Maximizing yield of liquid-lignin from membrane filtration retentate of kraft black liquor

Removal of lignin from kraft pulping black liquor has the potential to increase pulp production, and at the same time lignin can serve as raw material for renewable fuels. There are well-known challenges associated with the outtake of lignin from kraft black liquor. Examples of such challenges are acid consumption and effects on the Na/S balance in the pulp mill, which can be decreased by a new process concept combining membrane filtration and liquid-lignin separation. In this study, we investigated the yield of liquid-lignin in separation from kraft black liquor retentate, specifically concerning how it was influenced by the presence of xylose and galactoglucomannan. For example, the liquid lignin yield after carbon dioxide separation decreased from 89.3 % to 85.2 % and 86.1 % with the addition of 20 g/L xylose and galactoglucomannan, respectively. The results showed that the yield of liquid-lignin decreased with an increasing amount of sugar added. Similarly to previous research, our results showed an improvement of liquid-lignin yield associated with a decrease in the content of sugars in cases of heat treatment prior to separation. We suggest a new explanation for the interaction between lignin and hemicellulose, namely that the interactions occur at a molecular level, with lignin in solution, and that these interactions possibly involve the formation of covalent lignin-carbohydrate bonds. This makes the lignin more polar and less prone to form a liquid-lignin phase.