Patent ID: 7270725

Claim:
A method for the manufacture of cellulose pulp, in which wood chips are treated in several stages including at least one cooking stage, comprising: warming the wood chips with steam to a first temperature; prior to feeding the steamed wood chips to a treatment stage, adding an alkali rich impregnation fluid to the steamed wood chips in a pre-treatment stage, the pre-treatment stage being prior to and upstream of the treatment stage, the added alkali rich impregnation fluid constituting at least 50% of a total charge of fresh alkali required to cook cellulose pulp; the alkali rich impregnation fluid and the alkali treated wood chips forming a slurry in the pretreatment stage; feeding the slurry to the treatment stage; prior to impregnating the wood chips with a black liquor in the treatment stage, the black liquor being withdrawn from a cooking stage, separating a main portion of the alkali rich impregnation fluid from the alkali treated wood chips; replacing the separated alkali rich impregnation fluid with the black liquor withdrawn from the cooking stage; prior to the cooking stage, impregnating the wood chips with the black liquor withdrawn from the cooking stage and impregnating the wood chips with the black liquor in the treatment stage, the black liquor withdrawn from the cooking stage having a temperature being at least 20° C. below a cooking temperature of between 130-170° C.; withdrawing the black liquor from the treatment stage; after the withdrawal of the black liquor from the treatment stage, adding back the separated alkali rich impregnation fluid to the black liquor impregnated wood chips disposed in the treatment stage; conveying the black liquor impregnated wood chips and the separated alkali rich impregnation fluid to the cooking stage; withdrawing black liquor from the cooking stage and conveying the black liquor withdrawn from the cooking stage to a beginning of the treatment stage to replace the separated alkali rich impregnation fluid; and cooking the wood chips at the cooking temperature to form cellulose pulp.