The Influence of the Bleaching Medium on Caustic Extraction Efficiency (II): Oxidized Lignin Solubility

Abstract

Caustic extractions performed with >50% ethanol solutions severely hindered lignin removal from chlorine dioxide (D₀) treated pulps. In this study, we show that the alkali solubility of an isolated D₀ lignin decreased as the concentration of ethanol in the medium was increased from 50 to 75%. In addition, the amount of dissolved lignin in the alkaline extraction effluents approached its solubility limit when the medium contained more than 50% ethanol. These results indicate that alkaline extractions of D₀-stage pulps with ethanol-rich media were limited by the lower solubility of ionized lignin fragments in these solutions as compared to water. An empirical solubility parameter analysis predicted that non-ionized D₀ lignins should be more soluble in ethanol than in water. Indeed, delignification with an acid extraction stage was improved when the water was replaced with ethanol.