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.