Coal cleaning techniques take advantage of the differences in specific gravity of the coal and its impurities. Hydrocyclones and gravity concentration devices are examples of such systems. When coal is finely ground, physical processes that take advantage of the surface properties of the coal materials can be used. For instance, froth flotation exploits the hydrophilic surface characteristics of mineral impurities and the hydrophobic nature of coal particles to achieve separation. Hydrocyclone, heavy media separation, and froth flotation technologies are illustrated below.

Here is a link that might be useful: Steps to Cleaning Coal ■