Lignin fractionation from E. Globulus kraft liquor by ultrafiltration in a three stage membrane sequence

The aim of this work is to evaluate the fractionation of E. globulus industrial kraft liquor by ultrafiltration. A sequential membrane process, using three tubular membranes with nominal cut-offs of 5, 15 and 50 kDa, was applied and the results showed that ultrafiltration is an efficient process for kraft liquor fractionation according to its molecular weight. The obtained ultrafiltration fractions, retentates from 5, 15 and 50 kDa membranes and also permeate from 5 kDa membrane, were studied and the differences between its composition and properties were assessed. In addition, the lignins in fractions were isolated by acid precipitation and studied through nitrobenzene oxidation and 13C and 31P NMR, being able to establish the differences in their composition and structural characteristics and consequently find the more adequate commercial application as high added-value products. Lignin isolated from the retentate of 50 kDa membrane shows the higher molecular weight and the higher content of carbohydrates and inorganics. On the other hand, from the permeate of 5 kDa membrane it was obtained the less contaminated lignin and the one with the highest content of uncondensed structures and the highest DC, representing an important source of phenolic monomers through oxidative depolymerization.