Use of a Novel Extremophilic Xylanase for an Environmentally Friendly Industrial Bleaching of Kraft Pulps

Xylanases can boost pulp bleachability in Elemental Chlorine Free (ECF) processes, but their industrial implementation for producing bleached kraft pulps is not straightforward. It requires enzymes to be active and stable at the extreme conditions of alkalinity and high temperature typical of this industrial process; most commercial enzymes are unable to withstand these conditions. In this work, a novel highly thermo and alkaline-tolerant xylanase from Pseudothermotoga thermarum was overproduced in E. coli and tested as a bleaching booster of hardwood kraft pulps to save chlorine dioxide (ClO2) during ECF bleaching. The extremozyme-stage (EXZ) was carried out at 90 °C and pH 10.5 and optimised at lab scale on an industrial oxygen-delignified eucalyptus pulp, enabling us to save 15% ClO2 to reach the mill brightness, and with no detrimental effect on paper properties. Then, the EXZ-assisted bleaching sequence was validated at pilot scale under industrial conditions, achieving 25% ClO2 savings and reducing the generation of organochlorinated compounds (AOX) by 18%, while maintaining pulp quality and papermaking properties. Technology reproducibility was confirmed with another industrial kraft pulp from a mix of hardwoods. The new enzymatic technology constitutes a realistic step towards environmentally friendly production of kraft pulps through industrial integration of biotechnology.

Effect of hot-water extraction (HWE) severity on bleached pulp based biorefinery performance of eucalyptus during the HWE-Kraft-ECF bleaching process

The effectiveness of a biorefinery based on an HWE-Kraft-ECF bleaching process and the end use of pulp was systematically evaluated. Using a P-factor of 198, nearly 30% of xylan-based sugars were recovered. The resulting pulp and paper properties were found to be comparable with the control. A maximum xylan-based sugar recovery of nearly 50% was achieved at a P-factor of 738. Although the strength of this P-factor induced handsheet was lower than that of the control by about 20%, the corresponding pulp was sufficient for dissolving pulp application. However, once the P-factor rose above 1189, hemicellulose sugars were significantly degraded into furans; pulp and paper properties were also deteriorated due to cellulose degradation, lignin deposition and condensation. Thus, considering the different end use of pulps, the performance of an HWE-based biorefinery could be balanced by its HWE severity.