Lignin Exraction from Black Liquor: Precipitation, filtration and washing
Lignin extraction is a very interesting option for pulp mills today. Lignin is one major component, present as a dissolved organic compound, in black liquor, which is a by-product in the alkaline pulping of wood. After the digestion of wood chips, the black liquor is separated from the cellulose. In a modern, energy-optimised pulp mill, there is an energy surplus which can be seen as an excess of combustible materials present in the black liquor. Lignin has a high heating value and, hence, the energy surplus can be considered to be a surplus of lignin. The lignin product obtained from the extraction process can be used either as a biofuel or as a feed stock to produce other products, e.g. carbon fibres or phenols.
In this work, the lignin was precipitated from five different softwood black liquors solely by absorption of CO2. The absorption operation was studied and a mathematical model was proposed that can describe the decrease in pH during the precipitation process by adjusting the design parameters to the experimental data. The design parameters used in the theoretical model are: fkOH, a correction factor which substitutes the ionic strength in black liquor, δ, the film thickness surrounding each CO2 bubble during absorption, and K2, the equilibrium constant of the rate-determining reaction. Several black liquors from different mills were investigated in the absorption study. The film thickness obtained from the mathematical model varied from 0.110-9 to 0.310-9 m, showing that the mass transport of CO2 through the film to the reaction zone is very fast. This may indicate that the system is controlled principally by the reactions and chemical composition of the black liquor rather than the mass transport rate.
A novel lignin extraction process for precipitation, filtration and washing of lignin was investigated. The result obtained from the laboratory studies was found to be excellent in terms of final purity of the lignin product and very low yield losses during separation. The separation of the precipitated lignin was labelled as easy-filtered. The process was also studied in both bench and pilot-scale, with the focus being on the separation characteristics. Filtration and washing characteristics were determined and found to be similar in the different scales studied. In the pilot-scale experiments 8 tonnes of lignin were produced, with low levels of contamination (<0.5 %-wt sodium), dry solids content (>60%) and calorific value (LHV 25.4 MJ/kg).