Effect of addition of various salts on coal extraction with carbon disulfide/N-methyl-2-pyrrolidinone mixed solvent

The effect of addition of various salts on the extraction of seven different kinds of coals with carbon disulfide-N-methyl-2-pyrrolidinone (CS2-NMP) mixed solvent (1:1 by volume) was investigated. Lithium and tetrabutylammonium salts with various anions were used as additives for the coal extraction. Addition of some salts considerably increased the extraction yield for several coals, For Upper Freeport coal, in particular, the addition of a very small amount (0.25 mol/lig-coal) of tetrabutylammoniumfluoride increased the extraction yield from 60 to 84%. The results for the halogenide salts indicate that a kind of halogenide anion affects the extraction yields. The yields increased in the order, F- > Cl- > Br- > I-, indicating that the anions with the small ion radius or large electronegativity are effective for the enhancement of the yield. Some qualitative explanations for the observations were made using the Hard and Soft Acids and Bases principle. The role of anions for the enhancement of coal solubility was discussed on the basis of the assumption that acid-base interactions between coal acidic sites and anions are responsible for the increment of the extraction yields.