Laccase–HBT bleaching of eucalyptus kraft pulp: Influence of the operating conditions

Different operating conditions (viz. pulp consistency, oxygen pressure and treatment time) in the biobleaching of eucalyptus kraft pulp with the laccase–HBT system was tested in order to describe their effect and normalize a biobleaching protocol. A high O2 pressure (0.6 MPa) was found to result in improved laccase-assisted delignification of the pulp. Also, a high pulp consistency (10%) and a short treatment time (2 h) proved the best choices with a view to obtaining good pulp properties (kappa number and ISO brightness) under essentially mild conditions. The laccase–HBT treatment was found to result in slight delignification (in the form of a 20–27% decrease in kappa number); however, an alkaline extraction stage raised delignification to 41–45%, a much higher level than those obtained in the control tests (16–23%). Also, the use of hydrogen peroxide in the extraction stage resulted in improved brightness (14–19%), but in scarcely improved delignification (4–7%). Treating the pulp with the laccase–HBT system reduced the amount of hydrogen peroxide required for subsequent alkaline bleaching by a factor of 3–4 relative to control tests.