Method For Preparing Dissolving Pulp By Totally Chlorine-Free Bleaching Of Poplar Kraft Pulp

Abstract

The application relates to a method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp, including: preparing poplar kraft pulp by pre-hydrolysis kraft process with poplar as raw material; ZOQP bleaching of poplar kraft pulp to obtain dissolving pulp, wherein Z denotes ozone bleaching, O denotes oxy delignification, Q denotes chelation treatment, and P denotes hydrogen peroxide bleaching. In this way, washing and bleaching wastewater can be recycled in entire preparation process described above, achieving “zero” discharge. The dissolving pulp prepared by the method has no residual toxic substances, and exhibits that all indexes exceed the requirements of qualified products in dissolving pulp industry standard (QB/T 4898-2015), and most of indexes meet those of excellent products; thus, it can replace imported dissolving pulp. The use of the method significantly solved the problems of large pollution in the production and shortage of raw materials for viscose fibers.

What is claimed is: 
     
         1 . A method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp, comprising:
 (1) preparing poplar kraft pulp by pre-hydrolysis kraft process with poplar as a raw material; and   (2) performing ZOQP bleaching on the poplar kraft pulp to obtain dissolving pulp;   wherein Z denotes ozone bleaching, O denotes oxy delignification, Q denotes chelation treatment, and P denotes hydrogen peroxide bleaching.   
     
     
         2 . The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to  claim 1 , wherein the pre-hydrolysis kraft process comprises procedures of hot water pretreatment and the kraft cooking. 
     
     
         3 . The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to  claim 1 , wherein the hot water pretreatment comprises steps of:
 mixing poplar pieces with water according to a liquid ratio of 1:4-1:7, heating up to 150-180° C., and holding for 60-100 min.   
     
     
         4 . The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to  claim 1 , wherein the kraft cooking comprises steps of:
 performing cooking for 60-100 min at the liquid ratio of 1:4-1:7, the temperature of 150-180° C., the amount of alkali (calculated as Na 2 O) of 20-25%, and the degree of vulcanization of 20-25%.   
     
     
         5 . The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to  claim 1 , wherein Z-stage comprises steps of: adjusting poplar kraft pulp to the concentration of 25-30% and pH of 1.5-3.0, adding 1.0-3.0% of ozone and 0.3-0.7% of diethyltriamine pentaacetic acid (DTPA), mixing them uniformly, and then performing the reaction at 20-35° C. for 10-30 min; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 8-15%. 
     
     
         6 . The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to  claim 1 , wherein O-stage comprises steps of: placing the pulp bleached in Z-stage in a closed environment, introducing oxygen to an oxygen pressure of 0.4-0.8 Mpa, adding 1.0-3.0% of NaOH and 0.3-0.7% of MgCO 3 , and then and performing the reaction at 90-110° C. for 30-50 min while stirring at a speed of 60 r/min; after the reaction is completed, the pulp is washed to neutrality and is adjusted to the concentration of 8-15%. 
     
     
         7 . The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to  claim 1 , wherein Q-stage comprises steps of: placing the pulp treated in O-stage in a closed environment, adding 0.2-0.6% of chelating agent DTPA, mixing the pulp and DTPA uniformly, and then performing a chelate treatment at 60-80° C. for 20-40 min. 
     
     
         8 . The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to  claim 1 , wherein P-stage comprises steps of: after the reaction in Q-stage is completed, adding 1.0-3.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.4-0.9% of NaOH, 1.0-2.0% of sodium silicate and 0.1-0.3% of magnesium sulfate, and then performing the bleaching reaction for 70-100 min at the temperature of 80-100° C.; after the reaction is completed, the pulp is took out, and then is washed to neutrality and dried to obtain dissolving pulp. 
     
     
         9 . A dissolving pulp prepared by the method according to  claim 1 . 
     
     
         10 . An application of dissolving pulp according to  claim 9  in the preparation of viscose fiber, acetate fiber, nitrocellulose or cellulose ether.