BEHAVIOR OF RESIDUAL LIGNIN IN KRAFT PULP DURING BLEACHING

The behaviour of residual lignin in softwood kraft pulp during bleachi ng has been investigated Chlorine, chlorine dioxide, oxygen, hydrogen peroxide and ozone treatments have been carried our on unbleached pulp and the residual lignin has been extracted by enzymatic hydrolysis of the carbohydrates. The main functional groups have been measured by F -19 NMR spectroscopy and the molecular weight distribution has been st udied New information on the chemistry of the bleaching agents has bee n obtained, which explains some of the differences observed in their b leaching effect. For example, chlorine has the capability to depolymer ize the residual lignin while forming new free phenolic groups, which makes it without any equivalent among the bleaching agents. On the con trary ozone does not cleave the residual lignin very efficiently. Cons equently, it must be associated with another chemical which has this c apability, like oxygen or chlorine dioxide.