Multi-step pulp bleaching

Abstract

A method for bleaching cellulosic pulp is disclosed, with one embodiment revealing a three-step bleaching method for cellulosic pulp comprising an activating bleach step, an alkaline peroxide step, and a reductive bleach step. The cellulosic pulp treated according to the present invention may possess a greater GE brightness gain than if treated according to conventional methods, while not suffering a significant loss of lignin. The cellulosic pulp may comprise recycled fibers, including those with high groundwood content. The bleaching method may also decrease the pH of the pulp.

1 . A method for bleaching recycled fibers, comprising (1) providing recycled fibers with a Kappa number of less than about 40; and (2) contacting the recycled fibers with at least an activating bleach step, an alkaline hydroxide step, and a reductive bleach step, wherein the activating bleach step comprises contacting the recycled fibers with at least one activating bleaching agent chosen from peracetic acid, peroxymonosulfuric acid, and an acylamide. 
     
     
         2 . The method according to  claim 1 , wherein the recycled fibers contain groundwood. 
     
     
         3 . The method according to  claim 1 , wherein the method results in an increase of at least 10 GE brightness points from the initial recycled fibers. 
     
     
         4 . The method according to  claim 1 , wherein the activating bleach step further comprises contacting the recycled fibers with oxygen. 
     
     
         5 . The method according to  claim 4 , wherein contacting the recycled fibers with oxygen comprises contacting the recycled fibers with oxygen peroxide. 
     
     
         6 . The method according to  claim 3 , wherein the acylamide comprises N,N,N′,N′-tetraacetylethylenediamine. 
     
     
         7 . The method according to  claim 6 , wherein the acylamide further comprises a peroxy compound. 
     
     
         8 . The method according to  claim 7 , wherein the peroxy compound is hydrogen peroxide. 
     
     
         9 . The method according to  claim 1 , wherein the activating bleaching agent is present in an amount of about 0.1 to 0.2 percent by weight of fibers. 
     
     
         10 . The method according to  claim 1 , wherein the alkaline hydroxide step comprises contacting the recycled fibers with at least one alkaline peroxide agent chosen from a mixture of an alkaline hydroxide and a peroxy compound. 
     
     
         11 . The method according to  claim 10 , wherein the alkaline hydroxide is sodium hydroxide. 
     
     
         12 . The method according to  claim 10 , wherein the peroxy compound is chosen from at least one of hydrogen peroxide and oxygen peroxide. 
     
     
         13 . The method according to  claim 10 , wherein the alkaline hydroxide step further comprises contacting the recycled fibers with at least one of peracetic acid, peroxymonosulfuric acid, and an acylamide. 
     
     
         14 . The method according to  claim 13 , wherein the acylamide comprises N,N,N′,N′-tetraacetylethylenediamine. 
     
     
         15 . The method according to  claim 1 , wherein the reductive bleach step comprises contacting the recycled fibers with a reductive bleaching agent chosen from at least one of formamidine sulfinic acid, hydroxy methyl sulfinic acid, sodium borohydride, and sodium hydrosulfite. 
     
     
         16 . The method according to  claim 1 , wherein the activating bleach step occurs before the alkaline hydroxide step. 
     
     
         17 . The method according to  claim 16 , wherein the recycled fibers are carried from the activating bleach step to the alkaline hydroxide step without removal of the activating bleaching agent. 
     
     
         18 . The method according to  claim 1 , wherein the activating bleach step occurs after the alkaline hydroxide step. 
     
     
         19 . The method according to  claim 1 , wherein the recycled fibers are washed before the reductive bleaching step. 
     
     
         20 . The method according to  claim 19 , wherein the activating bleach step is followed by the alkaline hydroxide step, a wash of the recycled fibers, and then the reductive bleach step. 
     
     
         21 . The method according to  claim 20 , wherein the activating bleach step comprises contacting the recycled fibers with peracetic acid. 
     
     
         22 . The method according to  claim 20 , wherein the alkaline hydroxide step comprises contacting the recycled fibers with a mixture of sodium hydroxide and hydrogen peroxide. 
     
     
         23 . The method according to  claim 20 , wherein the reductive bleach step comprises contacting the recycled fibers with sodium hydrosulfite. 
     
     
         24 . The method according to  claim 1 , wherein the activating bleach step and the alkaline hydroxide step occur simultaneously. 
     
     
         25 . The method according to  claim 1 , wherein the time, temperature, and concentration of the activating bleach step and the alkaline hydroxide step are controlled such that the change in Kappa number of the recycled fibers is no more than about 10 points but the increase in GE brightness of the recycled fibers is at least about eight points. 
     
     
         26 . The method according to  claim 1 , wherein the recycled fibers experience a decrease in Kappa number of no more than about 10. 
     
     
         27 . The method of  claim 1 , wherein the recycled fibers contact the activating bleaching agent at a temperature of at least 170° F. 
     
     
         28 . The method of  claim 28 , wherein the method further comprises a filtrate step. 
     
     
         29 . The method of  claim 26 , wherein the filtrate step comprises contacting the recycled fibers with a mixture of filtrate from the alkaline hydroxide step and an acylamide. 
     
     
         30 . The method of  claim 29 , wherein the acylamide comprises N,N,N′,N′-tetraacetylethylenediamine. 
     
     
         31 . A method for bleaching recycled fibers, comprising (1) providing recycled fibers with a Kappa number of less than about 40, and (2) contacting the recycled fibers with peracetic acid, an alkaline oxygen peroxide, and a reductive bleaching agent in one or more steps. 
     
     
         32 . The method of  claim 31 , wherein the peracetic acid is produced by the reaction of N,N,N′,N′-tetraacetylethylenediamine, hydrogen peroxide, and a caustic, optionally including a chelant. 
     
     
         33 . The method of  claim 32 , wherein the caustic is sodium hydroxide. 
     
     
         34 . The method of  claim 32 , wherein the chelant is dimyristoleoyl phosphonomethyl trimethyl ammonium. 
     
     
         35 . The method of  claim 31 , wherein the alkaline oxygen peroxide comprises at least one of hydrogen peroxide and sodium peroxide. 
     
     
         36 . The method of  claim 31 , wherein the contacting occurs at a temperature of 170° F. or less. 
     
     
         37 . The method of  claim 33 , wherein said contacting occurs at a temperature of 140° F. or less. 
     
     
         38 . The method of  claim 31 , wherein the reductive bleaching agent comprises at least one of sodium hydrosulfite and hydroxymethane sulfinic acid. 
     
     
         39 . The method of  claim 31 , wherein the recycled fibers are washed before contacting the reductive bleaching agent. 
     
     
         40 . The method of  claim 31 , wherein the alkaline oxygen peroxide contacts the recycled fibers after the peracetic acid. 
     
     
         41 . The method of  claim 40 , wherein the reductive bleach agent contacts the recycled fibers after the alkaline oxygen peroxide. 
     
     
         42 . A method for bleaching cellulosic pulp, comprising:
 providing cellulosic pulp with a Kappa number of less than about 40;   contacting the cellulosic pulp with a first step chosen from one of an activating bleach step and an alkaline hydroxide step;   contacting the cellulosic pulp with a second step, different from the first step, chosen from one of an activating bleach step and an alkaline hydroxide;   collecting the filtrate from the second step; and, contacting the cellulosic pulp with a reductive bleach step after the second step,   
       wherein the first step including contacting the cellulosic pulp with the filtrate from the second step, and wherein the activating bleach step comprises contacting the cellulosic pulp with at least one activating bleaching agent chosen from peracetic acid, peroxymonosulfuric acid, and an acylamide. 
     
     
         43 . The method of  claim 42 , wherein the cellulosic pulp is recycled pulp. 
     
     
         44 . The method of  claim 42 , wherein the at least one activating bleaching agent is peracetic acid. 
     
     
         45 . The method of  claim 44 , wherein the peracetic acid is produced by the combination of N,N,N′,N′-tetraacetylethylenediamine, hydrogen peroxide, a caustic, and optionally a chelant. 
     
     
         46 . The method of  claim 45 , wherein the caustic is hydrogen peroxide. 
     
     
         47 . The method of  claim 45 , wherein the chelant is dimyristoleoyl phosphonomethyl trimethyl ammonium. 
     
     
         48 . The method of  claim 45 , wherein the filtrate from the second step comprises N,N,N′,N′-tetraacetylethylenediamine. 
     
     
         49 . The method of  claim 42 , wherein the alkaline hydroxide step comprises contacting the cellulosic pulp with at least one alkaline peroxide agent. 
     
     
         50 . The method of  claim 49 , wherein the at least one alkaline peroxide agent is sodium hydroxide. 
     
     
         51 . The method of  claim 49 , wherein the alkaline hydroxide step further comprises contacting the cellulosic pulp with at least one peroxy compound. 
     
     
         52 . The method of  claim 51 , wherein the at least one peroxy compound is chosen from hydrogen peroxide and oxygen peroxide. 
     
     
         53 . The method of  claim 42 , wherein the reductive bleach step comprises contacting the cellulosic pulp with at least one reductive bleaching agent. 
     
     
         54 . The method of  claim 53 , wherein the at least one reductive bleaching agent is chosen from sodium hydrosulfite and hydroxymethane sulfinic acid. 
     
     
         55 . A method for reducing the pH of a cellulosic pulp, comprising:
 providing cellulosic pulp with a pH of greater than about 10;   contacting the cellulosic pulp with a first step chosen from one of an activating bleach step and an alkaline hydroxide step;   contacting the cellulosic pulp with a second step, different from the first step, chosen from one of an activating bleach step and an alkaline hydroxide; and, contacting the cellulosic pulp with a reductive bleach step after the second step,   
       wherein the pH of the cellulosic pulp after the reductive bleach step is less than about 8, and wherein the activating bleach step comprises contacting the cellulosic pulp with at least one activating bleaching agent chosen from peracetic acid, peroxymonosulfuric acid, and an acylamide. 
     
     
         56 . A process for providing pulp to a paper-making machine, comprising:
 subjecting a cellulosic pulp to an alkaline peroxide step, comprising contacting cellulosic pulp at a pH greater than about 8 with at least one alkaline peroxide agent;   thereafter subjecting the cellulosic pulp to an activating bleach step comprising contacting the cellulosic pulp with peracetic acid, wherein the activating bleach step reduces the pH of the cellulosic pulp;   thereafter subjecting the cellulosic pulp to a reductive bleach step comprising contacting the cellulosic pulp with sodium hydrosulfite, wherein the reductive bleach step reduces the pH of the cellulosic pulp to about 6.5 to about 8; and, thereafter passing the cellulosic pulp to a paper-making machine.   
     
     
         57 . The process of  claim 56 , wherein the pH of the cellulosic pulp monotonically declines from at least about 10.5 to from about 6.5 to about 8. 
     
     
         58 . A process for providing cellulosic pulp to a paper-making machine, comprising:
 (i) subjecting a cellulosic pulp to an alkaline peroxide step at a pH greater than about 8;   (ii) thereafter subjecting the cellulosic pulp to an activating bleach step, wherein the activating bleach step reduces the pH of the cellulosic pulp;   (iii) thereafter subjecting the cellulosic pulp to a reductive bleach step, wherein the reductive bleach step reduces the pH of the cellulosic pulp to about 6.5 to about 8; and,   (iv) thereafter passing the cellulosic pulp to a paper-making machine.   
     
     
         59 . The process of  claim 58 , wherein the pH of the cellulosic pulp before the alkaline peroxide step is at least about 9.5. 
     
     
         60 . The process of  claim 58 , wherein the activating bleach step reduces the pH of the cellulosic pulp to about 9.5. 
     
     
         61 . The process of  claim 58 , wherein the activating bleach step reduces the pH of the cellulosic pulp to about 9 to about 10.5. 
     
     
         62 . The process of  claim 58 , wherein the pH of the cellulosic pulp before alkaline peroxide step is at least about 10.5 and the pH of the cellulosic pulp monotonically declines to from about 6.5 to about 8 before passing to the paper-making machine. 
     
     
         63 . The process of  claim 58 , wherein the alkaline peroxide step comprises contacting the cellulosic pulp with at least one alkaline peroxide agent. 
     
     
         64 . The process of  claim 63 , wherein the activating bleaching step comprises contacting the cellulosic pulp with peracetic acid. 
     
     
         65 . The process of  claim 64 , wherein the reductive bleaching step comprises contacting the cellulosic pulp with sodium hydrosulfite. 
     
     
         66 . The process of  claim 58 , wherein the pH of the cellulosic pulp before the alkaline peroxide step is about 10.5, the pH of the cellulosic pulp before the activating bleach step is about 9.5, and the pH of the cellulosic pulp before the reductive bleach step is about 8.5. 
     
     
         67 . A process for bleaching cellulosic pulp, comprising subjecting a cellulosic pulp with a GE brightness of less than about 67 and a pH of greater than about 9 to an activating bleach step and at least one step chosen from an alkaline peroxide step and a reductive bleach step, whereafter the cellulosic pulp has a GE brightness of at least about 74 and a pH of less than about 7.5. 
     
     
         68 . The process of  claim 67 , wherein the steps are an activating bleach step and an alkaline peroxide step. 
     
     
         69 . The process of  claim 68 , wherein the activating bleach step comprises contacting the cellulosic pulp with peracetic acid and the alkaline peroxide step comprises contacting the cellulosic pulp with at least one alkaline peroxide agent. 
     
     
         70 . The process of  claim 67 , wherein the pH of the cellulosic pulp monotonically declines while the cellulosic pulp is subjected to the activating bleach step and the at least one step. 
     
     
         71 . The process of  claim 67 , wherein the cellulosic pulp comprises recycled fibers.