Influence of the oxidative thermochemical treatment on the chemical composition of hardwood kraft black liquor

We thermochemically treated hardwood kraft black liquor to evaluate viscosity reduction with and without NaOH addition (1% of black liquor dry solids) at 115°C and 145°C for 45 min using three oxygen dosage levels (0.83, 1.24, and 2.07 mol/kg d.s.). All black liquors were analyzed for viscosity. In addition, selected liquors were characterized by residual effective alkali, molecular mass distribution of lignin, and chemical composition of aliphatic carboxylic acids. In these cases, we determined the total content of lignin, aliphatic carboxylic acids, and polysaccharides. The most significant viscosity reduction (about 60% of the initial viscosity at 70% d.s. content) occurred at 145°C with NaOH addition using an oxygen dosage of 1.24 mol/kg black liquor d.s. No clear changes in the average molecular mass of lignin or the content of polysaccharides were detected during the treatments. However, a decrease in the highest fraction (M > 10,000 Da) of lignin observed was considered to be the main reason for the viscosity decrease.