Effect of ash existence in the coal particles on the dewatering performance of ne coal

The dewatering experiments of ne coal with different ash contents in the particle size range of 0.125 mm − 0 mm were investigated in this study. Structures of coal samples were characterized by X-ray diffractometer (XRD) and surface functional groups were detected by Fourier transform infrared (FTIR). Wettability and wetting heats of coal samples were determined by contact angle measurements and micro-calorimeter system, respectively. In this study, the dewatering results indicate that the ash content of ne coal had less effect on the coal dewatering than the coalication degree in the dewatering process. However, for the given coal sample the moisture content was signicantly affected by the ash content while the coal particle size was less than 0.125 mm. The decrease of moisture content in coal sample after the ash was removed indicating that the hydrophobic property of coal surface was enhanced based on contact angle measurements and wetting heats. In addition, kaolinite played a primary role of minerals in coal for the coal dewatering.


Introduction
Flotation is the most effective method of separating ne coal from clay, sul de, carbonate, silica, sulfate and other minerals (Xing et  . Among these methods, ltration methods such as pressure lters, belt presses, vibrating basket and vacuum lters, have been widely used in the dewatering of coal product (Alam et al., 2011). Several process parameters ( occulant conditioning time, cake formation and drying time, size distribution etc.) were investigated in the dewatering process of ultra ne coal via vacuum ltration. The results indicate that the coal dewatering performance can be enhanced by optimizing these process parameters (Tao et al., 2000). Effects of lter cakes, equilibrium desaturation and ltration rates were also studied in the dewatering process of ne coal (Gala et al., 1981). The vacuum ltration was developed in the dewatering of ne coal (Roux et al., 2003). Tests on a bench scale vacuum lter demonstrated that the increase of the air ow passing a lter cake was favorable to the dewatering of ne coal.
Further fundamental investigations and auxiliary measures of coal dewatering were reported in the literature. Some surfactants were used in the tests of increasing coal dewatering performance (Hassas et al., 2014).The results indicate that low HLB (hydrophile-lipophilic balance) surfactants were e cient in the dewatering of bituminous and anthracite coals while not e cient in the dewatering of low rank coals such as lignite. The dewatering performance of ne coal from otation was investigated and the dewatering process was investigated by the oscillatory rheology of froth (Zhang et al., 2018). Their results indicated that the presence of stabilized froth on top of water was not favorable to the decrease of the moisture of lter cake. Surface hydrophobicity and air bubble entrapment on the ltration were studied in the ne coal dewatering (Asmatulu, 2008). It can be found that after hydrophobic reagents and air bubbles were introduced into the coal slurry the amount of moisture in ne coal was decreased and the e ciency of the ltration was increased.
Besides the above dewatering methods, coal ash from minerals in the coal may play an important role in the dewatering process although a large part of the minerals in the coal were removed by otation. There are few detailed studies about the effects on ltration behavior and the ne coal dewatering. In this study, the effect of ash content on the dewatering performance of the ne coal was investigated based on coal samples in different coali cation degrees, ash contents for a given coal sample and mineral types. The obtained results were hopefully provided for understanding the dewatering process of ne coal in the coal preparation plants.

Materials
Coal samples are collected from the coal preparation plant in Shanxi China. Coal particle size is less than 0.125 mm. Two sub-bituminous coal samples are denoted as C 1 and C 2 according to the increase of coal deterioration. The bituminous coal is denoted as C 3 . The results of the proximate analysis of coal samples are listed in Table 1. Kerosene (Xi'an Chemical Reagent Factory) was used as a collector and 2octanol was used as a frother (CP. 97%, Xi'an Chemical Reagent Factory).

Sample characterization
Before the proximate analysis was carried out, the coal samples were placed in a 105 ℃ vacuum drying oven and dried for 2 h. The ultimate analysis of coal was performed by the elemental analyzer (vario MACRO cube elementar, Germany). The crystalline structures of the prepared samples were detected by a Rigaku X-ray diffractometer (XRD) equipped with Cu-K α radiation at 40 kV and 40 mA. The powder diffraction pattern was studied in scanning range from 5° to 60° at 4°/min and 0.02 (2q) step size.
Fourier transform infrared (FTIR) spectra were collected on a Bruker Tensor 27 spectrometer at room temperature and the mass ratio of sample powder to KBr (100 mg) was 1:100. The FTIR spectra were ranged from 4000cm -1 to 400 cm -1 .Contact angle measurements (JC 2000C) of coal samples were performed for wettability of coal samples. Coal samples (<74μm ) were pressed to form a plate under a 40 MPa pressure, and then a water droplet was contacted with the coal plate. Wetting heats of coal samples (<74μm) weredetermined by using a micro-calorimeter system (C80, Setaram, France).The thermostatic mode at 303 K and membrane-mixing stainless steel cell (volume: 8.5 mL) were adopted.
The aluminum foil lm was xed between deionized water (2 ml) and coal powder sample (100 mg) in stainless steel cells. When the system of apparatus has no change at room temperature, the wetting heat was obtained after the aluminum foil lm was pierced by integration of the heat-ow curve.

Flotation, ash removal and dewatering tests
The otation test was performed in a 140 mL micro-otation cell with the speed of 1800 r/min (XFG, Wuhan Exploring Machinery Factory), the air ow rate was 0.2 L/min, and the pulp density was 100 g/L. The conditioning before otation lasted for 3 min. Kerosene as a collector(600g/t, 800g/t, 1500g/t) was added after 2 min of conditioning and 2-octanol as a frother (100g/t)was added after 3 min of conditioning. The mixing time was further for 10 s, and then clean coal product was collected. The ash in coal (20g) was removed by using 80 ml hydrochloric acid (37%, Tianjin Chemical Reagent Factory) and 80 ml hydro uoric acid (40%, Tianjin Chemical Reagent Factory)at 60 °C for 1 h, respectively. And then the sample was again mixed with 80 ml hydrochloric acid (37%) at 60 °C for 1 h. The obtained sample was rinsed with distilled water three times and dried in a vacuum drying box at 60 ℃ for 4h.The coal slurry was prepared via pouring the coal particles with a given mass into the distilled water, and then the coal slurry was ltered by a vacuum pump at 50 kPa. The coal sample was dried in an oven at 105 °C for 90 min and the moisture content in coal was obtained.
Results And Discussion

Proximate and Ultimate analysis
The proximate analysis and ultimate analysis of coal are listed in Table 1. From the results, it is found that the ash content in the C3 coal sample is highest among the three samples reaching up to 37.52%. The moisture content decreases with the increase of coal rank (C1 < C2 < C3), which is related to the abundant oxygen-containing functional groups on the surface of the C1 coal sample as shown in IR spectra (Figure1). The oxygen content of C1 was more than those of C2 and C3 based on the ultimate analysis indicating that more oxygen-containing functional groups on the surface of the C1 coal sample were existed. The volatile matter content increased with the increase of coal rank (C1 < C2 < C3). The xed carbon content was signi cantly affected by the above three contents of ne coal, and thus the xed carbon content of C3 sample was lowest due to the ash content of 37.52%.

XRD and IR characterization
XRD spectra of coal samples are shown in Fig. 1

Effect of coal samples with different ash contents
Firstly three coal samples with different ash contents were employed in the dewatering tests. As shown in Fig. 3, the moisture content in the lter cake changes in the time range 2.5 min to 8.5 min. The solid content in the slurry is 200 g/L and the particle size is less than0.125mm. For C1 coal sample, the moisture content was up to 30.7% on 2.5 min and decreased with the ltration time increase. The decrease about 3.8%was gained on 8.5 min. For C2 coal sample, the moisture content in the lter cake decreased in the experimental time and the nal moisture content was 17.8%. For C3 coal sample, the moisture content decreased from 22.4% to 17.1%. When the ltration time was 8.5 min, the decrease of moisture content was hardly found. According to the above results, it can be found that the moisture content in the lter cake deceases with the increase of coali cation degree and ltration time. Although the ash content of C3 was high about 37.52%, the nal moisture content in the lter cake was only 17% indicating that the coali cation degree had more important role in the moisture content in the lter cake than ash content in the coal.

Effect of ash contents in the coal samples
The presence of clay minerals in coal is not bene cial to the solid-liquid process (Ofori et al., 2011;Rong et al., 1995;Tao et al., 2000). Therefore, the effects of ash contents in coals should be investigated in the dewatering process. The ash content of coal was achieved by the coal otation using different dosage of collector. The C3 coal sample was used in this model test due to its high ash content. The solid content in the slurry is 200 g/L and the particle size is less than 0.125mm. The ltration time is 6.5 min. Flotation and dewatering results are shown in Table 2. The ash content in the coal decreases about 10% for collector dosage of 600 g/t, and increases slightly for collector dosage of 800 g/t and 1500g/t. From Table 2, it can be seen that the formation time of cake decreases from 46.1 s to 28.8 s and the moisture content of cake decreases from 17.10% to 14.18% when the ash was removed from coal by the otation.
However, for three coal samples by the otation similar formation times were gained and the moisture content of cake increased about1.2% and 1.4% for collector dosage of 800 g/t and 1500 g/t, respectively. The obtained results indicate that the presence of ash in coal is harmful to the dewatering of ne coal.
In order to investigate the further effect of ash on the dewatering of ne coal, the ash in coal was mostly removed by using hydrochloric acid and hydro uoric acid. Residual ash contents are 1.20% for C1, 1.01% for C2 and 3.85%for C3, respectively. The solid content in the slurry is 200 g/L and the particle size is less than 0.125 mm. The ltration time is 6.5 min. Moisture contents in the lter cakes before and after ash removal are shown in Fig. 4. From this gure, the moisture content decreased after the ash was removed from coal. The decrease of moisture content was about 2.3% for C1, 2.6% for C2 and 3.8% for C3 indicating the ash in the coal is responsible for the bad dewatering performance.
Images of water droplet contacting with coal surface before and after ash removal are shown in Fig. 5.
With the increasing coali cation degree, the contact angle increased from 7.8° to 46.8° (Fig. 5 a, c, e) demonstrating the hydrophobic property of coal surface became enhanced. When the ash in the coal was removed, all hydrophobic properties of three coal samples were enhanced and the C3 coal sample still presented the strongest hydrophobic property (Fig. 5 b, d, f). Moreover, based on the dewatering result in Figure 4, the decrease of moisture content in coal sample after the ash was removed indicating that the hydrophobic property of coal surface was enhanced, which may be responsible for the decrease of moisture content in the lter cake.
The resulting heat ow curves are shown in Fig. 6. With the increase of coali cation degree, the wetting heat ow signi cantly decreased indicating that the hydrophilicity of coal became weak (C1:33.9J/g, C2: 8.9 J/g, C3: 3.6J/g). After the removal of ash, the wetting heat ows were 29.4J/g for C1, 6.4 J/g for C2, 1.6 J/g for C3, respectively. According to the results of XRD, it can be known that kaolinite and quartz are predominant minerals in the coal sample. Therefore, heat ow curves of kaolinite and quartz were given in Fig. 7. Form this gure, the wetting heat value of kaolinite was 2.6 J/g and larger than that of quartz (0.4 J/g). And thus kaolinite played a primary role of minerals in coal for the coal dewatering. Based on the above results, three coal samples presented the wetting heat decrease after ash removal indicating that hydrophobic properties of three coal samples were enhanced. Therefore, coal samples exhibited the decrease of moisture content in the lter cake.

Conclusion
Three coal samples were used in the dewatering tests and the particle size was less than 0.125 mm. The proximate analysis and ultimate analysis of coal indicate that the ash content in the C3 coal sample was highest among the three samples reaching up to 37.52%, mainly including kaolinite and quartz, a small amount of pyrite supported by XRD characterization. IR result indicates that the amount of oxygencontaining functional group decreases with the increase of coali cation degree. According to the obtained results from three coal samples, it can be found that the moisture content in the lter cake deceases with the increase of coali cation degree and ltration time. The ash content had less effect on the moisture content in the lter cake than the coali cation degree of the coal sample. After the ash was removed by otation or by using hydrochloric acid and hydro uoric acid, the moisture content of lter cake decreased indicating that the presence of ash in coal is not bene cial to the dewatering of ne coal. Contact angle increased and wetting heats decreased after ash removal in coal samples indicate that coal samples presented stronger hydrophobic property. Based on the XRD characterization and wetting heat values of the minerals in the coal sample, it can be found that kaolinite played a primary role of minerals in coal for the coal dewatering.

Declarations
Compliance with Ethical Standards

Con ict of Interest
The authors declare that they have no con ict of interest.  Effect of ltration time on cake moisture content, (a) C1 (b) C2 (c) C3; (The error bars represent one standard deviation obtained from three independent runs.)