Recovering liquid-liginin fractions with well-defined molecular properties via the SLRP® Process

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 the SLRP^®Process) 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 carbonation 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 carbonation step, we are able to produce low-viscosity 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 processed both hardwood and softwood black liquors over a wide range of operating conditions, including temperature, pressure, and pH, for producing liquid-lignin fractions within narrow bands (0.5-1.0 units) of pH.  Well-behaved relationships between processing conditions and molecular properties of the lignin fractions, including molecular weight, phenolic and carboxylic acid content, and metals content (e.g., sodium and ash), were obtained.  For example, for a softwood black liquor the phenolic content of a fraction recovered at pH = 9.5 was 50% higher than one obtained at pH = 11.6; at the same time, the molecular weight of the 9.5 fraction was ~60% of the latter.