(441f) Extracting Ultrapure Lignin from Biomass By-Product Streams with Tunable, Renewable Solvent Systems

Lignin is one of the most common organic compounds on earth, comprising about 30% of all organic carbon. Lignin is also unique among biopolymers in having significant aromatic character, which makes it potentially attractive for a wide range of uses, including as a replacement for petroleum-derived polymers and as a precursor for carbon fibers. Unfortunately, the commercial-grade Kraft lignins available today are severely limited in their applications because of their high metals (primarily sodium) and ash content.  Thus, less than 0.2% of the 50 million tons/yr of available lignin is being recovered.  Cost-effective separations processes must be devised to dramatically improve the purities of today’s commercial lignins, if lignin is to achieve its potential as a biopolymer.  

Clemson researchers have discovered a powerful, versatile, and renewable solvent system that can be “tuned” to reduce the sodium content of lignin derived from the Kraft pulping process from several thousand ppm down to less than 50 ppm in two simple steps.  Furthermore, this process can also be used to control the molecular weight of the recovered lignin.  Properties such as temperature, solvent composition, and solvent-to-lignin ratio have been studied to determine their effect on yields, metals selectivities, and molecular weights.  

A softwood lignin derived from a Kraft paper-mill black liquor with a sodium content of 1500 ppm and a polydispersity index of 4.1 was used as the feed in this work.  Low-ash lignins with sodium contents ranging from 25 to 200 ppm and PDIs from 1.9 to 3.1 were obtained, with the recovered lignin fractions encompassing a range of molecular weights, from under 1000 to over 10,000 Daltons.

This process has also been shown to work for both batch and semi-batch processes, and continuous processing tests are also planned.