Origin of yield loss in chemical pulp bleaching

Abstract

The goal of this thesis was to determine the origin of the yield loss of kraft pulps in each bleaching stage in an Elemental Chlorine Free (ECF) bleaching sequence, including oxygen delignification (O), acid hydrolysis (A), chlorine dioxide bleaching (D), alkaline extraction with oxidative reinforcement (Ep) and peroxide bleaching (P). The literature review was elaborated on reactions of different pulps components, namely lignin, carbohydrates and hexenuronic acid (HexA), with bleaching chemicals, with emphasis on carbohydrates degradation. Effects of bleaching conditions on pulp yield loss were also considered.

Laboratory bleaching experiments (O-D0+Ep-D1-P) of softwood kraft pulps (85% pine, 15% spruce) provided by a Finnish pulp mill were done at the conditions similar to those used at the supplying mill. For each stage, pre- and post-bleached pulps were analysed for HexA content, viscosity and kappa number. Bleaching effluents were analysed for Total Organic Carbon (TOC) and lignin contents. Yield losses during bleaching were 2.4, 2.8, 0.6 and 0.5% for O-, D0+Ep-, D1- and P-stages, respectively. Combined yield losses of lignin and HexA were translated into kappa number reduction, which was close to the change in measured kappa numbers. Significant carbohydrate degradation was observed in Ep- and P-stages due to the wash-away of Mg2+ ions.