Eucalyptus Kraft Pulp Bleaching: State-Of-The-Art and New Developments, 2005 Engineering, Pulping & Environmental Conference

Bleached eucalyptus kraft pulps are largely used for manufacturing tissue and printing and writing paper grades. High final brightness and brightness stability are important for the latter since they affect optical brighteners demand. Bleaching chemical demand, yield, water consumption, effluent load and 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. High pulp HexA´s content and poor oxygen stage selectivity limits lowering kappa number below 9-10 in single or double O-stage. Application of the PMo stage allows further reduction of kappa number to 3-4. Efficient post-oxygen washing is the key for low cost bleaching. A three stage sequence (D-(EP)-D) suffices for bleaching eucalyptus kraft pulps; a fourth stage is desirable for production of high brightness/ low reversion pulps. The choice of ECF bleaching technology (D0, DHT) affects chemical consumption only slightly. Formaldehyde saves more chlorine dioxide when used in D1 than in D0/DHT stages. A final peroxide stage significantly improves pulp brightness stability. The production of organically bound chlorine decreases by 30% using DHT, but this gain disappears after effluent biological treatment.