Methods of treating chemical cellulose pulp

Abstract

A single pulp bleaching stage comprises an acid treatment, A, a neutralization treatment, n, and a chlorine dioxide treatment, D 0 . The AnD 0 bleaching stage may optionally include washing between treatments, either between each treatment or only some treatments, represented by A-n-D 0 which indicates there is washing between the acid (A) and the neutralization (n) treatments and the neutralization (n) and the chlorine dioxide (D) treatments, or An-D 0 which indicates there is washing only between the neutralization (n) and the chlorine dioxide (D) treatments. The AnD 0 stage can be followed by washing and further bleaching stages. Preferably, the (AnD 0 ) stage is followed by an extraction stage E op with caustic in the presence of oxygen and a peroxide.

1 . A method of treating chemical cellulose pulp produced by alkaline delignification, comprising: 
 (a) acid treating the pulp at a pH of between about 1 to about 5 and at a temperature of between about 40 to about 100° C.;    (b) neutralizing the acid treated pulp according to step (a) with an alkaline chemical to achieve a pH of between about 7 to about 14; and thereafter    (c) bleaching the neutralized pulp.    
   
   
       2 . The method of  claim 1 , wherein step (c) is practiced by subjecting the neutralized pulp to at least one chlorine dioxide (D 0 ) bleaching stage.  
   
   
       3 . The method of  claim 1 , further comprising (d) washing the pulp after the acid treatment stage.  
   
   
       4 . The method of  claim 3 , wherein the step of (d) washing the pulp is practiced between steps (a) and (b).  
   
   
       5 . The method of  claim 3 , wherein the step of (d) washing the pulp is practiced between steps (b) and (c).  
   
   
       6 . The method of  claim 3 , wherein the step of (d) washing the pulp is practiced between steps (a) and (b) and between steps (b) and (c).  
   
   
       7 . The method of  claim 1 , wherein step (c) comprises bleaching the pulp with chlorine dioxide at a relatively high pH of between about 4 to about 7 (D HI ), and thereafter bleaching the pulp with chlorine dioxide at a relatively low pH of between 1 to about 4 (D LO ).  
   
   
       8 . The method of  claim 7 , wherein D HI  and D LO  are practiced without intermediate washing.  
   
   
       9 . The method of  claim 1 , wherein the chemical cellulose pulp has a kappa number of less than 24, and a solids consistency of between about 0.1 to about 50%, and wherein step (a) is practiced for a time sufficient to remove at least 50% of the hexenuronic acid and to reduce the kappa number of the pulp by at least 2 units.  
   
   
       10 . A method of treating chemical cellulose pulp produced by alkaline delignification, consisting essentially of: 
 (a) acid treating the chemical cellulose pulp at a pH of between about 1 to about 5 and at a temperature of between about 40 to about 100° C.;    (b) neutralizing the pulp treated in step (a) with an alkaline chemical to achieve a pH of between about 7 to about 14;    (c) washing the neutralized pulp; and thereafter (d) bleaching the washed and neutralized pulp.    
   
   
       11 . The method of  claim 10 , further consisting essentially of (a1) washing the pulp between steps (a) and (b).