Method for two-stage oxygen bleaching and delignification of chemical pulp
AssigneeValmet Fibertech Aktiebolag
InventorsMonica Bokström, Pia Mellander, Solveig Nordén
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Abstract
Methods for oxygen delignification of a pulp are disclosed, including initially delignifying the pulp at a delignification temperature of less than 90° C., adding oxygen to the pulp so that the oxygen is present during the initial delignification step, and further delignifying the pulp at a delignification temperature of greater than 90° C., the difference between the two delignification temperatures being less than about 20° C., and the pressure being greater in the initial delignification step. The method also includes adding alkali solely to the initial delignification step in order to obtain high alkalinity therein.
Claims11
A method of bleaching chemical pulp comprising the steps of:
- (a) delignifying the pulp in a first oxygen delignification stage to which oxygen and alkali are added at a first alkali charge, at a first pressure of 4 to 10 bar and at a first temperature less than 90° C.;
- and then
- (b) further delignifying the pulp in a second oxygen delignification stage at a second pressure lower than said first pressure and at a second temperature greater than 90° C., said second temperature being 10° C. to less than 20° C. higher than said first temperature, the alkalinity in the second stage being lower than in the first stage.
Dependencies are inferred from the claim text.