General aspects of coal solubility

Coals of different rank were treated with a selection of common solvents. Whereas the solubilities in the pure solvents were very limited, it was possible to dissolve considerably more in a mixture of solvents. Using the simplification that a solvent mixture with coal is quasi binary, a correlation between interaction and the ‘solubility parameters’ of these solvents could be found. The ‘solubility parameters’ of 6 coals were determined. A comparison with calculated parameters which were found in the literature showed significantly lower values. Application of the theory explains several experimental results of coal solubility. Extractability and solubility are frequently though wrongly used interchangeably in coal chemistry. To avoid confusion the thermodynamic expression ‘solubility parameter’ (a common expression in polymer chemistry) has been adopted and is used in quotation marks.