Process for bleaching kraft pulp

Abstract

A method for bleaching lignocellulose containing pulp which has been chemically pulped is described wherein the method comprises the steps of sequentially treating the pulp with a reducing agent, a chelating agent and an oxidative agent.

I claim: 
     
       1. A method for bleaching lignocellulose containing pulp which has been chemically pulped wherein said method comprises the steps of sequentially treating the pulp with a reducing agent, a chelating agent and an oxidative agent. 
     
     
       2. A method as claimed in claim 1 which comprises: (a) adjusting the consistency of the pulp to be from 1-20 percent;   (b) heating the pulp to be at a temperature in the range of 40 degrees C.-100 degrees C.;   (c) adjusting the pH of the pulp to be in the range of from 4.5-10.0;   (d) adding a reducing agent in an amount of from about 0.1-3 percent based on the oven dried weight of the pulp for a retention time of from 10-360 minutes;   (e) washing said pulp;   (f) adjusting the pH of the pulp to be in the range of 2.5-11;   (g) treating said pulp with a chelating chemical in an amount of from 0.01-1.0 percent by weight based on the oven dried weight of the pulp while maintaining the pulp at a temperature of from 20 degrees-100 degrees C.;   (h) washing said pulp;   (i) treating said pulp with an oxidative chemical; and   (j) neutralizing said pulp to remove excess oxidative chemical.   
     
     
       3. A method as claimed in claim 1 or 2 wherein the reducing agent is selected from the group consisting of sodium hydrosulfite, zinc hydrosulfite, sodium borohydride, sodium formaldehyde bisulfite, sodium glyoxal bisulfite, formamidine sulfinic acid, sulfur dioxide and sodium bisulfite. 
     
     
       4. A method as claimed in claim 1 or 2 wherein said reducing agent is sodium hydrosulfite. 
     
     
       5. A method as claimed in claim 1 or 2 wherein the pH of the pulp for the treating with the chelating chemical is adjusted to be in the range of 4.5 to 9.0. 
     
     
       6. A method as claimed in claim 5 wherein said chelating chemical is selected from the group consisting of nitrogenous polycarboxylic acids, nitrogenous polyphosphonic acids and nitrogenous polyalcohols. 
     
     
       7. A method as claimed in claim 6 wherein said chelating chemical is selected from the group consisting of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid and diethylenetriaminepentamethylphosphonic acid. 
     
     
       8. A method as claimed in claim 2 where in step (i) said oxidative chemical is selected from the group consisting of: (a) inorganic peroxides;   (b) organic peracids and salts thereof selected from the group consisting of C 1  -C 3  aliphatic peracids and C 1  -C 3  aromatic peracids;   (c) inorganic peracids and salts thereof selected from the group consisting of peroxycarbonates, perborates and persulfates, wherein the salts are formed with a cation selected from the group consisting of sodium and potassium; and   (d) chlorine containing compounds selected from the group consisting of chlorine, chlorine dioxide, sodium hypochlorite and hypochlorous acid.   
     
     
       9. A method as claimed in claim 8 wherein the inorganic peroxide is selected from the group consisting of hydrogen peroxide and sodium peroxide. 
     
     
       10. A method as claimed in claim 8 wherein the organic acid is selected from the group consisting of peracetic and performic acids. 
     
     
       11. A method as claimed in claim 8 wherein the inorganic peracid is peroxymonosulfuric acid. 
     
     
       12. A method as claimed in claim 9 further comprising the step of adding at the oxidative step from 0.05-4 percent based on the oven dried weight of the pulp of an alkali selected from the group comprising sodium hydroxide, potassium hydroxide, sodium carbonate and potassium carbonate. 
     
     
       13. A method as claimed in claim 12 for using hydrogen peroxide as the oxidative stage wherein: (a) the pulp consistency in step (i) is from 10 to 20;   (b) the pulp temperature in step (i) is from 75 to 90 degrees C.;   (c) the oxidative chemical is hydrogen peroxide in an amount of 0.1 to 5.0 percent based on the oven dried weight of the pulp and includes the addition of sodium hydroxide in an amount of 0.5 to 3.5 percent by weight based on the oven dried weight of the pulp to yield a pulp pH in the range of 9.5 to 11.5; and   (d) magnesium ion is added at a dosage of 0.01 to 1 percent by weight based on the oven dried weight of the pulp for a retention time of 90-120 minutes.