(591d) Controlling the Molecular Properties of a Liquid-Lignin Phase Recovered from Kraft Black Liquor

The recovery and purification of lignin from biomass side-streams is essential for the development of lignin-based chemicals, biofuels, and higher-value products.  In collaboration with Liquid Lignin Company, we have developed an elevated temperature, elevated pressure acidification process (known as SLRP®) for recovering the lignin from kraft black liquors in the form of a low-viscosity, liquid phase. 

The fact that a so-called “liquid-lignin phase” is produced in the CO₂-acidification step has significant advantages in comparison with other lignin recovery processes that are either in operation or being proposed.  By adjusting the pH of the acidification step, we are able to produce low-viscosity, easy-to-process, liquid-lignin fractions of different molecular weights, phenolic contents, and melting points.  Thus, the SLRP process can be an effective way of dealing with the complex heterogeneity (i.e., having a wide range of chemical structures and molecular weights) of lignin. 

In this study, we investigated both hardwood and softwood black liquors over a wide range of SLRP processing conditions, including temperature, pressure, starting black liquor, and pH.  Well-behaved relationships between processing conditions and molecular properties of the lignin, including molecular weight, phenolic and aliphatic hydroxyl content, and metals content (e.g., sodium, potassium, and ash), were obtained. For example, the selectivity for excluding metals from the liquid-lignin phase derived from a black liquor with 20% solids is 30% higher than that of the liquid lignin derived from a black liquor with 40% solids; the phenolic content of a fraction at pH=9.5 was 50% higher than that at pH=11.6; at the same time, the molecular weight of the 9.5 fraction was ~60% of the latter.      

Implications of the results obtained for the production of clean, low-ash lignins for applications such as biofuels and value-added bioproducts will also be discussed.