Precipitation of Kraft Lignin from Aqueous Solutions

Doctoral thesis, 2019

One way of improving the materials yield of a kraft paper pulp process, and simultaneously providing the industry with aromatic macromolecular structures, is to extract lignin from the black liquor. This can be done by lowering the pH of the black liquor, which will result in the precipitation of the lignin. The suspension of particles is then filtered and washed to retrieve a purified lignin in solid form. For these operations to be conducted efficiently, the solubility of kraft lignin and the course of the particle formation process needs to be understood. This thesis investigates experimentally the particle formation process on the micron-scale. Furthermore, a model describing kraft lignin particle-particle interactions on the nano-scale has been developed.

During acid-induced precipitation in model systems, the particle formation process was studied as a function of relevant process parameters such as pH, salt concentration level, temperature, presence of xylan and anionic specificity. The precipitation experiments were carried out in a precipitation vessel, where the (particle-size related) chord length distribution, in the range 1-1000 µm, was measured in situ as the kraft lignin particles were formed. The technique used (FBRM®) enabled the precipitation to be monitored as the pH was lowed. This allowed the particle concentration of various size classes to be analysed as a function of the precipitation conditions in a range relevant to industrial use.

The onset of particle formation (≥ 1 µm) was found to depend on the process conditions. Moreover, beyond the onset condition, the sizes of the particles increased as the pH decreased or the salt concentration increased; the total volume of the particles formed (≥ 1 µm) followed the same trend. The results indicate that electrostatics influence the particle formation significantly due to the ionisable phenolic and carboxylic groups on kraft lignin, in a wide range of conditions: 1-4 mol Na+/kg water and pH 13-4. The temperature dependency was also significant (45-77 °C): the particles were largest at 77 °C whilst at 45 °C, the system even underwent gelation at some conditions (pH 9, 1 M Na+). Additionally, the presence of xylan during co-precipitation with kraft lignin (5 g / 95 g lignin) retarded the build-up of agglomerates, with a larger effect being observed at 77 °C than at 65 °C. The xylan was found to be distributed evenly in the precipitated lignin.

Numerical predictions of the dispersion stability of kraft lignin nanoparticles (10-1000 nm) were made using a modified Poisson-Boltzmann model within the DLVO framework. For NaCl solutions, the predictions agreed reasonably well with the onsets of particle formation (> 1 µm) found experimentally. They were, however, less accurate for sodium sulphate based aqueous solutions, although they predicted the same anionic relative salting-out ability observed in the experiments (i.e. Cl > Sulphate) at high salt concentrations (2-4 M Na+).