Precipitation of Kraft Lignin under Alkaline Conditions

by Sundin, Jonas

In this work, the influence of metal cations on the removal of lignin during kraft pulp washing has been studied. The interaction of metal cations with kraft lignin in alkaline solutions has been elucidated by, among other things, the application of ion-selective electrodes. The effect of the interaction of metal cations with kraft lignin in pulp washing, as well as its consequences in oxygen delignification and hydrogen peroxide bleaching stages are reported. An attempt is made to relate the laboratory findings to industrial process conditions. The high molecular mass fraction ( > 1000 g/mol) of kraft lignin was precipitated by metal cations (Na i +, Ca2+, Mg2+, Al3+) at pH 9, but only calcium and magnesium ions caused any noticeable precipitation at pH 11-13. Since only the high molecular mass fraction of the lignin was precipitated, the precipitation can be regarded as the coagulation of a colloid. The critical coagulation concentration (ccc), that is, the concentration of the metal cation when the coagulation occurs, was about 3 mmol/L for Ca2+ at pH 11 and ambient temperature. The ccc(Ca2+) was the same for birch and spruce kraft lignin but higher for dissolved lignin after oxygen delignification. A method was developed to estimate the free calcium ion concentration in lignin solutions during titration with calcium ions by the use of ion-selective electrodes. An end point was detected for the reaction between calcium ions and lignin. At calcium ion concentrations below the end point, almost all the added Ca2+ was bound to the lignin. The maximum content of calcium ions bound to the lignin was found to be about 30 Ca2+/100 C-9 units in the kraft lignin. This was confirmed by two other methods. The precipitation of kraft lignin by calcium ions during washing of a laboratorycooked kraft pulp resulted in a pulp with a kappa number 15 units higher than a pulp washed in the absence of calcium ions. The degree of swelling of the fibres in these pulps was constant. The corresponding difference in kappa number for laboratory washing of an industrial unbleached softwood kraft pulp was 5 kappa number units for a pulp produced for bleached products and 10 kappa number units for a pulp produced for unbleached products (sackpaper grade). The precipitated lignin was darker than the other residual lignin in the pulp. Pulps containing calcium-precipitated lignin yielded results on oxygen delignification similar to those of pulps without precipitated lignin. In an alkaline hydrogen peroxide stage, the precipitated lignin seemed to be more reactive than the other part of the residual lignin. This is probably due to a higher content of phenolic groups in the precipitated lignin. An attempt to estimate the effect of precipitation of lignin by calcium ions during industrial brownstock washing in a mill, where the calcium and magnesium concentration in a number of filtered liquors was determined, resulted in 1-2 units increase in kappa number for a pulp produced for bleached products in a fibre line with an oxygen stage. The estimate of the effect of magnesium ions is uncertain, but it may be considerably higher due to the higher concentrations of magnesium in process liquors.