XYLANASES IN BLEACHING - FROM AN IDEA TO THE INDUSTRY

The utilization of hemicellulases in bleaching of kraft (sulphate) pul p is considered as one of the most important new large-scale industria l applications of enzymes. This is partly due to the great potential o f an environmentally safe method. This method has in a short period al so proven to be economically realistic. The main enzymes needed in the enzyme-aided bleaching have been shown to belong to the group of endo -beta-xylanases. Xylanases act mainly on the relocated, reprecipitated xylan on the surface of the pulp fibres. Enzymatic hydrolysis of this specific type of xylan renders the structure of the fibres more perme able. The hydrolysis of xylan or mannan in the inner fibre layers may also enhance the bleachability. In practical process conditions, prope rties of the enzymes such as substrate specificity and the pH and temp erature optima are of utmost importance. The benefits obtained by enzy mes are dependent on the chemical bleaching sequence used as well as o n the residual lignin content of the pulp. The main goals in the enzym e-aided bleaching of kraft pulps have been the reduction of consumptio n of chlorine chemicals in the bleaching process and consequently lowe ring the AOX of the effluents. Enzymes have been applied as a pretreat ment both in conventional (C/D)EDED and in ECF (elementary chlorine-fr ee) bleaching sequences. In the production of TCF (totally chlorine-fr ee) pulps, enzymes have also been successfully used for increasing the brightness of pulp.