Separation of lignin in kraft cooking liquor from a continuous digester by ultrafiltration at temperatures above 100°C

Lignin was extracted by ultrafiltration from kraft cooking liquors taken directly from a continuous digester. The temperature of the liquors was 145°C (softwood) or 135°C (hardwood), the total dry solids content was 18% and the pH 13–14. Methods for start-up, shut-down and cleaning of conventional UF equipment modified for this demanding application, were developed. The influence of transmembrane pressure, cross-flow velocity and temperature on the performance of two ceramic membranes with cut-offs of 5,000 and 15,000 Da was studied. The flux of softwood liquor at 145°C and a TMP of 400 kPa was 100 l/m2 h for the membrane with the 15,000 Da cut-off and 50 l/m2 h for the membrane with the 5,000 Da cut-off. The retention of lignin was 20% and 30% for the 15,000 Da and 5,000 Da membranes, respectively.