Development of Xylanase Based Pulp Bleaching Process

Abstract

As per the industries norms in regulating and maintaining the environmental conditions, current interest in the implementing of biological treatment in pulp bleaching processes has arisen very fast in the last few years globally. Due to use of huge quantities of hazardous chemicals in bleaching process, generation of pollutants is rapidly increased in environment. To overcome this problem researchers and industrialist are working continuously and finding alternative solutions of these hazardous chemicals. Use of biotechnology in pulp bleaching process shows its high impact towards green technology. Enzymes manufactured from microbial sources are very helpful and useful for reducing the use of chemicals; resulting in reducing huge percentage of pollutants generation to save our environment. There are many commercial enzymes are easily available in the market now, which is highly alkalistable and thermostable in nature which are the best desired conditions for enzymatic bleaching. Laccases and xylanases suits and helps a lot in enzymatic bleaching for achieving higher pulp qualities with cost effective process as compared to conventional process. Xylanase bleaching is very much popular rather than laccase and delivers high quality pulp at very low cost. Due to a mediator needed for the use of laccase in the process; it leads to increase production cost as compared to xylanase bleaching process. In this study, we used four different commercial enzymes and studied their effect on bleaching of two different hardwood eucalyptus (E. globulus) and acacia (A. mangium) kraft pulps using different bleaching sequences. Out of these four enzymes; two enzymes were manufactured and gifted by Novozyme, India, one from DuPont Genencor Sciences, USA, and another from Dyadic International, USA and coded as EnzyA, EnzyB, EnzyC and EnzyD respectively for the study. Screening of these four enzymes was done on the basis of hexenuronic acid (Hex-A) removal on eucalyptus pulps using elemental chlorine-free (ECF) bleaching sequence (XD0EPD1D2). Enzyme (X) stage resulted in removal of hexenuronic acid up to 20.0% with EnzyC in addition to improved brightness by 3.4 units and 19.7% reduction in kappa number as compared to control. Hexenuronic acid content was reduced by 21.3% in the final bleached pulp with 1.5 units of brightness gain over control. Due to reduced Hex-A components in bleached pulps, post color number (brightness reversion) was also reduced up to 45.0% with EnzyC. In comparison to control, EnzyC resulted in maximum pollution load reduction of chemical oxygen demand (COD) and biological oxygen demand (BOD) up to 27.1 and 23.8% respectively in the bleach effluent. Out of these four enzymes; EnzyC was also helpful in maximum saving 22.0 and 31.8% of chlorine dioxide (ClO2) and sodium hydroxide (NaOH) respectively with improvement in optical properties and reduction in pollution load. The observations, thus, demonstrated Hex-A reduction profile, brightness improvement and reduction in pollution during the enzymatic bleaching with xylanases. Further, a comparative study was carried out with EnzyC in enzymatic stage followed by ECF bleaching sequence (D0EPD1D2) on two different hardwood kraft pulps (eucalyptus and acacia) for improvement in optical and physical strength properties. Reduction in bleach chemicals consumption was observed after the use of xylanase in enzymatic treatment. At equivalent charge of bleaching chemicals, a brightness improvement of 1.7 and 2.1 units and whiteness gain of 2.3 and 2.7 units were obtained with enzyme treatment in eucalyptus and acacia pulps, respectively. A final brightness of 89%ISO and 90%ISO was achieved easily for eucalyptus and acacia pulps respectively by following ECF bleaching sequence. During this study, brightness reversion was also studied and 45% reduction in post color number was observed for both hardwood pulps as compared to control. After xylanase pre-bleaching, a 17% and 23% reduction in chlorine dioxide (ClO2) and caustic (NaOH) were observed for both hardwood pulps; indicating an environmentally friendly approach to the process. Out of these two hardwood pulps, acacia was observed highly beneficial for the effect of bleachability over the use of eucalyptus. The effect of xylanase stage incorporation at different places of bleaching sequences was also studied to examine the outcome on the various parameters. For this study, nine different bleaching sequences were chosen and compared using eucalyptus hardwood kraft pulps and divided in three categories of bleaching sequences (pre-treatment, intermediate, and post-treatment). Efficacy of xylanase stage on final pulp brightness, whiteness, brightness reversion and bleach effluent characteristics were studied with EnzyC. Pre-treatment bleaching sequences resulted in increased final pulp brightness by 1.6 units with 32% reduction in adsorbable organic halogens (AOX) at 0.5 kgt-1 doses of xylanase, in addition to improved ratio of biological oxygen demand (BOD) to chemical oxygen demand (COD), which shows better bio-degradability of discharge effluents in a secondary treatment stage. Post-treatment bleaching sequences were found helpful in boosting of final pulp whiteness by 3.4 units and to improve brightness stability by 48% at the same dose of xylanase as in pre-treatment bleaching. Post-treatment bleaching sequences also showed a significant improvement in BOD-to-COD ratio at 0.5 kgt-1 xylanase doses. Intermediate incorporations of enzyme stage have not showed any significant benefits over pre and post-treatment bleaching sequences. This study, thus, presents findings regarding the application of xylanases in the bleaching of pulp, with emphasis on the mechanism and effects of xylanase treatment on pulp and paper and the factors affecting the bleaching process and its efficiency.