Production of a pure lignin product, Part 3: Distribution and removal of inorganics from softwood lignin

Future high value lignin-based products, such as carbon fibers, 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 process parameters, such as pH and retention time in re-suspension, on the content of inorganic impurities in the LignoBoost lignin. Black liquor from pulping of softwood was used in this study. Specific attention was paid to the elements Na, K, Al, Ba, Ca, Fe, Mg, Mn and Si. It was found that the softwood lignin contained significantly less Ca, resulting in lower ash content, than eucalyptus lignin separated in earlier studies made in this series. Inorganic particles, containing for example Ca-O and Na-Al-Si, were observed in the washed lignin. Prolonged retention time in the re-suspension stage reduced the content of Ca, Mn, Mg, Fe and Si in the washed lignin. A reduction of the content by 30-50% after 24 h was possible. The rate at which these elements are removed from the agglomerates was found to be dependent on the mixing conditions, which indicates that the mass transfer in the liquid film around the particles is of importance for the total mass transfer resistance.