Eucalyptus kraft pulp bleaching: state-of-the-art and new developments

Abstract

Chemical demand, yield, water consumption, effluent load and treatability, pulp organic chlorine compounds (OX), brightness stability, refinability, and strength are drivers for choosing bleaching technology. This work critically reviews the state-of-the-art processes for oxygen delignification, first stage, second stage, and final bleaching of eucalyptus kraft pulp in light of those drivers. Emerging technologies, such as the PMo stage and formaldehyde assisted bleaching, are discussed. Implementation of single- or double-stage oxygen delignification is determined by the pulp “true” lignin content. High pulp HexAs content and poor selectivity limits dropping kappa number under 9-10 in single or double O-stage. Mo-catalyzed acid peroxide delignification after O-stage allows further reduction of kappa number to 3-4. Efficient post-oxygen washing is the key for low cost bleaching, with a kilogram of COD/o.d. ton consuming the equivalent to 0.085% active chlorine. A/D-(EP)-D type three-stage sequence suffices for bleaching eucalyptus kraft pulps. The inclusion of a fourth stage is desirable for high brightness/low reversion pulps. Chemical consumption is strongly influenced by brown pulp origin, with variations of 3.2% to 7.7% active Cl2 demand, depending upon the pulp type. The type of elemental chlorine free (ECF) bleaching technology, based on chlorine dioxide, affects chemical consumption only slightly. Hot acid/hot chlorine dioxide bleaching technology saves small amounts of active chlorine for high bleachability pulps, but none for low bleachability ones. Atmospheric extraction (EP) suffices for eucalyptus kraft pulp bleaching. Formaldehyde saves more chlorine dioxide when used in D1 than in D0/DHT stages. A final peroxide stage improves pulp brightness stability. The production of organically bound chlorine decreases by 30% with hot chlorine dioxide bleaching, but this gain disappears after effluent biological treatment.