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The Research on Acid-Base Buffer System in Pulp Bleachingprocess

Author: WangLun
Tutor: HanQing
School: Shaanxi University of Science and Technology
Course: Pulp and Paper Engineering
Keywords: Non-wood pulp ECF bleaching Chlorine dioxide bleaching Peroxide bleaching Acid-base buffer system
CLC: TS745
Type: Master's thesis
Year: 2011
Downloads: 22
Quote: 0
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Abstract


This paper used wheat straw NaOH-AQ pulp as unbleached pulp, the influencing factors like quantity of bleaching medicine; bleaching temperature and time; pH value of mid-consistency DEP bleaching and OEP bleaching were studied. The results showed that during ClO2 bleaching and H2O2 bleaching, the acid-base buffer system is a effective carrier and can provide a good environment for ClO2 bleaching and H2O2 bleaching. During ClO2 bleaching, through analyzing the bleaching effects and mechanism of different types of acid-base buffer systems, find out sodium phosphate dibasic-citric acid; citric acid- trisodium citrate dehydrate; sodium phosphate dibasic-phosphoric acid buffer systems which have great buffering capacity and high bleaching effective. During H2O2 bleaching, find out borax-sodium hydroxide; sodium carbonate-sodium bicarbonate; disodium hydrogen phosphate-sodium hydroxide; boric acid-potassium chloride-sodium hydroxide acid-base buffer systems which have great buffering capacity and high bleaching effective.The optimal process conditions of wheat straw NaOH-AQ pulp DEP bleaching were: ClO2 charge 1.2%, bleaching time 120min, bleaching temperature 85℃, pH target 2.5. After ClO2 bleaching the pulp brightness can reach to 51.96%ISO; H2O2 charge 2.5%, bleaching time 120min, bleaching temperature 85℃, pH target 10.4. After H2O2 bleaching the pulp brightness can reach to 79.32%ISO.The optimal process conditions of wheat straw NaOH-AQ pulp OEP bleaching were: H2O2 charge 2.0%, bleaching time 100min, bleaching temperature 80℃, pH target 10.5. After H2O2 bleaching the pulp brightness can reach to 80.52%ISO.In acidic or alkaline condition, sodium phosphate dibasic-citric acid; citric acid- trisodium citrate dehydrate; sodium phosphate dibasic-phosphoric acid-base buffer systems have great buffering capacity, pH target is less affected by temperature, could be used in the ClO2 bleaching for pH control. Add sodium phosphate dibasic-phosphoric acid buffer system in the ClO2 bleaching, after bleaching the pulp brightness can incress from 44.81%ISO to 52.21%ISO, the rate of delignification can incress from 39.3% to 61.1%, the COD of bleaching liquor can reduce from 36.8 kg/t to 27.8 kg/t. The results also showed that adding acid buffer system in the ClO2 bleaching have no influence on paper property.In acidic or alkaline condition, borax-sodium hydroxide; sodium carbonate-sodium bicarbonate; disodium hydrogen phosphate-sodium hydroxide; boric acid-potassium chloride-sodium hydroxide acid-base buffer systems have great buffering capacity, pH target is less affected by temperature, could be used in the H2O2 bleaching for pH control. Add borax-sodium hydroxide acid buffer system in the H2O2 bleaching, after bleaching the pulp brightness can incress from 79.32%ISO to 83.58%ISO, the rate of delignification can incress from 25.8% to 37.8%, the degree of pulp viscosity is not obvious. The results also showed that adding acid buffer system in the H2O2 bleaching have no influence on pulp quality, it could be used to make cultural paper.The FT-IR spectrogram showed that after ClO2 bleaching, the absorption characteristic of benzene ring in the lignin wears off, both S type lignin and G type lignin suffered some damage, keep on bleaching with H2O2, more benzene ring was damaged and failure, most of the lignin was removed. The SEM pictures showed that after ClO2/H2O2 bleaching without using acid-base buffer systems, fiber configuration became flattened and more fluffy, but there are many pits on the surface of fiber, the air voids between fibers are very seldom; after ClO2/H2O2 bleaching using acid-base buffer systems, fibers have larger diameter, smooth surface, solid structure, so it means that the fibers suffered very little damage after bleaching.

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CLC: > Industrial Technology > Light industry,handicrafts > Paper Industry > Pulping process > Bleach
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