(535e) pH-Based Fractionation and Characterization of CO2-Precipitated Lignin From Kraft Black Liquor

Selected fractions of lignin were precipitated from a Kraft black liquor using a pH-based fractionation process, with the objective being to produce various lignin cuts for characterization.  A semi-batch operating mode was used, with CO₂ being bubbled into approximately 2 L of a black-liquor charge in order to bring about the precipitation process.  Experiments were carried out in a stirred, 2-L Parr reactor setup modified for our experiments.  The CO₂ feed was carefully controlled such that the pH of the black liquor was decreased in units of 0.5, with the first precipitation experiment taking place at a pH of 13.5 and the last at a pH of 9.5.  For each pH decrement, a lignin precipitate was collected for further analysis.  It is important to note that each collected fraction of lignin was obtained over a narrow pH change of only 0.5.  This was accomplished by removing the lignin precipitate and spent black liquor from the reactor after a given pH decrement (e.g., the 1^st run from 13.5 to 13.0 pH), and then returning the spent black liquor back to the reactor for the next CO₂ addition – and for the next pH decrement (e.g., from 13.0 to 12.5 pH for the 2^nd run).  This process was continued until a pH of 9.5 was reached.  A total of seven “narrow-pH” lignin fractions were produced. 

Matrix-Assisted Laser Desorption/Ionization Time of Flight (MALDI) mass spectrometry was used to determine the molecular weight distribution of the pH-fractionated lignin cuts described above.  The effects of different kinds of matrices, of sample preparation techniques, and of sample/lignin salt content on MALDI analysis was investigated and will also be discussed.  For determining the specific metals content of the various lignin cuts, which contribute to their ash content, Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) was used.

Finally, selected physical properties of the lignin cuts, such as softening points, densities, and viscosities, were also measured.