Na_2CO_3 as Alkali in Hydrogen Peroxide Bleaching of Softwood Kraft Pulp

摘要

An investigation was made to determine the impact of sodium carbonate, Na_2CO_3, as an alternative to caustic soda as the alkali source in the hydrogen peroxide ("peroxide") bleaching of kraft pulp. In this study the conditions of the alkaline extraction are pushed as an aggressive oxidative extraction stage in order to identify differences that may not have been apparent in earlier studies. Scandinavian softwood pulp taken was used as raw material. The study shows that complete replacement of caustic soda by sodium carbonate results in poor lignin removal, slower brightness development and lower whiteness. In addition to other factors, such as the lower pH achievable with carbonate, considerably lower peroxide residual was found. Only partial replacement can be considered, in case Na_2CO_3 is more economically available than NaOH. The differences in pulp tear and tensile indices were insignificant. This study shows that extractions using sodium carbonate were more sensitive to the presence of transition metal ions. Increased peroxide consumption, may limit the applicability of sodium carbonate in aggressive peroxide-reinforced extraction stages unless metal content is carefully managed. The application of peroxide performance enhancement additives may mitigate the loss of peroxide to wasteful decomposition side reactions and enable the use of more aggressive process conditions that will enable the consideration of alternative alkali sources while maintaining bleaching performance.