Sequential bleaching of kraft pulp with chlorine dioxide followed by chlorine

Abstract

There is provided a process for the sequential bleaching of kraft pulp comprising reacting kraft pulp with chlorine dioxide until the chlorine dioxide is .[.at least partially.]. .Iadd.more than 50% .Iaddend.depleted, reacting the kraft pulp with chlorine .Iadd.without an intermediate wash.Iaddend., and treating the kraft pulp with caustic. .[.Via this process bleached kraft pulps with exceptionally good brightness and strength characteristics are obtained..].

What is claimed is: 
     
       1. A process for the sequential bleaching of kraft pulp, comprising the steps of sequentially: (a) reacting kraft pulp dispersed in aqueous slurry with chlorine dioxide until the chlorine dioxide is .[.at least partially.]. .Iadd.more than 50 percent .Iaddend.depleted, and then reacting the kraft pulp with chlorine, .Iadd.without an intermediate wash, .Iaddend.wherein: (1) the total available chlorine added by the chlorine dioxide and chlorine is from about 1 to about 10 percent by weight of the kraft pulp,   (2) from about 5 to about 90 percent of the total available chlorine is furnished by chlorine dioxide, the remaining available chlorine being furnished by chlorine,   (3) the chlorine dioxide is reacted with the kraft pulp for from about 2 to about 75 minutes, and the chlorine is reacted with the kraft pulp for from about 2 to about 120 minutes, and   (4) the reactions are run at a temperature of from about 0 to about 100 degrees centigrade, thereby forming a neutral to acidic slurry of pulp material with a consistency of from about 2 to about 20 percent (by weight of pulp); and     (b) treating said slurry of pulp material with caustic at a temperature of from about 0 to about 100 degrees centigrade for about 15 minutes to about 3 hours until the pH of the slurry is from about 7 to about 12. .[.2. The process of claim 1, wherein the chlorine dioxide and chlorine are sequentially reacted with the kraft pulp without an intermediate wash..].   
     
     
         . The process of claim .[.2.]. .Iadd.1.Iaddend., wherein: (a) from about 10 to about 80 percent of total available chlorine is furnished by chlorine dioxide, the total available chlorine added by the chlorine dioxide and chlorine being from about 3 to about 10 percent by weight of the kraft pulp;   (b) the reactions of the kraft pulp with chlorine dioxide and chlorine and the treatment of the kraft pulp with caustic are conducted at a temperature of from about 5 to about 70 degrees centigrade; and   (c) the caustic treatment is conducted for from about 30 to about 120   
     
     
        minutes until the pH of the slurry is from about 8 to about 11. 4. The process of claim 3, wherein the chlorine dioxide and chlorine are sequentially reacted with the kraft pulp without an intermediate alkali 
     
     
        extraction. 5. The process of claim 4, wherein, after the kraft pulp is treated with caustic, it is subsequently bleached with from about 0.1 to about 2 percent (by weight of .[.available chlorine content.]. .Iadd.pulp.Iaddend.) of a bleaching reagent selected from the group 
     
     
        consisting of chlorine dioxide and hypochlorite. 6. The process of claim 5, wherein, after the kraft pulp has been treated with caustic, it is subsequently bleached with chlorine dioxide at a temperature of from about 50 to about 90 degrees centigrade for from about 1 to about 5 hours until 
     
     
        the pH of the slurry is from about 2 to about 6. 7. The process of claim 5, wherein, after the kraft pulp has been treated with caustic, it is subsequently bleached with hypochlorite at a temperature of from about 20 to about 70 degrees for from about 1 to about 4 hours until the pH of the 
     
     
        slurry is from about 8 to about 12. 8. The process of claim 6, wherein the subsequent chlorine dioxide bleaching is conducted at a temperature of from about 65 to about 75 degrees centigrade until the pH of the slurry is 
     
     
        from about 3 to about 5. 9. The process of claim 7, wherein the subsequent hypochlorite bleaching is conducted at a temperature of from about 30 to about 50 degrees centigrade until the pH of the slurry is from about 9 to 
     
     
        about 11. 10. The process of claim 8, wherein the caustic treatment is conducted for from about 1 to about 2 hours. .Iadd. 11. The process of claim 1, wherein the sequential chlorine dioxide and chlorine bleachings are effected in a first bleaching stage and 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.