Methods for the oxygen-based delignification of pulp

Abstract

Pulp is delignified by forming a mixture of pulp and caustic, adding oxygen and feeding this oxygen-containing mixture to a first step reactor. The lignin in the oxygen-containing mixture will be partially delignified and will be fed to a second step reactor where the remainder of the lignin in the pulp will be delignified. In this manner, fast reacting lignin can be treated in the first step reactor and slow reacting lignin can be treated in the second reactor. The delignified pulp is recovered from the apparatus and after washing can be forwarded to a bleaching unit.

Having thus described the invention, what I claim is: 
     
         1 . A method for delignifying pulp comprising the steps of:
 (a) Forming a mixture of the pulp and caustic;   (b) Feeding oxygen to the mixture and feeding the oxygen-containing mixture to a first step reactor wherein lignin will delignify;   (c) Feeding the oxygen-containing mixture to a second step reactor wherein lignin will delignify; and   (d) Recovering the delignified pulp.   
     
     
         2 . The method as claimed in  claim 1  wherein the lignin in step b) is fast reacting lignin. 
     
     
         3 . The method as claimed in  claim 1  wherein the lignin in step c) is slow reacting lignin. 
     
     
         4 . The method as claimed in  claim 1  wherein the pulp is selected from the group consisting of medium and high consistency pulp. 
     
     
         5 . The method as claimed in claim wherein the caustic is sodium hydroxide 
     
     
         6 . The method as claimed in  claim 1  wherein the oxygen is greater than 80% purity. 
     
     
         7 . The method as claimed in  claim 1  wherein steam is added to the mixture of pulp and caustic before being fed into the first step reactor. 
     
     
         8 . The method as claimed in  claim 1  wherein the oxygen-containing mixture comprises a gas phase and a liquid phase. 
     
     
         9 . The method as claimed in  claim 8  wherein the first step reactor is a reactor where the gas phase is the continuous phase and the liquid phase is the dispersed phase. 
     
     
         10 . The method as claimed in  claim 1  wherein supplemental oxygen may be added to the mixture of pulp and caustic after step (b). 
     
     
         11 . The method as claimed in  claim 1  wherein the oxygen-containing mixture is present in the first step reactor for about 0.25 to 5 minutes. 
     
     
         12 . The method as claimed in  claim 1  wherein the second step reactor is a reactor column. 
     
     
         13 . The method as claimed in  claim 1  wherein the oxygen-containing mixture is present in the second step reactor for about 2 to 40 minutes. 
     
     
         14 . The method as claimed in  claim 1  wherein the delignified pulp is fed to a washing unit. 
     
     
         15 . The method as claimed in  claim 13  wherein water is added to the washing unit. 
     
     
         16 . The method as claimed in  13  wherein the delignified pulp is fed to a pulp bleaching operation. 
     
     
         17 . An apparatus for delignifying pulp comprising a mixer, a first step reactor and a second step reactor. 
     
     
         18 . The apparatus as claimed in  claim 17  wherein the first step reactor is a reactor where the gas phase is the continuous phase and the liquid phase is the dispersed phase. 
     
     
         19 . The apparatus as claimed in  claim 17  wherein the second step reactor is a reactor column. 
     
     
         20 . The apparatus as claimed in  claim 17  wherein the mixer is in fluid communication with the first step reactor. 
     
     
         21 . The apparatus as claimed in  claim 17  wherein the first step reactor is in fluid communication with the second step reactor. 
     
     
         22 . The apparatus as claimed in  claim 17  further comprising a washing unit.