Precipitation of Lignin During Pulp Washing, 1998 Pulping Conference Proceedings

Colloids agglomerate and precipitate when ions are added in a concentration above the critical coagulation concentration specific for the system. In this work, we have studied lignin in the pH-range 9-13 of interest in alkaline delignification systems. Kraft pulps were washed in the laboratory at different temperatures and pH’s, with the addition of multivalent ions in different concentrations. It was found that, with calcium or magnesium ions present in concentrations above a certain threshold level, the kappa number of the washed pulp increased to up to 14 units. The pulp also showed a considerably higher light absorption coefficient, indicating a larger amount of chromophoric groups in the lignin precipitated with calcium.

Liquor samples were prepared by washing the kraft pulps, and the precipitation of lignin in these liquors was studied by establishing the concentration thresholds for precipitation. At ambient temperature, the threshold limit for calcium is 3 mmol/L at pH 11 with a shift upwards at higher pH’s and downwards at higher temperatures. Only higher molecular weight lignin precipitates, with a lower limit of approximately 1000 g/mol. Monovalent sodium ions precipitate lignin only at lower pH’s. At the pH’s studied here, the effect of sodium ions is small and is practically non-existent at pH 13.

A few measurements were made in mill systems. It was found that the concentration of calcium throughout the brown stock washing was lower than the previously established threshold limit, and the concentration decreased during the washing. The concentration of magnesium was found to be somewhat higher and to increase during the washing. This is understood to be due to the fact that magnesium is added in the oxygen stage and counter-current washing is applied from oxygen delignification to brown stock washing.

More mill measurements must be carried out before definite conclusions can be drawn.