270,703. I. G. Farbenindustrie Akt.- Ges. May 7, 1926, [Convention date]. Fuel. - Pulverulent fuel for use in internalcombustion engines is treated with hydrofluoric acid to free it wholly or partly from silicic acid and thereby reduce the grinding action of mineral impurities. A gas or air current charged with hydrofluoric acid vapours may be passed against the fuel dust descending a tower or inclined rotating pipe, the gas current preferably being returned to the acid container. The apparatus is made or lined with lead or other material resistant to the acid. Aqueous solutions of the acid or compounds such as ammonium fluoride evolving it may be employed.