Xylanase prebleaching of kraft pulps derived from three softwood species
Creators: name: Nelson, Sandra L.
Publisher: name: University of British Columbia
Year: 2009

description: For over a decade, the kraft pulp and paper industry has been challenged to find bleaching sequences that produce bright, high quality pulp without causing detrimental effects on the receiving environment. Historically kraft pulp was bleached with elemental chlorine and other chlorine containing compounds. However, after the discovery of toxic chlorinated organics in bleached kraft mill effluents, industry has tried to limit the use of these chemicals. Pretreatment of kraft pulps with the enzyme xylanase, a hemicellulase that degrades the xylan component of hemicellulose, has been shown to enhance the efficiency of the subsequent bleaching stage. In this study the application of xylanase in a totally chlorine-free (TCF) bleaching sequence was examined using three commercial xylanase preparations and kraft pulps derived from Douglas-fir, western hemlock and cedar woods. The activity of the xylanases Pulpzyme HB, Pulpzyme HC and Irgazyme 40 was compared in order to determine the enzymatic loadings required for the bleaching trials. The kraft pulps were treated with these enzymes (X) in two types of multiple peroxide (P) bleaching sequences (XQPP or QPXP) which included one chelation (Q) stage. The same experiment was repeated with oxygen delignified kraft pulps (OXQPP or OQPXP). The enzyme activity on the pulp was verified by monitoring increases in carbohydrate and UV-absorbing material in the filtrates collected immediately following the xylanase treatment. It was apparent that neither oxygen delignification nor peroxide bleaching inhibited the solubilization of sugar and UV-absorbing materials by the enzymes. Although the xylanases caused a range of brightening effects, direct brightening could be achieved for all of the partially bleached pulps, but not for the kraft brownstocks. In most cases, xylanase treatment directly delignified the brownstock, oxygen delignified and partially bleached pulps. Of the three xylanases, pretreatment with Irgazyme 40 elicited the most consistent bleaching improvements. Kraft pulp derived from cedar reached the highest brightnesses after oxygen delignification for both bleaching sequences tested. After xylanase pretreatment there appeared to be no detrimental effect on fiber strength for any of the bleaching trials. The final component of this work examined the toxicity, as monitored by Daphnia magna and Microtox, of the filtrates derived from the pulps after xylanase treatments and peroxide bleaching. The filtrates collected immediately after the xylanase stage were predominantly non toxic, the exceptions being those collected following the OQPX sequence. The toxic response elicited by the combined filtrates was found to be caused primarily by residual peroxide. In modern mills the residual peroxide is neutralized and is therefore not usually a problem. In many cases, xylanase pretreatment, in conjunction with the TCF bleaching sequence used for this study, improved final pulp brightness by 1-1.5 % ISO. Although this improvement in final brightness was variable, optimization of enzyme conditions could further enhance the applicability of xylanase in industry.