Dissolving pulp and a method for production thereof

Abstract

A method of preparing dissolving pulp. The method includes physically separating a kraft pulp or a kraft hydrolysis pulp into first and second fractions, the first fraction having a relatively low lignin content and the second fraction having a relatively high lignin content.

1 . A method of preparing dissolving pulp, the method comprising the steps of:
 (i) physically separating a kraft pulp into first and second fractions, the first fraction having a relatively low lignin content and the second fraction having a relatively high lignin content; and   (ii) processing the first fraction to produce a dissolving pulp.   
     
     
         2 . The method of  claim 1 , wherein step (i) includes separating pulp particles on the basis of size, wherein particles of the first fraction are relatively large and particles of the second fraction are relatively small. 
     
     
         3 . The method of  claim 2 , wherein the second fraction has a lignin content of at least about 15% by weight. 
     
     
         4 . The method of  claim 3 , wherein the first fraction has a lignin content of no more than about 6% by weight. 
     
     
         5 . The method of  claim 4 , wherein the unseparated kraft pulp has a kappa number of between 10 and 50. 
     
     
         6 . The method of  claim 5 , wherein the kappa number of the first fraction is no more than about 80% the kappa number of the unseparated pulp. 
     
     
         7 . The method of  claim 6 , further comprising the step of, prior to step (i), kraft pulping wood chips to produce the pulp. 
     
     
         8 . The method of  claim 7 , wherein said wood chips are subjected to steam and/or water prehydrolysis prior to said kraft pulping. 
     
     
         9 . The method of  claim 7 , wherein the kraft pulp is obtained from a conventional kraft process without a prehydrolysis step, and the pulp subject of separating step (i) has a kappa number of between 10 and 40. 
     
     
         10 . The method of  claim 9 , wherein the lignin content (wt %) of the second fraction is at least 1.1 the lignin content of the first fraction. 
     
     
         11 . The method of  claim 10 , wherein between 85 and 95 wt % of the unfractionated kraft pulp is contained in the first fraction obtained in step (i). 
     
     
         12 . The method of  claim 11 , further comprising the step of (iii) exposing the first pulp fraction to a cellulase, xylanase, and/or mannase enzyme to decrease the intrinsic viscosity of the pulp fraction by between 50 and 700 ml/g (0.5% cellulose in a cupriethylenediamine (CED) solution), optionally exposing the pulp fraction to one or more cationic polymers wherein said polymer can be selected from the group consisting of cationic polyacrylamide (CPAM), polydiallyldimethylammonium chloride (DADMAC), polyethylenimine (PEI), polyaluminum chloride (PAC), cationic starch, a dual polymer system, wherein the cationic polymer can further comprise an anionic polymer, microparticle-containing system, and/or a silica-based cationic polymers. 
     
     
         13 . The method of  claim 12 , wherein the enzyme comprises FiberCare D™, present in a dosage 0.1 to 10 u/g of dry pulp; further comprising the step of (iv) bleaching the first pulp fraction before, after or as part of step (iii); further comprising the step of (v) exposing the bleached pulp obtained in step (iv) to a xylanase; further comprising the step of (vi) subjecting the pulp obtained in step (v) to alkali; and wherein step (vi) comprises a hot alkali treatment, or step (vi) comprises a cold alkali treatment and the caustic soda concentration is in the range of 1 to 12 wt %. 
     
     
         14 . The method of  claim 13 , further comprising recovering active enzyme of step (iii) and cycling the recovered enzyme back into step (iii), wherein amount of enzyme recycled is in the range of 30 to 90%. 
     
     
         15 . The method of  claim 6 , wherein step (i) comprises centrifugal separation. 
     
     
         16 . The method of  claim 6 , wherein step (i) comprises screening of the kraft pulp into the first and second fractions. 
     
     
         17 . The method of  claim 6 , wherein the kraft pulp of step (i) is a prehydrolysis kraft pulp. 
     
     
         18 . The method of  claim 6 , further comprising the step of refining the fiber fraction to produce the dissolving pulp. 
     
     
         19 . The method of  claim 18 , wherein the dissolving pulp produced has a lignin content of no more than about 0.3 wt %. 
     
     
         20 . The method of  claim 19 , wherein the dissolving pulp produced has an α-cellulose content of at least about 88 wt %. 
     
     
         21 . The method of  claim 20 , wherein the dissolving pulp produced has a pentosan content of no more than about 8 wt %. 
     
     
         22 . The method of  claim 21 , wherein the Fock reactivity of the dissolving pulp produced is at least 50%.