Influence of membrane filtration on extraction and characteristics of lignin from a kraft dissolving pulp mill pre-hydrolysate

During dissolving pulp production, the extraction of hemicellulose and other organics from hardwood chips prior to the kraft pulping process can be performed using steam or hot water. The pre-hydrolysate from this step is rich in sugars and contains a considerable amount of lignin. It is estimated that the lignin content in this stream can be as high as 28 t d−1 for a kraft dissolving pulp mill producing about 550 t d−1 of pulp. In this article, we report on the recovery of lignin and its characterization. The use of a membrane filtration system has been experimentally validated as an effective method for concentrating the pre-hydrolysate and extracting lignin. Three membranes with different molecular weight cut-offs (100, 10 and 1 kDa) have been tested. In a subsequent step, sulfuric acid was added to the concentrated pre-hydrolysate to precipitate the lignin. The recovered lignin was characterized with respect to purity, molecular weight distribution and other chemical properties. Approximately 99% retention of both sugars and lignin was achieved using the 1 kDa membrane, with about 44.4% of the total lignin being extracted by sulfuric acid precipitation method. The lignin was found to have a low molecular weight (2810 Da), high homogeneity (Mw/Mn = 1.15), and high purity (91%). These characteristics make the pre-hydrolysate lignin a very attractive candidate for niche applications.