Lignin recovery from black liquor
The precipitation of lignin from black liquor was initially studied as a single step process in which a dilute solution of sulphuric acid (5 % wt (1.05 N), 25 % wt (6.01 N), 50 % wt (14.22 N), and 72 % wt (23.99 N)) was added to the black liquor with the pH adjusted to the desired value at a temperature of 50°C. Then, 100 ml of the black liquor was treated with different amounts of diluted acid to obtain a final pH value of 5 while keeping the temperature constant at 50°C.
After precipitation, the content of each flask was filtered through a pre-weighed oven-dried filter paper using a vacuum filtration unit. The precipitated lignin was washed twice with hot water (total volume 400 ml, pH= 6.8) to remove impurities. The lignin was then dried at 25°C under a pressure of 0.5 mbar using lyophilisation equipment (LYOVAC (GT2, Germany) until reaching a constant weight. Different techniques were used to establish the physicochemical characteristics of the obtained lignins (Tab. 1). These results were published in our paper (Jablonský et al. 2015a, b; Šurina et al. 2015).