Towards a pure kraft lignin
The most widely used production method for paper pulp in the world is kraft pulping. In chemical pulping the polymer, lignin, which binds the wood fibres together, is partly depolymerized. This allows the wood fibres to be separated from each other. Lignin and other organic by-products are dissolved in the black liquor and is subsequently combusted in the pulp mills recovery boiler to regenerate chemicals and recover the energy. Many kraft pulp mills have an energy surplus. This surplus means that parts of the organic by-products, such as lignin, can be separated and sold to generate an additional revenue stream for the pulp mill. Lignin with low inorganic content can today be separated from the kraft pulp mill with the LignoBoost process. This lignin can be used as solid fuel in e.g. lime kilns. Another alternative is to utilize the separated lignin as a raw material for high value materials such as carbon fibres. Such applications require a lignin raw material with even lower content of inorganic impurities than what can be produced today.
This study focuses on further development of the LignoBoost process in order to facilitate a reduced content of the elements Na, K, Al, Ba, Ca, Fe, Mg, Mn and Si in the separated lignin. In this work parameters such as black liquor composition, residence time and pH in re-suspension were evaluated. Industrial black liquors were used in this study. This study confirms the previous findings on the possibilities to remove Na by washing, introduction of an extra wash stage resulted in 80-95% lower content of both Na and K. Other elements have in earlier studies been found to be more difficult to remove by washing. These findings were confirmed as an extra wash stage only marginally affected the content of the other studied elements, compared to a normal LignoBoost lignin. By use of SEM-EDS, inorganic particles, such as Ca-oxalate, were observed in washed lignin. This partially explains why some inorganics are harder to remove from lignin than others. The pH in the re-suspension stage did not affect the content of Al, Fe, Mg, Mn and Si in the lignin. However, the Ca content was affected in samples where Ca-oxalate occurred in large amounts, the solubility of this salt is pH dependent at pH < 4. Prolonged residence time in the re-suspension stage reduced the content of Ca, Mn, Mg, Fe and Si in the washed lignin; a reduction by 30-80% after 24 hours was possible. The mass transfer resistance in the liquid film surrounding the particles affected the rate of which these elements are removed from the lignin.
This work has shed new light on why certain elements are hard to remove from lignin separated from the LignoBoost process. Furthermore, industrially applicable methods for obtaining lignin with lower inorganic content have been developed. By combining some of these methods it was possible to reduce the cationic content in the studied eucalyptus lignin from 5.7 to 0.6 g/kg lignin and the studied softwood lignin from 4.5 to 0.6 g/kg lignin. Findings in this work can be used as guidelines for LignoBoost operation and a base for further development towards a pure kraft lignin.