Soda–AQ delignification of poplar wood. Part 1: Reaction mechanism and pulp properties

Abstract

Abstract Soda and soda-anthraquinone (AQ) pulpings of poplar (Populus deltoides) wood performed in a flow-through reactor gives rise to the formation of coniferyl alcohol (1) sinapyl alcohol (2) and other low molecular weight (LMW) compounds in different stages of delignification. During the heating-up period, the formation of these compounds increases until the maximum pulping temperature (t max, 170°C) is reached. Afterward, their concentration in the spent liquor decreases sharply. This effect is accompanied by solubilization of high molecular weight (HMW) lignin fractions. Compared with soda pulping, the soda-AQ pulping shows a higher production of LMW compounds because of the presence of AQ-anthrahydroquinone (AHQ) redox system. During the heating-up period, the formation of 1 and 2 – apparently originated from non-etherified β-aryl-ether moieties in the lignin – is accompanied by solubilization of lignin fragments with relative LMW. Lignin cores, having β-aryl-ether bonds as major hydrolyzable inter-unit linkages, remain in the fiber, and they give rise to solubilized lignin fragments with HMW at later delignification stages. The dissolved fragments, in particular those formed during heating-up period, undergo further degradation in the liquor. The degradation occurs via oligomers at the end of the pulping, while the residual lignin cores are more resistant. Pulps with low kappa number can be produced by maintaining higher alkali concentrations than the usual in the last delignification phase. When the pulping liquor is replaced in this stage with liquors of higher alkali concentration, extended delignification will be the result. As a consequence, less degraded lignin fragments will be deposited on fibers.