Lignin Separation from Kraft Black Liquors by Tangential Ultrafiltration

I n chemical pulping a large part of the wood is dissolved during digestion. The dry matter content of the generated black liquor is typically 12 to 18 wt% [1, 2]. In the case of kraft black liquors four main groups of organic constituents are present, 30-45 wt% ligneous material, 25-35 wt% saccharinic acids, about 10 wt% formic and acetic acid, and 3-5 wt% extractives. It also contains about 1 wt% methanol and many inorganic elements, mainly sodium (17-20 wt%) and sulphur (3-5 wt%). The exact composition of the black liquor varies considerably between different mills depending on the cooking conditions and feedstock. The vast majority of lignin in the spent liquors of pulp mills is used as inhouse fuel for the recovery of chemicals. If liquid fuel production from biomass and solvent pulping reach commercial maturity the production of lignins will be enormous. Therefore, current and future availability of lignins in spent liquors together with their characteristic polydispersity will make mandatory the use of fractionation schemes for the production of streams with uniform characteristics and composition. Fractionation of cooking liquor from kraft and sulphite pulp mills by ultrafiltration (UF) has been studied since ca. thirty years ago [219]. In most of these applications the objective of UF processes was to purify the lignin fraction in order to use it as a valuable LIGNIN SEPARATION FROM KRAFT BLACK LIQUORS BY TANGENTIAL ULTRAFILTRATION