Influence of acid treatment on the structure and extraction performance of Xinjiang Hefeng low-rank coal

The acid treatment experiment to remove the inorganic minerals in Xinjiang Hefeng low-rank coal was conducted, and the effects of deashing treatment on the main structure of coal sample and its extraction performance by petroleum ether and CS₂ were analyzed by instrumental characterizations. The FT-IR results show that the main structure of the treated coal changes slightly compared with the untreated one. And the acid-washed coal sample (AC) presents a very weak absorption peak at 1712 cm^-1 attributed to carboxylic acid (C=O), which could not be observed in the raw coal sample (RC). It can also be seen from TG-DTG characterization that the process of acid treatment results in a cleavage of small molecular bonds in the coal without destroying the macromolecular network structure. The performance of two-stage ultrasonic extraction of RC and AC samples with petroleum ether (PE) and CS₂ as solvents proposes that the extraction proportions of PE and CS₂ of AC sample are higher than that of RC sample, from 0.16% and 0.53% (RC) to 0.17% and 0.64%, respectively, and the extraction rate of AC sample is larger than that of RC sample, reducing the number of solvent extraction operation significantly.FT-IR and GC-MS analysis of the extracts shows that the acid treatment not only effectively removes the heteroatoms in the coal sample, but also increases the type of CS₂ extracts. In addition, from the results of TG-DTG for the residues, it can be noted that the ultrasonic extraction is a physical swelling process, and does not destroy the macromolecular structure of the coal sample.