Washing lignin precipitated from kraft black liquor

Journal article, 2006

The washing properties of lignin precipitated from kraft black liquor were studied at varying precipitation pH, wash water pH, wash pressures and wash water temperatures. Significant experimental difficulties were encountered in several cases where the filter cake and/or the filter media were plugged, probably by re-dissolved/precipitated lignin. A peak in lignin concentration in the wash water between wash ratios 1-2 was detected, which may be attributed to a lowered ionic strength and a rather high pH in this area. The problems were minimised by applying a low wash water pH (approximately 1) and a low wash water temperature (20°C). After a wash ratio of approximately 1.75-2 is reached, no major effects on the remaining sodium levels can be achieved by additional washing. The final purity of the lignin varied considerably. These variations can be explained in some cases by a partial plugging of the filter cake, leading to an uneven distribution of wash water. In the successful experiments, the amount of impurities was reduced to very low levels, with sodium concentrations down to 0.065%. The sulphur content after washing was approximately 1.2% while the nitrogen content was below 0.86 mg/g. The lignin washing process could be modelled in a rather satisfactory way, with the advection-dispersion equation accounting for an average of 84% of the washing process, and with effective dispersion coefficients varying between 3.6·10-7 to 4.7·10-6. The equilibrium constant was very small in most cases. The modelling could be improved somewhat by adding an inlet mixing step to the model.