Method for controlling the delignfication and bleaching of a pulp suspension

Abstract

The invention relates to a process for the bleaching of an aqueous lignocellulosic pulp under acidic conditions in the bleaching plant of a pulp mill. The bleached pulp is used for the production of paper or pulp. The selectivity of the bleaching agent towards primary reactions with lignin is enhanced by the use of a carbon dioxide providing medium in the pulp.

1 . A process for bleaching an aqueous lignocellulosic pulp under acidic conditions in the bleaching plant of a pulp mill, said process comprising the following stages: 
 providing an aqueous lignocellulosic pulp suspension in said bleaching plant,    adding bleaching agent to said pulp suspension and providing an acidic pH therein for causing a bleaching reaction between lignin and said bleaching agent,    prior to or simultaneously therewith adding a carbon dioxide providing medium to said lignocellulosic pulp suspension to provide carbon dioxide in said acidic pulp suspension for controlling the degradation of lignin or its derivatives in their reactions with said bleaching agent,    subsequently subjecting said pulp suspension to an alkaline extraction step to solubilize and remove reacted lignin compounds from said pulp.    
     
     
         2 . Process according to  claim 1 , wherein said carbon dioxide providing medium comprises carbon dioxide in gaseous, liquid or solid form.  
     
     
         3 . A process according to  claim 2 , wherein said medium comprises gaseous carbon dioxide which is injected into a flowing stream of said pulp suspension just prior to the addition of said bleaching agent.  
     
     
         4 . A process according to  claim 1 , wherein said carton dioxide providing medium comprises a gas or a liquid capable of providing carbonate and/or bicarbonate ions in said aqueous suspension under alkaline or neutral conditions.  
     
     
         5 . A process according to any one of the preceding  claims 1  to  4 , wherein said carbon dioxide providing medium provides an effective amount of carbon dioxide in said pulp suspension for influencing the equilibrium and/or progression of the reaction between lignin or its derivatives and said bleaching agent or its derivatives to suppress the degradation of said lignin derivatives.  
     
     
         6 . A process according to any one of the preceding  claims 1  to  4 , wherein said carbon dioxide providing medium provides an effective amount of carbon dioxide to suppress the degradation of the carbohydrates in said pulp suspension by influencing the equilibrium and/or progression of any carbohydrate degradation reaction chain, thus increasing the selectivity of the bleaching agent towards a reaction with lignin.  
     
     
         7 . A process according to claims  5  or  6 , wherein said effective amount of carbon dioxide is one that provides a carbon dioxide content in the gas phase of the pulp system which is substantially equal to the carbon dioxide content generated in the gas phase of a similar pulp system bleached under the same conditions but without the addition of a carbon dioxide providing medium.  
     
     
         8 . A process according to claims  6  or  7 , wherein a preferred effective amount of carbon dioxide to be added is obtained by monitoring the COD o the filtrate of the bleached and unextracted pulp at varying carbon dioxide addition at amounts and locating the COD minimum for the pulp to be bleached.  
     
     
         9 . A process according to  claim 1 , wherein said aqueous lignocellulosic pulp suspension is at an alkaline pH prior to the addition of said carbon dioxide providing medium.  
     
     
         10 . A process according to  claim 9 , wherein the pH of said pulp suspension is adjusted to pH 6.5 to 7.5 by the addition of gaseous carbon dioxide.  
     
     
         11 . A process according to  claim 1 , wherein said carbon dioxide providing medium is added in and/or after any washing stage preceding said addition of bleaching agent.  
     
     
         12 . A process according to  claim 1 , wherein said bleaching agent is used in a bleaching stage selected from a chlorine dioxide (D) stage, a peracetic acid (Paa) stage, an ozone (Z) stage, and a multichemical stage with a combination of bleaching agents.  
     
     
         13 . A process according to  claim 12 , wherein said bleaching stage comprises a D or Z stage and the final bleaching pH of said pulp suspension is adjusted to pH 1.5 to 4.4.  
     
     
         14 . A process according to  claim 12 , wherein said bleaching stage comprises a Paa ,stage and the final bleaching pH of said pulp suspension is adjust d to pH 4 to 6.  
     
     
         15 . A process according to claim any one of the preceding  claims 1  to  14 , 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.  
     
     
         16 . Process according to  claim 1 , wherein said aqueous lignocellulosic pulp suspension comprises oxygen delignified pulp.  
     
     
         17 . Process according to  claim 1 , wherein said aqueous lignocellulosic pulp suspension has a consistency of 1 to 18%, preferably 3 to 15%.  
     
     
         18 . Process according to any one of the preceding  claims 1  to  17 , wherein said pulp suspension is subsequently processed into bleached paper or pulp.  
     
     
         19 . Use of carbon dioxide for controlling the degradation of ligning in a process for bleaching an aqueous lignocellulosic pulp under acidic Conditions in the bleaching plant of a pulp mill, wherein a carbon dioxide providing medium is added to a lignocellulosic pulp suspension prior to or simultaneously with a bleaching agent to provide carbon dioxide in said pulp suspension at an acidic pH for controlling the degradation of lignin or its derivatives in their reactions with said bleaching agent, reacted lignin compounds subsequently being solubilized and removed by alkaline extraction.  
     
     
         20 . A use according to  claim 19 , wherein said carbon dioxide providing medium is used in an amount effective to suppress the degradation of said lignin derivatives.