Abstract: A continuous process of removing silica from spent pulping liquors (black liquors) which have been obtained by the alkaline digestion of annual plants. The spent liquor is preconcentrated and contacted with a CO.sub.2 -containing gas. The CO.sub.2 -containing gas is supplied at a rate of 30 to 40 m.sup.3 s.t.p. per m.sup.3 of spent liquor. The precipitated silica is removed from the treated liquor. The silica-containing precipitate which has been separated is diluted and washed with water and causticized by an addition of lime or milk of lime. Solid and liquid phases are separated from each other and the resulting residue is combusted.