Totally chlorine free (TCF) bleaching of wheat straw soda - Anthraquinone pulp

Totally chlorine free (TCF) bleaching of wheat straw soda - anthraquinone (AQ) pulp was investigated. Previous pulping experiments, performed under different conditions had revealed that soda - AQ pulping with 16% active alkali for 90 minutes, at 160°C, yields pulp suitable for bleaching experiments. TCF bleaching was performed in an O Q (OP) or O Q (OP) P sequence. Oxygen delignification with an alkali charge of 2.5 % NaOH at 90°C and 90 min resulted in a delignification rate over 50 %. In the subsequent (OP) stage, drastic conditions (4% H₂O₂, 2.75% NaOH, 90 min at 98°C) were required to exceed an 80% ISO brightness. With a final P stage, brightness could be improved to 84-85% ISO. Bleaching led to only small losses in tensile and burst strength for unchanged tear strength. Short beating in a Jokro mill was sufficient to attain a breaking length well above 7 km. The results of the study indicate that TCF bleaching can be successfully applied to wheat straw soda - AQ pulps to produce pulp for writing and printing paper without detrimentally affecting the environment.