Biobleaching

Abstract

Use of enzymes in pulp bleaching has attracted considerable attention in recent years and achieved interesting results. Enzymes of the hemicellulolytic type, particularly xylanases, are used commercially for pulp bleaching. Xylanase enzymes have proven to be a cost-effective way for mills to realize a variety of bleaching benefits including reducing AOX discharges primarily by decreasing chlorine gas usage, debottlenecking mills limited by chlorine dioxide generator capacity, eliminating chlorine gas usage for mills at high chlorine dioxide substitution levels, increasing the brightness of the ceiling particularly for mills contemplating ECF and TCF bleaching sequences, and decreasing the cost of bleaching chemicals, particularly for mills using large amounts of peroxide or chlorine dioxide. These benefits are achieved over the long term when the enzymes are selected and applied properly in the mill. The use of oxidative enzymes from white-rot fungi can directly attack lignin. These enzymes are highly specific toward lignin; there is no damage or loss of cellulose and can produce larger chemical savings than xylanase but has yet not been developed to the full scale. It is being studied in several laboratories all over the world. Certain white-rot fungi can delignify kraft pulps increasing their brightness and their responsiveness to brightening with chemicals. The fungal treatments are too slow but the enzymes, manganese peroxidase and laccase, can also delignify pulps and enzymatic processes are likely to be easier to optimize and apply than the fungal treatments. Development work on laccase and manganese peroxidase continues. The overview of developments in the application of xylanase enzymes, lignin-oxidizing enzymes, and white-rot fungi in bleaching of chemical pulps is presented.