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Kvarnlöf, N., and Germgård, U. (2015). "Oxygen delignification of acid sulfite and bisulfite softwood pulps," BioRes. 10(3), 3934-3947.

Abstract

This study discusses alternatives for the production of sulfite pulps cooked to high kappa numbers, and a subsequent oxygen treatment by using the same cation in the cooking and in the oxygen stages. Magnesium was used as a cation during both the cooking and oxygen delignification stages. By using the same cation in the cook and in oxygen delignification, it is theoretically possible to recover filtrates and send them to the chemical recovery system, meaning that the discharge of pollutants will be very low. Acid sulfite and bisulfite pulps at yields between 56 and 57% (kappa number between 50 and 55) were produced and then delignified with oxygen and magnesium hydroxide to a kappa number between 25 to 30. It was found that the delignification process time was shorter and the yield and viscosity increased, compared with delignification to the same kappa number (25 to 30) for a single cooking stage. The tear and tensile strengths were however unchanged, regardless of delignification process used.
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