Increase of the extraction yields of coals by the addition of aromatic amines

The effect of the addition of aromatic amines on the yields of the extraction of Argonne Premium coals with carbon disulfide–N-methyl-2-pyrrolidinone (CS2–NMP) mixed solvent (1:1 by volume) was investigated. The addition of a very small amount (25 mg/g-coal) of p-phenylenediamine (p-PDA) increased the extraction yield of Upper Freeport (UF) coal from 51.4 to 81.3%. The solvent fractionation of the extracts obtained shows that the increase in the yield was caused mainly by the increase of the heaviest fraction, i.e. pyridine insoluble fraction (PI), and not that of the lighter pyridine soluble fraction. p-PDA was also found to increase the solubility of PI in the CS2–NMP mixed solvent. These behaviors of p-PDA are very similar to those already observed by the addition of tetracyanoethylene, and can be explained by the breaking of the associates among coal molecules through the interaction of the amines with coal. The extraction yields of other Argonne Premium coals than UF coal also increased their extraction yields, but only slightly. The results obtained from the addition of various aromatic amines with different structure and electron donatability suggest that the hydrogen bonds, not electron donor–acceptor interaction, of the amines with coal seem to be responsible for the solubility enhancements of the coals and extracts. Effect of water on the extraction yield was also examined.