Process for the bleaching of mechanical paper pulp

Abstract

The invention relates to a process for the bleaching of paper pulp with one or more peroxide oxidizing agents, in which the pulp is first treated in a pretreatment step with at least one inorganic magnesium derivative combined with at least one carboxylic acid or its salt, except its magnesium salt, or with at least one carboxylic acid at least partially saponified with Mg 2+ , the carboxylic acid being substituted by at least one electrodonating group or comprising at least one carbon-carbon double bond.

1 . A process for the bleaching of paper pulp with one or more peroxide oxidizing agents, comprising first treating the pulp in a pretreatment step with one or more chelating agents chosen selected from the group consisting of
 (i) carboxylic acids or their salts, except their magnesium salt, substituted by at least one electrodonating group or comprising at least one carbon-carbon double bond, in the presence of at least one inorganic magnesium derivative, and   (ii) carboxylic acids at least partially saponified with Mg 2+  and substituted by at least one electrodonating group or comprising at least one carbon-carbon double bond.   
     
     
         2 . The process according to  claim 1 , wherein the electrodonating group is selected from the group consisting of amines, alcohols, thiols, O-alkyl, and S-alkyl. 
     
     
         3 . The process according to  claim 1 , wherein the carboxylic acid has the following general formula
   HOOC—C n H m (OR 1 ) p (SR 2 ) q (NR 3 R 4 ) r (COOH) s —R′
   wherein   C n H m  is linear or branched and saturated or unsaturated;   n is an integer from 1 to 8;   m is an integer from 0 to 2n;   p is an integer from 0 to n;   q is an integer from 0 to n;   r is an integer from 0 to n;   s is an integer from 0 to n;   p+q+r+s is an integer from 0 to 2n, and if p+q+r+s is 0, m is ≦2n−2;   R′ is selected from the group consisting of —H, —CH 2 OH, and —COOH; and   R 1 , R 2 , R 3 , R 4  are the same or different and are selected from the group consisting of —H and linear or branched groups of formula —C n′ H m′ —R″, wherein n′ is an integer from 1 to 8, m′ is an integer from 0 to 2n, and R″ is selected from the group consisting of —H, —CH 2 OH, and —COOH.   
     
     
         4 . The process according to  claim 1 , wherein the carboxylic acid is selected from the group consisting of lactic acid, citric acid, tartaric acid, glucaric acid, maleic acid, nitrilotriacetic acid, methylglycine diacetic acid, their salts, and mixtures thereof. 
     
     
         5 . The process according to  claim 1 , wherein the inorganic magnesium derivative is selected from the group consisting of magnesium oxide, magnesium hydroxide, and magnesium sulfate. 
     
     
         6 . The process according to  claim 1 , wherein the at least one chelating agent is the carboxylic acid at least partially saponified with Mg 2+  (ii). 
     
     
         7 . The process according to  claim 1 , wherein the pretreatment step is performed at the natural pH of the paper pulp. 
     
     
         8 . The process according to  claim 1 , wherein the concentration of the inorganic magnesium derivative, expressed as Mg 2+  ions, is less than or equal to 1% by weight of the dry paper pulp and is more than or equal to 0.01% by weight of the dry paper pulp. 
     
     
         9 . The process according to  claim 1 , wherein the concentration of the carboxylic acid or its salt (i) or of the carboxylic acid at least partially saponified with Mg 2+  (ii) is less than or equal to 1% by weight of the dry pulp and is more than or equal to 0.2% by weight of the dry pulp. 
     
     
         10 . The process according to  claim 1 , being applied to a mechanical paper pulp. 
     
     
         11 . The process according to  claim 1 , wherein the pretreatment step is carried out in the presence of water in order to reach a consistency of 1 to 20% by weight of dry pulp. 
     
     
         12 . The process according to  claim 1 , comprising the additional steps of:
 (a) pressing the mixture resulting from the pretreatment step until a consistency of from 25 to 50% by weight of dry pulp is reached, and   (b) bleaching the pressed mixture resulting from step (a) with a peroxide oxidizing agent.   
     
     
         13 . The process according to  claim 1 , wherein the peroxide oxidizing agent is selected from the group consisting of hydrogen peroxide, organic peracids, and a combination thereof. 
     
     
         14 . A method of use as chelating agents of
 (i) a mixture of at least one carboxylic acid or salt thereof, except its magnesium salt, combined with at least one inorganic magnesium derivative, or   (ii) at least one carboxylic acid at least partially saponified with Mg 2+ ,   the carboxylic acids being substituted by at least one electrodonating group or comprising at least one carbon-carbon double bond.   
     
     
         15 . The method according to  claim 14 , being carried out in a pretreatment step of a process for the bleaching of paper pulp with one or more peroxide oxidizing agents.