Effect of demineralization on lignite structure from Yinmin coalfield by FT-IR investigation

The raw coal and demineralized coal obtained from Yimin lignite were studied by Fourier transform infrared spectroscopy (FT-IR) with curve-fitting analysis to obtain the structure change information after demineralization. The results show that demineralization has little effect on aliphatic hydrogen and hydroxyl. The absorption intensity of CH₂ asymmetric stretching vibration changed little, that of CH stretching vibration decreases, and that of CH₂ symmetric stretching vibration and CH₃ asymmetric stretching vibration increases. The absorption intensity of OH-N, OH-OH and OH-π hydrogen bonds decreases and that of ring hydroxyl and OH-O intensity increases. Demineralization has great effect on aromatic structures and oxygen-containing groups. Aromatic structures change from three hydrogens per ring dominating to three and four hydrogens per ring dominates. The absorption intensity of alkyl ether and aliphatic carboxylic acids decreases significantly after demineralization, which may be caused by hydrolysis reaction. The absorption intensity of phenolic hydroxyl and carboxylic acids increases greatly after demineralization.