Membrane technology allows the separation from the black liquor of different lignin fractions with a specific molecular weight being able to be used in the synthesis of several high value-added products (chemical reactants, resins and biocomposites, antioxidant agents, etc.). In the present study, black liquor resulting from the alkaline pulping of the Miscanthus sinensis (7.5% NaOH, 90 min and 90 °C) has been subjected to several stages of ultrafiltration, by using ceramics membranes of different cut-offs (5, 10 and 15 kDa). The different permeates obtained have been characterized (density, % TDS, inorganic and organic content and lignin recovery in each fraction), analyzing the effectiveness of the conditions of the pulping and ultrafiltration processes. Subsequently, the lignins have been isolated by acid precipitation and studied by means of different analytical techniques, being able to establish the differences in their chemical structure and thermal behaviour, and subsequently their more adequate commercial application.