Effect of Residual Alkali Level in Softwood Kraft Cooking

Abstract In washing of softwood pulp, pH is reduced to such low levels that re-precipitation of dissolved lignin may result. It was hypothesized that the lower the residual alkali after cooking, the higher the risk of lignin re-precipitation, which in turn would affect the subsequent oxygen delignification stage negatively. To test the hypothesis, kraft cooks were performed to different residual alkali levels, ranging from 5 to 15 g/l and the delignified chips were subjected to different washing strategies and then oxygen delignified. However, the results show that even at low residual alkali, lignin stays in solution and only minor amounts of lignin re-precipitate during washing. The amount was irrespective of the residual alkali level in the cook and amounted to 1-2 kappa number units. No effect of residual alkali level was observed on the performance of the oxygen delignification stage. The residual alkali level affected the lignin concentration in the lumen after cooking. At the low residual alkali level, the lignin concentration in the lumen liquor was equal to the free liquor and as the residual alkali level increased, the lignin concentration in the lumen increased, while the lignin concentration in the free black liquor remained on the same level at all residual alkali levels.