The action of solvents on coal at low temperatures

The dissolution behaviour of three medium-rank and three high-rank coals in various solvents has been studied. Aqueous KOH is a non-solvent up to 180 °C. Dissolution of medium-rank coals into aliphatic amines and pyridine is not influenced by mass transfer effects in the 120–180 °C temperature range-although it is at room temperature-and does not involve the breaking of chemical bonds. This sets them apart from low-rank coals, where ester bonds have to be broken before dissolution can take place. An extraction limit of ≈50% seems to exist. Of the solvents studied, nonoethanolamine is the best for low-rank bituminous coals but rather poor for higher-rank coals, while the reverse is true of pyridine. Extract yields in hexylamine and ethylenediamine remain approximately constant in the medium-rank range. Low-volatile bituminous coals and semi-anthracites still contain some soluble matter, the extraction of which is diffusion-limited, at least up to 180 °C. From the results a hypothesis concerning the basic physical constitution of coal is derived and a model of the coalification process rationalizing the conjectured changes in constitution with rank is suggested.