3.1 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 fixed carbon content was significantly affected by the above three contents of fine coal, and thus the fixed carbon content of C3 sample was lowest due to the ash content of 37.52%.
3.2 XRD and IR characterization
XRD spectra of coal samples are shown in Fig. 1. The XRD characterization result indicates that there are three main kinds of minerals such as kaolinite, quartz and pyrite besides amorphous organics in coal samples. According to the XRD spectrum of C1 coal sample, kaolinite and quartz are main minerals and a small amount of pyrite is presented in the coal. The XRD spectrum of C2 coal sample is similar to that of C1 coal sample. However, three kinds of characteristic peaks in relatively strong intensities could be found in the XRD spectra of C3 coal sample due to the presence of minerals, mainly including kaolinite, quartz and a small amount of pyrite (Lin et al., 2017; Zhou et al., 2019; Zhou et al., 2019), which can be supported by the above proximate analysis. In addition, form XRD spectra pyrite content of C3 coal sample was more than those of C1 coal sample and C2 coal sample, which was illuminated by the sulfur content in the proximate analysis.
IR spectra of coal sample in different coalification degrees are shown in Fig. 2. The peaks at 3438 cm−1and 1097 cm-1 are ascribed to the presence of hydroxyl groups such as alcohol or phenolic hydroxyl, and this peak intensity decreases with the increasing coal rank (C1 >C2 >C3) (Zhen et al., 2018).Three characteristic peaks at2920 cm-1，1440 cm-1 and 1375 cm-1 are due to the stretching vibration of methyl and methylene groups that increase with the increasing coal rank(C1 <C2 <C3) (Chen et al., 2019). The strong absorption peak at 1600 cm-1 may be attributed to the overlapping of carbonyl group and C=C double bonds in the aromatic ring (Zhao et al., 2018). Five peaks at 3695 cm-1, 3617 cm-1, 1032cm-1, 1010cm-1and 910 cm-1are assigned to the minerals in the coal and the peaks of C3 coal sample is strongest among three samples, which indicates that the highest content of minerals is inC3 coal sample supported by the above proximate analysis (Valeria et al., 2000; Yang et al., 2019; Zhao et al., 2017). In addition, the amount of oxygen-containing functional group decreases with the increase of coalification degree, which may result in the hydrophobicity increase of coal and may be advantageous for the dewatering of fine coal (Xia et al., 2019).
3.3 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 filter 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 filtration time increase. The decrease about 3.8%was gained on 8.5 min. For C2 coal sample, the moisture content in the filter cake decreased in the experimental time and the final moisture content was 17.8%. For C3 coal sample, the moisture content decreased from 22.4% to 17.1%. When the filtration 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 filter cake deceases with the increase of coalification degree and filtration time. Although the ash content of C3 was high about 37.52%, the final moisture content in the filter cake was only 17% indicating that the coalification degree had more important role in the moisture content in the filter cake than ash content in the coal.
3.4 Effect of ash contents in the coal samples
The presence of clay minerals in coal is not beneficial 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 flotation 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 filtration 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 flotation. However, for three coal samples by the flotation 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 fine coal.
In order to investigate the further effect of ash on the dewatering of fine coal, the ash in coal was mostly removed by using hydrochloric acid and hydrofluoric 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 filtration time is 6.5 min. Moisture contents in the filter cakes before and after ash removal are shown in Fig. 4. From this figure, 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 coalification 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 filter cake.
The resulting heat flow curves are shown in Fig. 6. With the increase of coalification degree, the wetting heat flow significantly 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 flows 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 flow curves of kaolinite and quartz were given in Fig. 7. Form this figure, 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 filter cake.