Production of a pure lignin product, part 1: Distribution and removal of inorganics in Eucalyptus globulus kraft lignin

摘要

Future high-value, lignin-based products, such as carbon fibers, are expected to require a lignin raw material essentially free from inorganic impurities. Lignin of low inorganic content can be separated from kraft black liquor by the LignoBoost process. This laboratory scale study focuses on the effect of different process parameters, such as final precipitation pH and resuspension pH, on the content of inorganic impurities in the processed lignin. Black liquor from pulping of Eucalyptus globulus was used in this study. Specific attention has been paid to the distribution of the inorganics aluminum (Al), calcium (Ca), iron (Fe), magnesium (Mg), manganese (Mn), potassium (K), silicon (Si), and sodium (Na). By lowering the resuspension pH and wash pH to 1.5, the ash content could be greatly reduced, primarily because of the dissolution of calcium-oxalate (Ca-oxalate) particles present in the lignin. By slightly increasing the retention time in the resuspension stage, the content of Al, Fe, Mg, Mn, and Si could be reduced by approximately 50%. Repeated resuspension followed by washing resulted in a lignin with very low Na and K content. The total amount of inorganics could be reduced from 5.7 g/kg lignin to 0.6 g/kg lignin by combining a low resuspension/wash pH and a repeated resuspension and wash.