The influence of the bleaching medium on caustic extraction efficiency .1. The role of fibre swelling

Delignification of a chlorine-dioxide-treated kraft pulp was inhibited when the alkaline extraction medium contained >50% (v/v) ethanol. Alkaline extraction experiments were performed with various media at a given fibre swollen volume to determine if fibre deswelling caused the entrapment of lignin fragments. Extractions conducted with ethanol and aqueous BaCl2 solutions showed similar levels of delignification. However, extractions performed with aqueous NaCl solutions yielded conflicting results. Estimates of the effective diffusion coefficients did not differ appreciably among the media investigated Further analyses indicated that the average lignin fragment diameter is smaller than the reduced average pore. width. These results suggested that factors other than fibre deswelling are responsible for the lower extent of delignification in ethanol-water and aqueous BaCl2 media.