Process for the bleaching of mechanical paper pulp

Abstract

A process for the bleaching of mechanical paper pulp with one or more peroxide oxidizing agents, in which the pulp is pretreated in a pretreatment step with one or more chelating agents chosen from aspartic acid and its derivatives, substituted on the nitrogen atom, characterized in that the initial pH of the pretreatment step, just before adding the one or more chelating agents, is above 8.

1 - A process for the bleaching of mechanical paper pulp with one or more peroxide oxidizing agents, in which the pulp is treated in a pretreatment step with one or more chelating agents selected from the group consisting of aspartic acid and its derivatives, substituted on the nitrogen atom, wherein the initial pH of the pretreatment step, just before adding the one or more chelating agents, is above 8. 
   
   
       2 - The process according to  claim 1 , characterized in that wherein the pH of the pretreatment step is not measured between the addition of one or more chelating agents, and the end of the pretreatment step. 
   
   
       3 - The process according to  claim 1 , wherein the pH is not controlled by the addition of pH modifying compounds during the pretreatment step. 
   
   
       4 - The process according to  claim 1 , wherein the chelating agent used in the pretreatment step is selected from the group consisting of N-carboxymethylaspartic acid, N-(1,2-dicarboxyethyl)-aspartic acid, N-(1,2-dicarboxy-2-hydroxyethyl aspartic acid, compounds of formula: 
     
       
         
         
             
             
         
       
     
     where n is 1-3, m is 0-3, p is 1-3, R 1 , R 2 , R 3  and R 4  are H, Na, K, Ca or Mg, R 5  and R 6  are H, CH 2 OH, CH 2 CH 2 OH or CH 2 O(CH 2 CH 2 O) 1-10 CH 2 CH 2 OH, and mixtures thereof. 
   
   
       5 - The process according to  claim 3 , wherein the chelating agent is selected from the group consisting of ethylenediamine-N,N′-disuccinic acid, its isomers, its salts, 2,2′-iminodisuccinic acid, its isomers, its salts, and mixtures thereof. 
   
   
       6 - The process according to  claim 1 , wherein the initial pH of the pretreatment step is from 8.1 to 11. 
   
   
       7 - The process according to  claim 1 , wherein the pretreatment step is carried out at a temperature from 40 to 95° C. 
   
   
       8 - The process according to  claim 1 , wherein the pretreatment step is carried out in the presence of water in order to reach a consistency from 0.1 to 45% by weight of dry pulp. 
   
   
       9 - The process according to  claim 1 , wherein the chelating agent used in the pretreatment step is used at a concentration from 0.1 to 1.5% by weight of dry pulp. 
   
   
       10 - The process according to  claim 1 , wherein the oxidizing agent used in the bleaching step is selected from the group consisting of hydrogen peroxide, organic peracids and combinations thereof. 
   
   
       11 - The process according to  claim 10 , wherein the organic peracid is peracetic acid.