Extraction under very mild conditions - a few minutes at room temperature with poor solvents - yields results characteristic of surface properties for a variety of coals._x000D_
Such superficial extractions preferentially remove long-chain aliphatic compounds. As coal becomes more oxidized, less of these yellow- colored compounds is extracted, forming the basis for a simple colorimetric test for coal surface quality. The fact that these aliphatic compounds are responsible for surface hydrophobicity is supported by good correlation between extraction results and floatability, which decreases with decreasing extractability._x000D_
This extraction test is very sensitive to oxi dationb even the effect of heating for an hour at 100 C can be detected. A second extraction test using mild alkali conditions has shown good correlation with plant dewatering Operations.