Refining and Urea Pretreatments to Enhance Biobleaching of Eucalyptus Kraft Pulp

Some pretreatments that swell and/or open the structure of wood fibers could increase the effectiveness of a biobleaching process, allowing for an industrial application. To this end, a chemical pretreatment (urea, U), a physical pretreatment (refining, R), and their combinations (RU and UR) were optimized to evaluate and compare their enhancement of the LE biobleaching sequence (laccase-mediator treatment plus alkali extraction). The urea pretreatment before biobleaching (ULE) provided the highest delignification (37.5%) and the highest increase in brightness (6.1 points % ISO). As expected, adding a refining process before or after the urea pretreatment increased paper strength. However, when the refining was applied after the urea pretreatment (URLE), the delignification was higher than that obtained after RULE. Thus, URLE provided a similar Kappa number and an increase of 97%, 149%, and 98% in the tensile, tear, and burst indexes, respectively, compared with ULE treatment, but it had a reduction of 2.8 points (% ISO) in brightness, caused by the action of refining. Therefore, depending on the final use of the paper (which can require high optical properties or high strength), either ULE or URLE would be the optimal sequence.