If the aim of the process is to produce a lignin powder, the lignin in the solution with high lignin concentration is precipitated. In the precipitation, acid is added to the lignin-rich solution in order to decrease the pH. By decreasing the pH, the lignin starts to precipitate and form particles. Due to the previous recovery of the major part of the cooking chemicals, only a small amount of acid is needed compared to if the precipitation would be performed on the black liquor directly.
After the precipitation, the lignin slurry is filtered through a filter cloth in order to separate the lignin particles from the liquor. The filter cake of lignin is then washed before it is dried. The dried lignin powder can finally be fed into different downstream processes, e.g. a pyrolysis unit for production of a bio-oil.