Spent sulphite liquor fractionation into lignosulphonates and fermentable sugars by ultrafiltration

• Ultrafiltration is used to separate lignosulphonates and sugars from sulphite liquor.
• Physico-chemical properties of the resulting streams are evaluated.
• Series membrane processes are proposed to improve the separation.
• More purified lignosulphonates and sugars are achieved to be valorised.

Lignosulphonates (LS) and fermentable sugars are the main components of spent sulphite liquors (SSL) produced in acid sulphite pulping. Ultrafiltration (UF) technology allows the separation of these components from the SSL due to their different molecular size. In the present study, ceramic membranes with different cutoffs (15, 5 and 1 kDa) were used. The membranes were evaluated according to not only their efficiency parameters, but also their efficacy of separation as well as their physico-chemical properties. Each membrane offered the best conditions in function of the studied parameter. Hence, different membrane series systems were proposed. System 3 (15 and 5 kDa membranes) and System 4 (15, 5 and 1 kDa membranes) showed the best conditions taking into account all the studied parameters. The description in more detail of these systems was carried out to determine the possible potential of the resulting streams for their further valorisation by biorefinery processes.