Pilot plant bleaching trials with laccase and mediator

The objective of the trials was to demonstrate that a laccase mediator discovered by Wacker-Chemie, N-hydroxyacetanilide (NHA), or its precursor diacetate (Diac), could be applied to kraft pulp at a pilot scale, in order to save bleaching chemicals in a typical North American elemental chlorine free (ECF) bleaching sequence. An eastern Canadian softwood pulp was chosen for the pilot trial based on laboratory results with laccase and the mediator NHA. The mill bleaching sequence was D(0)EopD(1)EpD(2) with a total chlorine dioxide consumption of 36 kg/torme. The pilot trial was run with the sequence LaEpD(0)EoD(1), where La indicates laccase with diacetate mediator. The diacetate was converted in situ to NHA by performing the laccase stage at 65 C and pH 6.8 over a reaction time of 4.5 h. Under these conditions laccase from Myceliophthora thermophila was found to oxidize NHA, even though its redox potential was sub-optimal. Initial trials indicated that the enzyme was sensitive to high mechanical shear when added to pulp at a high shear mixer. Higher activity was retained when the enzyme was added prior to the mixer. Chelating agent (DTPA) was included in the laccase stage in order to remove metals from the pulp prior to the Ep stage. The LaEpD(0)EoD(1) pilot bleaching sequence required 24% less chlorine dioxide than the mill sequence, although the resulting pulp was slightly lower in brightness (88% versus 90% at the mill). Strength properties were maintained in the pilot bleached pulp. Further work is required to optimize extraction stages and also to develop an enzyme optimal for the Diac mediator reaction conditions.