Process for bleaching a lignocellulosic pulp

Abstract

The invention relates to improvements in the bleaching of a lignocellulosic pulp by adding carbon dioxide to provide a beneficial pH profile in the chlorine dioxide bleaching or delignification. The resulting bleached pulp is used in the production of paper or pulp. The invention also relates to the use of carbon dioxide for providing a desired pH profile in a chlorine dioxide bleaching stage.

1 . A process for bleaching a lignocellulosic pulp with chlorine dioxide in an aqueous suspension at different pH values, said process comprising the following stages: 
 providing an aqueous lignocellulosic pulp suspension in a chlorine dioxide bleaching stage after cooking,    adding carbon dioxide to said lignocellulosic pulp,    subsequently adding a chlorine dioxide bleaching agent to said lignocellulosic pulp suspension,    said carbon dioxide being added in an amount sufficient for providing in said suspension a buffering system causing an initial chlorine dioxide bleaching period at a pH≧4.5 and after said initial period a subsequent chlorine dioxide bleaching period at a pH below 4.5,    and continuing said chlorine dioxide bleaching at a pH below 4.5 and thereby achieving an increased brightness of the bleached pulp compared to a similar bleach without carbon dioxide.    
     
     
         2 . A process according to  claim 1 , wherein the duration of said initial period is at least one minute, preferably 1 to 5 minutes.  
     
     
         3 . A process according to  claim 1  or  2 , wherein the buffering capacity of the carbon dioxide is lost due to a component of said buffering system reverting to another chemical and/or physical form.  
     
     
         4 . A process according to  claim 3 , wherein carbon dioxide is regenerated in the aqueous and/or gaseous phase in said suspension as said buffering capacity is lost.  
     
     
         5 . A process according to  claim 1 , wherein said carbon dioxide is added in gaseous, liquid, aqueous or solid form.  
     
     
         6 . A process according to  claim 5 , wherein said carbon dioxide is fed to said pulp suspension in a tank or a pipe or into a liquid to be added to said pulp prior to said chlorine dioxide addition.  
     
     
         7 . A process according to  claim 5  or  6 , wherein gaseous carbon dioxide which is injected into a flowing stream of said pulp suspension just prior to the addition of said bleaching agent.  
     
     
         8 . A process according to any one of the preceding  claims 1  to  7 , wherein the amount of carbon dioxide is adjusted so as to cause the buffering capacity of said buffering system to be consumed after a suitable length of said initial period by the acidic reaction products formed due to the action of the chlorine dioxide on said pulp.  
     
     
         9 . A process according to any one of the preceding  claims 1  to  8 , wherein said aqueous cellulosic pulp suspension comprises a chemical pulp, an organosolv pulp, a mechanical pulp, a chemi-mechanical pulp, a semi-chemical pulp, a pulp containing recycled fibers or broke, or a mixture of any of these in a bleaching plant of a pulp mill.  
     
     
         10 . A process according to  claim 1 , wherein said aqueous cellulosic pulp suspension comprises oxygen delignified pulp.  
     
     
         11 . A process according to  claim 1 , wherein said aqueous cellulosic pulp suspension has a concentration of 1 to 18%.  
     
     
         12 . A process according to  claim 1 , wherein the temperature at said chlorine dioxide bleaching is between 35° C. and 105° C., preferably between 45° C. and 75° C.  
     
     
         13 . A process according to  claim 1 , wherein said pulp suspension after said chlorine dioxide bleaching is processed into bleached paper or pulp.  
     
     
         14 . A process according to  claim 13 , wherein said processing includes an alkaline treatment.  
     
     
         15 . A process according to  claim 13  or  14 , wherein said processing includes an oxygen delignification step and/or an extraction step.  
     
     
         16 . Use of carbon dioxide in the chlorine dioxide bleaching of an aqueous lignocellulosic pulp suspension for providing in said suspension a buffering system causing, after an addition of chlorine dioxide to said suspension, an initial chlorine dioxide bleaching period at a pH≧4.5 and after said initial period a subsequent chlorine dioxide bleaching period at a pH below 4.5.