Extraction of Lignin from Kraft Cooking Liquor by Ultrafiltration

Abstract: Lignin is a potential biofuel which can be extracted from kraft cooking liquor. Lignin can also prove valuable as a speciality chemical. By extracting lignin potential economical revenue can be gained by pulp mills, as well as technical benefits such as easier delignification and bleaching. The subject of this work was the extraction of lignin from kraft cooking liquor using ceramic ultrafiltration membranes. Experiments have been performed in both the laboratory, at temperatures below 100°C, and in a pulp mill at temperatures between 90°C and 145°C. In order to operate membranes at high temperatures new methods which ensure safe and reliable operation were developed. Membranes with cut-offs of 5,000 Da and 15,000 Da have been used in the experimental studies. In the laboratory investigation the average flux at 90°C during concentration to 90% volume reduction of kraft cooking liquor was 55 l/m² h and 110 l/m² h using the 5,000 Da and 15,000 Da cut-off membranes, respectively. At the same time the lignin retention was 60% and 30%. Lignin purity with respect to total dry solids content was increased from 30% to 60% using the membrane with a cut-off of 5,000 Da. Reaching higher lignin purities by ultrafiltration alone proved difficult, and it is therefore suggested that diafiltration or acid precipitation be used on the ultrafiltration retentate to achieve higher lignin purities. It was shown that it is technically feasible to operate ceramic ultrafiltration membranes at temperatures above the normal boiling point when treating highly alkaline solutions. The flux was lower when treating kraft cooking liquor in a pulp mill than in the laboratory investigation. It was found that the lignin retention is temperature dependent, decreasing from 55% at 90°C to 30% at 145°C (at 400 kPa transmembrane pressure). Fouling and cleaning of the membranes were investigated both in laboratory and in mill trials. A successful cleaning method based on rinsing with cooking liquor permeate followed by alkaline cleaning was developed in the laboratory investigation. It was found in the mill trials that the membranes could be operated for up to 70 hours without the fouling becoming too severe and that the capacity of the membranes was restored after chemical cleaning.