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| XRD Structural Analysis of Raw Material Used as Coal-Based Needle Coke in the Coking Process |
| FAN Qing-jie, SONG Yan, LAI Shi-quan*, YUE Li, ZHU Ya-ming, ZHAO Xue-fei |
School of Chemical Engineering, University of Science and Technology Liaoning, Anshan 114051, China
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Abstract Soft coal tar pitch (SCTP) with low QI content is the preferred raw material for preparing coal-based needle coke. The study on its structure changes in the cooking process is helpful to prepare high-quality needle coke. In this paper, the CarbX software developed by the Smarsly team was used to fit the full spectrum X-ray diffraction (XRD) data of the samples toquantify the microcrystalline structure parameters of SCTP at different carbonization temperatures (400, 500, 600, 800, 1 000, 1 200 and 1 400 ℃), and then investigate the thermally induced structural changes of SCTP at the nanoscale. The results show that the average graphene layer size of microcrystalline stack La gradually increases from 10.3 Å for the pristine pitch to 47.9 Å at 1 400 ℃ with the rising of the carbonization temperature, but La increases slowly before 500 ℃. A significant increase of La is found only when the temperature exceeds 800 ℃, indicating that high temperatures above 800 ℃ are needed to recombine the atoms in the cross-linked graphene layers and lead to the growth of the microcrystals. However, the C—C bond length (lcc) of the graphene carbon network is slightly affected by temperature and varies in the range of 1.41~1.42 Å. Because of mesophase transformation during the liquid-phase carbonization of SCTP into semi-coke, the average stack size Lc gradually increases before 500 ℃ and reaches the maximum at 500 ℃ (Lc=31.1 Å). Subsequently, due to further pyrolysis and polycondensation of semi-coke, Lc gradually decreases and reaches the lowest point (Lc=15.4 Å) at 1 000 ℃, and increases again after 1 000 ℃. Similar to Lc, the average number of graphene layers per stack N increases from 2.66 layers for the raw pitch to 9.05 layers at 500 ℃, then decreases to 4.55 layers at 1 000 ℃, and then begins to increase after 1 000 ℃. The samples are still in the pitch state before 500 ℃ the average graphene interlayer spacing a3 is large, about 3.50 Å at this stage. When the pitch becomes semi-coke at ca. 500 ℃, a3 rapidly decreases to 3.44 Å, continues to decrease, reaches the minimum at 1 000 ℃ (a3=3.39 Å), and begins to increase again after 1 000 ℃, indicating that the coke has undergone a shrinkage and re-expansion process. By using CarbX software to fit the XRD data of the sample, the main size (La, Lc, N, a3) of carbon microcrystals of the sample can be obtained, as well as the dispersion (ka，kc，σ3，ε3) of these parameters and the orientation (q), homogeneity (η) of per stack and disordered carbon content (cun). It is helpful to deeply understand the sample’s microstructure and to produce high-quality needle coke.
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Received: 2021-05-04
Accepted: 2021-06-13
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Corresponding Authors:
LAI Shi-quan
E-mail: yuelilsq@163.com
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