Woodpulp bleaching process

Abstract

There is provided a process for the bleaching of woodpulp comprising reacting the pulp with chlorine dioxide until the chlorine dioxide is .[.at least partially.]. .Iadd.more than 50% .Iaddend.depleted and, thereafter, .Iadd.without an intermediate wash, .Iaddend.reacting the woodpulp with chlorine. .[.The process provides bleached pulps with excellent strength brightness, and color reversion properties..].

What is claimed is: 
     
       1. A process for the bleaching of woodpulp, comprising: (a) reacting woodpulp with chlorine dioxide until said chlorine dioxide is .[.at least partially.]. .Iadd.more than 50 percent .Iaddend.depleted and .[.,.]. thereafter, .Iadd.without an intermediate wash, .Iaddend.   (b) reacting the woodpulp with chlorine, wherein (1) the woodpulp is dispersed in an aqueous slurry.Iadd., .Iaddend.which slurry, after being treated with said chlorine dioxide.Iadd., .Iaddend.is rendered neutral to acidic;   (2) the temperature during said chlorine dioxide and chlorine reactions is maintained at from about 5 to about 100 degrees centigrade; and   (3) from about 2 to about 90 percent of the total available chlorine added is added as chlorine dioxide.     
     
     
       2. The process of claim 1, wherein: (a) from 1 to about 10 percent (by weight of woodpulp) of available chlorine is provided by the chlorine and chlorine dioxide; and   (b) the total reaction time for said process is up to about three hours.   
     
     
       3. The process of claim 2, wherein: (a) from about 20 to about 80 percent of the total available chlorine is added as chlorine dioxide;   (b) the chlorine dioxide is reacted with the woodpulp for from two to about 75 minutes; and   (c) the chlorine is reacted with the woodpulp for from about two minutes to two hours.   
     
     
       4. The process of claim 3, wherein: (a) the total reaction time for said process is from about 5 minutes to about two hours; and   (b) the chlorine is reacted approximately twice as long as is the chlorine dioxide. .Iadd. 5. The process of claim 4, wherein:   (a) the total reaction time for said process is about one hour;   (b) the chlorine dioxide is reacted with the woodpulp for about twenty minutes; and   (c) the chlorine is reacted with the treated woodpulp in the same stage for about 40 minutes. .Iaddend..Iadd. 6. The process of claim 5, wherein:   (a) the sequential chlorine dioxide and chlorine bleaching in the same stage is effected in the first bleaching stage; and   (b) such stage is followed by a washing, a caustic treatment and at least one additional bleaching stage wherein the bleaching agent is selected from the group consisting of chlorine dioxide, chlorine, hypochlorite, peroxide and mixtures thereof. .Iaddend..Iadd. 7. The process of claim 6, wherein:   (a) a total of about 7 percent of available chlorine is added in the first stage, with about 77 percent thereof being from chlorine dioxide and the balance being from chlorine;   (b) the reaction temperature is at about 10 to about 50 degrees centigrade in the first stage;   (c) caustic treatment is effected with sodium hydroxide in a second stage; and   (d) said treatment is followed by a third stage chlorine dioxide bleach, a fourth stage sodium hydroxide extraction and a fifth stage chlorine dioxide bleach. .Iaddend. .Iadd. 8. The process of claim 1, wherein:   (a) the sequential chlorine dioxide and chlorine bleachings are effected in a first bleaching stage; and   (b) such stage is followed by a caustic extraction and at least one additional bleaching stage wherein the bleaching agent is selected from the group consisting of chlorine dioxide, chlorine, hypochlorite, peroxide and mixtures thereof. .Iaddend..Iadd. 9. A process of claim 8, wherein:   (a) the caustic extraction after the first stage sequential chlorine dioxide and chlorine bleachings is effected with sodium hydroxide; and   (b) said extraction is followed by a plurality of bleaching stages and sodium hydroxide extraction stages intermediate said bleaching stages. .Iaddend..Iadd. 10. The process of claim 9, wherein:   (a) a total of about 7 percent of available chlorine is added in the first stage;   (b) the chlorine reaction time is greater than the chlorine dioxide reaction time; and   (c) the reaction temperature is at about 10 to 50 degrees centigrade in said stage. .Iaddend.