Effects of the partial removal of wood hemicelluloses on the properties of kraft pulp

摘要

The objective of this work was to study the effects of partial removal of wood hemicelluloseson the properties of kraft pulp.The work was conducted by extracting hemicelluloses (1) by asoftwood chip pretreatment process prior to kraft pulping, (2) by alkaline extraction frombleached birch kraft pulp, and (3) by enzymatic treatment, xylanase treatment in particular, ofbleached birch kraft pulp. The qualitative and quantitative changes in fibers and paperproperties were evaluated. In addition, the applicability of the extraction concepts andhemicellulose-extracted birch kraft pulp as a raw material in papermaking was evaluated in apilot-scale papermaking environment. The results showed that each examined hemicelluloseextraction method has its characteristic effects on fiber properties, seen as differences in boththe physical and chemical nature of the fibers.A prehydrolysis process prior to the kraft pulping process offered reductions in cooking time,bleaching chemical consumption and produced fibers with low hemicellulose content that aremore susceptible to mechanically induced damages and dislocations. Softwood chippretreatment for hemicellulose recovery prior to cooking, whether acidic or alkaline, had animpact on the physical properties of the non-refined and refined pulp. In addition, all thepretreated pulps exhibited slower beating response than the unhydrolyzed reference pulp.Both alkaline extraction and enzymatic (xylanase) treatment of bleached birch kraft pulpfibers indicated very selective hemicellulose removal, particularly xylan removal.Furthermore, these two hemicellulose-extracted birch kraft pulps were utilized in a pilot-scalepapermaking environment in order to evaluate the upscalability of the extraction concepts.Investigations made using pilot paper machine trials revealed that some amount of alkalineextractedbirch kraft pulp, with a 24.9% reduction in the total amount of xylan, could be usedin the papermaking stock as a mixture with non-extracted pulp when producing 75 g/m2paper. For xylanase-treated fibers there were no reductions in the mechanical properties of the180 g/m2 paper produced compared to paper made from the control pulp, although there was a14.2% reduction in the total amount of xylan in the xylanase-treated pulp compared to thecontrol birch kraft pulp.This work emphasized the importance of the hemicellulose extraction method in providingnew solutions to create functional fibers and in providing a valuable hemicellulose co-productstream. The hemicellulose removal concept therefore plays an important role in the integratedforest biorefinery scenario, where the target is to the co-production of hemicellulose-extractedpulp and hemicellulose-based chemicals or fuels.