Valorisation of sodium lignosulfonate by ultrafiltration of spent sulphite liquor using commercial polyethersulfone membrane

ABSTRACT Ultrafiltration of spent sulphite liquor, generated from a laboratory sulphite pulping using chopped stems of Prosopis juliflora, was carried out to recover sodium lignosulfonate (SLS). A commercial hydrophilised polyethersulfone flat-sheet membrane with a molecular weight cut-off of 5 kD was used for the experiments. Effects of the feed dilution factor on the permeate flux were observed, and actual solute rejection coefficients were investigated due to the formation of the concentration polarised layer on the membrane surface. In addition, flux decline and the fouling index of the membrane were determined under the present experimental conditions. SLS obtained in the reject stream was characterised following UV–VIS and nuclear magnetic resonance spectroscopy. The spectroscopic analysis confirmed the reduction of the lignin and hemicelluloses component of wood sample effectively into SLS during sulphite pulping with a considerable degree of similarity with the control sample. The results could be useful as guidelines for the scale-up of SLS production and simultaneous treatment of sulphite liquor from the paper and pulp industry. GRAPHICAL ABSTRACT