Concentration and purification of lignin in hardwood kraft pulping liquor by ultrafiltration and nanofiltration

Abstract

Today, there is considerable interest in integrated forest biorefineries which, besides pulp, produce high-value-added products, such as, polymers and carbon fibres. Lignin is the major non-cellulosic constituent of wood. It is a complex, amorphous highly crosslinked polyphenolic. Lignosulphonate in spent liquor in sulphite mills has long been isolated using ultrafiltration. Generally, precipitation is used to extract the lignin from kraft black liquor. The liquor is commonly withdrawn from the evaporation unit in the recovery system, where the dry substance is optimal for precipitation. Lately, interest in the isolation of lignin by ultrafiltration of kraft black liquor has arisen. The concentration of the liquor is not that critical, and there is therefore considerable freedom in the choice of liquor for treatment with ultrafiltration. In this work, the influence of membrane cut-off, transmembrane pressure and cross-flow velocity during ultrafiltration of cooking liquor and black liquor was studied. The extraction of lignin from hardwood black liquor withdrawn before the evaporation unit was investigated. A hybrid ultrafiltration/nanofiltration process was used. The lignin concentration was 60 g/l in the black liquor and 165 g/l in the product stream (the nanofiltration retentate). A cost estimate indicates a production cost of €33 per tonne of lignin.