Xylanase treatment of chemical pulp
The present invention discloses methods of bleaching chemical pulp that use xylanase enzymes after chemical bleaching. The method comprises the steps of carrying out a chlorine dioxide stage to produce a partially bleached pulp, treating the partially bleached pulp with a xylanase enzyme, optionally in the presence of oxygen and hydrogen peroxide, in a mild extraction stage, then bleaching the pulp with a second chlorine dioxide stage. The method allows the mill to decrease the usage of sodium hydroxide or other alkali, while decreasing the use of chlorine dioxide, and possibly improving the yield and strength of the pulp, while maintaining a similar level of bleached brightness of the pulp. The pulp bleaching method of the present invention may be performed in a pulp mill as part of a complex pulp bleaching process.
1. A method of bleaching chemical pulp comprising the steps of:
a) treating chemical pulp in a first chlorine dioxide bleaching stage and then washing the pulp with a first water wash to produce a partially bleached pulp;
b) treating said partially bleached pulp with a xylanase directly subsequent to said first chlorine dioxide bleaching stage and said first water wash to hydrolyze xylan present in said partially bleached pulp, and then washing the pulp with a second water wash to produce an enzymatically treated pulp, wherein said treating is carried out at a pH of between about 3 and about 8; and
c) treating said enzymatically treated pulp to a second chlorine dioxide bleaching stage directly subsequent to the xylanase treatment stage and said second water wash (step b).
2. The method of claim 1 , wherein said chemical pulp comprises kraft pulp, soda pulp or sulfite pulp.
3. The method of claim 1 , wherein at least one of said first and second chlorine dioxide bleaching stages comprises chlorine dioxide and chlorine.
4. The method of claim 1 , further comprising an oxygen delignification stage, a xylanase treatment stage, or a combination thereof, prior to said step of treating chemical pulp in the first chlorine dioxide bleaching stage (step a).