Extraction of lignin from black liquor
As demonstrated in the Scheme (Fig. 1), first, 1000 mL pristine black liquor(pH = 12, the solid content was 49.5 wt% as measured after the black liquor wasdried in an oven at 100 °C for 24 h) was purged with carbon dioxide gas for around30 min at 60 °C.
After treatment with carbon dioxide gas, the pH of the solution was reduced to 9, thereby the lignin started to precipitate, and the black color of theblack liquor changed to dark yellow (Fig. 1).
In the next step, 5000 mL of 0.2 Maluminum potassium sulfate dodecahydrate (AlK(SO4)212H2O) was added quicklyto reduce the pH to around 4 with vigorous mechanical stirring at room temperature.
The mixture was then left to stand overnight to ensure the precipitation of lignin inthe solution. The precipitate was then observed clearly and it can be easily collected by filtering or centrifugation.
The slurry was carefully washed with DI water severaltimes to eliminate impurities. The powder was finally vacuum-dried for 48 h.