be drawn into the glass capillary tube beforehand.When the bottom of the capillary tube was immersed in the water in the measurement vessel through the controller, an air bubble with controllable size could be generated at the end of the capillary tube using a microsyringe.However, it should be noted that when an oily bubble is needed, a small volume of (modified) diesel oil should be sucked into the capillary tube before being immersed in the water.Then, a (modified) diesel oily bubble could be generated at the end of the capillary tube by pushing air using the microsyringe.The generated air/oily bubble was brought into contact with the particle bed for a preset period, as shown in Figure 3.In this way, the corresponding induction time can be obtained.A more detailed description of the measurement process could be found in previous reports [36][37][38].The measurements were carried out at room temperature.Each experiment was repeated five times, and the average value was reported.

Conventional Flotation Tests
For conventional flotation, an XFD-Ⅳ flotation machine with a flotation cell volume of 0.5 L was selected.The concentration of the slurry was 40 g/L.The impeller speed and the airflow rate were set at 1996 r/min and 0.02 m 3 /min, respectively.Sec-octanol was used as a frother, and the dosage was fixed at 0.90 kg/t.Diesel oil was used as a collector, and the dosages were 0.75, 1.50, 1.98, 2.25, 3.00, 3.75, 4.50, 5.25, and 6.00 kg/t.The skimming time was 3 min.The froth concentrates and the flotation tailings were filtered, dried, weighed and calculated, separately.

Oily Bubble Flotation and Modification
To obtain a better flotation performance, a custom-made device for oily bubble generation was designed based on the atomization and flash evaporation methods [19].The schematic diagram of oily bubble flotation setup is shown in Figure 4.According to the characteristics of the test device, the collector (diesel oil) used for oily bubble flotation was added through the inflation tube of the flotation machine at dosages of 0.66, 1.32, 1.98, 2.64, and 3.30 kg/t.The rated air flow rate of atomizer was 6 L/min, and the atomizing rate of collector was kept at 0.299 g/min.The skimming time was 40 s, and the other parameters were the same as those used in the conventional flotation tests.A detailed description of the oily bubble flotation procedure can be found in previous reports [20,36].
Based on the optimal dosage determined from the oily bubble flotation test results (1.98 kg/t diesel oil), diethyl phthalate (DP) was selected as the modifier, and a modified diesel oil was prepared by blending DP and diesel oil at a weight ratio of 2:100.The modified oily bubble flotation processes are the same as those used for oily bubble flotation, except that the diesel oil was replaced with diesel oil modified by DP.
The method for adding and controlling the amount of collector (diesel oil or DP-modified diesel oil) during the flotation experiment was as follows: When the gas valve of the flotation machine was completely closed, the air and collector gasified by heating device would all enter the excess gas recovery device through the gas flowmeter, and the maximum gas flow value (V0) could be obtained from the gas flowmeter.When the test started, part of the gasified collector and air (V2) were allowed to flow in the flotation cell for oily bubble flotation, and the remaining collector and air (V1) would be discharged through the gas flowmeter by adjusting the gas valve of the flotation machine.As a result, the actual required collector and air in the flotation was obtained, i.e., V2 = V0-V1.The atomized collector undergoes rapid expansion upon gasification through the heating device, maintaining a constant mass.It is then uniformly dispersed throughout both the cavity and pipeline.Specifically, the collector drawn from the gas valve of the flotation cell matches the discharge from the gas flowmeter, differing only in volume.During the whole test, sufficient air was sucked in by the atomizer, and there was no air intake in the rest of the process.Therefore, the actual dosage of collector (Mobf) in oily bubble flotation could be calculated using equation (1): where Mobf is the actual dosage of collector in oily bubble flotation, kg/t; V0 is the maximum volume of air and collector after atomization and heating flash evaporation, L/min; V1 and V2 are the volume of air and collector discharged from the gas flowmeter and sucked into flotation cell, L/min, respectively; Ma is the atomizing rate of collector, which was fixed at 0.299 g/min; t is the oily bubble flotation time, i.e., the skimming time, which was kept at 40 s; mc is the amount of coal sample used in a single flotation test, which was fixed at 20 g in this study.In addition, the enriched vitrinite-rich and inertinite-rich coals were approximately treated as pure coal, and their flotation yields were used as the recoveries of concentrate.For both conventional flotation and oily bubble flotation, each experiment was repeated three times, and the average values were reported.

Analysis of FT-IR Spectra
Figure 5 shows the FT-IR spectra of the coal samples.It can be seen from the infrared absorption peak characteristic of coal [39][40][41] observed in Figure 5 that not only hydrophilic oxygen-containing functional groups (-OH, C-O, C=O, etc.) but also hydrophobic -CH3 and -CH2 groups, organic sulfur-containing groups (-SH), and some mineral groups (Si-O-Si and/or Si-O-Al, etc.) are present in each coal sample.Functional groups play a vital role in the adsorption of reagents and determine the surface wettability and floatability of minerals.The contents of hydrophilic oxygen-containing functional groups and hydrophobic groups are discussed in subsequent sections.

XPS Analysis
Table 4 shows the contents of C, N, and O on the surface of the DLT5 −2 and BLT2 −2 coals.There are clear differences between the element contents of vitrinite-rich coal and inertinite-rich coal, which match both DLT5 −2 and BLT2 −2 well.For DLT5 −2 coal, the content of C in the vitrinite-rich coal is approximately 2.59% greater than that in the inertinite-rich coal.Conversely, the O and N contents in the vitrinite-rich coal are approximately 2.33% and 0.27% lower than those in the inertinite-rich coal, respectively.For BLT2 −2 coal, similar experimental results were observed.Clearly, the O/C value of vitrinite-rich coal is lower than that of inertinite-rich coal, which is consistent with the results of proximate and ultimate analyses.and 7 show the C1s fitting spectra of vitrinite-rich and inertinite-rich coal for both the DLT5 −2 and BLT2 −2 coal samples.Within the peak range of the functional group containing C, the fitting curves of the vitrinite-rich and inertinite-rich coal samples exhibit a high degree of correspondence with the experimental curves, indicating that the relevant parameters were properly selected and that the fitting process was reliable.Table 5 shows the contents of the C-containing functional groups in the vitrinite-rich and inertinite-rich coal for both the DLT5 −2 and BLT2 −2 coal samples.The surface groups of vitrinite-rich and inertinite-rich coal are mainly C-C and C-H, and the oxygen-containing functional groups are mainly hydroxyl (C-OH) and ether groups (C-O-C).In addition, a limited quantity of carboxyl (O-C=O) and carbonyl groups (C=O) are also present.The results show that there are significant differences in the functional groups present in the vitrinite-rich and inertinite-rich coal fractions.For DLT5 −2 coal, the total content of C-C and C-H groups in vitrinite-rich coal is approximately 7.00% higher than that of inertinite-rich coal, and the total content of hydroxyl (C-OH) and ether groups (C-O-C) in vitrinite-rich coal is only 1.08% higher than that of inertinite-rich coal.However, the content of carboxyl (O-C=O) and carbonyl (C=O) groups in vitrinite-rich coal is approximately 0.24% lower than that of inertinite-rich coal.A similar trend was observed for BLT2 −2 coal.It can be concluded that the surface of inertinite-rich coal contains more oxygen-containing functional groups and thus is more hydrophilic, reducing its floatability.

Wettability Analysis
Figure 8 shows the contact angles of water, diesel oil, and DP-modified diesel oil droplets on coal macerals.The contact angle of the water droplets on the surface of the vitrinite-rich coal and inertinite-rich coal of DLT5 −2 is, respectively, 81.7° and 71.5°, and that for BLT coal is 73.1° and 57.9°, indicating that the vitrinite-rich coal is more hydrophobic, which is highly consistent with the results of the functional group analysis.In addition, for DLT 5 −2 coal, the contact angle of the diesel oil droplets on the surface of vitrinite-rich coal and inertinite-rich coal is separately 17.9° and 23.7°.Similarly, for BLT2 −2 coal, the contact angle of the diesel oil droplets on the surface of vitrinite-rich coal and inertinite-rich coal is 19.7° and 26.4°, respectively.The results show that diesel oil droplets can easily spread on the surface of vitrinite-rich coal, which is beneficial for enhancing its hydrophobicity.Moreover, the contact angle of modified diesel oil droplets containing DP on the surface of vitrinite-rich coal and inertinite-rich coal for DLT5 −2 coal is 16.1° and 18.0°, and that for BLT2 −2 coal is 16.3° and 18.3°, respectively, indicating that while the addition of DP could further increase the hydrophobicity of both vitrinite-rich coal and inertinite-rich coal, the hydrophobicity of vitrinite-rich coal is still higher than that of inertinite-rich coal.In summary, vitrinite-rich coal exhibits higher hydrophobicity than inertinite-rich coal.Consequently, the (modified) diesel oil droplets easily spread on vitrinite-rich coal, facilitating rapid bubble-particle attachment.

Induction Times of Air and Oily Bubbles on Coal Macerals
Figure 9 compares the induction times of air, diesel oily, and modified diesel oily bubbles on the surfaces of coal macerals.As shown in Figure 9, the induction time of air (diesel oily or modified diesel oily) bubbles on the surface of vitrinite-rich coal is shorter than that on the surface of inertinite-rich coal.In addition, for both vitrinite-rich and inertinite-rich coal, the induction times of air bubbles on the coal surfaces are greater than 200 ms.However, the induction times of diesel oily bubbles on the surfaces of both vitrinite-rich and inertinite-rich coal are less than 50 ms.Furthermore, for both vitrinite-rich and inertinite-rich coal, the induction times of modified diesel oily bubbles on the surfaces further decrease to less than 45 ms.It could be concluded that the time of adhesion on the coal surface is significantly reduced by the diesel oily bubble, and the addition of DP further reduces the adhesion time between the coal surface and the bubble.Under the same flotation conditions, a shorter induction time may result in rapid collection of the coal macerals during froth production.As a result, higher recoveries of coal macerals could be achieved via (modified) diesel oily bubble flotation due to the shorter induction time.The wettability analysis showed that the modified diesel oil droplet spreads more rapidly than the diesel oil droplet, and both of these spread more rapidly than the water droplet, which consistent with the findings of previous studies [20,22].Hence, the induction time results are reliable, and a shorter induction time for vitrinite-rich coal contributes to better flotation performance.

Conventional Flotation Tests
Figure 10 shows the results of conventional flotation experiments for DLT5 −2 and BLT2 −2 coal macerals.For DLT5 −2 and BLT2 −2 coals, the recoveries of vitrinite-rich coal were greater than that of inertinite-rich coal for the same diesel oil dosage.This difference may be ascribed to the higher hydrophobicity of the vitrinite-rich coal, which leads to a shorter induction time of air on the coal macerals and greater flotation recovery.In addition, it was found that when the dosage of diesel oil exceeded 4.50 kg/t, the growth trend of recoveries of vitrinite-rich and inertinite-rich coal disappeared.At a dosage of diesel oil of 6 kg/t, the recoveries of vitrinite-rich and inertinite-rich coal for DLT5 −2 are approximately 81.58% and 72.82%, and those for BLT2 −2 are 83.80% and 73.62%, respectively.It is likely that a further increase in the dosage of diesel oil would not enhance the hydrophobicity and reduce the induction time of air on coal macerals.In summary, the higher recoveries of vitrinite-rich and inertinite-rich coal could not be achieved via air bubble flotation.

Oily Bubble Flotation Tests
Figure 11 shows the results of oily bubble flotation of DLT5 −2 and BLT2 −2 coal macerals using diesel oil.The results show that the highest recoveries of the coal macerals were at a collector dosage of 1.98 kg/t.With the present experimental procedure, the aeration rate increased with collector dosage.For collector dosages up to 1.98 kg/t, the air introduced in the flotation cell produced small bubbles.However, above this collector dosage, our observations show that large air bubbles are generated in the flotation cell.Since flotation is an interfacial phenomenon that depends on bubble surface area, the production of large air bubbles results in a decrease in flotation recovery.Based on the above-described results, the oily bubbles were further modified by the surfactant DP to increase the recovery differences between the vitrinite-rich and inertinite-rich samples for the same coal (DLT or BLT) in order to improve the selectivity of the same collector for the two coal macerals.
The recoveries of conventional flotation and (modified) oily bubble flotation of vitrinite-rich and inertinite-rich coal are compared in Figure 12. Figure 12 shows that the flotation recovery of the vitrinite-rich coal is greater than that of the inertinite-rich coal for both conventional flotation and (modified) oily bubble flotation.At a diesel oil dosage of 1.98 kg/t, the recoveries of vitrinite-rich and inertinite-rich coal of DLT5 −2 by conventional flotation were 66.27% and 63.43%, and those of BLT2 −2 were 59.68% and 55.06%, respectively.However, the recoveries of vitrinite-rich and inertinite-rich coal by oily or modified oily bubble flotation were 92.91% and 86.26% for DLT5 −2 , and that were 90.13% and 82.88% for BLT2 −2 , respectively.Compared with those of conventional flotation, when oily or modified oily bubble flotation was used, the recoveries of vitrinite-rich and inertinite-rich coal for DLT5 −2 increased by more than 26.64% and 30.45%, respectively.For modified oily bubble flotation, the recoveries of vitrinite-rich and inertinite-rich coal for BLT2 −2 were 22.83% and 27.82% higher than those of conventional flotation, respectively.The above results are in agreement with the measurements of the induction times and contact angles between the (modified) diesel oily bubbles and coal macerals.Hence, oily bubble flotation is proven to be an effective method for improving the recoveries of vitrinite-rich coal and inertinite-rich coal.

Conclusions
In this study, the effect of oily bubbles on the flotation of Shendong Jurassic coal macerals was investigated using FT-IR analysis, XPS analysis, wettability analysis, induction time measurements, and flotation experiments.The following conclusions can be drawn: 1. Enrichment rates exceeding 84% for both vitrinite-rich and inertinite-rich coal can be obtained by the density gradient centrifugation method with two rounds of enrichment.2. FT-IR and XPS analyses confirmed that there are clear differences in the contents of the C-C, C-H, and oxygen-containing functional groups between vitrinite-rich and inertinite-rich coal, and the O/C value of vitrinite-rich coal is lower than that of inertinite-rich coal.3. Vitrinite-rich coal has a higher hydrophobicity than inertinite-rich coal, and (modified) diesel oil droplets can further increase the hydrophobicity.4. The induction time on the coal surface is significantly reduced by diesel oily bubbles, and a further reduction in the induction time between coal macerals and oily bubbles can be obtained by the addition of diethyl phthalate (DP). 5. Oily bubble flotation can significantly improve the recoveries of vitrinite-rich and inertinite-rich coal.

Abbreviations
All abbreviations used in this paper are explained as follows:

Figure 3 .
Figure 3. Attachment process of oily bubbles and coal particles.

Figure 4 .
Figure 4.The schematic diagram of oily bubble flotation setup.

Figure 8 .
Figure 8. Contact angles of water, diesel oil, and DP-modified diesel oil droplets on coal macerals.

Figure 9 .
Figure 9.Comparison of the induction times of air, diesel oily and modified diesel oily bubbles on coal macerals.

Figure 12 .
Figure 12.Comparison of conventional flotation and oily bubble flotation of vitrinite-rich and inertinite-rich coal with a diesel dosage of 1.98 kg/t, where CF-V and CF-I represent conventional (air) flotation of vitrinite-rich and inertinite-rich coal, respectively.OBF-V and OBF-I represent oily bubble flotation of vitrinite-rich and inertinite-rich coal, respectively.MOBF-V and MOBF-I represent modified oily bubble flotation of vitrinite-rich and inertinite-rich coal, respectively.

Table 2 .
Proximate and ultimate analyses of the enriched coal macerals.
1O is calculated by the subtraction method.

Table 4 .
The contents of C, N, and O elements on the surface of the DLT5 −2 and BLT2 −2 coals.