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Oxygen delignification as a pretreatment for acetosolv pulps bleaching.

Application: The information from this study can help mills in optimizing the bleaching process based on oxygen pre-treatments.

A preliminary study of Eucalyptus globulus acetosolv pulp bleaching suggested the possible benefits of an oxygen delignification stage following the one with acetic acid. Experiments on oxygen pre-bleaching looked at the influence of temperature, soda concentration, and time on lignin and xylan contents and on brightness of E. globulus acetosolv pulps. In time interval of conditions assayed, the [O.sub.2] treatment increases the degree of delignification and brightness of the pulp without noticeable changes in viscosity.

We conducted delignification experiments of E. globulus wood with acetic acid, varying temperature, time and catalyst [hydrogen chloride (HCl) or potassium iodidide (KI)]. The pulps were then used to test different bleaching sequences with hydrogen peroxide after being washed with water and/or a sodium hydroxide solution or treated with oxygen. We found that KI improves the selectivity of the process and the mechanical characteristics of the pulp are less damaged. Consequently, we focused on the acetosolv pulp obtained using KI as catalyst.

The bleached pulps showed acceptable kappa numbers, but none had satisfactory tensile, burst, and teat' indexes due to low fiber length and viscosity. These results suggest the convenience of applying a pretreatment with oxygen in alkaline medium before hydrogen peroxide bleaching.

The results indicate that oxygen delignification has led to an important reduction of lignin content of acetosolv pulp, together with an increase in brightness, without significantly affecting their viscosity. With the aim of preserving hemicelluloses with acceptable kappa number and brightness degree, [O.sub.2] delignitication at lower temperature, shorter time, and higher NaOH concentration than those of the assayed values should be carried out before hydrogen peroxide bleaching.

Vazquez and Antorrena are professors, Gonzalez is associate professor, and Freire is assistant professor at the University of Santiago, Department of Chemical Engineering, E-15706, Santiago de Compostela, Spain. Contact Antorrena by email at
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Title Annotation:Bleaching: summary of peer-reviewed material
Author:Antorrena, Gervasio
Publication:Solutions - for People, Processes and Paper
Date:Feb 1, 2002
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