Lignin Extraction from Black Liquor - Properties of the Liquors and Sulphur Content in the Lignin

Abstract: Kraft lignin isolated by precipitation can be considered as being a valuable renewable source of energy and material. Removing lignin from black liquor is also advantageous as it decreases the load on the recovery boiler, which is usually the limiting factor in the production of kraft pulp. This thesis is divided into two parts: in the first, the changes in the viscosity and boiling point elevation of black liquor were investigated when lignin was partly extracted from black liquor, and a correlation for the viscosity of black liquor was developed. In the second part, the isolated lignin was modified by the addition of calcium in combination with heat treatment in order to decrease the sulphur content in the lignin and as well as reduce the emission of SO2 in the flue gases after combustion.The liquors studied in the first part were both softwood and hardwood black liquors taken from several lignin extraction runs with varying concentrations of lignin. The viscosity of the filtrate obtained after lignin extraction (i.e. when approx. 70 % of the lignin had been removed from the black liquor) was between 2 and 100 times (depending on the dry content and temperature) lower than the corresponding value of the original black liquor. The new correlation presented in this work improves the prediction of the viscosity of lignin-depleted black liquors compared to previous models since it takes the concentration of lignin and hemicelluloses into account. The results from boiling point elevation measurements showed that the removal of lignin does not change the boiling point elevation drastically. In the second part of this thesis, the precipitated lignin was modified by the addition of calcium (0-20 w/w-%) at both room temperature and elevated temperatures (100, 140 and 180 °C). The experimental results indicated that at room temperature the lignin was saturated at about 0.8 mmol Ca/g lignin: at elevated temperatures, however, no saturation level could be detected. The flue gases from combustion of the modified and unmodified lignins were analysed; the results showed that the addition of calcium, followed by heat treatment, could reduce emissions of SO2 by at least 70 %. XRD analyses of the ashes showed that calcium sulphate was formed after the combustion of the modified lignin.