SIGNIFICANCE OF PULP METAL PROFILE ON ENZYME-AIDED TCF BLEACHING

The type of metal counter-ions and their dissociation in pulps play an important role in attempts to optimize enzymatic pretreatments of pul p, especially in TCF bleaching. Removal of metal counter-ions rendered kraft pulps practically non-hydrolysable by Trichoderma reesei xylana se pI 9 and reduced the hydrolytic effectiveness of T. reesei xylanase pI 5.5 and mannanase. The observation that metal-free pulp is poorly hydrolysable is of practical importance in TCF bleaching sequences, in which the metal removal stage is essential for retention of pulp stre ngth properties. By optimizing the enzyme treatment and metal removal stages, a further improvement in enzymatic treatment and subsequent bl eaching was obtained. Furthermore, the enzymatic hydrolysis and bright ness improvement in peroxide delignification were found to be heavily dependent on the content and type of the counter-ion of carboxyl group s. The content and type of metal cations in pulp was in turn influence d by the pH history Of the pulp, i.e. the washing pH and the metal hyd roxide used to adjust the pH of the metal-free pulp.