Abstract: Methods of treating an oil derived by liquefaction of coal particles to separate unreacted solid matter therefrom and collect it in water or other aqueous medium, which comprise controlling the specific gravity of the oil to provide an oleaginous fluid having a substantially lower specific gravity than the aqueous medium, as by mixing the oil with a liquid that is miscible therewith and has a lower specific gravity, or by controlling the temperature of the oil, or both; contacting the fluid with the aqueous medium; moving the fluid in such a manner as to provide a substantial acceleration thereto in a direction to drive most of the solid matter away from the fluid and into the aqueous medium as by moving the fluid in a swirling path around an axis while maintaining a portion of the aqueous medium around at least a portion of the periphery of the path; and separating the aqueous medium with the solid matter contained therein from the fluid.