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CONVERSION OF PAPER-GRADE KRAFT PULP INTO DISSOLVING-GRADE PULP FOR CELLULOSE ACETATE PRODUCTION
Abstract
Dissolving pulp is widely used for the manufacture of functionalized materials competing with cotton and chemical fibers. One of the most widespread applications of dissolving pulp is the production of cellulose acetates. To produce high quality acetates the pulp with high cellulose content, high reactivity, and narrow molecular weight distribution is required. Conventional dissolving pulp process is based on the sulfite pulping or pre-hydrolysis kraft pulping. Further processing includes cold or hot alkali extraction and viscosity reducing during multi-stage bleaching. One of the advanced and promising methods for dissolving pulp production is the conversion of kraft paper-grade pulp to dissolving-grade pulp by repeated enzymatic and alkali treatments. The previously proposed schemes have demonstrated the feasibility of such conversion, but enzymes and chemicals dosage used were rather high. In this paper we propose a conversion scheme of paper-grade bleached kraft hardwood pulp into dissolving-grade pulp that includes stages of enzymatic treatment, cold alkali extraction and bleaching. Combined cellulases and xylanases treatment prior to alkali extraction was used for both, cellulose reactivity activation and xylan removing. The viscosity of the pulp was reduced by chlorine-containing bleaching reagents or cellulase treatment. The pulp samples were tested for xylan and ?-cellulose content and viscosity. The fiber surface structure was assessed by scanning electron microscopy, and pulp reactivity was determined by direct cellulose acetylation in a supercritical carbon dioxide. After testing of various enzymatic and chemical treatment combinations it was found that it is possible to convert kraft hardwood bleached paper-grade pulp into dissolving-grade pulp acetate with a relatively small enzyme dosage of 0.5-1 kg per ton of the pulp. Dissolving pulp with ?-cellulose content of 92 ... 94 %, xylan content of 5пїЅ6 %, and viscosity of 7пїЅ15 mPa?s was obtained. Reactivity in the supercritical carbon dioxide acetylation process was comparable with conventional sulphite dissolving pulp.
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