Evolution in hardwood kraft pulp bleaching - A comparison of options

Optimization of the bleaching sequences' stages is imposed by the introduction of strict environmental requirements. It is considered that a prolonged reaction time and high temperature in the acid D-0 stage would assure a more effective carrying out of the next bleaching stages, as a result of improved hexenuronic acids' removal, and of a lower Kappa number. The question of optimal chemical distribution in the bleaching sequence, for obtaining pulp with high final brightness, strength properties and viscosity, is also under discussion. The present investigation studies the influence of the conditions in D stages, and the chemical distribution in the D-0 - EOP - D-1 sequence on bleached pulp's properties. Industrial, oxygen-delignified beech and poplar pulps were used; the ECF sequences were in compliance with the Svilosa Company options. It was found out that the conditions in the D-0 stage were essential for the bleaching processes in the next stages and for pulp properties' fort-nation. A prolonged hot D-0 stage resulted in pulp with more than a unit Kappa number reduction, as due to hexenuronic acid destruction. As a result, pulp brightness was up to 1.5 % higher in bleaching sequences with a high chlorine dioxide charge at the D-1 stage, yet unfavourable to pulp strength properties. The most optimal charge of chlorine dioxide was of 2.0-2.5 % in the D-0 stage, a minimal charge of this being recommended for use in the D-1 stage for attaining the desired, higher pH value. Short lower temperature D-0 bleaching resulted in pulp with equal brightness, but with better strength properties in comparison with the hot prolonged D-0 bleaching for this chemical distribution. Addition of hydrogen peroxide in the 0.25-0.75 % range in the EO stage led to significant chlorine dioxide saving, final brightness increases and improvement of some strength properties.