Process for the bleaching of paper pulp

Abstract

Process for the bleaching of paper pulp in which an aqueous suspension of paper pulp is subjected to a bleaching treatment with hydrogen peroxide in the presence of magnesium hydroxide and of at least one poly-α-hydroxyacrylic acid, its salt or their mixture.

1 . A process for the bleaching of paper pulp in which an aqueous suspension of paper pulp is subjected to a bleaching treatment with hydrogen peroxide in the presence of at least one alkaline earth hydroxide selected from the group consisting of magnesium hydroxide and calcium hydroxide and in the presence of at least one poly-α-hydroxyacrylic acid, its salt or their mixture. 
     
     
         2 . The process according to  claim 1 , wherein the alkaline earth hydroxide is used in an amount of  0 . 1  to  10 % by weight of dry pulp. 
     
     
         3 . The process according to  claim 1 , wherein the bleaching step is carried out in the absence of sodium hydroxide. 
     
     
         4 . The process according to  claim 1 , wherein the bleaching step is carried out in the absence of sodium silicate. 
     
     
         5 . The process according to  claim 1 , wherein, prior to the bleaching treatment, the aqueous suspension of the paper pulp is first subjected to a chelating treatment with at least one aminopolycarboxylic acid, its salt or their mixtures. 
     
     
         6 . The process according to  claim 1 , wherein the poly-α-hydroxyacrylic acid is in the form of its sodium salt. 
     
     
         7 . The process according to  claim 1 , wherein the bleaching treatment is carried out at a temperature equal to or less than 100° C. 
     
     
         8 . The process according to  claim 1 , wherein the bleaching step is carried out at an initial pH of from 8 to 14. 
     
     
         9 . The process according to  claim 1 , wherein the final pH, at the end of the bleaching step, is of from 6 to 11. 
     
     
         10 . The process according to  claim 1 , wherein the duration of the bleaching step is of from 1 to 6 hours. 
     
     
         11 . The process according to  claim 1 , wherein the hydrogen peroxide is used in an amount of 0.1 to 8% by weight of dry pulp. 
     
     
         12 . The process according to  claim 1 , wherein the poly-α-hydroxyacrylic acid, its salt or their mixture is used in an amount of from 0.1 to 2.0% by weight of dry pulp. 
     
     
         13 . The process according to  claim 2 , 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. 
     
     
         14 . The process according to  claim 1  applied to the bleaching of mechanical paper pulp. 
     
     
         15 . A method for controlling the pH of an aqueous suspension of paper pulp during the bleaching treatment of the paper pulp with hydrogen peroxide, wherein said bleaching treatment is conducted in the presence of at least one poly-α-hydroxyacrylic acid, its salt or their mixture, said method comprising using at least one alkaline earth hydroxide selected from the group consisting of magnesium hydroxide and calcium hydroxide. 
     
     
         16 . The process according to  claim 1 , wherein the bleaching treatment is carried out at a temperature from 60 to 95° C.