XRD Structural Analysis of Raw Material Used as Coal-Based Needle Coke in the Coking Process

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 degrees C), arid then investigate the thermally induced structural changes of SCTP at the nanoscale. The results show that the average graphene layer size of microcrystalline stack L-a gradually increases from 10. 3 A for the pristine pitch to 47. 9 angstrom at 1 400 degrees C with the rising of the carbonization temperature, but L-a increases slowly before 500 degrees C. A significant increase of L-a is found only when the temperature exceeds 800 degrees C, indicating that high temperatures above 800 degrees C 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 (l(cc)) of the graphene carbon network is slightly affected by temperature and varies in the range of 1. 41 similar to 1. 42 angstrom. Because of mesophase transformation during the liquid-phase carbonization of SCTP into semi-coke, the average stack size L-c gradually increases before 500 degrees C and reaches the maximum at 500 degrees C (L-c =31. 1 angstrom). Subsequently, due to further pyrolysis and polycondensation of semicoke, L-c gradually decreases and reaches the lowest point (L-c =15. 4 angstrom) at 1 000 degrees C, and increases again after 1 000 degrees C. Similar to L-c, the average number of graphene layers per stack N increases from 2. 66 layers for the raw pitch to 9. 05 layers at 500 degrees C, then decreases to 4. 55 layers at 1 000 degrees C, and then begins to increase after 1 000 degrees C. The samples are still in the pitch state before 500 degrees C the average graphene interlayer spacing a(3) is large, about 3. 50 angstrom at this stage. When the pitch becomes semi-coke at ca. 500 a(3) rapidly decreases to 3. 44 angstrom, continues to decrease, reaches the minimum at 1 000 degrees C (a(3) = 3. 39 angstrom), and begins to increase again after 1 000 degrees C, 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 (L-a, L-c, N, a(3)) of carbon microcrystals of the sample can be obtained, as well as the dispersion (k(a), k(c), sigma(3), epsilon(3)) of these parameters and the orientation (q) , homogeneity (eta) of per stack and disordered carbon content (c(un)). It is helpful to deeply understand the sample's microstructure and to produce high-quality needle coke.

Automated summary

This paper investigates the structural changes of soft coal tar pitch (SCTP) at different carbonization temperatures using CarbX software and X-ray diffraction (XRD) data. The results provide valuable insights into the microstructure of SCTP and its potential for producing high-quality needle coke.