Process for the bleaching of paper pulp

Abstract

Process for the bleaching of paper pulp in which : (a) an aqueous suspension of the paper pulp is first subjected to a chelating treatment with at least one aminopolycarboxylic acid, its salt or their mixtures in order to reduce at least partially the metallic ion, especially manganese, content of the suspension, and (b) the aqueous suspension of the paper pulp thus obtained is subjected to a bleaching treatment with hydrogen peroxide in the presence of i. at least one poly-α-hydroxyacrylic acid, its salt or their mixtures, the function of which is to chelate at least partially iron present in the aqueous suspension, and ii. at least one aminopolycarboxylic acid, its salt or their mixtures, the function of which is to chelate calcium present in the aqueous suspension thereby improving the performance of the poly-α-hydroxyacrylic acid, its salt or their mixtures.

1 . A process for the bleaching of paper pulp comprising :
 (a) first subjecting an aqueous suspension of the paper pulp to a chelating treatment with at least one aminopolycarboxylic acid, its salt or their mixtures in order to reduce at least partially the metallic ion content of the suspension, and   (b) subjecting the aqueous suspension of the paper pulp thus obtained to a bleaching treatment with hydrogen peroxide and an alkali source in the presence of
 i. at least one poly-α-hydroxyacrylic acid, its salt or their mixtures, the function of which is to chelate at least partially iron present in the aqueous suspension, and 
 ii. at least one aminopolycarboxylic acid, its salt or their mixtures, the function of which is to chelate calcium present in the aqueous suspension thereby improving the performance of the poly-α-hydroxyacrylic acid, its salt or their mixtures. 
   
   
   
       2 . The process according to  claim 1 , wherein the poly-α-hydroxyacrylic acid is in the form of its sodium salt. 
   
   
       3 . The process according to  claim 1 , wherein step (b) is carried out at a temperature equal to or less than 100° C. 
   
   
       4 . The process according to  claim 1 , wherein step (a) is carried out at the natural pH of the pulp in the range of 4 to 8 using an amount of aminopolycarboxylic acid, its salt or their mixture of at least 0.1% by weight of dry pulp and at a temperature of at least 40° C. 
   
   
       5 . The process according to  claim 1 , wherein step (b) is carried out at a pH of from 8 to 14, at a consistency of from 10 to 40% of dry pulp based on the weight of pulp suspension, and at a temperature of from 60 to 95° C. 
   
   
       6 . The process according to  claim 1 , wherein step (b) is carried out by using an amount of poly-α-hydroxyacrylic acid, its salt or their mixture of from 0.1 to 2.0 wt-% based on the weight of dry pulp, and an amount of aminopolycarboxylic acid, its salt or their mixtures of from 0.05 to 1.0 wt-% based on the weight of dry pulp. 
   
   
       7 . The process according to  claim 1 , wherein, in step (b), the weight ratio between the poly-α-hydroxyacrylic acid, its salt or their mixture and the aminopolycarboxylic acid, its salt or their mixture is of from 1:10 to 10:1. 
   
   
       8 . The process according to  claim 1 , wherein the aminopolycarboxylic acid, its salt or their mixture is selected from the group consisting of ethylene diamine tetra-acetic acid, diethylene triamine penta-acetic acid, triethylene tetramine hexa-acetic acid, cyclohexane diamine tetra-acetic acid, methylglycine di-acetic acid, nitrilo tri-acetic acid, their salts, and mixtures thereof. 
   
   
       9 . The process according to  claim 1 , wherein the aminocarboxylic acid used in steps (a) and (b) is diethylene triamine penta-acetic acid or its sodium salt. 
   
   
       10 . The process according to  claim 1 , wherein step (b) is carried out in the absence of any other stabilizer or chelating agent. 
   
   
       11 . The process according to  claim 1  which is applied to the bleaching of mechanical paper pulp. 
   
   
       12 . The process according to  claim 1 , wherein step (a) is carried out in order to at least partially reduce the manganese content of the suspension.