Abstract: A process, for removing solid impurities from impure phosphoric acid containing solids, comprising:(a) passing said acid at an elevated temperature through a filter cake of a phosphoric acid-resistant filter aid supported on a porous medium, thereby to cause at least a portion of said solid impurities of said acid to be filtered therefrom and to be retained by said filter cake;(b) contacting at least a portion of said filter cake with a displacing liquid, maintained at a temperature and in an amount sufficient to cause at least a portion of the acid which remains in said filter cake to pass through said filter cake without fracturing said filter cake;(c) collecting said filtered acid which passes through said filter cake; and(d) removing an impurity-containing portion of said filter cake after contact with said acid and displacing liquid, thereby to present a fresh surface for subsequent filtration.