In the extraction of coal using a liquid solvent oil, filtration can be assisted by using a filter aid which is of coal origin. The filter aid is produced by high temperature processing of coal or an ash-rich coal-derived residue under conditions at which the ash component of the coal or residue does not fuse. A preferred starting material is filter cake from a coal extraction process.
In the extraction of coal the liquid coal extract is filtered using a filter aid produced by washing the filter cake with a low viscosity, high boiling point, low vapor pressure ester and then calcining the cake to remove the wash solvent and other combustible constituents from the cake.
