Low consistency oxygen delignification process

Abstract

The present invention provides for a method for delignifying softwood or hardwood pulp in a fiber line of a pulp and papermaking system having a slurry at low consistency by adding to the pulp slurry oxygen and adding this pulp slurry reaction mixture to a reaction system comprising an efficient mixer and contact tank. The reaction system is preferably physically located after the knotters and before the brown stock washers to make more effective use the residual alkali and heat content in the incoming black liquor.

1 . A method for delignifying pulp in a slurry at low consistency from a fiber line of a papermaking system comprising adding oxygen to said slurry and mixing said slurry under conditions of temperature and pressure whereby said oxygen reacts with the lignin present in said pulp.  
     
     
         2 . The method as claimed in  claim 1  wherein the temperature of said slurry is about 85° C. to about 120° C.  
     
     
         3 . The method as claimed in  claim 1  wherein the pressure of said mixing is about 20 to about 160 psig.  
     
     
         4 . The method as claimed in  claim 1  wherein said mixing occurs for up to 2 minutes.  
     
     
         5 . The method as claimed in  claim 1  wherein said pulp slurry contains black liquor.  
     
     
         6 . The method as claimed in  claim 1  wherein said mixing occurs in a gas liquid mixer.  
     
     
         7 . The method as claimed in  claim 6  wherein said mixer is a controlled cavitation device.  
     
     
         8 . The method as claimed in  claim 6  wherein said mixer further comprises a residence tank.  
     
     
         9 . The method as claimed in  claim 1  wherein said treated slurry is returned to said bleach line.  
     
     
         10 . The method as claimed in  claim 1  wherein said pulp slurry is drawn from said fiber line from a point selected from the group consisting of after the knotters and before the brown stock washers; after the digester and before the brown stock washers; and between any two stages of a multi-stage brownstock washing step.  
     
     
         11 . A method for delignifying pulp slurry from a papermaking process fiber line comprising the steps: 
 a) directing said slurry to a mixing device;    b) adding oxygen to said slurry;    c) mixing said slurry and oxygen under temperature and pressure conditions to cause delignification;    d) directing said delignified reaction mixture to a holding tank; and    e) directing said delignified reaction mixture to said papermaking process.    
     
     
         12 . The method as claimed in  claim 11  wherein said pulp slurry is drawn from said fiber line from a point selected from the group consisting of after the knotters and before the brown stock washers; after the digester and before the brown stock washers; and between any two stages of a multi-step brownstock washing step.  
     
     
         13 . The method as claimed in  claim 11  further comprising adding caustic to said slurry.  
     
     
         14 . The method as claimed in  claim 11  further comprising adding steam to said slurry.  
     
     
         15 . The method as claimed in  claim 11  wherein said pulp slurry contains about 2 to about 8% by weight pulp.  
     
     
         16 . The method as claimed in  claim 11  wherein the pH of said pulp slurry is about at least 11.  
     
     
         17 . The method as claimed in  claim 11  wherein the temperature of said pulp slurry is about 85 to about 120° C.  
     
     
         18 . The method as claimed in  claim 11  wherein said mixing is at a pressure of about 20 to about 160 psig for up to 2 minutes.  
     
     
         19 . The method as claimed in  claim 11  wherein said pulp slurry is maintained at a pressure of about 20 to about 160 psig for about 5 to about 30 minutes in said holding tank.  
     
     
         20 . The method as claimed in  claim 11  wherein said pulp slurry contains black liquor.  
     
     
         21 . The method as claimed in claims  20  further comprising reacting the Na 2 S in said black liquor with oxygen.  
     
     
         22 . The method as claimed in  claim 21  wherein said oxygen reacts with lignin present in said black liquor.