Bleaching of mechanical pulps to very high brightness

Increasing demands in paper quality require increased brightness levels for mechanical pulps, too. Laboratory bleaching trials with SGW and TMP demonstrated that softwood mechanical pulps can be bleached to brightness levels as high as 84 - 85% ISO at reasonable chemical charges if optimum wood quality at high unbleached brightness is used. The results obtained in different processes are compared to each other: single-stage high-consistency peroxide bleaching 2-stage peroxide bleaching 2-stage peroxide bleaching followed by a dithionite stage For the TMP sample the influence of bleaching on optical and strength properties is shown. In the first commercial installation of a 3-stage bleach plant for SGW, which was started-up end of 1997, the pulp is bleached to a brightness of up to 84% ISO at economically acceptable costs. The plant concept is described and both, actual mill data are presented and operating costs for different processes are compared.