Kontinuerlig kokning av cellulosahaltigt material med förbehandling med alkaliska vätskor i två steg

A method of treating wood chips allows chemical pulp (e. g. kraft pulp) to be produced having enhanced fiber strength properties. In a first zone or stage the chips are cool impregnated (e. g. a temperature of about 60-120 DEG C.) with a first alkali-containing liquid (25-35 g/l expressed as NaOH). In the top of a continuous digester in a second zone or stage the chips are then treated with a second alkali-containing liquid having an alkali concentration at least 5 g/l less than the first liquid (e. g. 10-20 g/l) and a temperature of between about 120-160 DEG C. (e. g. 130-150 DEG C.). An extraction typically takes place to effect the change in treatment zones. After the second zone the chips are cooked at about 140-180 DEG C. (e. g. 150-170 DEG C.), and higher than in the second zone.