Fractionation of spent sulphite liquor into monomeric sugars and lignosulphonates by an ultrafiltration series system

Abstract

Spent sulphite liquor (SSL) is the main byproduct of acid sulphite pulping. Its main components are lignosulphonates (LS) and monomeric sugars. SSL is typically intended as binder, emulsifier or additive in concretes due to its high amount of LS. Sugars can be converted into high value-added products such as xylitol, PHB or biofuels among others; by means of fermentation processes. However, a separation step to remove the compounds that can act as fermentation inhibitors, as LS, is needed. Membrane technology allows the separation of sugars and LS from the SSL because of their different size of molecular weight. In the present study, ceramic membranes with different cut-offs (15 kDa, 5 kDa and 1 kDa) were used. 15 kDa membrane offered the best results according to LS concentration in the retentate stream, 5 kDa membrane the highest LS removal in permeate stream, and 1 kDa membrane the minimum sugar losses in the permeate stream. Because of this, the combination of the three membranes in a series system was proposed. In this way, an increase in LS removal in the final permeate was achieved (72.78 %), but also an increase in the sugar losses (26.80 %). Hence, further studies will determine the optimal configuration system of this process.