Use of peroxide bleaching filtrate to reinforce oxygen delignification of softwood kraft pulp

An addition of alkaline peroxide bleaching filtrate to oxygen-alkali delignification was evaluated in a laboratory reactor using real mill filtrates and fresh softwood kraft pulp. Compared with some earlier studies on laboratory pulps, no major positive impacts on the degree of delignification and pulp quality (brightness, viscosity, Pulmac fiber strength) were detected. This is most probably due to the use of real mill filtrates, lack of laboratory-type washing and chelation pretreatment for the pulps. However, the experiments indicated a slight decrease in the degree of delignification and minor increase in brightness at constant pulp kappa number when using peroxide bleaching filtrate in oxygen delignification of kraft pulp. The results suggest, that an addition of alkaline peroxide bleaching filtrate into oxygen delignification unlikely translates into clear benefits, at least when applied to a system that is closely connected to brown stock filtrate circulation, and is thus not very attractive in mill scale paractice.