Process of bleaching hardwood pulps in a D1 or D2 stage in a presence of a weak base

Abstract

This invention relates to an improved bleaching process for bleaching pulp comprising at least one bleaching stage which comprises treating a hardwood pulp with a bleaching agent comprising ClO2 in the presence of a weak base such as, for example, Mg(OH) 2 preferably at pH from about 3.5 to about 6.5. The invention is also relates a bleaching process for bleaching pulp having two or more bleaching stages, at least one of which and preferably two of which comprises treating a pulp with a bleaching agent comprising ClO2 in the presence of a weak base such as, for example, Mg(OH) 2 preferably at pH from about 3.5 to about 6.5.

1 . An improved bleaching process for bleaching pulp comprising:
 at least one bleaching stage which comprises treating a hardwood pulp with a bleaching agent comprising ClO2 in the presence of a weak base at pH from about 3.5 to about 6.5.   
     
     
         2 . The bleaching process of  claim 1  wherein the weak base is selected from the group consisting of NaH 2 PO 3 , Ca(OH) 2 , NH 4 OH, NaHCO3, HOCCH3- and Mg(OH) 2 . 
     
     
         3 . The bleaching process of  claim 1  wherein the weak base is Mg(OH) 2 . 
     
     
         4 . The bleaching process of  claim 1  wherein the pulp pH is from about 4.5 to about 5.50. 
     
     
         5 . The bleaching process of  claim 1  wherein the pulp consistency is from about 10% to about 20%. 
     
     
         6 . The bleaching process of  claim 1  wherein the retention time is from about 10 min. to about 300 min. 
     
     
         7 . The bleaching process of  claim 1  wherein the temperature is from about 55° C. to about 85° C. 
     
     
         8 . The bleaching process of  claim 1  wherein the pulp beaching efficiency is about 5% greater than the same bleaching process in which NaOH is used in at least one bleaching rather than Mg(OH) 2 . 
     
     
         9 . The bleaching process of  claim 1  wherein an amount of dirt resulting from the at least one bleaching stages is at least 15% greater than the same or substantially the same bleaching processes which do not include the Mg(OH) 2 . 
     
     
         10 . The bleaching process of  claim 1  wherein pulp viscosity is at least about 3% greater than the viscosity of the pulp made by the same or substantially the same bleaching processes without using NaOH. 
     
     
         11 . The bleaching process of  claim 1  wherein pulp brightness is at least about 10% greater than the brightness of the pulp made by the same or substantially the same bleaching processes using NaOH. 
     
     
         12 . The bleaching process of  claim 1  wherein the amount of ClO 2  used in the at least one bleaching stage is from about 0.1% to about 0.5%. 
     
     
         13 . The bleaching process of  claim 1  further comprising at least one extraction stage carried out in a D o  stage, E stage, Eo stage, Ep stage, and Eop stage and combination thereof. 
     
     
         14 . The bleaching process of  claim 13  having a bleaching sequence selected from the group consisting of DoEopD n , ODoEopD n , DoEopD 1 D 2 , ODoEopD 1 D 2 , DoEopD 1 EpD 2 , ODoEopD 1 EpD 2 , DoEopD 1 P, O(D 0 /C)EopD 1 , D 0 EopD 1 , D 0 EOPD 1 , D 0 EopED 1 , D 0 ED 1 EpEopD 2 , ZED 0 Eop, ZD o EopD 1 , D 0 EpZEop, D 0 EpZD 1 Z, D 0 D 1 EopPP, D 0 D 1 EopZ, DoEopD 1 , ODoEopD 1 , DoEopD 1 , ODoEopD 1 , DoEopD 1 EpD 2 , ODoEopD 1 EpD 2 , and DEopD 1 P, wherein E, Eo, Ep, Eop, Z, O are defined as: Eo is defined as treating the pulp with oxygen in presence of a base, E is defined as treating the pulp in the presence of a base, Ep is defined as treating the pulp with peroxide in presence of a base, Eop is defined as treating the pulp with oxygen and peroxide in presence of a base, Z is ozone, and O is Oxygen. 
     
     
         15 . An improved bleaching process for bleaching pulp comprising:
 at least one extraction stage and at least one bleaching stage wherein the least one bleaching stage comprises bleaching a hardwood pulp with a bleaching agent comprising ClO2 in the presence of a weak base at pH of about 3.5 to about 6.5.   
     
     
         16 . The bleaching process of  claim 1  wherein the weak base is selected from the group consisting of NaH 2 PO 3 , Ca(OH) 2 , NH 4 OH, NaHCO3, HOCCH3- and Mg(OH) 2 . 
     
     
         17 . The bleaching process of  claim 15  wherein the weak base is Mg(OH) 2 . 
     
     
         18 . An improved bleaching process for bleaching pulp having two or more bleaching stages, at least one of which comprises treating a pulp with a bleaching agent comprising ClO2 in the presence of a weak base. 
     
     
         19 . The bleaching process of  claim 17  wherein the weak base is Mg(OH) 2 . 
     
     
         20 . The bleaching process of  claim 17  wherein the pulp pH is from about 4.5 to about 5.50.