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Progress in eucalyptus kraft pulp bleaching

Profile image of MARCOS PAUL ORADO RABELOMARCOS PAUL ORADO RABELO

2005, … sobre celulosa kraft …

Abstract

Bleached eucalyptus kraft pulps are largely used for manufacturing tissue and print & writing (P&W) paper grades. High final brightness (92 + % ISO) and brightness stability (<2% ISO) are rather significant for P&W for it affects optical brighteners demand. Bleaching chemical demand, bleaching yield, water consumption, effluent load & treatability, pulp 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 the light of the aforementioned drivers. The potential of new bleaching technologies such as the P Mo stage and the use of formaldehyde in D-stages are also presented. Implementation of single or double-stage oxygen delignification is determined by the true pulp lignin content (discounted the HexA´s). The high pulp HexA´s content and poor oxygen stage selectivity limits dropping kappa number under 9-10 in single or double Ostage. Application of 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 kg of COD/odt consuming the equivalent to 0.085% active chlorine. A D-(EP)-D type three stage sequences suffices for bleaching eucalyptus kraft pulps. The inclusion of a fourth stage is desirable for production of high brightness/ low reversion pulps. Bleaching chemical consumption is strongly influenced by brown pulp origin, with variations of 3.2 to 7.7% active Cl 2 demand depending upon the pulp type. The type of 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 D 1 than in D 0 /D HT stages. A final peroxide stage significantly improves pulp brightness stability. The production or organically bound chlorine decreases by 30% with hot chlorine dioxide bleaching but this gain disappears after effluent biological treatment.

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