Influence of Soluble Anthraquinone Compound on Bleaching Load during Kraft Cooking of Various Woods

In this study, the influence of soluble anthraquinone compound (SAQ) addition to the kraft cooking process on decreasing the load of elemental chlorine free (ECF) bleaching was investigated by analyzing unbleached pulps. As a cooking additive, 1, 4-dihydro-9, 10-dihydroxyanthracene sodium salt in alkaline solution (SAQ) has been used to accelerate delignification in many kraft pulp (KP) mills. The addition of SAQ resulted in increased pulp yield as well as increased pulp brightness among all investigated hardwood and softwood pulps. Furthermore, the hexenuronic acid (HexA) content in unbleached pulp was not affected by the addition of SAQ under the same cooking conditions. Therefore, it is likely that the load of ECF bleaching will not increase with the addition of SAQ. The most important benefit of the addition of SAQ was found to be a decrease in the kappa number along with an increase in the pulp brightness to induce a decrease in the bleaching load.