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 (60%) and calorific value (LHV 25.4 MJ/kg)

Effect of precipitation conditions on properties of lignin from the LignBoost process

Lignin extraction from black liquor is today an attractive option for pulp mills. Besides the fact that the pulp production may be increased without expanding the recovery boiler the extracted lignin can be used as a fuel internally or be sold as a product, to an external company; either as biofuel or as a feed stock in the production of various chemicals e.g. phenols. This investigation deals with the precipitation conditions and theirs influence on the lignin properties, when using the LignoBoost concept. The precipitation of lignin was tested in the pH range between 11.5 and 9.0. Both softwood black liquor and hardwood black liquor was precipitated to evaluate the possibility to produce lignin with different chemical properties. The results indicate that a high quality lignin, i.e. low sodium content and low carbohydrate content can be produced without problems from the softwood black liquor. The hardwood black liquor was however harder to separate and wash. The sodium content was held at low levels but the carbohydrate content was still present in the lignin after washing

Effect of precipitation conditions on properties of lignin from the LignBoost process

Lignin extraction from black liquor is today an attractive option for pulp mills. Besides the fact that the pulp production may be increased without expanding the recovery boiler the extracted lignin can be used as a fuel internally or be sold as a product, to an external company; either as biofuel or as a feed stock in the production of various chemicals e.g. phenols. This investigation deals with the precipitation conditions and theirs influence on the lignin properties, when using the LignoBoost concept. The precipitation of lignin was tested in the pH range between 11.5 and 9.0. Both softwood black liquor and hardwood black liquor was precipitated to evaluate the possibility to produce lignin with different chemical properties. The results indicate that a high quality lignin, i.e. low sodium content and low carbohydrate content can be produced without problems from the softwood black liquor. The hardwood black liquor was however harder to separate and wash. The sodium content was held at low levels but the carbohydrate content was still present in the lignin after washing

Lignin precipitation from kraft black liquors: kinetics and carbon dioxide absorption

Lignin extraction from black liquor is today an interesting option for pulp mills since it makes it possible to increase the production capacity of pulp without increasing the load in the recovery boiler. Extracted lignin can be used either as a solid biofuel or as a feed stock producing various chemicals after it has been separated from the liquor and properly washed. The lignin must first, however, be precipitated from the black liquor. Although this can be performed in several ways, this investigation deals only with precipitation using CO2 as the acidifying agent. In this study, the gas absorption process and the kinetics of the chemical reactions have been evaluated. Several precipitation experiments were carried out on five kraft black liquors from different mills. A theoretical model, based on the film theory, was developed and fitted to experimental data. The film thickness obtained varied between 0.1•10-9 and 0.3•10-9 m, which showed that the mass transport of CO2 through the film to the reaction zone is fast. This may indicate that the course of events is dominated by the chemical composition of the black liquor rather than the parameters that influence absorption, e.g. mixing conditions