The influence of the bleaching medium on caustic extraction efficiency (II): Oxidized lignin solubility

Caustic extractions performed with >50% ethanol solutions severely hindered lignin removal from chlorine dioxide (D-0) treated pulps. In this study, w e show that the alkali solubility of an isolated D-0 lignin decreased as th e concentration of ethanol in the medium was increased from 50 to 75%. In a ddition, the amount of dissolved lignin in the alkaline extraction effluent s approached its solubility limit when the medium contained more than 50% e thanol. These results indicate that alkaline extractions of D-0-stage pulps with ethanol-rich media were limited by the lower solubility of ionized li gnin fragments in these solutions as compared to water. An empirical solubi lity parameter analysis predicted that non-ionized D-0 lignins should be mo re soluble in ethanol than in water. Indeed, delignification with an acid e xtraction stage was improved when the water was replaced with ethanol.