Generating ultrapure lignins from Kraft pulp mill lignins via the ALPHA technique

Lignins can now be recovered from paper mill Kraft black liquors by several processes that have recently become commercial or are under advanced development. Three of these processes are (1) LignoForce, (2) LignoBoost, and (3) SLRP (Sequential Lignin Recovery and Purification). The “Kraft lignins” obtained from these processes have ash contents of 1-2% and total metals contents of several thousand ppm. Of interest to us is the isolation of lignins that are greatly reduced in metals and ash content and could serve as biorenewable precursors for materials such as carbon fibers and composites. 

We have developed a technique for purifying Kraft lignins that can drive the metals content down to very low levels (<100ppm) in a few simple steps called ALPHA (Aqueous Lignin Purification using Hot Acids). Hot acetic acid-water mixtures that partition the lignin between a solvent-rich liquid phase and a lignin-rich liquid phase is the key feature that makes this technique work. ALPHA has been successfully applied to both LignoBoost and SLRP lignins. The technique requires only minor modification to the basic process, depending on the raw material source, in order to obtain the desired ultrapure (<100 ppm total metals) lignins (UPLs) and has been developed to operate in both batch and continuous modes. Also, a method for measuring this unique phase behavior was developed utilizing a technique known as electrochemical impedance spectroscopy. In conclusion, ALPHA has shown itself to be a robust technique for producing UPLs from different lignin sources encompassing different purities.