Xylanase in Kraft pulp bleaching

Abstract

The use of xylanase in pulp bleaching allows a reduction in consumption of chemicals and hence reduces the absorbable organic halogens load in the effluents. The aim of this thesis was to optimize the working conditions of the commercial xylanase Ecopulp® TX (hereafter referred to as TX) and to determine the potential savings of chemicals associated with the enzyme treatment.

TX had good storage stability at 22 °C and 37 °C. Over 80% of the original activity still remained after a two-month storage period. The operational stability of TX depended on both temperature and pH. When applied at lower pH it showed better stability at higher temperature.

TX behaved quite similarly on two different substrates: (a) 1% birch xylan, and (b) wood pulps. Optimal conditions for activity were 70 °C and pH 6.5. Under acidic conditions, the performance of TX was enhanced by increasing the temperature. Under alkaline conditions it was preferable to use TX at low temperature to maintain its stability. TX exhibited good activity at 90 °C over short reaction times. A low pH environment (pH 4) was unfavourable for TX. The selection of enzyme treatment time very much depended on the stability of the enzyme under the prevailing conditions. Both 0.2 l TX/t and 0.3 l TX/t pulp increased the bleachability of the pulp significantly. However, the selection of dosage has to be based on economic considerations.

At a dosage of 0.3 l TX/t pulp and a 60 minutes reaction time, TX was equally effective at 55-85 °C and pH 5-8 in softwood pulp ECF bleaching (X/DEDED). For a target pulp brightness of 89-90% the use of TX resulted in savings of chlorine chemicals of 10.6 aCl kg/t pulp without affecting pulp strength properties.