High PH treatment of pulp in a bleach sequence to produce pulp having low D.P. and low copper number for use in lyocell manufacture

Abstract

A high pH and a low pH process for reducing the degree of polymerization of a pulp having a hemicellulose content of at least 7%. The high pH is greater than 8, and the low pH process is 2 to 8. The high pH process reduces the degree of polymerization without substantially increasing the copper number. The low pH process requires a subsequent treatment with alkali to reduce the copper number of the pulp to less than 2. The process can be practiced in pulp mills with a bleaching sequence having one or more E or D stages. At the end of the bleach sequence, a pulp having a degree of polymerization of 200 to 1100, a copper number of less than 2, and a hemicellulose content of at least 7% is provided. The pulp can be used to make lyocell fibers.

1 . A process for making a composition for conversion to lyocell fiber, said process comprising: 
 contacting a pulp comprising cellulose and at least 7% hemicellulose with an oxidant and a catalyst for a period of time sufficient to reduce the average degree of polymerization of the cellulose at a pH greater than 8, the process occurring without substantially reducing the hemicellulose content of the pulp and without substantially increasing the copper number of the pulp.    
     
     
         2 . The process of  claim 1 , wherein the catalyst comprises a metal ion from at least one of Fe(II), Fe(III), Mn(II), Cu(II), Cu(I), Co(II), Co(III), Ni(II), the salts thereof, or any combination thereof.  
     
     
         3 . The process of  claim 1 , wherein the amount of oxidant is about 1% to about 4% based on the weight of the pulp.  
     
     
         4 . The process of  claim 1 , wherein the temperature during the reduction of D.P. is about 50° C. to about 100° C.  
     
     
         5 . The process of  claim 1 , further comprising adding a chelating agent.  
     
     
         6 . The process of  claim 5 , wherein the chelating agent is at least one of aminopolycarboxylic acid (APCA), ethylenediaminetetraacetic acid (EDTA), diethylene triamine pentaacetic acid (DTPA), nitrilotriacetic acid (NTA), phosphonic acid, ethylenediaminetetramethylene-phosphonic acid (EDTMP), diethylenetriaminepentamethylenephosphonic acid (DTPMP), nitrilotrimethylenephosphonic acid (NTMP), a polycarboxylic acid, a gluconate, a citrate, a polyacrylate, a polyaspartate or any combination thereof.  
     
     
         7 . The process of  claim 6 , wherein the amount of chelating agent is about 0.1% based on the weight of the pulp.  
     
     
         8 . The process of  claim 1 , wherein the reduction in degree of polymerization is about 20% to about 40%.  
     
     
         9 . The process of  claim 1 , wherein the period of time is at least 60 minutes.  
     
     
         10 . The process of  claim 1 , wherein the oxidant is hydrogen peroxide.  
     
     
         11 . A pulp bleaching sequence, comprising: 
 an E stage, wherein the pulp is bleached in the E stage in the presence of a catalyst at a pH greater than 8, without substantially increasing the copper number.