Patent ID: 9580864
Date: 2017-02-28
CPC Classifications: D21C

Claim:
1. A method for the preparation of kraft pulp with increased pulping yield from lignin-containing cellulosic material using polysulfide cooking liquor in a continuous cooking system, comprising: providing an impregnation vessel in operative engagement with a digester via a high-pressure conduit, the impregnation vessel having a first impregnation stage; heating a lignin-containing cellulosic material to a temperature in a range of 50-100° C. followed by adding polysulfide cooking liquor to the first impregnation stage which in turn is followed by cooking stages in the digester; adding knots to the high-pressure conduit extending between the impregnation vessel and the digester; and conducting the first impregnation stage at high alkali concentration above 60 g/l (effective alkali (EA) as NaOH basis) when adding the polysulfide cooking liquor, wherein the polysulfide concentration is above 3 g/l, or above 0.09 mol/l, when adding the polysulfide cooking liquor, wherein the first impregnation stage has a liquor-to-wood ratio in a range of 2.0 to 3.2 in order to increase a relative carbohydrate stability, the liquor-to-wood ratio calculated as containing polysulfide cooking liquor and wood moisture, and that the temperature is between 80-120° C. during a retention time resulting in a H-factor in a range of 2-20 of the first impregnation stage.