Lignin Extraction from Black Liquor - Precipitation, filtration and washing

Doctoral thesis, 2008

Various process streams in the pulp and/or paper mill contain valuable substances which can be converted into new products, e.g. lignin and xylane. Implementation of the most recently available technology allows sub-processes not only to produce new products but also to enhance the environmental performance of the mill as well as improve utilization of renewable resources. Extracting lignin from black liquor is one way of recovering valuable organic material. Lignin, which can be found in spent cooking liquor, is currently combusted in the recovery boiler to produce electricity and steam. Removing lignin decreases the overall heating value of the black liquor and, since the recovery boiler often is limited by the heat load, the throughput of black liquor can therefore be increased. Moreover, should the recovery boiler be a bottleneck in increasing the production of pulp, the removal of lignin would increase the capacity of the mill. Refined lignin can either be used as a solid biofuel or as a raw material for the production of other chemicals, e.g. active carbon and phenols. In this thesis, the extraction of lignin from black liquor using the “LignoBoost” process was investigated, with the primary focus being on increasing knowledge of the precipitation of lignin. The chemical reactions and the kinetics of the gas absorption process were studied and a mathematical model proposed that can describe the decrease in pH during the precipitation process by adjusting design parameters to experimental data. Several black liquors were used to study how different precipitation parameters and the chemical composition of the black liquors affected the absorption of gas into the black liquor and the filtration properties of the precipitated particles. In addition, the conditioning step after precipitation was also studied. During precipitation, it was found that the mixing speed affected the pH lowering rate below a pH value of about 10.5, indicating that above this level, the pH lowering rate may be controlled by the chemical reaction rate. This finding was consistent with the conclusions drawn from the work carried out on the mathematical model. It was also found that a higher mixing speed during precipitation increased the average specific filtration resistance of the precipitated particles. When a gas mixture with a low concentration CO2 was used, the pH lowering rate decreased significantly. The experiments using hardwood black liquors showed that the average specific filtration resistance was lowered considerably when the hemicelluloses were removed prior to precipitation. By using the “LignoBoost” concept on laboratory scale it was found that the lignin produced was excellent in terms of final purity and the yield losses during separation were very low. The process was also studied on both bench and pilot scales, with the focus being on the separation characteristics. 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).