Production of a pure lignin product : part 1: distribution and removal of inorganics in Eucalyptus Globulus kraft lignin

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 re-suspension pH, on the content of inorganic impurities in the LignoBoost lignin. Black liquor from pulping of Eucalyptus globulus was used in this study. Specific attention has been paid to the distribution of the inorganics Na, K, Ca, Al, Mn, Mg, Si and Fe. It was found that by lowering the re-suspension pH and wash pH to 1.5 the ash content could be greatly reduced, primarily due to the dissolution of Ca-oxalate particles present in the lignin. By slightly increasing the retention time in the re-suspension stage, the content of Mg, Al, Mn, Si and Fe could be reduced by approximately 50 %. Repeated re-suspension 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 re-suspension/wash pH and a repeated re-suspension and wash.