Kraft Green Liquor Pretreatment of Softwood Chips. Part III: Lignin Chemical Modifications

Abstract

Summary Quantitative 31P NMR analysis of residual lignin structures isolated from pulps pretreated by green liquor (GL) as a function of various operating parameters are discussed. Uncondensed phenolics and condensed hydroxyl groups decreased as the GL charge increased, but increased with increasing pretreatment temperature. During the cooking of pulps employing variable sulfidities and the addition of anthraquinone (AQ) or polysulfide (PS), various hydroxyl groups exhibited no obvious differences in the case of AQ addition, but dramatic changes were observed at variable sulfidities when the PS was used. The results implied that PS can exert a stronger catalytic effect under a higher sulfidity and result in more cleavage of the lignin macrostructure. The results also demonstrated a correlation between the nature of residual lignin and the efficiency of oxygen delignification based on the finding that the residual lignin in GL modified pulp exhibited a higher reactivity toward oxygen delignification.