Multistage peroxide bleaching of French hemp

Alkaline-sulphite French hemp mill pulp of 48-54% ISO brightness has been bleached with different totally chlorine free (TCF) sequences (multistage peroxide without and with activator (tetraacetylethylenediamine, TAED) and with and without pre-treatment with ultrasound to 85% ISO brightness of the mill conventional CEH bleaching. The effects of temperature, retention time, chemical charge, TAED/peroxide ratio and alkalinity have been studied in order to maximise the brightness gain at lowest viscosity loss. The chemistry and flexibility of TAED made it possible to activate the peroxide under conditions for which the peroxide alone is ineffective as bleaching agent, such as low alkalinity or low temperature. The presence of shives hindered the bleaching, but the TCF bleached non-screened pulp was stronger than the mill screened bleached pulp and after screening should be with at least 2% higher brightness. Pre-treatment with ultrasound for 4 min gave 3-4% gain in ISO brightness and facilitated further delignification and peroxide bleaching. (C) 2003 Published by Elsevier Science B.V.