Minimizing Viscosity Loss during Totally Chlorine-Free Bleaching of Hardwood Kraft Pulp

Frédéric Pouyet, Dominique Lachenal, Satyajit Das, Christine Chirat

Abstract


By applying increasing amounts of ozone (Z stage bleaching) on eucalyptus oxygen-delignified pulps, it was observed that both lignin and hexenuronic acids (HexA) are attacked early during the treatment. While the HexA were progressively removed, however, the oxidized lignin tended to stay in the pulp. An acidic stage (A stage) at pH 3 and a temperature of 90 to 95°C removed a high proportion of the HexA, which eventually reduced the ozone requirement. In spite of the negative impact of the A stage on the DP of cellulose, totally chlorine-free sequences containing A led to pulps of higher viscosity. Increasing the temperature in A to 120°C did not seem to have an effect other than increasing the rate of the hydrolysis reactions. A very efficient metal removal was observed when a chelating agent was added in A (AQ). Some trials showed that splitting the Z stage in a ZEZE-like process (E for alkaline extraction) minimized the impact on pulp viscosity, and that AQPZE-like sequences can reach a quality comparable to some commercial elemental chlorine-free (ECF) pulps.

Keywords


Pulp bleaching; Chlorine-free bleaching sequence; Acid hydrolysis; Ozone; Hydrogen peroxide

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