Effect of black liquor replacement in wheat straw soda-AQ cooking and lignin structure of pulps

Abstract

The effects of black liquor replacement cooking (BLRC) on wheat straw soda-anthraquinone (AQ) pulps and their lignin structures were investigated by using different black liquor replacement ratios (BLRR) and alkaline nitrobenzene oxidation and ozonation. The residual AQ, alkali, and dissolved lignin, as well as carbohydrates in the wheat straw black liquor, greatly influence wheat straw soda-AQ cooking. The influence could be controlled by different BLRR. The BLRC with around 60% BLRR resulted in superior delignification selectivity. The residual AQ and dissolved carbohydrates in the black liquor are beneficial to delignification selectivity, while the dissolved and degraded lignin can slow the delignification rate. No obvious differences were found in the lignin condensation and β-O-4 structure degradation by the BLRC at different BLRR compared to control soda-AQ cooking. The BLRC technology has potential application because it reduces chemical charge and chemical recovery load and results in higher yield and quality of pulp.