THE EFFECT OF LIQUOR HEAT TREATMENT ON THE LIGNIN CARBOHYDRATE CONTENT Master’s thesis for the degree of Master of Science in Technology

This thesis had three different objectives: i) to discover different methods for kraft lignin purification that could be used alongside acid precipitation, ii) test the most suitable treatment on laboratory scale and evaluate the potentiality of this method as a kraft lignin purification method, and iii) to compare the feasibility of lignin purification alongside the acid precipitation compared to conventional acid precipitation. Three different methods were discovered for kraft lignin purification: hydrolysis, solvent extraction and liquor heat treatment (LHT). The LHT was evaluated to be most suitable based on the existing literature. The effectiveness of the LHT was tested in laboratory scale while using cooking black liquor (CBL) and weak black liquor (WBL), which were obtained from softwood kraft pulp mill. The lignin samples were isolated from the liquors with CO2 precipitation procedure. The LHT changed the precipitation behavior of the lignin, which led to a formation of pitchlike lignin. This unwanted behavior could be prevented by decreasing the precipitation temperature to 50 °C. Both the liquors and the end product lignins were characterized. The characterized properties were dry matter content, residual alkali content, sodium sulfide content, lignin concentration, carbohydrate content and sulfur content for the liquors and purity, sodium content and elemental composition for the end product lignins. The LHT proved to be fairly effective method for lignin carbohydrate reduction. The highest carbohydrate reduction, circa 90%, was achieved with 190 °C treatment temperature, while the heat treatment at 170 °C for 45 minutes resulted to 75.3% and 65.6% carbohydrate content reduction for WBL and CBL respectively. The feasibility of the LHT lignin precipitation process was compared to the conventional acid precipitation. This comparison was based on the operating costs of each process. The inclusion of LHT increased the operating cost by 162 176 €/t of lignin for WBL and 148 167 €/t of lignin for CBL depending on the treatment temperature. These additional operating costs increased the total operating costs by roughly 50% when compared to the conventional acid precipitation, therefore the LHT lignin should only be used for high value applications. Further research is still required in order to optimize the LHT process.