The Research On Acid-Base Buffer System In Pulp Bleachingprocess

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.