Kraft cooking method using polysulfide cooking liquor

Abstract

The method is for the preparation of kraft pulp with increased pulping yield from lignin-containing cellulosic material using polysulfide cooking liquor. In order to increase carbohydrate stabilization and hence the yield from a kraft cooking process a first impregnation stage is using polysulfide cooking liquor implemented at high alkali and polysulfide concentration and at a low temperature.

1 . A method for the preparation of kraft pulp with increased pulping yield from lignin-containing cellulosic material using polysulfide cooking liquor, wherein the lignin-containing cellulosic material is heated to a temperature in the range 50-100° C. followed by adding polysulfide cooking liquor to a first impregnation stage which in turn is followed by cooking stages resulting in a kraft pulp with a kappa number below 40 characterized in that the impregnation stage is conducted at high alkali concentration, low temperature and high polysulfide concentration using polysulfide cooking liquor at a liquor-to-wood ratio in the range 2.0 to 3.2, and that the temperature is between 80-120° C. during a retention time resulting in a H-factor in the range 2-20 and preferably 2-10 of the impregnation stage. 
     
     
         2 . The method according to  claim 1  wherein the effective alkali concentration during the impregnation stage is above 60 g/l when adding the polysulfide cooking liquor. 
     
     
         3 . The method according to  claim 2  wherein the polysulfide concentration during the impregnation stage is above 3 g/l, or above 0.09 mol/l, when adding the polysulfide cooking liquor. 
     
     
         4 . The method according to  claim 3  wherein more than 90% of the total charge of cooking liquor needed for completion of the cooking stages to the intended kappa number below 40 (ändra I text) is charged to the first impregnation stage, and that at least 175 kg of effective alkali (EA as NaOH) for softwood and 160 kg of effective alkali for hardwood per ton of chips is charged. 
     
     
         5 . The method according to  claim 4  wherein the alkali concentration is reduced by at least 8 g/l by adding additional cooking liquids having lower alkali concentration than the alkali concentration prevailing at end of the first impregnation stage when increasing the temperature to cooking temperature, said cooking liquids in at least part thereof includes black liquor. 
     
     
         6 . The method according to  claim 5  wherein no black liquor is added to the first impregnation stage. 
     
     
         7 . The method according to  claim 6  wherein the white liquor added to the first impregnation stage has an alkali concentration above 100 g/l and a polysulfide concentration above 4 g/l .