Totally chlorine free (TCF) bleaching of wheat straw soda-anthraquinone (AQ) pulp with a low initial kappa number

Abstract

The totally chlorine free (TCF) bleaching was studied for wheat straw soda-anthraquinone (AQ) pulp with a low kappa number of 8.3 using the sequence of OQPO and OQPOPO. It was found that the oxygen delignification on wheat straw pulp was still effective despite its low initial kappa number. A delignification extent of 42% and an ISO brightness increase of 10.9% were achieved. In the OQPO sequence, an ISO brightness ceiling of 87.2% was achieved as the hydrogen peroxide (H2O2) charge was increased from 2% to 6% in the PO stage. In the OQPOPO sequence, 2% H2O2 was added in the second PO stage and the pulp brightness exceeded 88% with a total charge of 4% H2O2 in the POPO bleaching. Higher brightness values of 89.5% and 90.1% were obtained under the total charges of 6% and 8% H2O2, respectively. Compared to the OQPO sequence, the higher brightness and equal viscosity of the bleached pulps were obtained under the same total charge of H2O2 in the OQPOPO sequence. The bleached pulps with high ISO brightness values (greater than 88%) retained acceptable strength properties. A slight decrease of fiber length occurred as the bleaching sequences proceeded, while the fiber width and curl index somewhat increased.