Cost estimates of kraft lignin recovery by ultrafiltration

Climate change and endeavours towards a sustainable society have brought the interest of wood-based biorefineries to the fore. Lignin is the main energy carrier in chemical pulp mills. Today, the heat value of lignin is commonly recovered in the recovery boiler of the mill. However, interest in the extraction of lignin for use as an external biofuel and in speciality chemicals is growing. Lignosulphonates have been isolated by ultrafiltration from spent liquor in sulphite pulp mills for decades, whereas kraft lignin is mainly extracted by precipitation. The isolation of kraft lignin by ultrafiltration is currently attracting increasingly greater interest because lignin can be extracted from pulping liquors at almost all positions in a pulp mill using ceramic membranes, without altering the pH or temperature of the liquor. In this work hardwood lignin was recovered from two process streams in a kraft pulp mill by ultrafiltration with a ceramic membrane with a cut-off of 15,000 Da. It was shown that lignin can be recovered from a cooking liquor at a cost of about 60 € per tonne of lignin and from black liquor withdrawn from the evaporation unit at 33 € per tonne.