Method for bleaching mechanical and chemithermomechanical pulp g

Abstract

The present invention relates to a method for bleaching mechanical and chemithermomechanical pulp including that an advancing pulp suspension obtained after that the fibres are laid free is prebleached by adding reductive bleaching agent to the pulp suspension in a location just after the fibres are laid free and that the bleaching is carried out under given conditions in the form of high temperature and minimized oxygen access in respect of said adding location and immediately downstream of said location, characterized in, that the in the described manner prebleached pulp is subjected to at least one further bleaching treatment including that the pulp is bleached with an oxidizing bleaching agent, preferably peroxide bleaching agent, reinforced with borohydride.

1 . Method for bleaching mechanical and chemithermomechanical pulp including that an advancing pulp suspension obtained after that the fibres are laid free is prebleached by adding reductive bleaching agent to the pulp suspension in a location just after the fibres are laid free and that the bleaching is carried out under given conditions in the form of high temperature and minimized oxygen access in respect of said adding location and immediately downstream of said location, characterized in that the in the described manner prebleached pulp is subjected to at least one further bleaching treatment including that the pulp is bleached with an oxidizing bleaching agent, preferably peroxide bleaching agent, reinforced with borohydride.  
     
     
         2 . Method according to  claim 1 , characterized in, that borohydride is added to the pulp and is permitted to react with this during a short period of time in the form of a pretreatment before the pulp is bleached with peroxide bleaching agent.  
     
     
         3 . Method according to  claim 1 , characterized in, that borohydride is added to the pulp at the beginning of the peroxide bleaching treatment.  
     
     
         4 . Method according to  claim 1  characterized in that borohydride is added to the pulp at the end of the peroxide bleaching treatment.  
     
     
         5 . Method according to claims  1 - 4 , characterized in, that the prebleached pulp is bleached with peroxide bleaching agent and thereafter with peroxide bleaching agent reinforced with borohydride.  
     
     
         6 . Method according to  claim 5 , characterized in, that the first peroxide bleaching stage is carried out at medium consistency and that the second peroxide bleaching stage is carried out at high pulp consistency and that spent liquor containing residue peroxide from the second bleaching stage is returned and mixed into the pulp before or in connection with the first peroxide bleaching stage.  
     
     
         7 . Method according to claims  1 - 6 , characterized in, that to the pulp is delivered at least one of the chemicals alkali, water glass (sodium silicate) and magnesium compound as completion to borohydride.  
     
     
         8 . Method according to claims  1 - 7 , characterized in, that to the pulp is delivered at least one of the chemicals alkali, water glass (sodium silicate) and magnesium compound as completion to peroxide bleaching agent.  
     
     
         9 . Method according to  claim 1 , characterized in, that the reductive bleaching agent is sodium dithionite.  
     
     
         10 . Method according to claims  1 - 9 , characterized in, that the borohydride is sodium borohydride, preferably in the form of the commodity Borol.  
     
     
         11 . Method according to claims  1 - 10  characterized in, that the peroxide bleaching agent is hydrogen peroxide.