Qualitative and quantitative analyses of ultrafine anthracite by Fourier transform infrared spectroscopy after mechanochemical preparation

Comminution is widely used in the deep processing of coal resources. Different chemical structures in coals can influence follow-up procedures significantly. A planetary ball mill was used as a comminution device, and its effects on the grain size and chemical structure of anthracite from Ningxia, China were studied. Comminution kinetics were studied and qualitative and quantitative analyses of functional groups were done by IR spectrometry. The results demonstrate that product size distribution of anthracite from Ningxia conformed to the n-order kinetic model and the grain size reached −42 nm after 24 h. The peak intensity of the IR spectra increased markedly, and the surface activity of anthracite powder was enhanced after ultrafine comminution. Penta-substituted benzene rings of aromatic hydrogen were the dominant group in raw anthracite, while tris-substituted benzene rings of aromatic hydrogen became the dominant form after 24 h of ultrafine comminution. During ultrafine comminution, cyclization of aliphatic chains, dehydroaromatization of cycloparaffin, substitution reactions of orienting groups on aromatic rings, and decarboxylic reactions of benzene rings might occur. Although raw anthracite shows a relatively high degree of metamorphism, its particles show stronger activity and can absorb more oxygen after ultrafine comminution, thus increasing oxygen enrichment. The CH in the aliphatic hydrocarbons of ultrafine anthracite basically disappeared, and CH2 was more easily lost than CH3 during ultrafine comminution. Oxygen-containing functional groups in anthracite were altered or some of their elements reacted with oxygen, which decreased the amount of free OH groups on anthracite particle surfaces. During ultrafine comminution, the stability of hydrogen bonds was ranked OH…π > OH…OH > free OH groups > OH…ether > OH…N > cyclic OH tetramers. These changes indicate that the ultrafine anthracite comminution process is also accompanied with chemical reactions. The results provide a reference and guidance for exploring changes in the molecular structures of ultrafine anthracite and follow-up deep processing. In particular, the occurrence of mercaptan -SH and FeS2 implies that sulfur exists in anthracite in both organic and inorganic forms. The absorption peak of ultrafine pulverized kaolinite was intensified, indicating that minerals were dissociated and more impurities were exposed after comminution. 

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