Bleaching stage using xylanase with hydrogen peroxide, peracids, or a combination thereof

Abstract

The present invention discloses methods of bleaching chemical pulp that combine xylanase enzymes with hydrogen peroxide, peracids, or a mixture. The method comprises the steps of carrying out a chemical pulping operation, optionally followed by delignifying the pulp with oxygen, then combining xylanase enzymes with hydrogen peroxide, peracids, or a mixture to bleach the pulp. The method allows the mill to use both xylanase and peracids in a single bleaching tower to decrease the usage of chlorine dioxide and other bleaching chemicals. The pulp bleaching method of the present invention may be performed in a pulp mill as part of a complex pulp bleaching process.

The embodiments of the invention in which an exclusive property of privilege is claimed are defined as follows:  
     
         1 . A method of bleaching chemical pulp comprising, exposing said chemical pulp to a bleaching stage comprising at least one xylanase enzyme and at least one oxidizing chemical, said oxidizing chemical comprising hydrogen peroxide, one or more peracids, or a mire of hydrogen peroxide and one or more peracids.  
     
     
         2 . The method of  claim 1 , wherein said bleaching stage takes place at a pH between about 5.0 and about 9.0.  
     
     
         3 . The method of  claim 2  wherein said peracid is selected from the group consisting of peracetic acid, performic acid, peroxymonosulfuric acid, peroxymonophosphoric acid, peroxynitric acid, and a combination thereof.  
     
     
         4 . The method of  claim 3  wherein said peracid is peracetic acid or performic acid.  
     
     
         5 . The method of  claim 3  wherein said peracid is peroxymonosulfuric acid.  
     
     
         6 . The method of  claim 3  wherein said peracid is produced external to said bleaching stage, and then is added to said bleaching stage.  
     
     
         7 . The method of  claim 3  wherein said peracid is produced within said bleaching stage.  
     
     
         8 . The method of  claim 3  wherein said bleaching stage is conducted for a period of 20 to 60 minutes at a temperature of: about 40° C. to about 70° C., and at a pulp consistency of about 4% to about 12%.  
     
     
         9 . The method of  claim 1 , wherein said chemical pulp comprises kraft pulp, soda pulp or sulfite pulp.  
     
     
         10 . The method of  claim 1 , wherein said method is performed in a pulp mill.  
     
     
         11 . The method of  claim 3  wherein said pH is between about 6.5 to about 8.  
     
     
         12 . The method of  claim 1  wherein said xylanase is selected from the group consisting of BioBrite™ EB xylanase, HTX-18, and wild-type  Trichoderma reesei  xylanase II.  
     
     
         13 . The method of  claim 1  wherein said bleach stage is preceded by one or more alkaline oxygen delignification stages.  
     
     
         14 . The method of  claim 1  wherein said bleach stage is preceded by a ozone-peracid treatment.  
     
     
         15 . The method of  claim 1  wherein said bleach stage is followed by a bleaching sequence.  
     
     
         16 . The method of  claim 15 , wherein said bleaching sequence comprises a chlorine dioxide bleach sequence.  
     
     
         17 . The method of  claim 15 , wherein said bleaching sequence comprises an ozone-peracid bleach sequence.  
     
     
         18 . The method of  claim 1  wherein said oxidizing chemical is hydrogen peroxide, said hydrogen peroxide present at an amount of between about 1 and about 10 kg per tonne pulp.  
     
     
         19 . A method of reducing kappa number of a chemical pulp comprising: 
 vii) exposing said chemical pulp to a bleaching stage comprising at least one xylanase enzyme and at least one oxidizing chemical, said oxidizing chemical comprising hydrogen peroxide, one or more peracids, or a combination of hydrogen peroxide and one or more peracids, to produce a treated pulp;    viii) washing said treated pulp; and    ix) bleaching said treated pulp using chlorine dioxide, ozone, oxygen peracid, alkaline extraction, or a combination thereof;    
     
     
         20 . A method of reducing the amount of chlorine dioxide required during a bleaching sequence of a chemical pulp comprising: 
 x) exposing said chemical pulp to a bleaching stage comprising at least one xylanase enzyme and at least one oxidizing chemical, said oxidizing chemical comprising hydrogen peroxide, one or more peracids, or a combination of hydrogen peroxide and one or more peracids, to produce a treated pulp;    xi) washing said treated pulp; and    xii) bleaching said treated pulp using chlorine dioxide    
     
     
         21 . A method of bleaching a chemical pulp comprising: 
 i) exposing said chemical pulp to a bleaching stage comprising at least one xylanase enzyme and peracetic acid, said bleaching stage at a pH between about 5.0 and about 9.0, and comprising 0.5 to about 5 kg/t pulp of peracetic acid, for about 20 to about 60 min. at a temperature from about 40° C. to about 70° C., and with pulp consistency between about 4% to about 12%, to produce a treated pulp    iv) washing said treated pulp; and    v) bleaching said treated pulp using a chlorine dioxide bleach sequence.    
     
     
         22 . The method of  claim 21 , wherein in said step of exposing (step i)), said pH is about 6.5.  
     
     
         23 . The method of  claim 22 , wherein m said step of exposing (step i)), said temperature is about 60° C.  
     
     
         24 . The method of  claim 22 , wherein in said step of exposing (step i)), said pH of said chemical pulp is set, followed by addition of said peracetic acid, followed by addition of said xylanase.