Local and average filtration properties of kraft softwood lignin

The extraction of lignin from black liquor is of interest in order to increase the overall material yield when producing paper pulp by the kraft process. The extracted lignin can be used either as a solid biofuel or a potential alternative raw material to fossil oil for the production of various green chemicals and high valuable products such as carbon fibres. The LignoBoost process is a new approach in which the kraft lignin extracted reaches a high level of purity. In this study, the local and average dead-end filtration properties during dewatering of a softwood lignin extracted from black liquor using the LignoBoost process were investigated; the material was also characterized. It was found that the investigated lignin was an easy to filter material that formed weakly compressible filter cakes in the solid compressive pressure range of 0.03 to 1.5 MPa. The concentration of the lignin slurry did not influence the filtration properties; the resulting cakes final heights were in the industrial size range. Moreover, the two different filter media used yielded similar results. Semi-empirical relationships were fitted successfully to the experimental local data and a good estimation of the average filtration properties was also obtained using the relationships fitted parameters. This indicates a good agreement between the local and average experimental data.