Conversion of black liquors from the kraft process of a Eucalyptus pulp and paper mill factory into pure lignin. Then pure lignin will be used as sustainable raw-material for green processes able to generate promising added-value compounds products such as fibre precursors. First, the liquid stream generated from the kraft process, known as black liquor, will be thoroughly characterized using specific and tailored analytical methodologies. Second, the extraction of lignin from the black liquor will be realized by means of acidification. Reducing the pH of the initial black liquor, it is then possible to proceed with the filtration of particles of lignin which will form, by means of membrane filtration or ultrafiltration. Two procedures can be used for reducing the pH around to 9, LignoBoost process (acidification takes place, enhanced by means of CO2 presence); and LignoForce process, where the black liquor is first oxidized (O2), and then acidified (CO2). Finally, the lignin obtained from black liquor by acidification, filtration and vacuum drying will be trialled to produce polyurethanes (PUs). Later on, PUs can be used for molecular weight upgrade of the lignins, or for forming blends to be trialled in the spinning process.