Bleaching process with at least one extraction stage

Abstract

An improved pulp bleaching process having at least one Do delignification stage followed by at least one extraction stage is selected from the group consisting of Eo, Eop, Ep, and E extraction stage. The process comprises treating the delignified pulp Do pulp from the Do stage in a M DE mixing stage with a filtrate from at least one extraction stage after the Do stage prior to treatment of the delignified pulp in the least one extraction stage, preferably with interstage washing.

1 . A pulp bleaching process comprising at least one D 0  stage and at least one extraction stage selected from a group consisting of Eo, Eop, Ep, and E extraction stage, the process comprising:
 treating a D 0  pulp from the D 0  in a M DE  mixing stage with a extraction filtrate from the at least one extraction stage prior to treatment of the D 0  pulp in the least one extraction stage wherein:   
     Do is a delignification stage in which pulp is treated with an agent comprising chlorine dioxide preferably no or substantially no elemental chlorine dioxide (ECF);
 E 0  is an extraction stage in which a pulp is extracted with base in the presence of oxygen; 
 E is an extraction stage in which a pulp is extracted with a base; 
 Eop is an extraction stage in which pulp is treated with a composition comprising oxygen, base, and peroxide; 
 Ep is an extraction stage in which pulp is treated with a composition comprising base and peroxide; 
 and 
 M DE  is a mixing stage in which delignified pulp is treated with a filtrate from the at least one extraction stage. The M DE  can be accomplished in the bottom dilution zone of the D 0  tower (for a down flow D 0  tower) or a pipe to provide contact and mixing of extraction filtrate and D 0  pulp slurry. 
 
   
   
       2 . The pulp bleaching process of  claim 1  wherein the M DE  mixing stage is conducted in a separate tower at a point between a chlorination washing step and a step of mixing the pulp with an alkaline extracting solution. 
   
   
       3 . The pulp bleaching process of  claim 1  wherein the M DE  mixing stage is conducted in the dilution zone at bottom of a down flow D 0  tower or a pipe to provide contact and mixing of the extraction filtrate and D 0  pulp. 
   
   
       4 . The pulp bleaching process of  claim 1  wherein the pulp has a pH of greater than 2.5. 
   
   
       5 . The pulp bleaching process of  claim 1  wherein the pulp in the M DE  mixing stage has a pH of from about 4.5 and to about 2.5. 
   
   
       6 . The pulp bleaching process of  claim 1  wherein the pulp in the M DE  mixing stage has a pH from about 3.5 to about 6.0. 
   
   
       7 . The pulp bleaching process of  claim 1  wherein the pulp in the M DE  mixing stage has a pH from about 3.5 to about 4.5. 
   
   
       8 . The pulp bleaching process of  claim 1  wherein the pulp in the M DE  mixing stage has a consistency from about 5 to about 30. 
   
   
       9 . The pulp bleaching process of  claim 1  wherein the pulp in the M DE  mixing stage has a consistency from about 10 to about 20. 
   
   
       10 . The pulp bleaching process of  claim 1  wherein the pulp in the M DE  mixing stage has a consistency from about 10 to about 15. 
   
   
       11 . The pulp bleaching process of  claim 1  wherein the pulp in the M DE  mixing stage has a retention time of at least about 2 minutes. 
   
   
       12 . The pulp bleaching process of  claim 1  wherein the pulp in the M DE  mixing stage has a retention time from about 2 minutes to about 120 minutes. 
   
   
       13 . The pulp bleaching process of  claim 1  wherein the pulp in the M DE  mixing stage has a retention time from about 5 minutes to about 60 minutes. 
   
   
       14 . The pulp bleaching process of  claim 1  wherein the pulp in the M DE  mixing stage has a retention time from about 5 minutes to about 15 minutes. 
   
   
       15 . The pulp bleaching process of  claim 1  wherein the pulp in the M DE  mixing stage has a treatment temperature from about 50° C. to about 85° C. 
   
   
       16 . The pulp bleaching process of  claim 1  wherein the pulp in the M DE  mixing stage has a treatment temperature from about 60° C. to about 80° C. 
   
   
       17 . The pulp bleaching process of  claim 1  wherein the pulp in the M DE  mixing stage has a treatment temperature from about 65° C. to about 70° C. 
   
   
       18 . The pulp bleaching process of  claim 1  further comprising bleaching the pulp from the extraction stage in at least one D 1  bleaching stage, the process comprising chorine dioxide. 
   
   
       19 . The pulp bleaching process of  claim 1  wherein an amount of NaOH used in the extraction stage is reduced as compared to an amount of NaOH used in the same or substantially the same bleaching processes which do not include the M DE  mixing stage. 
   
   
       20 . The pulp bleaching process of  claim 19  wherein the reduction in the amount of NaOH is from about 15% to about 30% less than the amount of NaOH used in the same or substantially the same bleaching processes which do not include the M DE  mixing stage. 
   
   
       21 . The pulp bleaching process of  claim 18  wherein an amount of ClO 2  used in the D 1  bleaching stage is reduced as compared to the amount of ClO 2  used in the same or substantially the same bleaching processes which do not include the M DE  mixing stage. 
   
   
       22 . The pulp bleaching process of  claim 21  wherein a reduction in the amount of ClO 2  is from about 5% to about 20% less than the amount of ClO 2  used in the same or substantially the same bleaching processes which do not include the mixing stage (M DE ) to obtain the same or substantially level of pulp brightness. 
   
   
       23 . The improved pulp bleaching process of  claim 1  wherein an amount of AOX resulting from the extraction stage is reduced as compared to the amount of AOX used in the same or substantially the same bleaching processes which do not include the M DE  mixing stage. 
   
   
       24 . The improved pulp bleaching process of  claim 23  wherein the reduction in the amount of AOX is from about 10% to about 50%.