Introduction of an enzyme stage in bleaching of hardwood kraft pulp

Oxygen, ozone, hydrogen peroxide, enzymes may be used as an alternative in improving the environmental impact. The aim of the present investigations i s to establish whether it is possible to introduce a stage of enzyme treatm ent in the process of bleaching of hardwood kraft pulp and to determine the conditions required for its processing. The kraft pulp used is subjected to preliminary treatment by the enzyme xyl anase, an industrial product of Bulgaria, under the following conditions: p H 3.5, temperature 40 degrees, duration 3 h, pulp consistency 10%. The enzy me quantity is varied within the range of 0.2.10(-3) to 6.6.10(-3) g/g kraf t pulp with the aim of elucidating its effect on the degree of delignificat ion (Kappa number) and pulp's viscosity. The optimum enzyme charge is found to be 2.10(-3) g/g of the pulp used. Multistage graft pulp bleaching of the following sequence is carried out: I: X-D-1-E/P-D-2-E-D II: O-X-D-1-E/P-D-2. The results of the physico-chemical analysis show that, following the enzym e treatment, brightness is increased from 85 to 89% and correspondingly fro m 89 to 91% (according to ISO) in accordance with the first and second sequ ence. It should be underlined that bleaching with oxygen as well as reducti on of active chlorine charge of 1.2% are included in the second case. Visco sity of bleached kraft pulp is maintained unchanged. The studies carried out and the results obtained show the positive effect o f the introduction of an enzyme stage to the bleaching of hardwood kraft pu lp.